Industrial and academic approaches to the search for alternative melatonin receptor ligands: An historical survey

The search for melatonin receptor agonists formed the main part of melatonin medicinal chemistry programs for the last three decades. In this short review, we summarize the two main aspects of these programs: the development of all the necessary tools to characterize the newly synthesized ligands at the two melatonin receptors MT1 and MT2, and the medicinal chemist's approaches to find chemically diverse ligands at these receptors. Both strategies are described. It turns out that the main source of tools were industrial laboratories, while the medicinal chemistry was mainly carried out in academia. Such complete accounts are interesting, as they delineate the spirits in which the teams were working demonstrating their strength and innovative character. Most of the programs were focused on nonselective agonists and few of them reached the market. In contrast, discovery of MT1-selective agonists and melatonergic antagonists with proven in vivo activity and MT1 or MT2-selectivity is still in its infancy, despite the considerable interest that subtype selective compounds may bring in the domain, as the physiological respective roles of the two subtypes of melatonin receptors, is still poorly understood. Poly-pharmacology applications and multitarget ligands have also been considered.

Melatonin receptor structure and signaling

Melatonin (5-methoxy-N-acetyltryptamine) binds with high affinity and specificity to membrane receptors. Several receptor subtypes exist in different species, of which the mammalian MT1 and MT2 receptors are the best-characterized. They are members of the G protein-coupled receptor superfamily, preferentially coupling to Gi/o proteins but also to other G proteins in a cell-context-depending manner. In this review, experts on melatonin receptors will summarize the current state of the field. We briefly report on the discovery and classification of melatonin receptors, then focus on the molecular structure of human MT1 and MT2 receptors and highlight the importance of molecular simulations to identify new ligands and to understand the structural dynamics of these receptors. We then describe the state-of-the-art of the intracellular signaling pathways activated by melatonin receptors and their complexes. Brief statements on the molecular toolbox available for melatonin receptor studies and future perspectives will round-up this review.

Melatonin MT1 receptors as a target for the psychopharmacology of bipolar disorder: A translational study

The treatment of bipolar disorder (BD) still remains a challenge. Melatonin (MLT), acting through its two receptors MT1 and MT2, plays a key role in regulating circadian rhythms which are dysfunctional in BD. Using a translational approach, we examined the implication and potential of MT1 receptors in the pathophysiology and psychopharmacology of BD. We employed a murine model of the manic phase of BD (Clock mutant (ClockΔ19) mice) to study the activation of MT1 receptors by UCM871, a selective partial agonist, in behavioral pharmacology tests and in-vivo electrophysiology. We then performed a high-resolution Nuclear Magnetic Resonance study on isolated membranes to characterize the molecular mechanism of interaction of UCM871. Finally, in a cohort of BD patients, we investigated the link between clinical measures of BD and genetic variants located in the MT1 receptor and CLOCK genes. We demonstrated that: 1) UCM871 can revert behavioral and electrophysiological abnormalities of ClockΔ19 mice; 2) UCM871 promotes the activation state of MT1 receptors; 3) there is a significant association between the number of severe manic episodes and MLT levels, depending on the genetic configuration of the MT1 rs2165666 variant. Overall, this work lends support to the potentiality of MT1 receptors as target for the treatment of BD.

Agomelatine inhibits platelet aggregation through melatonin receptor-dependent and independent mechanisms

AIMS

Melatonin is known to inhibit platelet aggregation induced by arachidonic acid (AA). In the present study we investigated whether agomelatine (Ago), an antidepressant with agonist activity at melatonin receptor 1 (MT1) and MT2 could reduce platelets aggregation and adhesion.

MAIN METHODS

Human platelets from healthy donors were used to test the in vitro effects of Ago in the presence of different platelet activators. We performed aggregation and adhesion assays, thromboxane B₂ (TxB₂), cAMP and cGMP measurements, intra-platelet calcium registration and flow cytometry assays.

