Synergistic Effect on Thrombolysis of Sequential Infusion of Tissue-Type Plasminogen Activator (t-PA) Single-Chain Urokinase-Type Plasminogen Activator (scu-PA) and Urokinase in the Rabbit Jugular Vein Thrombosis Model

Summary In a quantitative model of thrombolysis, consisting of rabbits with a 125T-fibrin labeled blood clot in the jugular vein, simultaneous intravenous infusion over 4 hours of t-PA and scu-PA or of t-PA and urokinase had a significantly greater (p <0.01) thrombolytic effect than could be anticipated on the basis of the added effects of each agent alone. In order to further investigate the mechanism of this in vivo synergism, recombinant t-PA (rt-PA) and scu-PA in synergistic amounts were infused: 1) simultaneously over 4 hours, 2) rt-PA over 1 hour, then 15 min later scu-PA over 2 hours and 3) scu-PA over 1 hour, then 15 min later rt-PA over 2 hours. Simultaneous infusion of 0.1 mg/kg rt-PA and 0.2 mg/kg scu-PA gave 48°2 percent thrombolysis (mean ° SEM, n = 5) and of 0.2 mg/kg rt-PA and 0.4 mg/kg scu-PA 67°5 percent (n = 5). When these infusions were given sequentially, rt-PA followed by scu-PA gave 32 °5 (n = 4) and 49 °8 (n = 4) percent lysis, but scu-PA followed by rt-PA yielded only 14° 1 (n = 4) and 21 ° 1 (n = 4) percent lysis, indicating that synergism occurs when rt-PA is followed by scu-PA but not when scu-PA is followed by rt-PA. In order to investigate the hypothesis that rt-PA predigests the clot resulting in more efficient plasminogen activation by scu-PA at the clot surface, partial thrombolysis was induced by injection of urokinase. Subsequent infusion of 0.4 mg/ kg of scu-PA did however not result in more thrombolysis than expected for additive effects. Infusion of 0.5 mg/kg of urokinase followed by 0.1 mg/kg rt-PA was not synergistic (24 °3 percent lysis, n = 3) whereas lysis by rt-PA followed by urokinase was 34°3 percent (p <0.1, n = 3). Sequential therapy with rt-PA followed by scu-PA might constitute an alternative to simultaneous infusion of synergistic thrombolytic agents.