Mavoglurant Augmentation in OCD Patients Resistant to Selective Serotonin Reuptake Inhibitors: A Proof-of-Concept, Randomized, Placebo-Controlled, Phase 2 Study

Mavoglurant reduces cocaine use in patients with cocaine use disorder in a phase 2 clinical trial

Metabotropic glutamate receptor 5 (mGluR5) is involved in cocaine reward processing and addiction. Preclinical studies suggest that blocking this receptor inhibits cocaine self-administration and seeking behavior in rodents. We assessed a selective noncompetitive antagonist of mGluR5 called mavoglurant in a phase 2 randomized, placebo-controlled clinical trial of 68 adults with cocaine use disorder. Study participants were randomly assigned in a 1:1 ratio to an up-titrating schedule of oral mavoglurant twice daily up to 200 mg for 98 days or placebo. The primary end point was the proportion of cocaine use days over the treatment period assessed by a retrospective self-report using Timeline Followback. Secondary end points were urine analysis of the cocaine metabolite benzoylecgonine and alcohol use measured by Timeline Followback assessment. Exploratory end points included testing for cocaine and alcohol metabolites in hair samples. The posterior probability of mavoglurant reducing cocaine use at the end of treatment was ≥99.0% for a treatment difference <0 and ≥36.6% for a treatment difference <-10%. The difference between mavoglurant and placebo was also assessed using analysis of covariance (P = 0.021). Urine benzoylecgonine concentration was lower in the mavoglurant-treated group versus placebo (P = 0.025); there was reduced alcohol consumption in the treatment group (P = 0.072). Seventy-six percent (randomized set) and 79% (safety analysis set) of patients completed the final treatment visit. Adverse events in the treatment group were headache, dizziness, and nausea. In this small and short trial, mavoglurant reduced cocaine and alcohol use in patients with chronic cocaine use disorder.

Discovery of 4-(5-Membered)Heteroarylether-6-methylpicolinamide Negative Allosteric Modulators of Metabotropic Glutamate Receptor Subtype 5

This Letter details our efforts to develop novel, non-acetylene-containing metabotropic glutamate receptor subtype 5 (mGlu5) negative allosteric modulators (NAMs) with improved pharmacological properties. This endeavor involved replacing the ether-linked pyrimidine moiety, a metabolic liability, with various 5-membered heterocycles. From this exercise, we identified VU6043653, a highly brain penetrant and selective mGlu5 NAM which displayed moderate potency against both human and rat mGlu5. Moreover, VU6043653 has overall improved pharmacological and drug metabolism and pharmacokinetic profiles when compared to its predecessor compounds. Most notably, VU6043653 exhibits low predicted human hepatic clearance, a clean cytochrome P450 profile, and minimal inhibition of the dopamine transporter.

Pharmacological augmentation of serotonin reuptake inhibitors in patients with obsessive-compulsive disorder: A network meta-analysis

OBJECTIVES

The augmentation of serotonin reuptake inhibitors (SRIs) can be achieved by add-on therapy with different pharmacological agents in obsessive-compulsive disorder (OCD) for a better clinical outcome. This network meta-analysis (NMA) was conducted to evaluate and compare the effects of available augmentation agents for SRIs in OCD.

METHOD

The data was extracted from 59 relevant clinical trials after a literature search on MEDLINE/PubMed, Scopus, Cochrane databases and clinical trial registries. PRISMA guidelines were followed in data extraction, analysis and reporting. Random effects Bayesian NMA was done to pool the effects across the interventions for the change in Yale-Brown Obsessive-Compulsive Scale (YBOCS) scoring from baseline to the end of the study. Network graph was built, consistency model was run, node splitting analysis was performed, treatments were ranked as per SUCRA score and meta-regression was done for refractoriness to SRIs and duration of augmentation therapy as the predictor variables.

