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

Discovery of MK-8768, a Potent and Selective mGluR2 Negative Allosteric Modulator

Glutamate plays a key role in cognition and mood, and it has been shown that inhibiting ionotropic glutamate receptors disrupts cognition, while enhancing ionotropic receptor activity is pro-cognitive. One approach to elevating glutamatergic tone has been to antagonize presynaptic metabotropic glutamate receptor 2 (mGluR2). A desire for selectivity over the largely homologous mGluR3 motivated a strategy to achieve selectivity through the identification of mGluR2 negative allosteric modulators (NAMs). Extensive screening and optimization efforts led to the identification of a novel series of 4-arylquinoline-2-carboxamides. This series was optimized for mGluR2 NAM potency, clean off-target activity, and desirable physical properties, which resulted in the identification of improved C4 and C7 substituents. The initial lead compound from this series was Ames-positive in a single strain with metabolic activation, indicating that a reactive metabolite was likely responsible for the genetic toxicity. Metabolic profiling and Ames assessment across multiple analogs identified key structure-activity relationships associated with Ames positivity. Further optimization led to the Ames-negative mGluR2 negative allosteric modulator MK-8768.

Negative allosteric modulators of group II metabotropic glutamate receptors: A patent review (2015 - present)

INTRODUCTION

Group II metabotropic glutamate (mGlu) receptors have emerged as an attractive potential target for the development of novel CNS therapeutics in areas such as Alzheimer's disease (AD), anxiety, cognitive disorders, depression, and others. Several small molecules that act as negative allosteric modulators (NAMs) on these receptors have demonstrated efficacy and/or target engagement in animal models, and one molecule (decoglurant) has been advanced into clinical trials.

AREAS COVERED

This review summarizes patent applications published between January 2015 and November 2020. It is divided into three sections: (1) small molecule nonselective mGlu_2/3 NAMs, (2) small molecule selective mGlu₂ NAMs, and (3) small molecule selective mGlu₃ NAMs.

EXPERT OPINION

Much progress has been made in the discovery of novel small molecule mGlu₂ NAMs. Still, chemical diversity remains somewhat limited and room for expansion remains. Progress with mGlu₃ NAMs has been more limited; however, some promising molecules have been disclosed. The process of elucidating the precise role of each receptor in the diseases associated with group II receptors has begun. Continued studies in animals with selective NAMs for both receptors will be critical in the coming years to inform researchers on the right compound profile and patient population for clinical development.

Negative allosteric modulators of metabotropic glutamate receptor 3 target the stem-like phenotype of glioblastoma

Glioblastoma is an invariably deadly disease. A subpopulation of glioma stem-like cells (GSCs) drives tumor progression and treatment resistance. Two recent studies demonstrated that neurons form oncogenic glutamatergic electrochemical synapses with post-synaptic GSCs. This led us to explore whether glutamate signaling through G protein-coupled metabotropic receptors would also contribute to the malignancy of glioblastoma. We found that glutamate metabotropic receptor (Grm)3 is the predominantly expressed Grm in glioblastoma. Associations of GRM3 gene expression levels with survival are confined to the proneural gene expression subtype, which is associated with enrichment of GSCs. Using multiplexed single-cell qRT-PCR, GSC marker-based cell sorting, database interrogations, and functional assays in GSCs derived from patients' tumors, we establish Grm3 as a novel marker and potential therapeutic target in GSCs. We confirm that Grm3 inhibits adenylyl cyclase and regulates extracellular signal-regulated kinase. Targeting Grm3 disrupts self-renewal and promotes differentiation of GSCs. Thus, we hypothesize that Grm3 signaling may complement oncogenic functions of glutamatergic ionotropic receptor activity in neuroglial synapses, supporting a link between neuronal activity and the GSC phenotype. The novel class of highly specific Grm3 inhibitors that we characterize herein have been clinically tested as cognitive enhancers in humans with a favorable safety profile.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Regiospecific Synthesis and Structural Studies of 3,5-Dihydro-4H-pyrido[2,3-b][1,4]diazepin-4-ones and Comparison with 1,3-Dihydro-2H-benzo[b][1,4]diazepin-2-ones

