Continuous Localized Monitoring of Plasmin Activity Identifies Differential and Regional Effects of the Serine Protease Inhibitor Aprotinin: Relevance to Antifibrinolytic Therapy

Abstract

Antifibrinolytic therapy, such as the use of the serine protease inhibitor aprotinin, was a mainstay for hemostasis after cardiac surgery. However, aprotinin was empirically dosed, and although the pharmacological target was the inhibition of plasmin activity (PLact), this was never monitored, off-target effects occurred, and led to withdrawn from clinical use. The present study developed a validated fluorogenic microdialysis method to continuously measure PLact and tested the hypothesis that standardized clinical empirical aprotinin dosing would impart differential and regional effects on PLact. Pigs (30 kg) were instrumented with microdialysis probes to continuously measure PLact in myocardial, kidney, and skeletal muscle compartments (deltoid) and then randomized to high-dose aprotinin administration (2 mKIU load/0.5 mKIU/hr infusion; n = 7), low-dose aprotinin administration (1 mKIU load/0.250 mKIU/hr infusion; n = 6). PLact was compared with time-matched vehicle (n = 4), and PLact was also measured in plasma by an in vitro fluorogenic method. Aprotinin suppressed PLact in the myocardium and kidney at both high and low doses, indicative that both doses exceeded a minimal concentration necessary for PLact inhibition. However, differential effects of aprotinin on PLact were observed in the skeletal muscle, indicative of different compartmentalization of aprotinin. Using a large animal model and a continuous method to monitor regional PLact, these unique results demonstrated that an empirical aprotinin dosing protocol causes maximal and rapid suppression in the myocardium and kidney and in turn would likely increase the probability of off-target effects and adverse events. Furthermore, this proof of principle study demonstrated that continuous monitoring of determinants of fibrinolysis might provide a novel approach for managing fibrinolytic therapy.