Drug Resistance: Mechanisms and Strategies to
Overcome It
Introduction
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
Drug resistance remains a major public health challenge compromising
treatment of infectious diseases and cancer. It arises through adaptive
genetic/epigenetic changes enabling survival and proliferation despite drug
selective pressures. This review summarizes key drug resistance
mechanisms bacteria, viruses, parasites and tumors employ, and
investigates therapeutic strategies seeking to bypass resistance. Elucidating
resistance attributes advances precision therapeutics curtailing resistance
onset and clinical impacts.
Bacterial Drug Resistance Mechanisms
Efflux Pumps: Multi-drug efflux pumps like AcrAB-TolC in Enterobacteriaceae
eject structurally diverse antimicrobials decreasing intracellular
accumulation. Overexpression arises from mutations increasing pump
expression/activity, impairing transcriptional repressors or altering
membrane composition favoring efflux activity.
Permeability and Target Alterations: Mutations in porins modifying outer
membrane permeability limit drug influx. Altering DNA gyrase/topoisomerase
targets decreases drug binding affinities. Enzyme mutations hydrolyzing β-
lactams/aminoglycosides confer resistance. rRNA changes decrease
macrolide affinities.
β-lactamases: Extracellular β-lactamases hydrolyzing β-lactam rings are
encoded on plasmids/transposons spreading rapidly. Novel resistance genes
emerge through recombinatorial evolution of existing variants. Mutations
broaden substrate profiles facilitating resistance to multiple antibiotics
simultaneously.
Aminoglycoside-Modifying Enzymes: Phosphotransferases,
nucleotidyltransferases and acetyltransferases modify amino groups involved
in aminoglycoside binding, abrogating antibacterial activity. Genes co-
localize on mobile genetic elements enhancing dissemination.
Methicillin Resistance: Penicillin-binding protein 2a (PBP2a) shows reduced
affinity for β-lactams, continuing cell wall synthesis. mecA/mecC genes
encoding PBP2a spread Staphylococcus aureus resistance globally. Moreover,
additional resistance mechanisms co-select in MRSA.
Strategies Targeting Bacterial Resistance
Combination Therapy: Synergistic drug pairs circumvent resistance through
complementary, non-overlapping mechanisms, preventing/delaying
emergence. Multi-drug dosage optimizes pharmacodynamics against
heteroresistant populations enriched under monotherapy.
Repurposing Approaches: Antimicrobials used for other indications possess
undiscovered Gram-positive or Gram-negative activities. Their resistance
profiles differ, bypassing existing mechanisms. Sub-MIC levels target cellular
functions augmenting established antibiotics.
Virulence Factor Inhibitors: Blocking virulence factors stressing pathogens
heightens susceptibility to antimicrobials through fitness costs. However,
compensatory mutations may arise negating advantages. Combinations
mitigate resistance potential.
Immunotherapeutics: Antibodies, oligonucleotides and vaccines potentiating
immune defenses arm hosts to clear infections, lessening selective pressures
driving resistance evolution. Combinations address resistance offsetting
immune clearance.
Persister Cell Targeting: Non-dividing, antibiotic-tolerant persister states
contribute to chronic, recurrent infections defying therapy. Drugs/compounds
activating dormant cells resensitize them to conventional antibiotics,
improving eradication rates.
Antisense Approaches: Blocking resistance gene/operon expression utilizing
antisense oligonucleotides, DNAzymes or CRISPR limits phenotypic
resistance from reduced target levels, overcoming many resistance
mechanisms. However, compensatory evolution may circumvent antisense
activity.
Host-Directed Therapies: Controlling infection-induced host responses
dampening inflammation and oxidative/nitrosative stress mitigates selective
evolutionary pressures driving antibiotic resistance. Anti-virulence and
immunomodulatory adjuncts support host defenses too for indirect
antibacterial effects.
Viral Drug Resistance
Mutation rates: High replication rates and error-prone polymerases generate
virus mutant clouds that rapidly enrich resistant variants under drug
selective pressures before host defenses clear infection. Resistance arises
from single point mutations altering drug binding sites.
Codon deoptimization: Viruses evolve codon usage optimizing expression
and replication. Codon deoptimized synthetic vaccines/therapeutics reduce
optimal replication kinetics limiting natural variant clouds enriching rare
resistance mutations during treatment.
Error correction mechanisms: Proofreading polymerases and packaging
constraints reduce viral mutation rates, slowing resistance evolution.
However, compensatory mutations restoring fitness costs from deoptimized
codon usage or loss of proofreading arise rapidly.
Genetic recombination: RNA viruses frequently recombine segments from co-
infecting strains rapidly generating multidrug resistant mosaic genomes.
DNA viruses less often directly recombine but capture resistance genes
through host genome integration.
Strategies Targeting Viral Resistance
Combination antiviral therapy: Inhibiting distinct viral targets minimizes
chances of resistance by requiring multiple, simultaneous compensatory
changes to emerge. Synergy improves adherence reducing infection
recurrence/chance selection of resistance.
Host-targeting drugs: Inhibiting host factors essential for multiple virus
stages slows resistance by increasing genetic barriers. Compensatory
adaptations restoring dependence on restricted host factors are evolutionary
less likely.
Immune therapies: Immunotherapeutics like vaccines empowering innate
and acquired immunity complement direct-acting antivirals by limiting
clinical replication kinetics restricting development of resistance.
