Cyclic .beta.-Casomorphin Analogs with Mixed .mu. Agonist/.delta. Antagonist Properties: Synthesis, Pharmacological Characterization, and Conformational Aspects

Abstract

Analogues of the potent and moderately mu-opioid-receptor-selective cyclic beta-casomorphin-5 derivative H-Tyr-c[-D-Orn-Phe-D-Pro-Gly-] (2) were prepared by conventional solution synthesis. Replacement of the Phe3 residue by 2-naphthylalanine (2-Nal) led to a peptide (4) with high affinity for both mu and delta opioid receptors. This compound turned out to be an agonist in the mu-receptor-representative guinea pig ileum (GPI) assay but a moderately potent antagonist against various delta agonists in the delta-receptor-representative mouse vas deferens (MVD) assay. It thus represents the first known cyclic opioid peptide analogue with mixed mu agonist/delta antagonist properties. Interestingly, replacement of 2-Nal3 in compound 4 with 1-naphthylalanine (1-Nal) resulted in an analogue (5) showing high affinity for mu receptors and a full agonist effect in the MVD assay that was mediated by both mu and delta receptors. Substitution of Trp for Phe3 in 2 (compound 8) was well tolerated at both receptors and led to an analogue with agonist activity in both the GPI and MVD assays. Variation of the peptide ring size in 4 was achieved by substitution of D-Orn2 with D-Lys (compound 6) or D-2,4-diaminobutyric acid (compound 7). Analogue 6 was also a mixed mu agonist/delta antagonist with somewhat lower potency than 4, whereas compound 7 displayed mu agonist and partial delta agonist properties. Further reduction of the peptide ring size, as achieved by deletion of the Gly5 residue, produced a compound (9) which was a full agonist in both bioassays. Conformational analysis of analogues 2, 4, and 5 by 1H NMR spectroscopy and molecular mechanics studies suggested that the overall conformation of parent compound 2 and the 2-Nal-containing peptide 4 was similar, while the side-chain orientation of 1-Nal in peptide 5 was different. These results suggest that the delta antagonist properties of analogue 4 may not be due to a difference in its overall conformation as compared to the agonist 2 but may be a direct effect of the 2-naphthyl moiety per se preventing proper alignment of the peptide for receptor activation.

The solution conformation of cyclic β-casomorphin-5 analogues

“…In this way, we could determine the very likely pharmacophore conformations of active t3-casomorphin analogues [21] Fig. 6) is not only in complete agreement with the early proposed/~-opioid receptor binding conformation but shows also high similarity to the solution and pharmacophore conformation of H-Tyr-c[-D-Orn-2-Nal-Pro-Gly-] [19,21]. In agreement with our previously defined model, the bioactive conformers of the cyclic /~-casomorphin analogues permit good overlap of the defined pharmacophoric moieties [20].…”

“…It has been shown that Tyr-c[-D-Orn-Phe-ProGly-] 1 is a selective/~-opioid receptor ligand, whereas the L-Ore analogue is nearly inactive at both the/~-and ~5-opioid receptor [18,19]. In an earlier paper [20] we described a uniform model based on the spatial comparison of the pharmacophoric moieties and electrostatic potentials of various opiates.…”

“…In an earlier paper [20] we described a uniform model based on the spatial comparison of the pharmacophoric moieties and electrostatic potentials of various opiates. This model was used as a basis in the discussion of the pharmacophore conformations of the cyclic/3-casomorphin analogues and of their structure-activity relationships [ 18,19,21]. All the stable conformations obtained were compared with the proposed pharmacophore conformation of PET and fentanyl.…”

Opioid ligands with mixed μ/δ opioid receptor interactions: An emerging approach to novel analgesics

“…52 Among the analogs of the moderately selective ␤ -casomorphin-5, H-Tyr-c[D-Orn-Phe-D-Pro-Gly-], replacement of the Phe 3 residue by 2-naphthylalanine (2-Nal) gave the peptide H-Tyr-c[D-Orn-2-Nal-D-Pro-Gly-], which displayed high affi nity for both and ␦ receptors. This compound was an agonist in GPI smooth muscle assay and a moderately potent antagonist against various ␦ agonists in the MVD assay.…”

“…This compound was an agonist in GPI smooth muscle assay and a moderately potent antagonist against various ␦ agonists in the MVD assay. 52 Ligands with a more balanced agonist/ ␦ antagonist profi le of activity were discovered among analogs of the tetrapeptide amide H-Tyr-Tic-Phe-Phe-NH 2 (TIPP-NH 2 ). Substitution of Dmt for Tyr 1 in TIPP-NH 2 and reduction of the peptide bond between Tic 2 and Phe 3 led to a compound, H-Dmt-Tic [CH 2 -NH]Phe-Phe-NH 2 (DIPP-NH 2 [ ]), which showed high agonist potency and very high ␦ antagonist activity in the GPI and MVD bioassays.…”