Drugs Targeting Other Microorganisms

An important quality for an antimicrobial drug is selective toxicity, meaning that it selectively

kills or inhibits the growth of microbial targets while causing minimal or no harm to the host.

Most antimicrobial drugs currently in clinical use are antibacterial because the prokaryotic cell

provides a greater variety of unique targets for selective toxicity, in comparison to fungi,

parasites, and viruses. Each class of antibacterial drugs has a unique mode of action (the way

in which a drug affects microbes at the cellular level), and these are summarized in

Figure 14.3.114.3.1 and Table 14.3.114.3.1.

Figure 14.3.114.3.1: There are several classes of antibacterial compounds that are typically

classified based on their bacterial target.

Inhibitors of Cell Wall Biosynthesis

Several different classes of antibacterials block steps in the biosynthesis of peptidoglycan,

making cells more susceptible to osmotic lysis (Table 14.3.2

). Therefore, antibacterials that target cell wall biosynthesis are bactericidal in their action.

Because human cells do not make peptidoglycan, this mode of action is an excellent example

of selective toxicity.

Penicillin, the first antibiotic discovered, is one of several antibacterials within a class called

β-lactams. This group of compounds includes the penicillins, cephalosporins, monobactams,

and carbapenems, and is characterized by the presence of a β-lactam ring found within the

central structure of the drug molecule (Figure 14.3.2

). The β-lactam antibacterials block the crosslinking of peptide chains during the biosynthesis

of new peptidoglycan in the bacterial cell wall. They are able to block this process because the

β-lactam structure is similar to the structure of the peptidoglycan subunit component that is

recognized by the crosslinking transpeptidase enzyme, also known as a penicillin-binding

protein (PBP). Although the β-lactam ring must remain unchanged for these drugs to retain

their antibacterial activity, strategic chemical changes to the R groups have allowed for

development of a wide variety of semisynthetic β-lactam drugs with increased potency,

expanded spectrum of activity, and longer half-lives for better dosing, among other

characteristics.

Penicillin G and penicillin V are natural antibiotics from fungi and are primarily active against

gram-positive bacterial pathogens, and a few gram-negative bacterial pathogens such as

Pasteurella multocida. Figure 14.3.2

summarizes the semisynthetic development of some of the penicillins. Adding an amino group

(-NH2) to penicillin G created the aminopenicillins (i.e., ampicillin and amoxicillin) that have

increased spectrum of activity against more gram-negative pathogens. Furthermore, the

addition of a hydroxyl group (-OH) to amoxicillin increased acid stability, which allows for

improved oral absorption. Methicillin is a semisynthetic penicillin that was developed to

address the spread of enzymes (penicillinases) that were inactivating the other penicillins.

Changing the R group of penicillin G to the more bulky dimethoxyphenyl group provided

protection of the β-lactam ring from enzymatic destruction by penicillinases, giving us the first

penicillinase-resistant penicillin.

Similar to the penicillins, cephalosporins contain a β-lactam ring (Figure 14.3.2

) and block the transpeptidase activity of penicillin-binding proteins. However, the β-lactam

ring of cephalosporins is fused to a six-member ring, rather than the five-member ring found

in penicillins. This chemical difference provides cephalosporins with an increased resistance

to enzymatic inactivation by β-lactamases. The drug cephalosporin C was originally isolated

from the fungus Cephalosporium acremonium in the 1950s and has a similar spectrum of

activity to that of penicillin against gram-positive bacteria but is active against more gram-

negative bacteria than penicillin. Another important structural difference is that cephalosporin

C possesses two R groups, compared with just one R group for penicillin, and this provides for

greater diversity in chemical alterations and development of semisynthetic cephalosporins. The

family of semisynthetic cephalosporins is much larger than the penicillins, and these drugs

have been classified into generations based primarily on their spectrum of activity, increasing

in spectrum from the narrow-spectrum, first-generation cephalosporins to the broad-spectrum,

fourth-generation cephalosporins. A new fifth-generation cephalosporin has been developed

that is active against methicillin-resistant Staphylococcus aureus (MRSA).

