Action of Antimicrobials

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At the basis of all antimicrobial chemotherapy lies the concept of selec-

tive toxicity. The necessary selectivity can be achieved in several ways:

vulnerable targets within the microbe may be absent from the cells

of the host or, alternatively, the analogous targets within the host cellsmay be sufficiently different, or at least sufficiently inaccessible, for

selective attack to be possible. With agents like the polymyxins, the

organic arsenicals used in trypanosomiasis, the antifungal polyenes

and many antiviral compounds, the gap between toxicity to the

microbe and to the host is small, but in most cases antimicrobial

agents are able to exploit fundamental differences in structure and

function within the microbial cell, and host toxicity generally results

from unexpected secondary effects.

ANTIBACTERIAL AGENTS

The minute size and capacity for very rapid multiplication of

bacteria ensures that they are structurally and metabolicallyvery different from mammalian cells and, in theory, there are

numerous ways in which bacteria can be selectively killed or

disabled. In the event, it turns out that only the bacterial cell

wall is structurally unique; other subcellular structures, includ-

ing the cytoplasmic membrane, ribosomes and DNA, are built

on the same pattern as those of mammalian cells, although suf-

ficient differences in construction and organization do exist at

these sites to make exploitation of the selective toxicity prin-

ciple feasible.

Antibacterial agents have been discovered – rarely designed

– that attack each of these vulnerable sites; the most success-

ful compounds seem to be those that interfere with the con-

struction of the bacterial cell wall, the synthesis of protein, or

the replication and transcription of DNA. Relatively few clin-

ically useful agents act at the level of the cell membrane or by

interfering with specific metabolic processes of the bacterial

cell (Table 2.1).

Unless the target is located on the outside of the bacterial

cell, antimicrobial agents must be able to penetrate to the site

of action. Access through the cytoplasmic membrane is usually

achieved by passive or facilitated diffusion, or (as, for example,

with aminoglycosides and tetracyclines) by active transport

processes. In the case of Gram-negative organisms the anti-

biotic must also negotiate an outer membrane, consisting of a

characteristic lipopolysaccharide–lipoprotein complex, which

is responsible for preventing many antibiotics from reaching

an otherwise sensitive intracellular target. This lipophilic outer

membrane contains aqueous transmembrane channels

(porins), which selectively allow passage of hydrophilic mole-

cules depending on their molecular size and ionic charge

(Figure 2.1). Many antibacterial agents use porins to gain

D. Greenwood and R. Whitley

Modes of action

2

C H A P T E R

Table 2.1 Sites of action of antibacterial agents

Site Agent Principal target

Cell wall Penicillins TranspeptidaseCephalosporins Transpeptidase

Bacitracin Isoprenylphosphate

Glycopeptides Acyl-D-alanyl-D-alanine

Cycloserine Alanine racemase/synthetase

Fosfomycin Pyruvyl transferase

Isoniazid Desaturation of fatty acids

Ethambutol Arabinosyl transferases

Ribosome Chloramphenicol Peptidyl transferase

Tetracyclines Ribosomal A site

Aminoglycosides Initiation complex/translation

Macrolides Translocation

Lincosamides ?Peptidyl transferase

Fusidic acid Elongation factor G

Oxazolidinones 70S initiation complex formation

Mupirocin Isoleucyl-tRNA s ynthetase

Nucleic acid Quinolones DNA gyrase (a subunit)/

topoisomerase IV

Novobiocin DNA gyrase (b subunit)

Rifampicin RNA polymerase

5-Nitroimidazoles DNA strands

Nitrofurans DNA strands

Cell membrane Polymyxins Phospholipids

Ionophores Ion transport

Folate s ynthesis Sulfonamides Pteroate synthetase

Diaminopyrimidines Dihydrofolate reductase

02-Finch-ch2-pp 20/5/2002 12:50 pm Page 11



access to Gram-negative organisms, although other pathways

are also used.1

INHIBITORS OF BACTERIAL CELL WALLSYNTHESIS

Peptidoglycan forms the rigid, shape-maintaining layer of all

medically important bacteria except mycoplasmas. Its struc-

ture is basically similar in Gram-positive and Gram-negative

organisms, although there are important differences. In bothtypes of organism the basic macromolecular chain consists of

N -acetylglucosamine alternating with its lactyl ether, N -acetyl-

muramic acid. Each muramic acid unit carries a pentapeptide,

the third amino acid of which is L -lysine in most Gram-positive

cocci and meso-diaminopimelic acid in Gram-negative bacilli.

The cell wall is given its rigidity by cross-links between this

amino acid and the penultimate amino acid (which is always

D-alanine) of adjacent chains, with loss of the terminal amino

acid (also D-alanine) (Figure 2.2). Gram-negative bacilli have

a very thin peptidoglycan layer, which is loosely cross-linked;

Gram-positive cocci, in contrast, possess a very thick peptido-

glycan coat, which is tightly cross-linked through interpeptide

bridges. The walls of Gram-positive bacteria also differ in con-

taining considerable amounts of polymeric sugar alcohol phos-

phates (teichoic and teichuronic acids), while Gram-negative

bacteria possess an external membrane-like envelope as

described above.

Penicillins, cephalosporins and other b-lactam agents, as

well as fosfomycin, cycloserine, bacitracin and the glycopep-

tides vancomycin and teicoplanin, selectively inhibit different

stages in the construction of the peptidoglycan (Figure 2.3).

In addition, the unusual structure of the mycobacterial cell wall

is exploited by a number of antituberculosis agents.

Fosfomycin

The N -acetylmuramic acid component of the bacterial cell wall

is derived from N -acetylglucosamine by the addition of a lactic

acid substituent derived from phosphoenolpyruvate.

Fosfomycin blocks this reaction by inhibiting the pyruvyl trans-

ferase enzyme involved. The antibiotic enters bacterial cells by

active transport mechanisms for a-glycerophosphate andglucose-6-phosphate. Glucose-6-phosphate induces the hexose

phosphate transport pathway in some organisms (notably

Escherichia coli ) and potentiates the activity of fosfomycin

against those bacteria.

The related phosphonic acid, fosmidomycin, uses the same

transport pathways and is also potentiated by glucose-6-phos-

phate. However, fosmidomycin acts differently by interfering

with the formation of isopentyl diphosphate during isoprenoid

biosynthesis.2 The vulnerable enzyme is part of an alternative

mevalonate-independent pathway of isoprenoid biosynthesis

present in E. coli , malaria parasites and the chloroplasts of

higher plants, but Staphylococcus aureus uses the normal meval-

onate pathway, which probably explains the intrinsic resistance

to fosmidomycin of Gram-positive bacteria.3

Cycloserine

The first three amino acids of the pentapeptide chain of

muramic acid are added sequentially, but the terminal D-

alanyl-D-alanine is added as a unit (Figure 2.3). To form this

unit the natural form of the amino acid, L -alanine, must first

12 CHAPTER 2 MODES OF ACTION

Protein

Phospholipid

Protein

Lipoprotein

LipopolysacchandaCell wall

Slime layer

Outer

membrane

Periplasmic

space

Cyloplasmic

membrane

Porin protein

Fig. 2.1 Diagrammatic representation of the Gram-negative cell envelope. The

periplasmic space contains the peptidoglycan and some enzymes. (Reproduced with

permission from Russel AD, Quesnel LB, eds, Antibiotics: assessment of antimicrobial

activity and resistance. The Society for Applied Bacteriology Technical Series no. 18,

London: Academic Press, p. 62.)




be racemized to D-alanine and two molecules are then joined

together by D-alanine synthetase. Both of these reactions are

blocked by the antibiotic cycloserine, which is a structural

analog of D-alanine.

