Therapeutic potential of non-competitive, subtype-selective metabotropic glutamate receptor ligands

Discovery of potential negative allosteric modulators of mGluR5 from natural products using pharmacophore modeling, molecular docking, and molecular dynamics simulation studies

mGluR5, which belongs to the G-protein-coupled receptor superfamily, is believed to be associated with many human diseases, such as a wide range of neurological disorders, gastroesophageal reflux disease, and cancer. Comparing with compounds that target on the orthosteric binding site, significant roles have been established for mGluR5 negative allosteric modulators (NAMs) due to their higher subtype selectivity and more suitable pharmacokinetic profiles. Nevertheless, to date, none of them have come to market for various reasons. In this study, a 3D quantitative pharmacophore model was generated by using the HypoGen module in Discovery Studio 4.0. With several validation methods ultilized, the optimal pharmacophore model Hypo2 was selected to discover potential mGluR5 NAMs from natural products. Two hundred and seventeen potential NAMs were obtained after being filtered by Lipinski’s rule (≥4). Then, molecular docking was used to refine the pharmacophore-based screening results and analyze the binding mode of NAMs and mGluR5. Three compounds, aglaiduline, 5-O-ethyl-hirsutanonol, and yakuchinone A, with good ADMET properties, acceptable Fit value and estimated value, and high docking score, were reserved for a molecular dynamics simulation study. All of them have stability of ligand binding. From our computational results, there might exhibit drug-like negative allosteric moderating effects on mGluR5 in these natural products. This work provides a reliable method for discovering mGluR5 NAMs from natural products.

Piperidyl amides as novel, potent and orally active mGlu5 receptor antagonists with anxiolytic-like activity

High throughput screening led to the identification of nicotinamide derivative 2 as a structurally novel mGluR5 antagonist. Optimization of the modular scaffold led to the discovery of 16m, a compound with high affinity for mGluR5 and excellent selectivity over other glutamate receptors. Compound 16m exhibits a favorable PK profile in rats, robust anxiolytic-like effects in three different animal models of fear and anxiety, as well as a good PK/PD correlation.

Antagonists at metabotropic glutamate receptor subtype 5: structure activity relationships and therapeutic potential for addiction

As a result of intensive investigation, particularly in the pharmaceutical industry, a number of potent and selective metabotropic glutamate receptor subtype 5 (mGluR5) antagonists have been discovered. The structure activity relationship studies that led to the discovery of these mGluR5 antagonists are presented in this review. Results from studies on selected mGluR5 antagonists in animal models that simulate drug reward, reinforcement, and relapse appear promising. The comorbidity between drug abuse and anxiety and depression make drugs active in these disorders of great interest. Clinical studies showed that the mGluR5 antagonist fenobam was an active anxiolytic drug. Several new mGluR5 antagonists produced anxiolytic and antidepressant-like effects in animal models of these disorders. The results from the clinical and animal studies provide information for new approaches to finding mechanistically distinct pharmacotherapies to help patients achieve and maintain abstinence from cocaine, methamphetamine, opiates, ethanol, and nicotine (smoking).

Synthesis and characterization of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives: Part 3. New potent non-competitive metabotropic glutamate receptor 2/3 antagonists

A series of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives was evaluated as non-competitive mGluR2/3 antagonists. Replacement of the (2-aryl)-ethynyl-moiety in 8-position with smaller less lipophilic substituents produced compounds inhibiting the binding of [3H]-LY354740 to rat mGluR2 with low nanomolar affinity and consistent functional effect at both mGluR2 and mGluR3. These compounds were able to reverse LY354740-mediated inhibition of field excitatory postsynaptic potentials in the rat dentate gyrus and in vivo activity could be demonstrated by reversal of the LY354740-induced hypoactivity in mice after oral administration.

Synthesis and characterization of 8-ethynyl-1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives: part 2. New potent non-competitive metabotropic glutamate receptor 2/3 antagonists

A series of 1,3-dihydro-benzo[b][1,4]diazepin-2-one derivatives was evaluated as non-competitive mGluR2/3 antagonists. Replacement of a cyano group by a five-membered heterocycle produced compounds inhibiting the binding of [(3)H]-LY354740 to rat mGluR2 with low nanomolar affinity and consistent functional effect at both mGluR2 and mGluR3. Further modification to improve the physicochemical properties led eventually to compounds with the ability to reverse LY354740-mediated inhibition of field excitatory postsynaptic potentials in the rat dentate gyrus.

