Effect of Time From Onset of Major Depressive Disorder on the Therapeutic Response to Esmethadone (REL-1017).

Objective: The relationship between the duration of major depressive disorder (MDD) and therapeutic response to standard antidepressant treatment (SAT) is unknown. N-methyl-D-aspartate receptor uncompetitive antagonists are emerging drugs for MDD. We investigated whether the antidepressant effect of esmethadone (REL-1017) could be related to the duration of depression.Methods: We analyzed data from a Phase 2a study of adjunctive treatment with esmethadone in MDD patients (DSM-5) with inadequate response to ongoing SAT (May 2018-August 2019). Patients were randomized to treatment with esmethadone 25 mg, esmethadone 50 mg, or placebo for 7 days, followed by an observation period (Days 7-14). Duration of depression was derived from 2 measures: (1) time from onset (TFO), calculated as the difference in years between age at trial enrollment and age at the onset of the first major depressive episode (MDE), and (2) TFO index, calculated by computing the years of illness duration (number of years from the beginning of MDD), divided by age and multiplied by 100. First, bivariate correlations between TFO and change from baseline (CFB) were calculated by Spearman ρ. Linear mixed-model analyses were also conducted.Results: A total of 62 patients participated in the trial. The median values of time from MDD onset for the 62 patients were 11 years (absolute value) and 22% (percentage of life-years). Duration of depression was significantly correlated with Montgomery-Asberg Depression Rating Scale (MADRS) CFB on Day 14, even when controlling for the effect of current depression severity (MADRS baseline). In the linear mixed-model analyses, we found a significant effect of duration on reduction in MADRS score from T0 to subsequent assessments (P < .05). Number of previous MDEs and effect of esmethadone 50 mg when compared to 25 mg were not significant.Conclusion: Esmethadone 25 and 50 mg were more effective in reducing MADRS scores in patients with shorter time from first MDE onset.Trial Registration: ClinicalTrials.gov identifier: NCT03051256.

Sex-dependent effects of the uncompetitive N-methyl-D-aspartate receptor antagonist REL-1017 in G93A-SOD1 amyotrophic lateral sclerosis mice.

Introduction: The pathogenesis of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease caused by the demise of motor neurons has been linked to excitotoxicity caused by excessive calcium influx via N-methyl-D-aspartate receptors (NMDARs), suggesting that uncompetitive NMDAR antagonism could be a strategy to attenuate motor neuron degeneration. REL-1017, the dextro-isomer of racemic methadone, is a low-affinity uncompetitive NMDAR antagonist. Importantly, in humans REL-1017 has shown excellent tolerability in clinical trials for major depression. Methods: Here, we tested if REL-1017 improves the disease phenotypes in the G93A SOD1 mouse, a well-established model of familial ALS, by examining survival and motor functions, as well as the expression of genes and proteins involved in neuroplasticity. Results: We found a sex-dependent effect of REL-1017 in G93A SOD1 mice. A delay of ALS symptom onset, assessed as 10%-decrease of body weight (p < 0.01 vs. control untreated mice) and an extension of lifespan (p < 0.001 vs. control untreated mice) was observed in male G93A SOD1 mice. Female G93A SOD1 mice treated with REL-1017 showed an improvement of muscle strength (p < 0.01 vs. control untreated mice). Both males and females treated with REL-1017 showed a decrease in hind limb clasping. Sex-dependent effects of REL-1017 were also detected in molecular markers of neuronal plasticity (PSD95 and SYN1) in the spinal cord and in the GluN1 NMDAR subunit in quadricep muscles. Conclusion: In conclusion, this study provides preclinical in vivo evidence supporting the clinical evaluation of REL-1017 in ALS.

Are Gender Differences Important for Autoimmune Liver Diseases?

Gender Medicine has had an enormous expansion over the last ten years. Autoimmune liver diseases include several conditions, i.e., autoimmune hepatitis (AIH), primary biliary cholangitis (PBC), primary sclerosing cholangitis (PSC), and conditions involving the liver or biliary tree overlapping with AIH, as well as IgG4-related disease. However, little is known about the impact of sex in the pathogenesis and natural history of these conditions. The purpose of this review is to provide an update of the gender disparities among the autoimmune liver diseases by reviewing the data published from 1999 to 2023. The epidemiology of these diseases has been changing over the last years, due to the amelioration of knowledge in their diagnosis, pathogenesis, and treatment. The clinical data collected so far support the existence of sex differences in the natural history of autoimmune liver diseases. Notably, their history could be longer than that which is now known, with problems being initiated even at a pediatric age. Moreover, gender disparity has been observed during the onset of complications related to end-stage liver disease, including cancer incidence. However, there is still an important debate among researchers about the impact of sex and the pathogenesis of these conditions. With this review, we would like to emphasize the urgency of basic science and clinical research to increase our understanding of the sex differences in autoimmune liver diseases.

