Recent Advances in Melatonin Receptor Ligands

Synthesis and biological evaluation of paeoveitol D derivatives as new melatonin receptor agonists with antidepressant activities

Our previous study demonstrated that paeoveitol D, a benzofuran compound isolated from Paeonia veitchii, displayed activity on MT₁ and MT₂ receptors with agonistic ratios of 57.5% and 51.6% at a concentration of 1 mM. To explore the structure-activity relationships, 34 paeoveitol D derivatives were synthesized and evaluated for their MT₁ and MT₂ agonistic activities using the Fluo-8 calcium assay. Among them, 16 and 18 derivatives increased agonistic activities on the MT₁ and MT₂ receptors, respectively. Compound 18 indicated EC₅₀ values of 21.0 and 298.9 μM on MT₁ and MT₂ receptors in agonistic dose response curves with Tango assays and shortened immobility time in the forced swim test. The preliminary mechanism-of-action investigation manifested that the antidepressant activity of compound 18 may be mediated by promoting serotonin (5-HT) and dopamine (DA) levels in the mice brain. Compound 18 also showed favorable pharmacokinetic profiles and low toxicity in vivo. These results suggest that compound 18 could be a potential antidepressant agent.

Discovery of : A Novel Potent and Peripherally Preferred Melatonin Receptor Agonist that Reduces Liver Triglycerides and Steatosis in Diet-Induced Obese Rats

The modulation of melatonin signaling in peripheral tissues holds promise for treating metabolic diseases like obesity, diabetes, and nonalcoholic steatohepatitis. Here, several benzimidazole derivatives have been identified as novel agonists of the melatonin receptors MT1 and MT2. The lead compounds 10b, 15a, and 19a demonstrated subnanomolar potency at MT1/MT2 receptors, high oral bioavailability in rodents, peripherally preferred exposure, and excellent selectivity in a broad panel of targets. Two-month oral administration of 10b in high-fat diet rats led to a reduction in body weight gain similar to dapagliflozin with superior results on hepatic steatosis and triglyceride levels. An early toxicological assessment indicated that 10b (also codified as ACH-000143) was devoid of hERG binding, genotoxicity, and behavioral alterations at doses up to 100 mg/kg p.o., supporting further investigation of this compound as a drug candidate.

Pineal hormone melatonin as an adjuvant treatment for COVID‑19 (Review)

The beneficial properties of the pineal hormone, melatonin, as a neuroprotective and cardioprotective agent, have been previously identified. Furthermore, melatonin plays essential roles in biological rhythms resynchronization, sleep initiation/maintenance and metabolic, ocular, rheumatological diseases. In addition to these functions, melatonin is known to exert immunomodulation, anti-inflammatory and anti-oxidative effects. Due to these properties, coupled with its non-toxic nature, melatonin has been suggested to limit viral infections; however, melatonin cannot be classified as a viricidal drug. In addition, the recent increase in the number of clinical trials on melatonin's role, as an adjuvant treatment for COVID-19, has resurged the interest of the scientific community in this hormone. The present short review aimed to improve the understanding of the antiviral/anti-COVID-19 profile of melatonin and the clinical trials that have recently been conducted, with respect to its co-administration in treating individuals with COVID-19.

Melatonin receptor ligands: A pharmaco-chemical perspective

Melatonin MT₁ and MT₂ receptor ligands have been vigorously explored for the last 4 decades. Inspection of approximately 80 publications in the field revealed that most melatonergic ligands were structural analogues of melatonin combining three essential features of the parent compound: an aromatic ring bearing a methoxy group and an amide side chain in a relative arrangement similar to that present in melatonin. While several series of MT₂ -selective agents-agonists, antagonists, or partial agonists-were reported, the field was lacking MT₁ -selective agents. Herein, we describe various approaches toward the development of melatonergic ligands, keeping in mind that most of the molecules/pharmacophores obtained were essentially melatonin copies, even though diverse tri- or tetra-cyclic compounds were explored. In addition to lack of structural diversity, only few studies examined the activity of the reported melatonergic ligands in vivo. Moreover, an extensive pharmacological characterization including biopharmaceutical stability, pharmacokinetic properties, specificity toward other major receptors to name a few remained scarce. For example, many of the antagonists described were not stable in vivo, were not selective for the melatonin receptor subtype of interest, and were not fully characterized from a pharmacological standpoint. Indeed, virtual screening of large compound libraries has led to the recent discovery of potent and selective melatonin receptor agonists and partial agonists of new chemotypes. Having said this, the melatonergic field is still lacking subtype-selective melatonin receptor antagonists "active" in vivo, which are critical to our understanding of melatonin and melatonin receptors' role in basic physiology and disease.

