King Hang Chan

Synthesis of substituted N-[3-(3-methoxyphenyl)propyl] amides as highly potent MT2-selective melatonin ligands

A series of substituted N-[3-(3-methoxyphenyl)propyl] amides were synthesized and their binding affinities towards human melatonin MT(1) and MT(2) receptors were evaluated. It was discovered that a benzyloxyl substituent incorporated at C6 position of the 3-methoxyphenyl ring dramatically enhanced the MT(2) binding affinity and at the same time decreased MT(1) binding affinity.

A Molecular and Chemical Perspective in Defining Melatonin Receptor Subtype Selectivity

Melatonin is primarily synthesized and secreted by the pineal gland during darkness in a normal diurnal cycle. In addition to its intrinsic antioxidant property, the neurohormone has renowned regulatory roles in the control of circadian rhythm and exerts its physiological actions primarily by interacting with the G protein-coupled MT1 and MT2 transmembrane receptors. The two melatonin receptor subtypes display identical ligand binding characteristics and mediate a myriad of signaling pathways, including adenylyl cyclase inhibition, phospholipase C stimulation and the regulation of other effector molecules. Both MT1 and MT2 receptors are widely expressed in the central nervous system as well as many peripheral tissues, but each receptor subtype can be linked to specific functional responses at the target tissue. Given the broad therapeutic implications of melatonin receptors in chronobiology, immunomodulation, endocrine regulation, reproductive functions and cancer development, drug discovery and development programs have been directed at identifying chemical molecules that bind to the two melatonin receptor subtypes. However, all of the melatoninergics in the market act on both subtypes of melatonin receptors without significant selectivity. To facilitate the design and development of novel therapeutic agents, it is necessary to understand the intrinsic differences between MT1 and MT2 that determine ligand binding, functional efficacy, and signaling specificity. This review summarizes our current knowledge in differentiating MT1 and MT2 receptors and their signaling capacities. The use of homology modeling in the mapping of the ligand-binding pocket will be described. Identification of conserved and distinct residues will be tremendously useful in the design of highly selective ligands.

Development of substituted N-[3-(3-methoxylphenyl)propyl] amides as MT2-selective melatonin agonists: Improving metabolic stability

A series of novel and selective N-[3-(6-benzyloxy-3-methoxyphenyl)propyl] amides has recently been shown to possess sub-nanomolar range binding affinity to the type 2 melatonin receptor (MT(2)). Pharmacokinetics studies suggested that these compounds were subject to vigorous CYP450-mediated metabolism, resulting in a series of metabolites with significantly decreased or diminished binding affinities toward MT(2) receptor. The ether bonds were found to be the major positions susceptible to metabolism. In this study, the benzyl ether bond was either removed or replaced with a carbon-carbon bond in an attempt to improve metabolic stability and enhance their resistance towards phase I oxidation. The synthesis, receptor binding affinity, intrinsic potency and metabolic stability of modified structures are reported in this article. By removal or replacement of metabolic labile ether linkerage with carbon linkers, a novel compound was identified with good potency and MT(2) selectivity, and with increased metabolic stability.

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

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 MT(1) and MT(2) 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 (MT(2) EC(50) 10-90 pM) revealed that a benzyloxyl substituent incorporated at C6 position of the 3-methoxyphenyl ring dramatically enhanced the MT(2) potency and at the same time decreased MT(1) potency. Incorporation of structural moieties conferring the subtype selectivity produced several extremely potent MT(2)-selective ligands. The most potent subtype-selective ligand, 2q had a substantially higher potency for MT(2) receptor than melatonin for elevation of [Ca(2+)]i and inhibition of forskolin-elevated cAMP. Representative MT(2)-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.

Synthesis and functional characterization of substituted isoquinolinones as MT2-selective melatoninergic ligands.

A series of substituted isoquinolinones were synthesized and their binding affinities and functional activities towards human melatonin MT1 and MT2 receptors were evaluated. Structure-activity relationship analysis revealed that substituted isoquinolinones bearing a 3-methoxybenzyloxyl group at C5, C6 or C7 position respectively (C5>C6>C7 in terms of their potency) conferred effective binding and selectivity toward the MT2 receptor, with 15b as the most potent compound. Most of the tested compounds were MT2-selective agonists as revealed in receptor-mediated cAMP inhibition, intracellular Ca2+ mobilization and phosphorylation of extracellular signal-regulated protein kinases. Intriguingly, compounds 7e and 7f bearing a 4-methoxybenzyloxyl group or 4-methylbenzyloxyl at C6 behaved as weak MT2-selective antagonists. These results suggest that substituted isoquinolinones represent a novel family of MT2-selective melatonin ligands. The position of the substituted benzyloxyl group, and the substituents on the benzyl ring appeared to dictate the functional characteristics of these compounds.