Monoamine Oxidase B Total Distribution Volume in the Prefrontal Cortex of Major Depressive Disorder: An [11C]SL25.1188 Positron Emission Tomography Study

Synthesis and evaluation of a targeted PET radioligand [18F]FCOB02 for monoamine oxidase B

Monoamine oxidase B (MAO-B) is a membrane-bound flavinase that plays an important role in the regulation of monoamine neurotransmission. Positron emission tomography (PET) provides a way to study the molecular mechanisms of MAO-B-related diseases and to evaluate the effects of drugs. In this study, we designed and synthesized [18F]FCOB02, a 4-methylcoumarin-like targeting probe.[18F]FCOB02 is straightforward to synthesize and has a high affinity for MAO-B with an IC50 = 10.68 ± 3.25 nM. Successful radiolabeling with fluorine-18 was achieved, resulting in a labeling rate of 35 % along with favorable lipid solubility (log D7.4 = 2.4). Automated radiolabelling was achieved after optimization of the conditions. The radiochemical yield was 9.6 % ± 1.2 %(n = 3) with good radiochemical purity (>98 %), good stability in saline for 4 h and high specific activity (105.08 ± 19GBq/μmol，n = 3). Biodistribution studies conducted in mice revealed significant initial brain uptake of 8.22 ± 0.86 % ID/g at 2 min post-injection, followed by rapid metabolism primarily via the liver and kidneys. Brain uptake was comparable to the same type of probe [18F]SMBT-1 (brain 2min = 7.85 % ID/g). PET-CT images of [18F]FCOB02 in SD rats showed significant differences in brain uptake before and after inhibition by the inhibitor L-deprenyl. Whole brain uptake was reduced by 20 % after inhibition, indicating specific uptake of the probe in the brain, with a 40-min brain clearance rate of 81 %. The potential utility of [18F]FCOB02 for achieving specific MAO-B imaging as well as quantitative analysis in vivo warrants further investigation regarding its clinical translational value.

Evidence of altered monoamine oxidase B, an astroglia marker, in early psychosis and high-risk state

A novel radiotracer, [11C]SL25.1188, targets monoamine oxidase-B (MAO-B) enzyme, found primarily in astrocytes, which metabolizes monoamines (including dopamine), particularly in subcortical regions. Altered astrocyte function in schizophrenia is supported by convergent evidence from post-mortem, genetic, transcriptomic, peripheral and preclinical findings. We aimed to test whether levels of MAO-B, an index of astrocyte function are low in the living brains of early psychosis and their high-risk states. Thirty-eight participants including antipsychotic-free/minimally exposed clinical participants with first-episode psychosis (FEP), clinical high-risk (CHR) individuals and healthy volunteers (HVs) underwent a 90-min positron emission tomography (PET) scan with [11C]SL25.1188, to measure MAO-B VT, an index of MAO-B concentration. Participants were excluded if tested positive on urine drug screen (except for cannabis). This study of 14 FEP (mean[SD] age, 25.7[5.7] years; 6 F), 7 CHR (mean[SD] age, 20.9[3.7] years; 4 F) and 17 HV (mean[SD] age, 31.2[13.9] years; 9 F) demonstrated significant group differences in regional MAO-B VT (F(2,37.42) = 4.56, p = 0.02, Cohen's f = 0.49), controlling for tobacco (F (1,37.42) = 5.37, p = 0.03) and cannabis use (F(1,37.42) = 5.11, p = 0.03) with significantly lower MAO-B VT in CHR compared to HV (Cohen's d = 0.99). We report a significant cannabis effect on MAO-B VT (F(1,39.19) = 12.57, p = 0.001, Cohen's f = 0.57), with a significant group-by-cannabis interaction (F(2,37.30) = 3.82, p = 0.03, Cohen's f = 0.45), indicating lower MAO-B VT in cannabis-using clinical groups. Lower MAO-B VT levels were more robust in striatal than cortical regions, in both clinical groups (F(12,46.84) = 2.08, p = 0.04, Cohen's f = 0.73) and in cannabis users (F(6,46.84) = 6.42, p < 0.001, Cohen's f = 0.91). Lower MAO-B concentration supports astrocyte dysfunction in cannabis-using CHR and FEP clinical populations. Lower MAO-B is consistent with replicated striatal dopamine elevation in psychosis, as well as astrocyte dysfunction in schizophrenia.

Astrocytic pleiotrophin deficiency in the prefrontal cortex contributes to stress-induced depressive-like responses in male mice

Astrocytes are closely linked to depression, and the prefrontal cortex (PFC) is an important brain region involved in major depressive disorder (MDD). However, the underlying mechanism by which astrocytes within PFC contribute to MDD remains unclear. Using single-nucleus RNA sequencing analyses, we show a significant reduction in astrocytes and attenuated pleiotrophin-protein tyrosine phosphatase receptor type Z1 (PTN-PTPRZ1) signaling in astrocyte-to-excitatory neuron communication in the PFC of male MDD patients. We find reduced astrocytes and PTN in the dorsomedial PFC of male mice with depression induced by chronic restraint and social defeat stress. Knockdown of astrocytic PTN induces depression-related responses, which is reversed by exogenous PTN supplementation or overexpression of astrocytic PTN. The antidepressant effects exerted by astrocytic PTN require interaction with PTPRZ1 in excitatory neurons, and PTN-PTPRZ1 activates the AKT signaling pathway to regulate depression-related responses. Our findings indicate the PTN-PTPRZ1-AKT pathway may be a potential therapeutic target for MDD.

Recent progress towards the development of fluorescent probes for the detection of disease-related enzymes

Normal physiological functions as well as regulatory mechanisms for various pathological conditions depend on the activity of enzymes. Thus, determining the in vivo activity of enzymes is crucial for monitoring the physiological metabolism and diagnosis of diseases. Traditional enzyme detection methods are inefficient for in vivo detection, which have different limitations, such as high cost, laborious, and inevitable invasive procedures, low spatio-temporal resolution, weak anti-interference ability, and restricted scope of application. Because of its non-destructive nature, ultra-environmental sensitivity, and high spatiotemporal resolution, fluorescence imaging technology has emerged as a potent tool for the real-time visualization of live cells, thereby imaging the motility of proteins and intracellular signalling networks in tissues and cells and evaluating the binding and attraction of molecules. In the last few years, significant advancements have been achieved in detecting and imaging enzymes in biological systems. In this regard, the high sensitivity and unparalleled spatiotemporal resolution of fluorescent probes in association with confocal microscopy have garnered significant interest. In this review, we focus on providing a concise summary of the latest developments in the design of fluorogenic probes used for monitoring disease-associated enzymes and their application in biological imaging. We anticipate that this study will attract considerable attention among researchers in the relevant field, encouraging them to pursue advances in the development and application of fluorescent probes for the real-time monitoring of enzyme activity in live cells and in vivo models while ensuring excellent biocompatibility.