Polybenzyls from Gastrodia elata, their agonistic effects on melatonin receptors and structure-activity relationships

Gastrodia elata is a famous traditional Chinese herb with medicinal and edible application. In this study, nine polybenzyls (1-9), including six new ones (2-5, 7 and 9), were isolated from the EtOAc extract of G. elata. Five compounds 1, 3, 4, 6 and 8 were found to activate melatonin receptors. Especially, compound 1 showed agonistic effects on MT₁ and MT₂ receptors with EC₅₀ values of 237 and 244 μM. For better understanding their structure-activity relationships (SARs), ten polybenzyl analogs were further synthesized and assayed for their activities on melatonin receptors. Preliminary SARs study suggested that two para-hydroxy groups were the key pharmacophore for maintaining activity. Molecular docking simulations verified that compound 1 could strongly interact with MT₂ receptor by bonding to Phe 118, Gly 121, His 208, Try 294 and Ala 297 residues.

Bioactivity-guided synthesis of tropine derivatives as new agonists for melatonin receptors

Twenty-three tropine derivatives as new melatonin receptor (MT₁ and MT₂) agonists were synthesized and evaluated on HEK293 cells in vitro.

N-Bridged 5,6-bicyclic pyridines: Recent applications in central nervous system disorders

The search for novel heterobicyclic compounds within the drug-like chemical space continues to be an area of interest in medicinal chemistry. Unsaturated N-bridgehead heterocycles are well represented in marketed drugs for a variety of therapeutic areas, and continue to play an important role in central nervous system (CNS) drug discovery programs. Examples of medicinal chemistry strategies that make use of N-bridged 5,6-bicyclic pyridines are discussed here in this Minireview, which covers the literature from 2010 up to 2014. B1-class imidazopyridines and B3-class pyrazolopyridines have proven to be at the forefront of molecular prototypes that are capable of interacting with disease relevant targets in neurodegeneration and neuropsychiatry.

Agomelatine in the treatment of major depressive disorder: an assessment of benefits and risks

Agomelatine (AGM) was approved for the treatment of major depressive disorder (MDD) in adults by the European Medicines Agency (EMA) in February 2009. It is an analogue of melatonin and features a unique pharmacodynamic profile with agonism on both types of melatonergic receptors (MT1/MT2) and antagonism at serotonergic 5-HT2C receptors. There is, however, an ongoing debate regarding the efficacy and safety of this novel antidepressant agent, originally evoked by claims of a significant publication bias underlying the assessment of AGM being an effective antidepressant. Indeed, two recent comprehensive metaanalyses of published and unpublished clinical trials found evidence for a relevant publication bias. However, due to its statistically significant advantage over placebo based on the results of these metaanalyses AGM must be referred to as an effective antidepressant agent in the acute phase of MDD. However, the effect sizes of AGM in the treatment of MDD were evaluated as being small in comparison to other antidepressant agents. In addition, there is insufficient evidence for the efficacy of AGM in relapse prevention of MDD. Apart from efficacy issues, AGM appears to have the potential to exhibit severe hepatotoxicity (the EMA has identified AGM-associated “hepatotoxic reactions” as a new safety concern in September 2013) that is currently poorly understood. Considering these aspects, it seems inappropriate to evaluate AGM as an antidepressant agent of first choice. Nevertheless, its unique mechanism of action with particular sleep modulating effects may represent a specific treatment strategy for patients with particular characteristics; further studies with thorough characterization of patients are needed to test this hypothesis.

Anxiolytic-like effects of a new 1-N substituted analog of melatonin in pinealectomized rats

In spite of the wide variety of drugs available for treating anxiety, this disorder continues to represent a worldwide health problem that is classified within the first 10 causes of disability. Therefore, the search continues for new antianxiety agents, particularly those not related to benzodiazepines. Even though melatonin has been prescribed as an anxiolytic drug, its use is currently limited due to its short half-life and photo-sensitivity, among other disadvantages. The present study explores the antianxiety properties of a new 1-N substituted melatonin analog, M3C, in pinealectomized rats submitted to two behavioral tests (the cumulative burying behavior paradigm and the elevated plus-maze). Results from both tests show that M3C is effective as an anxiolytic-like agent, at doses lower than any other melatonin analog previously reported. The blocking of these actions by luzindole together with the available data suggests that the anxiolytic properties of M3C are mediated by MT1 and MT2 receptors.

