A Unique Recombinant Plasmin Molecule Lacking Kringles 2-5 Possesses Equivalent Hemostatic Safety as Full-Length Human Plasmin.

Background: All thrombolytic agents in current clinical use are plasminogen activators (PA). While effective, PA have a downside because of unavoidable and potentially lifethreatening bleeding complications in vulnerable patients. We previously demonstrated a striking margin of hemostatic safety for the “direct-acting” thrombolytic enzyme, plasmin. Now, a novel recombinant derivative of plasmin has been synthesized which lacks kringles 2-5, consisting only of kringle 1 linked to the serine protease domain.

Methods: We evaluated escalating dosages of recombinant D (K2-K5) plasmin in a randomized and blinded manner in comparison with molar equivalent dosages of full-length plasmin in an animal model of fibrinolytic hemorrhage. Hemostatic safety was measured by primary bleeding time (PBT) and plasma concentrations of a-2 antiplasmin, factor VIII and fibrinogen.

Results: A dose-related nadir in a-2 antiplasmin, factor VIII and fibrinogen followed administration of 4, 6, 8 and 10 mg/kg of D (K2-K5) plasmin, with residual fibrinogen of 65%, 40%, 30% and 0% of initial pre-treatment concentrations. Hemostasis was undisturbed with 4 mg/kg of D (K2-K5) plasmin (mean PBT 2.2 +/- 0.7 min), minimally so at the 6 or 8 mg/kg dosages (5 +/- 2.9 and 4.4 +/- 2.2 min). Hemostasis was markedly abnormal at the purposefully toxic 10 mg/kg dose (12.8 +/- 18.8 min), in association with complete depletion of plasma fibrinogen. Comparison of agents at 10 mg/kg showed no difference, but at 4 mg/kg (2-4-fold more than needed for efficacy), D (K2-K5) plasmin showed slightly less prolongation of PBT (2.2 +/- 0.7 vs 3.3 +/- 2.1 min). Both agents were equally potent for in vitro whole blood clot lysis, indicating that the safety advantage of D (K2-K5) plasmin was not due to decreased fibrinolytic activity.

Conclusions: Recombinant D (K2-K5) plasmin has equivalent hemostatic safety as full-length plasmin in an animal model of fibrinolytic hemorrhage and warrants evaluation in clinical thrombotic disease.