Imaging neuroglia

Imaging can help us understand the role neuroglia plays in health and during the course of neurologic disorders. In vivo microscopy has had a great impact on our understanding of how neuroglia behaves during health and disease. While initially the technique was hindered by the limited penetration depth in brain tissue, recent advancements lead to increasing possibilities for imaging of deeper brain structures, even at super-resolution. Unfortunately, in vivo microscopy cannot be applied in a clinical setting and thus cannot be used to study neuroglia in patient populations. However, noninvasive imaging techniques like positron emission tomography (PET) and magnetic resonance imaging (MRI) can. PET has provided valuable information on the involvement of neuroglia in neurologic disorders. To more specifically image microglia and astrocytes, many new PET biomarkers have been defined for which PET tracers are continuously developed, evaluated, and improved. A cell-type specific PET tracer with favorable imaging characteristics can have a huge impact on neuroglia research. While being less sensitive than PET, MRI is a more accessible imaging technique. Initially, only general neuroinflammation processes could be imaged with MRI, but newly developed methods and sequences allow for increasing cell-type specificity. Overall, while each imaging method comes with limitations, improvements are continuously made, all with the aim to truly understand the role that neuroglia play in health and disease.

Glia as a New Target for Therapeutic Actions of Electroconvulsive Therapy

Although electroconvulsive therapy (ECT) has immediate and profound effects on severe psychiatric disorders compared to pharmacotherapy, the mechanisms underlying its therapeutic effects remain elusive. Increasing evidence indicates that glial activation is a common pathogenetic factor in both major depression and schizophrenia, raising the question of whether ECT can inhibit glial activation. This article summarizes the findings from both clinical and experimental studies addressing this key question. Based on the findings, it is proposed that the suppression of glial activation associated with neuroinflammation may be involved in the mechanism by which ECT restores brain homeostasis and exerts its therapeutic effects.

PET imaging of neuroinflammation: any credible alternatives to TSPO yet?

Over the last decades, the role of neuroinflammation in neuropsychiatric conditions has attracted an exponentially growing interest. A key driver for this trend was the ability to image brain inflammation in vivo using PET radioligands targeting the Translocator Protein 18 kDa (TSPO), which is known to be expressed in activated microglia and astrocytes upon inflammatory events as well as constitutively in endothelial cells. TSPO is a mitochondrial protein that is expressed mostly by microglial cells upon activation but is also expressed by astrocytes in some conditions and constitutively by endothelial cells. Therefore, our current understanding of neuroinflammation dynamics is hampered by the lack of alternative targets available for PET imaging. We performed a systematic search and review on radiotracers developed for neuroinflammation PET imaging apart from TSPO. The following targets of interest were identified through literature screening (including previous narrative reviews): P2Y12R, P2X7R, CSF1R, COX (microglial targets), MAO-B, I2BS (astrocytic targets), CB2R & S1PRs (not specific of a single cell type). We determined the level of development and provided a scoping review for each target. Strikingly, astrocytic biomarker MAO-B has progressed in clinical investigations the furthest, while few radiotracers (notably targeting S1P1Rs, CSF1R) are being implemented in clinical investigations. Other targets such as CB2R and P2X7R have proven disappointing in clinical studies (e.g. poor signal, lack of changes in disease conditions, etc.). While astrocytic targets are promising, development of new biomarkers and tracers specific for microglial activation has proven challenging.

A Systematic Review of Delta-9-Tetrahydrocannabinol (∆9-THC) in Astrocytic Markers

BACKGROUND

Astrocytic reactivity in substance use disorders (SUDs) has been extensively studied, yet the molecular effect of delta-9-tetrahydrocannabinol (∆9-THC, the main psychoactive compound in cannabis) on glial cells, especially astrocytes, remains poorly understood. Exploring ∆9-THC's impact on astrocytic markers can provide insight into its effects on brain functions such as homeostasis, synaptic transmission, and response to neuronal injury. This systematic review synthesizes findings from studies investigating ∆9-THC's impact on astrocytic markers.

METHODS

A systematic review was conducted using EMBASE, Medline, and PsychoInfo via the OvidSP platform. Studies reporting astrocytic markers following ∆9-THC exposure in animals and humans were included. Data were extracted from twelve eligible full-text articles, and the risk of bias was assessed using the Systematic Review Center for Laboratory Animal Experimentation.

RESULTS

This research identified several astrocytic markers, including glial fibrillary acidic protein (GFAP), nestin, and glutamate-aspartate transporter (GLAST). Both GFAP and nestin expressions increased in adulthood following adolescence and adult ∆9-THC exposure. An increase in GLAST expression was also noted during early development after ∆9-THC exposure.

CONCLUSIONS

This review indicates varying levels of astrocytic reactivity to ∆9-THC across different developmental stages, including adolescence and adulthood. ∆9-THC appears to impact maturation, particularly during early developmental stages, and exhibits sex-dependent effects.

Preconception ethanol exposure changes anxiety, depressive and checking-like behavior and alter the expression levels of MAO-B in male offspring

Alcohol use, which alters the epigenome, increases the probability that it could affect subsequent generations, even if they were never directly exposed to ethanol or even in utero. We explored the effects of parental ethanol exposure before conception on behavioral changes in the offspring. Considering the role of Monoamine oxidase-B (MAO-B) in dopamine turnover in the prefrontal cortex (PFC) and its influence on behavior, and taking into account that ethanol exposure could alter MAO-B, we assessed the protein levels in the offspring. Male and female rats were exposed to ethanol for 30 days and then allowed ten days of abstinence. Afterward, they were mated with either control or ethanol-exposed rats. The F1 and F2 male offspring underwent tests to assess behavioral changes. Additionally, the levels of MAO-B in the PFC were evaluated. Results revealed that in the F1, anxiety increased only in the bi-parental ethanol-exposed male offspring in the elevated plus maze test (p < 0.05), while depressive-like behavior rose only in maternal and bi-parental ethanol-exposed offspring (p < 0.01). However, compulsive-like behavior increased in all ethanol-exposed offspring (p < 0.01). No significant phenotypic changes were observed in the F2. The levels of MAO-B in the PFC increased in the maternal (p < 0.05) and bi-parental ethanol-exposed offspring (p < 0.01). Our study demonstrates that parental ethanol exposure, even in the days preceding mating, adversely affects behaviors and induces molecular changes in the brain. Given these findings, it becomes imperative to monitor children exposed to parental (especially maternal) ethanol for the prevention of mental disorders.

The development status of PET radiotracers for evaluating neuroinflammation

Neuroinflammation is associated with the pathophysiologies of neurodegenerative and psychiatric disorders. Evaluating neuroinflammation using positron emission tomography (PET) plays an important role in the early diagnosis and determination of proper treatment of brain diseases. To quantify neuroinflammatory responses in vivo, many PET tracers have been developed using translocator proteins, imidazole-2 binding site, cyclooxygenase, monoamine oxidase-B, adenosine, cannabinoid, purinergic P2X7, and CSF-1 receptors as biomarkers. In this review, we introduce the latest developments in PET tracers that can image neuroinflammation, focusing on clinical trials, and further consider their current implications.

Monoamine oxidase B activatable red fluorescence probe for bioimaging in cells and zebrafish

A real-time and specific for the detection of Monoamine Oxidase B (MAO-B) to investigate the MAO-B-relevant disease development and treatment process is urgently desirable. Here, we utilized MAO-B to catalyze the conversion of propylamino groups to aldehyde groups, which was then quickly followed by a β-elimination process to produce fluorescent probes (FNJP) that may be used to detect MAO-B in vitro and in vivo. The FNJP probe possesses unique properties, including favorable reactivity (Km = 10.8 μM), high cell permeability, and NIR characteristics (λem = 610 nm). Moreover, the FNJP probe showed high selectivity for MAO-B and was able to detect endogenous MAO-B levels from a mixed population of NIH-3 T3 and HepG2 cells. MAO-B expression was found to be increased in cells under lipopolysaccharide-stimulated cellular oxidative stress in neuronal-like SH-SY5Y cells. In addition, the visualization of FNJP for MAO-B activity in zebrafish can be an effective tool for exploring the biofunctions of MAO-B. Considering these excellent properties, the FNJP probe may be a powerful tool for detecting MAO-B levels in living organisms and can be used for accurate clinical diagnoses of related diseases.

