Summary Direct fibrinolytics are proteolytic enzymes that degrade fibrin without requiring an intermediate step of plasminogen activation. This review summarizes the current information available for five such agents, namely, plasmin (the prototypical form), three derivatives of plasmin (mini-plasmin, micro-plasmin, and delta-plasmin), and alfimeprase, a recombinant variant of a snake venom alpha-fibrinogenase, fibrolase. Biochemical attributes of molecular size, fibrin binding and inhibitor neutralization are compared. Pre-clinical foundations that assess potential for thrombolytic efficacy in vitro and in animal models of vascular occlusion and for hemostatic safety in animal models of bleeding are detailed. Clinical potential has been assessed in patients with peripheral arterial and graft occlusion, acute ischemic stroke, access catheter and hemodialysis shunt occlusions. The direct fibrinolytic agents have impressive biochemical and pre-clinical foundation for ultimate clinical application. However, clinical trial results for micro-plasmin and alfimeprase have not measured up to their anticipated benefit. Plasmin has thus far shown encouraging hemostatic safety, but efficacy data await completion of clinical trials. Whether direct fibrinolytics will impart clinical superiority in major thrombotic disorders over currently utilized indirect fibrinolytics such as tissue plasminogen activator remains to be determined.

We investigated the in vitro fibrinolytic properties of microplasmin, the isolated proteinase domain of plasmin, and its thrombolytic efficacy in a coronary artery thrombosis model in dogs. The amidolytic and fibrinolytic activity of recombinant microplasmin was compared with natural human plasmin. The thrombolytic efficacy of microplasmin was studied in a canine model of copper coil induced coronary artery thrombosis. Animals were randomly assigned to one of six treatment regimens, each with five animals per cohort. Four treatment groups received an intravenous bolus of microplasmin followed by an intravenous infusion of microplasmin for 1 h (1 mg/kg + 1.5 mg/kg/h with or without abciximab or 2 mg/kg + 3 mg/kg/h). In two treatment groups, microplasmin was administered intracoronary. Bolus administration was followed by a 1-h infusion if coronary flow was incompletely restored after the initial bolus administration (1 mg/kg + 1.5 mg/kg/h or 2 mg/kg + 3 mg/kg/h, respectively). The thrombolytic efficacy was documented by repeated angiographies and the coronary perfusion was assessed with the Thrombolysis in Myocardial Infarction (TIMI) grading. No significant differences between plasmin and microplasmin were observed with respect to the catalytic efficiencies towards the synthetic chromogenic substrates S-2403 or S-2444. The concentration required for 50% lysis of purified fibrin clots in 3 h, was ~100 nM for microplasmin compared to 20 nM for natural plasmin. Intravenous bolus administration of microplasmin restored TIMI 3 coronary flow in 0/5, 0/5, 1/5 and 2/5, respectively, whereas intracoronary bolus administration restored TIMI 3 coronary flow in 1/5 and 4/5 (1 mg/kg and 2 mg/kg, respectively)(ANOVA P < 0.05). TIMI 3 coronary flow was obtained in 0/5, 2/5, 2/5 and 3/5, respectively, during subsequent intravenous administration and in 5/5 and 4/5 in case of intracoronary administration (ANOVA P < 0.05). When compared to natural plasmin, the catalytic efficiency of microplasmin towards chromogenic substrates was similar, but the fibrinolytic potency of microplasmin towards fibrin clots was lower. Intracoronay administration of microplasmin effectively lysed coronary thrombosis.

