Recent Advances in Melatonin Receptor Ligands

Melatonin receptors, heterodimerization, signal transduction and binding sites: what's new?

Melatonin is a neurohormone that has been claimed to be involved in a wide range of physiological functions. Nevertheless, for most of its effects, the mechanism of action is not really known. In mammals, two melatonin receptors, MT1 and MT2, have been cloned. They belong to the G-protein-coupled receptor (GPCR) superfamily. They share some specific short amino-acid sequences, which suggest that they represent a specific subfamily. Another receptor from the same subfamily, the melatonin-related receptor has been cloned in different species including humans. This orphan receptor also named GPR50 does not bind melatonin and its endogenous ligand is still unknown. Nevertheless, this receptor has been shown to behave as an antagonist of the MT1 receptor, which opens new pharmacological perspectives for GPR50 despite the lack of endogenous or synthetic ligands. Moreover, MT1 and MT2 interact together through the formation of heterodimers at least in cells transfected with the cDNA of these two receptors. Lastly, signalling complexes associated with MT1 and MT2 receptors are starting to be deciphered. A third melatonin-binding site has been purified and characterized as the enzyme quinone reductase 2 (QR2). Inhibition of QR2 by melatonin may explain melatonin's protective effect that has been reported in different animal models and that is generally associated with its well-documented antioxidant properties.

Synthesis, Enantiomeric Resolution, and Structure–Activity Relationship Study of a Series of 10,11-Dihydro-5H-Dibenzo[a,d]cycloheptene MT2 Receptor Antagonists

Racemic N-(8-methoxy-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-10-ylmethyl)acetamide (compound 5) was previously identified as a novel selective MT(2) antagonist fulfilling the requirements of pharmacophore and 3D QSAR models. In this study the enantiomers of 5 were separated by medium-pressure liquid chromatography and behaved as the racemate. Compound 5 was modified at the acylaminomethyl side chain and at position C8. The resulting analogues generally behaved as melatonin receptor antagonists (GTPgammaS test) with a modest degree of selectivity (up to 10-fold) for the MT(2) receptor. Changes at the amide side chain led to a decrease in binding affinity, whereas 8-acetyl and 8-methyl derivatives 12 and 11, respectively, were as potent as the 8-methoxy parent compound 5. Docking experiments with an MT(2) receptor model suggested binding modes consistent with the observed SARs and with the lack of selectivity of the enantiomers of 5.

7-Azamelatonin: Efficient Synthetic Routes, Excited-State Double Proton Transfer Properties and Biomedical Implications

On the basis of a seven-step synthetic route, the total synthesis of 7-azamelatonin, an analogue of melatonin, has been achieved with an overall yield of approximately 9.2%. In aqueous solution, 7-azamelatonin exhibits a unique excited-state double proton transfer (ESDPT) property, resulting in dual emission bands (405 and 560 nm). The ESDPT property makes 7-azamelatonin superb as a potential molecular probe for future bioapplication and for pharmacological research.

7-Substituted-melatonin and 7-substituted-1-methylmelatonin analogues: Effect of substituents on potency and binding affinity

A series of 7-substituted melatonin and 1-methylmelatonin analogues were prepared and tested against human and amphibian melatonin receptors. 7-Substituents reduced the agonist potency of all the analogues in the Xenopus laevis melanophore assay, 7-bromomelatonin (5d) and N-butanoyl 7-bromo-5-methoxytryptamine (5f) being the most active compounds, but both were 42-fold less potent than melatonin (1). Whereas all the analogues bind with lower affinity at the human MT(1) receptor than melatonin, 5d, 5f and N-propanoyl 7-bromo-5-methoxytryptamine (5e) show a similar binding affinity to melatonin at the MT(2) receptor and consequently show some MT(2) selectivity. These results suggest that the receptor pocket around C-7 favours binding by an electronegative group, suggesting an electropositive region in this area of the receptor.

Drug Insight: the use of melatonergic agonists for the treatment of insomnia-focus on ramelteon

Melatonin, a chronobiotic that participates in the control of the circadian system, is known for its sleep-promoting effects, which include shortening of sleep latency and lengthening of sleep duration. As a result of its short half-life, melatonin does not exhibit undesirable side effects, and its broad applicability for a variety of sleep problems has been the focus of numerous scientific studies. Melatonin has not, however, received regulatory approval from the US FDA as a drug, because it can be sold freely as a food supplement. Consequently, there has been an active search for patentable melatonin receptor ligands in recent years. Ramelteon, an agonist that acts solely on melatonin MT(1) and MT(2) receptors, is of particular interest, and preliminary research indicates that it holds considerable promise for clinical applications. Ramelteon has been shown to induce sleep initiation and maintenance in various animal models and in clinical trials. In chronic insomnia, ramelteon decreases sleep latency and increases total sleep time and sleep efficiency, without causing hangover, addiction or withdrawal effects. Ramelteon is thought to promote sleep by influencing homeostatic sleep signaling mediated by the suprachiasmatic nucleus. Although ramelteon's metabolism and pharmacokinetics differ from those of melatonin, its safety seems to be sufficient for short-term application. Its long-term effects remain to be determined.

