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Mechanisms of actions of antimicrobial drugs

الرحمن الله بسمالرحيم

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• Strategies to combat microbes:• Public health measures

• Screening procedures, early detection

• Vaccines

• Drugs

• Drugs therapy targets selective differences between microbes and host cells

Introduction

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Selective targeting of microorganisms• Targets unique to microbes & lack in host cells:• Bacterial cell wall contains peptidoglycan strands

e.g. Penicillins ↓ cross linking of peptidoglycans • Fungal cell membrane contains ergosterol

e.g. Azole antifungals ↓ ergoserol synthesis• Targets similar to microbes & have quantitative

differences from the host cells:• Different enzyme or receptor isoform

e.g.trimethoprim inhibits bacterial DHFR & pyrimethamine inhibits malarial DHFR

• For selective targeting important to understand mechanisms of actions of drugs

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Mechanism of action of antimicrobials (overview)

50S & 30S

Inhibitors of cell wallsynthesis

Inhibitors of DNA & cell division

Inhibitors of cell membranesynthesis

Inhibitors of proteinsynthesis

PABA

Pteridine

Folic acidFolinic acid

Purines Pyrimidines

DNA

Ribosome

mRNA

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Inhibitors of cell wall synthesis

• A. Inhibitors of synthesis of peptidoglycan: • Bacterial cell wall contains peptidoglycan strands • Peptidoglycan is composed of UDP-N-acetyl-muramic

acid, UDP-N-acetyl-glucosamine & a pentapeptide• Cycloserine, resembels to alanine & inhibits addition

of alanine into peptide chain• Vancomycin inhibits transglycosidase enzyme &

prevents peptidoglycan chain elongation

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Inhibitors of cell wall synthesis (Cont.) • B. Inhibitors of cross linking of peptidoglycan

strands:• Beta-lactam antibiotics: e.g. Penicillins &

cephalosporins• Inhibit transpeptidase enzyme, involved in cross

linking of peptidoglycan strands • Also called as transpeptidation reaction &

strengthens cell wall. • Defects in the synthesis of cell wall cause wholes in

cell wall & chnages in permeability• Leading to bacterial swelling & lysis• Hence cell wall synthesis inhibitors are bactericidal

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Inhibitors of cell wall synthesis (Cont.)• Inhibitors of mycobacterial cell wall:• Mycobacterial cell wall is composed of arabino-

galactan chains• Ethambutol inhibits arabinosyl-transferase enzyme

involved in addition of arabinose in arabino-galactan chains

• Isoniazid & pyrazinamide inhibit synthesis of mycolic acid for mycobacterial cell wall by inhibiting fatty-acid synthase enzyme (FAS)

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Inhibitors of cell membranes• Inhibitors of fungal cell membrane• A. Inhibitors of ergosterol synthesis • Azole anti-fungals: e.g. fluconazole

inhibit fungal P-450 enzyme (14α- demethylase)• Defects in synthesis causes wholes in cell membrane• B. Ergosterol binding compounds • Polyene anti-fungals: e.g. amphotericin B

Bind to ergosterol in the cell membrane & increase membrane permeability

• Leakage of essential elements, cell lysis, fungicidal

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Inhibitors of protein synthesis• Protein synthesis takes place in ribosomes• Bacterial ribosome consists of 30S & 50S sub-units

whereas, mammalian ribosomes have 40S & 60S• Tetracyclines ↓ 30S ribosomal subunit & are

bacteriostatic• Macrolides & chloramphenicol ↓ 50S ribosomal

subunit & are bacteriostatic• Aminoglycosides ↓ 30S ribosomal subunit, but some

abnormal proteins insert in cell membrane & form pores, thus become bactericidal

• Rifampin ↓ DNA dependent RNA polymerase & is bactericidal

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Inhibitors of DNA & cell division

• Antimicrobial drugs can affect microbial DNA & cell division in following ways:

• Inhibit DNA synthesis

Inhibitors of folate synthesis

Inhibitors of thymidylate synthesis

• Inhibit DNA replication

• Inhibit microtubules & mitosis

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Inhibitors of DNA synthesisFolate precursors

(PABA, pteridine, glutamic acid)

Folic acid

Folinic acid

Purines(adenine & guanine)

Pyrimidines(cytosine & thymine)

Ribonucleotides

Deoxy-ribonucleotides

DNA

Sulfonamides & sulfonesTrimethoprim

--

Flucytosine-

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Inhibitors of DNA synthesis (Cont.)

• A. Inhibitors of folate metabolism • Folic acid (dihydro-folate) synthesis inhibitors:

Sulfonamides & sulfones inhibit dihydro-pteroate synthase (DHPS) enzyme

• Folinic acid (tetrahydro-folate) synthesis inhibitors:

Trimethoprim inhibits dihydro-folate reductase (DHFR) enzyme (bacterial DHFR more than human)

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• Synergism of folic acid (dihydrofolate) & folinic acid (tetrahydro-folate) synthesis inhibitors:

Inhibitors of DNA synthesis (Cont.)

Folic acid

Folinic acid

PABA + pteridine

Dihydro-pteroate synthase

Dihydro-folatereductase

DNA

Sulfamethoxazole

Trimethoprim

Glutamic acid

• Sulfamethoxazole + Trimethoprim

(anti-bacterial)

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Inhibitors of DNA synthesis (Cont.)• B. Inhibitors of thymidylate synthesis• Flucytosine: is a cytosine analogue• Converted in fungi & GI flora to 5-fluorouracil (5-FU)• 5-FU converted to 5-FdUMP

• 5-FdUMP incorporated to DNA

& inhibits thymidylate synthase (TS)

• ↓ DNA synthesis

dUMP

dTMP

DNA

T S

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Inhibitors of DNA replication & mitosis• During cell division chromosomes line up in the

equator of cell• Each divides into two (DNA- replication) by

topoisomerases (topoisomerase I to IV)• Each part is pulled to opposite pole of the cell by

mitotic spindles made up of microtubules • Quinolones inhibit bacterial topisomerase-II [ in

gram (-) bacteria] & topoisomerase IV [in gram (+) bacteria], inhibit DNA replication & are bactericidal

• Griseofulvin inhibits microtubule assembly, mitotic spindle formation, cell division & is bacteriostatic