Evaluation of the Safety, Tolerability, and Pharmacokinetic Profiles of TP0473292 (TS-161), A Prodrug of a Novel Orthosteric mGlu2/3 Receptor Antagonist TP0178894, in Healthy Subjects and Its Antidepressant-Like Effects in Rodents

Glutamatergic Modulators for Major Depression from Theory to Clinical Use

Major depressive disorder (MDD) is a chronic, burdensome, highly prevalent disease that is characterized by depressed mood and anhedonia. MDD is especially burdensome as approved monoamine antidepressant treatments have weeks-long delays before clinical benefit and low remission rates. In the past 2 decades, a promising target emerged to improve patient outcomes in depression treatment: glutamatergic signaling. This narrative review provides a high-level overview of glutamate signaling in synaptogenesis and neural plasticity and the implications of glutamate dysregulation in depression. Based on this preclinical evidence implicating glutamate in depression and the rapid improvement of depression with ketamine treatment in a proof-of-concept trial, a range of N-methyl-D-aspartate (NMDA)-targeted therapies have been investigated. While an array of treatments has been investigated in registered phase 2 or 3 clinical trials, the development of most of these agents has been discontinued. Multiple glutamate-targeted antidepressants are actively in development, and two are approved. Nasal administration of esketamine (Spravato®) was approved by the US Food and Drug Administration (FDA) in 2019 to treat adults with treatment-resistant depression and in 2020 for adults with MDD with acute suicidal ideation or behavior. Oral combination dextromethorphan-bupropion (AXS-05, Auvelity® extended-release tablet) was FDA approved in 2022 for the treatment of MDD in adults. These approvals bolster the importance of glutamate in depression and represent an exciting breakthrough in contemporary psychiatry, providing new avenues of treatment for patients as first-line therapy or with either poor response or unacceptable side effects to monoaminergic antidepressants.

Advancing past ketamine: emerging glutamatergic compounds for the treatment of depression

Changes in glutamatergic neuroplasticity has been proposed as one of the core mechanisms underlying the pathophysiology of depression. In consequence components of the glutamatergic synapse have been explored as potential targets for antidepressant treatment. The rapid antidepressant effect of the NMDA receptor antagonist ketamine and subsequent approval of its S-enantiomer (i.e. esketamine), have set the precedent for investigation into other glutamatergic rapid acting antidepressants (RAADs). In this review, we discuss the potential of the different glutamatergic targets for antidepressant treatment. We describe important clinical outcomes of several key molecules targeting components of the glutamatergic synapse and their applicability as RAADs. Specifically, here we focus on substances beyond (es)ketamine, for which meaningful data from clinical trials are available, including arketamine, esmethadone, nitrous oxide and other glutamate receptor modulators. Molecules only successful in preclinical settings and case reports/series are only marginally discussed. With this review, we aim underscore the critical role of glutamatergic modulation in advancing antidepressant therapy, thereby possibly enhancing clinical outcomes but also to reducing the burden of depression through faster therapeutic effects.

Ketamine in neuropsychiatric disorders: an update

The discovery of ketamine as a rapid-acting antidepressant led to a new era in the development of neuropsychiatric therapeutics, one characterized by an antidepressant response that occurred within hours or days rather than weeks or months. Considerable clinical research supports the use of-or further research with-subanesthetic-dose ketamine and its (S)-enantiomer esketamine in multiple neuropsychiatric disorders including depression, bipolar disorder, anxiety spectrum disorders, substance use disorders, and eating disorders, as well as for the management of chronic pain. In addition, ketamine often effectively targets symptom domains associated with multiple disorders, such as anxiety, anhedonia, and suicidal ideation. This manuscript: 1) reviews the literature on the pharmacology and hypothesized mechanisms of subanesthetic-dose ketamine in clinical research; 2) describes similarities and differences in the mechanism of action and antidepressant efficacy between racemic ketamine, its (S) and (R) enantiomers, and its hydroxynorketamine (HNK) metabolite; 3) discusses the day-to-day use of ketamine in the clinical setting; 4) provides an overview of ketamine use in other psychiatric disorders and depression-related comorbidities (e.g., suicidal ideation); and 5) provides insights into the mechanisms of ketamine and therapeutic response gleaned from the study of other novel therapeutics and neuroimaging modalities.

