A soldier started on doxycycline for scrub typhus self-medicates with antacid tablets for dyspepsia. Ten days later his fever persists and repeat serology shows inadequate response. The most likely explanation is:

• A Induction of hepatic metabolism of doxycycline by antacids
• B Chelation of doxycycline by divalent cations forming insoluble complexes ✓
• C Antacid-induced acceleration of gastric emptying reducing contact time
• D Competitive inhibition of doxycycline uptake by intestinal transporters

Explanation

Tetracyclines chelate divalent and trivalent cations such as calcium, magnesium, aluminium and iron, forming insoluble complexes that are poorly absorbed. Antacids, dairy products and iron supplements therefore markedly reduce bioavailability and can cause treatment failure. Doxycycline is less affected than older tetracyclines but clinically significant reduction still occurs. The interaction is physicochemical in the gut lumen, not metabolic or transporter mediated.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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