A randomized trial of a low-trapping nonselective N-methyl-D-aspartate channel blocker in major depression

Adjunctive Lanicemine (AZD6765) in Patients with Major Depressive Disorder and History of Inadequate Response to Antidepressants: A Randomized, Placebo-Controlled Study

The objective of this study was to investigate the efficacy and safety of adjunctive lanicemine (NMDA channel blocker) in the treatment of major depressive disorder (MDD) over 12 weeks. This phase IIb, randomized, parallel-arm, double-blind, placebo-controlled study was conducted at 49 centers in four countries between December 2011 and August 2013 in 302 patients aged 18-70 years, meeting criteria for single episode or recurrent MDD and with a history of inadequate treatment response. Patients were required to be taking an allowed antidepressant for at least four weeks prior to screening. Patients were randomized equally to receive 15 double-blind intravenous infusions of adjunctive lanicemine 50 mg, lanicemine 100 mg, or saline over a 12-week course, in addition to ongoing antidepressant. The primary efficacy end point was change in Montgomery-Åsberg Depression Rating Scale (MADRS) total score from baseline to week 6. Secondary efficacy outcome variables included change in MADRS score from baseline to week 12, response and remission rates, and changes in Clinical Global Impression scale, Quick Inventory of Depressive Symptomology Self-Report score, and Sheehan Disability Scale score. Of 302 randomized patients, 240 (79.5%) completed treatment. Although lanicemine was generally well tolerated, neither dose was superior to placebo in reducing depressive symptoms on the primary end point or any secondary measures. There was no significant difference between lanicemine and placebo treatment on any outcome measures related to MDD. Post hoc analyses were performed to explore the possible effects of trial design and patient characteristics in accounting for the contrasting results with a previously reported trial.

What is the mechanism of Ketamine's rapid-onset antidepressant effect? A concise overview of the surprisingly large number of possibilities

WHAT IS KNOWN AND OBJECTIVE

Abundant clinical data now confirm that ketamine produces a remarkable rapid-onset antidepressant effect - hours or days - in contrast to the delayed onset (typically weeks) of current antidepressant drugs. This surprising and revolutionary finding may lead to the development of life-saving pharmacotherapy for depressive illness by reducing the high suicide risk associated with the delayed onset of effect of current drugs. As ketamine has serious self-limiting drawbacks that restrict its widespread use for this purpose, a safer alternative is needed. Our objective is to review the proposed mechanism(s) of ketamine's rapid-onset antidepressant action for new insights into the physiological basis of depressive illness that may lead to new and novel targets for antidepressant drug discovery.

METHODS

A search was conducted on published literature (e.g. PubMed) and Internet sources to identify information relevant to ketamine's rapid-acting antidepressant action and, specifically, to the possible mechanism(s) of this action. Key search words included 'ketamine', 'antidepressant', 'mechanism of action', 'depression' and 'rapid acting', either individually or in combination. Information was sought that would include less well-known, as well as well-known, basic pharmacologic properties of ketamine and that identified and evaluated the several hypotheses about ketamine's mechanism of antidepressant action.

RESULTS

Whether the mechanistic explanation for ketamine's rapid-onset antidepressant action is related to its well-known antagonism of the NMDA (N-Methyl-d-aspartate) subtype of glutamate receptor or to something else has not yet been fully elucidated. The evidence from pharmacologic, medicinal chemistry, animal model and drug-discovery sources reveals a wide variety of postulated mechanisms.

WHAT IS NEW AND CONCLUSION

The surprising discovery of ketamine's rapid-onset antidepressant effect is a game-changer for the understanding and treatment of depressive illness. There is some convergence on NMDA receptor antagonism as a likely, but to date unproven, common mechanism. The surprising number of other mechanisms, and the several novel biochemical aetiologies of depression proposed, suggests exciting new drug-discovery targets.

Mood Therapeutics: Novel Pharmacological Approaches for Treating Depression

INTRODUCTION

Real-world effectiveness trials suggest that antidepressant efficacy is limited in many patients with mood disorders, underscoring the urgent need for novel therapeutics to treat these disorders. Areas covered: Here, we review the clinical evidence supporting the use of novel modulators for the treatment of mood disorders, including specific glutamate modulators such as: 1) high-trapping glutamatergic modulators; 2) subunit (NR2B)-specific N-methyl-D-aspartate (NMDA) receptor antagonists; 3) NMDA receptor glycine-site partial agonists; and 4) metabotropic glutamate receptor (mGluR) modulators. We also discuss other promising, non-glutamatergic targets for potential rapid antidepressant effects in mood disorders, including the cholinergic system, the glucocorticoid system, and the inflammation pathway, as well as several additional targets of interest. Clinical evidence is emphasized, and non-pharmacological somatic treatments are not reviewed. In general, this paper only explores agents available in the United States. Expert commentary: Of these novel targets, the most promising - and the ones for whom the most evidence exists - appear to be the ionotropic glutamate receptors. However, moving forward will require us to fully embrace the goal of personalized medicine and will require health professionals to pre-emptively identify potential responders.

Beyond the therapeutic shackles of the monoamines: New mechanisms in bipolar disorder biology

Multiple novel biological mechanisms putatively involved in the etiology of bipolar disorders are being explored. These include oxidative stress, altered glutamatergic neurotransmission, mitochondrial dysfunction, inflammation, cell signaling, apoptosis and impaired neurogenesis. Important clinical translational potential exists for such mechanisms to help underpin development of novel therapeutics - much needed given limitations of current therapies. These new mechanisms also help improve our understanding of how current therapeutics might exert their effects. Lithium, for example, appears to have antioxidant, immunomodulatory, signaling, anti-apoptotic and neuroprotective properties. Similar properties have been attributed to other mood stabilizers such as valproate, lamotrigine, and quetiapine. Perhaps of greatest translational value has been the recognition of such mechanisms leading to the emergence of novel therapeutics for bipolar disorders. These include the antioxidant N-acetylcysteine, the anti-inflammatory celecoxib, and ketamine - with effects on the glutamatergic system and microglial inhibition. We review these novel mechanisms and emerging therapeutics, and comment on next steps in this space.

Therapeutic Modulation of Glutamate Receptors in Major Depressive Disorder

Current pharmacotherapies for major depressive disorder (MDD) have a distinct lag of onset that can prolong distress and impairment for patients, and realworld effectiveness trials further suggest that antidepressant efficacy is limited in many patients. All currently approved antidepressant medications for MDD act primarily through monoaminergic mechanisms, e.g., receptor/reuptake agonists or antagonists with varying affinities for serotonin, norepinephrine, or dopamine. Glutamate is the major excitatory neurotransmitter in the central nervous system, and glutamate and its cognate receptors are implicated in the pathophysiology of MDD, as well as in the development of novel therapeutics for this disorder. Since the rapid and robust antidepressant effects of the N-methyl-D-aspartate (NMDA) antagonist ketamine were first observed in 2000, other NMDA receptor antagonists have been studied in MDD. These have been associated with relatively modest antidepressant effects compared to ketamine, but some have shown more favorable characteristics with increased potential in clinical practice (for instance, oral administration, decreased dissociative and/or psychotomimetic effects, and reduced abuse/diversion liability). This article reviews the clinical evidence supporting the use of glutamate receptor modulators with direct affinity for cognate receptors: 1) non-competitive NMDA receptor antagonists (ketamine, memantine, dextromethorphan, AZD6765); 2) subunit (NR2B)-specific NMDA receptor antagonists (CP- 101,606/traxoprodil, MK-0657); 3) NMDA receptor glycine-site partial agonists (D-cycloserine, GLYX- 13); and 4) metabotropic glutamate receptor (mGluR) modulators (AZD2066, RO4917523/basimglurant). Several other theoretical glutamate receptor targets with preclinical antidepressant-like efficacy, but that have yet to be studied clinically, are also briefly discussed; these include α-amino-3-hydroxyl-5-methyl-4- isoxazoleproprionic acid (AMPA) agonists, mGluR2/3 negative allosteric modulators, and mGluR7 agonists.

Towards the Molecular Foundations of Glutamatergic-targeted Antidepressants

BACKGROUND

Depression affects over 120 million individuals of all ages and is the leading cause of disability worldwide. The lack of objective diagnostic criteria, together with the heterogeneity of the depressive disorder itself, makes it challenging to develop effective therapies. The accumulation of preclinical data over the past 20 years derived from a multitude of models using many divergent approaches, has fueled the resurgence of interest in targeting glutamatergic neurotransmission for the treatment of major depression.