RESULTS

The drugs showing significant reduction in YBOCS scoring were pregabalin (MD:-8.1;95% CrI: -16, -0.43), memantine (MD:-6.2;95% CrI: -9.9, -2.3), lamotrigine (MD:-6;95% CrI: -12, -0.47), ondansetron (MD:-5.7;95% CrI: -11, -0.67), granisetron (MD:-5.6;95% CrI: -11, -0.44), aripiprazole (MD:-5.4;95% CrI:-9.1, -1.6), risperidone (MD:-3.3;95% CrI: -6.4, -0.20) and topiramate (MD:-5.3;95% CrI: -9.6, -0.97). The node-split analysis showed that direct and indirect pooled effect sizes for all comparisons were comparable. Meta-regression showed a statistically non-significant association between YBOCS score reduction with the duration of augmentation therapy, but significant with SRI-refractory status. Finally, the results were sorted based on certainty of evidence.

CONCLUSION

Memantine was found to be most effective augmentation agent for SRIs in OCD, followed by lamotrigine, ondansetron and granisetron with moderate certainty of evidence. The augmentation agents showed better symptom reduction in patients with SRI-refractory OCD in comparison to non-refractory OCD.

PROSPERO REGISTRATION

CRD42022360110.

The metabotropic glutamate receptor 5 as a biomarker for psychiatric disorders

The role of glutamate system in the etiology and pathophysiology of psychiatric disorders has gained considerable attention in the past two decades, including dysregulation of the metabotropic glutamatergic receptor subtype 5 (mGlu5). Thus, mGlu5 may represent a promising therapeutic target for psychiatric conditions, particularly stress-related disorders. Here, we describe mGlu5 findings in mood disorders, anxiety, and trauma disorders, as well as substance use (specifically nicotine, cannabis, and alcohol use). We highlight insights gained from positron emission tomography (PET) studies, where possible, and discuss findings from treatment trials, when available, to explore the role of mGlu5 in these psychiatric disorders. Through the research evidence reviewed in this chapter, we make the argument that, not only is dysregulation of mGlu5 evident in numerous psychiatric disorders, potentially functioning as a disease "biomarker," the normalization of glutamate neurotransmission via changes in mGlu5 expression and/or modulation of mGlu5 signaling may be a needed component in treating some psychiatric disorders or symptoms. Finally, we hope to demonstrate the utility of PET as an important tool for investigating mGlu5 in disease mechanisms and treatment response.

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.

Targeting the Type 5 Metabotropic Glutamate Receptor: A Potential Therapeutic Strategy for Neurodegenerative Diseases?

The type 5 metabotropic glutamate receptor, mGlu₅, has been proposed as a potential therapeutic target for the treatment of several neurodegenerative diseases. In preclinical neurodegenerative disease models, novel allosteric modulators have been shown to improve cognitive performance and reduce disease-related pathology. A common pathological hallmark of neurodegenerative diseases is a chronic neuroinflammatory response, involving glial cells such as astrocytes and microglia. Since mGlu₅ is expressed in astrocytes, targeting this receptor could provide a potential mechanism by which neuroinflammatory processes in neurodegenerative disease may be modulated. This review will discuss current evidence that highlights the potential of mGlu₅ allosteric modulators to treat neurodegenerative diseases, including Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Furthermore, this review will explore the role of mGlu₅ in neuroinflammatory responses, and the potential for this G protein-coupled receptor to modulate neuroinflammation.

Quantification of Metabotropic Glutamate Receptor 5 Availability With Both [11C]ABP688 and [18F]FPEB Positron Emission Tomography in the Sapap3 Knockout Mouse Model for Obsessive-Compulsive-like Behavior

BACKGROUND

This study provides a first direct comparison between positron emission tomography radioligands targeting the allosteric site of the metabotropic glutamate receptor 5 (mGluR5): [¹¹C]ABP688 and [¹⁸F]FPEB. A blocking paradigm was set up to substantiate the common binding site of both radioligands. Second, both radioligands were applied in Sapap3 knockout (KO) mice showing compulsive-like behavior characterized by a lower in vivo mGluR5 availability.

METHODS

First, wild-type mice (n = 7) received four position emission tomography/computed tomography scans: a [¹¹C]ABP688 scan, a [¹⁸F]FPEB scan, and two blocking scans using cold FPEB and cold ABP688, respectively. A second experiment compared both radioligands in wild-type (n = 7) and KO (n = 10) mice. The simplified reference tissue model was used to calculate the nondisplaceable binding potential representing the in vivo availability of mGluR5 in the brain.

