Melagatran, An Oral Active-Site Inhibitor of Thrombin, Prevents or Delays Formation of Electrically Induced Occlusive Thrombus in the Canine Coronary Artery :

TAFI (thrombin-activatable fibrinolysis inhibitor) is a recently discovered plasma protein that can be activated by thrombin-catalyzed proteolysis to a carboxypeptidase B-like enzyme that inhibits fibrinolysis. This work shows that the thrombin-thrombomodulin complex, rather than free thrombin, is the most likely physiologic activator. Thrombomodulin increases the catalytic efficiency of the reaction by a factor of 1250, an effect expressed almost exclusively through an increase in kcat. The kinetics of the reaction conform to a model whereby thrombin can interact with either TAFI (Km = 1.0 microM) or thrombomodulin (Kd = 8.6 nM), and either binary complex so formed can then interact with the third component to form the ternary thrombin-thrombomodulin-TAFI complex from which activated TAFI is produced with kcat = 1.2 s-1. This work also shows that activated TAFI down-regulates tPA-induced fibrinolysis half-maximally at a concentration of 1.0 nM in a system of purified components. This concentration of TAFI is about 2% of the level of the zymogen in plasma, which indicates that ample activated TAFI could be generated to very significantly modulate fibrinolysis in vivo. Therefore, TAFI in vitro and possibly in vivo defines an explicit molecular connection between the coagulation and fibrinolytic cascades, such that expression of activity in the former down-regulates the activity of the latter.

Thrombin has a pivotal role in the pathogenesis of acute coronary thrombosis. We compared the clinical efficacy of a potent, direct thrombin inhibitor, recombinant hirudin, with that of heparin (an indirect antithrombin agent) in patients with unstable angina or acute myocardial infarction. At 373 hospitals in 13 countries, 12,142 patients with acute coronary syndromes were randomly assigned to 72 hours of therapy with either intravenous heparin or hirudin. Patients were stratified according to the presence of ST-segment elevation on the base-line electrocardiogram (4131 patients) or its absence (8011 patients), with the latter characteristic considered to indicate unstable angina or non-Q-wave myocardial infarction. At 24 hours, the risk of death or myocardial infarction was significantly lower in the group assigned to hirudin therapy than in the group assigned to heparin (1.3 percent vs. 2.1 percent, P = 0.001). The primary end point of death or nonfatal myocardial infarction or reinfarction at 30 days was reached in 9.8 percent of the heparin group as compared with 8.9 percent of the hirudin group (odds ratio for the risk of the end point in hirudin group,0.89; 95 percent confidence interval, 0.79 to 1.00; P = 0.06). The predominant effect of hirudin was on myocardial infarction or reinfarction and was not influenced by ST-segment status. There were no significant differences in the incidence of serious or life-threatening bleeding complications, but hirudin therapy was associated with a higher incidence of moderate bleeding (8.8 percent vs. 7.7 percent, P = 0.03). For acute coronary syndromes, recombinant hirudin provided a small advantage, as compared with heparin, principally related to a reduction in the risk of nonfatal myocardial infarction. The relative therapeutic effect was more pronounced early (at 24 hours) but dissipated over time. The small benefit was consistent across the spectrum of acute coronary syndromes and was not associated with a greater risk of major bleeding complications.

Acute myocardial infarction is triggered by coronary artery occlusion that may be recanalized by thrombolytic therapy with a success rate of up to 75% only. The resistance of coronary artery occlusion to thrombolysis may either be due to obstruction of the lumen by a nonthrombotic mechanism or by intrinsic resistance of thrombus to dissolution. Coronary arterial thrombi are composed of platelet-rich and erythrocyte-rich material in variable proportions. To evaluate the relative sensitivity of these thrombus components to thrombolysis, we have used two femoral arterial thrombosis models in the rabbit, consisting of erythrocyte-rich clot produced by injecting whole blood and thrombin in an isolated segment and of platelet-rich thrombus spontaneously formed on an everted (inside out) femoral arterial segment. Intravenous infusion of recombinant tissue-type plasminogen activator (rt-PA) at a rate of 30 micrograms/kg/min consistently reperfused arteries occluded with erythrocyte-rich clot (six of six animals compared with zero of six placebo-treated animals, p = 0.002), whereas infusion of 30 or 100 micrograms/kg/min was significantly less efficient for reperfusion of everted segments occluded with platelet-rich material (only four of 12 animals, p = 0.01). Intra-arterial infusion proximal to the occlusion, at a rate of 20 micrograms/kg/min reperfused six of seven rabbits with erythrocyte-rich clots but only one of seven rabbits with occluded everted segments (p = 0.03). A dose of 100 micrograms/kg/min was necessary to reperfuse platelet-rich occlusions in five of six rabbits.(ABSTRACT TRUNCATED AT 250 WORDS)