Aprotinin Inhibits Thrombin Generation by Inhibition of the Intrinsic Pathway, but is not a Direct Thrombin Inhibitor.

Background  Aprotinin is a broad-acting serine protease inhibitor that has been clinically used to prevent blood loss during major surgical procedures including cardiac surgery and liver transplantation. The prohemostatic properties of aprotinin likely are related to its antifibrinolytic effects, but other mechanisms including preservation of platelet function have been proposed. Aim  Here we assessed effects of aprotinin on various hemostatic pathways in vitro, and compared effects to tranexamic acid(TXA), which is an antifibrinolytic but not a serine protease inhibitor. Methods  We used plasma-based clot lysis assays, clotting assays in whole blood, plasma, and using purified proteins, and platelet activation assays to which aprotinin or TXA were added in pharmacological concentrations. Results  Aprotinin and TXA dose-dependently inhibited fibrinolysis in plasma. Aprotinin inhibited clot formation and thrombin generation initiated via the intrinsic pathway, but had no effect on reactions initiated by tissue factor. However, in the presence of thrombomodulin, aprotinin enhanced thrombin generation in reactions started by tissue factor. TXA had no effect on coagulation. Aprotinin did not inhibit thrombin, only weakly inhibited the TF-VIIa complex and had no effect on platelet activation and aggregation by various agonists including thrombin. Aprotinin and TXA inhibited plasmin-induced platelet activation. Conclusion  Pharmacologically relevant concentrations of aprotinin inhibit coagulation initiated via the intrinsic pathway. The antifibrinolytic activity of aprotinin likely explains the prohemostatic effects of aprotinin during surgical procedures. The anticoagulant properties may be beneficial during surgical procedures in which pathological activation of the intrinsic pathway, for example by extracorporeal circuits, occurs.