CHARACTERIZATION OF BINDING OF AN RGD MIMETIC, [H-3] 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, th e binding of [H-3]-SC-52012, a low molecular weight (483) mimetic of t he RGDF sequence present in fgn, was evaluated, This compound is a pot ent inhibitor of fgn binding to activated platelets, IC50 9.0 +/- 0.6 nM (mean +/- SEM), and inhibits ADP induced human platelet aggregation (IC50 44 +/- 5 nM). The dissociation constant (Kd) of [H-3]-SC-52012 was 21.6 +/- 4.7 nM (n = 13) in ADP-induced human washed platelets whi le the Kd for resting platelets was 156 +/- 8.3 nM (n = 3). The maximu m number of binding sites on ADP-activated and resting platelets were 60846 +/- 7158 and 59464 +/- 5898 molecules/platelet, respectively. By comparison, results with [I-125]-fgn binding to activated platelets g ave values of 363 +/- 73 nM and 58016 +/- 6386 molecules/platelet (n = 8) for the Kd and receptor number, respectively. These data suggest t hat the small molecule binds regardless of activation state of the pla telet with only a change in affinity. [H-3]-SC-52012 could be displace d by unlabelled SC-52012 with an IC50 of 135 +/- 20 nM.