Recent advances in the medicinal chemistry of the metabotropic glutamate receptor 1 (mGlu₁)

Negative allosteric modulator of Group Ⅰ mGluRs: Recent advances and therapeutic perspective for neuropathic pain

Neuropathic pain (NP) is a widespread public health problem that existing therapeutic treatments cannot manage adequately; therefore, novel treatment strategies are urgently required. G-protein-coupled receptors are important for intracellular signal transduction, and widely participate in physiological and pathological processes, including pain perception. Group I metabotropic glutamate receptors (mGluRs), including mGluR1 and mGluR5, are predominantly implicated in central sensitization, which can lead to hyperalgesia and allodynia. Many orthosteric site antagonists targeting Group I mGluRs have been found to alleviate NP, but their poor efficacy, low selectivity, and numerous side effects limit their development in NP treatment. Here we reviewed the advantages of Group I mGluRs negative allosteric modulators (NAMs) over orthosteric site antagonists based on allosteric modulation mechanism, and the challenges and opportunities of Group I mGluRs NAMs in NP treatment. This article aims to elucidate the advantages and future development potential of Group I mGluRs NAMs in the treatment of NP.

Subtype-selective mechanisms of negative allosteric modulators binding to group I metabotropic glutamate receptors

Group I metabotropic glutamate receptors (mGlu₁ and mGlu₅) are promising targets for multiple psychiatric and neurodegenerative disorders. Understanding the subtype selectivity of mGlu₁ and mGlu₅ allosteric sites is essential for the rational design of novel modulators with single- or dual-target mechanism of action. In this study, starting from the deposited mGlu₁ and mGlu₅ crystal structures, we utilized computational modeling approaches integrating docking, molecular dynamics simulation, and efficient post-trajectory analysis to reveal the subtype-selective mechanism of mGlu₁ and mGlu₅ to 10 diverse drug scaffolds representing known negative allosteric modulators (NAMs) in the literature. The results of modeling identified six pairs of non-conserved residues and four pairs of conserved ones as critical features to distinguish the selective NAMs binding to the corresponding receptors. In addition, nine pairs of residues are beneficial to the development of novel dual-target NAMs of group I metabotropic glutamate receptors. Furthermore, the binding modes of a reported dual-target NAM (VU0467558) in mGlu₁ and mGlu₅ were predicted to verify the identified residues that play key roles in the receptor selectivity and the dual-target binding. The results of this study can guide rational structure-based design of novel NAMs, and the approach can be generally applicable to characterize the features of selectivity for other G-protein-coupled receptors.

Progress toward allosteric ligands of metabotropic glutamate 7 (mGlu7) receptor: 2008-present

Metabotropic glutamate type 7 (mGlu7) receptor is a member of the group III family of mGlu receptors. It is widely distributed in the central nervous system (CNS) and is preferentially expressed on presynaptic nerve terminals where it is thought to play a critical role in modulating normal neuronal function and synaptic transmission, making it particularly relevant in neuropharmacology. The lack of small-molecule mGlu7 ligands with adequate potency, selectivity and drug-like properties has resulted in difficulties in the preclinical validation of mGlu7 modulation in disease models. In the last decade, allosteric modulators of mGlu7 receptors have emerged as valuable tools with good potency, selectivity and physicochemical properties to study and unleash the therapeutic potential of mGlu7 receptors. This review focusses on the medicinal chemistry of mGlu7 receptor allosteric ligands discovered since 2008.

Preclinical anaylses of [18F]cEFQ as a PET tracer for imaging metabotropic glutamate receptor type 1 (mGluR1)

Metabotropic glutamate receptor type 1 (mGluR1) is related with various neurological and psychiatric diseases, such as anxiety, depression, epilepsy, Parkinson's disease, and neuropathic pain. Hence, mGluR1 is an important target for drug development and imaging. We synthesized [¹⁸F]cEFQ (3-ethyl-2-[¹⁸F]fluoroquinolin-6-yl cis-(4-methoxycyclohexyl)methanone) as a PET tracer for selective mGluR1 imaging and evaluated its properties in rodents. A chloroquinoline precursor was labeled by a nucleophilic substitution reaction, and the resulting [¹⁸F]cEFQ was obtained with high radiochemical purity (>99%) and specific activity (63-246 GBq/µmol). The log D value was 3.24, and the initial brain uptake at 10 min was over 4% of injected dose per gram in BALB/c mice. According to PET/CT and autoradiography in SD rats, [¹⁸F]cEFQ showed wide distribution in the whole brain and the highest uptake in the cerebellum. Pre-treatment with unlabeled cEFQ or the mGluR1-specific antagonist JNJ16259685 blocked the uptake of [¹⁸F]cEFQ. However, the uptake was not blocked by pre-treatment with the mGluR5-specific antagonist ABP688. The trans isomer [¹⁸F]tEFQ did not show high uptake in the mGluR1-rich region. [¹⁸F]cEFQ was straightforwardly prepared using a chloro-derivative precursor. Its feasibility as a specific and selective PET agent for imaging mGluR1 was proved by in vitro and in vivo experiments using rodents.

