Effects of platelet glycoprotein IIb-IIIa number and glycoprotein IIIa Leu33Pro polymorphism on platelet aggregation and sensitivity to glycoprotein IIb-IIIa antagonists.

We investigated the influence of glycoprotein (GP) IIIa Leu33Pro polymorphism, platelet GP IIb-IIIa number, and plasma fibrinogen concentration on platelet aggregation and antiaggregatory action of GP IIb-IIIa antagonists. Healthy volunteers with GP IIIa Pro33(-) (Leu33Leu33, n = 20) and Pro33(+) (Leu33Pro33, n = 13, and Pro33Pro33, n = 2) genotypes were included into the study. GP IIIa Leu33Pro substitution was associated with the increase of the level and rate of platelet microaggregate formation induced by GP IIb-IIIa activating antibody CRC54 (100, 200, 400 microg/ml) against the epitope within 1-100 residues of GP IIIa N-terminal part (p from 0.001 to 0.047). No significant differences were detected between parameters of platelet aggregation induced by ADP (1.25, 2.5, 5.0, 20 microM) in GP IIIa Pro33(+) and Pro33(-) donors. GP IIb-IIIa antagonist Monafram (F(ab')(2) fragment of GP-IIb-IIIa blocking antibody CRC64) (1, 2, 3 microg/ml), but not eptifibatide (50, 100, 150 ng/ml) inhibited ADP-induced aggregation slightly less efficiently in GP IIIa Pro33(+) group (p < 0.05 at 1 and 2 microg/ml Monafram). GP IIb-IIIa number (evaluated as maximal binding of (125)I-labelled antibody CRC64) varied from 40.5 to 80.8 x 10(3) per platelet with no significant influence of GP IIIa genotype. Consistent correlations were revealed between GP IIb-IIIa quantity and the level and rate of ADP-induced aggregation (r from 0.353 to 0.583, p from <0.001 to 0.037) as well as resistance (level of residual aggregation) to both GP IIb-IIIa antagonists (r from 0.345 to 0.602, p from <0.001 to 0.042). ADP-induced aggregation was considerably increased and efficiency of GP IIb-IIIa antagonists decreased in donors with high in comparison with low GP IIb-IIIa quantity (>60 and 40-50 x 10(3) per platelet respectively, p < 0.01 for most tests). No correlations were observed between all tested parameters and plasma fibrinogen concentration. Our results indicate that inter-individual variability of platelet GP IIb-IIIa number significantly affects platelet aggregation and antiaggregatory effects of GP IIb-IIIa antagonists. Contribution of this factor is higher than that of GP IIIa Leu33Pro polymorphism and variations of fibrinogen concentration.