Monoamine oxidase: radiotracer chemistry and human studies

Recent developments and challenges in positron emission tomography imaging of gliosis in chronic neuropathic pain

ABSTRACT

Understanding the mechanisms that underpin the transition from acute to chronic pain is critical for the development of more effective and targeted treatments. There is growing interest in the contribution of glial cells to this process, with cross-sectional preclinical studies demonstrating specific changes in these cell types capturing targeted timepoints from the acute phase and the chronic phase. In vivo longitudinal assessment of the development and evolution of these changes in experimental animals and humans has presented a significant challenge. Recent technological advances in preclinical and clinical positron emission tomography, including the development of specific radiotracers for gliosis, offer great promise for the field. These advances now permit tracking of glial changes over time and provide the ability to relate these changes to pain-relevant symptomology, comorbid psychiatric conditions, and treatment outcomes at both a group and an individual level. In this article, we summarize evidence for gliosis in the transition from acute to chronic pain and provide an overview of the specific radiotracers available to measure this process, highlighting their potential, particularly when combined with ex vivo/in vitro techniques, to understand the pathophysiology of chronic neuropathic pain. These complementary investigations can be used to bridge the existing gap in the field concerning the contribution of gliosis to neuropathic pain and identify potential targets for interventions.

Molecular Imaging of Monoamine Oxidase A Expression in Highly Aggressive Prostate Cancer: Synthesis and Preclinical Evaluation of Positron Emission Tomography Tracers

The role of monoamine oxidase A (MAO-A) in the aggressiveness of prostate cancer (PCa) has been established in recent years. The molecular imaging of MAO-A expression could offer a noninvasive tool for the visualization and quantification of highly aggressive PCa. This study reports the synthesis and preclinical evaluation of 11C- and 18F-labeled MAO-A inhibitors as positron emission tomography (PET) tracers for proof-of-concept studies in animal models of PCa. Good manufacturing practice production and quality control of these radiotracers using an automated platform was achieved. PET imaging was performed in an LNCaP tumor model with high MAO-A expression. The tumor-to-muscle (T/M) uptake ratio of [11C]harmine (4.5 ± 0.5) was significantly higher than that for 2-[18F]fluoroethyl-harmol (2.3 ± 0.7) and [11C]clorgyline (2.0 ± 0.1). A comparable ex vivo biodistribution pattern in all radiotracers was observed. Furthermore, the tumor uptake of [11C]harmine showed a dramatic reduction (T/M = 1) in a PC3 tumor model with limited MAO-A expression, and radioactivity uptake in LNCaP tumors was blocked in the presence of nonradioactive harmine. Our findings suggest that [11C]harmine may serve as an attractive PET probe for the visualization of MAO-A expression in highly aggressive PCa. These radiotracers have the potential for clinical translation and may aid in the development of personalized therapeutic strategies for PCa patients.

Perfluorononanoic Acid Induces Neurotoxicity via Synaptogenesis Signaling in Zebrafish

Perfluorononanoic acid (PFNA), commonly used as an alternative polyfluorinated compound (PFC) of perfluorooctanoic acid (PFOA), has been widely detected in the aquatic environment. Previous ecotoxicological and epidemiological results suggested that some neurobehavioral effects were associated with PFC exposure; however, the ecological impacts and underlying neurotoxicity mechanisms remain unclear, particularly in aquatic organisms during sensitive, early developmental stages. In this study, zebrafish embryos were exposed to environmentally relevant concentrations of PFNA for 120 h, and the neurological effects of PFNA were comprehensively assessed using transcriptional, biochemical, morphological, and behavioral assays. RNA sequencing and advanced bioinformatics analyses predicted and characterized the key biological processes and pathways affected by PFNA exposure, which included the synaptogenesis signaling pathway, neurotransmitter synapse, and CREB signaling in neurons. Neurotransmitter levels (acetylcholine, glutamate, 5-hydroxytryptamine, γ-aminobutyric acid, dopamine, and noradrenaline) were significantly decreased in zebrafish larvae, and the Tg(gad67:GFP) transgenic line revealed a decreased number of GABAergic neurons in PFNA-treated larvae. Moreover, the swimming distance, rotation frequency, and activity degree were also significantly affected by PFNA, linking molecular-level changes to behavioral consequences.

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.

