Dr.Wafaa Ezz Elarab

Antimicrobial Drugs-3

3. Inhibitors of Nucleic Acid Synthesis

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3. Inhibition of nucleic acid synthesis

Acting on DNA replicationQuinolones

Metronidazole

Acting on RNA synthesisRifampin

Rifabutin

Inhibitors of RNA Synthesis

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Selectivity due to differences between prokaryotic and eukaryotic

RNA polymerase

Rifampin, Rifamycin, Rifampicin, Rifabutin

(bactericidal)• Mode of action - These antimicrobials bind to DNA-dependent RNA polymerase and inhibit initiation of mRNA synthesis.

• Spectrum of activity - Broad spectrum but is used most commonly in the treatment of tuberculosis

• Resistance - Common• Combination therapy - Since

resistance is common, rifampin is usually used in combination therapy.

Inhibitors of DNA Synthesis

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Selectivity due to differences between prokaryotic and eukaryotic enzymes

Quinolones (bactericidal)nalidixic acid, ciprofloxacin, ofloxacin, norfloxacin, levofloxacin, lomefloxacin,

sparfloxacin • Mode of action - Synthetic inhibitors of DNA-

Gyrase (= Topoisomerase II), a bacterial enzyme that winds and unwinds DNA (required for supercoiling the bacterial genome) => inhibition of DNA synthesis and transcription

• Spectrum of activity - Gram-positive cocci and urinary tract infections

• Resistance - Common for nalidixic acid; developing for ciprofloxacin

III. Mitronidazole:• Uses: It is antiprotozoal drug but also has antibacterial effects on anaerobic bacteria causing damage of DNA.• Increasingly, it is used as part of treatment of Helicobacter pylori infection of the stomach and duodenum associated with peptic ulcer disease.• It is used also to treat a variety of dental infections, particularly dental abscess.Adverse effects

• Nausea, anorexia and metallic taste, ataxia•Teratogenic if taken in the first trimester of pregnancy

4. Antimetabolite Antimicrobials

4. Antimetabolites

Sulfonamides √

Dapsone

Trimethoprim √

Paraaminosalicylic acid

Inhibitors of Folic Acid Synthesis

• Basis of Selectivity

• Review of Folic Acid Metabolism

p-aminobenzoic acid + Pteridine

Dihydropteroic acid

Dihydrofolic acid

Tetrahydrofolic acid

Pteridine synthetase

Dihydrofolate synthetase

Dihydrofolate reductase

ThymidinePurines

Methionine

Trimethoprim

Sulfonamides

Sulfonamides, Sulfones (bacteriostatic)

• Mode of action - These antimicrobials are analogues of para-aminobenzoic acid and competitively inhibit formation of dihydropteroic acid.

• Spectrum of activity - Broad range activity against gram-positive and gram-negative bacteria; used primarily in urinary tract and Nocardia infections.

• Resistance - Common

• Combination therapy - The sulfonamides are used in combination with trimethoprim; this combination blocks two distinct steps in folic acid metabolism and prevents the emergence of resistant strains.

Competitive Inhibitors Sulfonamides (Sulfa drugs)

• Inhibit folic acid synthesis• Broad spectrum

Figure 5.7

Clinical uses

• Sulfadiazine Sulfadimidine Sulfamethoxazole

Broad range activity against gram-positive and gram-negative bacteria; used primarily in urinary tract, infected burns, STDs, toxoplasmosis… infections

Trimethoprim, Methotrexate, Pyrimethamine (bacteriostatic)

• Mode of action - These antimicrobials binds to dihydrofolate reductase and inhibit formation of tetrahydrofolic acid.

• Spectrum of activity - Broad range activity against gram-positive and gram-negative bacteria; used primarily in urinary tract and Nocardia infections.

• Resistance - Common• Combination therapy - These antimicrobials

are used in combination with the sulfonamides; this combination blocks two distinct steps in folic acid metabolism and prevents the emergence of resistant strains.

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5 -Drugs that disrupt cell membrane function

Antibacterial medications that Injure Plasma

Membrane• Polymyxin B: binds to membrane of G- bacteria and alters permeability

• This leads to leakage of cellular contents and cell death

• These drugs also bind to eukaryotic cells to some extent, which limits their use to topical applications

The antimicrobial agent should act at a target site which

is present in the infecting organism but absent from

host cells. This is more likely to be achievable in

prokaryotes than eukaryotes, as they are structurally

more distinct from host cells.

Examples:

a- Cell wall synthesis inhibitor is selective toxic for

bacteria this is because peptidoglycan , a vital

component of the bacterial cell wall, is compound unique

to bacteria and thus provides an optimum target for

selective toxicity.

