Treatment options for schizophrenia that address all symptom categories (positive, negative, and cognitive) are lacking. Novel compounds that regulate signaling by the major excitatory neurotransmitter in the brain, glutamate, are emerging as a novel approach for the treatment of this disorder. Currently available medications ameliorate positive symptoms but do not have efficacy in reducing negative symptoms or cognitive disturbances. It is possible that agents that target glutamatergic signaling in the CNS could have efficacy in reducing all major symptom clusters, providing a more comprehensive treatment strategy, and also avoiding some of the adverse effects that are seen with currently available treatments. Three major approaches for targeting glutamate signaling are now advancing in preclinical and clinical development. First are inhibitors for a transporter for glycine termed GlyT1. Glycine is a co-agonist with glutamate for a specific subtype of glutamate receptor, termed the NMDA receptor, which is thought to be critically involved in brain circuits that are disrupted in schizophrenia patients. Inhibiting GlyT1 increases glycine levels and can selectively increase NMDA receptor signaling. Another promising approach is to increase activity of another family of glutamate receptors, termed metabotropic glutamate receptors (mGlus), which play important modulatory roles in brain circuits that are thought to be disrupted in schizophrenia patients. Activation of the group I (mGlu5) and the group II (mGlu2 and mGlu3) mGlus is hypothesized to normalize the disruption of aberrant signaling in these circuits. Novel drug-like molecules that increase activity of these receptors have robust efficacy in animal models that predict efficacy in treatment of schizophrenia. Early clinical studies provide some support for potential utility of these targets in reducing symptoms in schizophrenia patients. Clinical studies that are underway will provide further insights into the potential utility of these compounds in the treatment of multiple symptom domains in schizophrenia patients.
Recent evidence suggests that the functions of presynaptic metabotropic glutamate receptors (mGluRs) are tightly regulated by protein kinases. We previously reported that cAMP-dependent protein kinase (PKA) directly phosphorylates mGluR2 at a single serine residue (Ser843) on the C-terminal tail region of the receptor, and that phosphorylation of this site inhibits coupling of mGluR2 to GTP-binding proteins. This may be the mechanism by which the adenylyl cyclase activator forskolin inhibits presynaptic mGluR2 function at the medial perforant path-dentate gyrus synapse. We now report that PKA also directly phosphorylates several group III mGluRs (mGluR4a, mGluR7a, and mGluR8a), as well as mGluR3 at single conserved serine residues on their C-terminal tails. Furthermore, activation of PKA by forskolin inhibits group III mGluR-mediated responses at glutamatergic synapses in the hippocampus. Interestingly, β-adrenergic receptor activation was found to mimic the inhibitory effect of forskolin on both group II and III mGluRs. These data suggest that a common PKA-dependent mechanism may be involved in regulating the function of multiple presynaptic group II and group III mGluRs. Such regulation is not limited to the pharmacological activation of adenylyl cyclase but can also be elicited by the stimulation of endogenous Gs-coupled receptors, such as β-adrenergic receptors.
β-adrenergic; cAMP-dependent protein kinase; mGluR4; mGluR7; mGluR8; phosphorylation
Previous reports have shown that activation of N-methyl-D-aspartate (NMDA) receptors potentiates responses to activation of the group I metabotropic glutamate receptor mGluR5 by reversing PKC-mediated desensitization of this receptor. NMDA-induced reversal of mGluR5 desensitization is dependent on activation of protein phosphatases. However, the specific protein phosphatase involved and the precise mechanism by which NMDA receptor activation reduces mGluR desensitization are not known. We have performed a series of molecular, biochemical, and genetic studies to show that NMDA-induced regulation of mGluR5 is dependent on activation of calcium-dependent protein phosphatase 2B/calcineurin (PP2B/CaN). Furthermore, we report that purified calcineurin directly dephosphorylates the C-terminal tail of mGluR5 at sites that are phosphorylated by PKC. Finally, immunoprecipitation and GST fusion protein pull-down experiments reveal that calcineurin interacts with mGluR5, suggesting that these proteins could be colocalized in a signaling complex. Taken together with previous studies, these data suggest that activation of NMDA receptors leads to activation of calcineurin and that calcineurin modulates mGluR5 function by directly dephosphorylating mGluR5 at PKC sites that are involved in desensitization of this receptor. 2005 Elsevier Ltd. All rights reserved.