Glycopeptides

Once the muramylpentapeptide is formed in the cell cyto-

plasm, an N -acetylglucosamine unit is added, together with

any amino acids needed for the interpeptide bridge of Gram-

positive organisms. It is then passed to a lipid carrier molecule,

which transfers the whole unit across the cell membrane to be

added to the growing end of the peptidoglycan macromolecule

(Figure 2.3). Addition of the new building block is prevented

by the glycopeptides vancomycin and teicoplanin, which bind

to the acyl-D-alanyl-D-alanine tail of the muramylpentapep-

tide. Strains of enterococci that are able to replace the termi-

nal D-alanine with D-lactate exhibit resistance to

ANTIBACTER IAL AGENTS

Staphylococcus aureus

L- ala

D- glu

L- lys

D- ala

D- ala

peptide

D- ala

D- ala

D- glu

L- alapeptide

Escherichia coli

--NAG NAMA NAG NAMA NAG

(gly)5

L- lys (gly)5

peptide

--NAG NAMA NAG NAMA NAG

L- ala

D- glu

m- DAP

m- DAP

D- ala

D- ala

peptide

D- ala

D- ala

D- glu

L- alapeptide

NAMA NAG NAMA NAG NAMA--

NAMA NAG NAMA NAG NAMA--

peptide

Fig. 2.2 Schematic representations of the terminal stages of cell wall synthesis in Gram-positive (Staphylococcus aureus ) and Gram-

negative (Escherichia coli ) bacteria. See text for explanation. Arrows indicate formation of cross-links, with loss of terminal D-alanine; inGram-negative bacilli many D-alanine residues are not involved in cross-linking and are removed by D-alanine carboxypeptidase. NAG,

N -acetylglucosamine; NAMA, N -acetylmuramic acid; ala, alanine; glu, glutamic acid; lys, lysine; gly, glycine; m -DAP, meso-

diaminopimelic acid.

L- ala x 2

D- ala x 2

D- ala- D- alaAmino- acids

NAMA

NAG

NAMA- pentapeptide

Lipid carrier(membrane)

Transfer topeptidoglycan

Cross-linking

Fosfomycin

Cycloserine

Glycopeptides

Bacitracin

β- lactam

antibiotics

NAG

NAG= N- acetylglucosamine

NAMA= N- acetylmuramic acidDephosphorylation

of lipid

+

Fig. 2.3 Simplified scheme of bacterial cell wall synthesis, showing the sites of action of cell wall active antibiotics. NAG, N -

acetylglucosamine; NAMA, N -acetylmuramic acid. (Reproduced from Greenwood D, Antimicrobial Agents in: Greenwood D, Slack RCB,

Peutherer JF, eds, 2002 Medical Microbiology 16th edn. Edinburgh: Churchill Livingstone.)




glycopeptides.4 Because these glycopeptides are large polar

molecules they cannot penetrate the outer membrane of Gram-

negative organisms, which explains their restricted spectrum

of activity.

Bacitracin

The lipid carrier involved in transporting the cell wall build-

ing block across the membrane has been characterized as a C 55

isoprenyl phosphate. The lipid acquires an additional phos-

phate group in the transport process and must be dephospho-

rylated in order to regenerate the native compound for another

round of transfer. The cyclic peptide antibiotic bacitracin binds

to the isoprenyl pyrophosphate and prevents this dephospho-

rylation. Unfortunately, analogous reactions in eukaryotic cells

are also inhibited by bacitracin, and this may be the basis of

the toxicity of the compound.

b-Lactam antibiotics

The final cross-linking reaction that gives the bacterial cell wallits characteristic rigidity was pinpointed many years ago as the

primary target of penicillin and other b-lactam agents. These

compounds were postulated to inhibit formation of the

transpeptide bond by virtue of their structural resemblance to

the terminal D-alanyl-D-alanine unit that participates in the

transpeptidation reaction. This knowledge had to be recon-

ciled with various concentration-dependent morphological

responses that Gram-negative bacilli undergo on exposure to

penicillin and other b-lactam compounds: filamentation

(caused by inhibition of division rather than growth of the bac-

teria) at low concentrations, and the formation of osmotically

fragile spheroplasts (peptidoglycan-deficient forms that have

lost their bacillary shape) at high concentrations.

Three observations suggested that these morphologicalevents could be dissociated:

● The oral cephalosporin cefalexin (and some other b-lactam

agents, including cefradine, temocillin and the monobac-

tam, aztreonam) causes the filamentation response alone

over an extremely wide range of concentrations.

● Mecillinam (amdinocillin) does not inhibit division (and

hence does not cause filamentation in Gram-negative

bacilli), but has a generalized effect on the bacterial cell

wall.

● Combining cefalexin and mecillinam evokes the ‘typical’

spheroplast response in E. coli that neither agent induces

when acting alone.5

It was subsequently shown that isolated membranes of bac-

teria contain a number of proteins that are able to bind peni-

cillin and other b-lactam antibiotics. These penicillin-binding

proteins (PBPs) are numbered in descending order of their

molecular weight according to their separation by polyacry-

lamide gel electrophoresis. The number found in bacterial cells

varies from species to species: E. coli has at least seven and

Staph. aureus four. b-Lactam agents that induce filamentation

in Gram-negative bacilli bind to PBP3; similarly, mecillinam

binds exclusively to PBP2. Most b-lactam antibiotics, when

present in suf ficient concentration, bind to both these sites and

to others (PBP1a and PBP1b) that participate in the rapidly

lytic response of Gram-negative bacilli to many penicillins and

cephalosporins.

The low-molecular-weight PBPs (4, 5 and 6) of E. coli arecarboxypeptidases, which may operate to control the extent of

cross-linking in the cell wall. Mutants lacking these enzymes

grow normally and have thus been ruled out as targets for the

inhibitory or lethal actions of b-lactam antibiotics. The PBPs

with higher molecular weights (PBPs 1a, 1b, 2 and 3) possess

transpeptidase activity, and it seems that these PBPs represent

different forms of the transpeptidase enzyme necessary to

arrange the complicated architecture of the cylindrical or

spherical bacterial cell during growth, septation and division.

