Development of a Model for the δ Opioid Receptor Pharmacophore. 1. Conformationally Restricted Tyr¹ Replacements in the Cyclic Receptor Selective Tetrapeptide Tyr-c[d-Cys-Phe-d-Pen]OH (JOM-13)

A series of analogues of the conformationally restricted δ opioid receptor selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]OH (JOM 13) was prepared in which the conformationally labile Tyr residue was replaced with several less flexible tyrosine analogues. Among these tyrosine analogues were the bicyclic structures 1,2,3,4,-tetrahydro-7-hydroxyisoquinoline-3-carboxylic acid (HO-Tic), 2-amino-6-hydroxytetralin-2-carboxylic acid (Hat), and 2-amino-5-hydroxyindan-2-carboxylic acid (Hai) in which rotations about the C^α-C^β and C^β-C^γ bonds are restricted due to cyclization of the side chain to the backbone. Also examined were analogues in which tyrosine was replaced with either trans-3-(4′-hydroxyphenyl)proline (t-Hpp) or cis-3-(4′-hydroxyphenyl)proline (c-Hpp), residues in which rotations about C^α-C^β, but not C^β-C^γ are restricted. Both the t-Hpp¹ and c-Hpp¹ analogues displayed δ receptor binding affinity similar to the parent Tyr¹-containing peptide, while the D-Hat¹, L-Hat¹, and L-Hai¹ analogues exhibited somewhat lower affinity. The results observed for the t-Hpp¹ and c-Hpp¹ analogues are particularly significant since these two residues have little accessible conformational space in common. Since the binding conformation of residue 1 must be included in this limited conformational intersection, its elucidation is facilitated. Bioassay results from guinea pig ileum and mouse vas deferens preparations are in general agreement with the binding results; however some potency discrepancies are observed. These discrepancies may reflect different selectivities among δ receptor subtypes for the analogues or may represent differing efficacies among these conformationally restricted peptides. The conformational properties of the parent tetrapeptide and the residue 1-modified analogues were studied by molecular mechanics computations. All these peptides share a common rigid tripeptide cycle with a single energetically preferred backbone conformation and three different conformers of the D-Cys, D-Pen disulfide bridge, two of which are observed in the solid state and in aqueous solution, as previously determined from X-ray crystallography and ¹H NMR spectroscopy data (Lomize, A.; et al. J. Am. Chem. Soc. 1994, 116, 429-436). All the peptides have similar sets of lowenergy conformations of their common flexible elements, the Phe³ side chain and the peptide group between the first residue and the rigid tripeptide cycle. However, possible conformations of the first residue differ and depend on the covalent constraints incorporated into the side chain. Analysis of conformation-activity relationships obtained for these peptides allows the determination of some of the conformational requirements for their interaction with the δ opioid receptor. First, the side chain conformer of residue 1 in the δ receptor-bound state is determined to be trans (ϰ¹~ 180°). Second, an extended conformation of the exocyclic peptide group (both ψ of residue 1 and φ of D-Cys² torsion angles are ~ 160°) is identified as providing the mutual arrangement of the first residue and the tripeptide cycle required for δ receptor binding.