From bench to bedside: mGluR2 positive allosteric modulators as medications to treat substance use disorders

Clinical investigations of compounds targeting metabotropic glutamate receptors

Pharmacological modulation of glutamate has long been considered to be of immense therapeutic utility. The metabotropic glutamate receptors (mGluRs) are potential targets for safely altering glutamate-driven excitation. Data support the potential therapeutic use of mGluR modulators in the treatment of anxiety, depression, schizophrenia, and other psychiatric disorders, pain, epilepsy, as well as neurodegenerative and neurodevelopmental disorders. For each of the three mGluR groups, compounds have been constructed that produce either potentiation or functional blockade. PET ligands for mGlu5Rs have been studied in a range of patient populations and several mGlu5R antagonists have been tested for potential efficacy in patients including mavoglurant, diploglurant, basimglurant, GET 73, and ADX10059. Efficacy with mGlu5R antagonists has been reported in trials with patients with gastroesophageal reflux disease; data from patients with Parkinson's disease or Fragile X syndrome have not been as robust as hoped. Fenobam was approved for use as an anxiolytic prior to its recognition as an mGlu5R antagonist. mGlu2/3R agonists (pomaglumated methionil) and mGlu2R agonists (JNJ-40411813, AZD 8529, and LY2979165) have been studied in patients with schizophrenia with promising but mixed results. Antagonists of mGlu2/3Rs (decoglurant and TS-161) have been studied in depression where TS-161 has advanced into a planned Phase 2 study in treatment-resistant depression. The Group III mGluRs are the least developed of the mGluR receptor targets. The mGlu4R potentiator, foliglurax, did not meet its primary endpoint in patients with Parkinson's disease. Ongoing efforts to develop mGluR-targeted compounds continue to promise these glutamate modulators as medicines for psychiatric and neurological disorders.

Allosteric Modulators of Metabotropic Glutamate Receptors as Novel Therapeutics for Neuropsychiatric Disease

Metabotropic glutamate (mGlu) receptors, a family of G-protein-coupled receptors, have been identified as novel therapeutic targets based on extensive research supporting their diverse contributions to cell signaling and physiology throughout the nervous system and important roles in regulating complex behaviors, such as cognition, reward, and movement. Thus, targeting mGlu receptors may be a promising strategy for the treatment of several brain disorders. Ongoing advances in the discovery of subtype-selective allosteric modulators for mGlu receptors has provided an unprecedented opportunity for highly specific modulation of signaling by individual mGlu receptor subtypes in the brain by targeting sites distinct from orthosteric or endogenous ligand binding sites on mGlu receptors. These pharmacological agents provide the unparalleled opportunity to selectively regulate neuronal excitability, synaptic transmission, and subsequent behavioral output pertinent to many brain disorders. Here, we review preclinical and clinical evidence supporting the utility of mGlu receptor allosteric modulators as novel therapeutic approaches to treat neuropsychiatric diseases, such as schizophrenia, substance use disorders, and stress-related disorders.

Effect of mGluR2 positive allosteric modulation on frontostriatal working memory activation in schizophrenia

Negative symptoms and cognitive deficits contribute strongly to disability in schizophrenia, and are resistant to existing medications. Recent drug development has targeted enhanced NMDA function by increasing mGluR2/3 signaling. However, the clinical utility of such agents remains uncertain, and markers of brain circuit function are critical for clarifying mechanisms and understanding individual differences in efficacy. We conducted a double-blind, placebo-controlled, randomized cross-over (14 day washout) pilot study evaluating adjunctive use of the mGluR2 positive allosteric modulator AZD8529 (80 mg daily for 3 days), in chronic stable patients with schizophrenia (n = 26 analyzed). We focused on 3 T fMRI response in frontostriatal regions during an n-back working memory task, testing the hypothesis that AZD8529 produces fMRI changes that correlate with improvement in negative symptoms and cognition. We found that AZD8529 did not produce significant group-average effects on symptoms or cognitive accuracy. However, AZD8529 did increase n-back fMRI activation in striatum (p < 0.0001) and anterior cingulate/paracingulate (p = 0.002). Greater drug-versus-placebo effects on caudate activation significantly correlated with greater reductions in PANSS negative symptom scores (r = -0.42, p = 0.031), and exploratory correlations suggested broader effects across multiple symptom domains and regions of interest. These findings demonstrate that fMRI responses to mGluR2 positive modulation relate to individual differences in symptom reduction, and could be pursued for future biomarker development. Negative clinical results at the group level should not lead to premature termination of investigation of this mechanism, which may benefit an important subset of individuals with schizophrenia. Imaging biomarkers may reveal therapeutic mechanisms, and help guide treatment toward specific populations.

