DESIGN AND SYNTHESIS OF SULFUR-FREE CYCLIC HEXAPEPTIDES WHICH CONTAINTHE RGD SEQUENCE AND BIND TO THE FIBRINOGEN GP IIB IIIA RECEPTOR - A CONFORMATION-BASED CORRELATION BETWEEN PROPENSITY FOR IMIDE FORMATION AND RECEPTOR AFFINITY/

The Arg-Gly-Asp (RGD) sequence is the key recognition site in many adh esive interactions. To probe the structural and conformational require ments for potential antithrombotic agents, we have designed and synthe sized three cyclic hexapeptides (1, 5 and 6) containing the RGD sequen ce. In the ELISA GP IIb/IIIa-fibrinogen receptor assay, 1, 5 and 6 bou nd with IC50 values of 1, 0.1 and 0.016 mum, respectively. All three p eptides completely displaced fibrinogen from the receptor. No potent, sulfur-free cyclic hexapeptide had heretofore been described as a fibr inogen receptor antagonist. The enhanced binding affinity of 6, distin guished by the presence Of two D-amino acids, is likely to reflect an increased conformational resemblance to the natural peptide ligands. C yclization of H-Asp(OFm)-DSer-Phe-DPhe-Arg-Gly-OH with DPPA and NaHCO3 in DMF to afford 6 was attended by subsequent aspartimide formation w ith generation of 9-fluorenylmethanol. Interestingly, imide formation was not observed with any of the three linear hexapeptides (3, 8 and 9 ), with the all-L-CyCliC peptide 1, nor with 5, which contains only Se r-1 in the D-configuration. The observed imide formation led us to use catalytic transfer hydrogenation rather than piperidine to remove the 9-fluorenylmethyl ester protecting group at the beta-carbonyl of aspa rtic acid. Further investigation revealed that imide formation was min imized by careful exclusion of water, reducing dissolution of NaHCO3. Thus the distinguishing conformational features of 6 express themselve s both in receptor affinity and chemical propensity toward imide forma tion. (C) Munksgaard 1993.