Group I and group II metabotropic glutamate receptors are upregulated in the synapses of infant rats prenatally exposed to valproic acid

RATIONALE

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interaction and restricted/stereotyped behavior. Prenatal exposure to valproic acid (VPA) is associated with an increased risk of developing ASD in humans and autistic-like behaviors in rodents. Increasing evidence indicates that dysfunctions of glutamate receptors at synapses are associated with ASD. In the VPA rat model, an involvement of glutamate receptors in autism-like phenotypes has been suggested; however, few studies were carried out on metabotropic glutamate (mGlu) receptors.

OBJECTIVES

We examined the protein expression levels of group I (mGlu1 and mGlu5) and group II (mGlu2/3) mGlu receptors in rats prenatally exposed to VPA and evaluated the effect of mGlu receptor modulation on an early autism-like phenotype in these animals.

METHODS

We used western blotting analysis on synaptosomes obtained from forebrain of control and VPA rats at different ages (postnatal day P13, 35, 90) and carried out ultrasonic vocalization (USV) emission test in infant control and VPA rats.

RESULTS

The expression levels of all these receptors were significantly increased in infant VPA rats. No changes were detected in adolescent and adult rats. An acute treatment with the preferential mGlu2/3 antagonist, LY341495, attenuated the impairment in the USV emission in VPA rats. No effect was observed after a treatment with the mGlu5 selective antagonist, MTEP.

CONCLUSIONS

Our findings demonstrate that the expression of group I and group II mGlu receptors is upregulated at synapses of infant VPA rats and suggest that mGlu2/3 receptor modulation may have a therapeutic potential in ASD.

Courtes A, Jones G, Soares JC, Machado-Vieira R. Pharmacotherapies Targeting GABA-Glutamate Neurotransmission for Treatment-Resistant Depression

Treatment-resistant depression (TRD) is a term used to describe a particular type of major depressive disorder (MDD). There is no consensus about what defines TRD, with various studies describing between 1 and 4 failures of antidepressant therapies, with or without electroconvulsive therapy (ECT). That is why TRD is such a growing concern among clinicians and researchers, and it explains the necessity for investigating novel therapeutic targets beyond conventional monoamine pathways. An imbalance between two primary central nervous system (CNS) neurotransmitters, L-glutamate and γ-aminobutyric acid (GABA), has emerged as having a key role in the pathophysiology of TRD. In this review, we provide an evaluation and comprehensive review of investigational antidepressants targeting these two systems, accessing their levels of available evidence, mechanisms of action, and safety profiles. N-methyl-D-aspartate (NMDA) receptor antagonism has shown the most promise amongst the glutamatergic targets, with ketamine and esketamine (Spravato) robustly generating responses across trials. Two specific NMDA-glycine site modulators, D-cycloserine (DCS) and apimostinel, have also generated promising initial safety and efficacy profiles, warranting further investigation. Combination dextromethorphan-bupropion (AXS-05/Auvelity) displays a unique mechanism of action and demonstrated positive results in particular applicability in subpopulations with cognitive dysfunction. Currently, the most promising GABA modulators appear to be synthetic neurosteroid analogs with positive GABAA receptor modulation (such as brexanolone). Overall, advances in the last decade provide exciting perspectives for those who do not improve with conventional therapies. Of the compounds reviewed here, three are approved by the Food and Drug Administration (FDA): esketamine (Spravato) for TRD, Auvelity (dextromethorphan-bupropion) for major depressive disorder (MDD), and brexanolone (Zulresso) for post-partum depression (PPD). Notably, some concerns have arisen with esketamine and brexanolone, which will be detailed in this study.

mGlu2/3 receptor antagonists for depression: overview of underlying mechanisms and clinical development

