A patient on theophylline for asthma is started on cimetidine for peptic ulcer disease. He develops nausea, vomiting, and tachycardia. What is the biochemical mechanism of this drug interaction?
- A Cimetidine induces CYP1A2, increasing theophylline clearance
- B Cimetidine inhibits CYP1A2, decreasing theophylline clearance ✓
- C Cimetidine induces renal excretion of theophylline
- D Cimetidine displaces theophylline from albumin binding sites
Correct answer: B. Cimetidine inhibits CYP1A2, decreasing theophylline clearance
Explanation
Cimetidine is a non-specific inhibitor of the cytochrome P450 system, particularly CYP1A2, which metabolizes theophylline. Inhibition decreases theophylline clearance, leading to accumulation and toxicity. Cimetidine does not induce CYP enzymes or significantly alter protein binding to cause this interaction.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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