DU-176b is a novel potent, orally active and selective direct inhibitor of factor Xa (FXa). Direct FXa inhibitors have been reported to exert little effect on bleeding time at antithrombotic doses in animal studies. The aim of the present study was to compare the antithrombotic and hemorrhagic effects of DU-176b with unfractionated heparin (UFH), low molecular weight heparin (LMWH; dalteparin) and warfarin in rat models of thrombosis and hemorrhage. Rats were treated with DU-176b, UFH and LMWH by continuous intravenous infusion for 2 – 2.5 h, and with warfarin orally once daily for 4 days before thrombosis or hemorrhage. Thrombosis was induced by the insertion of a platinum wire into the inferior vena cava and left for 60 min. Tail template bleeding time was measured after an incision on the tail. DU-176b dose-dependently inhibited thrombus formation in the venous thrombosis model. The dose required for 50% inhibition (ED50) was 0.076 mg/kg/h. In contrast, the dose of DU-176b to double template bleeding time (BT2) was 0.75 mg/kg/h, indicating 10-fold dissociation of the doses of antithrombotic and hemorrhagic effects. UFH, LMWH and warfarin also prevented thrombus formation (ED50 = 56 U/kg/h, 66 U/kg/h and 0.16 mg/kg/day, respectively), but prolonged bleeding time at slightly higher doses (BT2 = 73 U/kg/h, 135 U/kg/h and 0.21 mg/kg/day, respectively) than the effective doses. The dissociation of the doses for these compounds was only 1.3, 2.0 and 1.3-fold, respectively. Moreover, the slope of dose-antithrombotic response curve of DU-176b was significantly slighter than those of UFH, LMWH and warfarin, indicating that the therapeutic dose range of DU-176b would be wider than those of the other anticoagulants. These results suggest that direct and selective inhibition of FXa by DU-176b is preferable for the treatment of thrombotic diseases in the aspect of lack of compromising primary hemostasis.

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