Triggered by the ground-breaking finding that ketamine exerts robust and rapid-acting antidepressant effects in patients with treatment-resistant depression, glutamatergic systems have attracted attention as targets for the development of novel antidepressants. Among glutamatergic systems, group II metabotropic glutamate (mGlu) receptors, consisting of mGlu2 and mGlu3 receptors, are of interest because of their modulatory roles in glutamatergic transmission. Accumulating evidence has indicated that mGlu2/3 receptor antagonists have antidepressant-like effects in rodent models that mirror those of ketamine and that mGlu2/3 receptor antagonists also share underlying mechanisms with ketamine that are responsible for these antidepressant-like actions. Importantly, contrary to their antidepressant-like profile, preclinical studies have revealed that mGlu2/3 receptor antagonists are devoid of ketamine-like adverse effects, such as psychotomimetic-like behavior, abuse potential and neurotoxicity. Despite some discouraging results for an mGlu2/3 receptor antagonist decoglurant (classified as a negative allosteric modulator [NAM]) in patients with major depressive disorder, clinical trials of two mGlu2/3 receptor antagonists, a phase 2 trial of TS-161 (an orthosteric antagonist) and a phase 1 trial of DSP-3456 (a NAM), are presently on-going. mGlu2/3 receptors still hold promise for the development of safer and more efficacious antidepressants.

Molecular mechanisms of rapid-acting antidepressants: New perspectives for developing antidepressants

Major depressive disorder (MDD) is a chronic relapsing psychiatric disorder. Conventional antidepressants usually require several weeks of continuous administration to exert clinically significant therapeutic effects, while about two-thirds of the patients are prone to relapse of symptoms or are completely ineffective in antidepressant treatment. The recent success of the N-methyl-D-aspartic acid (NMDA) receptor antagonist ketamine as a rapid-acting antidepressant has propelled extensive research on the action mechanism of antidepressants, especially in relation to its role in synaptic targets. Studies have revealed that the mechanism of antidepressant action of ketamine is not limited to antagonism of postsynaptic NMDA receptors or GABA interneurons. Ketamine produces powerful and rapid antidepressant effects by affecting α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors, adenosine A1 receptors, and the L-type calcium channels, among others in the synapse. More interestingly, the 5-HT2A receptor agonist psilocybin has demonstrated potential for rapid antidepressant effects in depressed mouse models and clinical studies. This article focuses on a review of new pharmacological target studies of emerging rapid-acting antidepressant drugs such as ketamine and hallucinogens (e.g., psilocybin) and briefly discusses the possible strategies for new targets of antidepressants, with a view to shed light on the direction of future antidepressant research.

Preclinical Metabolism and Disposition of TP0473292, a Novel Oral Prodrug of the Potent Metabotropic Glutamate 2/3 Receptor Antagonist TP0178894 for the Treatment of Depression

TP0473292 is an adamantane carboxylic acid (ACA) ester prodrug for enhancing the oral bioavailability of the hydrophilic glutamate analog TP0178894, a novel metabotropic glutamate 2 and 3 receptor antagonist, and being developed as an antidepressant. TP0473292 showed high membrane permeability and rapid hydrolysis to TP0178894 in rat, monkey, and human liver S9 fractions, with a conversion rate of such that complete conversion by first-pass metabolism was expected. TP0473292 was also hydrolyzed in the intestinal, renal, and lung S9 fractions, coinciding with the result that TP0473292 was activated by carboxylesterase (CES) 1 and more efficiently by CES2. Despite the rapid hydrolysis of TP0473292 in the intestinal S9 fraction, TP0473292 achieved good oral bioavailability of poorly permeable TP0178894 (approximately 60%) in rats and monkeys, with no TP0473292 detected in the plasma, revealing that rapid hydrolysis in the intestine is not necessarily a disadvantage. We also confirmed the penetration of TP0178894 into the cerebrospinal fluid and its unmetabolized excretion in urine. The ester promoiety, ACA, was metabolized to chemically stable acyl glucuronide and excreted in urine in rats and monkeys, suggesting a low risk of idiosyncratic drug toxicity. TP0473292 and its metabolites did not show a drug-drug interaction potential via cytochrome P450 in humans. These results suggested that TP0473292 functions as an ideal oral prodrug in humans; this was later confirmed to be true in phase 1 clinical trials. Furthermore, ACA was firstly confirmed to be a useful promoiety for hydrophilic drugs to enhance their oral bioavailability. SIGNIFICANCE STATEMENT: Hydrolysis in the intestine reportedly has negative effects on the oral bioavailability of hydrophilic active metabolites of ester prodrugs. This study reports the preclinical pharmacokinetics of a hydrophilic metabotropic glutamate 2/3 receptor antagonist, TP0178894, and its ester prodrug TP0473292, which was found to act as an oral prodrug despite being activated predominantly in the intestine. Furthermore, this study firstly reports that adamantane carboxylic acid is useful as the ester promoiety of a prodrug for increasing lipophilicity and oral bioavailability.