OBJECTIVE

The emergence of mechanistic studies are advancing our understanding of the molecular underpinnings of depression. While clearly far from complete and conclusive, they offer the potential to lead to the rational design of more specific therapeutic strategies and the development of safer and more effective rapid acting, long lasting antidepressants.

METHODS

The development of comprehensive omics-based approaches to the dysregulation of synaptic transmission and plasticity that underlies the core pathophysiology of MDD are reviewed to illustrate the fundamental elements.

RESULTS

This review frames the rationale for the conceptualization of depression as a "pathway disease". As such, it culminates in the call for the development of novel state-of-the-art "-omics approaches" and neurosystems biological techniques necessary to advance our understanding of spatiotemporal interactions associated with targeting glutamatergic-triggered signaling in the CNS.

CONCLUSION

These technologies will enable the development of novel psychiatric medications specifically targeted to impact specific, critical intracellular networks in a more focused manner and have the potential to offer new dimensions in the area of translational neuropsychiatry.

NMDA Receptors in the Central Nervous System

NMDA-type glutamate receptors are ligand-gated ion channels that mediate a major component of excitatory neurotransmission in the central nervous system (CNS). They are widely distributed at all stages of development and are critically involved in normal brain functions, including neuronal development and synaptic plasticity. NMDA receptors are also implicated in the pathophysiology of numerous neurological and psychiatric disorders, such as ischemic stroke, traumatic brain injury, Alzheimer's disease, epilepsy, mood disorders, and schizophrenia. For these reasons, NMDA receptors have been intensively studied in the past several decades to elucidate their physiological roles and to advance them as therapeutic targets. Seven NMDA receptor subunits exist that assemble into a diverse array of tetrameric receptor complexes, which are differently regulated, have distinct regional and developmental expression, and possess a wide range of functional and pharmacological properties. The diversity in subunit composition creates NMDA receptor subtypes with distinct physiological roles across neuronal cell types and brain regions, and enables precise tuning of synaptic transmission. Here, we will review the relationship between NMDA receptor structure and function, the diversity and significance of NMDA receptor subtypes in the CNS, as well as principles and rules by which NMDA receptors operate in the CNS under normal and pathological conditions.

Instant and Persistent Antidepressant Response of Gardenia Yellow Pigment Is Associated with Acute Protein Synthesis and Delayed Upregulation of BDNF Expression in the Hippocampus

Gardenia yellow pigment (GYP) is a collection of compounds with shared structure of crocin, which confers antidepressant activity. GYP is remarkably enriched in Gardenia jasminoides Ellis, implicated in rapid antidepressant effects that are exerted through enhanced neuroplasticity. This study aims to investigate the rapid antidepressant-like activity of GYP and its underlying mechanism. After the optimal dose was determined, antidepressant responses in tail suspension test or forced swim test were monitored at 30 min, 1 day, 3 days, and 7 days post a single GYP administration. Rapid antidepressant potential was tested using learned helplessness paradigm. The expression of proteins involved in hippocampal neuroplasticity was determined. The effect of blockade of protein synthesis on GYP's antidepressant response was examined. Antidepressant response was detected at 30 min, and lasted for at least 3 days post a single administration of GYP. A single administration of GYP also reversed the deficits in learned helplessness test. Thirty minutes post GYP administration, ERK signaling was activated, and its downstream effector phosphorylated eukaryotic elongation factor 2 was inhibited, contributing to increased protein translation. Expression of synaptic proteins GluR1 and synapsin 1 was upregulated. Blockade of protein synthesis with anisomycin blunted the immediate antidepressant response of GYP. CREB signaling and BDNF expression were upregulated at 24 h, but not at 30 min. In conclusion, GYP-induced immediate antidepressant response was dependent on synthesis of proteins, including synaptic proteins. This was followed by enhanced expression of CREB and BDNF, which likely mediated the persistent antidepressant responses.

Comparing the actions of lanicemine and ketamine in depression: key role of the anterior cingulate

Intravenous infusion of lanicemine (formerly AZD6765), a low trapping non-selective N-methyl-D-aspartate (NMDA) receptor antagonist, induces antidepressant effects with a similar time course to ketamine. We investigated whether a single dose lanicemine infusion would reproduce the previously reported decrease in subgenual anterior cingulate cortex (sgACC) activity evoked by ketamine, a potential mechanism of antidepressant efficacy. Sixty un-medicated adults meeting the criteria for major depressive disorder were randomly assigned to receive constant intravenous infusions of ketamine, lanicemine or saline during a 60min pharmacological magnetic resonance imaging (phMRI) scan. Both ketamine and lanicemine gradually increased the blood oxygen level dependent signal in sgACC and rostral ACC as the primary outcome measure. No decreases in signal were seen in any region. Interviewer-rated psychotic and dissociative symptoms were minimal following administration of lanicemine. There was no significant antidepressant effect of either infusion compared to saline. The previously reported deactivation of sgACC after ketamine probably reflects the rapid and pronounced subjective effects evoked by the bolus-infusion method used in the previous study. Activation of the ACC was observed following two different NMDA compounds in both Manchester and Oxford using different 3T MRI scanners, and this effect predicted improvement in mood 1 and 7 days post-infusion. These findings suggest that the initial site of antidepressant action for NMDA antagonists may be the ACC (NCT01046630. A Phase I, Multi-centre, Double-blind, Placebo-controlled Parallel Group Study to Assess the pharmacoMRI Effects of AZD6765 in Male and Female Subjects Fulfilling the Criteria for Major Depressive Disorder; http://clinicaltrials.gov/show/NCT01046630).

Ultimate Translation: Developing Therapeutics Targeting on N-Methyl-d-Aspartate Receptor

N-Methyl-d-aspartate receptors (NMDARs) are broadly distributed in the central nervous system (CNS), where they mediate excitatory signaling. NMDAR-mediated neurotransmission (NMDARMN) is the molecular engine of learning, memory and cognition, which are the basis for high cortical function. NMDARMN is also critically involved in the development and plasticity of CNS. Due to its essential and critical role, either over- or under-activation of NMDARMN can contribute substantially to the development of CNS disorders. The involvement of NMDARMN has been demonstrated in a variety of CNS disorders, including schizophrenia, depression, posttraumatic stress disorder, aging, mild cognitive impairment and Alzheimer's dementia, amyotrophic lateral sclerosis, and anti-NMDAR encephalitis. Several targets to "correct" or "reset" the NMDARMN in these CNS disorders have been identified and confirmed. With analogy to aminergic treatments, these targets include the glycine/d-serine co-agonist site, channel ionophore, glycine transporter-1, and d-amino acid oxidase. It is still early days in terms of developing novel therapeutics targeting the NMDAR. However, agents modulating NMDARMN hold promise as the next generation of CNS therapeutics.

Single-dose infusion ketamine and non-ketamine N-methyl-d-aspartate receptor antagonists for unipolar and bipolar depression: a meta-analysis of efficacy, safety and time trajectories

BACKGROUND

Ketamine and non-ketamine N-methyl-d-aspartate receptor antagonists (NMDAR antagonists) recently demonstrated antidepressant efficacy for the treatment of refractory depression, but effect sizes, trajectories and possible class effects are unclear.

METHOD

We searched PubMed/PsycINFO/Web of Science/clinicaltrials.gov until 25 August 2015. Parallel-group or cross-over randomized controlled trials (RCTs) comparing single intravenous infusion of ketamine or a non-ketamine NMDAR antagonist v. placebo/pseudo-placebo in patients with major depressive disorder (MDD) and/or bipolar depression (BD) were included in the analyses. Hedges' g and risk ratios and their 95% confidence intervals (CIs) were calculated using a random-effects model. The primary outcome was depressive symptom change. Secondary outcomes included response, remission, all-cause discontinuation and adverse effects.

RESULTS

A total of 14 RCTs (nine ketamine studies: n = 234; five non-ketamine NMDAR antagonist studies: n = 354; MDD = 554, BD = 34), lasting 10.0 ± 8.8 days, were meta-analysed. Ketamine reduced depression significantly more than placebo/pseudo-placebo beginning at 40 min, peaking at day 1 (Hedges' g = -1.00, 95% CI -1.28 to -0.73, p < 0.001), and loosing superiority by days 10-12. Non-ketamine NMDAR antagonists were superior to placebo only on days 5-8 (Hedges' g = -0.37, 95% CI -0.66 to -0.09, p = 0.01). Compared with placebo/pseudo-placebo, ketamine led to significantly greater response (40 min to day 7) and remission (80 min to days 3-5). Non-ketamine NMDAR antagonists achieved greater response at day 2 and days 3-5. All-cause discontinuation was similar between ketamine (p = 0.34) or non-ketamine NMDAR antagonists (p = 0.94) and placebo. Although some adverse effects were more common with ketamine/NMDAR antagonists than placebo, these were transient and clinically insignificant.

