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

Dysfunction in a wide array of systems—including the immune, monoaminergic, and glutamatergic systems—is implicated in the pathophysiology of depression. One potential intersection point for these three systems is the kynurenine (KYN) pathway. This study explored the impact of the prototypic glutamatergic modulator ketamine on the endogenous KYN pathway in individuals with bipolar depression (BD), as well as the relationship between response to ketamine and depression-related behavioral and peripheral inflammatory markers. Thirty-nine participants with treatment-resistant BD (23 F, ages 18–65) received a single ketamine infusion (0.5 mg/kg) over 40 min. KYN pathway analytes—including plasma concentrations of indoleamine 2,3-dioxygenase (IDO), KYN, kynurenic acid (KynA), and quinolinic acid (QA)—were assessed at baseline (pre-infusion), 230 min, day 1, and day 3 post-ketamine. General linear models with restricted maximum likelihood estimation and robust sandwich variance estimators were implemented. A repeated effect of time was used to model the covariance of the residuals with an unstructured matrix. After controlling for age, sex, and body mass index (BMI), post-ketamine IDO levels were significantly lower than baseline at all three time points. Conversely, ketamine treatment significantly increased KYN and KynA levels at days 1 and 3 versus baseline. No change in QA levels was observed post-ketamine. A lower post-ketamine ratio of QA/KYN was observed at day 1. In addition, baseline levels of proinflammatory cytokines and behavioral measures predicted KYN pathway changes post ketamine. The results suggest that, in addition to having rapid and sustained antidepressant effects in BD participants, ketamine also impacts key components of the KYN pathway.

This manuscript reviews the clinical evidence regarding single-dose intravenous (IV) administration of the novel glutamatergic modulator racemic ( R,S )-ketamine (hereafter referred to as ketamine) as well as its S -enantiomer, intranasal esketamine, for the treatment of major depressive disorder (MDD). Initial studies found that a single subanesthetic-dose IV ketamine infusion rapidly (within one day) improved depressive symptoms in individuals with MDD and bipolar depression, with antidepressant effects lasting three to seven days. In 2019, esketamine received FDA approval as an adjunctive treatment for treatment-resistant depression (TRD) in adults. Esketamine was approved under a risk evaluation and mitigation strategy (REMS) that requires administration under medical supervision. Both ketamine and esketamine are currently viable treatment options for TRD that offer the possibility of rapid symptom improvement. The manuscript also reviews ketamine’s use in other psychiatric diagnoses—including suicidality, obsessive–compulsive disorder, post-traumatic stress disorder, substance abuse, and social anxiety disorder—and its potential adverse effects. Despite limited data, side effects for antidepressant-dose ketamine—including dissociative symptoms, hypertension, and confusion/agitation—appear to be tolerable and limited to around the time of treatment. Relatively little is known about ketamine’s longer-term effects, including increased risks of abuse and/or dependence. Attempts to prolong ketamine’s effects with combined therapy or a repeat-dose strategy are also reviewed, as are current guidelines for its clinical use. In addition to presenting a novel and valuable treatment option, studying ketamine also has the potential to transform our understanding of the mechanisms underlying mood disorders and the development of novel therapeutics.

The N-methyl-D-aspartate antagonist ketamine has rapid antidepressant effects in patients with treatment-resistant major depression (TRD); these effects have been reported to last for 1 week in some patients. However, the extent and duration of this antidepressant effect over longer periods has not been well characterized under controlled conditions. Riluzole, a glutamatergic modulator with antidepressant and synaptic plasticity-enhancing effects, could conceivably be used to promote the antidepressant effects of ketamine. This study sought to determine the extent and time course of antidepressant improvement to a single-ketamine infusion over 4 weeks, comparing the addition of riluzole vs placebo after the infusion. Forty-two subjects (18-65) with TRD and a Montgomery-Asberg Depression Rating Scale (MADRS) score of ≥ 22 received a single intravenous infusion of ketamine (0.5 mg/kg). Four to six hours post-infusion, subjects were randomized to double-blind treatment with either riluzole (100-200 mg/day; n=21) or placebo (n=21) for 4 weeks. Depressive symptoms were rated daily. A significant improvement (P<0.001) in MADRS scores from baseline was found. The effect size of improvement with ketamine was initially large and remained moderate throughout the 28-day trial. Overall, 27% of ketamine responders had not relapsed by 4 weeks following a single ketamine infusion. The average time to relapse was 13.2 days (SE=2.2). However, the difference between the riluzole and placebo treatment groups was not significant, suggesting that the combination of riluzole with ketamine treatment did not significantly alter the course of antidepressant response to ketamine alone.

