Structure-activity relationships of cyclic beta-casomorphin-5 analogues.

Cyclic analogues of the beta-casein-derived opioid peptide beta-casomorphin-5 (H-Tyr-Pro-Phe-Pro-Gly-OH) were prepared through substitution of the Pro2 residue with various alpha,omega-diamino acid residues (lysine, ornithine, 2,4-diaminobutyric acid) and cyclization of the omega-amino group to the C-terminal carboxyl function. Compounds of this type, with D-configuration at the 2-position residue, showed high opioid receptor affinity with some preference for mu receptors over delta receptors, high potency in the guinea pig ileum assay and considerable activity in the mouse vas deferens assay. Configurational inversion at the 4-position in these cyclic analogues resulted in enhanced affinity for both mu and delta receptors, whereas N-methylation of the Phe3 residue produced a potency decrease.