COMPARATIVE EFFECTS OF 2 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 t his work was to compare the extent of prothrambinase inhibition produc ed by two factor Xa inhibitors: the antithrombin III-dependent synthet ic pentasaccharide (SR 90107/Org 31540) and DX-9065A, a direct factor Xa inhibitor. When incubated together with prothrombin, factor Xa, pho spholipids, antithrombin III and calcium, clots formed from human plas ma exhibited a prothrombinase activity as measured through fragment 1- 2 (F1+2) generation. Ten washes of the clot were required to achieve c omplete 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 fact or Xa and phospholipids but no factor V/Va. in all tested experimental conditions, clot-bound-factor Xa-induced F1+2 generation was inhibite d by SR 90107/AT and DX-9065A with IC50 in the same range of concentra tions (0.5 mu M). In contrast, the inhibition of prothrombinase formed with factor Xa, factor Va phospholipids and calcium in buffer was obs erved at significantly lower concentrations of DX-9065A than of SR 901 07/AT (respective IC50 concentrations: 0.1 and 70 mu M), In vivo, fibr in accretion onto a preformed thrombus as well as venous thrombosis in duced in the jugular vein of rabbits was inhibited by SR 90107 and DX- 9065A in the same range of concentrations therefore showing that inhib ition of clot-bound factor Xa is a predominant factor for the antithro mbotic activity of both direct and indirect inhibitors for factor Xa.