Nine 3,5-dihydro-4H-pyrido[2,3-b][1,4]diazepin-4-ones (17-25), some of which contain fluoro-substituents, have been regiospecifically prepared by reaction of 2,3-diaminopyridines with ethyl aroylacetates. In two cases, open intermediates have been isolated and these are related to the reaction pathway. The X-ray crystal structure of 1-methyl-4-phenyl-3,5-dihydro-4H-pyrido[2,3-b][1,4]diazepin-4-one (23) has been solved (formula, C₁₅H₁₃N₃O; crystal system, monoclinic; space group, C2/c). This is an asymmetric unit constituted by a single nonplanar molecule and its conformational enantiomer due to the presence of the seven-membered diazepin-2-one moiety, which introduces a certain degree of torsion in the adjacent pyridine ring. The ¹H, ¹³C, ¹⁵N, and ¹⁹F NMR spectra were obtained and the chemical shifts, together with those of the previously published 1,3-dihydro-2H-benzo[b][1,4]diazepin-2-ones (1-16), i.e., a total of 544 values, were successfully compared with the chemical shifts calculated at the gauge invariant atomic orbital (GIAO)/Becke, three-parameter, Lee-Yang-Parr (B3LYP)/6-311++G(d,p) level. The seven-membered ring inversion barrier in 5-benzyl-2-phenyl-3,5-dihydro-4H-pyrido[2,3-b][1,4]diazepin-4-one (25) was determined and, in conjunction with the data from the literature, compared with the B3LYP/6-311++G(d,p) computed values. This allowed the determination of several structural effects. The rotation about the exocyclic N1-CR bond was also calculated and its dynamic properties were discussed.

Further exploration of an N-aryl phenoxyethoxy pyridinone-based series of mGlu3 NAMs: Challenging SAR, enantiospecific activity and in vivo efficacy

This letter describes the further optimization of a series of mGlu₃ NAMs based on an N-aryl phenoxyethoxy pyridinone core. A multidimensional optimization campaign, with focused matrix libraries, quickly established challenging SAR, enantiospecific activity, differences in assay read-outs (Ca²⁺ flux via a promiscuous G protein (G_α15) versus native coupling to GIRK channels), identified both full and partial mGlu₃ NAMs and a new in vivo tool compound, VU6017587. This mGlu₃ NAM showed efficacy in tail suspension, elevated zero maze and marble burying, suggesting selective inhibition of mGlu₃ affords anxiolytic-like and antidepressant-like phenotypes in mice.

Design and Synthesis of N-Aryl Phenoxyethoxy Pyridinones as Highly Selective and CNS Penetrant mGlu3 NAMs

Herein, we detail the optimization of the mGlu₃ NAM, VU0650786, via a reductionist approach to afford a novel, simplified mGlu₃ NAM scaffold that engenders potent and selective mGlu₃ inhibition (mGlu₃ IC₅₀ = 245 nM, mGlu₂ IC₅₀ > 30 μM) with excellent central nervous system penetration (rat brain/plasma K_p = 1.2, K_p,uu = 0.40). Moreover, this new chemotype, exemplified by VU6010572, requires only four synthetic steps and displays improved physiochemical properties and in vivo efficacy in a mouse tail suspension test (MED = 3 mg/kg i.p.).

Metal-free oxidative ring contraction of benzodiazepinones: an entry to quinoxalinones

Herein is reported a practical method for the construction of 3-benzoylquinoxalinones from benzodiazepinones in the presence of DMSO which serves both as a solvent and an oxidant.

Practical Strategies and Concepts in GPCR Allosteric Modulator Discovery: Recent Advances with Metabotropic Glutamate Receptors

Allosteric modulation of GPCRs has initiated a new era of basic and translational discovery, filled with therapeutic promise yet fraught with caveats. Allosteric ligands stabilize unique conformations of the GPCR that afford fundamentally new receptors, capable of novel pharmacology, unprecedented subtype selectivity, and unique signal bias. This review provides a comprehensive overview of the basics of GPCR allosteric pharmacology, medicinal chemistry, drug metabolism, and validated approaches to address each of the major challenges and caveats. Then, the review narrows focus to highlight recent advances in the discovery of allosteric ligands for metabotropic glutamate receptor subtypes 1-5 and 7 (mGlu1-5,7) highlighting key concepts ("molecular switches", signal bias, heterodimers) and practical solutions to enable the development of tool compounds and clinical candidates. The review closes with a section on late-breaking new advances with allosteric ligands for other GPCRs and emerging data for endogenous allosteric modulators.