The nature of the lethal event In Gram-negative bacilli, the bactericidal effect of b-lactam

antibiotics can be quantitatively prevented by providing suf fi-

cient osmotic protection. In these circumstances the bacteriasurvive as spheroplasts, which readily revert to the bacillary

shape on removal of the antibiotic. It thus seems clear that cell

death in Gram-negative bacilli is a direct consequence of

osmotic lysis of cells deprived of the protective peptidoglycan

coat.

The nature of the lethal event in Gram-positive cocci is

more complex. Since these bacteria possess a much thicker,

tougher peptidoglycan layer than that present in the Gram-

negative cell wall, much greater damage has to be inflicted

before death of the cell ensues. However, one of the first events

that occurs on exposure of Gram-positive cocci to b-lactam

antibiotics is a release of lipoteichoic acid, an event which

appears to trigger autolysis of the peptidoglycan.

Optimal dosage effect For many strains of Gram-positive cocci, an optimal bacteri-

cidal concentration of b-lactam antibiotics can be identified

above which the killing effect is reduced, sometimes very strik-

ingly. The basis of this effect, often called the ‘Eagle phenom-

enon’ after its discoverer, has never been satisfactorily

explained. A plausible hypothesis is that the lethal event is trig-

gered by low concentrations of the antibiotic as a consequence

of binding to one particular target protein; binding at higher

concentrations to other targets (PBPs) stops the bacterial cell

from growing and this antagonizes the lethal effect, which

requires continued cell growth.

PersistersAbout 1 in 105 bacteria in a culture exposed to b-lactam antibi-

otics survive, even on prolonged exposure to an optimal bac-

tericidal concentration. These ‘persisters’ have not acquired

resistance, since, if the antibiotic is removed and they are

allowed to grow, most of their immediate progeny are killed

on re-exposure, just as the parent culture is. These bacteria

may be cells in which the peptidoglycan exists transiently as a





complete covalently linked macromolecule. Inhibition by the

antibiotic of autolytic enzymes needed to create growth points

in the peptidoglycan would effectively trap the cells in a state

in which they could not grow (and therefore could not be

killed) until the antibiotic is removed.

ToleranceIn some Gram-positive cocci there may be a marked dissoci-

ation between the concentrations of b-lactam agents (and gly-

copeptides) required to achieve a bacteristatic and a

bactericidal effect. The organisms are not ‘resistant’ since they

remain fully susceptible to the inhibitory activity of the antibi-

otic, although the bactericidal effect is reduced. Defective

autolysins remain the most likely explanation of the effect,

which has some similarities to the persister phenomenon.6

Antimycobacterial agents

Agents acting specifically against Mycobacterium tuberculosis and

other mycobacteria have been less well characterized than other

antimicrobial drugs. However, it is thought that several of themowe their activity to selective effects on the unique structure

of the mycobacterial envelope.7 Thus, although isoniazid has

been found to interfere with various cellular functions of bac-

teria, it is likely that it owes its specific bactericidal activity

against M. tuberculosis to interference with mycolic acid syn-

thesis. The effect is achieved by inhibition of a fatty acid desat-

urase after intracellular oxidation of isoniazid to an active

product.8 Ethionamide, prothionamide and pyrazinamide,

which are related nicotinic acid derivatives, are also thought

to undergo intracellular modification and to act in a similar

fashion.

Ethambutol, a slow acting and primarily bacteriostatic

antimycobacterial agent, inhibits arabinosyl transferases. These

enzymes bring about the polymerization of arabinose to formarabinan, a polysaccharide component of the core polymers of

the mycobacterial cell wall.8

INHIBITORS OF BACTERIAL PROTEINSYNTHESIS

The amazing process by which the genetic message in DNA is

translated into large and unique protein molecules is univer-

sal; in prokaryotic, as in eukaryotic cells, the workbench is the

ribosome, composed of two distinct subunits, each a complex

of ribosomal RNA (rRNA) and numerous proteins. However,

bacterial ribosomes are open to selective attack because they

differ from their mammalian counterparts in both protein and

RNA content; indeed they can be readily distinguished in the

ultracentrifuge: bacterial ribosomes exhibit a sedimentation

coefficient of 70S (composed of 30S and 50S subunits),

whereas mammalian ribosomes display a coefficient of 80S

(composed of 40S and 60S subunits).

In the first stage of bacterial protein synthesis, messenger

RNA (mRNA), transcribed from the appropriate region of

DNA, binds to the smaller ribosomal subunit and attracts N -

formylmethionyl transfer RNA (fMet-tRNA) to the initiator

codon AUG. The larger subunit is then added to form a com-

plete initiation complex. fMet-tRNA occupies the P (peptidyl

donor) site; adjacent to it is the A (aminoacyl acceptor) site

aligned with the next trinucleotide codon of the mRNA.

Transfer RNA (tRNA) bearing the appropriate anticodon, andits specific amino acid, enters the A site, and a peptidyl trans-

ferase joins N -formylmethionine to the new amino acid with

loss (via an exit site) of the tRNA in the P site; the first peptide

bond of the protein has been formed. A translocation event

then moves the remaining tRNA with its dipeptide to the P site

and concomitantly aligns the next triplet codon of mRNA with

the now vacant A site. The appropriate aminoacyl-tRNA enters

the A site and the transfer process and subsequent transloca-

tion are repeated. In this way, the peptide chain is built up in

precise fashion, faithful to the original DNA blueprint, until a

so-called ‘nonsense’ codon is encountered on the mRNA that

signals chain termination and release of the peptide chain. The

mRNA is disengaged from the ribosome, which dissociates into

its two subunits ready to form a new initiation complex. Withinbacterial cells, many ribosomes are engaged in protein syn-

thesis during active growth, and a single strand of mRNA may

interact with many ribosomes along its length to form a

polysome.

Many antibiotics interfere with the process of protein syn-

thesis (Figure 2.4). Some, like puromycin, which is an analog

of the aminoacyl tail of charged tRNA and causes premature

chain termination, act on bacterial and mammalian ribosomes

alike and are therefore unsuitable for systemic use in humans.

Therapeutically useful inhibitors of protein synthesis include

many of the naturally occurring antibiotics, such as chloram-

phenicol, tetracyclines, aminoglycosides, fusidic acid,

macrolides, lincosamides and streptogramins. Some newer

agents, including mupirocin and the oxazolidinones, also actat this stage.

Chloramphenicol

The molecular target for chloramphenicol is the peptidyl trans-

ferase enzyme that links amino acids in the growing peptide

chain. The effect of the antibiotic is thus to freeze the process

of chain elongation, bringing bacterial growth to an abrupt halt.

The process is completely reversible, and chloramphenicol is

fundamentally a bacteristatic agent. The binding of chloram-

phenicol to the 50S subunit of 70S ribosomes is highly specif-

ic. The basis for the rare, but fatal, marrow aplasia associated

with this compound is not therefore a generalized effect on

mammalian protein synthesis, although mitochondrial ribo-

somes, which are similar to those of bacteria, may be involved.

