A 24-year-old woman treated with tetracycline for acne takes her tablets with calcium-fortified orange juice and an aluminum hydroxide antacid. After four weeks her acne has not improved. The pharmacokinetic explanation is:
- A Induction of hepatic microsomal enzymes accelerating tetracycline clearance
- B Chelation of tetracycline by divalent and trivalent cations reducing its gastrointestinal absorption ✓
- C Competitive inhibition of tetracycline transport across the intestinal wall by calcium
- D Rapid conversion of tetracycline to an inactive epimer at gastric pH
Correct answer: B. Chelation of tetracycline by divalent and trivalent cations reducing its gastrointestinal absorption
Explanation
Tetracyclines chelate divalent and trivalent cations such as Ca2+, Mg2+, Al3+, and Fe2+, forming insoluble complexes that are not absorbed. Antacids and dairy products therefore markedly reduce serum concentrations and cause therapeutic failure. Doxycycline and minocycline are less affected because of their greater lipid solubility, which is why they may be taken with food. Epimerization occurs in solution during storage and is not relevant to absorption. Hepatic induction is a property of rifampin, not antacids.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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