INHIBITION OF HUMAN PLATELET-AGGREGATION BY GR91669, A PROTOTYPE FIBRINOGEN RECEPTOR ANTAGONIST

In order to produce more potent and specific fibrinogen receptor (GpII b/IIIa) antagonists, the Arg-Gly of a chemical series based upon Arg-G ly-Asp was replaced by alkyl chains of varying lengths. The most poten t in this series, GR91669, inhibited aggregation of human gel-filtered platelets (GFP) in vitro induced by ADP or the thromboxane A(2) mimet ic, U46619, with IC50 values of 200nM and 500nM respectively and was s elected for further studies. Its inhibitory effects on GFP were revers ed by addition of excess fibrinogen. The compound also inhibited ADP- or U46619- induced platelet aggregation in human whole blood (IC50 val ues of 700nM in both cases). I-125-Fibrinogen binding to ADP-stimulate d platelets was inhibited by GR91669 with an IC50 (65nM) similar to th at against platelet aggregation. GR91669 (1mM) did not inhibit U46619- induced platelet shape change or C-14-5HT secretion from platelets sti mulated by collagen, U46619 or thrombin. Therefore GR91669 inhibits ag gregation but has no significant effect on stimulus-response events, a profile consistent with fibrinogen receptor blockade. In addition, GR 98669 (1mM), unlike echistatin or Gly-Arg-Gly-Asp-Ser, did not disrupt vitronectin receptor-dependent attachment of cultured HUVECS in vitro and similarly did not inhibit Mac-1 dependent adhesion of human granu locytes. Thus, of the integrins tested, GR91669 appears to be specific for GpIIb/IIIa. Following intravenous administration to marmosets of 1 or 10mg/kg GR91669, ADP (10 mu M)-induced platelet aggregation ex vi vo was abolished for 15 and 60 minutes respectively. Greater than 50% inhibition was maintained for 30 minutes and 2 hours respectively. GR9 1669, therefore appears to be a potent, specific fibrinogen receptor a ntagonist in vitro and which is also active in vivo.