Amiodarone inhibits warfarin metabolism and is associated with major bleeding during warfarin therapy. Managing this drug-drug interaction (DDI) is challenging because of substantial interpatient variability in DDI magnitude. Because renal dysfunction induces changes in drug metabolism and protein binding that could alter cytochrome P450 inhibition mechanisms, we hypothesized that renal dysfunction alters the impact of the warfarin-amiodarone DDI. We tested this question in a propensity-matched cohort study of hospitalized patients with atrial fibrillation. Patients were queried from an electronic health record database. Renal function was estimated with creatinine clearance (CrCl). Warfarin response was measured with the warfarin sensitivity index (WSI), a dose-normalized international normalized ratio (INR) measure, and was modeled with multilevel mixed-effects linear regression. Time to supratherapeutic INR (>4) was modeled using Cox regression. Propensity score matching resulted in 4518 amiodarone patients and 4518 controls. Amiodarone’s effect on warfarin response varied three-fold across the renal function range, increasing WSI by 36% in patients with normal renal function (CrCl 115 ml/min), but by only 11.8% in patients with severe renal dysfunction (CrCl 15 ml/min). Similarly, amiodarone had a strong effect in patients with normal renal function, HR 1.80 (1.23,2.64), but a negligible effect on supratherapeutic INR hazard in patients with severe renal dysfunction, HR 1.01 (0.75,1.37). These results suggest that renal function is a novel factor that explains substantial variability in the warfarin-amiodarone DDI. This information could inform warfarin dosage adjustment and monitoring, and may have implications for the selection of oral anticoagulation agents in patients treated with amiodarone.
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