Apparent interaction between telaprevir and warfarin in a patient with chronic hepatitis C viral infection

A probable interaction between warfarin and a recently approved protease inhibitor used in a triple-drug regimen for hepatitis C virus (HCV) infection is reported.

A 45-year-old Hispanic man seen at an anticoagulation clinic was found to have an International Normalized Ratio (INR) of 6.0; for the preceding eight months, INR values in the therapeutic range (2.5-3.5) had been maintained on a stable regimen of warfarin sodium 6 mg daily. Two days before the clinic visit, triple therapy with peginterferon alfa-2a, ribavirin, and telaprevir had been initiated for chronic HCV infection. The patient was instructed to skip two warfarin sodium doses and then resume its use at a reduced daily dose (5 mg), but he reported missing five doses, resulting in a below-target INR. An increase in the weekly warfarin dose of 50% above the baseline dose was required to reattain a target INR. The warfarin dosing requirement began to decline only after the man finished the prescribed 12-week course of telaprevir.

The INR of an HCV-infected man who was on a stable warfarin regimen was found to be above the target range two days after triple therapy including telaprevir was begun. The INR fell below the target range after warfarin therapy was ceased for five days and returned to that range after warfarin was restarted and its dosage gradually increased to 1.5 times the baseline dosage. The warfarin dosage needed to maintain a target INR fell to nearly its baseline level after telaprevir was discontinued.