CONCLUSIONS

A single infusion of ketamine, but less so of non-ketamine NMDAR antagonists, has ultra-rapid efficacy for MDD and BD, lasting for up to 1 week. Development of easy-to-administer, repeatedly given NMDAR antagonists without risk of brain toxicity is of critical importance.

The qEEG Signature of Selective NMDA NR2B Negative Allosteric Modulators; A Potential Translational Biomarker for Drug Development

The antidepressant activity of the N-methyl-D-aspartate (NMDA) receptor channel blocker, ketamine, has led to the investigation of negative allosteric modulators (NAMs) selective for the NR2B receptor subtype. The clinical development of NR2B NAMs would benefit from a translational pharmacodynamic biomarker that demonstrates brain penetration and functional inhibition of NR2B receptors in preclinical species and humans. Quantitative electroencephalography (qEEG) is a translational measure that can be used to demonstrate pharmacodynamic effects across species. NMDA receptor channel blockers, such as ketamine and phencyclidine, increase the EEG gamma power band, which has been used as a pharmacodynamic biomarker in the development of NMDA receptor antagonists. However, detailed qEEG studies with ketamine or NR2B NAMs are lacking in nonhuman primates. The aim of the present study was to determine the effects on the qEEG power spectra of the NR2B NAMs traxoprodil (CP-101,606) and BMT-108908 in nonhuman primates, and to compare them to the NMDA receptor channel blockers, ketamine and lanicemine. Cynomolgus monkeys were surgically implanted with EEG radio-telemetry transmitters, and qEEG was measured after vehicle or drug administration. The relative power for a number of frequency bands was determined. Ketamine and lanicemine increased relative gamma power, whereas the NR2B NAMs traxoprodil and BMT-108908 had no effect. Robust decreases in beta power were elicited by ketamine, traxoprodil and BMT-108908; and these agents also produced decreases in alpha power and increases in delta power at the doses tested. These results suggest that measurement of power spectra in the beta and delta bands may represent a translational pharmacodynamic biomarker to demonstrate functional effects of NR2B NAMs. The results of these studies may help guide the selection of qEEG measures that can be incorporated into early clinical evaluation of NR2B NAMs in healthy humans.

Emerging treatment mechanisms for depression: focus on glutamate and synaptic plasticity

Major depression is a chronic and debilitating illness that effects approximately 1 in 5 people, but currently available treatments are limited by low rates of efficacy, therapeutic time lag, and undesirable side effects. Recent efforts have been directed towards investigating rapid-acting agents that reverse the behavioral and neuronal deficits of chronic stress and depression, notably the glutamate NMDA receptor antagonist ketamine. The cellular mechanisms underlying the rapid antidepressant actions of ketamine and related agents are discussed, as well as novel, selective glutamatergic receptor targets that are safer and have fewer side effects.

Role of calcium, glutamate and NMDA in major depression and therapeutic application

Major depression is a common, recurrent mental illness that affects millions of people worldwide. Recently, a unique fast neuroprotective and antidepressant treatment effect has been observed by ketamine, which acts via the glutamatergic system. Hence, a steady accumulation of evidence supporting a role for the excitatory amino acid neurotransmitter (EAA) glutamate in the treatment of depression has been observed in the last years. Emerging evidence indicates that N-methyl-D-aspartate (NMDA), group 1 metabotropic glutamate receptor antagonists and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) agonists have antidepressant properties. Indeed, treatment with NMDA receptor antagonists has shown the ability to sprout new synaptic connections and reverse stress-induced neuronal changes. Based on glutamatergic signaling, a number of therapeutic drugs might gain interest in the future. Several compounds such as ketamine, memantine, amantadine, tianeptine, pioglitazone, riluzole, lamotrigine, AZD6765, magnesium, zinc, guanosine, adenosine aniracetam, traxoprodil (CP-101,606), MK-0657, GLYX-13, NRX-1047, Ro25-6981, LY392098, LY341495, D-cycloserine, D-serine, dextromethorphan, sarcosine, scopolamine, pomaglumetad methionil, LY2140023, LY404039, MGS0039, MPEP, 1-aminocyclopropanecarboxylic acid, all of which target this system, have already been brought up, some of them recently. Drugs targeting the glutamatergic system might open up a promising new territory for the development of drugs to meet the needs of patients with major depression.

Negative Allosteric Modulators Selective for The NR2B Subtype of The NMDA Receptor Impair Cognition in Multiple Domains

Antidepressant activity of N-methyl-D-aspartate (NMDA) receptor antagonists and negative allosteric modulators (NAMs) has led to increased investigation of their behavioral pharmacology. NMDA antagonists, such as ketamine, impair cognition in multiple species and in multiple cognitive domains. However, studies with NR2B subtype-selective NAMs have reported mixed results in rodents including increased impulsivity, no effect on cognition, impairment or even improvement of some cognitive tasks. To date, the effects of NR2B-selective NAMs on cognitive tests have not been reported in nonhuman primates. The current study evaluated two selective NR2B NAMs, CP101,606 and BMT-108908, along with the nonselective NMDA antagonists, ketamine and AZD6765, in the nonhuman primate Cambridge Neuropsychological Test Automated Battery (CANTAB) list-based delayed match to sample (list-DMS) task. Ketamine and the two NMDA NR2B NAMs produced selective impairments in memory in the list-DMS task. AZD6765 impaired performance in a non-specific manner. In a separate cohort, CP101,606 impaired performance of the nonhuman primate CANTAB visuo-spatial Paired Associates Learning (vsPAL) task with a selective impairment at more difficult conditions. The results of these studies clearly show that systemic administration of a selective NR2B NAM can cause transient cognitive impairment in multiple cognitive domains.

[Psychopharmacology of anxiety and depression: Historical aspects, current treatments and perspectives]

Pharmacological treatment of acute anxiety still relies on benzodiazepines, while chronic anxiety disorders and depression are treated with different antidepressants, according to specific indications. The monoaminergic axis is represented by two families which are being developed: (i) serotonin-norepinephrine-dopamine reuptake inhibitors (SNDRI), also called triple reuptake inhibitors (TRI), for the treatment of depression (amitifadine), (ii) multimodal antidepressants for depression and anxiety disorders (generalized anxiety disorder mainly) (tedatioxetine, vortioxetine and vilazodone). Third-generation antipsychotics (aripiprazole, lurasidone, brexpiprazole, cariprazine) appear relevant in the treatment of resistant depression and some anxiety disorders. Among the modulators of the glutamatergic axis, promising compounds include: (i) ionotropic regulators of NMDA receptors: esketamine, AVP-923 and AVP-786, CERC-301, rapastinel (Glyx-13), NRX-1074 developed for depression, rapastinel and bitopertine developed for obsessive compulsive disorder, (ii) metabotropic glutamate receptors modulators: decoglurant and basimglurant developed for depression and mavoglurant developed for obsessive compulsive disorder.

Novel Glutamatergic Treatments for Severe Mood Disorders

All currently approved antidepressant medications for major depressive disorder (MDD) and bipolar disorder act primarily on the monoaminergic system and have varying affinities for serotonergic, norepinephrine-ergic, and/or dopaminergic receptors. Unfortunately, these drugs are only effective in approximately two-thirds of patients. Glutamate is the major excitatory neurotransmitter in the central nervous system, and the glutamatergic system has been implicated in the pathophysiology of MDD. Here, we review the putative involvement of the glutamate receptor subtypes-N-methyl-D-aspartate (NMDA), α-amino-3-hydroxyl-5-methyl-4-isoxazoleproprionic acid (AMPA), kainate, and the group I, II, and III metabotropic glutamate receptors (mGluRs)-in the development of novel and more effective treatments for MDD as well as preclinical and clinical trials of drugs targeting these receptors. The rapid and robust antidepressant effects of ketamine-an NMDA receptor antagonist-have been consistently replicated in multiple trials. Other glutamatergic drugs have been investigated with varying success. Here, we highlight some of the most interesting results, including: 1) repeated oral, intramuscular, and sublingual ketamine appears to be less robustly effective than intravenous ketamine, but also causes fewer significant adverse effects; 2) the glycine partial agonist GLYX-13 appears to be effective both as monotherapy and adjunctive treatment in the treatment of MDD. An oral analogue, NRX-1074, is currently under investigation; and 3) mGluR modulators targeting mGluR5 have demonstrated convincing preclinical results.