Age and gender effects of 11C-ITMM binding to metabotropic glutamate receptor type 1 in healthy human participants

We examined possible age- and gender-related changes in binding of the selective antagonist N-[4-[6-(isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]-4-¹¹C-methoxy-N-methylbenzamide (¹¹C-ITMM) to metabotropic glutamate receptor type 1 in healthy human brains. Dynamic ¹¹C-ITMM positron emission tomography scans (90 min) with serial arterial blood sampling were performed in 15 young and 24 older healthy adult volunteers. The total distribution volume (V_T) of several brain regions was estimated with 2-tissue compartment model analysis. The V_Ts of the cerebellar cortex, parietal cortex, putamen, amygdala, and hippocampus in older adult participants were significantly higher than in young participants. The age-related V_T increase was only observed in male participants. Our data suggest that an age-dependent increase in metabotropic glutamate receptor type 1 availability in several brain regions may exist predominantly in males.

Recent advances in the medicinal chemistry of group II and group III mGlu receptors

Metabotropic glutamate receptors (mGlu receptors) belong to the G-protein-coupled receptors superfamily. They are divided into three groups, in which group II and group III belong to presynaptic receptors that negatively modulate glutamate and γ-aminobutyric acid (GABA) release when activated. In this review, we introduce not only the functions of mGlu receptors, but also the group II and group III allosteric modulators and agonists/antagonists reported over the past five years according to a classification of their structures, with a specific focus on their biological activity and selectivity. In particular, the structure of these compounds and the future directions of ideal candidates are highlighted.

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.

Re-exploration of the mGlu₁ PAM Ro 07-11401 scaffold: Discovery of analogs with improved CNS penetration despite steep SAR

This letter describes the re-exploration of the mGlu1 PAM Ro 07-11401 scaffold through a multi-dimensional, iterative parallel synthesis approach. Unlike recent series of mGlu1 PAMs with robust SAR, the SAR around the Ro 07-11401 structure was incredibly steep (only ∼6 of 200 analogs displayed mGlu1 PAM activity), and reminiscent of the CPPHA mGlu5 PAM scaffold. Despite the steep SAR, two new thiazole derivatives were discovered with improved in vitro DMPK profiles and ∼3- to 4-fold improvement in CNS exposure (Kps 1.01-1.19); albeit, with a ∼3-fold diminution in mGlu1 PAM potency, yet comparable efficacy (∼5-fold leftward shift of the glutamate concentration-response curve at 10μM). Thus, this effort has provided additional CNS penetrant mGlu1 PAM tools in a different chemotype than the VU0486321 scaffold. These compounds will permit a better understanding of the pharmacology and therapeutic potential of selective mGlu1 activation, while highlighting the steep SAR challenges that can often be encountered in GPCR allosteric modulator discovery.

N-Alkylpyrido[1',2':1,5]pyrazolo-[4,3-d]pyrimidin-4-amines: A new series of negative allosteric modulators of mGlu1/5 with CNS exposure in rodents

Selective negative allosteric modulators (NAMs) of each of the group I metabotropic glutamate receptors (mGlu1 and mGlu5) have been well characterized in the literature and offer potential as therapeutics in several disorders of the central nervous system (CNS). Still, compounds that are potent mGlu1/5 NAMs with selectivity versus the other six members of the mGlu family as well as the balance of properties required for use in vivo are lacking. A medicinal chemistry effort centered on the identification of a lead series with the potential of delivering such compounds is described in this Letter. Specifically, a new class of pyrido[1',2':1,5]pyrazolo[4,3-d]pyrimidin-4-amines was designed as a novel isosteric replacement for 4-aminoquinazolines, and compounds from within this chemotype exhibited dual NAM activity at both group I mGlus. One compound, VU0467558 (29), demonstrated near equipotent activity at both receptors, selectivity versus other mGlus, a favorable ancillary pharmacology profile, and CNS exposure in rodents.