NO-donor melatonin derivatives: synthesis and in vitro pharmacological characterization

Numerous studies document that melatonin possesses a broad-spectrum antioxidant activity. It traps a number of reactive oxygen species (ROS) such as hydroxyl and peroxyl radicals, singlet oxygen and hypochlorous acid. It also inhibits peroxynitrite-induced reactions. It is known that atherosclerosis progression involves ROS-induced oxidation of low-density lipoproteins in sub-endothelial space and the depletion of nitric oxide (NO) in blood vessels, as well as a decreased sensitivity of the vessels to the actions of NO. Considering this, a series of new NO-donor antioxidants were designed and synthesized by joining melatonin with NO-donor nitrooxy and furoxan moieties as polyvalent agents potentially useful for the treatment of cardiovascular diseases involving atherosclerotic vascular changes. The in vitro antioxidant properties of the resulting products were assessed in the thiobarbituric acid reactive substances assay (TBARS), the ABTS(+.) as well as in the alkaline phosphatase (ALP) assay. The antioxidant capacities of NO-donor melatonins to inhibit lipoperoxidation (TBARS-IC(50)) was predominantly dependent on their lipophilicity, and therefore on their partitioning process into membranes. On the other hand, their comparable capacity to inhibit protein oxidation (ALP-IC(50)) was independent of their lipophilicity and was consistent with their similar ability to participate in electron transfer reactions. All the NO-donor melatonins were also evaluated for their ability to relax rat aorta strips precontracted with 1 microM phenylephrine. Finally, binding affinities and intrinsic activity studies, carried out at MT(1) and MT(2) receptor subtypes, showed a rather complex picture in need of further investigation.

Synthesis and biological activity of new melatonin dimeric derivatives

A new series of melatonin (MLT) dimers were obtained by linking together two melatonin units with a linear alkyl chain through the MLT acetamido group or through a C-2 carboxyalkyl function. The binding properties of these ligands were evaluated in in vivo experiments on cloned human MT(1) and MT(2) receptors expressed in NIH3T3 rat fibroblast cells. The class of 2-carboxyalkyl dimers was the most interesting one with compounds having good MT(1)/MT(2) nanomolar affinity. The data obtained suggest that the spacer length is crucial for optimal interaction at both receptor subtypes as well as to determine functional activity of the resulting dimers.

Drug treatment of depression in the 2000s: An overview of achievements in the last 10 years and future possibilities

During the past 10 years our knowledge about the pharmacotherapy of depression has been consolidated, and a variety of very interesting new compounds launched onto the market. The pipeline of the pharmaceutical industry is still filled with an assortment of new developments and very promising new approaches towards the pharmacotherapy of depressive disorders. Future pharmacological treatments of depression will not only enhance serotonergic and noradrenergic neurotransmission: other systems, such as the melatonergic receptor system and the hypothalamus-pituitary-adrenal axis, are also the targets of newly developed and upcoming substances with putative antidepressant effects. The main advantages of the currently available newer pharmacotherapeutic options are the broadening of the spectrum of possible antidepressant treatments, which is of particular importance for the growing number of patients suffering from difficult-to-treat depression, and a far better tolerability profile in comparison to older compounds such as tricyclic antidepressants. Unresolved issues are the unacceptably high rate of non-responsiveness during antidepressant treatment, a latency of sometimes several weeks until clinical improvement and remission can be achieved, and a variety of possible side effects also present during treatment with modern compounds. This review mainly presents the development of antidepressant pharmacotherapies during the past 10 years, together with pharmacokinetic and pharmacodynamic information and a comparison of different pharmacological treatment principles evaluated in randomized controlled clinical trials. In addition, new pharmacological strategies that are not yet available on the market and strategies currently under development are reviewed in detail. The study of new treatment options is of major importance to provide better strategies for the clinical management of depression in the future, and is thus also of great socio-economic importance.