Targeting Neuronal Networks with Combined Drug and Stimulation Paradigms Guided by Neuroimaging to Treat Brain Disorders

Improved therapy of brain disorders can be achieved by focusing on neuronal networks, utilizing combined pharmacological and stimulation paradigms guided by neuroimaging. Neuronal networks that mediate normal brain functions, such as hearing, interact with other networks, which is important but commonly neglected. Network interaction changes often underlie brain disorders, including epilepsy. "Conditional multireceptive" (CMR) brain areas (e.g., brainstem reticular formation and amygdala) are critical in mediating neuroplastic changes that facilitate network interactions. CMR neurons receive multiple inputs but exhibit extensive response variability due to milieu and behavioral state changes and are exquisitely sensitive to agents that increase or inhibit GABA-mediated inhibition. Enhanced CMR neuronal responsiveness leads to expression of emergent properties--nonlinear events--resulting from network self-organization. Determining brain disorder mechanisms requires animals that model behaviors and neuroanatomical substrates of human disorders identified by neuroimaging. However, not all sites activated during network operation are requisite for that operation. Other active sites are ancillary, because their blockade does not alter network function. Requisite network sites exhibit emergent properties that are critical targets for pharmacological and stimulation therapies. Improved treatment of brain disorders should involve combined pharmacological and stimulation therapies, guided by neuroimaging, to correct network malfunctions by targeting specific network neurons.

Current Status of Ketamine and Related Therapies for Mood and Anxiety Disorders

Major Depressive Disorder (MDD) is a leading cause of disability worldwide. Despite a plethora of established treatments, less than one-third of individuals with MDD achieve stable remission of symptoms. Given limited efficacy and significant lag time to onset of therapeutic action among conventional antidepressants, interest has shifted to treatments that act outside of the monoamine neurotransmitter systems (e.g., serotonin, norepinephrine, and dopamine). Preclinical and clinical research on the glutamate system has been particularly promising in this regard. Accumulating evidence shows support for a rapid antidepressant effect of ketamine - a glutamate N-methyl-d-aspartate (NMDA) receptor antagonist. The present article reviews the pharmacology, safety, and efficacy of ketamine as a novel therapeutic agent for mood and anxiety disorders. The majority of clinical trials using ketamine have been conducted in patients with treatment resistant forms of MDD; recent work has begun to examine ketamine in bipolar disorder, posttraumatic stress disorder, and obsessive-compulsive disorder. The impact of ketamine on suicidal ideation is also discussed. The current status and prospects for the identification of human biomarkers of ketamine treatment response and hurdles to treatment development are considered. We conclude by considering modulators of the glutamate system other than ketamine currently in development as potential novel treatment strategies for mood and anxiety disorders.

Towards new mechanisms: an update on therapeutics for treatment-resistant major depressive disorder

Depression is a devastating disorder that places a significant burden on both the individual and society. As such, the discovery of novel therapeutics and innovative treatments--especially for treatment-resistant depression (TRD)--are essential. Research into antidepressant therapies for TRD has evolved from explorations of antidepressants with primary mechanisms of action on the monoaminergic neurotransmitter system to augmentation agents with primary mechanisms both within and outside of the serotonin/norepinephrine system. Now the field of antidepressant research has changed trajectories yet again; this time, compounds with primary mechanisms of action on the glutamatergic, cholinergic and opioid systems are in the forefront of antidepressant exploration. In this review, we will discuss the most recent research surrounding these novel compounds. In addition, we will discuss novel device-based therapeutics, with a particular focus on transcranial magnetic stimulation. In many cases of antidepressant drug discovery, the role of serendipity coupled with meticulous clinical observation in drug development in medicine was crucial. Moving forward, we must look toward the combination of innovation plus improvements on the remarkable discoveries thus far to advance the field of antidepressant research.

Ketamine and other glutamate receptor modulators for depression in adults

BACKGROUND

Considering the ample evidence of involvement of the glutamate system in the pathophysiology of depression, pre-clinical and clinical studies have been conducted to assess the antidepressant efficacy of glutamate inhibition, and glutamate receptor modulators in particular. This review focuses on the use of glutamate receptor modulators in unipolar depression.

OBJECTIVES

To assess the effects - and review the acceptability - of ketamine and other glutamate receptor modulators in comparison to placebo (or saline placebo), other pharmacologically active agents, or electroconvulsive therapy (ECT) in alleviating the acute symptoms of depression in people with unipolar major depressive disorder.

SEARCH METHODS

We searched the Cochrane Depression, Anxiety and Neurosis Review Group's Specialised Register (CCDANCTR, to 9 January 2015). This register includes relevant randomised controlled trials (RCTs) from: the Cochrane Library (all years), MEDLINE (1950 to date), EMBASE (1974 to date), and PsycINFO (1967 to date). We did not apply any restrictions to date, language or publication status.

SELECTION CRITERIA

Double- or single-blind RCTs comparing ketamine, memantine, or other glutamate receptor modulators with placebo (or saline placebo), other active psychotropic drugs, or electroconvulsive therapy (ECT) in adults with unipolar major depression.

DATA COLLECTION AND ANALYSIS

Three review authors independently identified studies, assessed trial quality and extracted data. The primary outcomes for this review were response rate and adverse events.

MAIN RESULTS

We included 25 studies (1242 participants) on ketamine (9 trials), memantine (3), AZD6765 (3), D-cycloserine (2), Org26576 (2), atomoxetine (1), CP-101,606 (1), MK-0657 (1), N-acetylcysteine (1), riluzole (1) and sarcosine (1). Twenty-one studies were placebo-controlled and the majority were two-arm studies (23 out of 25). Twenty-two studies defined an inclusion criteria specifying the severity of depression; 11 specified at least moderate depression; eight, severe depression; and the remaining three, mild-moderate depression. Nine studies recruited only treatment-resistant patients.We rated the risk of bias as low or unclear for most domains, though lack of detail regarding masking of treatment in the studies reduced our certainty in the effect for all outcomes. We rated three studies as having high risk for selective outcome reporting. Many trials did not provide information on all the prespecified outcomes and we found no data, or very limited data, on very important issues like suicidality, cognition, quality of life, costs to healthcare services and dropouts due to lack of efficacy.Among all glutamate receptor modulators, only ketamine (administered intravenously) proved to be more efficacious than placebo, though the quality of evidence was limited by risk of bias and small sample sizes. There was low quality evidence that treatment with ketamine increased the likelihood of response after 24 hours (odds ratio (OR) 10.77, 95% confidence interval (CI) 2.00 to 58.00; 3 RCTs, 56 participants), 72 hours (OR 12.59, 95% CI 2.38 to 66.73; 3 RCTs, 56 participants), and one week (OR 2.58, 95% CI 1.08 to 6.16; 4 RCTs, 131 participants). The effect of ketamine was even less certain at two weeks, as data were available from only one trial (OR 0.93, 95% CI 0.31 to 2.83; 51 participants, low quality evidence). This was consistent across all efficacy outcomes. Ketamine caused more confusion and emotional blunting compared to placebo. There was insufficient evidence to determine if this increased the likelihood of leaving the study early (OR 1.90, 95% CI 0.43 to 8.47; 5 RCTs, 139 participants, low quality evidence).One RCT with 72 participants reported higher numbers of responders on ketamine than midazolam at 24 hours (OR 0.36, 95% CI 0.14 to 0.58), 72 hours (OR 0.37, 95% CI 0.16 to 0.59), and one week (OR 0.29, 95% CI 0.08 to 0.49). However, midazolam was better tolerated than ketamine in terms of blurred vision, dizziness, general malaise and nausea/vomiting at 24 hours post-infusion. The evidence contributing to these outcomes was of low quality.We found better efficacy of sarcosine over citalopram at four weeks (OR 6.93, 95% CI 1.53 to 31.38; 1 study, 40 participants), but not at two weeks (OR: 8.14, 95% CI 0.88 to 75.48); fewer participants in the sarcosine group experienced adverse events (OR 0.04, 95% CI 0.00 to 0.68; P = 0.03, 1 study, 40 participants). This was based on low quality evidence. No significant results were found for the remaining glutamate receptor modulators.In one study with 18 participants, ketamine was more effective than ECT at 24 hours (OR 28.00, 95% CI 2.07 to 379.25) and 72 hours (OR 12.25, 95% CI 1.33 to 113.06), but not at one week (OR 3.35, 95% CI 0.12 to 93.83), or two weeks (OR 3.35, 95% CI 0.12 to 93.83). No differences in terms of adverse events were found between ketamine and ECT, however the only adverse events reported were blood pressure and heart rate. This study was rated as very low quality.

