Ketamine: Promising Path or False Prophecy in the Development of Novel Therapeutics for Mood Disorders?

Sanacora and Schatzberg frame ketamine as a treatment that has challenged conventional views of antidepressant therapy by producing rapid-onset antidepressant and anxiolytic effects, including in patients with treatment-resistant depression. Earlier research has shown that most approved antidepressants require weeks to exert clinically meaningful effects and act primarily on monoaminergic systems; ketamine’s apparent ability to induce fast, robust, but temporally limited improvements has therefore reinvigorated interest in novel mechanisms of action for mood disorder therapeutics. This Circumspectives piece sets out to evaluate what is known and uncertain about ketamine’s antidepressant effects. Sanacora presents evidence and interpretation that emphasise NMDAR antagonism and downstream glutamatergic modulation as key mechanisms, whereas Schatzberg offers a countervailing view that other mechanisms (including opioid, sigma and monoaminergic effects) may be critical and that an exclusive focus on NMDAR antagonism risks misdirecting drug development. Together the authors review preclinical and clinical data, highlight methodological challenges in the field, and discuss implications for future therapeutic development and safety evaluation.

This paper is a narrative, opinion-style review (a Circumspectives piece) composed of two complementary essays by Sanacora and Schatzberg. Rather than reporting a systematic review or original experimental data, the authors synthesise and interpret animal and human preclinical and clinical literature to make contrasting arguments about ketamine’s mechanism(s) of action and clinical utility. The extracted text does not describe a formal literature search strategy, inclusion criteria, or systematic quality assessment. The authors draw on a variety of sources including rodent models (behavioural assays and molecular signalling studies), small clinical proof-of-concept trials, phase II and phase IIb studies of NMDAR-targeting compounds (for example CP-101,606 and AZD6765/lanicemine), and observational data related to safety and abuse. They also discuss mechanistic biomarkers and imaging modalities proposed for measuring target engagement (for example PET ligands sensitive to glutamatergic changes, 13C-MRS, fMRI and EEG), and consider genetic moderators such as the BDNF Val66Met polymorphism. Because this is an interpretive commentary, analytic methods are limited to qualitative synthesis and critique of existing findings rather than meta-analytic statistics.

Both authors agree on a central empirical observation: a single administration of ketamine produces a transient but often rapid improvement in depressive symptoms that typically endures for several days. Preclinical data consistently show that various NMDAR-antagonising agents produce antidepressant-like effects in rodents, and several downstream molecular cascades have been implicated: ketamine induces a transient glutamate surge, activation of AMPA receptors, engagement of mTORC1 signalling, increased synaptic protein synthesis, enhanced synaptogenesis, and BDNF-dependent plasticity. Evidence cited includes blockade of ketamine-like effects by AMPAR antagonists in animals, dose-dependent activation of mTORC1 and synaptic protein markers, and genetic data showing reduced ketamine effects in mice carrying the BDNF Val66Met allele; a preliminary human study is noted to suggest lower response rates in met allele carriers. Clinical trial data are mixed. Small trials suggest that NR2B-selective antagonists (for example CP-101,606) and low-trapping NMDAR blockers (AZD6765/lanicemine) can produce rapid, short-lived antidepressant effects. A phase IIb study administering repeated lanicemine infusions (three times per week for three weeks) reported significant antidepressant efficacy and maintenance over weeks, but a subsequent larger follow-up failed to replicate sustained clinical efficacy; that larger study encountered a high placebo response rate of 39% at the primary 6-week endpoint. Memantine, an NMDAR antagonist with many pharmacodynamic similarities to ketamine, has not produced consistent antidepressant effects in trials. Other glutamatergic approaches—partial agonists at the glycine site (GLYX13), glycine transporter inhibitors, and high-dose D-cycloserine—have shown suggestive results in animals or early-phase studies, but detailed human efficacy data remain limited or unconfirmed. Schatzberg highlights alternative or additional mechanisms. Ketamine interacts with sigma receptors, mobilises midbrain catecholamines (notably dopamine), potentiates prefrontal serotonin release via nicotinic receptors, and binds to opioid receptors (mu, lambda and kappa). Preclinical data indicate ketamine can increase mu-opioid receptor density and enhance mu-opioid signalling; analgesic effects in mice were blocked by mu and lambda, but not kappa, antagonists. These findings have led to concern that opioid receptor activity could contribute to both therapeutic and abuse-related effects. Clinical observations include frequent acute dissociation and psychotomimetic effects, and in a comparison trial ketamine produced dissociation in 60% of patients versus 30% with midazolam, an active control. Safety signals discussed include transient cardiovascular effects (elevated blood pressure and heart rate), occasional hypotension and bradycardia, and long-term risks suggested by abuse literature and animal models (cognitive effects, urinary cystitis associated with very high chronic use). Limited controlled data on repeated dosing report general tolerability, but the authors stress that long-term safety is insufficiently characterised. Other trial design and interpretive problems are emphasised: difficulty maintaining blinding because of distinctive subjective effects, high placebo response rates that can obscure small-to-moderate drug effects, and heterogeneity in dosing/regimens and outcome measures across studies. The authors also note that first placebo-controlled data showing sustained effects with repeated dosing were only recently presented but remain preliminary.

Sanacora interprets the accumulated evidence as broadly consistent with a model where NMDAR antagonism initiates a glutamate-dependent cascade—transient glutamate release, AMPAR activation, mTORC1 signalling, BDNF release and synaptogenesis—that underlies ketamine’s rapid antidepressant effects. From this vantage, NMDAR and glutamatergic modulation represent promising targets for developing new, rapid-acting antidepressants, but the complexity of glutamatergic transmission means that factors such as receptor trapping, subunit selectivity, half-life and effects on synaptic versus extrasynaptic receptors will influence whether candidate drugs can reproduce ketamine’s therapeutic profile. Schatzberg cautions that NMDAR antagonism may not be the sole or even primary mechanism. He highlights discordant findings—memantine’s lack of consistent efficacy, mixed outcomes with other glutamatergic agents, and signals that NMDA agonists or glycine-site modulators can produce antidepressant-like effects in some models. He emphasises alternative mechanisms including sigma receptor activity, catecholamine and serotonergic mobilisation, and opioid receptor agonism; the latter is especially consequential because it raises regulatory, abuse liability and ethical issues for developing ketamine-like treatments. Both authors therefore stress the need for more definitive mechanistic studies, including attempts to block putative pathways (for example with opioid antagonists), and better biomarkers of target engagement. Methodological limitations and uncertainties are explicitly acknowledged. The field faces problems of high placebo responses, functional unblinding due to dissociative effects, small sample sizes in many trials, variable replication (notably the lanicemine programme), and sparse long-term safety data for repeated administration. The authors call for well-controlled clinical trials that assess clinically meaningful short-term outcomes (for example reductions in suicidal behaviour or hospitalisation) and examine sustainable response beyond a few weeks. They also recommend a range of additional studies: abuse-liability assessments across populations, biomarker and imaging work (PET for mGluR5 or mu opioid binding, 13C-MRS, fMRI, EEG) to document target engagement, pharmacogenetic investigations (for example BDNF Val66Met), and head-to-head or mechanistic trials using antagonists such as naloxone to probe opioid involvement. In terms of translational implications, the authors agree that while ketamine demonstrates proof of principle for rapid-acting antidepressant effects, it is premature to conclude that any single mechanism explains the clinical response. Future drug development should remain agnostic about a unique mechanism and prioritise both safety and validated biomarkers of engagement. They also advocate open discussion about the potential opioid-related properties of ketamine and careful long-term monitoring if broader clinical use is pursued.