Termination of bleeding by a specific, anticatalytic antibody against plasmin

Abstract

BackgroundExcessive, plasmin‐mediated fibrinolysis augments bleeding and contributes to death in some patients. Current therapies for fibrinolytic bleeding are limited by modest efficacy, low potency, and off‐target effects. ObjectivesTo determine whether an antibody directed against unique loop structures of the plasmin protease domain may have enhanced specificity and potency for blocking plasmin activity, fibrinolysis, and experimental hemorrhage. MethodsThe binding specificity, affinity, protease cross‐reactivity and antifibrinolytic properties of a monoclonal plasmin inhibitor antibody (Pi) were examined and compared with those of epsilon aminocaproic acid (EACA), which is a clinically used fibrinolysis inhibitor. ResultsPi specifically recognized loop 5 of the protease domain, and did not bind to other serine proteases or nine other non‐primate plasminogens. Pi was ~7 logs more potent in neutralizing plasmin cleavage of small‐molecule substrates and >3 logs more potent in quenching fibrinolysis than EACA. Pi was similarly effective in blocking catalysis of a small‐molecule substrate as α2‐antiplasmin, which is the most potent covalent inhibitor of plasmin, and was a more potent fibrinolysis inhibitor. Fab or chimerized Fab fragments of Pi were equivalently effective. In vivo, in a humanized model of fibrinolytic surgical bleeding, Pi significantly reduced bleeding to a greater extent than a clinical dose of EACA. ConclusionsA mAb directed against unique loop sequences in the protease domain is a highly specific, potent, competitive plasmin inhibitor that significantly reduces experimental surgical bleeding in vivo.