The carbapenems and monobactams also have a β-lactam ring as part of their core structure,

and they inhibit the transpeptidase activity of penicillin-binding proteins. The only

monobactam used clinically is aztreonam. It is a narrow-spectrum antibacterial with activity

only against gram-negative bacteria. In contrast, the carbapenem family includes a variety of

semisynthetic drugs (imipenem, meropenem, and doripenem) that provide very broad-

spectrum activity against gram-positive and gram-negative bacterial pathogens.

The drug vancomycin, a member of a class of compounds called the glycopeptides, was

discovered in the 1950s as a natural antibiotic from the actinomycete Amycolatopsis orientalis.

Similar to the β-lactams, vancomycin inhibits cell wall biosynthesis and is bactericidal.

However, in contrast to the β-lactams, the structure of vancomycin is not similar to that of cell-

wall peptidoglycan subunits and does not directly inactivate penicillin-binding proteins. Rather,

vancomycin is a very large, complex molecule that binds to the end of the peptide chain of cell

wall precursors, creating a structural blockage that prevents the cell wall subunits from being

incorporated into the growing N-acetylglucosamine and N-acetylmuramic acid (NAM-NAG)

backbone of the peptidoglycan structure (transglycosylation). Vancomycin also structurally

blocks transpeptidation. Vancomycin is bactericidal against gram-positive bacterial pathogens,

but it is not active against gram-negative bacteria because of its inability to penetrate the

protective outer membrane.

The drug bacitracin consists of a group of structurally similar peptide antibiotics originally

isolated from Bacillus subtilis. Bacitracin blocks the activity of a specific cell-membrane

molecule that is responsible for the movement of peptidoglycan precursors from the cytoplasm

to the exterior of the cell, ultimately preventing their incorporation into the cell wall. Bacitracin

is effective against a wide range of bacteria, including gram-positive organisms found on the

skin, such as Staphylococcus and Streptococcus. Although it may be administered orally or

intramuscularly in some circumstances, bacitracin has been shown to be nephrotoxic

(damaging to the kidneys). Therefore, it is more commonly combined with neomycin and

polymyxin in topical ointments such as Neosporin.

The top of the image shows diagrams of various antibiotics. All have a beta-lactam ring wich

is a square made of 3 carbons and a nitrogen; one of the carbons has a double bonded O. The

antibiotics shown are penicillin, cephalosporin, monobactam and carbapenem Below is a table

with the rows: R group, Drug name, specrum of activity and route of administration. Penicillin

G has an R group of a carbon linked to a 6 carbon ring; it is active on G+ and a few G- cells,

and has a parenteral route of administration. Penicillin V has an R group of a carbon linked t

an oxygen linked to a carbon ring. IT affects G+ and a few G- and is administered orally.

Ampicillin has an R group of a Carbon attached to both an amine group and a carbon ring. It

is affective agains G+ and more G- than penicillin. It is administered orally and parenterally.

Amoxicillin has an R group similar to ampicillin but the carbon rign has an additional OH. It

has similar activity to ampicillin and is administerd orally (better than ampicillin). Methiciliin

has an R group of a carbon right with 2 CH3O attached to the ring. IT is affective against G+

only, including B-lactam producers. It is administered parenterally.

Figure 14.3.2

: Penicillins, cephalosporins, monobactams, and carbapenems all contain a β-lactam ring, the

site of attack by inactivating β-lactamase enzymes. Although they all share the same nucleus,

various penicillins differ from each other in the structure of their R groups. Chemical changes

to the R groups provided increased spectrum of activity, acid stability, and resistance to β-

lactamase degradation.

Inhibitors of Protein

Biosynthesis

The cytoplasmic ribosomes found in animal cells (80S) are structurally distinct from those

found in bacterial cells (70S), making protein biosynthesis a good selective target for

antibacterial drugs. Several types of protein biosynthesis inhibitors are discussed in this section

and are summarized in Figure 14.3.314.3.3.