Synthetic melatoninergic ligands: achievements and prospects

Pineal hormone melatonin is widely used in the treatment of disorders of circadian rhythms. The presence of melatonin receptors in various animal tissues motivates the use of this hormone in some other diseases. For this reason, in recent years investigators continued the search for synthetic analogues of melatonin which are metabolically stable and selective to receptors. This review includes recent information about the most famous melatonin analogues, their structure, properties, and physiological features of the interaction with melatonin receptors.

Design, synthesis, and biological evaluation of a new class of MT2-selective agonists

In this study, we have designed and synthesized a novel class of chiral 2,3-dihydro-1H-indene derivatives as highly selective MT₂ agonists.

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.

Age and Gender Effects on the Pharmacokinetics and Pharmacodynamics of Ramelteon, a Hypnotic Agent Acting via Melatonin Receptors MT1and MT2

Effects of age and gender on the pharmacokinetics and pharmacodynamics of ramelteon, a hypnotic acting via binding to melatonin MT(1) and MT(2) receptors, were evaluated in healthy young (18-34 years) and elderly (63-79 years) volunteers. Part 1 evaluated the pharmacokinetics of open-label oral ramelteon, 16 mg. Part 2 was a double-blind, randomized, 2-trial crossover pharmacodynamic study of 16-mg ramelteon and matching placebo. Ramelteon clearance was significantly reduced in elderly vs young volunteers (384 vs 883 mL/min/kg, P<.01) and half-life significantly increased (1.9 vs 1.3 h, P<.001). Gender did not significantly influence clearance or half-life. Ramelteon was extensively transformed to its hydroxylated M-II metabolite, with serum AUC values averaging about 30 times those of the parent drug. Compared to placebo, ramelteon increased self- and observer-rated sedation, but age and gender did not influence the magnitude of the ramelteon-placebo difference. Ramelteon did not significantly impair digit-symbol substitution test performance or impair information acquisition and recall. Thus, the reduced clearance and higher serum levels of ramelteon in elderly subjects were not associated with enhanced pharmacodynamic effects. The usually recommended clinical dose of ramelteon (8 mg) does not require modification based on age or gender.

Novel conformationally constrained analogues of agomelatine as new melatoninergic ligands

Novel conformationally restricted analogues of agomelatine were synthesized and pharmacologically evaluated at MT₁ and MT₂ melatoninergic receptors. Replacement of the N-acetyl side chain of agomelatine by oxathiadiazole-2-oxide (compound 3), oxadiazole-5(4H)-one (compound 4), tetrazole (compound 5), oxazolidinone (compound 7a), pyrrolidinone (compound 7b), imidazolidinedione (compound 12), thiazole (compounds 13 and 14) and isoxazole moieties (compound 15) led to a decrease of the melatoninergic binding affinities, particularly at MT₁. Compounds 7a and 7b exhibiting nanomolar affinity towards the MT₂ receptors subtypes have shown the most interesting pharmacological results of this series with the appearance of a weak MT₂-selectivity.

N-(Phenoxyalkyl)amides as MT(1) and MT(2) ligands: antioxidant properties and inhibition of Ca(2+)/CaM-dependent kinase II

Recently a series of chiral N-(phenoxyalkyl)amides have been reported as potent MT(1) and MT(2) melatonergic ligands. Some of these compounds were selected and tested for their antioxidant properties by measuring their reducing effect against oxidation of 2',7'-dichlorodihydrofluorescein (DCFH) in the DCFH-diacetate (DCFH-DA) assay. Among the tested compounds, N-[2-(3-methoxyphenoxy)propyl]butanamide displayed potent antioxidant activity that was stereoselective, the (R)-enantiomer performing as the eutomer. This compound displayed strong cytoprotective activity against H(2)O(2)-induced cytotoxicity resulting slightly more active than melatonin, and performed as Ca(2+)/calmodulin-dependent kinase II (CaMKII) inhibitor, too.

New quinoxaline derivatives as potential MT₁ and MT₂ receptor ligands

Ever since the idea arose that melatonin might promote sleep and resynchronize circadian rhythms, many research groups have centered their efforts on obtaining new melatonin receptor ligands whose pharmacophores include an aliphatic chain of variable length united to an N-alkylamide and a methoxy group (or a bioisostere), linked to a central ring. Substitution of the indole ring found in melatonin with a naphthalene or quinoline ring leads to compounds of similar affinity. The next step in this structural approximation is to introduce a quinoxaline ring (a bioisostere of the quinoline and naphthalene rings) as the central nucleus of future melatoninergic ligands.

