Plasma procarboxypeptidase B, also known as thrombin-activatable fibrinolysis inhibitor (TAFI), is converted by thrombin into the active enzyme, carboxypeptidase B (CPB)/activated TAFI. Plasma CPB down-regulates fibrinolysis by removing carboxy-terminal lysines, the ligands for plasminogen and tissue-type plasminogen activator (tPA), from partially degraded fibrin. To target thrombosis in a new way, we have identified and optimized a phosphinic acid-containing inhibitor of CPB,heptanoic acid] and determined both the pharmacological profile and pathophysiological role of CPB in rat thrombolysis. EF6265 specifically inhibited plasma CPB activity with an IC 50 (50% inhibitory concentration) of 8.3 nM and enhanced tPA-mediated clot lysis in a concentrationdependent manner. EF6265 decreased detectable thrombi (percentage of glomerular fibrin deposition; control, 98 Ϯ 1.1; EF6265, 0.1 mg/kg, 27 Ϯ 9.1) that had been generated by tissue factor in a rat microthrombosis model with concomitant increases in plasma D-dimer concentration (control, Ͻ0.5 g/ml; EF6265, 0.1 mg/kg, 15 Ϯ 3.5 g/ml). EF6265 reduced plasma ␣2-antiplasmin activity to a lesser extent than tPA. In an arteriovenous shunt model, EF6265 (1 mg/kg) enhanced exogenous tPA-mediated thrombolysis under the same conditions that neither EF6265 nor tPA (600 kIU/kg) alone reduced thrombi. EF6265 (1 and 30 mg/kg) did not affect the bleeding time in rats. Moreover, it did not prolong the bleeding time evoked by tPA (600 kIU/kg). These results confirm that circulating procarboxypeptidase B functions as a fibrinolysis inhibitor's zymogen and validates the use of CPB inhibitors as both an enhancer of physiological fibrinolysis in microcirculation and as a novel adjunctive agent to tPA for thromboembolic diseases while maintaining a small effect on primary hemostasis.Thrombosis-related diseases, including myocardial infarction, cerebral infarction, and disseminated intravascular coagulation are life-threatening and the search for treatments remains challenging. Although thrombolytics including tissue-type plasminogen activator (tPA) or urokinase-type plasminogen activator and anticoagulants including heparin have been developed over the last few decades, the risk of hemorrhaging with these antithrombotic agents restricts their clinical use. The dose of tPA must be high enough to overcome the inhibitory effects of plasminogen activator inhibitor-1 in the plasma, and results in the generation of plasmin in circulating blood (Rijken and Sakharov, 2001). Consequently, these large quantities of generated plasmin can induce thrombolysis and result in hemorrhaging as a side effect (Bloom et al., 1988). Many efforts have therefore been made to identify and develop pharmacologically distinct antithrombotic agents while maintaining a low risk of hemorrhaging.Plasma procarboxypeptidase B (proCPB; EC 3.4.17.20), also known as thrombin-activatable fibrinolysis inhibitor (TAFI) or procarboxypeptidase U, is produced in the liver and Article, publication date, and citation informa...

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