Exploring the binding mechanism of positive allosteric modulators in human metabotropic glutamate receptor 2 using molecular dynamics simulations

Positive allosteric modulators (PAMs) of human metabotropic glutamate receptor 2 (hmGlu₂) are well-known in the treatment of psychiatric disorders for their higher selectivity and lower tolerance risk. A variety of PAMs have been reported over the last decade and two compounds were in Phase II clinical trials for schizophrenia and anxiety. These trials were discontinued on account of the unsatisfactory therapeutic efficacy, but PAMs were explored as novel treatments for addiction and epilepsy. Thus, it is still important to explore novel hmGlu₂ PAMs in the near future. Nowadays, the challenges in optimizing drug potency and improving scaffold diversity for PAMs are the noncomprehensive character analyses of multiple scaffolds; the exploration of the binding modes of PAMs in the allosteric binding site have been proposed to reduce this difficulty. However, there has been no comprehensive research about the binding profiles of PAMs in the hmGlu₂ receptor. To address this issue, this work explores the binding characters of eight PAMs representing five chemical series by multiple computational methods. As a result, the shared binding modes of the eight studied PAMs interacting with 15 residues in the allosteric binding site were defined. In addition, the reduced hydrophobicity with low electronegativity of R₁, increased hydrophobicity with low negative electron density of R₂ and the electronegativity of the linker were identified as indicators that regulate the affinity of PAMs. This finding agrees well with the physicochemical properties of reported multiple series PAMs. This comprehensive work sheds additional light on the binding mechanism and physicochemical regularity underlining PAMs affinity and could be further utilized as a structural and energetic blueprint for discovering and assessing novel PAMs for hmGlu₂.

Development of Clickable Photoaffinity Ligands for Metabotropic Glutamate Receptor 2 Based on Two Positive Allosteric Modulator Chemotypes

The metabotropic glutamate receptor 2 (mGlu₂) is a transmembrane-spanning class C G protein-coupled receptor that is an attractive therapeutic target for multiple psychiatric and neurological disorders. A key challenge has been deciphering the contribution of mGlu₂ relative to other closely related mGlu receptors in mediating different physiological responses, which could be achieved through the utilization of subtype selective pharmacological tools. In this respect, allosteric modulators that recognize ligand-binding sites distinct from the endogenous neurotransmitter glutamate offer the promise of higher receptor-subtype selectivity. We hypothesized that mGlu₂-selective positive allosteric modulators could be derivatized to generate bifunctional pharmacological tools. Here we developed clickable photoaffinity probes for mGlu₂ based on two different positive allosteric modulator scaffolds that retained similar pharmacological activity to parent compounds. We demonstrate successful probe-dependent incorporation of a commercially available clickable fluorophore using bioorthogonal conjugation. Importantly, we also show the limitations of using these probes to assess in situ fluorescence of mGlu₂ in intact cells where significant nonspecific membrane binding is evident.

mGluR2 positive allosteric modulators: an updated patent review (2013-2018)

INTRODUCTION

Positive allosteric modulation of mGlu2 has attracted much interest as an alternative approach to classical orthosteric receptor activation. Two mGlu2 PAMS have advanced into the clinic. The results obtained in schizophrenia and MDD phase 2 clinical trials have tempered the high expectations put on selective mGlu2 receptor activation for treating these conditions; nevertheless, the search for novel therapeutic indications and novel chemotypes continues to be an active field of research.

AREAS COVERED

2013-2018 patent literature on mGlu2 receptor PAMs.

EXPERT OPINION

After a decade of intensive research, the mGlu2 PAM field has seen a deceleration in the last five years. Negative phase 2 schizophrenia clinical trials with JNJ-40411813 and AZD8529 seem to have tempered the high expectations of the scientific community on the utility of mGlu2 PAMs for the treatment of schizophrenia. Nevertheless, novel therapeutic indications continue to be explored and AZD8529 is currently in a phase 2 study for smoking cessation. The advances in medicinal chemistry and in pharmacology, with novel indications such as epilepsy, have set the stage in the field of mGlu2 receptor PAMs. Ongoing preclinical and clinical studies will contribute to define their optimal therapeutic indication and potential to become novel therapeutic agents.