BackgroundInflammation is associated with major depressive disorder (MDD). Translocator protein 18 kDa (TSPO), a putative biomarker of neuroinflammation, is quantified using positron emission tomography (PET) and 11C-PBR28, a TSPO tracer. We sought to (1) investigate TSPO binding in MDD subjects currently experiencing a major depressive episode, (2) investigate the effects of antidepressants on TSPO binding, and (3) determine the relationship of peripheral and central inflammatory markers to cerebral TSPO binding. Twenty-eight depressed MDD subjects (unmedicated (n = 12) or medicated (n = 16)) and 20 healthy controls (HC) underwent PET imaging using 11C-PBR28. Total distribution volume (VT, proportional to Bmax/Kd) was measured and corrected with the free fraction in plasma (fp). The subgenual prefrontal cortex (sgPFC) and anterior cingulate cortex (ACC) were the primary regions of interest. Peripheral blood samples and cerebrospinal fluid were analyzed to investigate the relationship between TSPO binding and peripheral and central inflammatory markers, including interleukins and neurotrophic factors previously linked to depression.ResultsTSPO binding was higher in MDD versus HC in the sgPFC (Cohen’s d = 0.64, p = .038, 95% CI 0.04–1.24) and ACC (d = 0.60, p = .049, 95% CI 0.001–1.21), though these comparisons missed the corrected threshold for statistical significance (α = .025). Exploratory analyses demonstrated that unmedicated MDD subjects had the highest level of TSPO binding, followed by medicated MDD subjects, who did not differ from HC. TSPO binding correlated with interleukin-5 in cerebrospinal fluid but with no other central inflammatory markers.ConclusionsThis study found a trend towards increased TSPO binding in the brains of MDD subjects, and post hoc analysis extended these findings by demonstrating that this abnormality is significant in unmedicated (but not medicated) MDD subjects.

The diagnosis of anxious depression is presently inconsistent. The many different definitions of anxious depression have complicated its diagnosis, leading to clinical confusion and inconsistencies in the literature. This article reviewed the extant literature in order to identify the varying definitions of anxious depression, which were then compared using Feighner's diagnostic criteria. Notably, these suggest a different clinical picture of patients with anxious depression. For instance, relying on The International Classification of Diseases (ICD) or Diagnostic and Statistical Manual of Mental Disorders (DSM) diagnoses yields a clinical picture of a comparatively mild or transient disorder; in contrast, using dimensional criteria such as DSM criteria combined with additional rating scales—most commonly the anxiety somatization factor score from the Hamilton Depression Rating Scale (HAM-D)—yields a more serious clinical picture. The evidence reviewed here suggests that defining anxious depression in a dimensional manner may be the most useful and clinically relevant way of differentiating it from other types of mood and anxiety disorders, and of highlighting the most clinically significant differences between patients with anxious depression versus depression or anxiety alone.

Bipolar disorder (BD) is characterized by extreme mood swings ranging from manic/hypomanic to depressive episodes. The severity, duration, and frequency of these episodes can vary widely between individuals, significantly impacting quality of life. Individuals with BD spend almost half their lives experiencing mood symptoms, especially depression, as well as associated clinical dimensions such as anhedonia, fatigue, suicidality, anxiety, and neurovegetative symptoms. Persistent mood symptoms have been associated with premature mortality, accelerated aging, and elevated prevalence of treatment-resistant depression. Recent efforts have expanded our understanding of the neurobiology of BD and the downstream targets that may help track clinical outcomes and drug development. However, as a polygenic disorder, the neurobiology of BD is complex and involves biological changes in several organelles and downstream targets (pre-, post-, and extra-synaptic), including mitochondrial dysfunction, oxidative stress, altered monoaminergic and glutamatergic systems, lower neurotrophic factor levels, and changes in immune-inflammatory systems. The field has thus moved toward identifying more precise neurobiological targets that, in turn, may help develop personalized approaches and more reliable biomarkers for treatment prediction. Diverse pharmacological and non-pharmacological approaches targeting neurobiological pathways other than neurotransmission have also been tested in mood disorders. This article reviews different neurobiological targets and pathophysiological findings in non-canonical pathways in BD that may offer opportunities to support drug development and identify new, clinically relevant biological mechanisms. These include: neuroinflammation; mitochondrial function; calcium channels; oxidative stress; the glycogen synthase kinase-3 (GSK3) pathway; protein kinase C (PKC); brain-derived neurotrophic factor (BDNF); histone deacetylase (HDAC); and the purinergic signaling pathway.

