A 62-year-old man with atrial fibrillation on warfarin (INR 2.5) is started on clarithromycin for a respiratory infection. After 5 days, his INR rises to 6.2 with no change in warfarin dose. Which mechanism best explains this interaction?

Explanation

## Warfarin–Clarithromycin Interaction: Mechanism ### Clinical Scenario Analysis **Key Point:** The INR rose significantly (2.5 → 6.2) within 5 days without a change in warfarin dose. This indicates **increased warfarin effect** due to **increased warfarin concentration**, not a pharmacodynamic antagonism. ### Mechanism: CYP3A4 Inhibition Warfarin is metabolized by **CYP2C9** (primary) and **CYP3A4** (secondary). Clarithromycin is a potent **CYP3A4 inhibitor** (and also inhibits CYP2C9 to a lesser extent). 1. **Normal warfarin metabolism:** Warfarin → hydroxylation by CYP2C9/CYP3A4 → inactive metabolites → renal/biliary excretion 2. **With clarithromycin:** CYP3A4 inhibited → ↓ warfarin metabolism → ↑ warfarin plasma concentration → ↑ anticoagulant effect → ↑ INR 3. **Timeline:** CYP inhibition is rapid (24–72 hours), explaining the 5-day rise in INR **High-Yield:** This is a **Type I (pharmacokinetic) interaction** — clarithromycin alters warfarin's metabolism, not its mechanism of action. ### Why Not the Other Options? | Mechanism | Why Incorrect | | --- | --- | | **Protein displacement** | Warfarin is highly protein-bound (99%), but acute displacement causes only a transient, small increase in free warfarin. The sustained INR rise over 5 days is inconsistent with acute displacement. | | **Enzyme induction** | Clarithromycin is an enzyme **inhibitor**, not inducer. Induction would decrease warfarin levels and lower INR, the opposite of what occurred. | | **Pharmacodynamic antagonism** | Clarithromycin has no direct anticoagulant or antiplatelet activity. A pharmacodynamic interaction would not explain the elevated INR without a dose change. | ### Clinical Management **Clinical Pearl:** When a CYP inhibitor is added to a warfarin patient: - Monitor INR closely (every 2–3 days initially) - Reduce warfarin dose by 20–30% - If clarithromycin must be used, consider switching to an alternative antibiotic without CYP inhibition (e.g., doxycycline, amoxicillin) - After clarithromycin is stopped, warfarin dose must be increased back to baseline **Mnemonic:** **CYP Inhibitors that ↑ Warfarin ("SICKFACES.COM")** — Sulfonamides, Isoniazid, Cimetidine, Ketoconazole, Fluconazole, Acute alcohol, Chloramphenicol, Erythromycin, **Clarithromycin**, Ciprofloxacin, Omeprazole, Metronidazole.