New diarylmethylpiperazines as potent and selective nonpeptidic δ opioid receptor agonists with increased in vitro metabolic stability

Nonpeptide delta opioid agonists are analgesics with a potentially improved side-effect and abuse liability profile, compared to classical opioids. Andrews analysis of the NIH nonpeptide lead SNC-80 suggested the removal of substituents not predicted to contribute to binding. This approach led to a simplified lead, N,N-diethyl-4-[phenyl( 1-piperazinyl)methyl]benzamide (1), which retained potent binding affinity and selectivity to the human delta receptor (IC50 = 11 nM, mu/delta = 740, kappa/delta > 900) and potency as a full agonist (EC50 = 36 nM) but had a markedly reduced molecular weight, only one chiral center, and increased in vitro metabolic stability. From this lead, the key pharmacophore groups for delta receptor affinity and activation were more clearly defined by SAR and mutagenesis studies. Further structural modifications on the basis of 1 confirmed the importance of the N,N-diethylbenzamide group and the piperazine lower basic nitrogen for delta binding, in agreement with mutagenesis data. A number of piperazine N-alkyl substituents were tolerated. In contrast, modifications of the phenyl group led to the discovery of a series of diarylmethylpiperazines exemplified by N,N-diethyl-4-[1-piperazinyl(8-quinolinyl)- methyl]benzamide (56) which had an improved in vitro binding profile (IC50 = 0.5 nM, mu/delta = 1239, EC50 = 3.6 nM) and increased in vitro metabolic stability compared to SNC-80.