Clinical pharmacological innovation in the treatment of depression

INTRODUCTION

Deficiencies in standard of care antidepressants are driving novel drug discovery. A new age of antidepressant medications has emerged with the introduction of rapid-acting antidepressants with efficacy in treatment-resistant patients.

AREAS COVERED

The newly approved medicines and those in clinical development for major depressive disorder (MDD) are documented in this scoping review of newly approved and emerging antidepressants. Compounds are evaluated for clinical efficacy, tolerability, and safety and compared to those of standard of care medicines.

EXPERT OPINION

A new age of antidepressant discovery relies heavily on glutamatergic mechanisms. New medicines based upon the model of ketamine have been delivered and are in clinical development. Rapid onset and the ability to impact treatment-resistant depression, raises the question of the best first-line medicines for patients. Drugs with improvements in tolerability are being investigated (e.g. mGlu_2/3 receptor antagonists, AMPA receptor potentiators, and novel NMDA receptor modulators). Multiple companies are working toward the identification of novel psychedelic drugs where the requirement for psychedelic activity is not fully known. Gaps still exist - methods for matching patients with specific medicines are needed, and medicines for the prevention of MDD and its disease progression need research attention.

Role of group II metabotropic glutamate receptors in ketamine's antidepressant actions

Major Depressive Disorder (MDD) is a serious neuropsychiatric disorder afflicting around 16-17 % of the global population and is accompanied by recurrent episodes of low mood, hopelessness and suicidal thoughts. Current pharmacological interventions take several weeks to even months for an improvement in depressive symptoms to emerge, with a significant percentage of individuals not responding to these medications at all, thus highlighting the need for rapid and effective next-generation treatments for MDD. Pre-clinical studies in animals have demonstrated that antagonists of the metabotropic glutamate receptor subtype 2/3 (mGlu_2/3 receptor) exert rapid antidepressant-like effects, comparable to the actions of ketamine. Therefore, it is possible that mGlu₂ or mGlu₃ receptors to have a regulatory role on the unique antidepressant properties of ketamine, or that convergent intracellular mechanisms exist between mGlu_2/3 receptor signaling and ketamine's effects. Here, we provide a comprehensive and critical evaluation of the literature on these convergent processes underlying the antidepressant action of mGlu_2/3 receptor inhibitors and ketamine. Importantly, combining sub-threshold doses of mGlu_2/3 receptor inhibitors with sub-antidepressant ketamine doses induce synergistic antidepressant-relevant behavioral effects. We review the evidence supporting these combinatorial effects since sub-effective dosages of mGlu_2/3 receptor antagonists and ketamine could reduce the risk for the emergence of significant adverse events compared with taking normal dosages. Overall, deconvolution of ketamine's pharmacological targets will give critical insights to influence the development of next-generation antidepressant treatments with rapid actions.

Antidepressants in the post-ketamine Era: Pharmacological approaches targeting the glutamatergic system

The efficacy of currently available medications for depression is unsatisfactory, and that has spurred the development of novel antidepressants based on a hypothesis other than the monoamine hypothesis. Recent studies have revealed the importance of the glutamatergic system as a drug target for depression, and the validity of this hypothesis has been underpinned by the discovery of the antidepressant effects of ketamine, leading to the market launch of Spravato® nasal spray which delivers (S)-ketamine (esketamine). However, both ketamine and esketamine have unwanted adverse effects that hinder their routine use in daily practice. Extensive studies have elucidated the mechanisms underlying the antidepressant effects of ketamine, and that has encouraged numerous drug discovery activities to search for agents that retain a ketamine-like antidepressant profile but with lesser adverse effect liabilities. The discovery activities have included attempts to identify 1) the active substance(s) in the circulation after ketamine administration and 2) agents that act on the proposed mechanisms of action of ketamine. Clinical trials of agents discovered in the course of these activities are underway, and in 2022, AUVELITY™ (AXS-05; dextromethorphan with bupropion) was approved by the United States Food and Drug Administration. Drug development of post-ketamine agents should provide novel antidepressants that are safer, but as potent and rapidly acting as ketamine.