Glutamate; Metabotropic; LTP; LTD; PKC; Plasticity
Presynaptic metabotropic glutamate receptors (mGluRs) often act as feedback inhibitors of synaptic transmission and serve important roles in defining the activity of glutamatergic synapses. Recent investigations have begun to identify novel interactions of presynaptic mGluRs, especially mGluR7, with multiple protein kinases and putative regulatory proteins that probably serve to further shape the overall activity of glutamatergic synapses. In the present study, we report that in addition to protein kinase C (PKC), cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG) can inhibit calmodulin (CaM) interactions with the carboxyl-terminal tail of mGluR7. These actions are mediated by PKC-, PKA-, or PKG-dependent phosphorylation of mGluR7 at a single serine residue, Ser862, in the carboxyl terminus of the receptor. Mutation of this residue inhibits kinase-mediated phosphorylation of the mGluR7 carboxyl terminus and reverses kinase-mediated inhibition of CaM binding to mGluR7. However, PKC-mediated inhibition of the functional coupling of mGluR7 to G protein-coupled inward rectifier potassium (GIRK) currents in a heterologous expression system is not affected by mutating Ser862. Furthermore, mutation of Ser862 to glutamate to mimic receptor phosphorylation and inhibit CaM interactions with mGluR7 does not affect receptor function. These studies demonstrate that the ability of these second messenger-dependent kinases to inhibit mGluR7-mediated activation of GIRK current is not dependent on the phosphorylation of Ser862 or the regulation of CaM binding to mGluR7. Furthermore, our studies suggest that CaM binding is not required for mGluR7-mediated activation of GIRK current.
G-Protein-coupled receptors (GPCRs) represent the largest class of drug targets, accounting for more than 40% of marketed drugs; however, discovery efforts for many GPCRs have failed to provide viable drug candidates. Historically, drug discovery efforts have focused on developing ligands that act at the orthosteric site of the endogenous agonist. Recently, efforts have focused on functional assay paradigms and the discovery of ligands that act at allosteric sites to modulate receptor function in either a positive, negative, or neutral manner. Allosteric modulators have numerous advantages over orthosteric ligands, including high subtype selectivity; the ability to mimic physiological conditions; the lack of densensitization, downregulation, and internalization; and reduced side effects. Despite these virtues, challenging issues have now arisen for allosteric modulators of metabotropic glutamate receptors (mGluRs): shallow SAR, ligand-directed trafficking, and the identification of subtle “molecular switches” that modulate the modes of pharmacology. Here, we will discuss the impact of modest structural changes to multiple mGluR allosteric ligands scaffolds that unexpectedly modulate pharmacology and raise concerns over metabolism and the pharmacology of metabolites.
Schizophrenia is a highly debilitating mental disorder which afflicts approximately 1% of the global population. Cognitive and negative deficits account for the lifelong disability associated with schizophrenia, whose symptoms are not effectively addressed by current treatments. New medicines are needed to treat these aspects of the disease. Neurodevelopmental, neuropathological, genetic, and behavioral pharmacological data indicate that schizophrenia stems from a dysfunction of glutamate synaptic transmission, particularly in frontal cortical networks. A number of novel pre- and postsynaptic mechanisms affecting glutamatergic synaptic transmission have emerged as viable targets for schizophrenia. While developing orthosteric glutamatergic agents for these targets has proven extremely difficult, targeting allosteric sites of these targets has emerged as a promising alternative. From a medicinal chemistry perspective, allosteric sites provide an opportunity of finding agents with better drug-like properties and greater target specificity. Furthermore, allosteric modulators are better suited to maintaining the highly precise temporal and spatial aspects of glutamatergic synaptic transmission. Herein, we review neuropathological and genomic/genetic evidence underscoring the importance of glutamate synaptic dysfunction in the etiology of schizophrenia and make a case for allosteric targets for therapeutic intervention. We review progress in identifying allosteric modulators of AMPA receptors, NMDA receptors, and metabotropic glutamate receptors, all with the aim of restoring physiological glutamatergic synaptic transmission. Challenges remain given the complexity of schizophrenia and the difficulty in studying cognition in animals and humans. Nonetheless, important compounds have emerged from these efforts and promising preclinical and variable clinical validation has been achieved.