[Psychopharmacotherapy with the MAO-inhibitor Tranylcypromine Key Aspects and Trends in Theory and Practice]

The irreversible monoamine oxidase inhibitor tranylcypromine has been known as an antidepressant drug for more than 60 years. The aim of this review was to make an assessment of the state of the art and therapy of tranylcypromine. The recent medical-scientific literature is analyzed and discussed with respect to key aspects of and general trends in practical psychopharmacotherapy. Meta-analyses of controlled clinical studies have shown that tranylcypromine is an established approach to treatment-resistant depression. Doses (maximum dose, maintenance dose) are increasingly adapted to the requirements of treatment-resistant depression. Monoamine oxidase is not only the primary pharmacological target of tranylcypromine but determines for the first doses also the pharmacokinetics of tranylcypromine because monoamine oxidase is also an enantioselective drug-metabolizing enzyme of the monoamine oxidase inhibitor. An increased diversity of the antidepressant pharmacotherapy suggests the need to rethink the continuing assessment of tranylcypromine as a therapeutic "ultima ratio" in depression. In conclusion, tranylcypromine as a drug of second choice remains a valuable option in antidepressant treatment. Criteria of a switch from other antidepressant drugs to tranylcypromine should be better defined.

The SARS-CoV-2 spike glycoprotein interacts with MAO-B and impairs mitochondrial energetics

SARS-CoV-2 infection is associated with both acute and post-acute neurological symptoms. Emerging evidence suggests that SARS-CoV-2 can alter mitochondrial metabolism, suggesting that changes in brain metabolism may contribute to the development of acute and post-acute neurological complications. Monoamine oxidase B (MAO-B) is a flavoenzyme located on the outer mitochondrial membrane that catalyzes the oxidative deamination of monoamine neurotransmitters. Computational analyses have revealed high similarity between the SARS-CoV-2 spike glycoprotein receptor binding domain on the ACE2 receptor and MAO-B, leading to the hypothesis that SARS-CoV-2 spike glycoprotein may alter neurotransmitter metabolism by interacting with MAO-B. Our results empirically establish that the SARS-CoV-2 spike glycoprotein interacts with MAO-B, leading to increased MAO-B activity in SH-SY5Y neuron-like cells. Common to neurodegenerative disease pathophysiological mechanisms, we also demonstrate that the spike glycoprotein impairs mitochondrial bioenergetics, induces oxidative stress, and perturbs the degradation of depolarized aberrant mitochondria through mitophagy. Our findings also demonstrate that SH-SY5Y neuron-like cells expressing the SARS-CoV-2 spike protein were more susceptible to MPTP-induced necrosis, likely necroptosis. Together, these results reveal novel mechanisms that may contribute to SARS-CoV-2-induced neurodegeneration.

PET Evaluation of the Novel F-18 Labeled Reversible Radioligand [18F]GEH200449 for Detection of Monoamine Oxidase-B in the Non-Human Primate Brain

Positron emission tomography (PET) using radioligands for the enzyme monoamine oxidase B (MAO-B) is increasingly applied as a marker for astrogliosis in neurodegenerative disorders. In the present study, a novel reversible fluorine-18 labeled MAO-B compound, [18F]GEH200449, was evaluated as a PET radioligand in non-human primates. PET studies of [18F]GEH200449 at baseline showed brain exposure (maximum concentration: 3.4-5.2 SUV; n = 5) within the range of that for suitable central nervous system radioligands and a regional distribution consistent with the known localization of MAO-B. Based on the quantitative assessment of [18F]GEH200449 data using the metabolite-corrected arterial plasma concentration as input function, the Logan graphical analysis was selected as the preferred method of quantification. The binding of [18F]GEH200449, as calculated based on regional estimates of the total distribution volume, was markedly inhibited (occupancy >80%) by the administration of the selective MAO-B ligands L-deprenyl (0.5 and 1.0 mg/kg) or rasagiline (0.75 mg/kg) prior to radioligand injection. Radioligand binding was displaceable by the administration of L-deprenyl (0.5 mg/kg) at 25 min after radioligand injection, thus supporting reversible binding to MAO-B. These observations support that [18F]GEH200449 is a reversible MAO-B radioligand suitable for applied studies in humans.

Synthetic cannabinoid receptor agonists are monoamine oxidase-A selective inhibitors

Synthetic cannabinoid receptor agonists (SCRAs) are one of the fastest growing classes of recreational drugs. Despite their growth in use, their vast chemical diversity and rapidly changing landscape of structures make understanding their effects challenging. In particular, the side effects for SCRA use are extremely diverse, but notably include severe outcomes such as cardiac arrest. These side effects appear at odds with the main putative mode of action, as full agonists of cannabinoid receptors. We have hypothesized that SCRAs may act as MAO inhibitors, owing to their structural similarity to known monoamine oxidase inhibitors (MAOI's) as well as matching clinical outcomes (hypertensive crisis) of 'monoaminergic toxicity' for users of MAOIs and some SCRA use. We have studied the potential for SCRA-mediated inhibition of MAO-A and MAO-B via a range of SCRAs used commonly in the UK, as well as structural analogues to prove the atomistic determinants of inhibition. By combining in silico and experimental kinetic studies we demonstrate that SCRAs are MAO-A-specific inhibitors and their affinity can vary significantly between SCRAs, most notably affected by the nature of the SCRA 'head' group. Our data allow us to posit a putative mechanism of inhibition. Crucially our data demonstrate that SCRA activity is not limited to just cannabinoid receptor agonism and that alternative interactions might account for some of the diversity of the observed side effects and that these effects can be SCRA-specific.

Positron Emission Tomography of Neuroimmune Responses in Humans: Insights and Intricacies

The brain's immune system plays a critical role in responding to immune challenges and maintaining homeostasis. However, dysregulated neuroimmune function contributes to neurodegenerative disease and neuropsychiatric conditions. In vivo positron emission tomography (PET) imaging of the neuroimmune system has facilitated a greater understanding of its physiology and the pathology of some neuropsychiatric conditions. This review presents an in-depth look at PET findings from human neuroimmune function studies, highlighting their importance in current neuropsychiatric research. Although the majority of human PET studies feature radiotracers targeting the translocator protein 18 kDa (TSPO), this review also considers studies with other neuroimmune targets, including monoamine oxidase B, cyclooxygenase-1 and cyclooxygenase-2, nitric oxide synthase, and the purinergic P2X7 receptor. Promising new targets, such as colony-stimulating factor 1, Sphingosine-1-phosphate receptor 1, and the purinergic P2Y12 receptor, are also discussed. The significance of validating neuroimmune targets and understanding their function and expression is emphasized in this review to better identify and interpret PET results.