Insights into Hepatocellular Carcinoma: From Pathophysiology to Novel Therapies.

Hepatocellular carcinoma (HCC), the most common primary liver cancer, accounts for 830,180 related deaths worldwide in 2020, according to GLOBOCAN, representing the fourth leading cause of cancer-related death, with a five-year survival rate of about 18% for advanced stage, and the second leading cause in men of cancer-related mortality worldwide [...].

Tyrosol attenuates NASH features by reprogramming the hepatic immune milieu.

Nonalcoholic steatohepatitis (NASH) is a leading cause of chronic liver disease, and no drugs have been approved for its therapy. Among plant-derived molecules, phenolic compounds of extra virgin olive oil like tyrosol (Tyr) had demonstrated multiple beneficial actions for liver health, including the modulation of inflammation in fibrosis. This study aims at assessing the protective effect and mechanism of Tyr in invitro and in vivo models of NASH, with a focus on the hepatic immune microenvironment and extrahepatic manifestations. The effect of Tyr was evaluated in cellular models of NASH, obtained by co-culturing palmitic and oleic acid-treated HepG2 cells with THP1-derived M1 macrophages and LX2 cells, and in a mouse model of NASH induced by a high fructose-high fat diet combined to CCl4 treatment. In vitro Tyr reduced fatty acid (FA) accumulation in HepG2 cells and displayed a beneficial effect on LX2 activation and macrophage differentiation. In vivo, beside reducing steatosis and fibrosis in NASH animals, Tyr prevented inflammation, as demonstrated by the reduction of hepatic inflammatory foci, and immune cells like CD86+ macrophages (p < 0.05), CD4+ (p < 0.05) and T helper effector CD4+ FoxP3- CD62L-lymphocytes (p < 0.05). Also, the prooxidant enzyme NOX1 and the mRNA expression of TGF-ß1 and IL6 (p < 0.05) were reduced by Tyr. Notably, in Tyr-treated animals, a significant increase of CD4+ FoxP3+ Treg cells (p < 0.05) was observed, involved in regenerative pathways. Moreover, Tyr attenuated the fatigue and anxious behavior observed in NASH mice. In conclusion, Tyr effectively reduced NASH-related steatosis, fibrosis, oxidative stress, and inflammation, displaying a beneficial effect on the hepatic immune infiltrate, indicating its possible development as a therapeutic agent for NASH due to its multifaceted mechanism.

Current Perspectives on the Molecular and Clinical Relationships between Primary Biliary Cholangitis and Hepatocellular Carcinoma.

Primary biliary cholangitis (PBC) is an autoimmune liver disease characterised by the immune-mediated destruction of small and medium intrahepatic bile ducts, with variable outcomes and progression. This review summarises the state of the art regarding the risk of neoplastic progression in PBC patients, with a particular focus on the molecular alterations present in PBC and in hepatocellular carcinoma (HCC), which is the most frequent liver cancer in these patients. Major risk factors are male gender, viral infections, e.g., HBV and HCV, non-response to UDCA, and high alcohol intake, as well as some metabolic-associated factors. Overall, HCC development is significantly more frequent in patients with advanced histological stages, being related to liver cirrhosis. It seems to be of fundamental importance to unravel eventual dysfunctional molecular pathways in PBC patients that may be used as biomarkers for HCC development. In the near future, this will possibly take advantage of artificial intelligence-designed algorithms.

Adipocyte Precursor-Derived NRG1 Promotes Resistance to FGFR Inhibition in Urothelial Carcinoma.