Protein Synthesis Inhibitors That Bind the 30S Subunit

Aminoglycosides are large, highly polar antibacterial drugs that bind to the 30S subunit of

bacterial ribosomes, impairing the proofreading ability of the ribosomal complex. This

impairment causes mismatches between codons and anticodons, resulting in the production of

proteins with incorrect amino acids and shortened proteins that insert into the cytoplasmic

membrane. Disruption of the cytoplasmic membrane by the faulty proteins kills the bacterial

cells. The aminoglycosides, which include drugs such as streptomycin, gentamicin, neomycin,

and kanamycin, are potent broad-spectrum antibacterials. However, aminoglycosides have

been shown to be nephrotoxic (damaging to kidney), neurotoxic (damaging to the nervous

system), and ototoxic (damaging to the ear).Another class of antibacterial compounds that bind

to the 30S subunit is the tetracyclines. In contrast to aminoglycosides, these drugs

are bacteriostatic and inhibit protein synthesis by blocking the association of tRNAs with

the ribosome during translation. Naturally occurring tetracyclines produced by various strains

of Streptomyces were first discovered in the 1940s, and several semisynthetic tetracyclines,

including doxycycline and tigecycline have also been produced. Although the tetracyclines are

broad spectrum in their coverage of bacterial pathogens, side effects that can limit their use

include phototoxicity, permanent discoloration of developing teeth, and liver toxicity with high

doses or in patients with kidney impairment.

Protein Synthesis Inhibitors That Bind the 50S Subunit

There are several classes of antibacterial drugs that work through binding to the 50S subunit of

bacterial ribosomes. The macrolide antibacterial drugs have a large, complex ring structure and

are part of a larger class of naturally produced secondary metabolites called polyketides,

complex compounds produced in a stepwise fashion through the repeated addition of two-

carbon units by a mechanism similar to that used for

fatty acid synthesis. Macrolides are broad-spectrum, bacteriostatic drugs that block elongation

of proteins by inhibiting peptide bond formation between specific combinations of amino

acids. The first macrolide was erythromycin It was isolated in 1952 from Streptomyces

erythreus and prevents translocation. Semisynthetic macrolides include azithromycin and

telithromycin. Compared with erythromycin , azithromycin

has a broader spectrum of activity, fewer side effects, and a significantly longer half-life (1.5

hours for erythromycin versus 68 hours for azithromycin ) that allows for once-daily dosing

and a short 3-day course of therapy (i.e., Zpac formulation) for most infections. Telithromycin

is the first semisynthetic within the class known as ketolides. Although telithromycin shows

increased potency and activity against macrolide-resistant pathogens, the US Food and Drug

Administration (FDA) has limited its use to treatment of community-acquired pneumonia and

requires the strongest “black box warning” label for the drug because of serious hepatotoxicity.

The lincosamides include the naturally produced lincomycin and semisynthetic clindamycin.

Although structurally distinct from macrolides, lincosamides are similar in their mode of

action to the macrolides through binding to the 50S ribosomal subunit and preventing peptide

bond formation. Lincosamides are particularly active against streptococcal and staphylococcal

infections.The drug chloramphenicol represents yet another structurally distinct class of

antibacterials that also bind to the 50S ribosome, inhibiting peptide

bond formation. Chloramphenicol, produced by Streptomyces venezuelae, was discovered in

1947; in 1949, it became the first broad-spectrum antibiotic that was approved by the FDA.

Although it is a natural antibiotic, it is also easily synthesized and was the first antibacterial

drug synthetically mass produced. As a result of its mass production, broad-spectrum coverage,

and ability to penetrate into tissues efficiently, chloramphenicol was historically used to treat a

wide range of infections, from meningitis to typhoid fever to conjunctivitis. Unfortunately,

serious side effects, such as lethal gray baby syndrome, and suppression of bone marrow

production, have limited its clinical role. Chloramphenicol also causes anemia in two different

ways. One mechanism involves the targeting of mitochondrial ribosomes within hematopoietic

stem cells, causing a reversible, dose-dependent suppression of blood cell production. Once

chloramphenicol dosing is discontinued, blood cell production returns to normal. This

mechanism highlights the similarity between 70S ribosomes of bacteria and the 70S ribosomes

within our mitochondria. The second mechanism of anemia is idiosyncratic (i.e., the

mechanism is not understood), and involves an irreversible lethal loss of blood cell production