Tetracyclines

Antibiotics of the tetracycline group are actively transported

into bacterial cells and attach to 30S ribosomal subunits in

such a manner as to prevent binding of incoming aminoacyl-





tRNA to the A site.9 The effect is to halt chain elongation and,

like chloramphenicol, these antibiotics are predominantly bac-

teristatic.

Tetracyclines also penetrate into mammalian cells (indeed,

the effect on chlamydiae depends on this) and can interfere

with protein synthesis on eukaryotic ribosomes. Fortunately,

cytoplasmic ribosomes are not affected at the concentrations

achieved during therapy, although mitochondrial ribosomes

are. The selective toxicity of tetracyclines thus presents some-

thing of a puzzle, the solution to which is presumably that these

antibiotics are not actively concentrated by mitochondria as

they are by bacteria and concentrations reached are insuf ficient

to deplete respiratory chain enzymes.9

Aminoglycosides

Much of the literature on the mode of action of aminoglyco-

sides has concentrated on streptomycin. However, the action

of gentamicin and other deoxystreptamine-containing amino-

glycosides is clearly not identical, since they bind at more than

one site on the ribosome, whereas streptomycin binds in a 1:1

ratio at a unique site. One consequence of this is that single-

step, high-level resistance to streptomycin, which is due to a

change in a specific protein of the 30S ribosomal subunit, does

not extend to other aminoglycosides.

Elucidation of the mode of action of aminoglycosides has

been complicated by the need to reconcile a variety of enig-

matic observations:

● Streptomycin and other aminoglycosides cause misreading

of mRNA on the ribosome while paradoxically halting pro-

tein synthesis completely by interfering with the formation

of functional initiation complexes.

● Inhibition of protein synthesis by aminoglycosides leads

not just to bacteriostasis as with, for example, tetracycline

or chloramphenicol, but also to rapid cell death.

● Susceptible bacteria (but not those with resistant ribo-

somes) quickly become leaky to small molecules on expo-

sure to the drug, apparently because of an effect on the cell

membrane.


INITIATION

accurate reading of genetic code and association of peptidyl- tRNA

with P site

ELONGATION

MacrolidesFusidic acid

Streptomycin

Gentamicin

Tobramycin

Amikacin

TERMINATION

AUG

Pre-initiation

complex

50S subunit

30S subunit

Initiation factors

AUG GCU CGC

fMet

AUG GCU CGC

fMet ala

AUG GCU CGC

ala

fMetfMet

AUG GCU CGC

ala

Peptidyl

(P) site

Acceptor

(A) site

mRNA

Initiator formylmethionine

tRNA

Linezolid

Formation of

70S initiation

complex

Tetracyclines

Peptide

product

Delivery of charged

tRNA e.g. Ala- tRNA

by elongation

factor Tu

Empty

A site

Growing

polypeptide

chain Spectinomycin

Translocation of

growing

polypeptide fromA to P site driven by

elongation factor G

Movement between

mRNA and

ribosome

Peptide bond

formation by

peptidyl transferase

Chloramphenicol

Lincosamides

Streptogramins

mRNA

ala

fMet

+

Fig. 2.4 The process of protein synthesis and the steps inhibited by various antibacterial agents. (Adapted from Chopra I, Greenwood

D, Antibacterial agents: basis of action, in: Encyclopedia of Life Sciences, Nature Publishing Group, London: www.els.net). PERMISSION

NOT YET RECEIVED.




● Mutations can occur that render the bacterial cell depen-

dent on streptomycin for growth.

● Susceptibility is dominant over resistance in merozygotes

that are diploid for the two allelic forms.

A well-lit path through this maze has not yet been defini-

tively charted, but the situation is slowly becoming clearer. Thetwo effects of aminoglycosides on initiation and misreading

may be explained by a concentration-dependent effect on ribo-

somes engaged in the formation of the initiation complex and

those in the process of chain elongation:10 in the presence of a

sufficiently high concentration of drug, protein synthesis is

completely halted once the mRNA is run off because re-initi-

ation is blocked; under these circumstances there is little or no

opportunity for misreading to occur. However, at concentra-

tions at which only a proportion of the ribosomes can be

blocked at initiation, some protein synthesis will take place and

the opportunity for misreading will be provided.

The dominance of susceptibility over resistance has been

tentatively explained by the fact that the non-functional initi-

ation complexes formed in the presence of aminoglycosidesare unstable, so that the ribosomes continuously dissociate

from mRNA and recycle in an inoperative form. These crip-

pled ribosomes (of which there are twice as many as there are

resistant ones in merozygotes) are hence continuously made

re-available to sequester newly formed mRNA and prevent the

resistant ribosomes from maintaining a supply of the polypep-

tides that the cell needs.10

The effects of aminoglycosides on membrane permeability,

and the potent bactericidal activity of these compounds,

remain enigmatic. However, the two phenomena may be

related.11 In bacteria, as in mammalian cells, some of the ribo-

somes (presumably those engaged in transmembrane protein

transfer) may be membrane bound. Moreover, aminoglyco-

sides enter bacteria by an active transport process (absent inanaerobes and streptococci, hence their inherent resistance)

that is dependent on protein synthesis. It is possible that site-

specific uptake of the drug at ribosomal attachment sites and

subsequent binding to the ribosome – membrane complex may

lead to membrane leakiness and cell death.12

SpectinomycinThe aminocyclitol antibiotic spectinomycin, often considered

alongside the aminoglycosides, binds in reversible fashion

(hence the bacteriostatic activity) to ribosomal RNA of the 30S

subunit. There it interrupts the translocation event that occurs

as the next codon of mRNA is aligned with the A site in readi-

ness for the incoming aminoacyl-tRNA.

Macrolides, lincosamides, streptogramins

These antibiotic groups are structurally very different, but bind

to closely related sites on the 50S ribosome of bacteria. One

consequence of this is that staphylococci exhibiting inducible

resistance to erythromycin, which is caused by methylation of

certain adenine residues in the rRNA, also become resistant

to other macrolides, lincosamides and streptogramin B in the

presence of erythromycin (p. 000).

The detailed mode of action of these antibiotics has not yet

been definitively worked out. Erythromycin (like type A strep-

togramins; see below) binds almost exclusively to free ribo-

somes and brings protein synthesis to a halt after formation of

the initiation complex, probably by interfering with the translo-cation reaction. The precise mechanism of action of lin-

cosamides is less clear, but they appear to interfere indirectly

with the peptidyl transferase reaction, possibly by blocking the

P site.