RESULTS

Using cold FPEB as a blocking compound for [¹¹C]ABP688 micro-positron emission tomography and vice versa, we observed averaged global reductions in mGluR5 availability of circa 98% for [¹¹C]ABP688 and 82%-96% for [¹⁸F]FPEB. For KOs, the [¹¹C]ABP688 nondisplaceable binding potential was on average 25% lower compared with wild-type control mice (p < .0001-.001), while this was about 17% for [¹⁸F]FPEB (p < .05).

CONCLUSIONS

The current findings substantiate a common binding site and suggest a strong relationship between mGluR5 availability levels measured with both radioligands. In Sapap3 KO mice, a reduced mGluR5 availability could therefore be demonstrated with both radioligands. With [¹¹C]ABP688, higher significance levels were achieved in more brain regions. These findings suggest [¹¹C]ABP688 as a preferable radiotracer to quantify mGluR5 availability, as exemplified here in a model for compulsive-like behavior.

Targeting mGlu5 for Methamphetamine Use Disorder

Methamphetamine abuse leads to devastating consequences, including addiction, crime, and death. Despite decades of research, no medication has been approved by the U.S. Food and Drug Administration for the treatment of Methamphetamine Use Disorder. Thus, there is a need for new therapeutic approaches. Animal studies demonstrate that methamphetamine exposure dysregulates forebrain function involving the Group-I metabotropic glutamate receptor subtype 5 (mGlu₅), which is predominantly localized to postsynaptic sites. Allosteric modulators of mGlu₅ offer a unique opportunity to modulate glutamatergic neurotransmission selectively, thereby potentially ameliorating methamphetamine-induced disruptions. Negative allosteric modulators of mGlu₅ attenuate the effects of methamphetamine, including rewarding/reinforcing properties of the drug across animal models, and have shown promising effects in clinical trials for Anxiety Disorder and Major Depressive Disorder. Preclinical studies have also sparked great interest in mGlu₅ positive allosteric modulators, which exhibit antipsychotic and anxiolytic properties, and facilitate extinction learning when access to methamphetamine is removed, possibly via the amelioration of methamphetamine-induced cognitive deficits. Clinical research is now needed to elucidate the mechanisms underlying the mGlu₅ receptor-related effects of methamphetamine and the contributions of these effects to addictive behaviors. The growing array of mGlu₅ allosteric modulators provides excellent tools for this purpose and may offer the prospect of developing tailored and effective medications for Methamphetamine Use Disorder.

Regional brain mGlu5 receptor occupancy following single oral doses of mavoglurant as measured by [11C]-ABP688 PET imaging in healthy volunteers

Mavoglurant binds to same allosteric site on metabotropic glutamate receptor 5 (mGluR5) as [¹¹C]-ABP688, a radioligand. This open-label, single-center pilot study estimates extent of occupancy of mGluR5 receptors following single oral doses of mavoglurant, using [¹¹C]-ABP688 positron emission tomography (PET) imaging, in six healthy males aged 20-40 years. This study comprised three periods and six subjects were divided into two cohorts. On Day 1 (Period 1), baseline clinical data and safety samples were obtained along with PET scan. During Period 2 (1-7 days after Period 1), cohort 1 and 2 received mavoglurant 25 mg and 100 mg, respectively. During Period 3 (7 days after Period 2), cohort 1 and 2 received mavoglurant 200 mg and 400 mg, respectively. Mavoglurant showed the highest distribution volumes in the cingulate region with lower uptake in cerebellum and white matter, possibly because myelinated axonal sheets maybe devoid of mGlu5 receptors. Maximum concentrations of mavoglurant were observed around 2-3.25 h post-dose. Mavoglurant passed the blood-brain barrier and induced dose- and exposure-dependent displacement of [¹¹C]-ABP688 from the mGluR5 receptors, 3-4 h post-administration (27%, 59%, 74%, 85% receptor occupancy for mavoglurant 25 mg, 100 mg, 200 mg, 400 mg dose, respectively). There were no severe adverse effects or clinically significant changes in safety parameters. This is the first human receptor occupancy study completed with Mavoglurant. It served to guide the dosing of mavoglurant in the past and currently ongoing clinical studies. Furthermore, it confirms the utility of [¹¹C]-ABP688 as a unique tool to study drug-induced occupancy of mGlu5 receptors in the living human brain.