Background Cyclooxygenase-2 (COX-2), which is rapidly upregulated by inflammation, is a key enzyme catalyzing the rate-limiting step in the synthesis of several inflammatory prostanoids. Successful positron emission tomography (PET) radioligand imaging of COX-2 in vivo could be a potentially powerful tool for assessing inflammatory response in the brain and periphery. To date, however, the development of PET radioligands for COX-2 has had limited success. Methods The novel PET tracer [ 11 C]MC1 was used to examine COX-2 expression [ 1 ] in the brains of four rhesus macaques at baseline and after injection of the inflammogen lipopolysaccharide (LPS) into the right putamen, and [ 2 ] in the joints of two human participants with rheumatoid arthritis and two healthy individuals. In the primate study, two monkeys had one LPS injection, and two monkeys had a second injection 33 and 44 days, respectively, after the first LPS injection. As a comparator, COX-1 expression was measured using [ 11 C]PS13. Results COX-2 binding, expressed as the ratio of specific to nondisplaceable uptake ( BP ND ) of [ 11 C]MC1, increased on day 1 post-LPS injection; no such increase in COX-1 expression, measured using [ 11 C]PS13, was observed. The day after the second LPS injection, a brain lesion (~ 0.5 cm in diameter) with high COX-2 density and high BP ND (1.8) was observed. Postmortem brain analysis at the gene transcript or protein level confirmed in vivo PET results. An incidental finding in an unrelated monkey found a line of COX-2 positivity along an incision in skull muscle, demonstrating that [ 11 C]MC1 can localize inflammation peripheral to the brain. In patients with rheumatoid arthritis, [ 11 C]MC1 successfully imaged upregulated COX-2 in the arthritic hand and shoulder and apparently in the brain. Uptake was blocked by celecoxib, a COX-2 preferential inhibitor. Conclusions Taken together, these results indicate that [ 11 C]MC1 can image and quantify COX-2 upregulation in both monkey brain after LPS-induced neuroinflammation and in human peripheral tissue with inflammation. Trial registration ClinicalTrials.gov NCT03912428. Registered April 11, 2019.

The serotonin-1A (5-HT1A) receptor is of particular interest in human positron emission tomography (PET) studies of major depressive disorder (MDD). Of the eight studies investigating this issue in the brains of patients with MDD, four reported decreased 5-HT1A receptor density, two reported no change, and two reported increased 5-HT1A receptor density. While clinical heterogeneity may have contributed to these differing results, methodological factors by themselves could also explain the discrepancies. This review highlights several of these factors, including the use of the cerebellum as a reference region and the imprecision of measuring the concentration of parent radioligand in arterial plasma, the method otherwise considered to be the ‘gold standard’. Other potential confounds also exist that could restrict or unexpectedly affect the interpretation of results. For example, the radioligand may be a substrate for an efflux transporter – like P-gp – at the blood–brain barrier; furthermore, the binding of the radioligand to the receptor in various stages of cellular trafficking is unknown. Efflux transport and cellular trafficking may also be differentially expressed in patients compared to healthy subjects. We believe that, taken together, the existing disparate findings do not reliably answer the question of whether 5-HT1A receptors are altered in MDD or in subgroups of patients with MDD. In addition, useful meta-analysis is precluded because only one of the imaging centers acquired all the data necessary to address these methodological concerns. We recommend that in the future, individual centers acquire more thorough data capable of addressing methodological concerns, and that multiple centers collaborate to meaningfully pool their data for meta-analysis. ► PET imaging of 5-HT1A receptor density in MDD patients has yielded mixed results. ► ‘Methodological’ rather than ‘clinical’ factors likely explain these discrepancies. ► One methodological confound is the use of the cerebellum as a reference region. ► Measuring parent radioligand concentrations in arterial plasma is imprecise. ► Other confounds are efflux transporter, radiometabolites, & 5-HT1A affinity states.

Treatment-resistant depression (TRD) is associated with worse clinical outcomes and longer course of illness. However, TRD is more difficult to model in animal phenotypes, suggesting that other experimental and translational models must be considered to properly address and research novel therapeutics. Reelin, an endogenous glycoprotein downregulated in depression, has shown rapid antidepressant-like effects akin to those of the N-methyl-D-aspartate receptor (NMDAR) antagonist ketamine. Interestingly, the antidepressant-like effects of both ketamine and reelin affect mechanistic target of rapamycin complex 1 (mTORC1) activity and that of its related downstream signalers. (2  R ,6  R )-hydroxynorketamine (HNK) is a major metabolite of ketamine that, at therapeutic levels, appears to activate mTORC1 without antagonizing NMDARs. To model the effects of (2  R ,6  R )-HNK and reelin on neurons from TRD participants, induced pluripotent stem cells (iPSCs) were reprogrammed from peripheral blood mononuclear cells collected from five females with TRD (mean=40.2 yrs) and then differentiated into cortical neurons. In iPSC-derived neurons from TRD participants, 50 nM reelin and 1 µM (2  R ,6  R )-HNK had similar effects on the protein expression of GluA1, PSD-95, Dab1, Synapsin I, and p-ERK, with concentration-dependent increases observed at one hour that significantly decreased by 24 h. RNA sequencing revealed similar changes in gene expression between 50 nM reelin and 1 µM (2  R ,6  R )-HNK at one hour, although only reelin upregulated mTORC1 signaling. While this work remains preliminary, the results suggest that iPSC-derived neurons could provide a valuable in vitro model to study TRD and hold promise for evaluating novel therapeutics such as (2  R ,6  R )-HNK and reelin. Clinicaltrials.gov : NCT02484456