Effect of MAOA DNA Methylation on Human in Vivo Protein Expression Measured by [11C]harmine Positron Emission Tomography

BACKGROUND

Epigenetic modifications like DNA methylation are understood as an intermediary between environmental factors and neurobiology. Cerebral monoamine oxidase A (MAO-A) levels are altered in depression, as are DNA methylation levels within the MAOA gene, particularly in the promoter/exon I/intron I region. An effect of MAOA methylation on peripheral protein expression was shown, but the extent to which methylation affects brain MAO-A levels is not fully understood.

METHODS

Here, the influence of MAOA promoter/exon I/intron I region DNA methylation on global MAO-A distribution volume (VT), an index of MAO-A density, was assessed via [11C]harmine positron emission tomography in 22 patients (14 females) suffering from seasonal affective disorder and 30 healthy controls (17 females).

RESULTS

No significant influence of MAOA DNA methylation on global MAO-A VT was found, despite correction for health status, sex, season, and MAOA variable number of tandem repeat genotype. However, season affected average methylation in women, with higher levels in spring and summer (Puncorr = .03). We thus did not find evidence for an effect of MAOA DNA methylation on brain MAO-A VT.

CONCLUSIONS

In contrast to a previous study demonstrating an effect of methylation of a MAOA promoter region located further 5' on brain MAO-A, MAOA methylation of the region assessed here appears to affect brain protein levels to a limited extent at most. The observed effect of season on methylation levels is in accordance with extensive evidence for seasonal effects within the serotonergic system.

CLINICALTRIALS.GOV IDENTIFIER

NCT02582398 (https://clinicaltrials.gov/ct2/show/NCT02582398).

Monoamine Oxidase B (MAO-B): A Target for Rational Drug Development in Schizophrenia Using PET Imaging as an Example

Monoamine oxidase B (MAO-B) is an important high-density enzyme involved in the generation of oxidative stress and central in the catabolism of dopamine, particularly in brain subcortical regions with putative implications in the pathophysiology of schizophrenia. In this chapter, we review postmortem studies, preclinical models, and peripheral and genetic studies implicating MAO-B in psychosis. A literature search in PubMed was conducted and 64 studies were found to be eligible for systematic review. We found that MAO-B could be identified as a potential target in schizophrenia. Evidence comes mostly from studies of peripheral markers, showing reduced platelet MAO-B activity in schizophrenia, together with preclinical results from MAO-B knock-out mice resulting in a hyperdopaminergic state and behavioral disinhibition. However, whether brain MAO-B is altered in vivo in patients with schizophrenia remains unknown. We therefore review methodological studies involving MAO-B positron emission tomography (PET) radioligands used to quantify MAO-B in vivo in the human brain. Given the limitations of currently available treatments for schizophrenia, elucidating whether MAO-B could be used as a target for risk stratification or clinical staging in schizophrenia could allow for a rational search for newer antipsychotics and the development of new treatments.

The MAOA rs979605 Genetic Polymorphism Is Differentially Associated with Clinical Improvement Following Antidepressant Treatment between Male and Female Depressed Patients

Major depressive disorder (MDD) is the leading cause of disability worldwide. Treatment with antidepressant drugs (ATD), which target monoamine neurotransmitters including serotonin (5HT), are only modestly effective. Monoamine oxidase (MAO) metabolizes 5HT to 5-hydroxy indoleacetic acid (5HIAA). Genetic variants in the X-chromosome-linked MAO-encoding genes, MAOA and MAOB, have been associated with clinical improvement following ATD treatment in depressed patients. Our aim was to analyze the association of MAOA and MAOB genetic variants with (1) clinical improvement and (2) the plasma 5HIAA/5HT ratio in 6-month ATD-treated depressed individuals. Clinical (n = 378) and metabolite (n = 148) data were obtained at baseline and up to 6 months after beginning ATD treatment (M6) in patients of METADAP. Mixed-effects models were used to assess the association of variants with the Hamilton Depression Rating Scale (HDRS) score, response and remission rates, and the plasma 5HIAA/5HT ratio. Variant × sex interactions and dominance terms were included to control for X-chromosome-linked factors. The MAOA rs979605 and MAOB rs1799836 polymorphisms were analyzed. The sex × rs979605 interaction was significantly associated with the HDRS score (p = 0.012). At M6, A allele-carrying males had a lower HDRS score (n = 24, 10.9 ± 1.61) compared to AA homozygous females (n = 14, 18.1 ± 1.87; p = 0.0067). The rs1799836 polymorphism was significantly associated with the plasma 5HIAA/5HT ratio (p = 0.018). Overall, CC/C females/males had a lower ratio (n = 44, 2.18 ± 0.28) compared to TT/T females/males (n = 60, 2.79 ± 0.27; p = 0.047). The MAOA rs979605 polymorphism, associated with the HDRS score in a sex-dependent manner, could be a useful biomarker for the response to ATD treatment.

Novel brain PET imaging agents: Strategies for imaging neuroinflammation in Alzheimer’s disease and mild cognitive impairment

Alzheimer’s disease (AD) is a devastating neurodegenerative disease with a concealed onset and continuous deterioration. Mild cognitive impairment (MCI) is the prodromal stage of AD. Molecule-based imaging with positron emission tomography (PET) is critical in tracking pathophysiological changes among AD and MCI patients. PET with novel targets is a promising approach for diagnostic imaging, particularly in AD patients. Our present review overviews the current status and applications of in vivo molecular imaging toward neuroinflammation. Although radiotracers can remarkably diagnose AD and MCI patients, a variety of limitations prevent the recommendation of a single technique. Recent studies examining neuroinflammation PET imaging suggest an alternative approach to evaluate disease progression. This review concludes that PET imaging towards neuroinflammation is considered a promising approach to deciphering the enigma of the pathophysiological process of AD and MCI.

Tryptophan Metabolism in Depression: A Narrative Review with a Focus on Serotonin and Kynurenine Pathways

Depression is a common and serious disorder, characterized by symptoms like anhedonia, lack of energy, sad mood, low appetite, and sleep disturbances. This disease is very complex and not totally elucidated, in which diverse molecular and biological mechanisms are involved, such as neuroinflammation. There is a high need for the development of new therapies and gaining new insights into this disease is urgent. One important player in depression is the amino acid tryptophan. This amino acid can be metabolized in two important pathways in the context of depression: the serotonin and kynurenine pathways. These metabolic pathways of tryptophan are crucial in several processes that are linked with depression. Indeed, the maintenance of the balance of serotonin and kynurenine pathways is critical for the human physiological homeostasis. Thus, this narrative review aims to explore tryptophan metabolism (particularly in the serotonin and kynurenine pathways) in depression, starting with a global overview about these topics and ending with the focus on these pathways in neuroinflammation, stress, microbiota, and brain-derived neurotrophic factor regulation in this disease. Taken together, this information aims to clarify the metabolism of tryptophan in depression, particularly the serotonin and kynurenine pathways.

Application of positron emission tomography in psychiatry-methodological developments and future directions

Mental disorders represent an increasing source of disability and high costs for societies globally. Molecular imaging techniques such as positron emission tomography (PET) represent powerful tools with the potential to advance knowledge regarding disease mechanisms, allowing the development of new treatment approaches. Thus far, most PET research on pathophysiology in psychiatric disorders has focused on the monoaminergic neurotransmission systems, and although a series of discoveries have been made, the results have not led to any material changes in clinical practice. We outline areas of methodological development that can address some of the important obstacles to fruitful progress. First, we point towards new radioligands and targets that can lead to the identification of processes upstream, or parallel to disturbances in monoaminergic systems. Second, we describe the development of new methods of PET data quantification and PET systems that may facilitate research in psychiatric populations. Third, we review the application of multimodal imaging that can link molecular imaging data to other aspects of brain function, thus deepening our understanding of disease processes. Fourth, we highlight the need to develop imaging study protocols to include longitudinal and interventional paradigms, as well as frameworks to assess dimensional symptoms such that the field can move beyond cross-sectional studies within current diagnostic boundaries. Particular effort should be paid to include also the most severely ill patients. Finally, we discuss the importance of harmonizing data collection and promoting data sharing to reach the desired sample sizes needed to fully capture the phenotype of psychiatric conditions.

