A 58-year-old asthmatic stabilized on oral theophylline is started on ciprofloxacin for acute prostatitis. Two days later she develops palpitations and tremor, and her serum theophylline level is found to be markedly elevated. The basis of this interaction is:

• A Ciprofloxacin inhibits CYP1A2 and reduces hepatic theophylline clearance ✓
• B Ciprofloxacin inhibits xanthine oxidase and blocks theophylline degradation
• C Ciprofloxacin induces CYP1A2 and increases theophylline clearance
• D Ciprofloxacin displaces theophylline from plasma protein binding sites

Explanation

Theophylline is metabolized mainly by hepatic CYP1C2. Ciprofloxacin is a clinically important CYP1C2 inhibitor, so coadministration raises theophylline levels and produces toxicity manifesting as arrhythmias, tremor and seizures. The same interaction applies to clozapine and caffeine. Theophylline dose reduction or alternative antibiotic selection is advised.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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