IMPROVED POTENCY AND SPECIFICITY OF ARG-GLY-ASP (RGD) CONTAINING PEPTIDES AS FIBRINOGEN RECEPTOR BLOCKING-DRUGS

A range of cyclic RGD based peptides have been developed to mimic the conformation of RGD within fibrinogen. These peptides, as well as echi statin (IC50 = 0.05 muM) and GRGDS (IC50 = 25 muM) fully inhibited ade nosine diphosphate (ADP)(10 muM)-induced platelet aggregation of human gel-filtered platelets (GFP). RGDF was the most potent linear peptide in inhibiting ADP-induced aggregation (IC50 = 8 muM) but cyclisation, using a 6,5 bicyclic coupling group to produce GR83895, led to an app roximately 10-fold increase in potency (IC50 0.9 muM). In GFP, ADP-ind uced I-125-fibrinogen binding was inhibited (>80%) by echistatin, GRGD S or GR83895 at concentrations (IC50 values 0.05 muM, 25 muM and 1.4 m uM respectively) similar to those needed to inhibit aggregation. All t hree compounds also completely inhibited ADP- and U46619-induced aggre gation in both platelet rich plasma (PRP) and whole blood. In contrast to platelet aggregation, U-46619-induced C-14-5HT secretion in PRP wa s not inhibited by GR83895 or echistatin, indicating that agonist-indu ced signal transduction is not affected by either agent, a profile con sistent with that predicted for a specific fibrinogen receptor blockin g drug. To test specificity of action, echistatin, GR83895 and GRGDS w ere also examined for their ability to detach cultured human umbilical vein endothelial cells attached to plastic through a vitronectin rece ptor dependent process. GR83895 only caused detachment at concentratio ns 100-fold greater than those required to inhibit platelet aggregatio n, in contrast to GRGDS and echistatin which caused cell detachment at concentrations similar to those inhibiting aggregation. In summary, c yclisation 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 fibrin ogen receptor blocking drug as an anti-thrombotic drug.