Reduced Platelet MAO-B Activity Is Associated with Psychotic, Positive, and Depressive Symptoms in PTSD

Post-traumatic stress disorder (PTSD) is a trauma-related disorder. Platelet monoamine oxidase (MAO-B) is a peripheral biomarker associated with various symptoms in different psychopathologies, but its role in PTSD or different symptoms in PTSD is not clear. This study elucidated the association between platelet MAO-B activity and clinical symptoms occurring in PTSD. Platelet MAO-B activity was determined in 1053 male Caucasian subjects: 559 war veterans with PTSD (DSM-5 criteria), 62 combat exposed veterans who did not develop PTSD, and 432 non-combat exposed healthy controls. Clinical symptoms in PTSD were determined using CAPS and PANSS. Platelet MAO-B activity, controlled for the effect of smoking, was significantly increased in PTSD with severe versus mild and moderate traumatic symptoms, and was significantly decreased in PTSD subjects with severe versus mild positive, psychotic, and depressive symptoms. This finding was further confirmed with reduced platelet MAO-B activity in PTSD veterans with severe versus mild individual items of the PANSS-depressed, PANSS-psychotic, and PANSS-positive subscales. Altered platelet MAO-B activity, controlled for the possible confounders, was associated with the development and severity of different symptoms occurring in PTSD. These findings confirmed the role of platelet MAO-B activity as a peripheral marker of various psychopathological symptoms.

Revisiting the Role of Astrocytic MAOB in Parkinson's Disease

Monoamine oxidase-B (MAOB) has been believed to mediate the degradation of monoamine neurotransmitters such as dopamine. However, this traditional belief has been challenged by demonstrating that it is not MAOB but MAOA which mediates dopamine degradation. Instead, MAOB mediates the aberrant synthesis of GABA and hydrogen peroxide (H2O2) in reactive astrocytes of Parkinson's disease (PD). Astrocytic GABA tonically suppresses the dopaminergic neuronal activity, whereas H2O2 aggravates astrocytic reactivity and dopaminergic neuronal death. Recently discovered reversible MAOB inhibitors reduce reactive astrogliosis and restore dopaminergic neuronal activity to alleviate PD symptoms in rodents. In this perspective, we redefine the role of MAOB for the aberrant suppression and deterioration of dopaminergic neurons through excessive GABA and H2O2 synthesis of reactive astrocytes in PD.

Development and Clinical Application of Positron Emission Tomography Imaging Agents for Monoamine Oxidase B

Monoamine oxidase B (MAO-B) is a high-density protein in the brain mainly found on outer mitochondrial membranes, primarily in astroglia, but additionally in serotonergic neurons and in the substantia nigra in the midbrain. It is an enzyme that participates in the oxidative metabolism of important monoamines including dopamine, norepinephrine, benzylamine, and phenylethylamine. Elevated MAO-B density may be associated with astrogliosis and inhibiting MAO-B may reduce astrogliosis. MAO-B density is elevated in postmortem sampling of pathology for many neuropsychiatric diseases including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and alcohol use disorder. Initial development of positron emission tomography (PET) imaging agents focused on analogs of [¹¹C]L-deprenyl, with the most commonly applied being the deuterium substituted [¹¹C]L-deprenyl-D2. This latter radiotracer was modeled with an irreversible trapping compartment reflecting its irreversible binding to MAO-B. Subsequently, [¹¹C]SL25.1188, a reversible binding MAO-B radioligand with outstanding properties including high specific binding and excellent reversibility was developed. [¹¹C]SL25.1188 PET was applied to discover a substantive elevation of MAO-B binding in the prefrontal cortex in major depressive disorder (MDD) with an effect size of more than 1.5. Longer duration of MDD was associated with greater MAO-B binding throughout most gray matter regions in the brain, suggesting progressive astrogliosis. Important applications of [¹¹C]L-deprenyl-D2 PET are detecting a 40% loss in radiotracer accumulation in cigarette smokers, and substantial occupancy of novel therapeutics like EVT301 and sembragiline. Given the number of diseases with elevations of MAO-B density and astrogliosis, and the advance of [¹¹C]SL25.1188, clinical applications of MAO-B imaging are still at an early stage.

Imaging of Reactive Astrogliosis by Positron Emission Tomography

Many neurodegenerative diseases are neuropathologically characterized by neuronal loss, gliosis, and the deposition of misfolded proteins such as β-amyloid (Aβ) plaques and tau tangles in Alzheimer’s disease (AD). In postmortem AD brains, reactive astrocytes and activated microglia are observed surrounding Aβ plaques and tau tangles. These activated glial cells secrete pro-inflammatory cytokines and reactive oxygen species, which may contribute to neurodegeneration. Therefore, in vivo imaging of glial response by positron emission tomography (PET) combined with Aβ and tau PET would provide new insights to better understand the disease process, as well as aid in the differential diagnosis, and monitoring glial response disease-specific therapeutics. There are two promising targets proposed for imaging reactive astrogliosis: monoamine oxidase-B (MAO-B) and imidazoline2 binding site (I2BS), which are predominantly expressed in the mitochondrial membranes of astrocytes and are upregulated in various neurodegenerative conditions. PET tracers targeting these two MAO-B and I2BS have been evaluated in humans. [18F]THK-5351, which was originally designed to target tau aggregates in AD, showed high affinity for MAO-B and clearly visualized reactive astrocytes in progressive supranuclear palsy (PSP). However, the lack of selectivity of [18F]THK-5351 binding to both MAO-B and tau, severely limits its clinical utility as a biomarker. Recently, [18F]SMBT-1 was developed as a selective and reversible MAO-B PET tracer via compound optimization of [18F]THK-5351. In this review, we summarize the strategy underlying molecular imaging of reactive astrogliosis and clinical studies using MAO-B and I2BS PET tracers.