Rational design, synthesis, and structure–activity relationship of benzoxazolones: New potent mglu5 receptor antagonists based on the fenobam structure

A novel class of potent and stable mGlu5 receptor antagonists was developed by combining information from a high-throughput screening campaign with the structure of the known anxiolytic fenobam. Representative compounds from this class show favorable pharmacokinetic properties and are active in an in vivo model of anxiety.

Comparison of the binding pockets of two chemically unrelated allosteric antagonists of the mGlu5 receptor and identification of crucial residues involved in the inverse agonism of MPEP

Fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea], a clinically validated non-benzodiazepine anxiolytic, has been shown to be a potent and non-competitive metabotropic glutamate (mGlu)-5 receptor antagonist. In the present study, we have used the site-directed mutagenesis coupled with three-dimensional receptor-based pharmacophore modelling to elucidate the interacting mode of fenobam within the seven-transmembrane domain (7TMD) of mGlu5 receptor and its comparison with that of 2-methyl-6-(phenylethynyl)pyridine (MPEP), the prototype antagonist. The common residues involved in the recognition of MPEP and fenobam include Pro654(3.36), Tyr658(3.40), Thr780(6.44), Trp784(6.48), Phe787(6.51), Tyr791(6.55) and Ala809(7.47). The differentiating residues between both modulators' interacting modes are Arg647(3.29), Ser657(3.39) and Leu743(5.47). Our data suggest that these chemically unrelated mGlu5 antagonists act similarly, probing a functionally unique region of the 7TMD. Using [3H]inositol phosphates accumulation assay, we have also identified the critical residues involved in the inverse agonist effect of MPEP. The mutation W784(6.48)A completely blocked the inverse agonist activity of MPEP; two mutations F787(6.51)A and Y791(6.55)A, caused a drastic decrease in the MPEP inverse agonism. Furthermore, these three mutations led to an increased efficacy of quisqualate without having any effect on its potency. The fact that the residues Trp784(6.48) and Phe787(6.51) are essential equally in antagonism and inverse agonism effects emphasizes again the key role of these residues and the involvement of a common transmembrane network in receptor inactivation by MPEP.

A new series of pyridinyl-alkynes as antagonists of the metabotropic glutamate receptor 5 (mGluR5)

Synthesis and some structure-activity relationships for a new series of propargyl ethers as mGluR5 antagonists are reported.

Arylmethoxypyridines as novel, potent and orally active mGlu5 receptor antagonists

Optimisation of affinity, chemical stability, metabolic stability and solubility led from a chemically labile HTS hit 1 to mGlu5 receptor antagonists (24-26) with high affinity for the allosteric MPEP binding site, improved microsomal metabolic stability and anxiolytic-like activity in vivo as assessed by the Vogel conflict drinking test.

Phenyl ureas of creatinine as mGluR5 antagonists. A structure–activity relationship study of fenobam analogues

Fenobam (1) was developed by McNeil Laboratories as an anxiolytic agent with an unknown molecular target in the late 1970s. In a recent publication, it was revealed that fenobam is a non-competitive mGluR5 antagonist. Herein, we present the structure-activity relationship of fenobam and its analogues and similarities between the SAR of mGluR5 antagonism and the SAR of CNS properties originally reported by McNeil are discussed.

Structure-activity relationship of thiopyrimidines as mGluR5 antagonists

Structure-activity relationship investigations of the thiopyrimidine (1), an HTS hit with micromolar activity as a metabotropic glutamate receptor 5 (mGluR5) antagonist, led to compounds with sub-micromolar activity.

Molecular pharmacology and therapeutic prospects of metabotropic glutamate receptor allosteric modulators

The metabotropic glutamate receptors (mGluR) consist of a family of eight G-protein-coupled receptors that differ in their function, distribution and physiological roles within the central nervous system. In recent years substantial efforts have been made towards developing selective agonists and antagonists which have proven useful for elucidating their potential as novel targets for the treatment of psychiatric and neurological diseases. In the present review we will provide an update of the recent developments of functional allosteric modulators of the mGluR family and explore their therapeutic potential for anxiety/depression, schizophrenia, epilepsy/stroke, pain and Alzheimer's, Parkinson's and Huntington's diseases.