Melatonin receptor agonists: new options for insomnia and depression treatment

The circadian nature of melatonin (MLT) secretion, coupled with the localization of MLT receptors to the suprachiasmatic nucleus, has led to numerous studies of the role of MLT in modulation of the sleep-wake cycle and circadian rhythms in humans. Although much more needs to be understood about the various functions exerted by MLT and its mechanisms of action, three therapeutic agents (ramelteon, prolonged-release MLT, and agomelatine) are already in use, and MLT receptor agonists are now appearing as new promising treatment options for sleep and circadian-rhythm related disorders. In this review, emphasis has been placed on medicinal chemistry strategies leading to MLT receptor agonists, and on the evidence supporting therapeutic efficacy of compounds undergoing clinical evaluation. A wide range of clinical trials demonstrated that ramelteon, prolonged-release MLT and tasimelteon have sleep-promoting effects, providing an important treatment option for insomnia and transient insomnia, even if the improvements of sleep maintenance appear moderate. Well-documented effects of agomelatine suggest that this MLT agonist offers an attractive alternative for the treatment of depression, combining efficacy with a favorable side effect profile. Despite a large number of high affinity nonselective MLT receptor agonists, only limited data on MT₁ or MT₂ subtype-selective compounds are available up to now. Administration of the MT₂-selective agonist IIK7 to rats has proved to decrease NREM sleep onset latency, suggesting that MT₂ receptor subtype is involved in the acute sleep-promoting action of MLT; rigorous clinical studies are needed to demonstrate this hypothesis. Further clinical candidates based on selective activation of MT₁ or MT₂ receptors are expected in coming years.

Application of the bridgehead fragments for the design of conformationally restricted melatonin analogues

Conformationally constrained analogues of the hormone melatonin with a side chain incorporated into the bicyclic bridgehead core were synthesized based on the homology modeling and molecular docking studies performed for the MT(2) melatonin receptor. The methoxy-indole derivative fused with exo-N-acetamino-substituted bicyclo[2.2.2]octane was found to possess nanomolar MT(2) receptor affinity.

Synthesis of a novel series of tricyclic dihydrofuran derivatives: discovery of 8,9-dihydrofuro[3,2-c]pyrazolo[1,5-a]pyridines as melatonin receptor (MT1/MT2) ligands

Novel tricyclic dihydrofuran derivatives were designed, synthesized, and evaluated as melatonin receptor (MT(1)/MT(2)) ligands based on the previously reported 1,6-dihydro-2H-indeno[5,4-b]furan 1a. By screening the central tricyclic cores, we identified 8,9-dihydrofuro[3,2-c]pyrazolo[1,5-a]pyridine as a potent scaffold with a high ligand-lipophilicity efficiency (LLE) value. Subsequent optimization of the side chains led to identification of the potent MT(1)/MT(2) agonist 4d (MT(1), K(i) = 0.062 nM; MT(2), K(i) = 0.420 nM) with good oral absorption and blood-brain barrier (BBB) penetration in rats. The oral administration of compound 4d exhibited a sleep-promoting action in freely moving cats at 0.1 mg/kg.

Ramelteon Treatment of Insomnia

Objective:

To provide an overview of the use of ramelteon for the treatment of insomnia.

Data Sources:

A literature review was conducted through PubMed using the terms ramelteon, insomnia, treatment, adverse events, elderly, and melatonin. Additionally, reference citations from publications identified were reviewed. Data from 1982 to 2009 were gathered.

Study Selection and Data Extraction:

All articles in English that were identified from the data sources were evaluated.

Data Synthesis:

Long-term clinical trials of ramelteon have demonstrated that it is effective and safe, causing minimal adverse effects and no potential for abuse. Additionally, discontinuation of ramelteon does not result in rebound insomnia, which makes it an ideal agent for the management of insomnia in the elderly population.

Conclusions:

Ramelteon offers a safe and effective new therapeutic option for patients experiencing transient or chronic insomnia.

Agomelatine: The evidence for its place in the treatment of depression

INTRODUCTION

Depressive disorders are among the main causes of disability due to disease. In spite of recent progress in the pharmacotherapy of depression, there is still a high nonresponse rate of approximately 30% to the first antidepressant treatment. Furthermore, the latency of several weeks until sufficient clinical improvement and the risk of side effects remain unresolved problems. Therefore, there is still further need for the development of new antidepressants. In the last years a variety of melatonin receptor agonists have been synthesized and evaluated for the treatment of sleep disorders. Animal studies suggested that agomelatine (S-20098), a synthetic melatonergic MT(1) and MT(2) receptor agonist with serotonin receptor antagonistic properties, may have additional activating properties and may represent a new approach in the treatment of depression.