The streptogramins are composed of two interacting com-

ponents designated A and B (p. 000). The type A molecules

bind to 50S ribosomal subunits and affect both donor and

acceptor functions of peptidyl transferase by blocking attach-

ment of aminoacyl-tRNA to the catalytic site of the enzyme

and subsequent transfer of the growing peptide chain. Type B

streptogramins occupy an adjacent site on the ribosome and

prevent formation of the peptide bond, leading to the prema-

ture release of incomplete polypeptides.13 Type A molecules

bind to free ribosomes, but not to polysomes engaged inprotein synthesis, whereas type B can prevent further synthe-

sis during active synthesis. The bactericidal synergy between

the two components arises mainly from conformational

changes induced by Type A molecules that improve the

binding af finity of Type B compounds.14

Fusidic acid

Fusidic acid forms a stable complex with an elongation factor

(EF-G) involved in translocation and with guanosine triphos-

phate (GTP), which provides energy for the translocation

process. One round of translocation occurs, with hydrolysis of

GTP, but the fusidic acid – EF – G – GDP – ribosome complex

blocks further chain elongation, leaving peptidyl-tRNA in theP site.

Although protein synthesis in Gram-negative bacilli – and,

indeed, mammalian cells – is susceptible to fusidic acid, the

antibiotic penetrates poorly into these cells and the spectrum

of action is virtually restricted to Gram-positive bacteria,

notably staphylococci.

Oxazolidinones

Linezolid and other oxazolidinones are bacteriostatic agents

that act at an earlier stage than other inhibitors of protein syn-

thesis. They prevent the process by which the 50S ribosomal

subunit and the 30S unit (charged with mRNA and fMet-

tRNA) come together to form the 70S initiation complex.15,16

This is achieved by binding to the 50S subunit, at a site close

to that of chloramphenicol and lincosamides.17

Mupirocin

Mupirocin also has a unique mode of action. The epoxide-

containing monic acid tail of the molecule (see p. 000) is an





analog of isoleucine and, as such, is a competitive inhibitor of

isoleucyl-tRNA synthetase in bacterial cells.18 It may be a

bifunctional inhibitor of the synthetase, since it also binds to

the ATP-binding site of the enzyme.19 The corresponding

mammalian enzyme is unaffected.

INHIBITORS OF NUCLEIC ACIDSYNTHESIS

Compounds that bind directly to the double helix are gener-

ally highly toxic to mammalian cells and only a few – those that

interfere with DNA-associated enzymic processes – exhibit suf-

ficient selectivity for systemic use as antibacterial agents. These

compounds include antibacterial quinolones, novobiocin and

rifampicin (rifampin). Diaminopyrimidines, sulfonamides, 5-

nitroimidazoles and (probably) nitrofurans also affect DNA

synthesis and will be considered under this heading.

Quinolones

The problem of packaging the enormous circular chromosome

of bacteria (>1 mm long) into a microscopic cell, while making

adequate arrangements for transcription and replication, has

necessitated some considerable ingenuity on the part of the

microbe. The solution has been to condense the DNA down

and to twist it into a ‘supercoiled’ state – a process aided by

the natural strain imposed on a covalently closed double helix.

The twists are introduced in the opposite sense to those of the

double helix itself and the molecule is said to be negatively

supercoiled. Relaxation and re-establishment of the supercoiled

state involves precisely regulated nicking and resealing of the

DNA strands, accomplished by enzymes called topoisomeras-

es. One topoisomerase, DNA gyrase, is a tetramer composed

of two pairs of a and b subunits, and the primary target of theaction of nalidixic acid and other quinolones is the a subunit

of DNA gyrase, although another enzyme, topoisomerase IV,

is also affected.20 Indeed, in Gram-positive bacteria, topoiso-

merase IV seems to be the main target.21 This enzyme does

not have supercoiling activity; it appears to be involved in

relaxation of the DNA chain and chromosomal segregation.

Fortunately, the corresponding mammalian topoisomerases

are less susceptible to quinolone attack.

Curiously, the coumarin antibiotic novobiocin, which acts

in a complementary fashion by binding specifically to the b-

subunit of DNA gyrase, displays an exactly opposite spectrum

of activity to that of nalidixic acid.

RifampicinRifampicin and other compounds of the ansamycin group

specifically inhibit DNA-dependent RNA polymerase; that is,

they prevent the transcription of RNA species from the DNA

template. Rifampicin is an extremely ef ficient inhibitor of the

bacterial enzyme, but fortunately eukaryotic RNA polymerase

is not affected. RNA polymerase consists of a core enzyme

made up of four polypeptide subunits, and rifampicin specifi-

cally binds to the b subunit. However, since isolated b subunit

does not bind rifampicin, the precise configuration in which it

is locked into the core enzyme is important.

Sulfonamides and diaminopyrimidines

These agents act at separate stages in the pathway of folic acid

synthesis and thus act indirectly on DNA synthesis, since the

active form of the co-enzyme, tetrahydrofolic acid, serves as

intermediate in the transfer of methyl, formyl and other single-

carbon fragments in the biosynthesis of purine nucleotides and

thymidylic acid, as well as of some amino acids.

Sulfonamides are analogues of p-aminobenzoic acid. They

competitively inhibit dihydropteroate synthetase, the enzyme

which condenses p-aminobenzoic acid with dihydropteroic acid

in the early stages of folic acid synthesis. Most bacteria need

to synthesize folic acid and cannot use exogenous sources of

the vitamin. Mammalian cells, in contrast, require preformed

folate and this is the basis of the selective action of sulfon-

amides. The antileprotic sulfone dapsone, and the antituber-culosis drug p-aminosalicylic acid, act in a similar way; the

basis for their restricted spectrum may reside in differences of

af finity for variant forms of dihydropteroate synthetase in the

bacteria against which they act.

Diaminopyrimidines act later in the pathway of folate syn-

thesis. These compounds inhibit dihydrofolate reductase, the

enzyme that generates the active form of the co-enzyme

tetrahydrofolic acid. In most of the reactions in which tetrahy-

drofolate takes part it remains unchanged, but in the biosyn-

thesis of thymidylic acid tetrahydrofolate acts as hydrogen

donor as well as a methyl group carrier and is thus oxidized to

dihydrofolic acid in the process. Dihydrofolate reductase is

therefore crucial in recycling tetrahydrofolate, and diaminopy-

rimidines act relatively quickly to halt bacterial growth.Sulfonamides, in contrast, cut off the supply of folic acid at

source and act slowly, since the existing folate pool can satisfy

the needs of the cell for several generations.

The selective toxicity of diaminopyrimidines comes about

because of differential affinity of these compounds for dihy-

drofolate reductase from various sources. Thus trimethoprim

has a vastly greater af finity for the bacterial enzyme than for its

mammalian counterpart, pyrimethamine exhibits a particular-

ly high af finity for the plasmodial version of the enzyme and,

in keeping with its anticancer activity, methotrexate has high

af finity for the enzyme found in mammalian cells.

5-Nitroimidazoles

The most intensively investigated compound in this group is

metronidazole, but other 5-nitroimidazoles are thought to act

in a similar manner. Metronidazole siphons off electrons from

ferredoxin (or other electron transfer proteins with low redox

potential) causing the nitro group of the drug to be reduced.

