Design and Synthesis of Sulfur-free cyclic Hexapeptides which Contain the RGD Sequence and Bind to the Fibrinogen GP IIb/IIIa Receptor

LENDER, A, ; YAO, W.; SPRENGLER, P. A.; SPANEVELLO, R. A.; FURST, G. T.; HIRSCHMANN, R.; SMITH III, A. B.

INTERNATIONAL JOURNAL OF PEPTIDE AND PROTEIN RESEARCH

Wiley

Año: 1993 vol. 42 p. 509 - 517

0367-8377

The Arg-Gly-Asp (RGD) sequence is the key recognition site in many adhesive interactions. To probe the structural and conformational requirements for potential antithrombotic agents, we have designed and synthesized three cyclic hexapeptides (1, 5 and 6) containing the RGD sequence. In the ELISA GP IIb/IIIafibrinogen receptor assay, 1, 5 and 6 bound with IC50 values of 1, 0.1 and 0.016 μm, respectively. All three peptides completely displaced fibrinogen from the receptor. No potent, sulfur-free cyclic hexapeptide had heretofore been described as a fibrinogen receptor antagonist. The enhanced binding affinity of 6, distinguished by the presence of two d-amino acids, is likely to reflect an increased conformational resemblance to the natural peptide ligands. Cyclization 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 with 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 Ser-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 β-carbonyl of aspartic acid. Further investigation revealed that imide formation was minimized by careful exclusion of water, reducing dissolution of NaHCO3. Thus the distinguishing conformational features of 6 express themselves both in receptor affinity and chemical propensity toward imide formation.