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.

Progression of obsessive compulsive disorder-like grooming in Sapap3 knockout mice: A longitudinal [11C]ABP688 PET study

We aimed to evaluate [3-(6-methyl-pyridin-2-ylethynyl)-cyclohex-2-enone-0-¹¹C-methyloxime] ([¹¹C]ABP688) small animal positron emission tomography (μPET) as a biomarker to visualize possible longitudinal changes in metabotropic glutamate receptor 5 (mGluR5) availability in the brain of SAP90/PSD-95 associated protein 3 (Sapap3) knockout (ko) mice, showing obsessive compulsive disorder (OCD)-like behavior.

METHODS

Alongside the assessment of grooming, we performed [¹¹C]ABP688 μPET/CT imaging in wildtype (wt; n=10) and ko (n=11) mice both at 3 and 9 months. Using the simplified reference tissue method (SRTM), the nondisplaceable binding potential (BP_ND) was calculated representing the in vivo availability of the metabotropic glutamate receptor 5 (mGluR5) in the brain with the cerebellum as a reference region. Longitudinal voxel-based statistical parametric mapping (SPM) was performed on BP_ND images. Results were verified using [¹¹C]ABP688 ex vivo autoradiography, [³H]ABP688 in vitro autoradiography, and mGluR5 immunohistochemistry.

RESULTS

Cross-sectional comparisons revealed significantly increased grooming parameters in ko animals, at both time points. A significant longitudinal increase in % grooming duration (+268.25%; p<0.05) reflected aggravation of this behavior in ko mice. [¹¹C]ABP688 μPET revealed significantly lower mGluR5 availability in the cortex, striatum, hippocampus, and amygdala of ko mice at both ages. A significant longitudinal BP_ND decline was present for ko mice (p<0.01: cortex -17.14%, striatum -19.82%, amygdala -23.57%; p<0.05: hippocampus -15.53%), which was confirmed by SPM (p<0.01).

CONCLUSION

Sapap3 ko mice show a decline in mGluR5 availability in OCD relevant brain regions parallel to the worsening of OCD-like behavior. This demonstrates a potential role for [¹¹C]ABP688 PET as a biomarker to monitor disease progression in vivo.

Structure-based discovery and development of metabotropic glutamate receptor 5 negative allosteric modulators

The metabotropic glutamate (mGlu) receptors are a family of eight class C G protein-coupled receptors (GPCRs) which modulate cell signaling and synaptic transmission to the major excitatory neurotransmitter l-glutamate (l-glutamic acid). Due to their role in modulating glutamate response, their widespread distribution in the central nervous system (CNS) and some evidence of dysregulation in disease, the mGlu receptors have become attractive pharmacological targets. As the orthosteric (glutamate) binding site is highly conserved across the eight mGlu receptors, it is difficult not only to generate ligands with subtype selectivity but, due to the nature of the binding site, with suitable drug-like properties to allow oral bioavailability and CNS penetration. Selective pharmacological targeting of a single receptor subtype can be achieved by targeting alternative (allosteric) binding sites. The nature of the allosteric binding pockets allows ligands to be developed that have good physical chemical properties as evidenced by several allosteric modulators of mGlu receptors entering clinical trials. The first negative allosteric modulators of the metabotropic glutamate 5 (mGlu₅) receptor were discovered from high throughput screening activities. An alternative approach to drug discovery is to use structural knowledge to enable structure-based drug design (SBDD), which allows the design of molecules in a more rational, rather than empirical, fashion. Here we will describe the process of SBDD in the discovery of the mGlu₅ negative allosteric modulator HTL0014242 and describe how knowledge of receptor structure can also be used to gain insights into the receptor activation mechanisms.

