Tricyclic alkylamides as melatonin receptor ligands with antagonist or inverse agonist activity

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.

Assembly of 5H-dibenzo[a,d]cycloheptenes by a formal [5 + 2] annulation of ortho-aryl alkynyl benzyl alcohols with arenes

A new synthetic methodology for the synthesis of 5H-dibenzo[a,d]cycloheptenes from ortho-aryl alkynyl benzyl alcohols and arenes via a Tf₂O-mediated formal [5 + 2] annulation reaction has been achieved. From this transformation, structurally diverse 5H-dibenzo[a,d]cycloheptenes were achieved in moderate to good yields. This transformation probably involves an intermolecular Friedel-Crafts-type alkylation and a subsequent intramolecular 7-endo-dig cyclization in one pot, highlighting the high efficiency, regioselectivity, and step-economy of this protocol.

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.

Synthesis of dibenzocyclohepta[1,2-a]naphthalene derivatives from phenylacetaldehyde and alkynyl benzyl alcohols via sequential electrophilic addition and double Friedel-Crafts reactions

A simple methodology has been developed for the synthesis of substituted 9H-dibenzo[3,4:6,7]-cyclohepta[1,2-a]naphthalenes from phenylacetaldehydes and ortho-alkynyl benzyl alcohols in the presence of a Lewis acid in moderate to good yields within a short reaction time. Interestingly, the reaction proceeds through a highly regioselective electrophilic addition followed by double Friedel-Crafts reaction to form uncommon dibenzo-fused seven-membered carbocycles.

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.

BF3·OEt2-mediated one pot synthesis of 10-indolyldibenzo[b,f]azepine derivatives via tandem ring expansion and C–C bond formation

A simple and efficient one pot protocol was developed for the synthesis of new structurally diverse 10-indolyldibenzo[b,f]azepine derivatives.

Carbamazepine derivatives with P2X4 receptor-blocking activity

Antagonists for the P2 receptor subtype P2X4, an ATP-activated cation channel receptor, have potential as novel drugs for the treatment of neuropathic pain and other inflammatory diseases. In the present study, a series of 47 carbamazepine derivatives including 32 novel compounds were designed, synthesized, and evaluated as P2X4 receptor antagonists. Their potency to inhibit ATP-induced calcium influx in 1321N1 astrocytoma cells stably transfected with the human P2X4 receptor was determined. Additionally, species selectivity (human, rat, mouse) and receptor subtype selectivity (P2X4 vs P2X1, 2, 3, 7) were investigated for selected derivatives. The most potent compound of the present series, which exhibited an allosteric mechanism of P2X4 inhibition, was N,N-diisopropyl-5H-dibenz[b,f]azepine-5-carboxamide (34, IC50 of 3.44μM). The present study extends the so far very limited knowledge on structure-activity relationships of P2X4 receptor antagonists.

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.

Anti-cancer activity of novel dibenzo[b,f]azepine tethered isoxazoline derivatives

Background

Dibenzoazepine (DB) derivatives are important and valuable compounds in medicinal chemistry. The synthesis and chemotherapeutic properties of naturally occurring DBs and different heterocyclic moiety tethered DBs are reported. Herein, we report the DB-fused hybrid structure that containing isoxazolines (DBIs) and their anti-cancer activity, which could throw light on the structural and functional features of new molecules.

Results and Conclusion

The synthesis and characterization of novel ring DB tethered isoxazoline derivatives (DBIs) were carried out. After the detailed structural characterization using 2D-NMR experiments, the compounds were identified as 5-substituted isoxazolines. The effect of newly synthesized DBIs against the invasion of murine osteosarcoma (LM8G7) cells was studied. Among the tested molecules, compound 4g (5-[−3-(4-chlorophenyl)-4,5-dihydroisoxazol-5-yl-methyl]-5 H-dibenzo[b,f]azepine), was found to inhibit the invasion of LM8G7 cells strongly, when compared to other structurally related compounds. Cumulatively, the compound 4g inhibited the invasion MDA-MB-231 cells completely at 10 μM. In addition to anti-invasion property the compound 4g also inhibited the migration of LM8G7 and human ovarian cancer cells (OVSAHO) dose-dependently. Compound 4g inhibited the proliferation of LM8G7, OVSAHO, human breast cancer cells (MCF-7) and human melphalan-resistant multiple myeloma (RPMI8226-LR5) cells that are comparable to cisplatin and suramin.

