CONFORMATIONAL-ANALYSIS OF A HIGHLY POTENT DICYCLIC GONADOTROPIN-RELEASING-HORMONE ANTAGONIST BY NUCLEAR-MAGNETIC-RESONANCE AND MOLECULAR-DYNAMICS

Structural analysis of constrained (monocyclic) analogues of gonadotro pin-releasing hormone (GnRH) has led to the development of a model for the receptor-bound conformation of GnRH and to the design of highly p otent, dicyclic GnRH antagonists. This is one of the first cases where a dicyclic backbone has been introduced into analogues of a linear pe ptide hormone with retention of high biological activity. Here we pres ent a conformational analysis of -D-Trp3-Asp4-Glu5-D-Arg6-Leu7-Lys8-Pr o9-Dpr10]-NH2 (I), using two-dimensional nuclear magnetic resonance (N MR) spectroscopy and molecular dynamics simulation. Compound I inhibit s ovulation in the rat at a dose of 5-10 mug (Rivier et al. In Peptide s: Chemistry, Structure ad Biology; Rivier, J. E., Marshall, G. R., Ed s.; ESCOM: Leiden, The Netherlands, 1990; pp 33-37). The backbone conf ormation of the 4-10 cycle in this dicyclic compound is very similar t o that found previously for a parent monocyclic (4-10) GnRH antagonist (Rizo et al. J. Am. Chem. Soc. 1992, 114, 2852-2859; ibid. 2860-2871) , which gives strong support to the hypothesis that GnRH adopts a simi lar conformation upon binding to its receptor. In this conformation, r esidues 5-8 form a ''beta-hairpin-like' structure that includes two tr ansannular hydrogen bonds and a Type II' beta turn around residues D-A rg6-Leu7. The ''tail' of the molecule formed by residues 1-3 is somewh at structured but does not populate a single major conformation. Howev er, the orientation of the tail on the same side of the 4-10 cycle as the 5-8 bridge favors interactions between this bridge and the tail re sidues. These observations correlate with results obtained previously for the parent monocyclic (4-10) antagonist, and have led to the desig n of a series of new dicyclic GnRH antagonists with bridges between th e tail residues and residues 5 or 8.