Psychopharmacological Treatment of Obsessive-Compulsive Disorder (OCD)

Background:

Obsessive-compulsive disorder (OCD) is associated with affective and cognitive symptoms causing personal distress and reduced global functioning. These have considerable societal costs due to healthcare service utilization.

Objective:

Our aim was to assess the efficacy of pharmacological interventions in OCD and clinical guidelines, providing a comprehensive overview of this field.

Methods:

We searched the PubMed database for papers dealing with drug treatment of OCD, with a specific focus on clinical guidelines, treatments with antidepressants, antipsychotics, mood stabilizers, off-label medications, and pharmacogenomics.

Results:

Prolonged administration of selective serotonin reuptake inhibitors (SSRIs) is most effective. Better results can be obtained with a SSRI combined with cognitive behavioral therapy (CBT) or the similarly oriented exposure and response prevention (ERP). Refractory OCD could be treated with different strategies, including a switch to another SSRI or clomipramine, or augmentation with an atypical antipsychotic. The addition of medications other than antipsychotics or intravenous antidepressant administration needs further investigation, as the evidence is inconsistent. Pharmacogenomics and personalization of therapy could reduce treatment resistance.

Conclusions:

SSRI/clomipramine in combination with CBT/ERP is associated with the optimal response compared to each treatment alone or to other treatments. New strategies for refractory OCD are needed. The role of pharmacogenomics could become preponderant in the coming years.

Placebo Effect in Obsessive-Compulsive Disorder (OCD). Placebo Response and Placebo Responders in OCD: The Trend Over Time

BACKGROUND

Placebo response appears to be increasing in antidepressant, antipsychotic and various internal medicine trials. A similar trend has been reported for OCD during 1989-1999. Placebo response is generally considered as the extent to which placebo treatment is associated with core symptom improvement. In this analysis, we used Joinpoint regression to assess the time trend of both placebo response and placebo responder rates according to the year of publication with no time restriction in OCD drug trials.

METHODS

We included drug and/or psychotherapy trials vs. placebo from PubMed, Embase, CINAHL, and PsycINFO retrieved through the search (placebo OR sham) AND (obsessive* OR OCD). We included studies through investigator consensus. We then performed on data of included studies log-linear joinpoint segmented regression models using a p<0.05 cutoff.

RESULTS

We included 113 studies from 112 published papers. Placebo mean annual response rates in OCD studies significantly increased from 1991 to 2017 with an annual percent change (APC) of 0.66%, while placebo mean annual responder rates also significantly increased from 2010 to 2017, with an APC of 5.45%. Drug mean annual response rates in OCD studies significantly increased from 1987 to 2012 with an APC of 0.72%, while the corresponding responder rates did not show statistically significant APC changes between 1984 and 2017.

CONCLUSION

We observed a tendency for placebo to increase both measures of response in OCD clinical drug trials through the years that tend to approximate the responses shown by drugs. Changes in the type of study (moving from classical head to head comparisons to add-on studies in treatmentresistant populations) and countries involved in experimentation may partially account for some portion of these results. It appears that placebo effects are becoming more elusive and out of control.

Fragment-Based Approaches for Allosteric Metabotropic Glutamate Receptor (mGluR) Modulators

Metabotropic glutamate receptors (mGluR) are members of the class C G-Protein Coupled Receptors (GPCR-s) and have eight subtypes. These receptors are responsible for a variety of functions in the central and peripheral nervous systems and their modulation has therapeutic utility in neurological and psychiatric disorders. It was previously established that selective orthosteric modulation of these receptors is challenging, and this stimulated the search for allosteric modulators. Fragment-Based Drug Discovery (FBDD) is a viable approach to find ligands binding at allosteric sites owing to their limited size and interactions. However, it was also observed that the structure-activity relationship of allosteric modulators is often sharp and inconsistent. This can be attributed to the characteristics of the allosteric binding site of mGluRs that is a water channel where ligand binding is accompanied with induced fit and interference with the water network, both playing a role in receptor activation. In this review, we summarize fragment-based drug discovery programs on mGluR allosteric modulators and their contribution identifying of new mGluR ligands with better activity and selectivity.