Deoptimized codon therapy: Synthetic live-attenuated vaccines
recombinantly deoptimized to rely on non-optimal codons effectively
outcompete wildtype clinical strains. Deoptimized codon dependencies
restrict compensatory adaptation potential.
Structural antiviral design: Conformationally flexible broad-spectrum
antivirals less prone to single point resistance through polyvalent, multi-
epitope engagements can overcome viral plasticity but depend on high
genetic barriers.
Parasitic Drug Resistance
Target masking: Parasites express surface variant antigens and proteins
shielding drug targets from recognition. Antigenic variation periodically
switches expressed variant types evading acquired immunity too.
Mutations altering drug binding: Single point mutations diminish drug binding
affinities/specificities at targets like cytochrome bc1 complex in malaria.
Multidrug resistance arises through stepwise selection.
Amplification of drug target: Increased target gene/protein copy numbers
dilute drug levels to sub-inhibitory concentrations at binding sites. However,
fitness costs from overexpression curb long-term selection.
Upregulation of drug exporters: ATP-binding cassette and mitochondrial
transporters actively efflux several antimalarials, trypanocides and anti-
leishmanials from intracellular parasites into host cells.
Strategies Against Parasitic Resistance
Combination therapies: Synergistic partner drugs block resistance by hitting
distinct targets reducing chances of simultaneous compensatory adaptations
emerging. Pharmacodynamic profiles minimize selective pressures.
Drug rotation/mixing: Periodically alternating/combining structurally
unrelated compounds with non-crossresistant profiles restricts sequential
accumulation of resistance determinants to individual agents.
Transmission blocking: Inhibitors targeting parasite sexual/mosquito stages
prevent onward transmission from treated hosts, curbing spread and
Darwinian selection of resistant variants in new hosts.
Host-targeted adjuncts: Immunomodulators like arteminisinin-primaquine
complement first-line therapies by limiting parasitemias and clinical disease
durations restricting within-host selection.
Next-generation candidates: Novel compound classes/targets limit cross-
resistance by circumventing known resistant mechanisms; designer drugs
can potentially restore activity against multidrug resistance.
Tumor Drug Resistance
Target overexpression: Amplified drug target levels expressed from
duplicated oncogenes saturate target binding sites diluting intracellular drug
levels below pharmacological thresholds.
Target mutation: Point mutations altering drug binding affinities emerge
under selective pressure. However, restored signaling maintains
proliferative/survival advantages.
Bypass signaling: Upregulated compensatory pathways independent of the
drug target restore proliferative/survival signaling when the targeted node is
inhibited.
Increased drug efflux: ATP-binding cassette transporters like P-glycoprotein
actively pump cytotoxic chemotherapy from cancer cells into extracellular
compartments, lowering intracellular accumulation.
Altered drug metabolism: Enhanced expression/activity of drug-metabolizing
cytochrome P450 enzymes metabolize/inactivate therapeutics before
reaching intended targets.
Drug inactivation: Enzymes like glutathione-S-transferases and UDP-
glucuronosyltransferases conjugating endogenous metabolites to drugs
render them physiologically inert prior to target engagement.
Repair of drug-induced DNA damage: Enhanced DNA damage
response/repair mechanisms like homologous recombination expedite
recovery from genotoxic anticancer therapies designed to induce apoptosis.
Strategies Overcoming Tumor Resistance
Combination therapy: Targeting distinct pathways synergistically using drugs
with non-overlapping resistance mechanisms prevents/delays emergence of
multi-factorial resistance. Dose optimization maintains synergistic exposures.
Third-generation EGFR inhibitors: Mutant-selective covalent inhibitors like
osimertinib overcome resistance from gatekeeper/bypass mutations in EGFR
that deactivate first/second-generation reversible inhibitors.
BCL-2 antagonists: Venetoclax overcomes apoptosis resistance by directly
inhibiting pro-survival BCL-2 protein a critical node in evading cell death.
Delivered alongside standard genotoxic therapies.
PARP inhibitors: Exploiting defective homologous recombination repair in
BRCA-mutated tumors, PARP inhibitors trigger synthetic lethality selectively
in HRR deficient cells. Clinically validated against platinum resistance.
Drug efflux pump inhibitors: Paclitaxel/irinotecan efficacy improved when
administered with P-gp inhibitors tariquidar or elacridar restoring intracellular
drug accumulations circumventing efflux.
Antibody-drug conjugates: Targeting cell surface antigens like HER2, CD19
and CD22 selectively delivers potent cytotoxins inside tumors while sparing
non-cancerous cells. Mechanism bypasses common resistance pathways.
Oncolytic viruses: Genetically engineered viruses directly killing tumor cells
and eliciting anti-tumor immunity provide therapeutic advantages less prone
to classical tumor survival/proliferation pathways driving drug resistance.
Immunotherapies: Checkpoint inhibitors and cancer vaccines empower anti-
tumor immunity through indirect mechanisms, circumventing many adaptive
signaling pathway alterations driving resistance to conventional
targeted/genotoxic agents.
Historically, therapeutic success has correlated with strategies directly
confronting intrinsic and acquired drug resistance mechanisms across major
disease areas. Future challenges involve predicting, monitoring and
preemptively overcoming emerging resistance, facilitated by systems
approaches deciphering multi-dimensional resistance networks. Combining
resistance-modulating strategies holds promise to maximize durable clinical
responses by restricting adaptive evolvability driving treatment failure.
