Characterization of Binding of an RGD Mimetic, [(3)H]-SC-52012, to Platelet GPIIb/IIIa

In order to compare binding of small peptide mimetics on activated vs resting platelets and with fibrinogen (fgn) on activated platelets, the binding of [(3)H]-SC-52012, a low molecular weight (483) mimetic of the RGDF sequence present in fgn, was evaluated. This compound is a potent inhibitor of fgn binding to activated platelets, IC, 9.0 ± 0.6 nM (mean ± SEM), and inhibits ADP induced human platelet aggregation (IC, 44 ± 5 nM). The dissociation constant (Kd) of [(3)H]-SC-52012 was 21.6 ± 4.7 nM (n = 13) in ADP-induced human washed platelets while the Kd for resting platelets was 156 ± 8.3 nM (n = 3). The maximum number of binding sites on ADP-activated and resting platelets were 60846 ± 7158 and 59464 ± 5898 molecules/platelet, respectively. By comparison, results with [(125)I]-fgn binding to activated platelets gave values of 363 ± 73 nM and 58046 ± 6386 molecules/platelet (n = 8) for the Kd and receptor number, respectively. These data suggest that the small molecule binds regardless of activation state of the platelet with only a change in affinity. [(3)H]-SC-52012 could be displaced by unlabelled SC-52012 with an IC(50) of 135 ± 20 nM.