Introduction: GP IIb/IIIa inhibitor doses high enough to inhibit platelet macroaggregation may not completely prevent formation of microaggregates. Platelets contain an internal pool of GP IIb/IIIa receptors which externalizes with activation and supports microaggregation. This study assesses microaggregation in the presence of the two GP IIb/IIIa inhibitors abciximab and tirofiban.
Methods: Citrated whole blood was preincubated with abciximab (5 microg/ml), tirofiban (50 ng/ml), or saline as control and activated with TRAP (5 microM) or ADP (2 microM) at 37 degrees C under constant stirring. Microaggregate formation and receptor expression were determined with flow cytometry.
Results: Within few seconds after TRAP-activation the platelet count dropped from 266,000/microl to 20,000/microl, the number of microaggregates increased from 3700/microl to 10,100/microl and the mean number of GP IIb/IIIa receptors increased from 53,000 to 65,000/platelet. With TRAP+abciximab the platelet count dropped from 259,000 to 113,000/microl, microaggregates increased from 2500 to 9300/microl, GP IIb/IIIa receptors from 56,000 to 77,000/platelet. Platelet microaggregate formation was reversible. With TRAP and tirofiban platelet count dropped to only 190,000/microl, there was no increase in platelet microaggregates, receptors increased to 66,000/platelet. Platelet activation with ADP gave similar results.
Conclusions: During the early phase of activation additional GP IIb/IIIa receptors externalize to the platelet surface. Abciximab does not block these new receptors sufficiently to prevent microaggregate formation. However, the number of unblocked receptors is not high enough to maintain a stable aggregate, microaggregation is reversible. With tirofiban there is no microaggregate formation, possibly because the inhibitor rapidly binds to newly externalized receptors.