Synthesis and biological activity of highly active alpha-aza analogues of luliberin

Analogues of luliberin containing an alpha-azaamino acid in position 6, 9, or 10 (I--XIV) have been synthesized by the solution method of peptide synthesis. Two nonaza analogues, [D-Phe6]- and [D-Ser(But)6,des-Gly-NH2(10),Pro-ethylamide9]luliberin, were also synthesized for comparison. The ovulation-inducing activity of the compounds was evaluated in androgen-sterilized constant-estrus rats. A combination of D-amino acid replacement in position 6 with an azaglycine residue at position 10 resulted in highly active compounds which were superior to the corresponding nonaza analogues. The most active compoungs, [D-Phe6,Azgly10-a1-, [D-Tyr(Me)6,Azgly10]-, and [D-Ser(But)6,Azgly10]luliberin, were about 100 times as potent as luliberin. N-Methylleucine substitution in position 7 in these compounds resulted in decreased activity; [D-Phe6,MeLeu7,Azgly10]- and [D-Tyr(Me)6,MeLeu7,Azgly10]luliberin were only 50 times as active as luliberin. The presence of either an azaproline residue in position 9, an azaphenylalanine or azaglycine residue in positions 6 and 10, or a tert-butyl ether protecting group on the hydroxyl group of the tyrosine residue in position 5 resulted in compounds with significantly reduced biological activity.