Development of a Model for the δ Opioid Receptor Pharmacophore. 2. Conformationally Restricted Phe³ Replacements in the Cyclic δ Receptor Selective Tetrapeptide Tyr-c[d-Cys-Phe-d-Pen]OH (JOM-13)

The in vitro pharmacological properties and conformational features of analogs of the δ opioid receptor selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]OH (JOM-13) in which the Phe³ residue was replaced by each of the four stereoisomers of β-methylphenylalanine (β-MePhe) were investigated. Both analogs in which the α carbon of the Phe³ replacement has L-stereochemistry display high affinity for δ receptors with the (2S,3S)-MePhe³ analog exhibiting approximately 8-fold higher affinity than the (2S,3S)-MePhe³ diastereomer. Surprisingly, one analog with D-stereochemistry in residue 3, the (2R,3R)-MePhe³ analog, also displays high affinity for the δ receptor and is extraordinarily selective for this receptor. All analogs were agonists in the mouse vas deferens (MVD) and guinea pig ileum (GPI) smooth muscle bioassays, displaying MVD and GPI potencies consistent with their δ and μ opioid receptor affinities, respectively. The use of β-MePhe as a replacement for Phe³ was based upon the desire to reduce the conformational flexibility of the Phe³ side chain by imposing a steric rotational constraint in the form of the β-methyl substituent and to thus deduce the residue 3 side chain orientation in the δ receptor-bound conformation from the correlation between δ receptor binding affinities and conformational preferences. Molecular mechanics computations revealed, however, that the conformational constraints imposed by the β-methyl group in the (2S,3S)-MePhe³ and (2S,3R)-MePhe³ analogs were too modest to allow unequivocal determination of δ receptor-bound residue 3 side chain conformation. However, analysis of the high-affinity (2R,3R)-MePhe³ analog revealed a strong preference for a single side chain conformer (ϰ¹ ~ 60°). Low-energy conformers of this analog could only be effectively superimposed with low-energy conformers of the parent peptide in which the Phe³ side chain conformation was limited to ϰ¹ ~ −60°. This observation eliminates the last remaining uncertainty regarding conformational features of the pharmacophore elements in the δ receptor-bound state, allowing the proposal of a complete model.