RPR120844, a novel, specific inhibitor of coagulation factor Xa inhibits venous thrombosis in the rabbit.

The in vivo antithrombotic activity of RPR 20844, a novel synthetic coagulation factor Xa (fXa) inhibitor (Ki = 7 nM), was assessed by its ability to inhibit thrombus formation in a damaged segment of the rabbit jugular vein. Intravenous dose-response studies were performed and thrombus mass (TM), activated partial thromboplastin time (APTT), prothrombin time (PT), inhibition of ex vivo fXa activity and plasma drug levels (PDL) were determined. TM, measured at the end of a 50 min infusion, was significantly reduced (p<0.05 vs. saline-treated animals) by RPR120844 at 30 and 100 microg/kg/min. At doses of 10, 30 and 100 microg/kg/min, APTT was prolonged by 2.1, 4.2 and 6.1-fold, and PT was prolonged by 1.4, 2.2 and 3.5-fold, respectively. PDL were determined by measuring anti-fXa activity using an amidolytic assay. Peak PDL were 0.8+/-0.3, 1.5+/-0.9 and 2.4+/-0.6 microM, respectively. The drug effect was reversible with APTT, PT and PDL returning toward pretreatment values 30 min after termination of treatment. The results suggest that RPR 120844, or similar compounds, may provide an efficacious, yet easily reversible, means of inhibiting thrombus formation.