Comparative effects of two direct and indirect factor Xa inhibitors on free and clot-bound prothrombinase.

Factor Xa, as with thrombin, binds to the clot and contributes to the propensity of thrombi to activate the coagulation system. The aim of this work was to compare the extent of prothrombinase inhibition produced by two factor Xa inhibitors: the antithrombin III-dependent synthetic pentasaccharide (SR 90107/Org 31540) and DX-9065A, a direct factor Xa inhibitor. When incubated together with prothrombin, factor Xa, phospholipids, antithrombin III and calcium, clots formed from human plasma exhibited a prothrombinase activity as measured through fragment 1-2 (F1+2) generation. Ten washes of the clot were required to achieve complete removal of unbound factor Xa. The absence of F1+2 generation brought about by washed clots in buffer when factor V was omitted, or in the presence of annexin V, indicated that they contained bound factor Xa and phospholipids but no factor V/Va. In all tested experimental conditions, clot-bound-factor Xa-induced F1+2 generation was inhibited by SR 90107/AT and DX-9065A with IC50 in the same range of concentrations (0.5 microM). In contrast, the inhibition of prothrombinase formed with factor Xa, factor Va phospholipids and calcium in buffer was observed at significantly lower concentrations of DX-9065A than of SR 90107/AT (respective IC50 concentrations: 0.1 and 70 microM). In vivo, fibrin accretion onto a preformed thrombus as well as venous thrombosis induced in the jugular vein of rabbits was inhibited by SR 90107 and DX-9065A in the same range of concentrations therefore showing that inhibition of clot-bound factor Xa is a predominant factor for the antithrombotic activity of both direct and indirect inhibitors for factor Xa.