Differential effects of c7E3 Fab on thrombus formation and rt-PA-Mediated thrombolysis under flow conditions.

Although the Fab fragment of the mouse-human chimeric anti-alphaIIbbeta3 (GP IIb/IIIa) monoclonal antibody (MoAb) c7E3 facilitates recombinant tissue-type plasminogen activator (rt-PA)-mediated thrombolysis, it is not clear whether this is due to inhibition of new clot formation and/or a direct effect on the lysis rate. We employed an in vitro flow (re)circulation model to investigate how c7E3 Fab affected (a) platelet adhesion to clotted fibrin substrates under laminar flow at wall shear rates of 100 or 500 s(-1) and (b) rt-PA-induced lysis of preformed mural platelet-fibrin substrates at 500 s(-1). c7E3 Fab dose-dependently (0.5-5 microg/ml) inhibited platelet adhesion from flowing whole blood onto fibrin substrates ( approximately 14 microm thick) at each wall shear rate. When at 5 min after the onset of flow, c7E3 Fab (0.1-10 microg/ml) and rt-PA (1 microg/ml) were coinjected in flowing blood, it was found that modest fibrinolysis caused major platelet release from fibrin substrates and there was no difference in the lysis rate in the presence of rt-PA + c7E3 Fab compared to rt-PA alone. Platelet pretreatment with c7E3 Fab (10 microg/ml) had no effect on the lysis rate of thin ( approximately 40 microm), and slightly delayed the lysis rate of thick (< 250 microm), platelet-fibrin substrates containing evenly dispersed platelets (10(9)/ml). When the platelets within thick platelet-fibrin substrates were organized in platelet-rich regions ("residual thrombi"), these substrates followed a nonuniform lysis pattern, where fibrin between the thrombi lysed first and the residual thrombi lysed at a slower rate. Platelet pretreatment with c7E3 Fab (10 microg/ml) abolished the formation of the lytic-resistant residual thrombi and the associated platelet-protected fibrin zones. Hence, treatment with c7E3 Fab has no direct effect on the rate of rt-PA-mediated lysis, but is expected to block platelet-fibrin interactions that lead to clot retraction, thus maintaining a fibrin architecture that is more susceptible to lysis.