AUTHORS' CONCLUSIONS

We found limited evidence for ketamine's efficacy over placebo at time points up to one week in terms of the primary outcome, response rate. The effects were less certain at two weeks post-treatment. No significant results were found for the remaining ten glutamate receptor modulators, except for sarcosine being more effective than citalopram at four weeks. In terms of adverse events, the only significant differences in favour of placebo over ketamine were in regards to confusion and emotional blunting. Despite the promising nature of these preliminary results, our confidence in the evidence was limited by risk of bias and the small number of participants. Many trials did not provide information on all the prespecified outcomes and we found no data, or very limited data, on very important issues like suicidality, cognition, quality of life, costs to healthcare services and dropouts due to lack of efficacy.All included studies administered ketamine intravenously, which can pose practical problems in clinical practice. Very few trials were included in the meta-analyses for each comparison; the majority of comparisons contained only one study. Further RCTs (with adequate blinding) are needed to explore different modes of administration of ketamine with longer follow-up, which test the comparative efficacy of ketamine and the efficacy of repeated administrations.

Potential of Glutamate-Based Drug Discovery for Next Generation Antidepressants

Recently, ketamine has been demonstrated to exert rapid-acting antidepressant effects in patients with depression, including those with treatment-resistant depression, and this discovery has been regarded as the most significant advance in drug development for the treatment of depression in over 50 years. To overcome unwanted side effects of ketamine, numerous approaches targeting glutamatergic systems have been vigorously investigated. For example, among agents targeting the NMDA receptor, the efficacies of selective GluN2B receptor antagonists and a low-trapping antagonist, as well as glycine site modulators such as GLYX-13 and sarcosine have been demonstrated clinically. Moreover, agents acting on metabotropic glutamate receptors, such as mGlu2/3 and mGlu5 receptors, have been proposed as useful approaches to mimicking the antidepressant effects of ketamine. Neural and synaptic mechanisms mediated through the antidepressant effects of ketamine have been being delineated, most of which indicate that ketamine improves abnormalities in synaptic transmission and connectivity observed in depressive states via the AMPA receptor and brain-derived neurotrophic factor-dependent mechanisms. Interestingly, some of the above agents may share some neural and synaptic mechanisms with ketamine. These studies should provide important insights for the development of superior pharmacotherapies for depression with more potent and faster onsets of actions.

Effect of NMDAR antagonists in the tetrabenazine test for antidepressants: comparison with the tail suspension test

OBJECTIVE

The N-methyl-d-aspartate receptor (NMDAR) antagonist ketamine, produces rapid and enduring antidepressant effect in patients with treatment-resistant depression. Similar dramatic effects have not been observed in clinical trials with other NMDAR antagonists indicating ketamine may possess unique pharmacological properties. Tetrabenazine induces ptosis (a drooping of the eyelids), and the reversal of this effect, attributed to a sympathomimetic action, has been used to detect first-generation antidepressants, as well as ketamine. Because the actions of other NMDAR antagonists have not been reported in this measure, we examined whether reversal of tetrabenazine-induced ptosis was unique to ketamine, or a class effect of NMDAR antagonists.

METHODS

The effects of ketamine and other NMDAR antagonists to reverse tetrabenazine-induced ptosis were examined and compared with their antidepressant-like effects in the tail suspension test (TST) in mice.

RESULTS

All the NMDAR antagonists tested produced a partial reversal of tetrabenazine-induced ptosis and, as expected, reduced immobility in the TST. Ketamine, memantine, MK-801 and AZD6765 were all about half as potent in reversing tetrabenazine-induced ptosis compared to reducing immobility in the TST, while an NR2B antagonist (Ro 25-6981) and a glycine partial agonist (ACPC) were equipotent in both tests.

CONCLUSION

The ability to reverse tetrabenazine-induced ptosis is a class effect of NMDAR antagonists. These findings are consistent with the hypothesis that the inability of memantine, AZD6765 (lanicemine) and MK-0657 to reproduce the rapid and robust antidepressant effects of ketamine in the clinic result from insufficient dosing rather than a difference in mechanism of action among these NMDAR antagonists.

Two cellular hypotheses explaining the initiation of ketamine's antidepressant actions: Direct inhibition and disinhibition

A single, low dose of ketamine evokes antidepressant actions in depressed patients and in patients with treatment-resistant depression (TRD). Unlike classic antidepressants, which regulate monoamine neurotransmitter systems, ketamine is an antagonist of the N-methyl-D-aspartate (NMDA) family of glutamate receptors. The effectiveness of NMDAR antagonists in TRD unveils a new set of targets for therapeutic intervention in major depressive disorder (MDD) and TRD. However, a better understanding of the cellular mechanisms underlying these effects is required for guiding future therapeutic strategies, in order to minimize side effects and prolong duration of efficacy. Here we review the evidence for and against two hypotheses that have been proposed to explain how NMDAR antagonism initiates protein synthesis and increases excitatory synaptic drive in corticolimbic brain regions, either through selective antagonism of inhibitory interneurons and cortical disinhibition, or by direct inhibition of cortical pyramidal neurons. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Nitrous Oxide for Treatment-Resistant Major Depression: A Proof-of-Concept Trial

BACKGROUND

N-methyl-D-aspartate receptor antagonists, such as ketamine, have rapid antidepressant effects in patients with treatment-resistant depression (TRD). We hypothesized that nitrous oxide, an inhalational general anesthetic and N-methyl-D-aspartate receptor antagonist, may also be a rapidly acting treatment for TRD.

METHODS

In this blinded, placebo-controlled crossover trial, 20 patients with TRD were randomly assigned to 1-hour inhalation of 50% nitrous oxide/50% oxygen or 50% nitrogen/50% oxygen (placebo control). The primary endpoint was the change on the 21-item Hamilton Depression Rating Scale (HDRS-21) 24 hours after treatment.

RESULTS

Mean duration of nitrous oxide treatment was 55.6 ± 2.5 (SD) min at a median inspiratory concentration of 44% (interquartile range, 37%-45%). In two patients, nitrous oxide treatment was briefly interrupted, and the treatment was discontinued in three patients. Depressive symptoms improved significantly at 2 hours and 24 hours after receiving nitrous oxide compared with placebo (mean HDRS-21 difference at 2 hours, -4.8 points, 95% confidence interval [CI], -1.8 to -7.8 points, p = .002; at 24 hours, -5.5 points, 95% CI, -2.5 to -8.5 points, p < .001; comparison between nitrous oxide and placebo, p < .001). Four patients (20%) had treatment response (reduction ≥50% on HDRS-21) and three patients (15%) had a full remission (HDRS-21 ≤ 7 points) after nitrous oxide compared with one patient (5%) and none after placebo (odds ratio for response, 4.0, 95% CI, .45-35.79; OR for remission, 3.0, 95% CI, .31-28.8). No serious adverse events occurred; all adverse events were brief and of mild to moderate severity.

CONCLUSIONS

This proof-of-concept trial demonstrated that nitrous oxide has rapid and marked antidepressant effects in patients with TRD.

Effect of multiple intravenous doses of lanicemine (AZD6765) on the pharmacokinetics of midazolam in healthy subjects

The objectives of the present study were to evaluate safety and tolerability as well as the effects of multiple doses of lanicemine on the pharmacokinetics of a CYP3A substrate, midazolam. A total of 46 healthy volunteers were enrolled in the open-label, fixed-sequence, nonrandomized study. All volunteers received an oral dose of 5 mg of midazolam alone or after 6 days of 150 mg daily intravenous infusion of lanicemine. Lanicemine reached a plasma Cmax of 1.51 μg/mL after 150 mg daily dosing to steady state. The geometric mean CL, Vss, and t1/2 of lanicemine were 8.1 L/h, 122.0 L, and 10.4 hours, respectively. The geometric least-squares mean ratios and 90% confidence intervals for midazolam AUC0- ∞ , and Cmax were within the 80% to 125% limits when lanicemine plus midazolam treatment was compared with midazolam alone, demonstrating that daily dosing with 150 mg of lanicemine for 6 days had no effect on CYP3A activity. Comprehensive physiologically based pharmacokinetic modeling using in vitro and in silico findings also indicated lanicemine would have little impact on the pharmacokinetics of CYP3A substrate, such as midazolam. In addition, lanicemine and midazolam administered alone or in combination were generally safe and well tolerated.

