Novel nociceptin analogues: Synthesis and biological activity

Syntheses are described of the nociceptin (1-13) amide [NC(1-13)-NH2] and of several analogues in which either one or both the phenylalanine residues (positions I and 4), the arginine residues (positions 8 and 12) and the alanine residues (positions 7 and 11) have been replaced by N-benzyl-glycine, N-(3-guanidino-propyl)-glycine and beta-alanine, respectively. The preparation is also described of NC(1-13)-NH2 analogues in which either galactose or N-acetyl-galactosamine are beta-O-glycosidically linked to Thr(5) and/or to Ser(10). Preliminary pharmacological experiments on mouse vas deferens preparations showed that Phe(4), Thr(5), Ala(7) and Arg(8) are crucial residues for OP4 receptor activation. Manipulation of Phe(1) yielded peptides endowed with antagonist activity but [Nphe(1)] NC(1-13)-NH2 acted as an antagonist still possessing weak agonist activity. Introduction of the beta A1a residue either in position 7 or 11 of the [Nphe(1)] NC(1-13)-NH2 sequence, abolished any residual agonist activity and [Nphe1, beta Ala(7)] NC(1-13)-NH2 and [Nphe(1), beta Ala11] NC(1-13)-NH2 acted as competitive antagonists only. Modification of both Ala(7) and Ala(11) abolished the antagonist activity of [Nphe(1)]NC(1-13)-NH2 probably by hindering receptor binding. Changes at positions 10 and 11 gave analogues still possessing agonist activity. [Ser(beta Gal)(10)] NC(1-13)-NH2 displayed an activity comparable with that of NC(1-13)-NH2, [Ser(beta GalNAc)(10)] NC(1-13)-NH2 and [beta Ala(11)] NC(1-13)-NH2 were five and 10 times less active, respectively.