Modifications of the cyclic mu receptor selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]NH2 (Et): effects on opioid receptor binding and activation.

The previously described cyclic mu opioid receptor-selective tetrapeptide Tyr-c[D-Cys-Phe-D-Pen]NH2 (Et) (JOM-6) was modified at residues 1 and 3 by substitution with various natural and synthetic amino acids, and/or by alteration of the cyclic system. Effects on mu and delta opioid receptor binding affinities, and on potencies and efficacies as measured by the [35S]-GTPgammaS assay, were evaluated. Affinities at mu and delta receptors were not influenced dramatically by substitution of Tyr1 with conformationally restricted phenolic amino acids. In the [35S]-GTPgammaS assay, all of the peptides tested exhibited a maximal response comparable with that of fentanyl at the mu opioid receptor, and all showed high potency, in the range 0.4-9nM. However, potency changes did not always correlate with affinity, suggesting that the conformation required for binding and the conformation required for activation of the opioid receptors are different. At the delta opioid receptor, none of the peptides were able to produce a response equivalent to that of the full delta agonist BW 373,U86 and only one had an EC50 value of less than 100nM. Lastly, we have identified a peptide, D-Hat-c[D-Cys-Phe-D-Pen]NH2 (Et), with high potency and > 1,000-fold functional selectivity for the mu over delta opioid receptor as measured by the [35S]-GTPgammaS assay.