The ups and downs of modelling mood disorders in rodents

The wide spectrum of disruptions that characterizes major depressive disorder (MDD) and bipolar disorder (BD) highlights the difficulties researchers are posed with as they try to mimic these disorders in the laboratory. Nonetheless, numerous attempts have been made to create rodent models of mood disorders or at least models of the symptoms of MDD and BD. Present antidepressants are all descendants of the serendipitous findings in the 1950s that the monoamine oxidase inhibitor iproniazid and the tricyclic antidepressant imipramine were effective antidepressants. Thus, the need for improved animal models to provide insights into the neuropathology underlying the disease is critical. Such information is in turn crucial for identifying new antidepressants and mood stabilisers. Currently, there is a shift away from traditional animal models to more focused research dealing with an endophenotype-style approach, genetic models, and incorporation of new findings from human neuroimaging and genetic studies. Such approaches are opening up more tractable avenues for understanding the neurobiological and genetic bases of these disorders. Further, such models promise to yield better translational animal models and hence more fruitful therapeutic targets. This overview focuses on such animal models and tests and how they can be used to assess MDD and BD in rodents.

Bipolar disorder and antibodies against the N-methyl-d-aspartate receptor: A gate to the involvement of autoimmunity in the pathophysiology of bipolar illness

The high prevalence of comorbidity between bipolar disorder (BD) and other medical conditions, including autoimmune diseases, supports the hypothesis of the nature of BD as a biological illness category. Hence, an immune dysregulation process may play an important role in the development of at least certain subtypes of BD. Increasing evidence also suggests that the N-methyl-d-aspartate receptor (NMDAR) may be relevant in the pathophysiology of BD. A possible key mechanism underlying the physiopathology of certain autoimmune diseases that may present with affective symptoms might be the production of anti-NMDAR auto-antibodies (auto-Abs). The best characterized autoimmune anti-NMDAR disease is the anti-NMDAR encephalitis. It has been found that 4% of these patients present isolated, mostly affective, psychiatric manifestations during their illness. An interesting suggestion emerged from this overview is that the same mechanisms that trigger affective symptoms in patients with increased anti-NMDAR auto-Abs levels could be involved in the physiopathology of at least a subgroup of BD. Future studies are needed to characterize the relationship between anti-NMDAR auto-Abs and BD.

Ketamine and other N-methyl-D-aspartate receptor antagonists in the treatment of depression: a perspective review

Current pharmacotherapies for major depressive disorder (MDD) and bipolar depression (BDep) have a distinct lag of onset that can generate great distress and impairment in patients. Furthermore, as demonstrated by several real-world effectiveness trials, their efficacy is limited. All approved antidepressant medications for MDD primarily act through monoaminergic mechanisms, agonists or antagonists with varying affinities for serotonin, norepinephrine and dopamine. The glutamate system has received much attention in recent years as an avenue for developing novel therapeutics. A single subanesthetic dose infusion of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine has been shown to have rapid and potent antidepressant effects in treatment-resistant MDD and BDep. In a reverse translational framework, ketamine's clinical efficacy has inspired many preclinical studies to explore glutamatergic mechanisms of antidepressant action. These studies have revealed enhanced synaptic plasticity/synaptogenesis via numerous molecular and cellular mechanisms: release of local translational inhibition of brain-derived neurotrophic factor and secretion from dendritic spines, mammalian target of rapamycin activation and glycogen synthase kinase-3 inhibition. Current efforts are focused on extending ketamine's antidepressant efficacy, uncovering the neurobiological mechanisms responsible for ketamine's antidepressant activity in biologically enriched subgroups, and identifying treatment response biomarkers to personalize antidepressant selection. Other NMDA receptor antagonists have been studied both preclinically and clinically, which have revealed relatively modest antidepressant effects compared with ketamine but potentially other favorable characteristics, for example, decreased dissociative or psychotomimetic effects; therefore, there is great interest in developing novel glutamatergic antidepressants with greater target specificity and/or decreased adverse effects.

60 years of advances in neuropsychopharmacology for improving brain health, renewed hope for progress

Pharmacotherapy is effective in helping many patients suffering from psychiatric and neurological disorders, and both psychotherapeutic and stimulation-based techniques likewise have important roles to play in their treatment. However, therapeutic progress has recently been slow. Future success for improving the control and prevention of brain disorders will depend upon deeper insights into their causes and pathophysiological substrates. It will also necessitate new and more rigorous methods for identifying, validating, developing and clinically deploying new treatments. A field of Research and Development (R and D) that remains critical to this endeavour is Neuropsychopharmacology which transformed the lives of patients by introducing pharmacological treatments for psychiatric disorder some 60 years ago. For about half of this time, the European College of Neuropsychopharmacology (ECNP) has fostered efforts to enhance our understanding of the brain, and to improve the management of psychiatric disorders. Further, together with partners in academia and industry, and in discussions with regulators and patients, the ECNP is implicated in new initiatives to achieve this goal. This is then an opportune moment to survey the field, to analyse what we have learned from the achievements and failures of the past, and to identify major challenges for the future. It is also important to highlight strategies that are being put in place in the quest for more effective treatment of brain disorders: from experimental research and drug discovery to clinical development and collaborative ventures for reinforcing "R and D". The present article sets the scene, then introduces and interlinks the eight articles that comprise this Special Volume of European Neuropsychopharmacology. A broad-based suite of themes is covered embracing: the past, present and future of "R and D" for psychiatric disorders; complementary contributions of genetics and epigenetics; efforts to improve the treatment of depression, neurodevelopmental and neurodegenerative disorders; and advances in the analysis and neuroimaging of cellular and cerebral circuits.

Learning from the past and looking to the future: Emerging perspectives for improving the treatment of psychiatric disorders

Modern neuropsychopharmacology commenced in the 1950s with the serendipitous discovery of first-generation antipsychotics and antidepressants which were therapeutically effective yet had marked adverse effects. Today, a broader palette of safer and better-tolerated agents is available for helping people that suffer from schizophrenia, depression and other psychiatric disorders, while complementary approaches like psychotherapy also have important roles to play in their treatment, both alone and in association with medication. Nonetheless, despite considerable efforts, current management is still only partially effective, and highly-prevalent psychiatric disorders of the brain continue to represent a huge personal and socio-economic burden. The lack of success in discovering more effective pharmacotherapy has contributed, together with many other factors, to a relative disengagement by pharmaceutical firms from neuropsychiatry. Nonetheless, interest remains high, and partnerships are proliferating with academic centres which are increasingly integrating drug discovery and translational research into their traditional activities. This is, then, a time of transition and an opportune moment to thoroughly survey the field. Accordingly, the present paper, first, chronicles the discovery and development of psychotropic agents, focusing in particular on their mechanisms of action and therapeutic utility, and how problems faced were eventually overcome. Second, it discusses the lessons learned from past successes and failures, and how they are being applied to promote future progress. Third, it comprehensively surveys emerging strategies that are (1), improving our understanding of the diagnosis and classification of psychiatric disorders; (2), deepening knowledge of their underlying risk factors and pathophysiological substrates; (3), refining cellular and animal models for discovery and validation of novel therapeutic agents; (4), improving the design and outcome of clinical trials; (5), moving towards reliable biomarkers of patient subpopulations and medication efficacy and (6), promoting collaborative approaches to innovation by uniting key partners from the regulators, industry and academia to patients. Notwithstanding the challenges ahead, the many changes and ideas articulated herein provide new hope and something of a framework for progress towards the improved prevention and relief of psychiatric and other CNS disorders, an urgent mission for our Century.

The role of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in depression: central mediators of pathophysiology and antidepressant activity?

