Irbesartan does not affect the steady-state pharmacodynamics and pharmacokinetics of warfarin

Objectives: To determine whether the initiation or titration of irbesartan alters the pharmacodynamics and/or pharmacokinetics of warfarin in a clinically significant manner, thereby requiring additional monitoring of the anticoagulant effect of warfarin. Methods: Daily doses of warfarin were administered to 16 healthy males for 21 days (10 mg on day 1 and 2.5-10 mg on days 2-21). Irbesartan (300 mg/day) or placebo was concomitantly administered on days 15-21. The pharmacodynamic parameters prothrombin time (PT) and prothrombin time ratio (PTR) were evaluated throughout the study. Plasma and urine samples were collected before and up to 24 h after administration on days 14, 15 and 21 for the determination of the maximum concentration (C-max), time to reach C-max (t(max)), the area under the concentration-time curve (AUC) of S-warfarin and the cumulative urinary excretion of warfarin and its metabolites. Pre-dose plasma samples were also collected to determine the C-min of S-warfarin (days 12, 13, 14 and 21) and irbesartan (days 19, 20 and 21). Results: Analysis of PTR data revealed no significant difference between the group mean PTR values at day 22 and those at day 15 (P = 0.699). S-warfarin concentrations in plasma and urine, as well as the urinary concentrations of the metabolites of warfarin, were not affected by concomitant single- or multiple-dose administration of irbesartan. Plasma C-min concentrations of S-warfarin and irbesartan were also not affected. Conclusions: No clinically important effect of irbesartan on the pharmacodynamics and pharmacokinetics of warfarin are likely to occur during concomitant administration; therefore, neither a dosage adjustment of irbesartan or warfarin nor any additional monitoring of the anticoagulant effect of warfarin is necessary.