Insights into the Structural Aspects of the mGlu Receptor Orthosteric Binding Site

The amino terminal domain (ATD) of the metabotropic glutamate (mGlu) receptors contains the orthosteric glutamate recognition site, which is highly conserved across the eight mGlu receptor subtypes. In total, 29 X-ray crystal structures of the mGlu ATD proteins have been reported to date. These structures span across 3 subgroups and 6 subtypes, and include apo, agonist- and antagonist-bound structures. We will discuss the insights gained from the analysis of these structures with the focus on the interactions contributing to the observed group and subtype selectivity for select agonists. Furthermore, we will define the full expanded orthosteric ligand binding pocket (LBP) of the mGlu receptors, and discuss the macroscopic features of the mGlu ATD proteins.

Augmentation agents to serotonin reuptake inhibitors for treatment-resistant obsessive-compulsive disorder: A network meta-analysis

BACKGROUND

Various agents for augmentation of serotonin reuptake inhibitors have been investigated for treatment-resistant obsessive-compulsive disorder (OCD). We aimed to comprehensively compare different augmentation agents for treatment-resistant OCD in adults.

METHODS

PubMed, Embase, Web of Science, CENTRAL, the WHO's ICTRP, and ClinicalTrials.gov were searched on February 20, 2018. Pairwise meta-analysis and Bayesian network meta-analysis were performed. The primary outcome was efficacy measured by the Yale-Brown Obsessive Compulsive Scale. The secondary outcomes were tolerability (side-effect discontinuation) and acceptability (all cause discontinuation). Mean differences (MDs) and odds ratios (ORs) were reported with 95% confidence intervals (CIs).

RESULTS

Thirty-three articles with 34 trials (1216 patients) were included. Memantine (MD, -8.94; 95% CI, -14.42 to -3.42), risperidone (-4.47, -8.75 to -0.17), topiramate (-6.05, -10.89 to -1.20), lamotrigine (-6.07, -11.61 to -0.50), and aripiprazole (-5.14, -9.95 to -0.28) were significantly superior to placebo. Antipsychotic (-4.09, -6.22 to -1.93) and glutamatergic (-5.22, -7.53 to -2.84) agents were significantly superior to placebo. Considerable heterogeneity was found across studies, and baseline symptom severity was identified as a significant moderator. After baseline severity adjustment, quetiapine (-5.00, -8.59 to -1.29) and olanzapine (-8.28, -15.34 to -1.13) became significantly superior to placebo.

CONCLUSIONS

Our study supports the use of antipsychotic or glutamatergic agents as augmentation agents for treatment-resistant OCD. Topiramate, lamotrigine, aripiprazole, olanzapine, risperidone, memantine, and quetiapine are alternative augmentation drugs; however, a definitive conclusion of the best drug remains undetermined because of the considerable heterogeneity and limited numbers of studies and patients for each agent.

Discovery of 4-alkoxy-6-methylpicolinamide negative allosteric modulators of metabotropic glutamate receptor subtype 5

This letter describes the further chemical optimization of VU0424238 (auglurant), an mGlu5 NAM clinical candidate that failed in non-human primate (NHP) 28 day toxicology due to accumulation of a species-specific aldehyde oxidase (AO) metabolite of the pyrimidine head group. Here, we excised the pyrimidine moiety, identified the minimum pharmacophore, and then developed a new series of saturated ether head groups that ablated any AO contribution to metabolism. Putative back-up compounds in this novel series provided increased sp3 character, uniform CYP450-mediated metabolism across species, good functional potency and high CNS penetration. Key to the optimization was a combination of matrix and iterative libraries that allowed rapid surveillance of multiple domains of the allosteric ligand.