Design and synthesis of N-(3,3-diphenylpropenyl)alkanamides as a novel class of high-affinity MT2-selective melatonin receptor ligands

A novel series of melatonin receptor ligands was discovered by opening the cyclic scaffolds of known classes of high affinity melatonin receptor antagonists, while retaining the pharmacophore elements postulated by previously described 3D-QSAR and receptor models. Compounds belonging to the classes of 2,3- and [3,3-diphenylprop(en)yl]alkanamides and of o- or [(m-benzyl)phenyl]ethyl-alkanamides were synthesized and tested on MT(1) and MT(2) receptors. The class of 3,3-diphenyl-propenyl-alkanamides was the most interesting one, with compounds having MT(2) receptor affinity similar to that of MLT, remarkable MT(2) selectivity, and partial agonist or antagonist behavior. In particular, the (E)-m-methoxy cyclobutanecarboxamido derivative 18f and the di-(m-methoxy) acetamido one, 18g, have sub-nM affinity for the MT(2) subtype, with more than 100-fold selectivity over MT(1), 18f being an antagonist and 18g a partial agonist on GTPgammaS test. Docking of 18g into a previously developed MT(2) receptor model showed a binding scheme consistent with that of other antagonists. The MT(2) expected binding affinities of the new compounds were calculated by a previously developed 3D-QSAR CoMFA model, giving satisfactory predictions.

Synthesis, NMR conformational analysis and pharmacological evaluation of 7,7a,13,14-tetrahydro-6H-cyclobuta[b]pyrimido[1,2-a:3,4-a′]diindole analogues as melatonin receptor ligands

A structure for the self-condensation product of 2-(1H-indol-2-yl)ethyl tosylate 2a, previously proposed as 6,7,14,15-tetrahydro-15aH-azocino[1,2-a:6,5-b]diindole 3a, was revised based on the (13)C-2D-INADEQUATE experiment, and proved to be 7,7a,13,14-tetrahydro-6H-cyclobuta[b]pyrimido[1,2-a:3,4-a']diindole 4a. A mechanism for the unexpected formation of this novel hexacyclic heterocycle was proposed and its NMR solution structure was elucidated. Five derivatives of the title ring skeleton 12-16 designed as melatonin receptor ligands were synthesized and their affinities for the human MT(1) and MT(2) receptors were determined. Both butyramides 13 and 15, as well as the non-methoxy acetamide 12 exhibited micromolar binding affinities for both receptors being slightly MT(2) selective. The methoxy acetamide 14 showed the best pharmacological profile exhibiting a five times higher affinity for MT(1) (K(i) = 49 nM) than for MT(2) (K(i) = 246 nM) receptor.

Melatonin receptors in the eye: location, second messengers and role in ocular physiology

The pineal hormone melatonin, an important regulator of circadian and seasonal rhythms, has a role in ocular pathophysiology. In addition to the pineal gland, melatonin synthesis is carried out in several ocular structures. Moreover, specific melatonin receptors have been located in the retina, cornea, ciliary body, lens, choroid and sclera, which suggests that cells in these tissues may be targets for melatonin action. This review summarizes the current knowledge about melatonin receptor subtypes with the emphasis on those melatonin receptors, which have been identified in ocular tissues and their possible roles in biochemical and physiological processes in the eye.

Towards the Development of Mixed MT1-Agonist/MT2-Antagonist Melatonin Receptor Ligands

Herein we report attempts to optimize the pharmacological properties of 5-(2-hydroxyethoxy)-N-acetyltryptamine (5-HEAT), a melatonin receptor ligand previously described by us. Several 5-substituted and 2,5-disubstituted N-acyltryptamines were synthesized and evaluated in vitro for the human cloned MT(1) and MT(2) receptors. From this series of N-acyltryptamines the 2-bromo derivative (5 c) retains the interesting efficacy profile of 5-HEAT and shows increased melatonin receptor affinities; it represents one of the first examples of a high-affinity MT(1) agonist/MT(2) antagonist. Some other full agonists for both melatonin receptors which exhibit similar or increased affinity relative to that of melatonin were obtained.

Differences in Binding Sites of Two Melatonin Receptors Help to Explain Their Selectivity to Some Melatonin Analogs: A Molecular Modeling Study

Numerous diseases have been linked to the malfunction of G-protein coupled receptors (GPCRs). Their adequate treatment requires rational design of new high-affinity and high-selectivity drugs targeting these receptors. In this work, we report three-dimensional models of the human MT(1) and MT(2) melatonin receptors, members of the GPCR family. The models are based on the X-ray structure of bovine rhodopsin. The computational approach employs an original procedure for optimization of receptor-ligand structures. It includes rotation of one of the transmembrane alpha-helices around its axis with simultaneous assessment of quality of the resulting complexes according to a number of criteria we have developed for this purpose. The optimal geometry of the receptor-ligand binding is selected based on the analysis of complementarity of hydrophobic/hydrophilic properties between the ligand and its protein environment in the binding site. The elaborated "optimized" models are employed to explore the details of protein-ligand interactions for melatonin and a number of its analogs with known affinity to MT(1) and MT(2) receptors. The models permit rationalization of experimental data, including those that were not used in model building. The perspectives opened by the constructed models and by the optimization procedure in the design of new drugs are discussed.

