Differential in vitro effects of the platelet glycoprotein IIb/IIIa inhibitors abixicimab or SR121566A on platelet aggregation, fibrinogen binding and platelet secretory parameters

The aim of this study was to compare fibrinogen binding, inhibition of plat elet aggregation and seceretory potential of the MAb abciximab (0.5-5 mu g/ mL) and the peptidomimetic compound SR121-566A (15-250 ng/mL) in vitro in w hole blood. Fibrinogen binding was followed by flow cytometry; platelet fun ction was evaluated by light transmittance and by impedance aggregometry. S ecretory functions of platelets were evaluated using ATP as marker for earl y secretion by dense granulae and P-selectin (CD62) for or-granular secreti on as well as CD63 for lysosomal degranulation. Results showed that fibrino gen binding induced by 5 mu M TRAP was maximally inhibited greater than 80% at 3 mu g/mL abciximab or at 250 ng/mL SR121566A. At these concentrations of antagonists, platelet aggregation induced by 5 mu M ADP or 2 mu g/mL col lagen was inhibited completely. Expression of CD62 was reduced 34% with abc iximab or 15% with SR121566A; CD63 expression was reduced 22% with both age nts. With both agents, the EC50 for inhibition of CD62 and CD63 expressions was in similar magnitudes than the EC50 for fibrinogen binding inhibition. With 3 mu g/mL abciximab, ATP secretion was maximally reduced to 50% of th e control, whereas SR121566A at 250 ng/mL had no inhibitory effect on this parameter. A slight increase in ATP secretion was seen with 0.5 mu g/mL abc iximab and with SR121566A in concentrations of less than 45 ng/mL, The data suggest a discoupling between the anti-aggregatory and the antisecretory e ffects of IIb/IIIa antagonists. Because it is not established to what exten d CD62 or CD63 expression can be reduced by any means, the reduction by 20- 30% obtained by 3 mu g/mL abciximab or 250 ng/mL SR121566A might already be the maximum possible inhibition by these agents. (C) 2000 Elsevier Science Ltd. All rights reserved.