THROMBOSIS RESEARCH 72; 231-245,1993 0049-3848/93 $6.00 + .OOPrinted in the USA. Copyright (c) 1993 Pergamon Press Ltd. All rights reserved. IMPROVED POTENCY AND SPECIFICITY OF ARG-GLY-ASP (RGD) CONTAINING PEPTIDES AS FIBRINOGEN RECEPTOR BLOCKING DRUGS Martyn R.Foster, Edward J.Hornby, Susan Brown, John Kitchin’, Michael Harm’ and Peter Ward’ Department of Cardiovascular and Respiratory Pharmacology and ‘Department of Medicinal Chemistry Glaxo Group Research Limited, Ware, Herts,SG12 ODP. (Received 10.1.1993; accepted in revised form 21.7.1993 by Editor M.F. Sculls) (Received by Executive Editorial Office 16.8.1993) Abstract A range of cyclic RGD based peptides have been developed to mimic the conformation of RGD within fibrinogen. These peptides, as well as echistatin (IC50 = 0.05pM) and GRGDS (IC50 = 25pM) fully inhibited adenosine diphosphate (ADP)( lOpM)-induced platelet aggregation of human gel-filtered platelets (GFP). RGDF was the most potent linear peptide in inhibiting ADP- induced aggregation (KS0 = 8pM) but cyclisation, using a 6,5 bicyclic coupling group to produce GR83895, led to an approximately lo-fold increase in potency (IC 5. = 0.9pM). In GFP, ADP-induced 1251-fibrinogen binding was inhibited (>80%) by echistatin, GRGDS or GR83895 at concentrations (IC50 values O.OSfl, 25 MM and 1.4bM respectively) similar to those needed to inhibit aggregation. All three compounds also completely inhibited ADP- and U46619-induced aggregation in both platelet rich plasma (PRP) and whole blood. In contrast to platelet aggregation, U-46619-induced 14C-5HT secretion in PRP was not inhibited by GR83895 or echistatin, indicating that agonist-induced signal transduction is not affected by either agent, a profile consistent with that predicted for a specific fibrinogen receptor blocking drug. To test specificity of action, echistatin, GR83895 and GRGDS were also examined for their ability to detach cultured human umbilical vein endothelial cells attached to plastic through a vitronectin receptor dependent process. GR83895 only caused detachment at concentrations lOO-fold greater than those required to inhibit platelet aggregation, in contrast to GRGDS and echistatin which caused cell detachment at concentrations similar to those inhibiting aggregation. In summary, cyclisation of RGD-containing peptides has led to both improved potency and specificity of action. Such specificity of action may prove to be an important consideration for the successful development of a fibrinogen receptor blocking drug as an anti-thrombotic drug. Key words: Human platelets, fibrinogen receptor blocking drugs, aggregation, Arg-Gly-Asp (RGD), GR83895, echistatin. 231