Allosterism; AMPA; glycine; glutamate; NAMS; NMDA; PAMS; schizophrenia
Treatment options for schizophrenia that address all symptom categories (positive, negative, and cognitive) are lacking in current therapies for this disorder. Compounds targeting the metabotropic glutamate (mGlu) receptors hold promise as a more comprehensive therapeutic alternative to typical and atypical antipsychotics and may avoid the occurrence of extrapyramidal side effects that accompany these treatments. Activation of the group II mGlu receptors (mGlu2 and mGlu3) and the group I mGlu5 are hypothesized to normalize the disruption of thalamocortical glutamatergic circuitry that results in abnormal glutamaterigic signaling in the prefrontal cortex (PFC). Agonists of mGlu2 and mGlu3 have demonstrated efficacy for the positive symptom group in both animal models and clinical trials with mGlu2 being the subtype most likely responsible for the therapeutic effect. Limitations in the chemical space tolerated by the orthosteric site of the mGlu receptors has led to the pursuit of compounds that potentiate the receptor’s response to glutamate by acting at less highly conserved allosteric sites. Several series of selective positive allosteric modulators (PAMs) for mGlu2 and mGlu5 have demonstrated efficacy in animal models used for the evaluation of antipsychotic agents. In addition, evidence from animal studies indicates that mGlu5 PAMs hold promise for the treatment of cognitive deficits that occur in schizophrenia. Hopefully, further optimization of allosteric modulators of mGlu receptors will yield clinical candidates that will allow full evaluation of the potential efficacy of these compounds in the treatment of multiple symptom domains in schizophrenia patients in the near future.
metabotropic; glutamate; schizophrenia; NMDA; allosteric
The development of receptor subtype-selective ligands by targeting allosteric sites of G protein-coupled receptors (GPCRs) has proven highly successful in recent years. One GPCR family that has greatly benefited from this approach is the metabotropic glutamate receptors (mGlus). These family C GPCRs participate in the neuromodulatory actions of glutamate throughout the CNS, where they play a number of key roles in regulating synaptic transmission and neuronal excitability. A large number of mGlu subtype-selective allosteric modulators have been identified, the majority of which are thought to bind within the transmembrane regions of the receptor. These modulators can either enhance or inhibit mGlu functional responses and, together with mGlu knockout mice, have furthered the establishment of the physiologic roles of many mGlu subtypes. Numerous pharmacological and receptor mutagenesis studies have been aimed at providing a greater mechanistic understanding of the interaction of mGlu allosteric modulators with the receptor, which have revealed evidence for common allosteric binding sites across multiple mGlu subtypes and the presence for multiple allosteric sites within a single mGlu subtype. Recent data have also revealed that mGlu allosteric modulators can display functional selectivity toward particular signal transduction cascades downstream of an individual mGlu subtype. Studies continue to validate the therapeutic utility of mGlu allosteric modulators as a potential therapeutic approach for a number of disorders including anxiety, schizophrenia, Parkinson’s disease, and Fragile X syndrome.