It is this reduced and highly reactive intermediate that is

responsible for the antimicrobial effect, probably by binding





to DNA, which undergoes strand breakage.22 The requirement

for interaction with low redox systems restricts the activity

largely to anaerobic bacteria and certain protozoa that exhibit

anaerobic metabolism. The basis for activity against micro-

aerophilic species such as Helicobacter pylori and Gardnerella

vaginalis remains speculative, though a novel nitroreductase,

which is altered in metronidazole-resistant strains, is implicat-ed in H. pylori .23

Nitrofurans

As with nitroimidazoles, the reduction of the nitro group of

nitrofurantoin and other nitrofurans is a prerequisite for

antibacterial activity. Micro-organisms with appropriate

nitroreductases act on nitrofurans to produce a highly reactive

electrophilic intermediate and this is postulated to affect DNA

as the reduced intermediates of nitroimidazoles do. Other evi-

dence suggests that the reduced nitrofurans bind to bacterial

ribosomes and prevent protein synthesis;24 inducible enzyme

synthesis seems to be particularly susceptible. An effect on

DNA has the virtue of explaining the known mutagenicity of these compounds in vitro and any revised mechanism relating

to inhibition of protein synthesis needs to be reconciled with

this property.

AGENTS AFFECTING MEMBRANEPERMEABILITY

Agents acting on cell membranes do not normally discriminate

between microbial and mammalian membranes, although the

fungal cell membrane has proved more amenable to selective

attack (see below). The only membrane-active antibacterial

agents to be administered systemically in human medicine,

polymyxin B (now rarely used systemically) and the closelyrelated compound colistin (polymyxin E), act like cationic

detergents; they disrupt the cytoplasmic membrane of the cell,

probably by attacking the exposed phosphate groups of the

membrane phospholipid. They also have an effect on the exter-

nal membrane of Gram-negative bacilli, which might explain

their preferential action on these organisms. The end result is

leakage of cytoplasmic contents and death of the cell. Various

factors, including growth phase and incubation temperature,

alter the balance of fatty acids within the bacterial cell mem-

brane, and this can concomitantly affect the response to

polymyxins.25

Several antibiotics, known collectively as ionophores, inter-

fere with cation transport in cell membranes. These include

the topical antibiotic gramicidin A, and some agents used in

veterinary medicine, such as the macrotetralide monensin and

the depsipeptide valinomycin. Naturally occurring antimicro-

bial peptides, such as the cecropins, magainins and defensins,

as well as the lanthionine-containing lantibiotics, disrupt cell

membranes, sometimes in a selective manner; some of these

peptides appear to form aggregates with ionophoric proper-

ties.26,27

ANTIFUNGAL AGENTS

In view of the scarcity of antibacterial agents acting on the cyto-

plasmic membrane, it is surprising to find that the most suc-

cessful groups of antifungal agents – the polyenes, azoles, and

allylamines – all achieve their effects in this way.28,29 However,

the echinocandins, the latest addition to the antifungal arma-mentarium, differ in affecting the fungal cell wall.

POLYENES

The polyenes bind only to membranes containing sterols; ergos-

terol, the predominant sterol of fungal membranes, appears to

be particularly susceptible. The effect is to make the membrane

leaky, probably by the formation of transmembrane pores. Since

bacterial cell membranes (except those of mycoplasmas) do not

contain sterols, they are unaffected by polyenes, even in high

concentration; unfortunately, this immunity does not extend to

sterol-containing mammalian cells, and polyenes consequent-

ly exhibit a low therapeutic index.

AZOLES

The activity of the antifungal azoles is also dependent on the

presence of ergosterol in the fungal cell membrane. These

compounds block ergosterol synthesis by interfering with the

demethylation of its precursor, lanosterol.30 Lanosterol

demethylase is a cytochrome P450 enzyme and, although azole

antifungals have much less influence on analogous mammalian

systems, some of the side effects are attributable to such action.

Antifungal azole derivatives are predominantly fungistatic

but some compounds, notably miconazole and clotrimazole,

kill fungi at concentrations higher than those which merelyinhibit growth, apparently by causing direct membrane

damage. Other, less well characterized, effects of azoles on

fungal respiration have also been described.31

ALLYLAMINES

The antifungal allylamine derivatives terbinafine and naftifine

inhibit squalene epoxidase, another enzyme involved in the

biosynthesis of ergosterol.32 The fungicidal effect may be due

to accumulation of squalene rather than a deficiency of ergos-

terol. In Candida albicans the effect is fungistatic and the yeast

form is less susceptible than is mycelial growth. In this species

there is less accumulation of squalene than in dermatophytes,

and ergosterol deficiency may be the limiting factor.33

ECHINOCANDINS

Caspofungin and related compounds inhibit the formation of

glucan, an essential polysaccharide of the cell wall of many

ANTIFUNGAL AGENTS




fungi, including Pneumocystis carinii . The vulnerable enzyme

is b-1,3-glucan synthase, which is located in the cell mem-

brane.34

FLUCYTOSINE

The spectrum of activity of flucytosine (5-fluorocytosine) is

virtually restricted to yeasts. In these fungi flucytosine is trans-

ported into the cell by a cytosine permease; a cytosine deam-

inase then converts flucytosine to 5-fluorouracil, which is

incorporated into RNA in place of uracil, leading to the for-

mation of abnormal proteins. There is also an effect on DNA

synthesis through inhibition of thymidylate synthetase.35

GRISEOFULVIN

The antidermatophyte antibiotic griseofulvin binds to the

microtubules of the mitotic spindle, interfering with their

assembly and function. However, the precise mechanism of action and the basis of the selectivity remain to be elucidated.

ANTIPROTOZOAL AGENTS

The actions of some antiprotozoal drugs overlap with, or are

analogous to, those seen with the antibacterial and antifungal

agents already discussed. Thus, the activity of 5-nitroimida-

zoles such as metronidazole extends to those protozoa that

exhibit an essentially anaerobic metabolism; the antimalarial

agents pyrimethamine and cycloguanil (the metabolic product

of proguanil), like trimethoprim, inhibit dihydrofolate reduc-

tase; some polyenes and antifungal imidazoles display suffi-

cient activity against Leishmania and certain other protozoa forthem to have received attention as potential therapeutic agents.

There seems to be deep uncertainty about how other

antiprotozoal agents actually work. Various sites of action have

been ascribed to many of them and, with a few notable excep-

tions, the literature reveals only desultory attempts to pin down

the primary target.

ANTIMALARIAL AGENTS

Quinoline antimalarials

Quinine and the various quinoline antimalarials were once

thought to achieve their effect by intercalation with plasmodi-

al DNA after concentration in parasitized erythrocytes.

However, these effects occur only at concentrations in excess

of those achieved in vivo; moreover, a non-specific effect on

DNA does not explain the selective action of these compounds

at precise points in the plasmodial life cycle, or the differential

activity of antimalarial quinolines.