In Vivo Non-radioactive Assessment of mGlu5 Receptor-Activated Polyphosphoinositide Hydrolysis in Response to Systemic Administration of a Positive Allosteric Modulator

mGlu5 receptor-mediated polyphosphoinositide (PI) hydrolysis is classically measured by determining the amount of radioactivity incorporated in inositolmonophosphate (InsP) after labeling of membrane phospholipids with radioactive inositol. Although this method is historically linked to the study of mGlu receptors, it is inappropriate for the assessment of mGlu5 receptor signaling in vivo. Using a new ELISA kit we showed that systemic treatment with the selective positive allosteric modulator (PAM) of mGlu5 receptors VU0360172 enhanced InsP formation in different brain regions of CD1 or C57Black mice. The action of VU0360172 was sensitive to the mGlu5 receptor, negative allosteric modulator (NAM), MTEP, and was abolished in mice lacking mGlu5 receptors. In addition, we could demonstrate that endogenous activation of mGlu5 receptors largely accounted for the basal PI hydrolysis particularly in the prefrontal cortex. This method offers opportunity for investigation of mGlu5 receptor signaling in physiology and pathology, and could be used for the functional screening of mGlu5 receptor PAMs in living animals.

Metabotropic Glutamatergic Receptor 5 and Stress Disorders: Knowledge Gained From Receptor Imaging Studies

The metabotropic glutamatergic receptor subtype 5 (mGluR5) may represent a promising therapeutic target for stress-related psychiatric disorders. Here, we describe mGluR5 findings in stress disorders, particularly major depressive disorder (MDD), highlighting insights from positron emission tomography studies. Positron emission tomography studies report either no differences or lower mGluR5 in MDD, potentially reflecting MDD heterogeneity. Unlike the rapidly acting glutamatergic agent ketamine, mGluR5-specific modulation has not yet shown antidepressant efficacy in MDD and bipolar disorder. Although we recently showed that ketamine may work, in part, through significant mGluR5 modulation, the specific role of mGluR5 downregulation in ketamine's antidepressant response is unclear. In contrast to MDD, there has been much less investigation of mGluR5 in bipolar disorder, yet initial studies indicate that mGluR5-specific treatments may aid in both depressed and manic mood states. The direction of modulation needed may be state dependent, however, limiting clinical feasibility. There has been relatively little study of posttraumatic stress disorder or obsessive-compulsive disorder to date, although there is evidence for the upregulation of mGluR5 in these disorders. However, while antagonism of mGluR5 may reduce fear conditioning, it may also reduce fear extinction. Therefore, studies are needed to determine the role mGluR5 modulation might play in the treatment of these conditions. Further challenges in modulating this prevalent neurotransmitter system include potential induction of significant side effects. As such, more research is needed to identify level and type (positive/negative allosteric modulation or full antagonism) of mGluR5 modulation required to translate existing knowledge into improved therapies.

Exploring the Binding Mechanism of Metabotropic Glutamate Receptor 5 Negative Allosteric Modulators in Clinical Trials by Molecular Dynamics Simulations

Metabotropic glutamate receptor 5 (mGlu₅) plays a key role in synaptic information storage and memory, which is a well-known target for a variety of psychiatric and neurodegenerative disorders. In recent years, the increasing efforts have been focused on the design of allosteric modulators, and the negative allosteric modulators (NAMs) are the front-runners. Recently, the architecture of the transmembrane (TM) domain of mGlu₅ receptor has been determined by crystallographic experiment. However, it has been not well understood how the pharmacophores of NAMs accommodated into the allosteric binding site. In this study, molecular dynamics (MD) simulations were performed on mGlu₅ receptor bound with NAMs in preclinical or clinical development to shed light on this issue. In order to identify the key residues, the binding free energies as well as per-residue contributions for NAMs binding to mGlu₅ receptor were calculated. Subsequently, the in silico site-directed mutagenesis of the key residues was performed to verify the accuracy of simulation models. As a result, the shared common features of the studied 5 clinically important NAMs (mavoglurant, dipraglurant, basimglurant, STX107, and fenobam) interacting with 11 residues in allosteric site were obtained. This comprehensive study presented a better understanding of mGlu₅ receptor NAMs binding mechanism, which would be further used as a useful framework to assess and discover novel lead scaffolds for NAMs.

