Characterization of Substituted Phenylpropylamides as Highly Selective Agonists at the Melatonin MT2 Receptor

King      H. Chan; Yueqing      Hu; Maurice      K. C. Ho; Yung      H. Wong

Abstract

Melatonin is a widely distributed hormone that regulates several major physiological processes, including the circadian rhythm and seasonal adaptation. The two subtypes of mammalian G protein-coupled melatonin receptors are primarily responsible for mediating the actions of melatonin. Because synthetic melatonin agonists have considerable therapeutic potentials in modulating insomnia and circadian- related sleep disorders, it is highly desirable to develop subtype-selective melatoninergic compounds. The pharmacological potencies of a series of substituted N-[3-(3-methoxyphenyl)propyl] amides towards human melatonin MT1 and MT2 receptors were evaluated by the FLIPR high-throughput screening assay, whilst their subtype-selectivity was subsequently verified with ERK phosphorylation and cAMP assays. Structure-activity relationship analysis of highly potent subtype-selective ligands (MT2 EC50 10-90 pM) revealed that a benzyloxyl substituent incorporated at C6 position of the 3-methoxyphenyl ring dramatically enhanced the MT2 potency and at the same time decreased MT1 potency. Incorporation of structural moieties conferring the subtype selectivity produced several extremely potent MT2-selective ligands. The most potent subtype-selective ligand, 2q had a substantially higher potency for MT2 receptor than melatonin for elevation of [Ca2+]i and inhibition of forskolin-elevated cAMP. Representative MT2-selective ligands also induced ERK phosphorylation in both recombinant and native cell lines, and no cross-reactivity to 17 other GPCRs could be detected. These ligands represent invaluable tools for delineating the functional roles of distinct melatonin receptor subtypes and are viable candidates for drug development.

Keywords: Melatonin, GPCR, MT1, MT2, subtype selectivity, hormone, physiological processes, circadian rhythm, seasonal adaptation, mammalian G protein-coupled