KEY FINDINGS

Our data revealed that different concentrations of Ago reduced AA- and collagen-induced human platelet aggregation in vitro. Ago also reduced AA-induced increase in thromboxane B₂ (TxB₂) production, intracellular calcium levels and P-selectin expression at plasma membrane. The effects of Ago in AA-activated platelets were likely dependent on MT1 as they were blocked by luzindole (a MT1/MT2 antagonist) and mimicked by the MT1 agonist UCM871 in a luzindole-sensitive manner. The MT2 agonist UCM924 was also able to inhibit platelet aggregation, but this response was not affected by luzindole. On the other hand, although UCM871 and UCM924 reduced collagen-induced platelet aggregation and adhesion, inhibition of collagen-induced platelet aggregation by Ago was not mediated by melatonin receptors because it was not affected by luzindole.

SIGNIFICANCE

The present data show that Ago suppresses human platelet aggregation and suggest that this antidepressant may have the potential to prevent atherothrombotic ischemic events by reducing thrombus formation and vessel occlusion.

In silico drug discovery of melatonin receptor ligands with therapeutic potential

INTRODUCTION

The neurohormone melatonin (N-acetyl-5-methoxytryptamine) regulates circadian rhythms exerting a variety of effects in the central nervous system and in periphery. These activities are mainly mediated by activation of MT₁ and MT₂ GPCRs. MT₁/MT₂ agonist compounds are used clinically for insomnia, depression, and circadian rhythm disturbances.

AREA COVERED

The following review describes the design strategies that have led to the identification of melatonin receptor ligands, guided by in silico approaches and molecular modeling. Initial ligand-based design, mainly relying on pharmacophore modeling and 3D-QSAR, has been flanked by structure-based virtual screening, given the recent availability of MT₁ and MT₂ crystal structures. Receptor ligands with different activity profiles, agonist/antagonist and subtype-selective compounds, are available.

EXPERT OPINION

An insight on the pharmacological characterization and therapeutic perspectives for relevant ligands is provided. In silico drug discovery has been instrumental in the design of novel ligands targeting melatonin receptors. Ligand-based approaches has led to the construction of a solid framework defining structure-activity relationships to obtain compounds with a tailored pharmacological profile. Structure-based techniques could integrate previous knowledge and provide compounds with novel chemotypes and pharmacological activity as drug candidates for disease conditions in which melatonin receptor ligands are currently being investigated, including cancer and pain.

Identification of a Quinone Derivative as a YAP/TEAD Activity Modulator from a Repurposing Library

The transcriptional regulators YAP (Yes-associated protein) and TAZ (transcriptional co-activator with PDZ-binding motif) are the major downstream effectors in the Hippo pathway and are involved in cancer progression through modulation of the activity of TEAD (transcriptional enhanced associate domain) transcription factors. To exploit the advantages of drug repurposing in the search of new drugs, we developed a similar approach for the identification of new hits interfering with TEAD target gene expression. In our study, a 27-member in-house library was assembled, characterized, and screened for its cancer cell growth inhibition effect. In a secondary luciferase-based assay, only seven compounds confirmed their specific involvement in TEAD activity. IA5 bearing a p-quinoid structure reduced the cytoplasmic level of phosphorylated YAP and the YAP–TEAD complex transcriptional activity and reduced cancer cell growth. IA5 is a promising hit compound for TEAD activity modulator development.

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.

Melatonin MT1 and MT2 Receptors Exhibit Distinct Effects in the Modulation of Body Temperature across the Light/Dark Cycle