Molecular mechanism of positive allosteric modulation of the metabotropic glutamate receptor 2 by JNJ-46281222

BACKGROUND AND PURPOSE

Allosteric modulation of the mGlu2 receptor is a potential strategy for treatment of various neurological and psychiatric disorders. Here, we describe the in vitro characterization of the mGlu2 positive allosteric modulator (PAM) JNJ-46281222 and its radiolabelled counterpart [(3) H]-JNJ-46281222. Using this novel tool, we also describe the allosteric effect of orthosteric glutamate binding and the presence of a bound G protein on PAM binding and use computational approaches to further investigate the binding mode.

EXPERIMENTAL APPROACH

We have used radioligand binding studies, functional assays, site-directed mutagenesis, homology modelling and molecular dynamics to study the binding of JNJ-46281222.

KEY RESULTS

JNJ-46281222 is an mGlu2 -selective, highly potent PAM with nanomolar affinity (KD = 1.7 nM). Binding of [(3) H]-JNJ-46281222 was increased by the presence of glutamate and greatly reduced by the presence of GTP, indicating the preference for a G protein bound state of the receptor for PAM binding. Its allosteric binding site was visualized and analysed by a computational docking and molecular dynamics study. The simulations revealed amino acid movements in regions expected to be important for activation. The binding mode was supported by [(3) H]-JNJ-46281222 binding experiments on mutant receptors.

CONCLUSION AND IMPLICATIONS

Our results obtained with JNJ-46281222 in unlabelled and tritiated form further contribute to our understanding of mGlu2 allosteric modulation. The computational simulations and mutagenesis provide a plausible binding mode with indications of how the ligand permits allosteric activation. This study is therefore of interest for mGlu2 and class C receptor drug discovery.

In vitro pharmacological and rat pharmacokinetic characterization of LY3020371, a potent and selective mGlu2/3 receptor antagonist

Metabotropic glutamate _2/3 (mGlu_2/3) receptors are of considerable interest owing to their role in modulating glutamate transmission via presynaptic, postsynaptic and glial mechanisms. As part of our ongoing efforts to identify novel ligands for these receptors, we have discovered (1S,2R,3S,4S,5R,6R)-2-amino-3-[(3,4-difluorophenyl)sulfanylmethyl]-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylic acid; (LY3020371), a potent and selective orthosteric mGlu_2/3 receptor antagonist. In this account, we characterize the effects of LY3020371 in membranes and cells expressing human recombinant mGlu receptor subtypes as well as in native rodent and human brain tissue preparations, providing important translational information for this molecule. In membranes from cells expressing recombinant human mGlu₂ and mGlu₃ receptor subtypes, LY3020371.HCl competitively displaced binding of the mGlu_2/3 agonist ligand [³H]-459477 with high affinity (hmGlu₂ K_i = 5.26 nM; hmGlu₃ Ki = 2.50 nM). In cells expressing hmGlu₂ receptors, LY3020371.HCl potently blocked mGlu_2/3 agonist (DCG-IV)-inhibited, forskolin-stimulated cAMP formation (IC₅₀ = 16.2 nM), an effect that was similarly observed in hmGlu₃-expressing cells (IC₅₀ = 6.21 nM). Evaluation of LY3020371 in cells expressing the other human mGlu receptor subtypes revealed high mGlu_2/3 receptor selectivity. In rat native tissue assays, LY3020371 demonstrated effective displacement of [³H]-459477 from frontal cortical membranes (K_i = 33 nM), and functional antagonist activity in cortical synaptosomes measuring both the reversal of agonist-suppressed second messenger production (IC₅₀ = 29 nM) and agonist-inhibited, K⁺-evoked glutamate release (IC₅₀ = 86 nM). Antagonism was fully recapitulated in both primary cultured cortical neurons where LY3020371 blocked agonist-suppressed spontaneous Ca²⁺ oscillations (IC₅₀ = 34 nM) and in an intact hippocampal slice preparation (IC₅₀ = 46 nM). Functional antagonist activity was similarly demonstrated in synaptosomes prepared from epileptic human cortical or hippocampal tissues, suggesting a translation of the mGlu_2/3 antagonist pharmacology from rat to human. Intravenous dosing of LY3020371 in rats led to cerebrospinal fluid drug levels that are expected to effectively block mGlu_2/3 receptors in vivo. Taken together, these results establish LY3020371 as an important new pharmacological tool for studying mGlu_2/3 receptors in vitro and in vivo. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Design of 4-Oxo-1-aryl-1,4-dihydroquinoline-3-carboxamides as Selective Negative Allosteric Modulators of Metabotropic Glutamate Receptor Subtype 2

