Opioid peptides: synthesis and biological activity of new endomorphin analogues

Syntheses are described of new endomorphin 1 and 2 peptoid-peptide hybrids in which Tyr(1), and either one or both Phe(3) and Phe(4) have been replaced by N-substituted-glycine. The preparation is also described of two glycosylated Hyp(2)-endomorphin 2 analogues in which either 2,3,4,6-tetra-O-acetyl glucose or glucose are beta-O-glycosidically linked to the hydroxyproline residue. The Hyp(2)-endomorphin sequences have also been elongate by adding a C-terminal P-alanine residue and several linear dimers have been prepared by coupling either the native peptides or the modified analogues. The cyclo endomorphin 2 has also been synthesized. Preliminary pharmacological experiments on isolated organ preparations showed that the agonist activities of both endomorphin 1 and 2 are not significantly affected by the Pro/Hyp substitution. Phe(4)/Nphe substitution in the endomorphin 1 reduced the potency on guinea pig ileum (GPI) by about 100 times and abolished the agonist activity on mouse vas deferens (MVD) preparation. The decrease of the agonist activity induced by modification of one phenylalanine residue only, either Phe(3) or Phe(4), is lower on endomorphin 2. Either modification of both Phe(3) and Phe(4) or glycosylation of the Hyp(2)-endomorphin 2 cancelled any agonist activity on both preparations. The linear peptide dimers [endomorphin 112, [endomorphin 2](2), [Hyp(2)-endomorphin 1](2), [Hyp(2)-endomorphin 2](2), [Hyp(2)-endomorphin 1-Hyp(2)-endomorphin 2](2) or [Hyp(2)-endomorphin 2-Hyp(2)-endomorphin 1](2), are 7-19 times less potent than endomorphin 1 on GPI and significantly less active than endomorphins 1 and 2 on MVD. The other afforded modifications significantly affected or abolished the agonist activity of the resulting endomorphin analogues on both GPI and MVD preparations.