Inhibition of Purified Factor Xa Amidolytic Activity May Not Be Predictive of Inhibition of In Vivo Thrombosis

Abstract

Objective— In this study we test the hypothesis that blood/plasma-based prothrombinase assays, rather than inhibition of purified factor Xa (fXa), are predictive of in vivo antithrombotic activity. Methods and Results— Six fXa inhibitors with equivalent nanomolar K _i were studied in thrombin generation assays using human plasma/blood and endogenous macromolecular substrate. In all assays, benzamidine inhibitors were more potent (100 to 800 nmol/L) than the aminoisoquinolines (5 to 58 μmol/L) or neutral inhibitors (3 to10 μmol/L). A similar rank order of compound inhibition was also seen in purified prothrombinase assays as well as in a rabbit model of deep vein thrombosis. Conclusions— Assays using prothrombinase with protein substrates are better predictors of in vivo efficacy than fXa K _i using amidolytic substrates.