Aberrations of the fibroblast growth factor receptor (FGFR) family members are frequently observed in metastatic urothelial cancer (mUC), and blocking the FGF/FGFR signaling axis is used as a targeted therapeutic strategy for treating patients. Erdafitinib is a pan-FGFR inhibitor, which has recently been approved by the FDA for mUC with FGFR2/3 alterations. Although mUC patients show initial response to erdafitinib, acquired resistance rapidly develops. Here, we found that adipocyte precursors promoted resistance to erdafitinib in FGFR-dependent bladder and lung cancer in a paracrine manner. Moreover, neuregulin 1 (NRG1) secreted from adipocyte precursors was a mediator of erdafitinib resistance by activating human epidermal growth factor receptor 3 (ERBB3; also known as HER3) signaling, and knockdown of NRG1 in adipocyte precursors abrogated the conferred paracrine resistance. NRG1 expression was significantly downregulated in terminally differentiated adipocytes compared with their progenitors. Pharmacologic inhibition of the NRG1/HER3 axis using pertuzumab reversed erdafitinib resistance in tumor cells in vitro and prolonged survival of mice bearing bladder cancer xenografts in vivo. Remarkably, data from single-cell RNA sequencing revealed that NRG1 was enriched in platelet-derived growth factor receptor-A (PDGFRA) expressing inflammatory cancer-associated fibroblasts, which is also expressed on adipocyte precursors. Together, this work reveals a paracrine mechanism of anti-FGFR resistance in bladder cancer, and potentially other cancers, that is amenable to inhibition using available targeted therapies. SIGNIFICANCE: Acquired resistance to FGFR inhibition can be rapidly promoted by paracrine activation of the NRG1/HER3 axis mediated by adipocyte precursors and can be overcome by the combination of pertuzumab and erdafitinib treatment. See related commentary by Kolonin and Anastassiou, p. 648.

Targeting the Adipose Tissue-Liver-Gut Microbiota Crosstalk to Cure MASLD.

The gut microbiota is a complex system, playing a peculiar role in regulating innate and systemic immunity. Increasing evidence links dysfunctional gut microbiota to metabolic dysfunction-associated steatotic liver disease (MASLD) due to the activation of multiple pathways in the gut and in the liver, including those mediated by Toll-like receptors (TLRs), that sustain hepatic inflammation. Thus, many efforts have been made to unravel the role of microbiota-associated dysfunction in MASLD, with the final aim of finding novel strategies to improve liver steatosis and function. Moreover, recent evidence underlines the role of adipose tissue in sustaining hepatic inflammation during MASLD development. In this review, we focus on the recently discovered strategies proposed to improve the alteration of gut microbiota observed in MASLD patients, with a particular insight into those known to modulate gut microbiota-associated dysfunction and to affect the complex crosstalk between the gut, the adipose tissue, and the liver.

Sex Drives Functional Changes in the Progression and Regression of Liver Fibrosis.

Liver fibrosis is a common and reversible feature of liver damage associated with many chronic liver diseases, and its onset is influenced by sex. In this study, we investigated the mechanisms of liver fibrosis and regeneration, focusing on understanding the mechanistic gaps between females and males. We injected increasing doses of carbon tetrachloride into female and male mice and maintained them for a washout period of eight weeks to allow for liver regeneration. We found that male mice were more prone to developing severe liver fibrosis as a consequence of early chronic liver damage, supported by the recruitment of a large number of Ly6Chigh MoMφs and neutrophils. Although prolonged liver damage exacerbated the fibrosis in mice of both sexes, activated HSCs and Ly6Chigh MoMφs were more numerous and active in the livers of female mice than those of male mice. After eight weeks of washout, only fibrotic females reported no activated HSCs, and a phenotype switching of Ly6Chigh MoMφs to anti-fibrogenic Ly6Clow MoMφs. The early stages of liver fibrosis mostly affected males rather than females, while long-term chronic liver damage was not influenced by sex, at least for liver fibrosis. Liver repair and regeneration were more efficient in females than in males.

Drug-Drug Interaction Studies of Esmethadone (REL-1017) Involving CYP3A4- and CYP2D6-Mediated Metabolism.

