Potent morphiceptin analogs: structure activity relationships and morphine-like activities.

Morphiceptin (Tyr-Pro-Phe-Pro-NH2), a nonenkephalin peptide, is an opioid agonist highly selective for mu opiate receptors. Chemical modification of Tyr-Pro-Phe-Pro-NH2 was carried out by substituting structurally related amino acids at residues 2, 3 and 4. The morphiceptin analogs synthesized were then examined for receptor binding activities using 125I-labeled FK 33,824 (Tyr-D-Ala-Gly-NMePhe-Met(O)-ol) as the mu-ligand and 125I-labeled D-Ala2,D-Leu5-enkephalin as the delta-ligand, and for inhibitory activities on electrically evoked smooth muscle contraction of mouse vas deferens and isolated myenteric plexus-longitudinal muscle strips of guinea-pig ileum. All of these analogs showed virtually no activity at delta opiate receptor binding sites. Methylation of the nitrogen atom of phenylalanine and the substitution at the C-terminal of L-proline by D-proline produced potent mu-agonists, the prototype analog being Tyr-Pro-NMePhe-D-Pro-NH2 (PL017). The IC50 values of morphiceptin and its analogs for mu receptor binding were correlated to the ED50 values in the guinea-pig ileum assay, suggesting that the ileum effects were mediated by mu receptor interactions. A similar correlation between mu receptor binding activity and the ED50 values in the mouse vas deferens assay suggested that morphiceptin and its analogs also acted on mu receptors in this tissue. This idea is supported by the observation that naloxone has a high pA2 value of 8.71 against PL017 in mouse vas deferens. In in vivo studies, PL017 administered centrally into the rostral portion of the 4th ventricle produced long-lasting, naloxone-reversible analgesia in rats.(ABSTRACT TRUNCATED AT 250 WORDS)