known as aplastic anemia. This mechanism of aplastic anemia is not dose dependent and can

develop after therapy has stopped. Because of toxicity concerns, chloramphenicol usage in

humans is now rare in the United States and is limited to severe infections unable to be treated

by less toxic antibiotics. Because its side effects are much less severe in animals, it is used in

veterinary medicine.The oxazolidinones, including linezolid, are a new broad-spectrum class

of synthetic protein synthesis inhibitors that bind to the 50S ribosomal subunit of both gram-

positive and gram-negative bacteria. However, their mechanism of action seems somewhat

different from that of the other 50S subunit-binding protein synthesis inhibitors already

discussed. Instead, they seem to interfere with formation of the initiation complex (association

of the 50S subunit, 30S subunit, and other factors) for translation, and they prevent

translocation of the growing protein from the ribosomal A site to the P site.

Table 14.3.314.3.3 summarizes the protein synthesis inhibitors.

Figure 14.3.314.3.3: The major classes of protein synthesis inhibitors target the 30S or 50S

subunits of cytoplasmic ribosomes.

Inhibitors of Membrane Function

A small group of antibacterials target the bacterial membrane as their mode of action (Table

14.3.4 ). The polymyxins are natural polypeptide antibiotics that were first discovered in 1947

as products of Bacillus polymyxa; only polymyxin B and polymyxin E (colistin) have been

used clinically. They are lipophilic with detergent-like properties and interact with the

lipopolysaccharide component of the outer membrane of gram-negative bacteria, ultimately

disrupting both their outer and inner membranes and killing the bacterial cells. Unfortunately,

the membrane-targeting mechanism is not a selective toxicity, and these drugs also target and

damage the membrane of cells in the kidney and nervous system when administered

systemically. Because of these serious side effects and their poor absorption from the digestive

tract, polymyxin B is used in over-the-counter topical antibiotic ointments (e.g., Neosporin),

and oral colistin was historically used only for bowel decontamination to prevent infections

originating from bowel microbes in immunocompromised patients or for those undergoing

certain abdominal surgeries. However, the emergence and spread of multidrug-resistant

pathogens has led to increased use of intravenous colistin in hospitals, often as a drug of last

resort to treat serious infections. The antibacterial daptomycin is a cyclic lipopeptide produced

by Streptomyces roseosporus that seems to work like the polymyxins, inserting in the bacterial

cell membrane and disrupting it. However, in contrast to polymyxin B and colistin, which target

only gram-negative bacteria, daptomycin specifically targets gram-positive bacteria. It is

typically administered intravenously and seems to be well tolerated, showing reversible

toxicity in skeletal muscles.

Inhibitors of Nucleic Acid Synthesis

Some antibacterial drugs work by inhibiting nucleic acid synthesis (Table 14.3.5

). For example, metronidazole is a semisynthetic member of the nitroimidazole family that is

also an antiprotozoan. It interferes with DNA replication in target cells. The drug rifampin is a

semisynthetic member of the rifamycin family and functions by blocking RNA polymerase

activity in bacteria. The RNA polymerase enzymes in bacteria are structurally different from

those in eukaryotes, providing for selective toxicity against bacterial cells. It is used for the

treatment of a variety of infections, but its primary use, often in a cocktail with other

antibacterial drugs, is against mycobacteria that cause tuberculosis. Despite the selectivity of

its mechanism, rifampin can induce liver enzymes to increase metabolism of other drugs being

administered (antagonism), leading to hepatotoxicity (liver toxicity) and negatively influencing

the bioavailability and therapeutic effect of the companion drugs.

One member of the quinolone family, a group of synthetic antimicrobials, is nalidixic acid. It

was discovered in 1962 as a byproduct during the synthesis of chloroquine, an antimalarial

drug. Nalidixic acid selectively inhibits the activity of bacterial DNA gyrase, blocking DNA

replication. Chemical modifications to the original quinolone backbone have resulted in the

production of fluoroquinolones, like ciprofloxacin and levofloxacin, which also inhibit the

activity of DNA gyrase. Ciprofloxacin and levofloxacin are effective against a broad spectrum

of gram-positive or gram-negative bacteria, and are among the most commonly prescribed

antibiotics used to treat a wide range of infections, including urinary tract infections, respiratory

infections, abdominal infections, and skin infections. However, despite their selective toxicity

against DNA gyrase, side effects associated with different fluoroquinolones include

phototoxicity, neurotoxicity, cardiotoxicity, glucose metabolism dysfunction, and increased

risk for tendon rupture.