BACKGROUND AND OBJECTIVE: Esmethadone (dextromethadone; d-methadone; S-methadone (+)-methadone; REL-1017) is the opioid inactive dextro-isomer of racemic methadone. Esmethadone is a low potency N-methyl-D-aspartate (NMDA) receptor channel blocker with higher affinity for GluN2D subtypes. Esmethadone showed robust, rapid, and sustained antidepressant effects in patients with major depressive disorder (MDD) with inadequate response to ongoing serotonergic antidepressant treatment. METHODS: Here we described the results of in vitro and phase 1 clinical trials aimed at investigating the esmethadone metabolism and possible drug-drug interactions. RESULTS: Esmethadone is primarily metabolized to EDDP (2-ethylene-1,5-dimethyl-3,3-diphenylpyrrolidine) by multiple enzymes, including CYP3A4/5 and CYP2B6. In vitro studies showed that esmethadone inhibits CYP2D6 with IC50 of 9.6 µM and is an inducer of CYP3A4/5. The clinical relevance of the inhibition of CYP2D6 and the induction of CYP3A4 were investigated by co-administering esmethadone and dextromethorphan (a substrate for CYP2D6) or midazolam (a substrate for CYP3A4) in healthy volunteers. The administration of esmethadone at the dosage of 75 mg (which is the loading dose administered to patients in MDD clinical trials) significantly increased the exposure (AUC) of both dextromethorphan and its metabolite dextrorphan by 2.71 and 3.11-fold, respectively. Esmethadone did not modify the pharmacokinetic profile of midazolam, while it increased Cmax and AUC of its metabolite 1'-hydroxymidazolam by 2.4- and 3.8-fold, respectively. A second study evaluated the effect of the CYP3A4 inhibitor cobicistat on the pharmacokinetics of esmethadone. Cobicistat slightly increase (+32%) the total exposure (AUC0-inf) of esmethadone. CONCLUSIONS: In summary, esmethadone demonstrated a negligible effect on CYP3A4 induction and its metabolism was not meaningfully affected by strong CYP3A4 inhibitors while it increased exposure of CYP2D6-metabolized drugs.

Anti-HER2 Super Stealth Immunoliposomes for Targeted-Chemotherapy.

Liposomes play an important role in the field of drug delivery by virtue of their biocompatibility and versatility as carriers. Stealth liposomes, obtained by surface decoration with hydrophilic polyethylene glycol (PEG) molecules, represent an important turning point in liposome technology, leading to significant improvements in the pharmacokinetic profile compared to naked liposomes. Nevertheless, the generation of effective targeted liposomes-a central issue for cancer therapy-has faced several difficulties and clinical phase failures. Active targeting remains a challenge for liposomes. In this direction, a new Super Stealth Immunoliposomes (SSIL2) composed of a PEG-bi-phospholipids derivative is designed that stabilizes the polymer shielding over the liposomes. Furthermore, its counterpart, conjugated to the fragment antigen-binding of trastuzumab (Fab'TRZ -PEG-bi-phospholipids), is firmly anchored on the liposomes surface and correctly orients outward the targeting moiety. Throughout this study, the performances of SSIL2 are evaluated and compared to classic stealth liposomes and stealth immunoliposomes in vitro in a panel of cell lines and in vivo studies in zebrafish larvae and rodent models. Overall, SSIL2 shows superior in vitro and in vivo outcomes, both in terms of safety and anticancer efficacy, thus representing a step forward in targeted cancer therapy, and valuable for future development.

Primary biliary cholangitis: primary autoimmune disease or primary secretory defect.

INTRODUCTION: Primary biliary cholangitis (PBC) is a chronic cholestatic liver disease characterized by the immune-mediated destruction of small and medium intrahepatic bile ducts, involving predominantly females. PBC has long been described as an autoimmune liver disease, also because it is very often associated with many autoimmune conditions. More recently, another pathogenic mechanism exploring the damage of cholangiocytes has been hypothesized, i.e. a defect in the biliary umbrella which is physiologically responsible for the exchange of the ions Cl- and HCO3- and maintains the integrity of glycocalyx. To provide a state-of-the-art analysis of this topic, a systematic review of literature in PubMed, Scopus, and Science Direct was conducted (inclusive dates: 1986-2023). AREA COVERED: Although the etiology remains unknown, pathogenesis consists of a complex immune-mediated process resulting from a genetic susceptibility. PBC can be triggered by an immune-mediated response to an autoantigen, which leads to a progressive destruction of bile ducts and eventually to a progressive fibrosis with cirrhosis. The defect in the 'bicarbonate umbrella' acts as a protection against the toxic hydrophobic bile acids, leading to a toxic composition of bile. EXPERT OPINION: This review offers a summary of the current knowledge about the pathogenesis of PBC, indicating that this is probably based on the mutual relationship between the immune insult and the unbalanced secretory mechanisms.