Depression is a major psychiatric disorder affecting more than 120 million people worldwide every year. Changes in monoaminergic transmitter release are suggested to take part in the pathophysiology of depression. However, more recent experimental evidence suggests that glutamatergic mechanisms might play a more central role in the development of this disorder. The importance of the glutamatergic system in depression was particularly highlighted by the discovery that N-methyl-D-aspartate (NMDA) receptor antagonists (particularly ketamine) exert relatively long-lasting antidepressant like effects with rapid onset. Importantly, the antidepressant-like effects of NMDA receptor antagonists, but also other antidepressants (both classical and novel), require activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Additionally, expression of AMPA receptors is altered in patients with depression. Moreover, preclinical evidence supports an important involvement of AMPA receptor-dependent signaling and plasticity in the pathophysiology and treatment of depression. Here we summarize work published on the involvement of AMPA receptors in depression and discuss a possible central role for AMPA receptors in the pathophysiology, course and treatment of depression.

A systematic review and meta-analysis of randomized, double-blind, placebo-controlled trials of ketamine in the rapid treatment of major depressive episodes

BACKGROUND

There is growing interest in glutamatergic agents in depression, particularly ketamine, a glutamate N-methyl-d-aspartate (NMDA) receptor antagonist. We aimed to assess the efficacy of ketamine in major depressive episodes.

METHOD

We searched EMBASE, PsycINFO, CENTRAL, and Medline from 1962 to January 2014 to identify double-blind, randomized controlled trials with allocation concealment evaluating ketamine in major depressive episodes. Clinical remission, response and depressive symptoms were extracted by two independent raters. The primary outcome measure was clinical remission at 24 h, 3 days and 7 days post-treatment. Analyses employed a random-effects model.

RESULTS

Data were synthesized from seven RCTs employing an intravenous infusion and one RCT employing intranasal ketamine, representing 73 subjects in parallel arms and 110 subjects in cross-over designs [n = 34 with bipolar disorder (BD), n = 149 with major depressive disorder (MDD)]. Ketamine was associated with higher rates of clinical remission relative to comparator (saline or midazolam) at 24 h [OR 7.06, number needed to treat (NNT) = 5], 3 days (OR 3.86, NNT = 6), and 7 days (OR 4.00, NNT = 6), as well as higher rates of clinical response at 24 h (OR 9.10, NNT = 3), 3 days (OR 6.77, NNT = 3), and 7 days (OR 4.87, NNT = 4). A standardized mean difference of 0.90 in favor of ketamine was observed at 24 h based on depression rating scale scores, with group comparisons revealing greater efficacy in unipolar depression compared to bipolar depression (1.07 v. 0.68). Ketamine was associated with transient psychotomimetic effects, but no persistent psychosis or affective switches.

CONCLUSION

Our meta-analysis suggests that single administrations ketamine are efficacious in the rapid treatment of unipolar and bipolar depression. Additional research is required to determine optimal dosing schedules, route, treatment schedules, and the potential efficacy of other glutamatergic agents.

Inflammation and immunity in schizophrenia: implications for pathophysiology and treatment

Complex interactions between the immune system and the brain might have important aetiological and therapeutic implications for neuropsychiatric brain disorders. A possible association between schizophrenia and the immune system was postulated over a century ago, and is supported by epidemiological and genetic studies pointing to links with infection and inflammation. Contrary to the traditional view that the brain is an immunologically privileged site shielded behind the blood-brain barrier, studies in the past 20 years have noted complex interactions between the immune system, systemic inflammation, and the brain, which can lead to changes in mood, cognition, and behaviour. In this Review, we describe some of the important areas of research regarding innate and adaptive immune response in schizophrenia and related psychotic disorders that, we think, will be of interest to psychiatric clinicians and researchers. We discuss potential mechanisms and therapeutic implications of these findings, including studies of anti-inflammatory drugs in schizophrenia, describe areas for development, and offer testable hypotheses for future investigations.

Randomized proof of concept trial of GLYX-13, an N-methyl-D-aspartate receptor glycine site partial agonist, in major depressive disorder nonresponsive to a previous antidepressant agent

BACKGROUND

Approximately 45% of patients with major depressive disorder (MDD) do not remit when treated with biogenic amine antidepressants. Consequently, there is a significant need for antidepressant agents with different mechanisms of action. Early proof of concept (POC) studies with such novel agents play a significant role in helping drug developers identify agents and mechanisms of action that merit more intensive research. Studies have demonstrated that high affinity N-methyl-Daspartate (NMDA) receptor blockers (eg, ketamine) can produce rapid antidepressant effects in patients who have not responded to currently available agents, but treatment with these agents is accompanied by psychotomimetic effects that make their use problematic. This column describes a POC study involving GLYX-13, an N-methyl-D-aspartate receptor glycine site functional partial agonist.

METHOD

In this double-blind, randomized, placebo-controlled study, a single intravenous (IV) dose of GLYX-13 (1, 5, 10, or 30 mg/kg) or placebo was administered to 116 subjects with MDD who had not benefitted from a trial of at least one biogenic amine antidepressant during the current episode. The primary outcome measure was score on the Hamilton Depression Rating Scale-17 (Ham-D17), which was used to rate overall depressive symptoms at baseline and at 24 hours and days 3, 7, 14, and, in some arms, days 21 and 28 after administration.

RESULTS

GLYX-13, 5 or 10 mg/kg IV, reduced depressive symptoms as assessed by the Ham-D17 at days 1 through 7. Onset of action as assessed using the Bech-6 occurred within 2 hours. GLYX-13 did not elicit psychotomimetic or other significant side effects.

CONCLUSION

In this early POC study, GLYX-13 reduced depressive symptoms within 2 hours and this effect was maintained for 7 days on average in subjects with MDD who had not responded to another antidepressant agent during the current depressive episode. The findings of this study support the hypothesis that modulation of the NMDA receptor is a valid target for the development of antidepressant drugs and the need for additional studies to further evaluate the effects of GLYX-13. POC studies such as the one described here play a pivotal role in allowing drug researchers to decide whether to move forward with larger and more expensive studies, and they enable them to focus available resources on those molecules that appear to have the most therapeutic promise. Based on the POC study described here, a multiple dose study has been completed which showed sustained therapeutic benefit with repeated dosing of GLYX-13 for more than 6 weeks. Phase 3 studies are now being planned.

The promise of ketamine for treatment-resistant depression: current evidence and future directions

Major depressive disorder (MDD) is one of the most disabling diseases worldwide and is becoming a significant public health threat. Current treatments for MDD primarily consist of monoamine-targeting agents and have limited efficacy. However, the glutamate neurotransmitter system has recently come into focus as a promising alternative for novel antidepressant treatments. We review the current data on the glutamate NMDA receptor antagonist ketamine, which has been shown in clinical trials to act as a rapid antidepressant in MDD. We also examine ketamine efficacy on dimensions of psychopathology, including anhedonia, cognition, and suicidality, consistent with the NIMH Research Domain Criteria initiative. Other aspects of ketamine reviewed in this paper include safety and efficacy, different administration methods, and the risks of misuse of ketamine outside of medical settings. Finally, we conclude with a discussion of glutamatergic agents other than ketamine currently being tested as novel antidepressants.

Cognition as a target in major depression: new developments

Major depressive disorder (MDD) is a highly prevalent and disabling psychiatric illness often accompanied of cognitive dysfunction which may persist even when patients achieve clinical remission. Currently, cognitive deficits emerge as a potential target because they compromise the functional outcome of depressed patients. The aim of this study was to review data for several potential pharmacological treatments targeting cognition in MDD, resulting from monotherapy or adjunctive treatment. An extensive and systematic Pubmed/Medline search of the published literature until March 2014 was conducted using a variety of search term to find relevant articles. Bibliographies of retrieved papers were further examined for publications of interest. Searches were limited to articles available in English language. We describe studies using modafinil, lisdexamfetamine, ketamine, lanicemine, memantine, galantamine, donepezil, vortioxetine, intranasal oxytocin, omega-3, s-adenosyl-methionine, scopolamine and erythropoietin. From these articles, we determined that there are a number of promising new therapies, pharmacological agents or complementary medicines, but data are just emerging. Drugs and therapies targeting cognitive dysfunction in MDD should prove effective in improving specific cognitive domains and functioning, while ruling out pseudospecificity.