This Letter describes the synthesis and SAR of two mGluR4 positive allosteric modulator leads, 6 and 7. VU001171 (6) represents the most potent (EC50 = 650 nM), efficacious (141% Glu Max) and largest fold shift (36-fold) of any mGluR4 PAM reported to date. However, this work highlights the challenges in hit-to-lead for mGluR4 PAMs, with multiple confirmed HTS hits displaying little or no tractable SAR.
mGluR4; PAM; Positive allosteric modulator; Parkinson’s Disease
Sponsored by the New York Academy of Sciences and with support from the National Institute of Mental Health, the Life Technologies Foundation, and the Josiah Macy Jr. Foundation, “Advancing Drug Discovery for Schizophrenia” was held March 9–11 at the New York Academy of Sciences in New York City. The meeting, comprising individual talks and panel discussions, highlighted basic, clinical, and translational research approaches, all of which contribute to the overarching goal of enhancing the pharmaceutical armamentarium for treating schizophrenia. This report surveys work by the vanguard of schizophrenia research in such topics as genetic and epigenetic approaches; small molecule therapeutics; and the relationships between target genes, neuronal function, and symptoms of schizophrenia.
schizophrenia; genetics; GWAS; neuronal function; small molecules; therapeutics
Activation of metabotropic glutamate receptor subtype 4 has been shown to be efficacious in rodent models of Parkinson’s disease. Artificial neural networks were trained based on a recently reported high throughput screen which identified 434 positive allosteric modulators of metabotropic glutamate receptor subtype 4 out of a set of approximately 155,000 compounds. A jury system containing three artificial neural networks achieved a theoretical enrichment of 15.4 when selecting the top 2% compounds of an independent test dataset. The model was used to screen an external commercial database of approximately 450,000 drug-like compounds. 1,100 predicted active small molecules were tested experimentally using two distinct assays of mGlu4 activity. This experiment yielded 67 positive allosteric modulators of metabotropic glutamate receptor subtype 4 that confirmed in both experimental systems. Compared to the 0.3% active compounds in the primary screen, this constituted an enrichment of 22 fold.
Metabotropic glutamate receptor subtype 4; Virtual high-throughput screening; Machine learning; Quantitative structure-activity relationship; Enrichment
We previously showed that muscarinic agonists with M1 and/or M4 receptor affinities attenuated cocaine discrimination and self-administration in wild type mice but not in M1/M4 double-knockout mice.
To elucidate the respective contributions of M1 and M4 receptors to this effect.
Knockout mice lacking either the M1 subtype (M1−/−) or the M4 subtype (M4−/−), and wild-type mice were trained to discriminate 10 mg/kg cocaine from saline. Muscarinic ligands were tested for modulation of cocaine discrimination: xanomeline (M1/M4-preferring agonist), VU0357017 (M1-selective partial agonist), 77-LH-28-1(M1 agonist), and BQCA (M1-selective positive allosteric modulator).
Xanomeline produced rightward shifts in the cocaine dose-effect curve in all three genotypes, but most robustly in wild-type mice. VU0357017 produced rightward shifts in the cocaine dose-effect curve in wild-type and M4−/− mice, but not M1−/− mice. Response rates were suppressed by xanomeline in wild-type and M1−/−, but not in M4−/− mice, and were unaltered by VU0357017. 77-LH-28-1 and BQCA also showed evidence of attenuating cocaine’s discriminative stimulus, but at doses that suppressed responding or had other undesirable effects. Intriguingly, both VU0357017 and 77-LH-28-1 exhibited U-shaped dose-effect functions in attenuating cocaine discrimination. None of the drugs substituted for the cocaine stimulus.
Attenuation of the cocaine stimulus by VU0357017 depended upon M1 receptors, and full effects of xanomeline depended upon both M1 and M4 receptors. Therefore M1-selective agonists and mixed M1/M4 agonists may be promising leads for developing medications that block cocaine’s effects.
Parkinson’s disease (PD) is a progressive neurological disorder predominantly characterized by motor symptoms including bradykinesia and resting tremor. The gold standard of treatment for PD remains dopamine replacement therapy, which eventually fails due to continued progression of the disease and the development of debilitating side effects. Recent breakthroughs are providing the first major advances in the development of fundamentally new pharmacological strategies for the treatment of PD that do not rely on dopamine replacement strategies, but rather aim to reduce the overactive indirect pathway within the basal ganglia. In this article, we will review the role of metabotropic glutamate receptors within the basal ganglia and discuss the potential for modulation of metabotropic glutamate receptors as a treatment for PD.
