Effect of SanOrg123781A, a synthetic hexadecasaccharide, on clot-bound thrombin and factor Xa in vitro and in vivo

Summary. Factor (F)Xa and thrombin bound to the clot during its formation contribute to the propensity of thrombi to activate the coagulation system. The aim of this work was to study the inhibition of clot‐bound FXa and clot‐bound thrombin by SanOrg123781A, a synthetic hexadecasaccharide that enhances the inhibition of thrombin and FXa by antithrombin (AT). SanOrg123781A, designed to exhibit low non‐specific binding to proteins other than AT, was compared with heparin. In buffer, heparin and SanOrg123781A inhibited FXa and thrombin at similar concentrations [concentration inhibiting 50% (IC₅₀) of Xa and IIa activity were, respectively: heparin 120 ± 7 and 3 ± 1 ng mL^?1; SanOrg123781A 77 ± 5 and 4 ± 1 ng mL^?1]. In human plasma, the activity of both compounds was reduced, although the activity of heparin was much more affected than that of SanOrg123781A (IC₅₀ values for inhibition of FXa and FIIa activity were, respectively: heparin 100 ± 5 and 800 ± 40 ng mL^?1; SanOrg123781A 10 ± 5 and 30 ± 3 ng mL^?1). We demonstrated, in agreement with our previous results, that the procoagulant activity of the clot is essentially due to clot‐bound FXa and to some extent to clot‐bound thrombin. We showed that heparin and SanOrg123781A were able to inhibit fragment F1+2 generation induced by clot‐bound FXa with IC₅₀ values of 2 ± 0.5 µg mL^?1 and 0.6 ± 0.2 µg mL^?1, respectively. Both compounds also inhibited clot‐bound thrombin activity, the IC₅₀ values of heparin and SanOrg123781A being 1 ± 0.01 µg mL^?1 and 0.1 ± 0.1 µg mL^?1, respectively. Moreover, both heparin and SanOrg123781A significantly inhibited fibrinopeptide A generated by the action of clot‐bound thrombin on fibrinogen but also by free thrombin generated from prothrombin by clot‐bound FXa with IC₅₀ values of 4 ± 0.6 and 1 ± 0.1 µg mL^?1, respectively. As with clot‐bound enzymatic activities, SanOrg123781A was three times more active than heparin in vivo on fibrinogen accretion onto a pre‐existing thrombus and as activators of recombinant tissue‐type plasminogen activator‐induced thrombolysis. In conclusion, due to the specific activities of SanOrg123781A, this compound is much more active than heparin in the presence of plasma proteins, on clot‐bound enzymes and in in vivo models of thrombosis/thrombolysis.