Both orthosteric and allosteric antagonists of the group II metabotropic glutamate receptors (mGlus) have been used to establish a link between mGlu2/3 inhibition and a variety of CNS diseases and disorders. Though these tools typically have good selectivity for mGlu2/3 versus the remaining six members of the mGlu family, compounds that are selective for only one of the individual group II mGlus have proved elusive. Herein we report on the discovery of a potent and highly selective mGlu2 negative allosteric modulator 58 (VU6001192) from a series of 4-oxo-1-aryl-1,4-dihydroquinoline-3-carboxamides. The concept for the design of this series centered on morphing a quinoline series recently disclosed in the patent literature into a chemotype previously used for the preparation of muscarinic acetylcholine receptor subtype 1 positive allosteric modulators. Compound 58 exhibits a favorable profile and will be a useful tool for understanding the biological implications of selective inhibition of mGlu2 in the CNS.

Discovery of a Selective and CNS Penetrant Negative Allosteric Modulator of Metabotropic Glutamate Receptor Subtype 3 with Antidepressant and Anxiolytic Activity in Rodents

Previous preclinical work has demonstrated the therapeutic potential of antagonists of the group II metabotropic glutamate receptors (mGlus). Still, compounds that are selective for the individual group II mGlus (mGlu2 and mGlu3) have been scarce. There remains a need for such compounds with the balance of properties suitable for convenient use in a wide array of rodent behavioral studies. We describe here the discovery of a selective mGlu3 NAM 106 (VU0650786) suitable for in vivo work. Compound 106 is a member of a series of 5-aryl-6,7-dihydropyrazolo[1,5-a]pyrazine-4(5H)-one compounds originally identified as a mGlu5 positive allosteric modulator (PAM) chemotype. Its suitability for use in rodent behavioral models has been established by extensive in vivo PK studies, and the behavioral experiments presented here with compound 106 represent the first examples in which an mGlu3 NAM has demonstrated efficacy in models where prior efficacy had previously been noted with nonselective group II antagonists.

Deletion of Metabotropic Glutamate Receptors 2 and 3 (mGlu2 & mGlu3) in Mice Disrupts Sleep and Wheel-Running Activity, and Increases the Sensitivity of the Circadian System to Light

Sleep and/or circadian rhythm disruption (SCRD) is seen in up to 80% of schizophrenia patients. The co-morbidity of schizophrenia and SCRD may in part stem from dysfunction in common brain mechanisms, which include the glutamate system, and in particular, the group II metabotropic glutamate receptors mGlu2 and mGlu3 (encoded by the genes Grm2 and Grm3). These receptors are relevant to the pathophysiology and potential treatment of schizophrenia, and have also been implicated in sleep and circadian function. In the present study, we characterised the sleep and circadian rhythms of Grm2/3 double knockout (Grm2/3^-/-) mice, to provide further evidence for the involvement of group II metabotropic glutamate receptors in the regulation of sleep and circadian rhythms. We report several novel findings. Firstly, Grm2/3^-/- mice demonstrated a decrease in immobility-determined sleep time and an increase in immobility-determined sleep fragmentation. Secondly, Grm2/3^-/- mice showed heightened sensitivity to the circadian effects of light, manifested as increased period lengthening in constant light, and greater phase delays in response to nocturnal light pulses. Greater light-induced phase delays were also exhibited by wildtype C57Bl/6J mice following administration of the mGlu2/3 negative allosteric modulator RO4432717. These results confirm the involvement of group II metabotropic glutamate receptors in photic entrainment and sleep regulation pathways. Finally, the diurnal wheel-running rhythms of Grm2/3^-/- mice were perturbed under a standard light/dark cycle, but their diurnal rest-activity rhythms were unaltered in cages lacking running wheels, as determined with passive infrared motion detectors. Hence, when assessing the diurnal rest-activity rhythms of mice, the choice of assay can have a major bearing on the results obtained.