Neurobiological and Neurophysiological Mechanisms Underlying Nicotine Seeking and Smoking Relapse

Tobacco-related morbidity and mortality continue to be a significant public health concern. Unfortunately, current FDA-approved smoking cessation pharmacotherapies have limited efficacy and are associated with high rates of relapse. Therefore, a better understanding of the neurobiological and neurophysiological mechanisms that promote smoking relapse is needed to develop novel smoking cessation medications. Here, we review preclinical studies focused on identifying the neurotransmitter and neuromodulator systems that mediate nicotine relapse, often modeled in laboratory animals using the reinstatement paradigm, as well as the plasticity-dependent neurophysiological mechanisms that facilitate nicotine reinstatement. Particular emphasis is placed on how these neuroadaptations relate to smoking relapse in humans. We also highlight a number of important gaps in our understanding of the neural mechanisms underlying nicotine reinstatement and critical future directions, which may lead toward the development of novel, target pharmacotherapies for smoking cessation.

Effect of Novel Allosteric Modulators of Metabotropic Glutamate Receptors on Drug Self-administration and Relapse: A Review of Preclinical Studies and Their Clinical Implications

Results from preclinical rodent studies during the last 20 years implicated glutamate neurotransmission in different brain regions in drug self-administration and rodent models of relapse. These results, along with evidence for drug-induced neuroadaptations in glutamatergic neurons and receptors, suggested that addiction might be treatable by medications that inhibit glutamatergic responses to drugs of abuse, drug-associated cues, and stressors. This idea is supported by findings in rodent and primate models that drug self-administration and relapse are reduced by systemic injections of antagonists of ionotropic glutamate receptors or metabotropic glutamate receptors (mGluRs) or orthosteric agonists of mGluR2/3. However, these compounds have not advanced to clinical use because of potential side effects and other factors. This state of affairs has led to the development of positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) of mGluRs. PAMs and NAMs of mGluRs, either of which can inhibit evoked glutamate release, may be suitable for testing in humans. We reviewed results from recent studies of systemically injected PAMs and NAMs of mGluRs in rodents and monkeys, focusing on whether they reduce drug self-administration, reinstatement of drug seeking, and incubation of drug craving. We also review results from rat studies in which PAMs or NAMs of mGluRs were injected intracranially to reduce drug self-administration and reinstatement. We conclude that PAMs and NAMs of mGluRs should be considered for clinical trials.

Metabotropic Glutamate Receptors 2 and 3 as Targets for Treating Nicotine Addiction

Tobacco smoking, driven by the addictive properties of nicotine, continues to be a worldwide health problem. Based on the well-established role of glutamatergic neurotransmission in drug addiction, novel medication development strategies seek to halt nicotine consumption and prevent relapse to tobacco smoking by modulating glutamate transmission. The presynaptic inhibitory metabotropic glutamate receptors 2 and 3 (mGluR2/3) are key autoreceptors on glutamatergic terminals that maintain glutamate homeostasis. Accumulating evidence suggests the critical role of mGluR2/3 in different aspects of nicotine addiction, including acquisition and maintenance of nicotine taking, nicotine withdrawal, and persistent nicotine seeking even after prolonged abstinence. The involvement of mGluR2/3 in other neuropsychiatric conditions, such as anxiety, depression, schizophrenia, Alzheimer's disease, Parkinson's disease, and pain, provides convincing evidence suggesting that mGluR2/3 may provide an effective therapeutic approach for comorbidity of smoking and these conditions. This focused review article highlights that mGluR2/3 provide a promising target in the search for smoking cessation medication with novel mechanisms of actions that differ from those of currently U.S. Food and Drug Administration-approved pharmacotherapies.

Animal Models of Addiction and Neuropsychiatric Disorders and Their Role in Drug Discovery: Honoring the Legacy of Athina Markou

Each of the co-authors worked with Athina Markou, at different stages of our careers and in different capacities, to develop, optimize, and use animal models of drug addiction and, more generally, mental health disorders such as anxiety, depression, and schizophrenia. Here, we briefly summarize some of our work with Athina, primarily involving the use of the intracranial self-stimulation and intravenous drug self-administration procedures. This work established that excessive consumption of addictive drugs can induce profound dysfunction in brain reward circuits. Such drug-induced reward deficits are likely to play a key role in precipitating the emergence of compulsive drug-seeking behaviors. We also summarize findings suggesting that perturbations in glutamatergic transmission contribute to brain reward deficits in drug-dependent animals and that metabotropic glutamate receptors are potential targets for the development of novel medications to facilitate long-term drug abstinence and prevention of relapse.