Determination of key amino acids implicated in the actions of allosteric modulation by 3,3′-difluorobenzaldazine on rat mGlu5 receptors

Several mutations in the seven-transmembrane region of rat metabotropic glutamate 5 (rmGlu5) receptors were produced by site-directed mutagenesis and expressed in CHO cells. Using functional intracellular calcium ([Ca(2+)](i)) mobilisation, we identified amino acids implicated in the positive allosteric modulation of quisqualate-induced response by 3,3'-difluorobenzaldazine (DFB). Human and rat mGlu5 receptors displayed a higher potency and a higher efficacy in the presence of DFB. Mutant receptors S657(3.39)C, T780(6.44)A and M801(7.39)T disrupted the DFB-mediated increase in functional response. DFB-induced increase in potency was abolished in mutant receptors N733(45.51)A, Y791(6.55)A, A809(7.47)V, P654(3.36)S/S657(3.39)C and P654(3.36)S/S657(3.39)C/L743(5.47)V without affecting the enhancement of efficacy observed in wild type receptors. Mutations at positions Leu-743(5.47) and Trp-784(6.48) resulted in significantly larger DFB-induced potentiation of EC(50) and E(max) values than in wild type receptors. DFB-mediated increase of efficacy was abolished and EC(50) values were right-shifted in mutant receptor F787A, resulting in DFB acting as a weak partial antagonist at this mutant receptor. Based on these findings, we constructed a homology model concluding that six key residues in transmembranes 3, 5, 6 and 7 are necessary for the allosteric modulation of rmGlu5a receptor by DFB. The model confirms an overlapping but distinct binding site to 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and in particular emphasises the key role of W784 in transmembrane (TM) 6 for controlling the receptor's activation state.

Antagonist-induced supersensitivity of mGlu1 receptor signalling in cerebellar granule cells

We investigated the effect of 24 h sustained treatment with the mGlu1 receptor antagonist CPCCOEt on mGlu1 receptor signalling in primary cultures of rat cerebellar granule cells. In the absence of ionotropic glutamate (iGlu) blockers, the maximal inositol phosphate (IP) response (E(max)) but not the potency of glutamate was significantly increased when cells were pre-exposed for 24 h with CPCCOEt. When the contribution of iGlu receptors to the glutamate-induced IP response was eliminated with the use of DNQX, the E(max) was again increased but also the concentration eliciting 50% of the maximal glutamate stimulus was significantly decreased. In the absence of iGlu receptor inhibitors, the E(max) of quisqualate, which likely mediates IP accumulation only via the mGlu1 receptor, was significantly increased in CPCCOEt-pretreated cells. Also, less quisqualate was needed to reach the same IP effect. The potency of R193845, a selective mGlu1 receptor antagonist, was significantly decreased in antagonist-pretreated cells. These findings demonstrate that 24 h sustained antagonist treatment can render mGlu1 receptors in neurons supersensitive to agonists, with a concomitant decrease in the effectiveness of antagonists.

Mutational Analysis and Molecular Modeling of the Binding Pocket of the Metabotropic Glutamate 5 Receptor Negative Modulator 2-Methyl-6-(phenylethynyl)-pyridine

Metabotropic glutamate (mGlu) 5 is a G-protein-coupled metabotropic glutamate receptor that plays an important role as a modulator of synaptic plasticity, ion channel activity, and excitotoxicity. 2-Methyl-6-(phenylethynyl)-pyridine (MPEP) is a highly potent, noncompetitive, selective, and systemically active antagonist of mGlu5 receptors. It binds to a novel allosteric site that resides within the seven-transmembrane domain of mGlu5 receptors. Using site-directed mutagenesis, [3H]MPEP binding, a functional Ca2+ mobilization assay, and rhodopsin-based homology modeling, we identified eight residues (Pro-6543.36, Tyr-6583.40, Leu-7435.47, Thr-7806.44, Trp-7846.48, Phe-7876.51, Tyr-7916.55, and Ala-8097.47) that are crucial for MPEP-binding to rat mGlu5 receptors. Four mutations, Y6583.40V, W7846.48A, F7876.51A, and A8097.47V, caused complete loss of [3H]MPEP binding and also blocked the MPEP-mediated inhibition of quisqualate-induced intracellular Ca2+ mobilization. To visualize these experimental findings, we have constructed a homology model based on the X-ray crystal of bovine rhodopsin and have suggested a possible binding mode of MPEP. We propose that MPEP via its interactions with a network of the aromatic residues including Phe-6583.40 in transmembrane (TM) 3 helix and Trp-7986.48, Phe-7876.51, and Tyr-7916.55 in TM6 helix prevents the movement of TM6 helix relative to TM3 helix, a step that is required for receptor activation, and consequently stabilizes the inactive conformation of mGlu5 receptor. In the TM6 region, we observed a striking similarity between the critical residues involved in MPEP-binding site with those of previously identified as 1-ethyl-2-methyl-6-oxo-4-(1,2,4,5-tetrahydro-benzo[d]azepin-3-yl)-1,6-dihydropyrimidine-5-carbonitrile-binding pocket of mGlu1, pointing to a common mechanism of inhibition shared by both antagonists.