Selective toxicity of antimicrobial agents. Selective toxicity of antimicrobial agents.

b- Tetracyclines inhibit protein synthesis by

preventing aminoacyltransfer RNA from entering the

acceptors sites on the ribosome. This action is not

selective. The selective action of tetracyclines is

based on their uptake by bacterial cells much greater

than by human cells.

c- Chloramphenicol does have some inhibitory

activity on human ribosomes and this may account

for some of the dose dependent toxicity to bone

marrow.

d- The selective toxicity of sulphonamides depends on

the fact that many bacteria synthesize THFA whereas

human cells lack this capacity and depend on an

exogenous supply of folic acid.

e- The selective toxicity of rifampicin is based on the far

greater affinity for bacterial polymerases than for human

enzymes.

f-The inhibition of bacterial gyrase by quinolone is

specific and does not affect the equivalent topisomerase

enzymes in mamalin cells.

Antimicrobial resistance

is the ability of microorganism to survive and reproduce in the presence of antimicrobial does that were previously thought effective against them

Cross resistance single mechanism confers resistance to multiple antimicrobial agents, is commonly seen with closely related antimicrobial agents

Cross Resistance:

multiple resistance that multiple mechanisms are involved to multiple antimicrobial agents is seen with unrelated antimicrobial agents.

Multiple resistance

• The Development of Resistance in Populations– Some pathogens are naturally resistant – Resistance by bacteria acquired in two ways

• New mutations of chromosomal genes• Acquisition of R-plasmids via transformation,

transduction, and conjugation.• Mechanisms of Resistance– At least five mechanisms of microbial

resistance1. Production of enzyme that destroys or deactivates drug2. Slow or prevent entry of drug into the cell3. Alter target of drug so it binds less effectively4. Alter their metabolic chemistry5. Pump antimicrobial drug out of the cell before it can act

1- Production of enzyme that destroys or deactivates drug

2- Slow or prevent entry of drug into the cellDecreased uptake of the drug

Alterations in porin proteins decrease permeability of cells

Prevents certain drugs from entering

3- Alter target of drug so it binds less effectivelyMinor structural changes in antibiotic target can

prevent bindingEx: Changes in ribosomal RNA prevent aminoglycosides from binding to ribosomal subunits

4- Alter their metabolic chemistry

5- Pump antimicrobial drug out of the cell before it can actSome organisms produce efflux pumps so increases

overall capacity of organism to eliminate drug

Enables organism to resist higher

concentrations of drug

Mechanism of resistance to common antimicrobial

agents 1- Betalactam

• Modification of target sites (Penicillin binding proteins)

• Decreased accumulation (Efflux pump)• Enzymatic inactivation (Beta-lactamase )

2- Non Betalactam antibiotics

A- Vancomycin

• D-alanyl-D-alanine residue ↓

D-alanyl-D-lactate moiety

• vancomycin cannot bind to this peptide

Alter target of drug so it can not bind to the new target

B- BacitracinMechanism of action: Inhibits dephosphorylation in cycling of bactoprenol that transfers peptidoglycan subunits to the growing cell wallMechanism of Resistance: Increase synthesis of the bactoprenol molecule

• The loss of outer membrane porin proteins, which

involved in the penetration of polymyxin B. • A reduction in binding of polymyxin to the cell envelope

as a result of changes in lipid and LPS composition.

3- Polymyxins

4- Aminoglycosides

• Production of aminoglycoside inactivating enzyme that chemically modifies drug (the most common mechanism)• Reduced uptake or decreased cell permeability:

• Altered ribosome binding Sites by mutations binding. This mechanism is very common for streptomycin√√

5- Tetracyclines

Cells become resistant to tetracycline by at least three mechanisms: 1. Enzymatic Inactivation is the rarest type of resistance, 2. Efflux: Resistance due to decreased accumulation by bacterial cells.3. Ribosomal protection: through certain reactive protein include: - blocking tetracyclines from binding to the ribosome. - binding to the ribosome and distorting the structure to still allow t-RNA binding while tetracycline is bound

6- ChloramphenicolThere are three mechanisms of resistance to chloramphenicol: • Reduced membrane permeability.• Mutation of the 50S ribosomal subunit which lead to structural changes in antibiotic target therefore prevent binding. • Production of chloramphenicol acetyltransferase which deactivate the drug. 7- Macrolides• Structural changes in the target ribosomal RNA.• Production of drug-inactivating enzymes (esterases or kinases).• Production of active ATP-dependent efflux proteins that transport the drug outside of the cell.

8- Quinolone

Resistance due to alteration of DNA gyrase.9- Rifamycins

Resistance due to mutation coding RNA polymerase

10- Sulfonamides and trimethoprim

Resistance due to plasmid codes for enzyme that has lower affinity to drug

• Retarding Resistance - Maintain high concentration of drug in

patient for sufficient time• Kills all sensitive cells and inhibits

others so immune system can destroy- Use antimicrobial agents in combination

• Synergism vs. antagonism

- Use antimicrobials only when necessary

- Develop new variations of existing drugsFourth generation drugs

- Search for new antibiotics, semi-synthetics, and synthetics

Design drugs complementary to the shape of microbial proteins to inhibit them

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Mechanisms drug resistance