Metabotropic glutamate receptor 3 as a potential therapeutic target for psychiatric and neurological disorders

Glutamate is a major excitatory neurotransmitter in the central nervous system (CNS) and abnormalities in the glutamatergic system underlie various CNS disorders. As metabotropic glutamate receptor 3 (mGlu3 receptor) regulates glutamatergic transmission in various brain areas, emerging literature suggests that targeting mGlu3 receptors can be a novel approach to the treatment of psychiatric and neurological disorders. For example, mGlu3 receptor negative allosteric modulators (NAMs) induce rapid antidepressant-like effects in both acute and chronic stress models. Activation of mGlu3 receptors can enhance cognition in the rodents modeling schizophrenia-like pathophysiology. The mGlu3 receptors expressed in the astrocytes induce neuroprotective effects. Although polymorphisms in GRM3 have been shown to be associated with addiction, there is not significant evidence about the efficacy of mGlu3 receptor ligands in rodent models of addiction. Collectively, drugs targeting mGlu3 receptors may provide an alternative approach to fill the unmet clinical need for safer and more efficacious therapeutics for CNS disorders.

Where do we go next in antidepressant drug discovery? A new generation of antidepressants: a pivotal role of AMPA receptor potentiation and mGlu2/3 receptor antagonism

INTRODUCTION

Major depressive disorder remains a prevalent world-wide health problem. Currently available antidepressant medications take weeks of dosing, do not produce antidepressant response in all patients, and have undesirable ancillary effects.

AREAS COVERED

The present opinion piece focuses on the major inroads to the creation of new antidepressants. These include N-methyl-D-aspartate (NMDA) receptor antagonists and related compounds like ketamine, psychedelic drugs like psilocybin, and muscarinic receptor antagonists like scopolamine. The preclinical and clinical pharmacological profile of these new-age antidepressant drugs is discussed.

EXPERT OPINION

Preclinical and clinical data have accumulated to predict a next generation of antidepressant medicines. In contrast to the current standard of care antidepressant drugs, these compounds differ in that they demonstrate rapid activity, often after a single dose, and effects that outlive their presence in brain. These compounds also can provide efficacy for treatment-resistant depressed patients. The mechanism of action of these compounds suggests a strong glutamatergic component that involves the facilitation of AMPA receptor function. Antagonism of mGlu2/3 receptors is also relevant to the antidepressant pharmacology of this new class of drugs. Based upon the ongoing efforts to develop these new-age antidepressants, new drug approvals are predicted in the near future.

Advances in translating mGlu2 and mGlu3 receptor selective allosteric modulators as breakthrough treatments for affective disorders and alcohol use disorder

Metabotropic glutamate (mGlu) receptors are promising targets for the treatment of affective disorders and alcohol use disorder (AUD). Nonspecific ligands for Group II (mGlu2 and mGlu3) mGlu receptors have demonstrated consistent therapeutic potential for affective disorders in preclinical models. Disentangling the specific roles of mGlu2 versus mGlu3 receptors in these effects has persisted as a major challenge, in part due to pharmacological limitations. However, the recent development of highly specific allosteric modulators for both mGlu2 and mGlu3 receptors have enabled straightforward and rigorous investigations into the specific function of each receptor. Here, we review recent experiments using these compounds that have demonstrated both similar and distinct receptor functions in behavioral, molecular, and electrophysiological measures associated with basal function and preclinical models of affective disorders. Studies using these selective drugs have demonstrated that mGlu2 is the predominant receptor subclass involved in presynaptic neurotransmitter release in prefrontal cortex. By contrast, the activation of postsynaptic mGlu3 receptors induces a cascade of cellular changes that results in AMPA receptor internalization, producing long-term depression and diminishing excitatory drive. Acute stress decreases the mGlu3 receptor function and dynamically alters transcript expression for both mGlu2 (Grm2) and mGlu3 (Grm3) receptors in brain areas involved in reward and stress. Accordingly, both mGlu2 and mGlu3 negative allosteric modulators show acute antidepressant-like effects and potential prophylactic effects against acute and traumatic stressors. The wide array of effects displayed by these new allosteric modulators of mGlu2 and mGlu3 receptors suggest that these drugs may act through improving endophenotypes of symptoms observed across several neuropsychiatric disorders. Therefore, recently developed allosteric modulators selective for mGlu2 or mGlu3 receptors show promise as potential therapeutics for affective disorders and AUD.

