Cyclic dermorphin-like tetrapeptides with delta-opioid receptor selectivity. 3. Effect of residue 3 modification on in vitro opioid activity

A series of residue 3-modified analogs of the cyclic, delta-opioid receptor-selective, dermorphin-like tetrapeptide Tyr-D-Cys-Phe-D-Pen and the corresponding residue 4-modified analog of the related delta receptor-selective cyclic pentapeptide [D-Pen2,D-Pen5] enkephalin were synthesized and evaluated in opioid receptor binding assays and in the in vitro mouse vas deferens (MVD) bioassay. In both series, substitutions that would be expected to alter the orientation of the phenylalanine-substituted aromatic side chain relative to the rest of the peptide, due to changes in the conformation of the peptide backbone, had deleterious effects on binding affinity and MVD potency. In general, these adverse effects were more pronounced in the pentapeptide series, owing, most likely, to the greater rigidity and, therefore, reduced susceptibility to conformational perturbation of the tetrapeptides. Substitution of phenylalanine by p-fluorophenylalanine enhances binding affinity in the pentapeptide series, consistent with previous observations in the enkephalins, but is without effect on binding in the tetrapeptide series. Substitution of phenylalanine by homophenylalanine, which alters the relationship of the aromatic phenyl ring to the remainder of the peptide by inserting an additional methylene group between the aromatic moiety and the backbone, greatly reduces binding affinity and MVD potency in the pentapeptide. The corresponding modification in the tetrapeptide series has little effect on delta receptor binding affinity and MVD potency and enhances binding to mu opioid receptors. Several possible interpretations of these results are discussed.