Relative Efficacy of Active Site-Blocked Factors IXa, Xa in Models of Rabbit Venous and Arterio-Venous Thrombosis

Summary In order to investigate the respective roles of prothrombinase and intrinsic tenase (IXa/VIIIa) in venous thrombosis, we compared the antithrombotic efficacy of inhibitors of these two coagulation complexes. The agents tested were dansyl-Glu-Gly-Arg chloromethyl ketone-inactivated bovine factor IXa (IXai) and Glu-Gly-Arg chloromethyl ketone- inactivated human factor Xa (Xai). In vitro formation of active complexes (prothrombinase or tenase) was inhibited by Xai and IXai resulting in IC50 values of 3 nM and 5 nM, respectively. Antithrombotic activity was measured by inhibition of clot accretion on cotton threads placed in the abdominal vena cava of anesthetized rabbits. Intravenous bolus dosing followed by infusion of Xai during the experimental protocol resulted in a dose dependent reduction of clot weight, a dosage of 16.0 μg/kg + 0.28 μg/kg/min being sufficient to produce a 96% inhibition of thrombosis. A much higher dose of IXai (1.0 mg/kg + 17.3 |ig/kg/min) resulted in a 39% reduction of clot weight. In a rabbit arterio-venous shunt model mimicking arterial thrombosis, the relative efficacy of the two agents was found to be more comparable. The doses required for optimum antithrombotic activity were 128.0 μg/kg + 2.2 μg/kg/min for Xai and 1.0 mg/kg+ 17.3 μg/kg/min for IXai. We conclude that, in this study, prothrombinase rather than tenase inhibition was more effective in reducing venous thrombosis and that these effects can be achieved without disruption of extravascular hemostasis.