Discovery of Aminobenzyloxyarylamides as κ Opioid Receptor Selective Antagonists: Application to Preclinical Development of a κ Opioid Receptor Antagonist Receptor Occupancy Tracer

Arylphenylpyrrolidinylmethylphenoxybenzamides were found to have high affinity and selectivity for K opioid receptors. On the basis of receptor binding assays in Chinese hamster ovary (CHO) cells expressing cloned human opioid receptors, (S)-3-fluoro-4-(4-((2-(3-fluorophenyppyrrolidin-1-yl)methyl)phenoxy)benzamide (25) had a K-i = 0.565 nM for kappa opioid receptor binding while having a K-i = 35.8 nM for mu opioid receptors and a K-i = 211 nM for delta opioid receptor binding. Compound 25 was also a potent antagonist of K opioid receptors when tested in vitro using a [S-35]-guanosine 5'O-[3-thiotriphosphate] ([S-35]GTP-gamma-S) functional assay in CHO cells expressing cloned human opioid receptors. Compounds were also evaluated for potential use as receptor occupancy tracers. Tracer evaluation was done in vivo, using liquid chromatography-tandem mass spectrometry (LC/MS/MS) methods, precluding the need for radiolabeling. (S)-3-Chloro-4-(4-((2-(pyridine-3-yl(pyrrolidin-1-yl)methyl)phenoxy)benzamide (18) was found to have favorable properties for a tracer for receptor occupancy, including good specific versus nonspecific binding and good brain uptake.