G Protein-Coupled Receptor; Seven Transmembrane Receptor; allosteric; positive allosteric modulator; negative allosteric modulator; silent allosteric modulator; ago-potentiator; partial antagonist; molecular switch; ligand-biased signaling
Activation of group II metabotropic glutamate receptors (mGlu2 and mGlu3) has been implicated as a potential therapeutic strategy for treating both motor symptoms and progressive neurodegeneration in Parkinson's disease (PD). Modulation of excitatory transmission in the basal ganglia represents a possible mechanism by which group II mGlu agonists could exert antiparkinsonian effects. Previous studies have identified reversible effects of mGlu2/3 activation on excitatory transmission at various synapses in the basal ganglia, including the excitatory synapse between the subthalamic nucleus (STN) and the substantia nigra pars reticulata (SNr). Using whole-cell patch clamp studies of GABAergic SNr neurons in rat midbrain slices, we have found that a prolonged activation of group II mGlus by the selective agonist LY379268 induces a long-term depression (LTD) of evoked excitatory postsynaptic current (EPSC) amplitude. Bath application of LY379268 (100 nM, 10 minutes) induced a marked reduction in EPSC amplitude, and excitatory transmission remained depressed for at least 40 minutes after agonist washout. The effect of LY379268 was concentration-dependent and was completely blocked by the group II mGlu-preferring antagonist LY341495 (500 nM). To determine the relative contributions of mGlu2 and mGlu3 to the LTD induced by LY379268, we tested the ability of LY379268 (100 nM) to induce LTD in wild type mice and mice lacking mGlu2 or mGlu3. LY379268 induced similar LTD in wild type mice and mGlu3 knockout mice, whereas LTD was absent in mGlu2 knockout mice, indicating that mGlu2 activation is necessary for the induction of LTD in the SNr. These studies suggest a novel role for mGlu2 in the long-term regulation of excitatory transmission in the SNr and invite further exploration of mGlu2 as a therapeutic target for treating the motor symptoms of PD.
metabotropic glutamate receptor; substantia nigra pars reticulata; long-term depression; synaptic plasticity; basal ganglia; Parkinson's disease
This Letter describes the discovery, SAR and in vitro and in vivo pharmacological profile of a novel non-MPEP derived mGlu5 positive allosteric modulator (PAM) based upon an N-aryl piperazine chemotype. This mGlu5 chemotype exhibits the ability to act as either a non-competitive antagonist/negative allosteric modulator (NAM) or potentiator of the glutamate response depending on the identity of the amide substituent, i.e., a ‘molecular switch’. A rapidly optimized PAM, 10e (VU0364289), was shown to be potent and specific for the rat mGlu5 receptor and subsequently demonstrated to be efficacious in a clinically relevant rodent model predictive of anti-psychotic activity, thus providing the first example of a centrally active mGluR5 PAM optimized from an HTS-derived mGluR5 competitive antagonist.
mGluR; potentiator; positive allosteric modulator; schizophrenia; hyperlocomotion
Although early clinical observations implicated dopamine dysfunction in the neuropathology of schizophrenia, accumulating evidence suggests that multiple neurotransmitter pathways are dysregulated. The psychotomimetic actions of NMDA receptor antagonists point to an imbalance of glutamatergic signaling. Encouragingly, numerous preclinical and clinical studies have elucidated several potential targets for increasing NMDA receptor function and equilibrating glutamatergic tone, including the metabotropic glutamate receptors 2, 3 and 5, the muscarinic acetylcholine receptors M1 and M4, and the glycine transporter GlyT1. Highly specific allosteric and orthosteric ligands have been developed that modify the activity of these novel target proteins, and in this review we summarize both the glutamatergic mechanisms and the novel compounds that are increasing promise for a multifaceted pharmacological approach to treat schizophrenia.