The role of mGlu2/3 receptor antagonists in the enhancement of the antidepressant-like effect of ketamine

MGlu_2/3 receptor antagonists produce antidepressant-like effects in animal models of depression. A number of mechanisms responsible for these actions are convergent to the mechanism of the fast antidepressant-like effect of ketamine. Furthermore, the data indicate that ketamine effect is related to the action of mGlu₂ receptors and may be reduced by their agonists. The above facts became the basis for the hypothesis that the antidepressant effect of low doses of ketamine might be enhanced by coadministration of a mGlu₂ receptor antagonist. This strategy was aimed not only at enhancing the therapeutic effect of ketamine but also at reducing the risk of undesirable effects by lowering its therapeutic dose. It is known that ketamine, effective in relieving depressive symptoms in patients suffering from treatment-resistant depression (TRD), is burdened with a number of side effects, which may be particularly dangerous in psychiatric patients. Data have confirmed that subeffective doses of ketamine and its enantiomer, (R)-ketamine, coadministered with an mGlu_2/3 receptor antagonist, induce antidepressant-like effects in the screening tests and in the chronic-stress-induced model of depression. At the same time, these drug combinations did not cause undesirable effects characteristic of higher doses of ketamine and (S)-ketamine, including those related to psychostimulatory effects. Further research is required to prove whether this strategy will also be effective in depressive patients.

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.

Investigational Drugs for the Treatment of Depression (Part 2): Glutamatergic, Cholinergic, Sestrin Modulators, and Other Agents

Many investigational drugs with antidepressant activity are currently explored in different phases of clinical research, with indications such as major depressive disorder, treatment-resistant major depression, bipolar depression, post-partum depression, and late-life depression. Although the vast majority of the antidepressants in clinical use are based on the monoaminergic hypothesis of depression, recent data supported the launching on the market of two new, non-monoamine-modulating drugs. Esketamine for treatment-resistant major depression and brexanolone for post-partum depression are two exceptions from the monoaminergic model, although their use is still limited by high costs, unique way of administration (only intravenously for brexanolone), physicians’ reluctance to prescribe new drugs, and patients’ reticence to use them. Glutamatergic neurotransmission is explored based on the positive results obtained by intranasal esketamine, with subanesthetic intravenous doses of ketamine, and D-cycloserine, traxoprodil, MK-0657, AXS-05, AVP-786, combinations of cycloserine and lurasidone, or dextromethorphan and quinidine, explored as therapeutic options for mono- or bipolar depression. Sestrin modulators, cholinergic receptor modulators, or onabotulinumtoxinA have also been investigated for potential antidepressant activity. In conclusion, there is hope for new treatments in uni- and bipolar depression, as it became clear, after almost 7 decades of monoamine-modulating antidepressants, that new pathogenetic pathways should be targeted to increase the response rate in this population.

Antidepressant-like actions of the mGlu2/3 receptor antagonist TP0178894 in the chronic social defeat stress model: Comparison with escitalopram

The metabotropic glutamate 2/3 (mGlu2/3) receptor antagonists are reported to produce ketamine-like rapid-acting and sustained antidepressant-like effects in rodents. In this study, we compared the effects of single administration of the new mGlu2/3 receptor antagonist TP0178894 and the selective serotonin reuptake inhibitor (SSRI) escitalopram in the chronic social defeat stress (CSDS) model of depression, a model which has been shown to be resistant to treatment with a single dose of SSRI. In the tail suspension test and forced swimming test, high dose (3.0 mg/kg) of TP0178894 significantly attenuated the increased immobility time of these tests in CSDS susceptible mice, compared with vehicle-treated mice. In contrast, low doses (0.3 and 1.0 mg/kg) of TP0178894 and escitalopram (10 mg/kg) did not alter the increased immobility time of these two tests. In the sucrose preference test, TP0178894 (3.0 mg/kg) significantly improved the reduced sucrose preference of CSDS susceptible mice, three and seven days after a single dose. In addition, Western blot analyses showed that TP0178894 (3.0 mg/kg), but not low doses of TP0178894 and escitalopram, significantly attenuated the reduced expression of synaptic proteins [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (GluA1) and postsynaptic density protein 95 (PSD-95)] in the prefrontal cortex from CSDS susceptible mice. This study suggests that TP0178894 shows rapid-acting and sustained antidepressant-like effects in CSDS model, as ketamine does.