A COMBINATION OF RECOMBINANT SINGLE-CHAIN UROKINASE-TYPE PLASMINOGEN-ACTIVATOR AND STREPTOKINASE HAS A SYNERGISTIC NON-FIBRIN SPECIFIC ACTION ON IN-VITRO CLOT LYSIS

Thrombolytic effects of streptokinase (SK), a recombinant single-chain urokinase-type plasminogen activator (rscu-PA), and their combination s were measured in an in vitro system composed of I-125-fibrin labelle d human plasma clot immersed in a citrated human plasma at 37 degrees C. Clot lysis was monitored by release of I-125-fibrin degradation pro ducts and an activation of the fibrinolytic system in the surrounding plasma was measured by fibrinogen, plasminogen and alpha(2)-antiplasmi n levels. SK and rscu-PA induced dose- and time-dependent clot lysis: 50% lysis in 4 h was obtained with 75 nM SK and 38 nM rscu-PA, respect ively. At these concentrations, no significant activation of the fibri nolytic system in plasma was observed with rscu-PA, whereas SK produce d fibrinogen breakdown. As was judged by a mathematical analysis of do se-response curves by the isobole method, simultaneous and sequential equimolar combinations of SK and rscu-PA caused a significant non-fibr in specific synergistic thrombolytic effect. The synergy of thrombolyt ic action of these combinations persisted for 24 h and was more pronou nced at the subsequent addition of SK followed by rscu-PA (in 1 h). 4 h after the addition of 25 nM of SK or rscu-PA, they had induced 6% an d 25% lysis respectively while simultaneous or sequential addition of both plasminogen activators at the same concentrations induced 55% and 80% clot lysis. The maximal achievable 5-fold increase in the thrombo lytic action of sequential combinations was observed in the molar rati o of plasminogen activators 1:1, as compared with the total effect of the agents at the same concentrations as used in simultaneous combinat ions. The synergistic thrombolytic effect of the sequential equimolar combinations was more fibrin-specific than that of the simultaneous co mbinations. It is concluded that the simultaneous and sequential equim olar combinations of SK and rscu-PA possessed synergistic thrombolytic action on clot lysis in vitro. The sequential combination of equimola r concentrations of these plasminogen activators induced a moderate ac tivation of the fibrinolytic system while the simultaneous combination produced a significant exhaustion of the fibrinolytic system.