Allosteric Binding Site and Activation Mechanism of Class C G-Protein Coupled Receptors: Metabotropic Glutamate Receptor Family

Metabotropic glutamate receptors (mGluR) are mainly expressed in the central nervous system (CNS) and contain eight receptor subtypes, named mGluR1 to mGluR8. The crystal structures of mGluR1 and mGluR5 that are bound with the negative allosteric modulator (NAM) were reported recently. These structures provide a basic model for all class C of G-protein coupled receptors (GPCRs) and may aid in the design of new allosteric modulators for the treatment of CNS disorders. However, these structures are only combined with NAMs in the previous reports. The conformations that are bound with positive allosteric modulator (PAM) or agonist of mGluR1/5 remain unknown. Moreover, the structural information of the other six mGluRs and the comparisons of the mGluRs family have not been explored in terms of their binding pockets, the binding modes of different compounds, and important binding residues. With these crystal structures as the starting point, we built 3D structural models for six mGluRs by using homology modeling and molecular dynamics (MD) simulations. We systematically compared their allosteric binding sites/pockets, the important residues, and the selective residues by using a series of comparable dockings with both the NAM and the PAM. Our results show that several residues played important roles for the receptors' selectivity. The observations of detailed interactions between compounds and their correspondent receptors are congruent with the specificity and potency of derivatives or compounds bioassayed in vitro. We then carried out 100 ns MD simulations of mGluR5 (residue 26-832, formed by Venus Flytrap domain, a so-called cysteine-rich domain, and 7 trans-membrane domains) bound with antagonist/NAM and with agonist/PAM. Our results show that both the NAM and the PAM seemed stable in class C GPCRs during the MD. However, the movements of "ionic lock," of trans-membrane domains, and of some activation-related residues in 7 trans-membrane domains of mGluR5 were congruent with the findings in class A GPCRs. Finally, we selected nine representative bound structures to perform 30 ns MD simulations for validating the stabilities of interactions, respectively. All these bound structures kept stable during the MD simulations, indicating that the binding poses in this present work are reasonable. We provided new insight into better understanding of the structural and functional roles of the mGluRs family and facilitated the future structure-based design of novel ligands of mGluRs family with therapeutic potential.

Molecular Insights into Metabotropic Glutamate Receptor Allosteric Modulation

The metabotropic glutamate (mGlu) receptors are a group of eight family C G protein-coupled receptors that are expressed throughout the central nervous system (CNS) and periphery. Within the CNS the different subtypes are found in neurons, both pre- and/or postsynaptically, where they mediate modulatory roles and in glial cells. The mGlu receptor family provides attractive targets for numerous psychiatric and neurologic disorders, with the majority of discovery programs focused on targeting allosteric sites, with allosteric ligands now available for all mGlu receptor subtypes. However, the development of allosteric ligands remains challenging. Biased modulation, probe dependence, and molecular switches all contribute to the complex molecular pharmacology exhibited by mGlu receptor allosteric ligands. In recent years we have made significant progress in our understanding of this molecular complexity coupled with an increased understanding of the structural basis of mGlu allosteric modulation.

Novel metabotropic glutamate receptor 2/3 antagonists and their therapeutic applications: a patent review (2005 - present)

INTRODUCTION

This review focuses on the medicinal chemistry efforts directed toward the identification of competitive and noncompetitive antagonists of glutamate at group II metabotropic glutamate receptors (mGluRII: mGlu2/3 and mGlu2). This class of compounds holds promise for the treatment of CNS disorders such as major depression, cognitive deficits and sleep-wake disorders, and several pharmaceutical companies are advancing mGluRII antagonists from discovery research into clinical development.

AREA COVERED

This review article covers for the first time the patent applications that were published on mGlu2/3 orthosteric and allosteric antagonists between January 2005 and September 2014, with support from the primary literature, posters and oral communications from international congresses. Patent applications published prior to 2005 for which compositions of matter were largely described in peer review articles are briefly discussed with main findings.

