Metronidazole is selectively toxic to anaerobes and not to aerobic host cells because its activation requires:

• A Cytochrome P450 3A4-mediated N-oxidation present only in anaerobes
• B Reduction of its nitro group to a cytotoxic radical intermediate, occurring only in low-redox-potential anaerobic environments ✓
• C Protonation in the acidic microenvironment of anaerobic abscesses
• D Binding to ferredoxin-independent reductases unique to anaerobic bacteria

Explanation

Metronidazole is a prodrug; intracellularly, its nitro group undergoes single-electron reduction to form short-lived cytotoxic radical intermediates (nitro-radical anion) that damage DNA. This reduction occurs only under low intracellular redox potential (oxygen-depleted environments), as oxygen would reoxidise the radical before it causes damage. Ferredoxin and other electron carriers in anaerobes have sufficiently low redox potentials to drive this reaction; aerobic organisms do not.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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