Dose-Dependent Antithrombotic Effects of Apixaban, an Oral Direct Factor Xa Inhibitor, in Prevention and Treatment of Thrombosis in Rabbit Models of Arteriovenous-Shunt and Venous Thrombosis at Doses That Preserve Hemostasis.
Authors: Wong Pancras C. , Crain Earl J. , Pinto Donald J. , Watson Carol A. 
Published: October 2019

Abstract
Apixaban is an oral, direct and highly selective factor Xa (FXa) inhibitor, which is currently in late stage clinical development for the prevention and treatment of thromboembolic diseases. The dose-dependent antithrombotic and antihemostatic profile of apixaban was determined in the rabbit models of arteriovenous-shunt thrombosis (AVST), venous thrombosis (VT), and cuticle bleeding time (BT), and compared to those of the direct thrombin inhibitor lepirudin, the indirect FXa inhibitor fondaparinux and the oral anticoagulant warfarin. We induced the formation of thrombus in the AVST and VT models by placing threads in the extracorporeal shunt and vena cava, respectively, and bleeding by the incision of cuticles in anesthetized rabbits. In the AVST and VT prevention models, apixaban (0.03 to 3 mg/kg/h), lepirudin (0.006 to 0.75 mg/kg/h) and fondaparinux (0.01 to 1 mg/kg/h) were infused IV 30–60 min before the initiation of thrombosis. Warfarin (0.1 to 3 mg/kg/day) was administered orally for 4 days before the study. Control thrombus weight averaged 290±11 mg and 64±2 mg in AVST and VT, respectively, and control BT averaged 179±5 s (n=6 per group). Apixaban exhibited similar dose-related efficacy as lepirudin, fondaparinux, and warfarin in preventing AVST and VT. At doses that prevented 80 to 90% of thrombus formation in AVST and VT, apixaban, fondaparinux, lepirudin and warfarin increased BT by 20±2, 30±5, 500±10, 502±20%, respectively (n=6 per group). Doses for 50% reduction of control thrombus weight in AVST, VT were 0.27±0.03, 0.11±0.02 mg/kg/h IV for apixaban, 0.04±0.01, 0.05±0.01 mg/kg/h IV for lepirudin, 0.05±0.01, 0.05±0.005 mg/kg/h IV for fondaparinux and 0.53±0.04, 0.27±0.02 mg/kg PO for warfarin, respectively. To increase BT by 3-fold required higher doses of apixaban and fondaparinux (>3 mg/kg/h IV), lepirudin (0.24±0.05 mg/kg/h IV) and warfarin (0.70±0.07 mg/kg PO). In a VT treatment model, apixaban, lepirudin and fondaparinux, administered IV as a bolus injection supplemented with a continuous infusion after thrombus formation, were all able to arrest thrombus growth. However clot regression was only observed following administration of apixaban (0.6 mg/kg+0.87 mg/kg/h IV) where the preformed thrombus decreased from an initial weight of 38±2 mg (n=6) to 26±4 mg (n=6; P<0.05). In summary, apixaban and fondaparinux were effective in the prevention and treatment of experimental thrombosis at doses that preserve hemostasis in rabbits. Warfarin and lepirudin also prevented thrombus formation but with greater increases in BT. Furthermore, these standard anticoagulant agents have well-known limitations including narrow therapeutic index, frequent laboratory monitoring, or the requirement of parenteral administration. The favorable preclinical antithrombotic and antihemostatic profile of apixaban demonstrated here is consistent with clinical efficacy and safety results in recent Phase II trials, and indicates that direct inhibition of FXa with apixaban is a promising approach for the prevention and treatment of venous thromboembolism.