Coronary artery often reoccludes after therapy of acute myocardial infarction with recombinant tissue plasminogen activator rt-PA, most likely due to in situ thrombin generation during thrombolysis. Previous studies with high molecular weight thrombin inhibitors, such as hirudin, have shown variable improvement in the frequency of sustained thrombolysis. This study was conducted to examine the modulation of thrombolysis, indices of thrombin generation and activated partial thromboplastin time (APTT) by a novel low molecular weight direct thrombin inhibitor, inogatran. A stable occlusive intracoronary thrombus was created in 19 dogs. Nine dogs were given an intravenous bolus of saline (group A), and 5 dogs were given inogatran (group B) followed by rapid infusion of rt-PA (1 mg/kg over 20 min), whereas saline or inogatran was continuously infused for 2 hr. Five other dogs were given inogatran (bolus and continuous infusion) only after thrombolysis by rt-PA was obtained (group C). Time to reflow was similar in all dogs. None of the reperfused coronary arteries reoccluded in group B dogs (vs. 75% and 40% reocclusion rates in groups A and C, respectively, P < .02). Accordingly, the mean duration of reflow was > 120 min in group B dogs (vs. 39 +/- 7 and 44 +/- 14 min in group A and C dogs, respectively, P < .05). After infusion of inogatran, APTT was increased to 1.6 to 1.9 times the base-line value, and the changes in APTT were similar in group B and C dogs. Thrombin generation and activity, assessed by thrombin-antithrombin complex and fibrinopeptide A levels, increased in all dogs during thrombus formation. The increase in thrombin-antithrombin complex and fibrinopeptide A levels during thrombolysis was not evident in group B dogs. These data show that direct thrombin inhibition with inogatran, when initiated before rt-PA, results in sustained thrombolysis and only a modest increase in APTT. However, inogatran given after thrombolysis only partially prevents reocclusion because large amounts of thrombin generation occur during the early stages of thrombolysis.