Differential Effects of TAK-442, a Factor Xa Inhibitor, and Ximelagatran, a Thrombin Inhibitor, on Bleeding: Possible Role of Differences in Effects on Factor V-Mediated Feedback on Blood Coagulation Cascade

Thrombin generation serves to amplify the coagulation cascade via positive feedback activation of factor V (FV) and factor VIII. We hypothesized that factor Xa (FXa) inhibitors, unlike thrombin inhibitors, would not block the feedback activation of the coagulation cascade but would have a favorable anticoagulating profile—sufficient to prevent thrombus formation, yet not interfere with hemostatic plug formation. TAK-442 is a newly synthesized, selective FXa inhibitor that strongly inhibits FXa (with a Ki value of 1.8 nM), and displays more than 440x selectivity toward FXa than other serine proteases. In the present study, we compared the effects of TAK-442 versus ximelagatran on FV-mediated positive feedback in vitro, and on their antithrombotic and hemorrhagic effects in a rat model of venous thrombosis.

In vitro, TAK-442 gradually inhibited thrombin generation and prolonged prothrombin time (PT) in a dose-dependent manner, while melagatran, an active form of ximelagatran, exhibited a steeper effect at higher doses tested. The PT prolonging potency was increased in FV–deficient human plasma, with CT2 values (the concentration that causes 2 times prolongation of clotting times) of 120 nM for TAK-442 and 32 nM for melagatran, compared with 500 nM and 360 nM for TAK-442 and melagatran, respectively in normal plasma. In the rat model of venous thrombosis, TAK-442 (10 mg/kg, po) prevented thrombus formation by 55% and prolonged PT by 1.3 times of control values; a similar effect was observed in ximelagatran-treated (3 mg/kg, po) animals, with 59% inhibition of thrombus formation and 1.2 times prolongation of PT. TAK-442 at 100 mg/kg, prolonged PT by 2.1 times, with no significant change in bleeding time (BT); in contrast, increasing the dose of ximelagatran to 10 mg/kg, po prolonged PT by 3.9 times and significantly (P<0.025) increased BT.

Our data suggest that the differential effects of the two agents on FV-mediated amplification of thrombin generation may underlie the observation of a wider therapeutic window for TAK-442 than for ximelagatran.