Action of theophylline is reduced by ?
**Core Concept**
Theophylline is a methylxanthine derivative that acts as a phosphodiesterase inhibitor, increasing intracellular cyclic AMP (cAMP) levels, which leads to relaxation of smooth muscle and bronchodilation. Its action is also influenced by its metabolism, which is primarily hepatic.
**Why the Correct Answer is Right**
Theophylline is metabolized by the cytochrome P450 enzyme system, specifically by CYP1A2, CYP3A4, and CYP2E1. Certain drugs can induce or inhibit these enzymes, affecting theophylline's plasma levels and its efficacy. A known inhibitor of CYP1A2 is ciprofloxacin, a fluoroquinolone antibiotic. Ciprofloxacin can significantly reduce theophylline's clearance, thereby increasing its plasma levels and potential toxicity.
**Why Each Wrong Option is Incorrect**
**Option A:** is not a relevant inhibitor of theophylline metabolism.
**Option B:** does not have a known interaction with theophylline metabolism.
**Option C:** is not a relevant inhibitor of theophylline metabolism; it's actually a substrate of CYP1A2.
**Clinical Pearl / High-Yield Fact**
When prescribing theophylline, it's essential to consider potential drug interactions, especially with other medications that can affect its metabolism, such as ciprofloxacin, erythromycin, and cimetidine.
**Correct Answer:** C. Cimetidine.