AIMS

Clinical trials that have demonstrated efficacy and safety of agomelatine for the treatment of depression are reviewed.

EVIDENCE REVIEW

In clinical trials, including phase III studies, superior efficacy compared to placebo and good efficacy compared to standard antidepressants was shown for agomelatine for the acute treatment of major depression. In all studies published so far agomelatine was safe and the overall tolerability profile was superior to selective serotonin reuptake inhibitors or selective serotonin and norepinephrine reuptake inhibitors.

PLACE IN THERAPY

Agomelatine may represent a novel perspective in the treatment of acute depression. The improvement of sleep disturbances, the tolerability in terms of sexual side effects, and the lack of withdrawal symptoms after abrupt discontinuation of treatment may represent important clinical benefits compared to established antidepressants.

Physiology and pharmacology of melatonin in relation to biological rhythms

Melatonin is an evolutionarily conserved molecule that serves a time-keeping function in various species. In vertebrates, melatonin is produced predominantly by the pineal gland with a marked circadian rhythm that is governed by the central circadian pacemaker (biological clock) in the suprachiasmatic nuclei of the hypothalamus. High levels of melatonin are normally found at night, and low levels are seen during daylight hours. As a consequence, melatonin has been called the "darkness hormone". This review surveys the current state of knowledge regarding the regulation of melatonin synthesis, receptor expression, and function. In particular, it addresses the physiological, pathological, and therapeutic aspects of melatonin in humans, with an emphasis on biological rhythms.

Critical appraisal and update on the clinical utility of agomelatine, a melatonergic agonist, for the treatment of major depressive disease in adults

This article describes the pharmacology of the novel atypical antidepressant drug agomelatine, critically reviews and evaluates its clinical use for the treatment of major depression, and suggests areas for further research. Agomelatine is a synthetic analog of the hormone melatonin. It stimulates the activity of melatonin MT1 and MT2 receptors and inhibits the activity of serotonin 5HT-2C receptor subtypes. Three acute trials demonstrated clinically modest, but statistically significant benefits over placebo. Three acute trials did not find agomelatine more effective than placebo. A meta-analysis of these six trials demonstrated a small, statistically significant, marginally clinically relevant difference between agomelatine and placebo. The only placebo-controlled study in elderly patients did not demonstrate a significant benefit for agomelatine. It was more effective than placebo in only one of two relapse prevention studies. Agomelatine was generally well tolerated compared to placebo. Its side-effect profile is different than and compares favorably to other antidepressant drugs. The overall tolerability of agomelatine in head-to-head comparisons was not substantially better than active drug comparators. Agomelatine is contraindicated in patients with impaired liver function and in patients taking drugs that potently inhibit CYP-1A2 metabolic enzymes. Because elevated liver enzymes are common, and there is a rare risk of more serious liver reactions, routine laboratory monitoring of liver function is recommended periodically throughout treatment. Agomelatine does not have clinically significant advantages compared to other antidepressant drugs, and it has certain limitations and disadvantages. Because of its unique pharmacology and relatively benign tolerability profile, however, it may be a useful alternative for patients who do not respond to or cannot tolerate other antidepressant drugs.

Sleep-promoting action of IIK7, a selective MT2 melatonin receptor agonist in the rat

Several novel melatonin receptor agonists, in addition to various formulations of melatonin itself, are either available or in development for the treatment of insomnia. Melatonin is thought to exert its effects principally through two high affinity, G-protein coupled receptors, MT₁ and MT₂, though it is not known which subtype is responsible for the sleep-promoting action. The present study used radiotelemetry to record EEG and EMG in un-restrained freely moving rats to monitor the sleep-wake behaviour and examined the acute sleep-promoting activity of an MT₂ receptor subtype selective melatonin analog, IIK7. IIK7 is a full agonist at the MT₂ receptor subtype but a partial agonist at the MT₁ receptor and has ∼90-fold higher affinity for MT₂ than MT₁. Like melatonin, IIK7 (10 mg/kg i.p.) significantly reduced NREM sleep onset latency and transiently increased the time spent in NREM sleep, but did not alter REM sleep latency or the amount of REM sleep. An analysis of the EEG power spectrum showed no change in delta (1–4 Hz) or theta activity (5–8 Hz) following IIK7 administration. Core body temperature was slightly decreased (∼0.3 °C) by IIK7 compared to vehicle-treated rats. The acute and transient changes in the sleep-wake cycle mimic the changes seen with melatonin and suggest that its sleep-promoting activity is mediated by activation of the MT₂ receptor subtype.