The thrombolytic properties of a novel modified human tissue plasminogen activator (E6010), in which cystein 84 in the epidermal growth factor domain is replaced by serine and that has a prolonged biological half-life, were examined. The thrombolytic efficacies of E6010 and recombinant human tissue plasminogen activator (rt-PA) on the duration of coronary artery thrombus were evaluated in a canine model (123 anesthetized dogs) with copper coil-induced left anterior descending coronary artery thrombus. Thrombi established for periods of 1, 3, or 6 h, as documented by coronary arteriography, were employed. A single bolus i.v. injection of E6010 or rt-PA and an i.v. infusion of rt-PA over 60 min were compared (n = 6). Thrombolytic efficacy was evaluated by three criteria: time to reperfusion (TR), reperfusion rate at 60 min (RR), and reocclusion rate at 60 min after reperfusion (OR). With a bolus i.v. injection of E6010 at a dose of 0.2 mg/kg or an i.v. infusion of rt-PA at a dose of 0.6 mg/kg/h, these parameters were as follows: TR, 30.0 +/- 15.3 and 27.5 +/- 4.8 min; RR, 100 and 100%; OR, 17 and 33% for 1-h aged thrombi; TR, 30.0 +/- 9.5 and 35.0 +/- 8.2 min; RR, 83 and 50%; OR, 20 and 67% for 6-h aged thrombi. These data indicate that a bolus injection of E6010 is almost equally efficacious in lysing thrombi aged both 1 and 6 h. On the other hand, in the case of rt-PA, the thrombi aged 6 h were lysed significantly less than the thrombi aged 1 h. Plasma half-lives of E6010 were t1/2 alpha, 4.8 +/- 0.95 (estimated by antigen level) and 3.0 +/- 0.78 min (estimated by activity), and t1/2 beta, 51 +/- 5.4 (antigen level) and 22 +/- 7.0 min (activity). The half-lives of rt-PA were t1/2 alpha, 3.6 +/- 0.23 (antigen level) and 2.1 +/- 0.61 min (activity), and t1/2 beta, 36 +/- 2.3 (antigen level) and 7.0 +/- 3.5 min (activity). We conclude that a bolus injection of E6010 may have a more potent and longer-lasting effect than i.v.-infused rt-PA in clot lysis therapy.