Apixaban, an oral, direct and highly selective factor Xa inhibitor: in vitro, antithrombotic and antihemostatic studies

Background: Apixaban is an oral, direct and highly selective Factor Xa (FXa) inhibitor in late-stage clinical development for the prevention and treatment of thromboembolic diseases.

Objective: We evaluated the in vitro properties of apixaban and its in vivo activities in rabbit models of thrombosis and hemostasis.

Methods: Studies were conducted in arteriovenous-shunt thrombosis (AVST), venous thrombosis (VT), electrically mediated carotid arterial thrombosis (ECAT) and cuticle bleeding time (BT) models.

Results: In vitro, apixaban is potent and selective, with a K(i) of 0.08 nm for human FXa. It exhibited species difference in FXa inhibition [FXa K(i) (nm): 0.16, rabbit; 1.3, rat; 1.7, dog] and anticoagulation [EC(2x) (microm, concentration required to double the prothrombin time): 3.6, human; 2.3, rabbit; 7.9, rat; 6.7, dog]. Apixaban at 10 microm did not alter human and rabbit platelet aggregation to ADP, gamma-thrombin, and collagen. In vivo, the values for antithrombotic ED(50) (dose that reduced thrombus weight or increased blood flow by 50% of the control) in AVST, VT and ECAT and the values for BT ED(3x) (dose that increased BT by 3-fold) were 0.27 +/- 0.03, 0.11 +/- 0.03, 0.07 +/- 0.02 and > 3 mg kg(-1) h(-1) i.v. for apixaban, 0.05 +/- 0.01, 0.05 +/- 0.01, 0.27 +/- 0.08 and > 3 mg kg(-1) h(-1) i.v. for the indirect FXa inhibitor fondaparinux, and 0.53 +/- 0.04, 0.27 +/- 0.01, 0.08 +/- 0.01 and 0.70 +/- 0.07 mg kg(-1) day(-1) p.o. for the oral anticoagulant warfarin, respectively.

Conclusions: In summary, apixaban was effective in the prevention of experimental thrombosis at doses that preserve hemostasis in rabbits.