In vitro measurement of platelet glycoprotein IIb/IIIa receptor blockade by abciximab: interindividual variation and increased platelet secretion.

BACKGROUND AND OBJECTIVES Inhibition of soluble fibrinogen binding to activated platelets represents the target of pharmacologic approach with antagonists of the glycoprotein IIb/IIIa (GPIIb/IIIa) complex. In this study we assessed the effects of abciximab, a recombinant chimeric Fab fraction of the antibody against GPIIb/IIIa, on several markers of platelet activation. DESIGN AND METHODS The platelet surface expression of GPIIb/IIIa was measured by a flow cytometry technique using a two-color assay. GPIIb/IIIa was detected by FITC-conjugated antibodies in whole blood, either unstimulated or exposed to platelet stimuli. The following antibodies were used: CD41, which recognizes the IIb/IIIa complex both in activated and non-activated conformers, and PAC-1, which is directed toward the activated conformer of GPIIb/IIIa. In addition, the same blood sample was incubated with CD62 antibody to measure P-selectin, as a marker of a-granule degranulation. The effect of abciximab was also assessed by experiments carried out on shear stress-induced platelet aggregation, a test that appears to be a predictor of platelet hemostatic function. RESULTS Abciximab inhibited CD41 binding to glycoprotein IIb (GPIIb) in a concentration-dependent manner and also inhibited the binding of PAC-1 to active GPIIb/IIIa. In contrast, membrane-associated P-selectin was significantly increased by the drug, which suggests that blockade of GPIIb/IIIa receptors results in an increased platelet degranulation in response to agonists. Shear stress-induced platelet aggregation was inhibited by abciximab, with a more pronounced effect on blood filtration, which represents an index of platelet aggregate formation. INTERPRETATION AND CONCLUSIONS Our results indicate that GPIIb/IIIa blockade by abciximab is accompanied by an increase of a-granule secretion, suggesting that different mechanisms regulate these aspects of platelet activation. The described flow cytometry technique, that allows the simultaneous in vitro detection of several platelet markers, is a suitable method for assessing the effects of agents which interfere with platelet function.