Brain injury and inflammation genes common to a number of neurological diseases and the genes involved in the genesis of GABAnergic neurons are altered in monoamine oxidase B knockout mice

Monoamine oxidase B (MAO B) oxidizes trace amine phenylethylamine (PEA), and neurotransmitters serotonin and dopamine in the brain. We reported previously that PEA levels increased significantly in all brain regions, but serotonin and dopamine levels were unchanged in MAO B knockout (KO) mice. PEA and dopamine are both synthesized from phenylalanine by aromatic L-amino acid decarboxylase in dopaminergic neurons in the striatum. A high concentration of PEA in the striatum may cause dopaminergic neuronal death in the absence of MAO B. We isolated the RNA from brain tissue of MAO B KO mice (2-month old) and age-matched wild type (WT) male mice and analyzed the altered genes by Affymetrix microarray. Differentially expressed genes (DEGs) in MAO B KO compared to WT mice were analyzed by Partek Genomics Suite, followed by Ingenuity Pathway Analysis (IPA) to assess their functional relationships. DEGs in MAO B KO mice are involved in brain inflammation and the genesis of GABAnergic neurons. The significant DEGs include four brain injury or inflammation genes (upregulated: Ido1, TSPO, AVP, Tdo2), five gamma-aminobutyric acid (GABA) receptors (down-regulated: GABRA2, GABRA3, GABRB1, GABRB3, GABRG3), five transcription factors related to adult neurogenesis (upregulated: Wnt7b, Hes5; down-regulated: Pax6, Tcf4, Dtna). Altered brain injury and inflammation genes in MAO B knockout mice are involved in various neurological disorders: attention deficit hyperactive disorder, panic disorder, obsessive compulsive disorder, autism, amyotrophic lateral sclerosis, Parkinson's diseases, Alzheimer's disease, bipolar affective disorder. Many were commonly involved in these disorders, indicating that there are overlapping molecular pathways.

Imaging of astrocytes in posttraumatic stress disorder: A PET study with the monoamine oxidase B radioligand [11C]SL25.1188

Posttraumatic stress disorder (PTSD) is a debilitating mental health condition that results from exposure to traumatic event(s). Decreased astrocyte-related proteins (e.g., glial fibrillary acidic protein, GFAP) and atrophic astrocytes in corticolimbic brain areas implicated in PTSD have been reported in experimental models suggesting that astrocyte pathology may be a feature of this disorder. We used positron emission tomography (PET) of the monoamine oxidase (MAO)-B probe [¹¹C]SL25.1188 to test the hypothesis that levels of MAO-B, an index of astrocyte levels is decreased in PTSD. MAO-B availability ([¹¹C]SL25.1188 distribution volume) was measured in 13 participants with PTSD (∼39 years, 6F) and 17 healthy controls (HC) (∼31 years, 9F). A magnetic resonance image was acquired to delineate 6 cortiolimbic brain regions. PTSD was associated with a trending reduction in [¹¹C]SL25.1188 availability across regions (8-17%; p = 0.067) implicating the ventral striatum (p uncorrected = 0.015) and medial prefrontal cortex (p uncorrected = 0.060). [¹¹C]SL25.1188 availability was ∼30% lower in corticolimbic regions in PTSD with co-morbid major depressive disorder (MDD) (n = 4) vs HC (p = 0.001) and vs PTSD without MDD (p = 0.005). Our preliminary results do not suggest astrogliosis (inferred from elevated availability) in PTSD, but rather point to a loss of astrocytes or an independent downregulation of MAO-B in PTSD with more severe negative affect. These exploratory findings, which are partly in line with preclinical literature and recent PET observations of decreased microglia marker, Translocator Protein, in PTSD, warrant replication in a larger PTSD cohort.

Depression in type 2 diabetes: A systematic review and meta-analysis of blood inflammatory markers

The prevalence of depression is higher among people with type 2 diabetes (T2DM). Individually, both conditions are associated with systemic inflammation. This study aimed to summarize the clinical data comparing peripheral inflammatory markers in blood between people with T2DM, with and without comorbid depression. From 2187 records, we identified 20 original peer-reviewed articles from which blood inflammatory marker concentrations could be combined and compared between people with T2DM and comorbid depression (D) vs. no depression (ND) as standardized mean differences (SMD) in random effects meta-analysis. Concentrations of C-reactive protein (CRP; ND/NND = 1742/15244, SMD = 0.31 95% confidence interval [0.16, 0.45], Z16 = 4.03, p < 0.01; I2 = 84.0%) and interleukin-6 (IL-6; ND/NND = 677/4349, SMD = 0.17 [0.04, 0.30], Z4 = 2.58, p = 0.01; I2 = 48.1%), were higher, and concentrations of brain derived neurotrophic factor (BDNF; ND/NND = 358/1512, SMD = -0.37 95% confidence interval [-0.64,-0.10], Z2 = -2.68, p = 0.01; I2 = 61.2%) were lower, among those with depression. Depression in T2DM was associated with systemic inflammation and lower peripheral blood BDNF concentrations. Inconsistency between studies suggests the need to explore further population heterogeneity and pathophysiological elements. PROSPERO (CRD42020188509).

PCOS and Depression: Common Links and Potential Targets

PCOS or polycystic ovary syndrome is a common endocrine disorder that occurs during the reproductive age in females. It manifests in the form of a wide range of symptoms including (but not limited to) hirsutism, amenorrhea, oligomenorrhea, obesity, acne vulgaris, infertility, alopecia, and insulin resistance. The incidence of depression in PCOS population is increasing as compared to the general population. Increased depression in PCOS significantly alters the quality of life (QOL) of affected females. Also, self-esteem is found to be low in both depression and PCOS. The loss in self-esteem in such patients can be largely attributed to the associated factors including (but not limited to) obesity, acne, androgenic alopecia, and hirsutism. The reason behind the occurrence of depression in PCOS remains elusive to date. Literature suggests that there is an overlap of clinical symptoms between depression and PCOS. As the symptoms overlap, there is a possibility of common associations between depression, PCOS, and PCOS-associated abnormalities including insulin resistance (IR), obesity, CVD, and androgen excess. Studies demonstrate that depression is an inflammatory disorder marked with increased levels of inflammatory markers. On the other hand, PCOS is also regarded as a pro-inflammatory state that is characterized by increased levels of pro-inflammatory markers. Thus, there is a possibility of an inflammatory relationship existing between depression and PCOS. It is also possible that the inflammatory markers in PCOS can cross the blood-brain barrier (BBB) leading to the development of depression. Through the present review, we have attempted to shed light on common associations/shared links between depression and PCOS with respect to the levels of cortisol, androgen, vitamin D, neurotransmitters, monoaminoxidase (MAO), and insulin-like growth factor-1 (IGF-1). Tracking down common associations between depression and PCOS will help find potential drug therapies and improve the QOL of females with depression in PCOS.

Excess tau PET ligand retention in elderly patients with major depressive disorder

Depression is one of the common psychiatric disorders in old age. Major depressive disorder (MDD) has been identified as a risk factor or prodrome for neurodegenerative dementias, suggesting neuropathological overlaps and a continuum between MDD and neurodegenerative disorders. In this study, we examined tau and amyloid-β (Aβ) accumulations in the brains of MDD and healthy controls using positron emission tomography (PET) to explore pathological substrates of this illness. Twenty MDD and twenty age-matched, healthy controls were examined by PET with a tau radioligand, [¹¹C]PBB3, and an Aβ radioligand, [¹¹C]PiB. Radioligand retentions were quantified as a standardized uptake value ratio (SUVR). We also assessed clinical manifestations of the patients using the 17-item Hamilton Depression Scale, the Geriatric Depression Scale, and psychotic symptoms. Mean cortical [¹¹C]PBB3 SUVRs in MDD patients were significantly higher than those of healthy controls. These values were higher in MDD patients with psychotic symptoms than in those without any. The present findings indicate that tau depositions may underlie MDD, and especially in patients with psychotic symptoms. PET detection of tau accumulations may provide mechanistic insights into neuronal dysfunctions in these cases and could serve as predictions of their clinical consequences.