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.

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.

Application of 3D-QSAR in the Rational Design of Receptor Ligands and Enzyme Inhibitors

Quantitative structure-activity relationships (QSARs) are frequently employed in medicinal chemistry projects, both to rationalize structure-activity relationships (SAR) for known series of compounds and to help in the design of innovative structures endowed with desired pharmacological actions. As a difference from the so-called structure-based drug design tools, they do not require the knowledge of the biological target structure, but are based on the comparison of drug structural features, thus being defined ligand-based drug design tools. In the 3D-QSAR approach, structural descriptors are calculated from molecular models of the ligands, as interaction fields within a three-dimensional (3D) lattice of points surrounding the ligand structure. These descriptors are collected in a large X matrix, which is submitted to multivariate analysis to look for correlations with biological activity. Like for other QSARs, the reliability and usefulness of the correlation models depends on the validity of the assumptions and on the quality of the data. A careful selection of compounds and pharmacological data can improve the application of 3D-QSAR analysis in drug design. Some examples of the application of CoMFA and CoMSIA approaches to the SAR study and design of receptor or enzyme ligands is described, pointing the attention to the fields of melatonin receptor ligands and FAAH inhibitors.

Structure-activity relationships of inverse agonists for G-protein-coupled receptors

It has been recently established that G-protein-coupled receptors (GPCRs) can be constitutively active, i.e., they can be active in the absence of an agonist. This activity can be inhibited by so-called inverse agonists. For a number of GPCRs, such inverse agonists have been developed and studied, now enabling for the first time a study into their structure-activity relationships.

Analysis of structure-activity relationships for MT2 selective antagonists by melatonin MT1 and MT2 receptor models

Three-dimensional homology models of human MT(1) and MT(2) melatonin receptors were built with the aim to investigate the structure-activity relationships (SARs) of MT(2) selective antagonists. A common interaction pattern was proposed for a series of structurally different MT(2) selective antagonists, which were positioned within the binding site by docking and simulated annealing. The proposed antagonist binding mode to the MT(2) receptor is characterized by the accommodation of the out-of-plane substituents in a hydrophobic pocket, which resulted as being fundamental for the explanation of the antagonist behavior and the MT(2) receptor selectivity. Moreover, to assess the ability of the MT(2) receptor model to reproduce the SARs of MT(2) antagonists, three new derivatives of the MT(2) selective antagonist N-[1-(4-chloro-benzyl)-4-methoxy-1H-indol-2-ylmethyl]-propionamide (7) were synthesized and tested for their receptor affinity and intrinsic activity. These compounds were docked into the MT(2) receptor model and were submitted to molecular dynamics studies, providing results in qualitative agreement with the experimental data. These results confirm the importance of the out-of-plane group in receptor binding and selectivity and provide a partial validation of the proposed G protein-coupled receptor model.

Recent Advances in Melatonin Receptor Ligands

Melatonin is a hormone exerting its multiple actions mainly through two G-protein-coupled receptors MT(1) and MT(2). Exploring the physiological role of each of these subtypes requires subtype selective MT(1) and MT(2) ligands. While several MT(2)-selective ligands were developed in the 1990s, no selective agonists and antagonists for the MT(1) subtype were described. The present article reviews mela toninergic ligands developed in the current millennium focusing on subtype selective agents and on drug candidates. Notable compounds are the MT(1)-selective agonists 35 and 134, MT(1)-selective antagonists 117 and 131, MT(2)-selective agonists 58, 70, 79, 97 and 125, MT(2)-selective antagonists 27, 73 and 119, and the highly potent non-selective agonist 120. The non-selective agonists agomelatine 2, and ramelteon 87 are drug candidates as antidepressive agent and for the treatment of insomnia and circadian rhythm disfunction, respectively.