A brief history of the development of antidepressant drugs: from monoamines to glutamate

Major depressive disorder (MDD) is a chronic, recurring, and debilitating mental illness that is the most common mood disorder in the United States. It has been almost 50 years since the monoamine hypothesis of depression was articulated, and just over 50 years since the first pharmacological treatment for MDD was discovered. Several monoamine-based pharmacological drug classes have been developed and approved for the treatment of MDD; however, remission rates are low (often less than 60%) and there is a delayed onset before remission of depressive symptoms is achieved. As a result of a "proof-of-concept" study in 2000 with the noncompetitive NMDA antagonist ketamine, a number of studies have examined the glutamatergic systems as viable targets for the treatment of MDD. This review will provide a brief history on the development of clinically available antidepressant drugs, and then review the possible role of glutamatergic systems in the pathophysiology of MDD. Specifically, the glutamatergic review will focus on the N-methyl-D-aspartate (NMDA) receptor and the efficacy of drugs that target the NMDA receptor for the treatment of MDD. The noncompetitive NMDA receptor antagonist ketamine, which has consistently produced rapid and sustained antidepressant effects in MDD patients in a number of clinical studies, has shown the most promise as a novel glutamatergic-based treatment for MDD. However, compounds that target other glutamatergic mechanisms, such as GLYX-13 (a glycine-site partial agonist at NMDA receptors) appear promising in early clinical trials. Thus, the clinical findings to date are encouraging and support the continued search for and the development of novel compounds that target glutamatergic mechanisms.

Antidepressant actions of ketamine: from molecular mechanisms to clinical practice

In the past decade the emergence of glutamate N-methyl-d-aspartate (NMDA) receptor blockers such as ketamine as fast-acting antidepressants fostered a major conceptual advance by demonstrating the possibility of a rapid antidepressant response. This discovery brings unique mechanistic insight into antidepressant action, as there is a substantial amount of basic knowledge on glutamatergic neurotransmission and how blockade of NMDA receptors may elicit plasticity. The combination of this basic knowledge base and the growing clinical findings will facilitate the development of novel fast acting antidepressants.

Beyond Monoamines-Novel Targets for Treatment-Resistant Depression: A Comprehensive Review

Major depressive disorder (MDD) is a leading cause of disability worldwide. Current first line therapies target modulation of the monoamine system. A large variety of agents are currently available that effectively alter monoamine levels; however, approximately one third of MDD patients remain treatment refractory after adequate trials of multiple monoamine based therapies. Therefore, patients with treatment-resistant depression (TRD) may require modulation of pathways outside of the classic monoamine system. The purpose of this review was thus to discuss novel targets for TRD, to describe their potential mechanisms of action, the available clinical evidence for these targets, the limitations of available evidence as well as future research directions. Several alternate pathways involved in the patho-etiology of TRD have been uncovered including the following: inflammatory pathways, the oxidative stress pathway, the hypothalamic-pituitary-adrenal (HPA) axis, the metabolic and bioenergetics system, neurotrophic pathways, the glutamate system, the opioid system and the cholinergic system. For each of these systems, several targets have been assessed in preclinical and clinical models. Preclinical models strongly implicate these pathways in the patho-etiology of MDD. Clinical trials for TRD have been conducted for several novel targets; however, most of the trials discussed are small and several are uncontrolled. Therefore, further clinical trials are required to assess the true efficacy of these targets for TRD. As well, several promising novel agents have been clinically tested in MDD populations, but have yet to be assessed specifically for TRD. Thus, their applicability to TRD remains unknown.

A role of Yueju in fast-onset antidepressant action on major depressive disorder and serum BDNF expression: a randomly double-blind, fluoxetine-adjunct, placebo-controlled, pilot clinical study

INTRODUCTION

Conventional antidepressants, including fluoxetine, have a major disadvantage in delayed onset of efficacy. Yueju, an herbal medicine used to treat mood disorders was recently found to exhibit rapid antidepressant effects. The present study was conducted to evaluate the role of Yueju in rapidly acting on major depressive disorder (MDD).

METHODS

Participants were MDD patients with scores of 24-item Hamilton Depression Rating Scale (HDRS-24) ≥20 and without history of antidepressant use. They randomly received daily oral doses of Yueju (23 g/day) plus fluoxetine (20 mg/day) (experimental group) or placebo plus fluoxetine (control group) for 7 days. HDRS-24 was used as the primary outcome measurement at baseline, and on days 1, 3, 5, and 7. Concentrations of serum brain-derived neurotrophic factor (BDNF) were assessed at baseline and on days 1 and 7.

RESULTS

In all, 18 participants met the criteria for data analysis. Compared to baseline level, only experimental group showed significant decrease of HDRS-24 score from day 3 to day 7 (P<0.05). Experimental group also showed significant improvement compared with control group from day 3 to day 7 (P<0.05). No correlation between treatment outcomes with serum BDNF levels was observed. However, experimental group showed significant correlation for serum BDNF level on day 1 with day 7 (r=0.721, P=0.028), whereas the control group did not.

CONCLUSION

Yueju likely contributes to fast-onset antidepressant effects on MDD. Further investigation is necessary to firmly establish the ancient formula as a safe, efficacious, and rapidly acting alternative medicine for MDD treatment.

Ketamine: promising path or false prophecy in the development of novel therapeutics for mood disorders?

Large 'real world' studies demonstrating the limited effectiveness and slow onset of clinical response associated with our existing antidepressant medications has highlighted the need for the development of new therapeutic strategies for major depression and other mood disorders. Yet, despite intense research efforts, the field has had little success in developing antidepressant treatments with fundamentally novel mechanisms of action over the past six decades, leaving the field wary and skeptical about any new developments. However, a series of relatively small proof-of-concept studies conducted over the last 15 years has gradually gained great interest by providing strong evidence that a unique, rapid onset of sustained, but still temporally limited, antidepressant effects can be achieved with a single administration of ketamine. We are now left with several questions regarding the true clinical meaningfulness of the findings and the mechanisms underlying the antidepressant action. In this Circumspectives piece, Dr Sanacora and Dr Schatzberg share their opinions on these issues and discuss paths to move the field forward.

Resting state networks in major depressive disorder

Resting state functional magnetic resonance imaging (fMRI) examines the spontaneous low frequency neural activity of the brain to reveal networks of correlated neural activity. A number of different methodologies, each with its own advantages and disadvantages, have been used to examine networks of neural activity that may be related to clinical presentation. Major depressive disorder (MDD) research has largely focused on the default mode network (DMN), which is most active at rest and may relate to negative rumination. However, other networks can be discerned in the resting state such as salience and affective and cognitive control networks, all of which may be relevant to MDD psychopathology. This article reviews the rapidly increasing literature on resting state networks. A number of state- and trait-dependent abnormalities have been reported in MDD in a wide variety of regions including the cerebellum, lingual gyrus, anterior cingulate cortex (ACC), middle frontal gyrus (MFG), dorsolateral prefrontal cortex (dlPFC), amygdala and insula. Current and chronic medication is often a potential confound. Few trials have examined the immediate or delayed effects of antidepressants on resting state networks. This article presents a novel approach to the analysis of drug effects, the identification of signatures of efficacy, and thus for drug development.

The integrative management of treatment-resistant depression: a comprehensive review and perspectives

BACKGROUND

Major depressive disorder is a prevalent and disabling illness. Notwithstanding numerous advances in the pharmacological treatment of depression, approximately 70% of patients do not remit after first-line antidepressant treatment.

METHODS

The MEDLINE/PubMed, EMBASE and ClinicalTrials.gov electronic databases were searched from inception to October 1, 2013, for randomized controlled trials (RCT), relevant open-label trials, meta-analyses and ongoing trials of pharmacological and psychotherapeutic approaches to treatment-resistant depression (TRD).

RESULTS

Switching to a different antidepressant is a useful option following nonresponse to a first-line agent. Although widely used in clinical practice, there is limited evidence to support antidepressant combination for TRD. Notwithstanding evidence for lithium or T3 augmentation to be successful in TRD, most studies were carried out when participants were treated with tricyclic antidepressants (TCA). Of the available strategies to augment the response to new-generation antidepressants, the use of some atypical antipsychotics is best supported by evidence. Several novel therapeutic options are currently discussed. Evidence suggests that cognitive therapy (CT) is an effective strategy for TRD.

CONCLUSIONS

The success of switching to a different antidepressant following a first-line agent is supported by evidence, but there is limited evidence for effective combination strategies. Lithium and T3 augmentation of TCA have the strongest evidence base for successful treatment of TRD. The use of augmentation of newer-generation antidepressants with atypical antipsychotics is supported by a growing evidence base. Current evidence supports CT as an effective strategy for TRD. There is a need for additional large-scale RCT of TRD. The development of new antidepressants targeting novel pathways opens a promising perspective for the management of TRD.