EXPERT OPINION

Recent advances in the prodrug approach of novel mGlu2/3 orthosteric antagonists combined with the design of novel mGlu2/3 and mGlu2 negative allosteric modulators provide new therapeutic opportunities for neurologic and psychiatric disorders.

mGlu(2) receptor-mediated modulation of conditioned avoidance behavior in rats

Inhibition of conditioned avoidance behavior in rats is generally considered predictive for antipsychotic activity in man. The present study investigated the mGlu2-mediated modulation of conditioned avoidance and compared mGlu2 agonists with available antipsychotics for their relative effects on conditioned avoidance behavior and locomotion. The mGlu2/3 orthosteric agonist 4-amino-2-thiabicyclo[3.1.0]hexane-4,6-dicarboxylic acid 2,2-dioxide (LY-404039) and mGlu2 positive allosteric modulator (PAM) 3-(cyclopropylmethyl)-7-(4-phenylpiperidin-1-yl)-8-(trifluoromethyl)[1,2,4]triazolo[4,3-a]pyridine (JNJ-42153605) inhibited avoidance and blocked escape behavior. The mGlu2/3 negative allosteric modulators (NAMs) 7-(dimethylamino)-4-(3-pyridin-3-ylphenyl)-8-(trifluoromethyl)-1,3-dihydro-2 H-1,5-benzodiazepin-2-one (JNJ-42112265) and 4-[3-(2,6-dimethylpyridin-4-yl)phenyl]-7-methyl-8-(trifluoromethyl)-1,3-dihydro-2H-1,5-benzodiazepin-2-one (RO-4491533) reversed the LY-404039-induced impairment of avoidance and escape. JNJ-42112265 also reversed the impairment of avoidance and escape induced by the mGlu2-specific PAM JNJ-42153605, suggesting that the effects on conditioned avoidance are specifically mGlu2-mediated. The mGlu2/3 antagonist (2-(2-carboxycyclopropyl)-3-(9H-xanthen-9-yl)-d-alanine (LY-341495; s.c.) reversed the LY-404039-induced escape impairment but failed to restore avoidance, suggesting interfering side effects. Like the tested antipsychotics, mGlu2/3 orthosteric and allosteric agonists inhibited avoidance behavior and locomotion at similar doses. Hence no clear-cut differences between mGlu2 modulators and currently available antipsychotics in the way they interfere with avoidance behavior in relation to inhibition of locomotion could be established.

Cognitive impairment in major depression and the mGlu2 receptor as a therapeutic target

Cognitive impairment, in particular of attention and memory, is often reported by patients suffering from major depressive disorder (MDD) and deficits in attention are part of the current diagnostic criteria of MDD. Objectively measured cognitive deficits associated with MDD have been described in many studies. They have been conceptualized as an integral facet and epiphenomenon of MDD. However, evidence accumulated in recent years has challenged this notion and demonstrated that in a subset of patients the degree of cognitive deficits cannot be accounted for by the severity of depression. In addition, in some patients cognitive deficits persist despite resolution of depressive symptomatology. It is plausible to assume that cognitive deficits contribute to functional impairment even though supportive data for such a relationship are lacking. However, the exact association between cognitive deficits and major depression and the clinical and neurobiological characteristics of patients with MDD in whom cognitive deficits seem partially or fully independent of the clinical manifestation of depressive symptoms remain poorly understood. This review focuses on objective measures of non-emotional cognitive deficits in MDD and discusses the presence of a subgroup of patients in whom these symptoms can be defined independently and in dissociation from the rest of the depressive symptomatology. The current understanding of brain circuits and molecular events implicated in cognitive impairment in MDD are discussed with an emphasis on the missing elements that could further define the specificity of cognitive impairment in MDD and lead to new therapeutics. Furthermore, this article presents in detail observations made in behavioral studies in rodents with potential novel therapeutic agents, such as negative allosteric modulators at the metabotropic glutamate receptor type 2/3 (mGlu2/3 NAM) which exhibit both cognitive enhancing and antidepressant properties. Such a compound, RO4432717, was tested in tests of short term memory (delayed match to position), cognitive flexibility (Morris water maze, reversal protocol), impulsivity and compulsivity (5-choice serial reaction time) and spontaneous object recognition in rodents, providing first evidence of a profile potentially relevant to address cognitive impairment in MDD. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Characterization of an mGluR2/3 negative allosteric modulator in rodent models of depression

