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 T yr-c[D-Cys-Phe-D-Pen]NH2 (Et) (JOM-6) was modified at residues 1 and 3 by s ubstitution with various natural and synthetic amino acids, and/or by alter ation of the cyclic system. Effects on mu and delta opioid receptor binding affinities, and on potencies and efficacies as measured by the [S-35]-GTP gamma S assay, were evaluated. Affinities at mu and delta receptors were no t influenced dramatically by substitution of Tyr(1) with conformationally r estricted phenolic amino acids. In the [S-35]-CTP gamma S assay, all of the peptides tested exhibited a maximal response comparable with that of fenta nyl 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, sug gesting that the conformation required for binding and the conformation req uired for activation of the opioid receptors are different. At the delta op ioid receptor, none of the peptides were able to produce a response equival ent to that of the full delta agonist BW 373,U86 and only one had an EC50 v alue of less than 100nM. Lastly, we have identified a peptide, D-Hat-c[D-Cy s-Phe-o-Pen]NH2 (Et), with high potency and >1000-fold functional selectivi ty for the mu over delta opioid receptor as measured by the [S-35]-GTP gamm a S assay.