PET Imaging of Neuroinflammation in Alzheimer's Disease

Neuroinflammation play an important role in Alzheimer's disease pathogenesis. Advances in molecular imaging using positron emission tomography have provided insights into the time course of neuroinflammation and its relation with Alzheimer's disease central pathologies in patients and in animal disease models. Recent single-cell sequencing and transcriptomics indicate dynamic disease-associated microglia and astrocyte profiles in Alzheimer's disease. Mitochondrial 18-kDa translocator protein is the most widely investigated target for neuroinflammation imaging. New generation of translocator protein tracers with improved performance have been developed and evaluated along with tau and amyloid imaging for assessing the disease progression in Alzheimer's disease continuum. Given that translocator protein is not exclusively expressed in glia, alternative targets are under rapid development, such as monoamine oxidase B, matrix metalloproteinases, colony-stimulating factor 1 receptor, imidazoline-2 binding sites, cyclooxygenase, cannabinoid-2 receptor, purinergic P2X7 receptor, P2Y12 receptor, the fractalkine receptor, triggering receptor expressed on myeloid cells 2, and receptor for advanced glycation end products. Promising targets should demonstrate a higher specificity for cellular locations with exclusive expression in microglia or astrocyte and activation status (pro- or anti-inflammatory) with highly specific ligand to enable in vivo brain imaging. In this review, we summarised recent advances in the development of neuroinflammation imaging tracers and provided an outlook for promising targets in the future.

Monoamine Oxidase as a Potential Biomarker of the Efficacy of Treatment of Mental Disorders

The review summarizes the results of our own studies and published data on the biological markers of psychiatric disorders, with special emphasis on the activity of platelet monoamine oxidase. Pharmacotherapy studies in patients with the mixed anxiety-depressive disorder and first episode of schizophrenia have shown that the activity of platelet monoamine oxidase could serve as a potential biomarker of the efficacy of therapeutic interventions in these diseases.

Macro- and Microscale Stress-Associated Alterations in Brain Structure: Translational Link With Depression

Major depressive disorder (MDD) is a stress-related disorder associated with many cytoarchitectural and neurochemical changes. However, the majority of these changes cannot be reliably detected in the living brain. The examination of animal stress models and postmortem human brain tissue has significantly contributed to our understanding of the pathophysiology of MDD. Ronald Duman's work in humans and in rodent models was critical to the investigation of the contribution of synaptic deficits to MDD and chronic stress pathology, their role in the development and expression of depressive-like behavior, and reversal by novel drugs. Here, we review evidence from magnetic resonance imaging in humans and animals that suggests that corticolimbic alterations are associated with depression symptomatology. We also discuss evidence of cytoarchitectural alterations affecting neurons, astroglia, and synapses in MDD and highlight how similar changes are described in rodent chronic stress models and are linked to the emotion-related behavioral deficits. Finally, we report on the latest approaches developed to measure the synaptic and astroglial alterations in vivo, using positron emission tomography, and how it can inform on the contribution of MDD-associated cytoarchitectural alterations to the symptomatology and the treatment of stress-related disorders.

The role of monoamine oxidase enzymes in the pathophysiology of neurological disorders

Monoamine oxidase enzymes are responsible for the degredation of serotonin, dopamine, and norepinephrine in the central neurvous system. Although it has been nearly 100 years since they were first described, we are still learning about their role in the healthy brain and how they are altered in various disease states. The present review provides a survey of our current understanding of monoamine oxidases, with a focus on their contributions to neuropsychiatric, neurodevelopmental, and neurodegenerative disease. Important species differences in monoamine oxidase function and development in the brain are highlighted. Sex-specific monoamine oxidase regulatory mechanisms and their implications for various neurological disorders are also discussed. While our understanding of these critical enzymes has expanded over the last century, gaps exist in our understanding of sex and species differences and the roles monoamine oxidases may play in conditions often comorbid with neurological disorders.

Imaging Biomarkers for Monitoring the Inflammatory Redox Landscape in the Brain

Inflammation is one key process in driving cellular redox homeostasis toward oxidative stress, which perpetuates inflammation. In the brain, this interplay results in a vicious cycle of cell death, the loss of neurons, and leakage of the blood–brain barrier. Hence, the neuroinflammatory response fuels the development of acute and chronic inflammatory diseases. Interrogation of the interplay between inflammation, oxidative stress, and cell death in neurological tissue in vivo is very challenging. The complexity of the underlying biological process and the fragility of the brain limit our understanding of the cause and the adequate diagnostics of neuroinflammatory diseases. In recent years, advancements in the development of molecular imaging agents addressed this limitation and enabled imaging of biomarkers of neuroinflammation in the brain. Notable redox biomarkers for imaging with positron emission tomography (PET) tracers are the 18 kDa translocator protein (TSPO) and monoamine oxygenase B (MAO–B). These findings and achievements offer the opportunity for novel diagnostic applications and therapeutic strategies. This review summarizes experimental as well as established pharmaceutical and biotechnological tools for imaging the inflammatory redox landscape in the brain, and provides a glimpse into future applications.

Preclinical Evaluation of TSPO and MAO-B PET Radiotracers in an LPS Model of Neuroinflammation

Discovery of novel PET radiotracers targeting neuroinflammation (microglia and astrocytes) is actively pursued. Employing a lipopolysaccharide (LPS) rat model, this longitudinal study evaluated the translocator protein 18-kDa radiotracer [¹⁸F]FEPPA (primarily microglia) and monoamine oxidase B radiotracers [¹¹C]L-deprenyl and [¹¹C]SL25.1188 (astrocytes preferred). Increased [¹⁸F]FEPPA binding peaked at 1 week in LPS-injected striatum whereas increased lazabemide-sensitive [¹¹C]L-deprenyl binding developed later. No increase in radiotracer uptake was observed for [¹¹C]SL25.1188. The unilateral intrastriatal LPS rat model may serve as a useful tool for benchmarking PET tracers targeted toward distinct phases of neuroinflammatory reactions involving both microglia and astrocytes.

Revisiting monoamine oxidase inhibitors for the treatment of depressive disorders: A systematic review and network meta-analysis

BACKGROUND

Monoamine oxidase inhibitors (MAOIs) were the first class of modern antidepressants; however, they are under-utilized as compared to the newer antidepressants.

METHODS

In this systematic review, network meta-analysis was used to investigate the comparative efficacy and acceptability of MAOIs for depressive disorders. Overall, the network meta-analysis included 52 double-blind, randomized controlled trials (RCTs) that compared 14 antidepressants or placebo. Across studies, the mean arm size was n = 58 participants from a total N = 6462 (5309 active drug; 1153 placebo).

RESULTS

Except fluvoxamine, all antidepressants demonstrated superior efficacy to placebo, and none demonstrated substantially better or worse all-cause dropout rates. Phenelzine demonstrated superior evidence for efficacy compared to all other treatments, and clomipramine demonstrated superior evidence for acceptability compared to all other treatments.

LIMITATIONS

The study is primarily limited by low estimate precision due to a relative paucity of studies for some of the included treatment conditions. Further evidence is required to study the relative efficacy of MAOIs against newer antidepressants.

CONCLUSIONS

The results of this analysis largely support the re-evaluation of the use of MAOIs as antidepressant agents in the treatment algorithm of depression.