There is growing evidence suggesting that antagonists of group II metabotropic glutamate receptors (mGluR2/3) exhibit antidepressant-like properties in several preclinical models of depression. However, all those studies have been performed using competitive group II non-selective orthosteric antagonists. In this study we extensively characterized a group II selective negative allosteric modulator (4-[3-(2,6-Dimethylpyridin-4-yl)phenyl]-7-methyl-8-trifluoromethyl-1,3-dihydrobenzo[b][1,4]diazepin-2-one, namely RO4491533, Woltering et al., 2010) in several in vitro biochemical assays and in vivo models of depression. In vitro, RO4491533 completely blocked the glutamate-induced Ca(2+) mobilization and the glutamate-induced accumulation in [(35)S]GTP(γS) binding in cells expressing recombinant human or rat mGluR2 and in native tissues. Results from Schild plot experiments and reversibility test at the target on both cellular and membrane-based assays confirmed the negative allosteric modulator properties of the compound. RO4491533 was equipotent on mGluR2 and mGluR3 receptors but not active on any other mGluRs. RO4491533 has acceptable PK properties in mice and rats, is bioavailable following oral gavage (F = 30%) and brain-penetrant (CSF conc/total plasma conc ratio = 0.8%). RO4491533 appeared to engage the central mGluR2 and mGluR3 receptors since the compound reversed the hypolocomotor effect of an mGluR2/3 orthosteric agonist LY379268 in a target-specific manner, as did the group II orthosteric mGluR2/3 antagonist LY341495. RO4491533 and LY341495 dose-dependently reduced immobility time of C57Bl6/J mice in the forced swim test. Also, RO4491533 and LY341495 were active in the tail suspension test in a line of Helpless (H) mice, a putative genetic model of depression. These data suggest that mGluR2/3 receptors are viable targets for development of novel pharmacotherapies for depression.

Development of a novel, CNS-penetrant, metabotropic glutamate receptor 3 (mGlu3) NAM probe (ML289) derived from a closely related mGlu5 PAM

Herein we report the discovery and SAR of a novel metabotropic glutamate receptor 3 (mGlu(3)) NAM probe (ML289) with 15-fold selectivity versus mGlu(2). The mGlu(3) NAM was discovered via a 'molecular switch' from a closely related, potent mGlu(5) positive allosteric modulator (PAM), VU0092273. This NAM (VU0463597, ML289) displays an IC(50) value of 0.66 μM and is inactive against mGlu(5).

Structural determinants of allosteric antagonism at metabotropic glutamate receptor 2: mechanistic studies with new potent negative allosteric modulators

BACKGROUND AND PURPOSE

Altered glutamatergic neurotransmission is linked to several neurological and psychiatric disorders. Metabotropic glutamate receptor 2 (mGlu₂) plays an important role on the presynaptic control of glutamate release and negative allosteric modulators (NAMs) acting on mGlu₂/₃ receptors are under assessment for their potential as antidepressants, neurogenics and cognitive enhancers. Two new potent mGlu₂/₃ NAMs, RO4988546 and RO5488608, are described in this study and the allosteric binding site in the transmembrane (TM) domain of mGlu₂ is characterized.

EXPERIMENTAL APPROACH

Site directed mutagenesis, functional measurements and β₂-adrenoceptor-based modelling of mGlu₂ were employed to identify important molecular determinants of two new potent mGlu₂/₃ NAMs.

KEY RESULTS

RO4988546 and RO5488608 affected both [³H]-LY354740 agonist binding at the orthosteric site and the binding of a tritiated positive allosteric modulator (³H-PAM), indicating that NAMs and PAMs could have overlapping binding sites in the mGlu₂ TM domain. We identified eight residues in the allosteric binding pocket that are crucial for non-competitive antagonism of agonist-dependent activation of mGlu₂ and directly interact with the NAMs: Arg³·²⁸, Arg³·²⁹, Phe³·³⁶, His(E2.52) , Leu⁵·⁴³, Trp⁶·⁴⁸, Phe⁶·⁵⁵ and Val⁷·⁴³. The mGlu₂ specific residue His(E2.52) is likely to be involved in selectivity and residues located in the outer part of the binding pocket are more important for [³H]-LY354740 agonist binding inhibition, which is independent of the highly conserved Trp⁶·⁴⁸ residue.

