Ketamine and other potential glutamate antidepressants

The paper is a narrative review of clinical and mechanistic studies of ketamine and other glutamate-modulating drugs; clinical studies are summarised in a Table referenced by the authors. Included clinical work comprised a mixture of designs: five double-blind crossover randomised controlled trials (RCTs), one double-blind parallel RCT, several non-double-blind RCTs, single-blind and open-label studies, and case reports. Most trials enrolled patients with MDD, though a few focused on bipolar depression or included mixed diagnostic groups. Ketamine was most commonly administered as 0.5 mg/kg by intravenous infusion over 40 minutes, with some studies using lower doses and others using 1.0–1.5 mg/kg in anaesthetic contexts (for ECT or surgery). Outcome measurement typically used standard clinician-rated depression scales (MADRS or HAM-D) as primary endpoints, with some studies also reporting Beck Depression Inventory or scales for dissociation/psychotomimetic effects. The review highlights recurrent methodological limitations across studies: small sample sizes (most active groups <20), difficulty maintaining blinding because ketamine produces characteristic acute subjective effects, variable allowance of concomitant medications, heterogeneity in patient characteristics (for example baseline severity, age, comorbidity), and varied timing of outcome assessments. The extracted text does not clearly report a standardised analysis approach across studies (for example whether intention-to-treat analyses were used) beyond noting these design features.

Across the clinical literature reviewed, a consistent finding is that a single intravenous ketamine infusion produces antidepressant effects within 24 hours and that these effects can persist for up to 14 days in some patients. Reported peak reductions on HAM-D scores varied widely (15% to 86%), and response rates after ketamine infusions ranged from 20% to 90%. In bipolar depression some studies observed very rapid improvements (as early as 40 minutes) but generally lower and less sustained response compared with MDD; for example, on day 1 response was 42% in bipolar depression versus 71% in MDD in one comparison. Repeated ketamine infusions have, in several studies, prolonged the interval to relapse (one report noted delay up to 19 days) and other protocols (for example slower infusions or continuous 5-day infusions) reported improved durability; one open-label report claimed remission maintained for at least 12 months after discontinuation, but such findings come from small, uncontrolled samples. Investigations of biomarkers and predictors produced heterogeneous and largely non‑replicated signals. Some studies linked ketamine response with increases in circulating brain‑derived neurotrophic factor (BDNF) or with changes in slow-wave sleep measures; others did not find BDNF changes. Lower pre‑treatment plasma D‑ and L‑serine concentrations were associated with response in one study. Neurophysiological measures (magnetoencephalography, MEG) pointed to greater prefrontal/ACC engagement or increased γ‑band responses as potential predictors in small samples. Neurochemical imaging ([1H]MRS) findings are inconsistent regarding glutamate/glutamine (Glx) changes. Clinical predictors reported across studies included family history of alcohol dependence, higher body mass index and fewer prior suicide attempts, but the authors emphasise these are not yet clinically useful. Mechanistic challenge and adjunctive trials yielded mixed results. Pre‑treatment with lamotrigine (which suppresses glutamate release) reduced some dissociative and psychotomimetic effects of ketamine in healthy volunteer studies but did not reliably block antidepressant effects in patient trials; riluzole (which modulates glutamate and enhances AMPA expression) generally failed to prolong ketamine’s antidepressant response in trials designed to test maintenance effects. Isomer and route comparisons are limited: small case series of S‑ketamine showed some benefit; animal data suggest R‑ketamine may be more potent and longer‑lasting than S‑ketamine, but these findings have not been translated to controlled human trials. Alternative administration routes (intranasal, intramuscular) produced antidepressant effects in small or early randomised trials. Evidence for other glutamatergic agents is variable. In a double‑blind RCT memantine did not separate from placebo on MADRS at 8 weeks, though small open‑label trials reported improvements. Riluzole produced benefit in several open‑label studies with response rates around 32% in a small clinical series, but controlled data are limited. Amantadine has only limited open‑label evidence. CP‑101,606 (a GluN2B‑selective antagonist) showed promising adjunctive results in a randomized add‑on study (60% response vs 20% placebo on HAM‑D, with 33% remission by day 5), though dosing changes for safety introduced potential bias. GLYX‑13 (a glycine‑site partial agonist at the NMDA receptor) completed Phase II trials and reported antidepressant effects within 24 hours lasting up to 7 days without dissociative side‑effects. MK‑0657 (an oral GluN2B antagonist) in a very small crossover study did not show MADRS improvement but had mixed signals on other scales; sample size (n=5) precludes firm conclusions. RO4917523/AZD2066 (mGlu5 antagonists) and older AMPAkines have produced largely negative or inconclusive results in clinical samples. Overall, no other agent consistently reproduces the rapidity and magnitude of antidepressant effect seen with ketamine.

Dutta and colleagues conclude that ketamine provides proof‑of‑concept that modulation of glutamate neurotransmission can produce rapid antidepressant effects. The review highlights a plausible mechanistic chain in which NMDA blockade by ketamine leads to increased glutamate release acting on AMPA receptors, downstream activation of intracellular pathways such as mammalian target of rapamycin (mTOR) that promote synaptic protein synthesis and synaptogenesis, and resultant clinical benefit that can outlast the drug’s plasma presence. Nevertheless, the authors emphasise substantial uncertainty: human neurochemical and post‑mortem data on glutamate system abnormalities in MDD are inconsistent, many candidate biomarkers of ketamine response lack replication, and the diversity of trial designs, small samples, poor blinding and frequent concomitant treatments limit confidence in effect estimates. The review notes that other glutamate‑modifying drugs show some antidepressant promise but have not matched ketamine’s rapid onset, and that mechanistic probes (for example lamotrigine, riluzole) have produced mixed or negative results. The authors call for more rigorous proof‑of‑concept studies in both animals and humans, better biomarkers of glutamate function, and investigations that link glutamate‑related mechanisms to inflammatory pathways implicated in depression. Such work, they argue, is needed before glutamate‑targeted treatments can be reliably translated into routine clinical practice.