Recent developments on PET radiotracers for TSPO and their applications in neuroimaging

The 18 kDa translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is predominately localized to the outer mitochondrial membrane in steroidogenic cells. Brain TSPO expression is relatively low under physiological conditions, but is upregulated in response to glial cell activation. As the primary index of neuroinflammation, TSPO is implicated in the pathogenesis and progression of numerous neuropsychiatric disorders and neurodegenerative diseases, including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), multiple sclerosis (MS), major depressive disorder (MDD) and obsessive compulsive disorder (OCD). In this context, numerous TSPO-targeted positron emission tomography (PET) tracers have been developed. Among them, several radioligands have advanced to clinical research studies. In this review, we will overview the recent development of TSPO PET tracers, focusing on the radioligand design, radioisotope labeling, pharmacokinetics, and PET imaging evaluation. Additionally, we will consider current limitations, as well as translational potential for future application of TSPO radiopharmaceuticals. This review aims to not only present the challenges in current TSPO PET imaging, but to also provide a new perspective on TSPO targeted PET tracer discovery efforts. Addressing these challenges will facilitate the translation of TSPO in clinical studies of neuroinflammation associated with central nervous system diseases.

Neuroinflammation in psychiatric disorders: PET imaging and promising new targets

Neuroinflammation is a multifaceted physiological and pathophysiological response of the brain to injury and disease. Given imaging findings of 18 kDa translocator protein (TSPO) and the development of radioligands for other inflammatory targets, PET imaging of neuroinflammation is at a particularly promising stage. This Review critically evaluates PET imaging results of inflammation in psychiatric disorders, including major depressive disorder, schizophrenia and psychosis disorders, substance use, and obsessive-compulsive disorder. We also consider promising new targets that can be measured in the brain, such as monoamine oxidase B, cyclooxygenase-1 and cyclooxygenase-2, colony stimulating factor 1 receptor, and the purinergic P2X7 receptor. Thus far, the most compelling TSPO imaging results have arguably been found in major depressive disorder, for which consistent increases have been observed, and in schizophrenia and psychosis, for which patients show reduced TSPO levels. This pattern highlights the importance of validating brain biomarkers of neuroinflammation for each condition separately before moving on to patient stratification and treatment monitoring trials.

Translocator Protein Distribution Volume Predicts Reduction of Symptoms During Open-Label Trial of Celecoxib in Major Depressive Disorder

BACKGROUND

Gliosis is common among neuropsychiatric diseases, but the relationship between gliosis and response to therapeutics targeting effects of gliosis is largely unknown. Translocator protein total distribution volume (TSPO VT), measured with positron emission tomography, mainly reflects gliosis in neuropsychiatric disease. Here, the primary objective was to determine whether TSPO VT in the prefrontal cortex (PFC) and anterior cingulate cortex (ACC) predicts reduction of depressive symptoms following open-label celecoxib administration in treatment-resistant major depressive disorder.

METHODS

A total of 41 subjects with treatment-resistant major depressive disorder underwent one [18F]FEPPA positron emission tomography scan to measure PFC and ACC TSPO VT. Open-label oral celecoxib (200 mg, twice daily) was administered for 8 weeks. Change in symptoms was measured with the 17-item Hamilton Depression Rating Scale (HDRS).

RESULTS

Cumulative mean change in HDRS scores between 0 and 8 weeks of treatment was plotted against PFC and ACC TSPO VT, showing a significant nonlinear relationship. At low TSPO VT values, there was no reduction in HDRS scores, but as TSPO VT values increased, there was a reduction in HDRS scores that then plateaued. This was modeled with a 4-parameter sigmoidal model in which PFC and ACC TSPO VT accounted for 84% and 92% of the variance, respectively.

CONCLUSIONS

Celecoxib administration in the presence of gliosis labeled by TSPO VT is associated with greater reduction of symptoms. Given the predictiveness of TSPO VT on symptom reduction, this personalized medicine approach of matching a marker of gliosis to medication targeting effects of gliosis should be applied in early development of novel therapeutics, in particular for treatment-resistant major depressive disorder.

Pyrimido[1,2-b]indazole derivatives: Selective inhibitors of human monoamine oxidase B with neuroprotective activity

Structurally diverse heterotricyclic compounds are recognized as monoamine oxidase (MAO) inhibitors and thus represent an appealing scaffold in development and optimization of novel MAO inhibitors. Herein we explored the chemical space of pyrimido[1,2-b]indazoles as MAO inhibitors by preparing a small library of (hetero)aryl derivatives. An efficient synthetic strategy was developed starting from commercially available 1H-indazol-3-amines, which were converted to various 3-bromoheterotricyclic derivatives and further functionalized via Suzuki-Miyaura coupling reaction. Derivatives 4a-t selectively inhibited human MAO-B isoform in a reversible and competitive manner as confirmed by kinetic experiments and docking studies. Selected derivatives were not cytotoxic to neuroblastoma SH-SY5Y cells. Moreover, analogue 4i protected human neuroblastoma SH-SY5Y cells against 6-hydroxydopamine-induced cell death, which confirms the applicability of the pyrimido[1,2-b]indazoles as potential antiparkinsonian agents.

The State of Our Understanding of the Pathophysiology and Optimal Treatment of Depression: Glass Half Full or Half Empty?

Major depressive disorder is a remarkably common and often severe psychiatric disorder associated with high levels of morbidity and mortality. Patients with major depression are prone to several comorbid psychiatric conditions, including posttraumatic stress disorder, anxiety disorders, obsessive-compulsive disorder, and substance use disorders, and medical conditions, including cardiovascular disease, diabetes, stroke, cancer, which, coupled with the risk of suicide, result in a shortened life expectancy. The goal of this review is to provide an overview of our current understanding of major depression, from pathophysiology to treatment. In spite of decades of research, relatively little is known about its pathogenesis, other than that risk is largely defined by a combination of ill-defined genetic and environmental factors. Although we know that female sex, a history of childhood maltreatment, and family history as well as more recent stressors are risk factors, precisely how these environmental influences interact with genetic vulnerability remains obscure. In recent years, considerable advances have been made in beginning to understand the genetic substrates that underlie disease vulnerability, and the interaction of genes, early-life adversity, and the epigenome in influencing gene expression is now being intensively studied. The role of inflammation and other immune system dysfunction in the pathogenesis of major depression is also being intensively investigated. Brain imaging studies have provided a firmer understanding of the circuitry involved in major depression, providing potential new therapeutic targets. Despite a broad armamentarium for major depression, including antidepressants, evidence-based psychotherapies, nonpharmacological somatic treatments, and a host of augmentation strategies, a sizable percentage of patients remain nonresponsive or poorly responsive to available treatments. Investigational agents with novel mechanisms of action are under active study. Personalized medicine in psychiatry provides the hope of escape from the current standard trial-and-error approach to treatment, moving to a more refined method that augurs a new era for patients and clinicians alike.

Classics in Neuroimaging: Development of PET Tracers for Imaging Monoamine Oxidases

In this Viewpoint, we highlight the history of positron emission tomography (PET) radiotracer development to quantify changes in monoamine oxidase (MAO)-A and -B enzyme expression or activity. MAO-A and MAO-B are critical for understanding monoaminergic pathways in psychiatric addiction disorders, and more recently in neurodegenerative disorders with MAO-B expression in astrogliosis. Unique radiochemical innovations have been shown for neuroimaging of MAOs including the clinical translation of irreversible propargylamine-based suicide inhibitors, application of deuterium-substitution to slow down metabolism, development of trapped metabolite imaging agents, and unique ¹¹C-carbonylation chemistry toward novel high-affinity reversibly binding inhibitors.