Metronidazole achieves selective toxicity against anaerobes and certain protozoa because:
- A It is selectively concentrated in anaerobic organisms via an ABC transporter absent in aerobic cells
- B Its nitro group is reduced to a toxic radical anion only in organisms with low intracellular redox potential (ferredoxin/flavodoxin-dependent reductive activation), which occurs in anaerobes and not in aerobic human cells ✓
- C It inhibits the anaerobic enzyme acetyl-CoA synthetase, which is absent in aerobic organisms
- D It is a prodrug activated by aerobic cytochrome P450 enzymes selectively expressed in anaerobic organisms
Correct answer: B. Its nitro group is reduced to a toxic radical anion only in organisms with low intracellular redox potential (ferredoxin/flavodoxin-dependent reductive activation), which occurs in anaerobes and not in aerobic human cells
Explanation
Metronidazole is a prodrug whose nitro group is reduced by low-redox electron carriers — ferredoxin or flavodoxin — present only in anaerobic/microaerophilic organisms and certain protozoa. The resulting nitro radical anion damages DNA by causing strand breaks. In aerobic cells, the intracellular environment oxidises metronidazole back before activation can occur, conferring selective toxicity.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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