Discovery of 6-(pyrimidin-5-ylmethyl)quinoline-8-carboxamide negative allosteric modulators of metabotropic glutamate receptor subtype 5

Based on previous work that established fused heterocycles as viable alternatives for the picolinamide core of our lead series of mGlu₅ negative allosteric modulators (NAMs), we designed a novel series of 6-(pyrimidin-5-ylmethyl)quinoline-8-carboxamide mGlu₅ NAMs. These new quinoline derivatives also contained carbon linkers as replacements for the diaryl ether oxygen atom common to our previously published chemotypes. Compounds were evaluated in a cell-based functional mGlu₅ assay, and an exemplar analog 27 was >60-fold selective versus the other seven mGlu receptors. Selected compounds were also studied in metabolic stability assays in rat and human S9 hepatic fractions and exhibited a mixture of P450- and non-P450-mediated metabolism.

Silent Allosteric Modulation of mGluR5 Maintains Glutamate Signaling while Rescuing Alzheimer's Mouse Phenotypes

Metabotropic glutamate receptor 5 (mGluR5) has been implicated in Alzheimer's disease (AD) pathology. We sought to understand whether mGluR5's role in AD requires glutamate signaling. We used a potent mGluR5 silent allosteric modulator (SAM, BMS-984923) to separate its well-known physiological role in glutamate signaling from a pathological role in mediating amyloid-β oligomer (Aβo) action. Binding of the SAM to mGluR5 does not change glutamate signaling but strongly reduces mGluR5 interaction with cellular prion protein (PrP^C) bound to Aβo. The SAM compound prevents Aβo-induced signal transduction in brain slices and in an AD transgenic mouse model, the APPswe/PS1ΔE9 strain. Critically, 4 weeks of SAM treatment rescues memory deficits and synaptic depletion in the APPswe/PS1ΔE9 transgenic mouse brain. Our data show that mGluR5's role in Aβo-dependent AD phenotypes is separate from its role in glutamate signaling and silent allosteric modulation of mGluR5 has promise as a disease-modifying AD intervention with a broad therapeutic window.

Discovery of imidazo[1,2-a]-, [1,2,4]triazolo[4,3-a]-, and [1,2,4]triazolo[1,5-a]pyridine-8-carboxamide negative allosteric modulators of metabotropic glutamate receptor subtype 5

Based on a hypothesis that an intramolecular hydrogen bond was present in our lead series of picolinamide mGlu₅ NAMs, we reasoned that an inactive nicotinamide series could be modified through introduction of a fused heterocyclic core to generate potent mGlu₅ NAMs. In this Letter, we describe the synthesis and evaluation of compounds that demonstrate the viability of that approach. Selected analogs were profiled in a variety of in vitro assays, and two compounds were evaluated in rat pharmacokinetic studies and a mouse model of obsessive-compulsive disorder. Ancillary pharmacology screening revealed that members of this series exhibited moderate inhibition of the dopamine transporter (DAT), and SAR was developed that expanded the selectivity for mGlu₅ versus DAT.

Discovery of N-(5-Fluoropyridin-2-yl)-6-methyl-4-(pyrimidin-5-yloxy)picolinamide (VU0424238): A Novel Negative Allosteric Modulator of Metabotropic Glutamate Receptor Subtype 5 Selected for Clinical Evaluation

Preclinical evidence in support of the potential utility of mGlu5 NAMs for the treatment of a variety of psychiatric and neurodegenerative disorders is extensive, and multiple such molecules have entered clinical trials. Despite some promising results from clinical studies, no small molecule mGlu5 NAM has yet to reach market. Here we present the discovery and evaluation of N-(5-fluoropyridin-2-yl)-6-methyl-4-(pyrimidin-5-yloxy)picolinamide (27, VU0424238), a compound selected for clinical evaluation. Compound 27 is more than 900-fold selective for mGlu5 versus the other mGlu receptors, and binding studies established a Ki value of 4.4 nM at a known allosteric binding site. Compound 27 had a clearance of 19.3 and 15.5 mL/min/kg in rats and cynomolgus monkeys, respectively. Imaging studies using a known mGlu5 PET ligand demonstrated 50% receptor occupancy at an oral dose of 0.8 mg/kg in rats and an intravenous dose of 0.06 mg/kg in baboons.