Complete inhibition of ADP-induced fibrinogen binding by abciximab despite GPIIb/IIIa receptor externalization.

(A) Fibrinogen binding induced by 30 μM ADP and 100 μM TRAP was measured in washed platelets 10 minutes after stimulation. Abciximab (10 μg/mL) was preincubated for 15 minutes prior to stimulation. (B) Washed platelets were stimulated with 0.3 to 300 μM ADP and 0.3 to 300 μM TRAP for 15 minutes, and surface expression of CD62P was measured. (C) Washed platelets were stimulated with 30 μM ADP and 100 μM TRAP for 15 minutes, and the expression levels of CD41 and CD61 were measured. (D) Washed platelets were stimulated with 30 μM ADP (▾) and 100 μM TRAP (●) for different time periods, and the expression of CD41 was measured. The time course of anti-CD41 binding was also measured in washed platelets pretreated for 10 minutes with 10 μg/mL abciximab after stimulation with 30 μM ADP (▿) and 100 μM TRAP (○). To obtain relative GPIIb/IIIa externalization rates, data from panel C were transformed to relative increased values (maximal expression at 5 minutes, 100%) and the externalization half-times (t_1/2) were deduced from each individual experiment. Data are means ± SEM from 4 to 6 independent experiments. The externalization half-times were not significantly different (P > .05, ANOVA-Bonferroni).