Effects of aprotinin on coagulation and fibrinolysis enzymes

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.

Effects of agents, used to treat bleeding disorders, on bleeding time prolonged by a very high dose of a direct thrombin inhibitor in anesthesized rats and rabbits

Melagatran is the active form of the oral, direct thrombin inhibitor, H 376/95, that is under evaluation in clinical trials for the prevention and treatment of thromboembolism. In this study, a single dose, calculated on body weight basis, of antifibrinolytic treatment, factor VIIa, factor VIII with and without von Willebrand factor (vWF), factor IX, activated (APCC) or nonactivated (PCC) prothrombin complex concentrates was given intravenously to rats and rabbits, in an attempt to reverse the prolonged bleeding time during intensive anticoagulation with melagatran (2 micromol/kg/h). The doses used were at or above human therapeutic doses. The cutaneous tail bleeding time in the rat, as well as the ear incision bleeding time and cuticle bleeding time, and the blood loss in the rabbit were used for evaluation of the hemostatic effects of these agents. In vivo Feiba (APCC) and Prothromplex-T (PCC) shortened the prolonged cutaneous bleeding times in rats (P<.05); Feiba and Autoplex (APCC) shortened the cutaneous bleeding times in rabbits (P<.05). In contrast, Prothromplex-T prolonged bleeding times and blood loss in the rabbits (P<.05). Ex vivo Feiba, Autoplex and NovoSeven (rF VIIa) significantly (P<.05) shortened the prolonged whole blood clotting time (WBCT). Prothromplex-T significantly prolonged WBCT, activated clotting time (ACT) and activated partial thromboplastin time (APTT). Feiba, Autoplex, and Prothromplex-T increased thrombin generation measured as increased thrombin-antithrombin complex (TAT) formation. In conclusion, APCCs were found to be the most effective agents for reversing bleeding time induced by a very high plasma concentration of melagatran. APCC and recombinant activated factor FVII (rF VIIa) effectively shortened the prolonged WBCT. Thus, stimulating thrombin generation with the use of APCC may counteract the anticoagulant effect observed with a very high dose of a thrombin inhibitor.