This Letter describes the hit-to-lead progression and SAR of a series of biphenyl acetylene compounds derived from an HTS screening campaign targeting the mGlu5 receptor. ‘Molecular switches’ were identified that modulated modes of pharmacology, and several compounds within this series were shown to be efficacious in reversal of amphetamine induced hyperlocomotion in rats after i.p. dosing, a preclinical model that shows similar positive effects with known antipsychotic agents.
Recent studies suggest that subtype specific activators of metabotropic glutamate receptors (mGluRs) have exciting potential for the development of novel treatment strategies for numerous psychiatric and neurological disorders. A number of positive allosteric modulators (PAMs) have been identified that are highly selective for mGluR1, including the compounds Ro 01-6128, Ro 67-4853, and Ro 67-7476. These PAMs have been previously found to interact with a site distinct from that of negative allosteric modulators (NAMs), typified by R214127. These mGluR1 PAMs do not have an effect on baseline calcium levels but induce leftward shifts in the concentration response of mGluR1 to agonists. However, their effects on a variety of signaling pathways and their mechanism of action have not been fully explored and are of critical importance for further development of mGluR1 allosteric modulators as novel drugs. In baby hamster kidney (BHK) cells, mGluR1 activates calcium mobilization, cAMP production, and extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation; signaling cascades which are distinct and differentially regulated. In contrast to their effects on calcium mobilization, these compounds were found to activate ERK1/2 phosphorylation in the absence of exogenously added agonist, an effect that was fully blocked by both orthosteric (LY341495) and allosteric (R214127) mGluR1 antagonists. The mGluR1 PAMs were also found to activate cAMP production in the absence of agonist. Thus, these mGluR1 PAMs have qualitatively different effects on a variety of mGluR1-mediated signal transduction cascades. Together, these data provide further evidence that allosteric compounds can differentially modulate the coupling of a single receptor to independent signaling pathways or act in a system-dependent manner.
mGluR1; allosteric potentiator; glutamate; ligand-induced differential signaling
Herein we report the discovery and SAR of a novel series of M1 agonists based on the MLPCN probe, ML071. From this, VU0364572 emerged as a potent, orally bioavailable and CNS penetrant M1 agonist with high selectivity, clean ancillary pharmacology and enantiospecific activity.
Muscarinic acetylcholine receptor 1; mAChR1 (M1); ML071; Allosteric agonist
Group III metabotropic glutamate receptors (mGluRs) reduce synaptic transmission at the Schaffer collateral-CA1 (SC-CA1) synapse in rats by a presynaptic mechanism. Previous studies show that low concentrations of the group III-selective agonist, L-AP4, reduce synaptic transmission in slices from neonatal but not adult rats, whereas high micromolar concentrations reduce transmission in both age groups. L-AP4 activates mGluRs 4 and 8 at much lower concentrations than those required to activate mGluR7, suggesting that the group III mGluR subtype modulating transmission is a high affinity receptor in neonates and a low affinity receptor in adults. The previous lack of subtype selective ligands has made it difficult to test this hypothesis. We have measured fEPSPs in the presence of novel subtype selective agents to address this question. We show that the effects of L-AP4 can be blocked by LY341495 in both neonates and adults, verifying that these effects are mediated by mGluRs. In addition, the selective mGluR8 agonist, DCPG, has a significant effect in slices from neonatal rats but does not reduce synaptic transmission in adult slices. The mGluR4 selective allosteric potentiator, PHCCC, is unable to potentiate the L-AP4-induced effects at either age. Taken together, our data suggest that group III mGluRs regulate transmission at the SC-CA1 synapse throughout development but there is a developmental regulation of the subtypes involved so that that both mGluR8 serves this role in neonates but not adults whereas mGluR7 is involved in regulating transmission at this synapse in throughout postnatal development.