CONCLUSIONS AND IMPLICATIONS

This is the first complete molecular investigation of the allosteric binding pocket of mGlu₂ and Group II mGluRs and provides new information on what determines mGlu₂ NAMs selective interactions and effects.

Metabotropic glutamate receptors for new treatments in schizophrenia

Metabotropic glutamate receptors (mGluRs) represent exciting targets for the development of novel therapeutic agents for schizophrenia. Recent studies indicate that selective activation of specific mGluR subtypes may provide potential benefits for not only the positive symptoms, but also the negative symptoms and cognitive impairments observed in individuals with schizophrenia. Although optimization of traditional orthosteric agonists may still offer a feasible approach for the activation of mGluRs, important progress has been made in the discovery of novel subtype-selective allosteric ligands, including positive allosteric modulators (PAMs) of mGluR2 and mGluR5. These allosteric mGluR ligands have improved properties for clinical development and have served as key preclinical tools for a more in-depth understanding of the potential roles of these different mGluR subtypes for the treatment of schizophrenia.

Investigation on quantitative structure activity relationships and pharmacophore modeling of a series of mGluR2 antagonists

MGluR2 is G protein-coupled receptor that is targeted for diseases like anxiety, depression, Parkinson’s disease and schizophrenia. Herein, we report the three-dimensional quantitative structure–activity relationship (3D-QSAR) studies of a series of 1,3-dihydrobenzo[ b][1,4]diazepin-2-one derivatives as mGluR2 antagonists. Two series of models using two different activities of the antagonists against rat mGluR2, which has been shown to be very similar to the human mGluR2, (activity I: inhibition of [³H]-LY354740; activity II: mGluR2 (1S,3R)-ACPD inhibition of forskolin stimulated cAMP.) were derived from datasets composed of 137 and 69 molecules respectively. For activity I study, the best predictive model obtained from CoMFA analysis yielded a Q² of 0.513, R²_ncv of 0.868, R²_pred = 0.876, while the CoMSIA model yielded a Q² of 0.450, R²_ncv = 0.899, R²_pred = 0.735. For activity II study, CoMFA model yielded statistics of Q² = 0.5, R²_ncv = 0.715, R²_pred = 0.723. These results prove the high predictability of the models. Furthermore, a combined analysis between the CoMFA, CoMSIA contour maps shows that: (1) Bulky substituents in R₇, R₃ and position A benefit activity I of the antagonists, but decrease it when projected in R₈ and position B; (2) Hydrophilic groups at position A and B increase both antagonistic activity I and II; (3) Electrostatic field plays an essential rule in the variance of activity II. In search for more potent mGluR2 antagonists, two pharmacophore models were developed separately for the two activities. The first model reveals six pharmacophoric features, namely an aromatic center, two hydrophobic centers, an H-donor atom, an H-acceptor atom and an H-donor site. The second model shares all features of the first one and has an additional acceptor site, a positive N and an aromatic center. These models can be used as guidance for the development of new mGluR2 antagonists of high activity and selectivity. This work is the first report on 3D-QSAR modeling of these mGluR2 antagonists. All the conclusions may lead to a better understanding of the mechanism of antagonism and be helpful in the design of new potent mGluR2 antagonists.

Recent progress in the synthesis and characterization of group II metabotropic glutamate receptor allosteric modulators

Group II metabotropic glutamate (mGlu) receptors consist of the metabotropic glutamate 2 (mGlu(2)) and metabotropic glutamate 3 (mGlu(3)) receptor subtypes which modulate glutamate transmission by second messenger activation to negatively regulate the activity of adenylyl cyclase. Excessive accumulation of glutamate in the perisynaptic extracellular region triggers mGlu(2) and mGlu(3) receptors to inhibit further release of glutamate. There is growing evidence that the modulation of glutamatergic neurotransmission by small molecule modulators of Group II mGlu receptors has significant potential for the treatment of several neuropsychiatric and neurodegenerative diseases. This review provides an overview of recent progress on the synthesis and pharmacological characterization of positive and negative allosteric modulators of the Group II mGlu receptors.

Allosteric modulation of metabotropic glutamate receptors

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.