Development of Novel, CNS Penetrant Positive Allosteric Modulators for the Metabotropic Glutamate Receptor Subtype 1 (mGlu1), Based on an N-(3-Chloro-4-(1,3-dioxoisoindolin-2-yl)phenyl)-3-methylfuran-2-carboxamide Scaffold, That Potentiate Wild Type and Mutant mGlu1 Receptors Found in Schizophrenics

The therapeutic potential of selective mGlu1 activation is vastly unexplored relative to the other group I mGlu receptor, mGlu5; therefore, our lab has focused considerable effort toward developing mGlu1 positive allosteric modulators (PAMs) suitable as in vivo proof of concept tool compounds. Optimization of a series of mGlu1 PAMs based on an N-(3-chloro-4-(1,3-dioxoisoindolin-2-yl)phenyl)-3-methylfuran-2-carboxamide scaffold provided 17e, a potent (mGlu1 EC50 = 31.8 nM) and highly CNS penetrant (brain to plasma ratio (Kp) of 1.02) mGlu1 PAM tool compound, that potentiated not only wild-type human mGlu1 but also mutant mGlu1 receptors derived from deleterious GRM1 mutations found in schizophrenic patients. Moreover, both electrophysiological and in vivo studies indicate the mGlu1 ago-PAMs/PAMs do not possess the same epileptiform adverse effect liability as mGlu5 ago-PAMs/PAMs and maintain temporal activity suggesting a broader therapeutic window.

The search for novel analgesics: targets and mechanisms

The management of the pain state is of great therapeutic relevance to virtually every medical specialty. Failure to manage its expression has deleterious consequence to the well-being of the organism. An understanding of the complex biology of the mechanisms underlying the processing of nociceptive information provides an important pathway towards development of novel and robust therapeutics. Importantly, preclinical models have been of considerable use in determining the linkage between mechanism and the associated behaviorally defined pain state. This review seeks to provide an overview of current thinking targeting pain biology, the use of preclinical models and the development of novel pain therapeutics. Issues pertinent to the strengths and weaknesses of current development strategies for analgesics are considered.

Identification of Nitazoxanide as a Group I Metabotropic Glutamate Receptor Negative Modulator for the Treatment of Neuropathic Pain: An In Silico Drug Repositioning Study

PURPOSE

Drug repositioning strategies were employed to explore new therapeutic indications for existing drugs that may exhibit dual negative mGluR1/5 modulating activities as potential treatments for neuropathic pain.

METHOD

A customized in silico-in vitro-in vivo drug repositioning scheme was assembled and implemented to search available drug libraries for compounds with dual mGluR1/5 antagonistic activities, that were then evaluated using in vitro functional assays and, for validated hits, in an established animal model for neuropathic pain.

RESULTS

Tizoxanide, the primary active metabolite of the FDA approved drug nitazoxanide, fit in silico pharmacophore models constructed for both mGluR1 and mGluR5. Subsequent calcium (Ca++) mobilization functional assays confirmed that tizoxanide exhibited appreciable antagonist activity for both mGluR1 and mGluR5 (IC50 = 1.8 μM and 1.2 μM, respectively). The in vivo efficacy of nitazoxanide administered by intraperitoneal injection was demonstrated in a rat model for neuropathic pain.

CONCLUSION

The major aim of the present study was to demonstrate the utility of an in silico-in vitro-in vivo drug repositioning protocol to facilitate the repurposing of approved drugs for new therapeutic indications. As an example, this particular investigation successfully identified nitazoxanide and its metabolite tizoxanide as dual mGluR1/5 negative modulators. A key finding is the vital importance for drug screening libraries to include the structures of drug active metabolites, such as those emanating from prodrugs which are estimated to represent 5-7% of marketed drugs.