Inhibitors of Metabolic Pathways

Some synthetic drugs control bacterial infections by functioning as antimetabolites,

competitive inhibitors for bacterial metabolic enzymes (Table 14.3.6

). The sulfonamides (sulfa drugs) are the oldest synthetic antibacterial agents and are structural

analogues of para-aminobenzoic acid (PABA), an early intermediate in folic acid synthesis

(Figure 14.3.4

). By inhibiting the enzyme involved in the production of dihydrofolic acid, sulfonamides

block bacterial biosynthesis of folic acid and, subsequently, pyrimidines and purines required

for nucleic acid synthesis. This mechanism of action provides bacteriostatic inhibition of

growth against a wide spectrum of gram-positive and gram-negative pathogens. Because

humans obtain folic acid from food instead of synthesizing it intracellularly, sulfonamides are

selectively toxic for bacteria. However, allergic reactions to sulfa drugs are common. The

sulfones are structurally similar to sulfonamides but are not commonly used today except for

the treatment of Hansen’s disease (leprosy).

Trimethoprim is a synthetic antimicrobial compound that serves as an antimetabolite within the

same folic acid synthesis pathway as sulfonamides. However, trimethoprim is a structural

analogue of dihydrofolic acid and inhibits a later step in the metabolic pathway (Figure 14.3.4

). Trimethoprim is used in combination with the sulfa drug sulfamethoxazole to treat urinary

tract infections, ear infections, and bronchitis. As discussed, the combination of trimethoprim

and sulfamethoxazole is an example of antibacterial synergy. When used alone, each

antimetabolite only decreases production of folic acid to a level where bacteriostatic inhibition

of growth occurs. However, when used in combination, inhibition of both steps in the metabolic

pathway decreases folic acid synthesis to a level that is lethal to the bacterial cell. Because of

the importance of folic acid during fetal development, sulfa drugs and trimethoprim use should

be carefully considered during early pregnancy.

The drug isoniazid is an antimetabolite with specific toxicity for mycobacteria and has long

been used in combination with rifampin or streptomycin in the treatment of tuberculosis. It is

administered as a prodrug, requiring activation through the action of an intracellular bacterial

peroxidase enzyme, forming isoniazid-nicotinamide adenine dinucleotide (NAD) and

isoniazid-nicotinamide adenine dinucleotide phosphate (NADP), ultimately preventing the

synthesis of mycolic acid, which is essential for mycobacterial cell walls. Possible side effects

of isoniazid use include hepatotoxicity, neurotoxicity, and hematologic toxicity (anemia).

PABA binds to an enzyme to produce dihydrofolic acid, which binds to another enzyme to

produce tetrahydrofolic acid and nucletides. Trimethoprim, a structural analog of dihydrofolic

acid, completely inhibits the synthesis of tetrtahydrofolic acid. Sulfonamide, a structural analog

of PABA, competitively inhibits the synthesis of dihydrofolic acid.

Figure 14.3.4

: Sulfonamides and trimethoprim are examples of antimetabolites that interfere in the bacterial

synthesis of folic acid by blocking purine and pyrimidine biosynthesis, thus inhibiting bacterial

growth.

Inhibitor of ATP Synthase

Bedaquiline, representing the synthetic antibacterial class of compounds called the

diarylquinolones, uses a novel mode of action that specifically inhibits mycobacterial growth.

Although the specific mechanism has yet to be elucidated, this compound appears to interfere

with the function of ATP synthases, perhaps by interfering with the use of the hydrogen ion

gradient for ATP synthesis by oxidative phosphorylation, leading to reduced ATP production.

Due to its side effects, including hepatotoxicity and potentially lethal heart arrhythmia, its use

is reserved for serious, otherwise untreatable cases of tuberculosis