Yueqing Hu

Astragaloside IV and Cycloastragenol Stimulate the Phosphorylation of Extracellular Signal-Regulated Protein Kinase in Multiple Cell Types

Two Chinese herb-derived small molecule telomerase activators, astragaloside IV (AG-IV) and cycloastragenol (CAG), have recently been shown to improve the proliferative response of CD8+ T lymphocytes from HIV-infected patients by upregulating telomerase activity. Here, we examined the signaling mechanism of AG-IV and CAG. Telomerase activity in human embryonic kidney HEK293 fibroblasts was increased upon treatment with increasing concentrations of AG-IV or CAG. Both compounds induced the phosphorylation of extracellular signal-regulated protein kinase (ERK) in a time- and dose-dependent manner in HEK293 cells and HEK-neo keratinocytes. AG-IV and CAG also stimulated ERK phosphorylation in other cell lines of lung, brain, mammary, endothelial, and hematopoietic origins. Use of selective inhibitors and dominant negative mutants revealed the involvement of c-Src, MEK (ERK kinase), and epidermal growth factor receptor in CAG-induced ERK phosphorylation. Our data indicate that AG-IV and CAG may exert their cellular effects through the activation of the Src/MEK/ERK pathway.

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.

Design, synthesis and evaluation of novel heterodimers of donepezil and huperzine fragments as acetylcholinesterase inhibitors.

Four series of novel heterodimers comprised of donepezil and huperzine A (HupA) fragments were designed, synthesized, and evaluated in search of potent acetylcholinesterase (AChE) inhibitors as potential therapeutic treatment for Alzheimers disease. Heterodimers comprised of dimethoxyindanone (from donepezil), hupyridone (from HupA), and connected with a multimethylene linker, were identified as potent and selective inhibitors of AChE. Diastereomeric heterodimers (RS,S)-17b (with a tetramethylene linker) exhibited the highest potency of inhibition towards AChE with an IC(50) value of 9 nM and no detectable inhibitory effect on butyrylcholinesterase at 1mM.

In Search of Novel and Therapeutically Significant Melatoninergic Ligands

Melatonin, the pineal gland hormone, is widely distributed in mammalian tissues and exerts its action via two melatonin receptor sub-types, MT(1) and MT(2). Melatonin is known to play functional roles in regulating circadian rhythms and seasonal reproduction. In recent years, growing evidence has also linked melatonin to a variety of other body systems and disease states, thus highlighting its significance as a therapeutic agent. However, due to its properties, melatonin is ineffective in clinical use, thus prompting the development of melatoninergic ligands that mimic the actions of melatonin but in a manner that is more potent and specific for melatonin receptors. An additional focus has been to develop ligands that exhibit receptor subtype selectivity. While there are over seventy patents on melatoninergic ligands, success in developing therapeutically effective melatoninergic ligands have been varied. However, the recent approval of Ramelteon for treatment of sleep disorders and the evaluation of other compounds in clinical trials have highlighted their clinical importance. In this review an overview of recently developed novel melatoninergic ligands is provided including recently filed patents and compounds undergoing clinical evaluation.

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.

3-Methoxylphenylpropyl amides as novel receptor subtype-selective melatoninergic ligands: characterization of physicochemical and pharmacokinetic properties

Developing subtype-selective melatoninergic ligands has been a subject of considerable interest in drug discovery. A series of 3-methoxyphenylpropyl amide derivatives showing selective binding capacity to type 2 melatonin receptor with subnanomolar range of affinities has been identified recently by our laboratory. In the present study, their physicochemical properties, Caco-2 cell and mdr1-MDCK cell permeability, plasma protein binding, and metabolic stability were investigated. The selected compounds are lipophilic in nature, exhibiting aqueous solubility ranging from 40 to 200 µg/mL. Cell permeability studies on Caco-2 and mdr1-MDCK model revealed that they were readily transported through intestinal epithelium and possessed high penetration potential through blood–brain barrier, implying good oral absorption and central nervous system (CNS) distribution potential. They also showed substantial binding to human plasma protein ranging from 78.5% to 92.3%. These compounds were, however, subjected to rapid cytochrome P450-mediated degradation in rat and human liver microsomes with in vitro half-life of 9.5–31.9 min in rat and 5.5–66.7 min in human, which were much shorter than that of melatonin (∼73 min). Metabolite profiling unveiled that C6-ether linkage and methoxy substituents were likely the major metabolic soft spots in their structures, which provided important information for further improvement of their structural stability.

Pharmacokinetics, oral bioavailability and metabolism of a novel isoquinolinone-based melatonin receptor agonist in rats.

7-Methoxy-6-(3-methoxy-benzyloxy)-2-methylisoquinolin-1(2H)-one (named as IS0042) is a newly identified melatoninergic agonist which exhibits selectivity to the type 2 melatonin receptor. Here, we examined the in vitro and in vivo pharmacokinetics properties of IS0042 in rats. IS0042 was considerably lipophilic with a modest aqueous solubility of 27.3 µg/mL. It was stable in simulated gastrointestinal fluid, and readily penetrated across differentiated Caco-2 cell model of intestinal barrier, suggesting good oral absorption. IS0042 underwent metabolism in rat intestinal and liver microsomes with an in vitro half-life of 367.5 ± 36.6 and 17.5 ± 2.7 min, respectively. Metabolite identification suggested that the major biotransformation pathways included the cleavage of ether bond, hydroxylation and demethylation. The same metabolites were also present in blood circulation following oral administration, indicating a good correlation between in vitro and in vivo metabolism. The pharmacokinetics parameters of IS0042 were evaluated after intravenous administration (10 or 25 mg/kg) and oral administration (100 mg/kg) of the drug to rats. IS0042 showed moderate clearance (0.73–1.02 L/h/kg), large volume of distribution (1.76–3.16 L/kg) and long elimination half-life (3.11–6.04 h) after intravenous administration. The absolute oral bioavailability of IS0042 was relatively low (9.8–18.6%). Overall, these results provide important parameters for the further development of this novel class of melatoninergic ligands.

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.