The novel uncompetitive NMDA receptor antagonist esmethadone (REL-1017) has no meaningful abuse potential in recreational drug users.

Esmethadone (REL-1017) is the opioid-inactive dextro-isomer of methadone and a low-affinity, low-potency uncompetitive NMDA receptor antagonist. In a Phase 2, randomized, double-blind, placebo-controlled trial, esmethadone showed rapid, robust, and sustained antidepressant effects. Two studies were conducted to evaluate the abuse potential of esmethadone. Each study utilized a randomized, double-blind, active-, and placebo-controlled crossover design to assess esmethadone compared with oxycodone (Oxycodone Study) or ketamine (Ketamine Study) in healthy recreational drug users. Esmethadone 25 mg (proposed therapeutic daily dose), 75 mg (loading dose), and 150 mg (Maximum Tolerated Dose) were evaluated in each study. Positive controls were oral oxycodone 40 mg and intravenous ketamine 0.5 mg/kg infused over 40 min. The Ketamine study included oral dextromethorphan 300 mg as an exploratory comparator. The primary endpoint was maximum effect (Emax) for Drug Liking, assessed using a bipolar 100-point visual analog scale (VAS). A total of 47 and 51 participants completed the Oxycodone Study and the Ketamine Study, respectively (Completer Population). In both studies, esmethadone doses ranging from therapeutic (25 mg) to 6 times therapeutic (150 mg) had a meaningful and statistically significantly (p < 0.001) lower Drug Liking VAS Emax compared with the positive control. Results were consistent for all secondary endpoints in both studies. In both studies, all doses of esmethadone were statistically equivalent to placebo on Drug Liking VAS Emax (p < 0.05). In the Ketamine Study, Drug Liking VAS Emax scores for esmethadone at all tested doses were significantly lower vs. dextromethorphan (p < 0.05) (exploratory endpoint). These studies indicate no meaningful abuse potential for esmethadone at all tested doses.

Toward a New Era in the Management of Hepatocellular Carcinoma: Novel Perspectives on Therapeutic Options and Biomarkers.

Liver cancer remains a global health challenge and its incidence is growing worldwide [...].

Esmethadone-HCl (REL-1017): a promising rapid antidepressant.

This review article presents select recent studies that form the basis for the development of esmethadone into a potential new drug. Esmethadone is a promising member of the pharmacological class of uncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonists that have shown efficacy for major depressive disorder (MDD) and other diseases and disorders, such as Alzheimer's dementia and pseudobulbar affect. The other drugs in the novel class of NMDAR antagonists with therapeutic uses that are discussed for comparative purposes in this review are esketamine, ketamine, dextromethorphan, and memantine. We present in silico, in vitro, in vivo, and clinical data for esmethadone and other uncompetitive NMDAR antagonists that may advance our understanding of the role of these receptors in neural plasticity in health and disease. The efficacy of NMDAR antagonists as rapid antidepressants may advance our understanding of the neurobiology of MDD and other neuropsychiatric diseases and disorders.

Tumor Mutational Burden for Predicting Prognosis and Therapy Outcome of Hepatocellular Carcinoma.

Hepatocellular carcinoma (HCC), the primary hepatic malignancy, represents the second-highest cause of cancer-related death worldwide. Many efforts have been devoted to finding novel biomarkers for predicting both patients' survival and the outcome of pharmacological treatments, with a particular focus on immunotherapy. In this regard, recent studies have focused on unravelling the role of tumor mutational burden (TMB), i.e., the total number of mutations per coding area of a tumor genome, to ascertain whether it can be considered a reliable biomarker to be used either for the stratification of HCC patients in subgroups with different responsiveness to immunotherapy, or for the prediction of disease progression, particularly in relation to the different HCC etiologies. In this review, we summarize the recent advances on the study of TMB and TMB-related biomarkers in the HCC landscape, focusing on their feasibility as guides for therapy decisions and/or predictors of clinical outcome.

The phenolic compounds tyrosol and hydroxytyrosol counteract liver fibrogenesis via the transcriptional modulation of NADPH oxidases and oxidative stress-related miRNAs.

