Design and Synthesis of Potent Tyr(OMe)5-Gonadotropin-Releasing Hormone (GnRH) Analogues with Modifications at Positions 6, 9 and 10

We have recently reported the synthesis and the conformational properties of some Gonadotropin-releasing hormone (GnRH) analogues in which the tyrosine residue at position 5 is substituted with tyrosine-O-methyl (Keramida et al., Let. Pept. Sci., 3 (1996) 257/Matsoukas et al., Eur. J. Med. Chem., 32 (1997) 927). The analogue [Tyr-(OMe)5]-GnRH was found to exert a lower degree of desensitization than the native GnRH peptides in terms of pituitary gonadotropin (GTH) release in goldfish. Compared to GnRH, however, [Tyr-(OMe)5]-GnRH exerted a lower GTH-release potency in cultured goldfish pituitary fragments, and was bound with a lower binding affinity to the rat pituitary GnRH receptors. In order to increase the potency of [Tyr-(OMe)5]-GnRH, we have synthesized a group of GnRH peptides containing Tyr-(OMe)5 in combination with other substitutions at positions 6, 9 and 10 and we have estimated their binding affinity for the rat pituitary receptors and gonadotropin (GTH) release potency in the goldfish pituitary. A selected number of these analogues was also tested for their ability to induce ovulation in seabass. The important structural modifications that increased the binding and gonadotropic activity of [Tyr(OMe)5]-GnRH in vitro and in vivo were found to include the replacement of the proline at position 9 with azetidine, glycine amide terminus with an alkyl amide group and Gly6 residue with hydrophilic D-amino acids such as D-Arg6. Overall, the findings provide SAR information on a group of novel GnRH peptides that can be also used to induce ovulation in teleosts.