Differential Stereochemical Requirements of μ vs. δ Opioid Receptors for Ligand Binding and Signal Transduction: Development of a Class of Potent and Highly δ-Selective Peptide Antagonists

Opioid peptide analogs consisting entirely of aromatic amino acid residues and containing conformationally restricted phenylalanine derivatives in position 2 of the peptide sequence were synthesized and pharmacologically characterized in vitro. Both diastereoisomers of H-Tyr-(D or L)-NMePhe-Phe-Phe-NH2(NMePhe is Nα-methylphenylalanine) were μ-receptor-selective, were full agonists in the μ-receptor-representative guinea pig ileum assay, and were partial agonists in the mouse vas deferens assay, with the L-NMePhe2analog displaying somewhat higher intrinsic activity than the D-NMePhe2analog. Further conformational restriction at position 2 in the sequence, as achieved through substitution of D- or L-tetrahydro-3-isoquinoline carboxylic acid (Tic), produced a configuration-dependent differential effect on receptor selectivity and intrinsic activity, leading to a potent μ-selective μ agonist (the D-Tic2analog) with increased intrinsic activity in the mouse vas deferens assay and to a potent δ-selective δ antagonist (the L-Tic2analog). These results demonstrate that imposition of conformational constraints in a peptide not only may alter receptor selectivity but also may decrease, totally abolish, or even enhance intrinsic activity. The tetrapeptide H-Tyr-Tic-Phe-Phe-NH2was a moderately potent full agonist in the guinea pig ileum assay and, thus, represents a compound with mixed μ-agonist/δ-antagonist properties. The corresponding peptide with a free C-terminal carboxyl group H-Tyr-Tic-Phe-Phe-OH showed high δ-receptor affinity (Kδ i= 1.2 nM), unprecedented δ selectivity (Kμ i/Kδ i= 1410), high potency as δ antagonist (Ke= 3-8 nM against various δ agonists in the mouse vas deferens assay) and, unlike other δ antagonists, had no μ-antagonist properties. The tripeptides H-Tyr-Tic-Phe-OH and H-Tyr-Tic-Phe-NH2were also δ antagonists.