More than 850,000 myocardial infarctions (MI) occur in the US annually, 300,000 of which are recurrences, carrying with them a two-fold increased risk of death. Recurrent MI take place owing to the high rates of reocclusion of cardiac arteries observed following initially successful fibrinolytic therapy with t-PA or streptokinase. Plasmin-induced activation of the intrinsic coagulation system, proceeding via two tightly- linked enzymes, factor XII (FXII) and plasma kallikrein (PK), may be the mechanism for generation of new thrombin during fibrinolysis. Mice deficient in FXII are highly resistant to the formation of occluding arterial thrombi in multiple models of injury-mediated arterial thrombus formation. The extent of PK activation regulates the amount of FXII activity, and thus inhibition of PK would provide a novel anti-thrombotic therapeutic approach for the treatment of acute MI, especially as adjunctive therapy to current standard-of- care fibrinolytic treatments. We have generated novel small molecule PK inhibitors displaying both high potency for PK inhibition and selectivity against other enzymes in the intrinsic pathways. In addition, sensitive plasma-based assays have been developed to quantitatively measure the extent of inhibition of the intrinsic coagulation pathway resulting from PK inhibition. Using these assays, we have demonstrated that inhibition of PK is more effective than depletion of FXII for inhibiting thrombin generation and coagulation by the intrinsic pathway. We propose in Phase I to develop PK inhibitors that have defined potency, selectivity and pharmacokinetic properties, then test the best compounds in a mouse model of arterial thrombosis, so as to provide proof-of-concept that selective inhibition of PK could provide a novel therapeutic approach for reducing risk of reocclusion and reinfarction following fibrinolytic therapy for MI. The successful completion of Phase I will also result in the identification of one or more lead compounds suitable for preclinical development in Phase II for adjunctive therapy for MI patients undergoing fibrinolytic therapy. This project proposes that inhibition of the intrinsic coagulation pathway, to be accomplished by inhibition of a key enzyme of this pathway, plasma kallikrein (PK), would have significant therapeutic impact in reducing the risk of reocclusion and reinfarction in patients with mycorcardial infarction (MI) who have undergone fibrinolytic treatment. It seeks to optimize prototype novel small molecule inhibitors of PK to obtain lead molecules that will be tested in animal models of thrombosis in Phase I, then proceed into preclinical development in Phase II for adjunctive therapy for MI patients receiving fibrinolytic therapy. [unreadable] [unreadable] [unreadable]