De novo design, synthesis, and biological activities of high-affinity and selective non-peptide agonists of the δ-opioid receptor

On the basis of the structure-activity relationships of δ-opioid- selective peptide ligands and on a model of the proposed bioactive conformation for a potent and selective, conformationally constrained δ- opioid peptide ligand [(2S,3R)-TMT¹]DPDPE, a series of small organic peptide mimetic compounds targeted for the δ-opioid receptor have been designed, synthesized, and evaluated in radiolabeled ligand binding assays and in vitro bioassays. The new non-peptide ligands use piperazine as a template to present the most important pharmacophore groups, including phenol and phenyl groups and a hydrophobic moiety. This hydrophobic group was designed to mimic the hydrophobic character of the D-Pen residues in DPDPE, which has been found to be extremely important for increasing the binding affinity and selectivity of these non-peptide ligands for the δ-opioid receptor over the μ-opioid receptor. Compound 6f (SL3111) showed 8 nM binding affinity and over 2000-fold selectivity for the δ-opioid receptor over the μ-opioid receptor. Both enantiomers of SL-3111 were separated, and the (-)-isomer was shown to be the compound with the highest affinity for the δ-opioid receptor found in our study (IC₅₀ = 4.1 nM), with a selectivity very similar to that observed for the racemic compound. The phenol hydroxyl group of SL-3111 turned out to be essential to maintain high affinity for the δ-opioid receptor, which also was observed in the case of the δ-opioid-selective peptide ligand DPDPE. Binding studies of SL-3111 and [p-C]Phe⁴]DPDPE on the cloned wild-type and mutated human δ-opioid receptors suggested that the new non-peptide ligand has a binding profile similar to that of DPDPE but different from that of (+)-4-[((αR)-α(2S,5R)-4-allyl-2,5dimethyl-1- piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-80), another δ- opioid-selective non-peptide ligand.