Design and synthesis of potent hexapeptide and heptapeptide gonadotropin-releasing hormone antagonists by truncation of a decapeptide analogue sequence

A novel strategy for designing reduced-size analogues of the decapeptide gonadotropin-releasing hormone (GnRH) was developed. As opposed to previous attempts to delete residues from either of the peptide's termini, our approach is based upon the known importance of both C- and N-terminals of GnRH analogues for receptor recognition, whereas the central part of the molecule is replaced by a short spacer. The present truncation strategy was successful for generation of reduced-size hexapeptide and heptapeptide antagonists possessing potent antagonistic capacity. The same methodology was not suitable for the generation of reduced-size agonists, suggesting different conformational characteristics for GnRH agonists and antagonists. A heptapeptide antagonist designed by this method was shown to inhibit serum levels of luteinizing hormone in castrated rats in vivo. Structure-activity studies suggested that the structural preferences for GnRH receptor recognition are similar to those reported for decapeptide antagonists. Our studies resulted in a heptapeptide GnRH antagonist (Ac-D-Nal2-D-Cpa-D-Pal- Gly-Arg-Pro-D-Ala-NH₂) with high receptor binding affinity (IC₅₀ = 7 nM), as compared to that of GnRH itself (IC₅₀ = 2 nM). The highest affinity of a hexapeptide antagonist that we have synthesized was somewhat lower (IC₅₀ = 45 nM).