Inductive Effect of a Ritonavir-Containing Hepatitis C Treatment Regimen on Warfarin in a Patient-A Case Report

The warfarin management strategy for a mechanical mitral valve patient initiated on a ritonavir-based hepatitis C treatment regimen is described. A 62-year-old male with a past medical history of hepatitis C genotype 1a and stable warfarin dose history was initiated on a concomitant Viekira Pak® (VP) regimen containing ritonavir. Prior to initiation of the VP for hepatitis C treatment, the patient was stable on a warfarin dose of 40 mg/wk for 5 months. During treatment with VP, the patient experienced a markedly decreased international normalized ratio (INR) and warfarin requirements ultimately increased 125% from baseline (90 mg/wk). Effective anticoagulation management throughout and surrounding the treatment period for hepatitis C involved frequent warfarin dose adjustments, including preemptive changes, close monitoring, and repeated use of enoxaparin to ensure adequate thrombotic prophylaxis. This is believed to be the first reported case describing the management of warfarin in a patient with hepatitis C who received VP and required a drastically increased weekly warfarin dose. The possible mechanisms suggestive of this interaction and similar case reports in the literature are discussed.