Chemical modulation of mutant mGlu1 receptors derived from deleterious GRM1 mutations found in schizophrenics

Schizophrenia is a complex and highly heterogeneous psychiatric disorder whose precise etiology remains elusive. While genome-wide association studies (GWAS) have identified risk genes, they have failed to determine if rare coding single nucleotide polymorphisms (nsSNPs) contribute in schizophrenia. Recently, two independent studies identified 12 rare, deleterious nsSNPS in the GRM1 gene, which encodes the metabotropic glutamate receptor subtype 1 (mGlu1), in schizophrenic patients. Here, we generated stable cell lines expressing the mGlu1 mutant receptors and assessed their pharmacology. Using both the endogenous agonist glutamate and the synthetic agonist DHPG, we found that several of the mutant mGlu1 receptors displayed a loss of function that was not due to a loss in plasma membrane expression. Due to a lack of mGlu1 positive allosteric modulators (PAM) tool compounds active at human mGlu1, we optimized a known mGlu4 PAM/mGlu1 NAM chemotype into a series of potent and selective mGlu1 PAMs by virtue of a double "molecular switch". Employing mGlu1 PAMs from multiple chemotypes, we demonstrate that the mutant receptors can be potentiated by small molecules and in some cases efficacy restored to that comparable to wild type mGlu1 receptors, suggesting deficits in patients with schizophrenia due to these mutations may be amenable to intervention with an mGlu1 PAM. However, in wild type animals, mGlu1 negative allosteric modulators (NAMs) are efficacious in classic models predictive of antipsychotic activity, whereas we show that mGlu1 PAMs have no effect to slight potentiation in these models. These data further highlight the heterogeneity of schizophrenia and the critical role of patient selection strategies in psychiatric clinical trials to match genotype with therapeutic mechanism.

Allosteric modulation of Class C GPCRs: a novel approach for the treatment of CNS disorders

Allosteric modulation has emerged as an innovative pharmacological approach to selectively activate or inhibit several Class C GPCRs. Of the Class C GPCRs, metabotropic glutamate (mGlu) receptors represent the most promising candidates for clinical success, and both positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) of mGluRs have demonstrated therapeutic potential for a range of psychiatric and neurological disorders such as pain, depression, anxiety, cognition, Fragile X syndrome, Parkinson’s disease and schizophrenia.

Molecular recognition of metabotropic glutamate receptor type 1 (mGluR1): synergistic understanding with free energy perturbation and linear response modeling

Metabotropic glutamate receptors (mGluRs) constitute an important family of the G-protein coupled receptors. Due to their widespread distribution in the central nervous system (CNS), these receptors are attractive candidates for understanding the molecular basis of various cognitive processes as well as for designing inhibitors for relevant psychiatric and neurological disorders. Despite many studies on drugs targeting the mGluR receptors to date, the molecular level details on the ligand binding dynamics still remain unclear. In this study, we performed in silico experiments for mGluR1 with 29 different ligands including known synthetic agonists and antagonists as well as natural amino acids. The ligand-receptor binding affinities were estimated by the use of atomistic simulations combined with the mathematically rigorous, Free Energy Perturbation (FEP) method, which successfully recognized the native agonist l-glutamate among the highly favorable binders, and also accurately distinguished antagonists from agonists. Comparative contact analysis also revealed the binding mode differences between natural and non-natural amino acid-based ligands. Several factors potentially affecting the ligand binding affinity and specificity were identified including net charges, dipole moments, and the presence of aromatic rings. On the basis of these findings, linear response models (LRMs) were built for different sets of ligands that showed high correlations (R(2) > 0.95) to the corresponding FEP binding affinities. These results identify some key factors that determine ligand-mGluR1 binding and could be used for future inhibitor designs and support a role for in silico modeling for understanding receptor ligand interactions.

Allosteric modulators for the treatment of schizophrenia: targeting glutamatergic networks

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.

Octahydropyrrolo[3,4-c]pyrrole negative allosteric modulators of mGlu1

Development of SAR in an octahydropyrrolo[3,4-c]pyrrole series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. The octahydropyrrolo[3,4-c]pyrrole scaffold was chosen as an isosteric replacement for the piperazine ring found in the initial hit compound. Characterization of selected compounds in protein binding assays was used to identify the most promising analogs, which were then profiled in P450 inhibition assays in order to further assess the potential for drug-likeness within this series of compounds.

N-Acyl-N'-arylpiperazines as negative allosteric modulators of mGlu1: identification of VU0469650, a potent and selective tool compound with CNS exposure in rats

Development of SAR in an N-acyl-N'-arylpiperazine series of negative allosteric modulators of mGlu1 using a functional cell-based assay is described in this Letter. Characterization of selected compounds in protein binding assays was used to aid in selecting VU0469650 for further profiling in ancillary pharmacology assays and pharmacokinetic studies. VU0469650 demonstrated an excellent selectivity profile and good exposure in both plasma and brain samples following intraperitoneal dosing in rats.

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).