Fibrinolytic activity has been shown to be reduced in many vascular diseases, including hepatic veno-occlusive disease after stem cell transplantation, a microangiopathy characterized by sinusoidal endothelial cell injury. Defibrotide is a polydisperse oligonucleotide with antithrombotic, profibrinolytic, anti-ischemic, and antiadhesive properties. Numerous clinical studies have shown promising activity of defibrotide in the treatment and prevention of veno-occlusive disease, with minimal toxicity. In corollary laboratory studies, defibrotide has been shown to decrease plasminogen activator inhibitor-1, increase tissue plasminogen activator levels, and increase overall plasma fibrinolytic activity in patients. Plasmin, a potent and nonspecific serine protease, plays a pivotal role in fibrinolysis by virtue of its ability to effectively degrade fibrin clots. In this study, defibrotide increases the activity of plasmin in hydrolyzing its substrate in a dose-dependent and length-dependent manner. Similar concentration-dependent effects of defibrotide were observed when plasmin was generated by tissue plasminogen activator or urokinase activation of plasminogen. In contrast, defibrotide had no direct effect on the activation of plasminogen to plasmin. Defibrotide was also able to enhance the activity of plasmin in degrading fibrin clot formed from fibrinogen, plasminogen, and thrombin. This effect was also concentration-dependent and directly correlated with the enzymatic activity of plasmin. This study therefore demonstrates that defibrotide is capable of enhancing the activity of plasmin and so contributes to its fibrinolytic activity. Taken together, these results support the effect of defibrotide in restoring the fibrinolytic vascular phenotype, in microangiopathic conditions such as veno-occlusive disease.

Plasmin is the prototype of a distinct class of "direct-acting" fibrinolytic agents, with biochemical and physiological attributes that are favorable for catheter-delivered thrombolytic therapy. Our studies indicate that plasmin is superior to plasminogen activators for hemostatic safety and thrombolytic efficacy in experimental models, and that plasmin has potential to avoid the bleeding risk that accompanies therapy of deep vein thrombosis with currently-used thrombolytic agents.

Several randomized trials of catheter-directed thrombolysis versus operative revascularization in patients with acute lower extremity ischemia were performed in the mid-1990s. Although the outcomes of these trials were not definitive, they did provide insight into potential uses and techniques of catheter-directed thrombolysis and lytic agents. This article reviews the outcomes of these randomized trials and describes advances in thrombolytic techniques and technology, including percutaneous mechanical thrombectomy devices and innovative catheter designs that accelerate lysis and the development of direct-acting lytic agents.

INTRODUCTION: Hemodialysis grafts often fail because of stenosis at the venous anastomosis resulting in thrombosis. Percutaneous intervention involves thrombolysis with plasminogen activators, mechanical removal of thrombus, and angioplasty of the stenotic lesions. OBJECTIVES: This article describes a phase I trial using plasmin (human) TAL 05-00018, a direct-acting fibrinolytic agent, to evaluate safety and, secondarily, to establish effective thrombolytic dosing. PATIENTS/METHODS: Six cohorts of five patients with acute HD graft occlusion documented by fistulagrams were treated with increasing dosages of plasmin (1, 2, 4, 8, 12, and 24 mg) infused within the graft over 30 min via two criss-crossed pulse-spray catheters. The primary efficacy endpoint was at least 50% thrombolysis, as determined by fistulography. RESULTS: There was no significant change in plasma alpha-2 antiplasmin or fibrinogen concentration, major bleeding did not occur, and there were no deaths. Serious adverse events in four patients were not related to the study drug. There was a dose-response relationship for the primary efficacy endpoint, all five subjects receiving 24 mg achieving more than 75% lysis. CONCLUSION: This first phase I study of plasmin (human) TAL 05-00018, infused into thrombosed HD grafts, documents safety at all doses and an effective thrombolytic dosage of 24 mg indicating that further investigation into locally delivered plasmin is warranted.

Catheter-directed thrombolysis is an important treatment option for many patients with acute lower extremity ischemia due to arterial and/or graft thrombosis. A growing body of evidence demonstrates that thrombolytic therapy for acute limb ischemia reduces the need for amputation; however, bleeding complications are more likely with lytic therapy. New direct-acting fibrinolytic compounds are being developed that facilitate thrombolysis while reducing the risk of bleeding. Plasmin, the active enzyme produced by all plasminogen activators, works directly upon thrombus and is neutralized instantly in the systemic circulation. This report reviews the principles for effective catheter directed thrombolysis for acute arterial occlusions and describes the development and evaluation of the new thrombolytic, plasmin.