Termination of bleeding by a specific, anticatalytic antibody against plasmin

Background Excessive, 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. Objectives To 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. Methods The 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. Results Pi 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 α 2 ‐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 . Conclusions A 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.