Polypeptides. Part 16. Synthesis and biological activity of α-aza-analogues of luliberin with high antagonist activity

Abstract

Analogues of luliberin (luteinising hormone-releasing hormone) were synthesised where the glycine residue in position 10 was replaced by either azaglycine (–NH–NH–CO–) or aza-alanine (–NH–NMe–CO–), the histidine residue in position 2 was either omitted, or replaced by D-phenylalanine or D-tryptophan, and the glycine residue in position 6 was substituted either by D-phenylalanine or D-tryptophan. These compounds were evaluated for their ability to prevent ovulation induced by luliberin in androgen-sterilised constant-oestrus rats. Compounds with the azaglycine residue in position 10 and other modifications in positions 2 and 6 showed good antagonist activity, whereas aza-alanine replacement in position 10 together with modifications in position 2 resulted in inactive compounds. The most potent analogue, [2-D-Phe-6-D-Phe-10-Azgly]-luliberin, completely inhibited ovulation induced by luliberin (0.5 µg/rat) at a dose of 15 µg/rat.