Melatonin (MLT) is a neurohormone that regulates many physiological functions including sleep, pain, thermoregulation, and circadian rhythms. MLT acts mainly through two G-protein-coupled receptors named MT₁ and MT₂, but also through an MLT type-3 receptor (MT₃). However, the role of MLT receptor subtypes in thermoregulation is still unknown. We have thus investigated the effects of selective and non-selective MLT receptor agonists/antagonists on body temperature (T_b) in rats across the 12/12-h light-dark cycle. Rectal temperature was measured every 15 min from 4:00 a.m. to 9:30 a.m. and from 4:00 p.m. to 9:30 p.m., following subcutaneous injection of each compound at either 5:00 a.m. or 5:00 p.m. MLT (40 mg/kg) had no effect when injected at 5 a.m., whereas it decreased T_b during the light phase only when injected at 5:00 p.m. This effect was blocked by the selective MT₂ receptor antagonist 4P-PDOT and the non-selective MT₁/MT₂ receptor antagonist, luzindole, but not by the α₁/MT₃ receptors antagonist prazosin. However, unlike MLT, neither the selective MT₁ receptor partial agonist UCM871 (14 mg/kg) nor the selective MT₂ partial agonist UCM924 (40 mg/kg) altered T_b during the light phase. In contrast, UCM871 injected at 5:00 p.m. increased T_b at the beginning of the dark phase, whereas UCM924 injected at 5:00 a.m. decreased T_b at the end of the dark phase. These effects were blocked by luzindole and 4P-PDOT, respectively. The MT₃ receptor agonist GR135531 (10 mg/kg) did not affect T_b. These data suggest that the simultaneous activation of both MT₁ and MT₂ receptors is necessary to regulate T_b during the light phase, whereas in a complex but yet unknown manner, they regulate T_b differently during the dark phase. Overall, MT₁ and MT₂ receptors display complementary but also distinct roles in modulating circadian fluctuations of T_b.

Carbamate Insecticides Target Human Melatonin Receptors

Carbaryl (1-naphthyl methylcarbamate) and carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) are among the most toxic insecticides, implicated in a variety of diseases including diabetes and cancer among others. Using an integrated pharmacoinformatics based screening approach, we have identified these insecticides to be structural mimics of the neurohormone melatonin and were able to bind to the putative melatonin binding sites in MT₁ and MT₂ melatonin receptors in silico. Carbaryl and carbofuran then were tested for competition with 2-[¹²⁵I]-iodomelatonin (300 pM) binding to hMT₁ or hMT₂ receptors stably expressed in CHO cells. Carbaryl and carbofuran showed higher affinity for competition with 2-[¹²⁵I]-iodomelatonin binding to the hMT₂ compared to the hMT₁ melatonin receptor (33 and 35-fold difference, respectively) as predicted by the molecular modeling. In the presence of GTP (100 μM), which decouples the G-protein linked receptors to modulate signaling, the apparent efficacy of carbaryl and carbofuran for 2-[¹²⁵I]-iodomelatonin binding for the hMT₁ melatonin receptor was not affected but significantly decreased for the hMT₂ melatonin receptor compatible with receptor antagonist/inverse agonist and agonist efficacy, respectively. Altogether, our data points to a potentially new mechanism through which carbamate insecticides carbaryl and carbofuran could impact human health by altering the homeostatic balance of key regulatory processes by directly binding to melatonin receptors.

Update on melatonin receptors: IUPHAR Review 20

Melatonin receptors are seven transmembrane-spanning proteins belonging to the GPCR superfamily. In mammals, two melatonin receptor subtypes exist - MT1 and MT2 - encoded by the MTNR1A and MTNR1B genes respectively. The current review provides an update on melatonin receptors by the corresponding subcommittee of the International Union of Basic and Clinical Pharmacology. We will highlight recent developments of melatonin receptor ligands, including radioligands, and give an update on the latest phenotyping results of melatonin receptor knockout mice. The current status and perspectives of the structure of melatonin receptor will be summarized. The physiological importance of melatonin receptor dimers and biologically important and type 2 diabetes-associated genetic variants of melatonin receptors will be discussed. The role of melatonin receptors in physiology and disease will be further exemplified by their functions in the immune system and the CNS. Finally, antioxidant and free radical scavenger properties of melatonin and its relation to melatonin receptors will be critically addressed.

MT1 and MT2 Melatonin Receptors: A Therapeutic Perspective

Melatonin, or 5-methoxy-N-acetyltryptamine, is synthesized and released by the pineal gland and locally in the retina following a circadian rhythm, with low levels during the day and elevated levels at night. Melatonin activates two high-affinity G protein-coupled receptors, termed MT1 and MT2, to exert beneficial actions in sleep and circadian abnormality, mood disorders, learning and memory, neuroprotection, drug abuse, and cancer. Progress in understanding the role of melatonin receptors in the modulation of sleep and circadian rhythms has led to the discovery of a novel class of melatonin agonists for treating insomnia, circadian rhythms, mood disorders, and cancer. This review describes the pharmacological properties of a slow-release melatonin preparation (i.e., Circadin®) and synthetic ligands (i.e., agomelatine, ramelteon, tasimelteon), with emphasis on identifying specific therapeutic effects mediated through MT1 and MT2 receptor activation. Discovery of selective ligands targeting the MT1 or the MT2 melatonin receptors may promote the development of novel and more efficacious therapeutic agents.