Clarification of the mode of action of these compounds has

proved elusive, but it now seems likely that chloroquine and

related compounds act primarily by inhibiting haem poly-

merase, thus preventing detoxification of ferriprotoporphyrin

IX (heme), which is produced from the red cell hemoglobin

in the food vacuole of the parasites.36 Ferriprotoporphyrin IX

is a toxic metabolite which is normally rendered innocuous by

polymerization; malarial pigment consists of granules of thispolymer.

Chloroquine achieves a very high concentration within the

food vacuole of the parasite and this greatly aids its activity.

However, quinine and mefloquine are not concentrated to the

same extent, and have much less effect on heme polymeriza-

tion, raising the possibility that other (possibly multiple) targets

are involved in the action of these compounds.37,38

8-Aminoquinolines like primaquine, which, at therapeuti-

cally useful concentrations exhibit selective activity against

liver-stage parasites and gametocytes, possibly inhibit mito-

chondrial enzyme systems after undergoing hepatic metabo-

lism. However, the precise mechanism of action is unknown.

Artemisinin

Artemisinin, the active principle of the Chinese herbal remedy

qinghaosu, has several effects on malaria parasites, but the

activity appears to be due chiefly to the reactivity of the

endoperoxide bridge. This is cleaved in the presence of heme

or free iron within the parasitized red cell to form a short-lived,

but highly reactive, free radical that irreversibly alkylates

malaria proteins.39

Atovaquone

The hydroxynaphthoquinone atovaquone, which exhibits anti-

malarial and antipneumocystis activity, is an electron transport

inhibitor that causes depletion of the ATP pool. The primaryeffect is on the iron flavoprotein dihydro-orotate dehydroge-

nase, an essential enzyme in the production of pyrimidines.

Mammalian cells are able to avoid undue toxicity by use of

preformed pyrimidines.40 Dihydro-orotate dehydrogenase from

Plasmodium falciparum is inhibited by concentrations of ato-

vaquone that are very much lower than those needed to inhibit

the pneumocystis enzyme, raising the possibility that the

antimicrobial consequences might differ in the two organ-

isms.41

OTHER ANTIPROTOZOAL AGENTS

Arsenical compounds, which are still the mainstay of treatment

of African sleeping sickness, poison the cell by an effect on

glucose catabolism and are consequently very toxic to the host.

The mechanism by which this is achieved and the basis for any

selective action are not well understood, though they are

known to bind to essential thiol groups. The primary target

may be trypanothione, which substitutes for glutathione in try-

panosomes, and this may aid the selective toxicity.42





The actions of other agents with antitrypanosomal activity,

including suramin and pentamidine, are also poorly charac-

terized.43 Various cell processes, mainly those involved in gly-

colysis within the specialized glycosomes of protozoa of the

trypanosome family, have been implicated in the action of

suramin.44 Pentamidine and other diamidines disrupt the try-

panosomal kinetoplast, a specialized DNA-containingorganelle, probably by binding to DNA, though they also inter-

fere with polyamine synthesis.45

Laboratory studies of leishmania are hampered by the fact

that in-vitro culture yields promastigotes that are morpholog-

ically and metabolically different from the amastigotes involved

in disease. Such evidence as is available suggests that the pen-

tavalent antimonials commonly used for treatment inhibit gly-

colysis in leishmanial glycosomes. Antifungal azoles take

advantage of similarities in sterol biosynthesis among fungi and

leishmanial amastigotes.46

Eflornithine (difluoromethylornithine) is a selective inhibitor

of ornithine decarboxylase and achieves its effect by depleting

the biosynthesis of polyamines such as spermidine, a precur-

sor of trypanothione.47

The corresponding mammalian enzymehas a much shorter half-life than its trypanosomal counterpart,

and this may account for the apparent selectivity of action. The

preferential activity against Trypanosoma brucei gambiense rather

than the related rhodesiense form may be due to reduced drug

uptake or differences in polyamine metabolism in the latter

subspecies.48

Several of the drugs used in amebiasis, including the plant

alkaloid emetine and diloxanide furoate appear to interfere with

protein synthesis within amebic trophozoites or cysts.49

ANTHELMINTIC AGENTS

Just as the cell wall of bacteria is a prime target for selectiveagents and the cell membrane is peculiarly vulnerable in

fungi, so the neuromuscular system appears to be the

Achilles’ heel of parasitic worms. Despite the fact that present

understanding of the neurobiology of helminths is extremely

meagre, a considerable number of anthelmintic agents have

been shown to work by paralysing the neuromusculature in

various ways. Such compounds include piperazine, prazi-

quantel, levamisole, pyrantel pamoate, ivermectin, metri-

fonate (trichlorfon) and dichlorovos.50 – 52 Praziquantel

induces schistosomes to disengage from their intravascular

attachment site and migrate to the liver, but there is also a

profound effect on schistosome metabolism and disruption of

the tegument, causing exposure of parasite antigens. All these

effects appear to be referable to alterations in calcium home-

ostasis.53

A notable exception to the general rule that anthelmintic

agents act on the neuromuscular systems of worms is provid-

ed by the benzimidazole derivatives, including mebendazole

and albendazole. These broad-spectrum anthelmintic drugs

seem to have at least two effects on adult worms and larvae:

inhibition of the uptake of the chief energy source, glucose;

and binding to tubulin, the structural protein of micro-

tubules.54

The basis of the activity of the antifilarial drug diethylcar-

bamazine has long been a puzzle, since the drug has no effect

on microfilaria in vitro. Consequently it seems likely that the

effect of drug observed in vivo is due to alterations in the

surface coat, making them responsive to immunologicalprocesses from which they are normally protected.55 The

source of its effect on adult filarial worms is unknown.

ANTIVIRAL AGENTS

The prospects for the development of selectively toxic antivi-

ral agents were long thought to be poor, since the life cycle of

the virus is so closely bound to normal cellular processes.

However, closer scrutiny of the relationship of the virus to the

cell reveals several points at which the viral cycle might be

interrupted.56 These include:

● Adsorption to and penetration of the cell.

● Uncoating of the viral nucleic acid.

● The various stages of nucleic acid replication.

● Assembly of the new viral particles.

● Release of infectious virions (if the cell is not destroyed).

NUCLEOSIDE ANALOGS

In the event, it is the process of viral replication (which is

extremely rapid relative to most mammalian cells) that has

proved to be the most vulnerable point of attack, and most

clinically useful antiviral agents are nucleoside analogs.

Among these, only aciclovir (acycloguanosine) and penci-

clovir (the active product of the oral agent famciclovir) exhibita genuine selectivity. In order to achieve their antiviral effect,

nucleoside analogs have to be converted within the cell to the

triphosphate derivative. In the case of aciclovir and penci-

clovir the initial phosphorylation, yielding aciclovir or penci-

clovir monophosphate, is accomplished by a thymidine kinase

coded for by the virus itself. The corresponding cellular

thymidine kinase phosphorylates these compounds very inef-

ficiently and consequently only cells harbouring the virus are

affected. Moreover, the triphosphates of aciclovir and penci-

clovir inhibit viral DNA polymerase more ef ficiently than the

cellular enzyme; this is another feature of their selective activ-

ity. As well as inhibiting viral DNA polymerase, aciclovir and

penciclovir triphosphates are incorporated into the growing

DNA chain and cause premature termination of DNA syn-

thesis.