metabotropic glutamate receptor; group III; hippocampus; DCPG; L-AP4
Glutamate is the major excitatory transmitter in the mammalian CNS, exerting its effects through both ionotropic and metabotropic glutamate receptors. The metabotropic glutamate receptors (mGlus) belong to family C of the G-protein-coupled receptors (GPCRs). The eight mGlus identified to date are classified into three groups based on their structure, preferred signal transduction mechanisms, and pharmacology (Group I: mGlu1 and mGlu5; Group II: mGlu2 and mGlu3; Group III: mGlu4, mGlu6, mGlu7, and mGlu8). Non-competitive antagonists, also known as negative allosteric modulators (NAMs), of mGlu5 offer potential therapeutic applications in diseases such as pain, anxiety, gastroesophageal reflux disease (GERD), Parkinson's disease (PD), fragile X syndrome, and addiction. The development of SAR in a (3-cyano-5-fluorophenyl)biaryl series using our functional cell-based assay is described in this communication. Further characterization of a selected compound, 3-fluoro-5-(2-methylbenzo[d]thiazol-5-yl)benzonitrile, in additional cell based assays as well as in vitro assays designed to measure its metabolic stability and protein binding indicated its potential utility as an in vivo tool. Subsequent evaluation of the same compound in a pharmacokinetic study using intraperitoneal dosing in mice showed good exposure in both plasma and brain samples. The compound was efficacious in a mouse marble burying model of anxiety, an assay known to be sensitive to mGlu5 antagonists. A new operant model of addiction termed operant sensation seeking (OSS) was chosen as a second behavioral assay. The compound also proved efficacious in the OSS model and constitutes the first reported example of efficacy with a small molecule mGlu5 NAM in this novel assay.
mGlu5; negative allosteric modulator; non-competitive antagonist; addiction
Glutamate is the major excitatory transmitter in the mammalian central nervous system (CNS), exerting its effects through both ionotropic and metabotropic glutamate receptors. The metabotropic glutamate receptors (mGlus) belong to family C of the G-protein-coupled receptors (GPCRs). The eight mGlus identified to date are classified into three groups based on their structure, preferred signal transduction mechanisms, and pharmacology (group I: mGlu1 and mGlu5; group II: mGlu2 and mGlu3; group III: mGlu4, mGlu6, mGlu7, and mGlu8). Noncompetitive antagonists, also known as negative allosteric modulators (NAMs), of mGlu5 offer potential therapeutic applications in diseases such as pain, anxiety, gastresophageal reflux disease (GERD), Parkinson’s disease (PD), fragile X syndrome, and addiction. The development of structure−activity relationships (SAR) in a (3-cyano-5-fluorophenyl)biaryl series using our functional cell-based assay is described in this communication. Further characterization of a selected compound, 3-fluoro-5-(2-methylbenzo[d]thiazol-5-yl)benzonitrile, in additional cell based assays as well as in vitro assays designed to measure its metabolic stability and protein binding indicated its potential utility as an in vivo tool. Subsequent evaluation of the same compound in a pharmacokinetic study using intraperitoneal dosing in mice showed good exposure in both plasma and brain samples. The compound was efficacious in a mouse marble burying model of anxiety, an assay known to be sensitive to mGlu5 antagonists. A new operant model of addiction termed operant sensation seeking (OSS) was chosen as a second behavioral assay. The compound also proved efficacious in the OSS model and constitutes the first reported example of efficacy with a small molecule mGlu5 NAM in this novel assay.
mGlu5; negative allosteric modulator; noncompetitive antagonist; addiction
Based on SAR in the alkyne class of mGlu5 receptor negative allosteric modulators and a set of amide-based positive allosteric modulators, optimized substitution of the aryl ‘b’ ring was used to create substituted N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamides. Results from an mGlu5 receptor functional assay, using calcium fluorescence, revealed varying efficacies and potencies that provide evidence that subtle changes in compounds within a close structural class can have marked effects on functional activity including switches in modes of efficacy (i.e. negative to positive allosteric modulation).