Novel difluoroacetamide analogues of agomelatine and melatonin: probing the melatonin receptors for MT1selectivity

The difluoroacetamide analogue of the standard MT₁-selective ligand, the dimeric (CH₂)₃-linked agomelatine, shows higher affinity and selectivity toward MT₁receptors than the parent compound.

Melatonergic drugs in development

Melatonin (N-acetyl-5-methoxytryptamine) is widely known as "the darkness hormone". It is a major chronobiological regulator involved in circadian phasing and sleep-wake cycle in humans. Numerous other functions, including cyto/neuroprotection, immune modulation, and energy metabolism have been ascribed to melatonin. A variety of studies have revealed a role for melatonin and its receptors in different pathophysiological conditions. However, the suitability of melatonin as a drug is limited because of its short half-life, poor oral bioavailability, and ubiquitous action. Due to the therapeutic potential of melatonin in a wide variety of clinical conditions, the development of new agents able to interact selectively with melatonin receptors has become an area of great interest during the last decade. Therefore, the field of melatonergic receptor agonists comprises a great number of structurally different chemical entities, which range from indolic to nonindolic compounds. Melatonergic agonists are suitable for sleep disturbances, neuropsychiatric disorders related to circadian dysphasing, and metabolic diseases associated with insulin resistance. The results of preclinical studies on animal models show that melatonin receptor agonists can be considered promising agents for the treatment of central nervous system-related pathologies. An overview of recent advances in the field of investigational melatonergic drugs will be presented in this review.

Synthesis and characterization of new bivalent agents as melatonin- and histamine H3-ligands

Melatonin is an endogenous molecule involved in many pathophysiological processes. In addition to the control of circadian rhythms, its antioxidant and neuroprotective properties have been widely described. Thus far, different bivalent compounds composed by a melatonin molecule linked to another neuroprotective agent were synthesized and tested for their ability to block neurodegenerative processes in vitro and in vivo. To identify a novel class of potential neuroprotective compounds, we prepared a series of bivalent ligands, in which a prototypic melatonergic ligand is connected to an imidazole-based H₃ receptor antagonist through a flexible linker. Four imidazolyl-alkyloxy-anilinoethylamide derivatives, characterized by linkers of different length, were synthesized and their binding affinity for human MT₁, MT₂ and H₃ receptor subtypes was evaluated. Among the tested compounds, 14c and 14d, bearing a pentyl and a hexyl linker, respectively, were able to bind to all receptor subtypes at micromolar concentrations and represent the first bivalent melatonergic/histaminergic ligands reported so far. These preliminary results, based on binding affinity evaluation, pave the way for the future development of new dual-acting compounds targeting both melatonin and histamine receptors, which could represent promising therapeutic agents for the treatment of neurodegenerative pathologies.

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.

Unveiling the role of melatonin MT2 receptors in sleep, anxiety and other neuropsychiatric diseases: a novel target in psychopharmacology

BACKGROUND

Melatonin (MLT) is a pleiotropic neurohormone controlling many physiological processes and whose dysfunction may contribute to several different diseases, such as neurodegenerative diseases, circadian and mood disorders, insomnia, type 2 diabetes and pain. Melatonin is synthesized by the pineal gland during the night and acts through 2 G-protein coupled receptors (GPCRs), MT1 (MEL1a) and MT2 (MEL1b). Although a bulk of research has examined the physiopathological effects of MLT, few studies have investigated the selective role played by MT1 and MT2 receptors. Here we have reviewed current knowledge about the implications of MT2 receptors in brain functions.