Other nucleoside analogues, including the anti-HIV agents

zidovudine, didanosine, zalcitabine, stavudine, lamivudine and

abacavir, and the anti-cytomegalovirus agents ganciclovir and

valganciclovir, act in a non-specific manner because they are

phosphorylated by cellular enzymes and/or are less selective

for viral versus host cell enzymes. Ribavirin is also a nucleo-

ANTIVIR AL AGENTS




side analog that acts through the inosine monophosphate

pathway. The anti-HIV compounds are thought to act pri-

marily to inhibit reverse transcriptase activity by causing pre-

mature chain termination during the transcription of DNA

from the single-stranded RNA template. Similarly, ganciclovir

acts as a chain terminator and DNA polymerase inhibitor

during the transcription of cytomegalovirus DNA. Since thesecompounds lack a hydroxyl group on the deoxyribose ring,

they are unable to form phosphodiester linkages in the DNA

chain.57 Ribavirin, in contrast, allows DNA synthesis to occur,

but prevents the formation of viral proteins, probably by inter-

fering with capping of viral mRNA.58 In vitro, ribavirin antag-

onizes the action of zidovudine, probably by feedback

inhibition of thymidine kinase so that the zidovudine is not

phosphorylated.59

NON-NUCLEOSIDE REVERSETRANSCRIPTASE INHIBITORS

Although they are structurally unrelated, the non-nucleosidereverse transcriptase inhibitors nevirapine, delavirdine, and

efavirenz (p. 000) all bind to HIV-1 reverse transcriptase in a

non-competitive fashion.

PROTEASE INHIBITORS

An alternative tactic to disable HIV is to inhibit the enzyme

that cleaves the polypeptide precursor of several essential viral

proteins.60 Such protease inhibitors in therapeutic use include

saquinavir, ritonavir, indinavir, nelfinavir, amprenavir and

lopinavir/ritonavir (see Chapter 00).

NUCLEOTIDE ANALOGS

One nucleotide analog is licensed for the treatment of

cytomegalovirus disease in AIDS patients: hydrox-

ypropoxymethyl cytosine (cidofovir). It is phosphorylated by

cellular kinases to the triphosphate derivative, which then

becomes a competitive inhibitor of DNA polymerase.

PHOSPHONIC ACID DERIVATIVES

The simple phosphonoformate salt foscarnet and its close

analog phosphonoacetic acid inhibit DNA polymerase activi-

ty of herpes viruses by preventing pyrophosphate exchange.61

The action is selective in that the corresponding mammalian

polymerase is much less susceptible to inhibition. Activity of

foscarnet against HIV seems to be due to a different mecha-

nism. Like the nucleoside analogs, it inhibits reverse tran-

scriptase activity of retroviruses, but it binds to the enzyme at

a site distinct from that of the nucleoside triphosphates. The

effect is non-competitive and reversible.

AMANTADINE AND RIMANTIDINE

The anti-influenza A compound amantadine and its close rel-

ative rimantadine appear to act at the stage of viral uptake by

preventing membrane fusion; these compounds also interfere

with virus disassembly. Both effects may be due to specific

interaction of the drugs with a membrane-associated proteinof the virus.62,63

NEURAMINIDASE INHIBITORS

Two drugs target the neuraminidase of influenza A and B

viruses: zanamivir and oseltamivir. Both directly bind to the

neuraminidase enzyme and prevent the formation of infectious

progeny virions.64 – 66

ANTISENSE DRUGS

Fomivirsen is the only licensed antisense oligonucleotide forthe treatment of cytomegalovirus retinitis.67 The nucleotide

sequence of fomivirsen is complementary to a sequence in the

messenger RNA transcript of the major immediate early region

2 of cytomegalovirus, which is essential for production of infec-

tious virus. The binding is reversible.

Further Information

Detailed information on the mode of action of anti-infective agents can be found

in the following sources:

Campbell WC, Few RS (eds) 19 86 Chemotherapy of parasitic disease. Plenum, New

York

Dax SL 1997 Antibacterial chemotherapeutic agents. Blackie Academic, London

Franklin TJ, Snow GA 1998 Biochemistry and molecular biology of antimicrobial

action, 5th edn. Kluwer Academic Publishers, DordrechtFrayha GJ, Smyth JD, Gobert JG, Savel J 1997 The mechanism of action of antipro-

tozoal and anthelmintic drugs in man. General Pharmacology 28: 273–299

Gale EF, Cundliffe E, Reynolds PE, Richmond MH, Waring MJ 1981 The molecular

basis of antibiotic action, 2nd edn. Wiley, Chichester

Greenwood D, O’Grady F (eds) 1985 The scientific basis of antimicrobial chemo-

therapy. Cambridge University Press, Cambridge

James DH, Gilles HM 1985 Human antiparasitic drugs: Pharmacology and usage.

Wiley, Chichester

Lancini G, Parenti F, Gallo G-C 1995 Antibiotics: a multidisciplinary approach.

Plenum, New York.

Russell AD, Chopra I 1996 Understanding antibacterial action and resistance, 2nd

edn. Ellis Horwood, London

Scholar EM, Pratt WB 2000 The antimicrobial drugs 2nd edn. Oxford University

Press, Oxford

Williams RAD, Lambert PA, Singleton P 1996 Antimicrobial drug action. bios

Scientific Publishers, Oxford

References

1. Hancock REW, Bellido F 1992 Antibiotic uptake: unusual results for unusual

molecules. Journal of Antimicrobial Chemotherapy 29: 235–239.

2. Lichtenthaler 2000







62. Hay AJ, Wolstenholme AJ, Skehel JJ, Smith MH 1985 The molecular basis of the

specific anti-influenza action of amantadine. EMBO Journal 4: 3021–3024

63. Skehel JJ 1992 Amantadine blocks the channel. Nature 358: 110–111

64. Waghorn SL, Goa KL 1998 Zanamivir. Drugs 55: 721–725

65. Bardsley-Elliot A, Noble S 1999 Oseltamivir Drugs 58: 851–860

66. Shitara E, Nishimura Y, Nerome K et al 2000 Synthesis of 6-acetamido-5-amino-

and 5-guanidino-3, 4-dehydro-N-(2-ethylbutyrul)-3-piperideinecarboxylic

acids related to zanamivir and oseltamivir, inhibitors of influenza virus neu-

raminidases. Organic Letters 2: 3837–3840

67. Perry CM, Balfour JA 1999 Fomivirsen. Drugs 57: 375–380

Hooper DC 1993 Quinolone mode of action Drugs 49 (Suppl. 2): 10–15.



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