, Volume 24, Issue 1, pp 43-51
Date: 24 Feb 2007

Razaxaban, a direct factor Xa inhibitor, in combination with aspirin and/or clopidogrel improves low-dose antithrombotic activity without enhancing bleeding liability in rabbits

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Coactivation of platelets and the blood coagulation cascade contributes to the pathophysiology of arterial thrombosis. Combination therapy with antiplatelet and anticoagulant drugs may be needed for maximizing the prevention and treatment of arterial thrombosis. Few studies have thoroughly investigated the combined antithrombotic and bleeding effects of these antithrombotic agents. We, therefore, evaluated the antithrombotic and bleeding profiles of dual and triple therapy with razaxaban, a direct factor Xa inhibitor, plus aspirin and/or clopidogrel in rabbit models of electrolytic injury-induced carotid artery thrombosis and cuticle bleeding time, respectively. Compounds were infused either IV or into the portal vein from 1 h before arterial injury or cuticle transection to the end of experiment. Carotid blood flow was used as a marker of antithrombotic effect. We first evaluated the antithrombotic potency of razaxaban, and examined its ex vivo effects on coagulation parameters to confirm its selectivity. Antithrombotic ED50 of razaxaban averaged 0.22 ± 0.05 mg/kg/h (n = 6). Razaxaban at 3 mg/kg/h IV produced full antithrombotic efficacy, increased significantly ex vivo activated partial thromboplastin time and prothrombin time by 2.2 ± 0.1- and 2.3 ± 0.1-fold, respectively, and inhibited ex vivo factor Xa activity significantly by 91 ± 5% (n = 6, P < 0.05) without affecting ex vivo thrombin activity. Razaxaban at concentrations up to 10 μM did not alter in vitro platelet aggregation responses to ADP, γ-thrombin or collagen. To identify additive or synergistic antithrombotic effects of the various combination therapies, we purposefully used marginally effective doses of razaxaban at 0.1 mg/kg/h, aspirin at 0.3 mg/kg/h and clopidogrel at 1 mg/kg/h. Dual combination of threshold doses of razaxaban and aspirin or clopidogrel produced an enhanced antithrombotic effect without further increases in bleeding time. When compared with dual therapy with aspirin and clopidogrel (38 ± 5% increase in blood flow), addition of razaxaban increased blood flow to 75 ± 5% without additional bleeding time effects (n = 6/group, P < 0.05). In summary, razaxaban was an effective antithrombotic agent in a rabbit model of arterial thrombosis. Low-dose razaxaban was useful in combination with sub-optimal doses of aspirin and/or clopidogrel for the prevention of occlusive arterial thrombosis without excessive bleeding.

Presented in part at the 45th Annual Meeting of the American Society of Hematology, December 6–9, 2003, San Diego, California (abstr 3011), and at the XXth Congress of the International Society on Thrombosis and Hemostasis, August 6–12, 2005, Sydney, Australia (abstr P1852).