METHODS

We searched PubMed, Web of Science, Scopus, Google Scholar and articles' reference lists for studies on MT2 receptor ligands in sleep, anxiety, neuropsychiatric diseases and psychopharmacology, including genetic studies on the MTNR1B gene, which encodes the melatonin MT2 receptor.

RESULTS

These studies demonstrate that MT2 receptors are involved in the pathophysiology and pharmacology of sleep disorders, anxiety, depression, Alzheimer disease and pain and that selective MT2 receptor agonists show hypnotic and anxiolytic properties.

LIMITATIONS

Studies examining the role of MT2 receptors in psychopharmacology are still limited.

CONCLUSION

The development of novel selective MT2 receptor ligands, together with further preclinical in vivo studies, may clarify the role of this receptor in brain function and psychopharmacology. The superfamily of GPCRs has proven to be among the most successful drug targets and, consequently, MT2 receptors have great potential for pioneer drug discovery in the treatment of mental diseases for which limited therapeutic targets are currently available.

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.

New radioligands for describing the molecular pharmacology of MT1 and MT2 melatonin receptors

Melatonin receptors have been studied for several decades. The low expression of the receptors in tissues led the scientific community to find a substitute for the natural hormone melatonin, the agonist 2-[125I]-iodomelatonin. Using the agonist, several hundreds of studies were conducted, including the discovery of agonists and antagonists for the receptors and minute details about their molecular behavior. Recently, we attempted to expand the panel of radioligands available for studying the melatonin receptors by using the newly discovered compounds SD6, DIV880, and S70254. These compounds were characterized for their affinities to the hMT1 and hMT2 recombinant receptors and their functionality in the classical GTPS system. SD6 is a full agonist, equilibrated between the receptor isoforms, whereas S70254 and DIV880 are only partial MT2 agonists, with Ki in the low nanomolar range while they have no affinity to MT1 receptors. These new tools will hopefully allow for additions to the current body of information on the native localization of the receptor isoforms in tissues.

Homology models of melatonin receptors: challenges and recent advances

Melatonin exerts many of its actions through the activation of two G protein-coupled receptors (GPCRs), named MT1 and MT2. So far, a number of different MT1 and MT2 receptor homology models, built either from the prototypic structure of rhodopsin or from recently solved X-ray structures of druggable GPCRs, have been proposed. These receptor models differ in the binding modes hypothesized for melatonin and melatonergic ligands, with distinct patterns of ligand-receptor interactions and putative bioactive conformations of ligands. The receptor models will be described, and they will be discussed in light of the available information from mutagenesis experiments and ligand-based pharmacophore models. The ability of these ligand-receptor complexes to rationalize structure-activity relationships of known series of melatonergic compounds will be commented upon.

Structure-based virtual screening of MT2 melatonin receptor: influence of template choice and structural refinement

Developing GPCR homology models for structure-based virtual screening requires the choice of a suitable template and refinement of binding site residues. We explored this systematically for the MT2 melatonin receptor, with the aim to build a receptor homology model that is optimized for the enrichment of active melatoninergic ligands. A set of 12 MT2 melatonin receptor models was built using different GPCR X-ray structural templates and submitted to a virtual screening campaign on a set of compounds composed of 29 known melatonin receptor ligands and 2560 drug-like decoys. To evaluate the effect of including a priori information in receptor models, 12 representative melatonin receptor ligands were placed into the MT2 receptor models in poses consistent with known mutagenesis data and with assessed pharmacophore models. The receptor structures were then adapted to the ligands by induced-fit docking. Most of the 144 ligand-adapted MT2 receptor models showed significant improvements in screening enrichments compared to the unrefined homology models, with some template/refinement combinations giving excellent enrichment factors. The discriminating ability of the models was further tested on the 29 active ligands plus a set of 21 inactive or low-affinity compounds from the same chemical classes. Rotameric states of side chains for some residues, presumed to be involved in the binding process, were correlated with screening effectiveness, suggesting the existence of specific receptor conformations able to recognize active compounds. The top MT2 receptor model was able to identify 24 of 29 active ligands among the first 2% of the screened database. This work provides insights into the use of refined GPCR homology models for virtual screening.