Liver fibrosis is the result of a chronic pathological condition caused by the activation of hepatic stellate cells (HSCs), which induces the excessive deposition of extracellular matrix. Fibrogenesis is sustained by an exaggerated production of reactive oxidative species (ROS) by NADPH oxidases (NOXs), which are overactivated in hepatic inflammation. In this study, we investigated the antifibrotic properties of two phenolic compounds of natural origin, tyrosol (Tyr) and hydroxytyrosol (HTyr), known for their antioxidant and anti-inflammatory effects. We assessed Tyr and HTyr antifibrotic and antioxidant activity both in vitro, by a co-culture of LX2, HepG2 and THP1-derived Mϕ macrophages, set up to simulate the hepatic microenvironment, and in vivo, in a mouse model of liver fibrosis obtained by carbon tetrachloride treatment. We evaluated the mRNA and protein expression of profibrotic and oxidative markers (α-SMA, COL1A1, NOX1/4) by qPCR and/or immunocytochemistry or immunohistochemistry. The expression of selected miRNAs in mouse livers were measured by qPCR. Tyr and HTyr reduces fibrogenesis in vitro and in vivo, by downregulating all fibrotic markers. Notably, they also modulated oxidative stress by restoring the physiological levels of NOX1 and NOX4. In vivo, this effect was accompanied by a transcriptional regulation of inflammatory genes and of 2 miRNAs involved in the control of oxidative stress damage (miR-181-5p and miR-29b-3p). In conclusion, Tyr and HTyr exert antifibrotic and anti-inflammatory effects in preclinical in vitro and in vivo models of liver fibrosis, by modulating hepatic oxidative stress, representing promising candidates for further development.

Improved Trimethylangelicin Analogs for Cystic Fibrosis: Design, Synthesis and Preliminary Screening.

A small library of new angelicin derivatives was designed and synthesized with the aim of bypassing the side effects of trimethylangelicin (TMA), a promising agent for the treatment of cystic fibrosis. To prevent photoreactions with DNA, hindered substituents were inserted at the 4 and/or 6 positions. Unlike the parent TMA, none of the new derivatives exhibited significant cytotoxicity or mutagenic effects. Among the synthesized compounds, the 4-phenylderivative 12 and the 6-phenylderivative 25 exerted a promising F508del CFTR rescue ability. On these compounds, preliminary in vivo pharmacokinetic (PK) studies were carried out, evidencing a favorable PK profile per se or after incorporation into lipid formulations. Therefore, the selected compounds are good candidates for future extensive investigation to evaluate and develop novel CFTR correctors based on the angelicin structure.

Obeticholic Acid for Primary Biliary Cholangitis.

Primary biliary cholangitis (PBC) is a rare autoimmune cholestatic liver disease that may progress to fibrosis and/or cirrhosis. Treatment options are currently limited. The first-line therapy for this disease is the drug ursodeoxycholic acid (UDCA), which has been proven to normalize serum markers of liver dysfunction, halt histologic disease progression, and lead to a prolongation of transplant-free survival. However, 30-40% of patients unfortunately do not respond to this first-line therapy. Obeticholic acid (OCA) is the only registered agent for second-line treatment in UDCA-non responders. In this review, we focus on the pharmacological features of OCA, describing its mechanism of action of and its tolerability and efficacy in PBC patients. We also highlight current perspectives on future therapies for this condition.

Esmethadone (REL-1017) and Other Uncompetitive NMDAR Channel Blockers May Improve Mood Disorders via Modulation of Synaptic Kinase-Mediated Signaling.

This article presents a mechanism of action hypothesis to explain the rapid antidepressant effects of esmethadone (REL-1017) and other uncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonists and presents a corresponding mechanism of disease hypothesis for major depressive disorder (MDD). Esmethadone and other uncompetitive NMDAR antagonists may restore physiological neural plasticity in animal models of depressive-like behavior and in patients with MDD via preferential tonic block of pathologically hyperactive GluN2D subtypes. Tonic Ca2+ currents via GluN2D subtypes regulate the homeostatic availability of synaptic proteins. MDD and depressive behaviors may be determined by reduced homeostatic availability of synaptic proteins, due to upregulated tonic Ca2+ currents through GluN2D subtypes. The preferential activity of low-potency NMDAR antagonists for GluN2D subtypes may explain their rapid antidepressant effects in the absence of dissociative side effects.