Mapping the Melatonin Receptor. 7. Subtype Selective Ligands Based on β-SubstitutedN-Acyl-5-methoxytryptamines and β-SubstitutedN-Acyl-5-methoxy-1-methyltryptamines

A series of beta-substituted and beta,beta-disubstituted N-acyl 5-methoxy-1-methyltryptamines and 5-methoxytryptamines have been prepared as melatonin analogues to investigate the nature of the binding site of the melatonin receptor. The affinity of analogues was determined in a radioligand binding assay using cloned human MT(1) and MT(2) receptor subtypes expressed in NIH 3T3 cells. Agonist and antagonist potency of all analogues was measured using the pigment aggregation response of a clonal line of Xenopus laevis melanophores. beta-Methylmelatonin (17a) and beta,beta-dimethylmelatonin (17b), though showing a slight decrease in binding at human receptors, show an increase in potency on Xenopus. N-Butanoyl 5-methoxy-1-methyl-beta,beta-trimethylenetryptamine (12c) is an antagonist at human MT(1) receptors but an agonist at MT(2), while N-butanoyl 5-methoxy-1-methyl-beta,beta-tetramethylenetryptamine (13c) is an antagonist at MT(1) but had no action at MT(2) and is one of the first examples of an MT(1) selective antagonist.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Reassessing the melatonin pharmacophore—Enantiomeric resolution, pharmacological activity, structure analysis, and molecular modeling of a constrained chiral melatonin analogue

3-(Acetylaminomethyl)-2-(ethoxycarbonyl)-6-methoxy-1,3,4,5-tetrahydrobenzo[cd]indole (2) is a rigid melatonin analogue that as a racemate displays about the same affinity and intrinsic activity of melatonin (1) in in vitro experiments. We report here the resolution of the racemate by preparative medium pressure liquid chromatography (MPLC) and the X-ray determination of the R absolute configuration of the (-)-enantiomer. The two enantiomers were separately tested as MT1 and MT2 ligands, and the (+)-(S)-2 showed a potency comparable to that of melatonin and about three orders of magnitude greater than that of its enantiomer. The information obtained by crystallographic analysis and NMR studies about the conformational preference for 2 and by the pharmacological characterization of (R)-2 and (S)-2 was employed in a molecular modeling study, aimed at reassessing the melatonin receptor pharmacophore model for agonist compounds. Chiral enantioselective agonists reported in the literature were also included in the study.

Melatonin receptors in humans: biological role and clinical relevance

In addition to its antioxidative effects melatonin acts through specific nuclear and plasma membrane receptors. To date, two G-protein coupled melatonin membrane receptors, MT(1) and MT(2), have been cloned in mammals, while the newly purified MT(3) protein belongs to the family of quinone reductases. Screening studies have shown that various tissues of rodents express MT(1) and/or MT(2) melatonin receptors. In humans, melatonin receptors were also detected in several organs, including brain and retina, cardiovascular system, liver and gallbladder, intestine, kidney, immune cells, adipocytes, prostate and breast epithelial cells, ovary/granulosa cells, myometrium, and skin. This review summarizes the data published so far about MT(1) and MT(2) receptors in human tissues and human cells. Established and putative functions of melatonin after receptor activation as well as the clinical relevance of these findings will be discussed.

Ramelteon

Ramelteon, approved in the US for the treatment of insomnia characterised by difficulty with sleep onset, is a highly selective agonist for the melatonin MT1/MT2 receptors, which are believed to mediate the circadian rhythm in mammals. Ramelteon has negligible affinity for the MT3 binding sites and other receptors in the brain, including the opiate, dopamine, benzodiazepine and serotonin receptors, which may explain the lack of significant adverse events and lack of abuse or dependence potential observed with ramelteon. In three clinical trials in patients with chronic insomnia, ramelteon 8mg was effective in reducing sleep latency, without being associated with any significant or clinically relevant residual effects. It also generally increased total sleep time and, where assessed, sleep efficiency. In a first-night-effect model of transient insomnia, ramelteon 8mg was significantly more effective than placebo at reducing sleep latency and increasing total sleep time. Ramelteon was generally well tolerated; the most commonly reported adverse events occurring in more ramelteon than placebo recipients were somnolence (5% vs 3%), fatigue (4% vs 2%) and dizziness (5% vs 3%). Adverse events were mostly mild or moderate in nature. Ramelteon has been shown to have no potential for abuse or dependence.