title: Synthesis and electrophysiological characterization of cyclic morphiceptin analogues.
abstract: A challenge in opioid peptide chemistry and pharmacology is the possibility to develop novel peptides with peripheral selectivity. An enzymatically stable opioid peptide could involve an antidiarrheal effect. For this reason, we constrained the highly selective and potent tetrapeptide morphiceptin with a 6-atom bridge, resulting in a cyclic amide and an ester analogue, 2 and 3, respectively. Taking advantage of the functional coupling of the opioid receptor with the heteromultimeric G-protein-coupled inwardly rectifying K+ (GIRK1/GIRK2) channel, either the wild-type mu-, kappa-, delta- or a mutated mu-opioid receptor (hMORS329A) was functionally co-expressed with GIRK1/GIRK2 channels and a regulator of G-protein signaling (RGS4) in Xenopus laevis oocytes. The two-microelectrode voltage clamp technique was used to measure the opioid receptor activated GIRK1/GIRK2 channel responses. Both cyclic analogues were equally potent via the wild-type mu-opioid receptor hMORwt (EC(50) value 976.5 +/- 41.7 for 2 and 1017.7 +/- 60.7 for 3), while the EC(50) value for Tyr-Pro-Phe-D-Pro-NH(2) measured 59.3 +/- 4.8 nM. These three agonists displayed a four to five times decreased potency via hMORS329A as compared to the wild type. Interestingly, no effect on kappa- and delta-opioid receptors was observed. The intramolecular bridge created by cyclization of morphiceptin prevents dipeptidyl peptidase IV from interacting with these analogues. We conclude that constraining morphiceptin with a 6-atom bridge resulted in enzymatically stable peptidomimetics that are exclusively active on mu-opioid receptors. These analogues provide an interesting template in the promising approach for the design of potential antidiarrheal agents.
authors.affiliation: Laboratory of Toxicology, Faculty of Pharmaceutical Sciences, University of Leuven, Van Evenstraat 4, 3000 Leuven, Belgium.
authors.forename: Joost
authors.initials: J
authors.lastname: Pil
authors.forename: Pieter
authors.initials: P
authors.lastname: Van der Veken
authors.forename: Gunther
authors.initials: G
authors.lastname: Bal
authors.forename: Koen
authors.initials: K
authors.lastname: Augustyns
authors.forename: Achiel
authors.initials: A
authors.lastname: Haemers
authors.forename: Jan
authors.initials: J
authors.lastname: Tytgat
mesh_terms: D000818:Animals; D004594:Electrophysiology; D004723:Endorphins / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D018847:Opioid Peptides / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology; D010456:Peptides, Cyclic / Q000138:chemical synthesis* / Q000737:chemistry / Q000494:pharmacology; D015221:Potassium Channels / Q000378:metabolism; D024661:Potassium Channels, Inwardly Rectifying; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: D004723:Endorphins; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D018847:Opioid Peptides; D010456:Peptides, Cyclic; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D017450:Receptors, Opioid, mu; C028889:morphiceptin