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-alpha (IIb)beta (3) (GP IIb/IIIa) monoclonal antibody (MoAb) c7E3 facilitates recombinant t issue-type plasminogen activator (rt-PA)-mediated thrombolysis, it is not c lear whether this is due to inhibition of new clot formation and/or a direc t effect on the lysis rate. We employed an in vitro flow (re)circulation mo del to investigate how c7E3 Fab affected (a) platelet adhesion to clotted f ibrin 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 mug/ml) inhibited platelet a dhesion from flowing whole blood onto fibrin substrates (similar to 14 mum thick) at each wall shear rate. When at 5 min after the onset of flow, c7E3 Fab (0.1-10 mug/ml) and rt-PA (1 mug/ml) were coinjected in flowing blood, it was found that modest fibrinolysis caused major platelet release from f ibrin substrates and there was no difference in the lysis rate in the prese nce of rt-PA+c7E3 Fab compared to rt-PA alone. Platelet pretreatment with c 7E3 Fab (10 mug/ml) had no effect on the lysis rate of thin (similar to 40 mum), and slightly delayed the lysis rate of thick (< 250 mum), platelet-fi brin substrates containing evenly dispersed platelets (10(9)/ml). When the platelets within thick platelet-fibrin substrates were organized in platele t-rich regions ("residual thrombi"), these substrates followed a nonuniform lysis pattern, where fibrin between the thrombi lysed first and the residu al thrombi lysed at a slower rate. Platelet pretreatment with c7E3 Fab (10 mug/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-PAmediated lysis, but is expect ed to block platelet-fibrin interactions that lead to clot retraction, thus maintaining a fibrin architecture that is more susceptible to lysis. (C) 2 001 Elsevier Science Ltd. All rights reserved.