Evaluation of advanced, pathophysiologic new targets for imaging of CNS

The inadequate information about the in vivo pathological, physiological, and neurological impairments, as well as the absence of in vivo tools for assessing brain penetrance and the efficiency of newly designed drugs, has hampered the development of new techniques for the treatment for variety of new central nervous system (CNS) diseases. The searching sites such as Science Direct and PubMed were used to find out the numerous distinct tracers across 16 CNS targets including tau, synaptic vesicle glycoprotein, the adenosine 2A receptor, the phosphodiesterase enzyme PDE10A, and the purinoceptor, among others. Among the most encouraging are [¹⁸ F]FIMX for mGluR imaging, [¹¹ C]Martinostat for Histone deacetylase, [¹⁸ F]MNI-444 for adenosine 2A imaging, [¹¹ C]ER176 for translocator protein, and [¹⁸ F]MK-6240 for tau imaging. We also reviewed the findings for each tracer's features and potential for application in CNS pathophysiology and therapeutic evaluation investigations, including target specificity, binding efficacy, and pharmacokinetic factors. This review aims to present a current evaluation of modern positron emission tomography tracers for CNS targets, with a focus on recent advances for targets that have newly emerged for imaging in humans.

Beyond monoamines: I. Novel targets and radiotracers for Positron emission tomography imaging in psychiatric disorders

With the emergence of positron emission tomography (PET) in the late 1970s, psychiatry had access to a tool capable of non-invasive assessment of human brain function. Early applications in psychiatry focused on identifying characteristic brain blood flow and metabolic derangements using radiotracers such as [¹⁵ O]H₂ O and [¹⁸ F]FDG. Despite the success of these techniques, it became apparent that more specific probes were needed to understand the neurochemical bases of psychiatric disorders. The first neurochemical PET imaging probes targeted sites of action of neuroleptic (dopamine D₂ receptors) and psychoactive (serotonin receptors) drugs. Based on the centrality of monoamine dysfunction in psychiatric disorders and the measured success of monoamine-enhancing drugs in treating them, the next 30 years witnessed the development of an armamentarium of PET radiopharmaceuticals and imaging methodologies for studying monoamines. Continued development of monoamine-enhancing drugs over this time however was less successful, realizing only modest gains in efficacy and tolerability. As patent protection for many widely prescribed and profitable psychiatric drugs lapsed, drug development pipelines shifted away from monoamines in search of novel targets with the promises of improved efficacy, or abandoned altogether. Over this period, PET radiopharmaceutical development activities closely paralleled drug development priorities resulting in the development of new PET imaging agents for non-monoamine targets. Part one of this review will briefly survey novel PET imaging targets with relevance to the field of psychiatry, which include the metabotropic glutamate receptor type 5 (mGluR5), purinergic P₂ X₇ receptor, type 1 cannabinoid receptor (CB₁ ), phosphodiesterase 10A (PDE10A), and describe radiotracers developed for these and other targets that have matured to human subject investigations. Current limitations of the targets and techniques will also be discussed.

Clinically Relevant Drug Interactions with Monoamine Oxidase Inhibitors

Monoamine oxidase inhibitors (MAOI) are a class of drugs that were originally developed for the treatment of depression but have since been expanded to be used in management of affective and neurological disorders, as well as stroke and aging-related neurocognitive changes. Ranging from irreversible to reversible and selective to non-selective, these drugs target the monoamine oxidase (MAO) enzyme and prevent the oxidative deamination of various monoamines and catecholamines such as serotonin and dopamine, respectively. Tyramine is a potent releaser of norepinephrine (NE) and is found in high concentrations in foods such as aged cheeses and meats. Under normal conditions, NE is unable to accumulate to toxic levels due to the presence of MAO-A, an enzyme that degrades neurotransmitters, including NE. When MAO-A is inhibited, the capacity to handle tyramine intake from the diet is significantly reduced causing the brain to be vulnerable to overstimulation of postsynaptic adrenergic receptors with as little as 8-10 mg of tyramine ingested and can result in life-threatening blood pressure elevations. In addition to adverse reactions with certain foods, both older and newer MAOIs can negatively interact with both sympathomimetic and serotonergic drugs. In general, patients on a MAOI want to avoid two types of medications: those that can elevate blood pressure via sympathomimetic actions (e.g., phenylephrine and oxymetazoline) and those that can increase serotonin levels via 5-HT reuptake inhibition (e.g., dextromethorphan, chlorpheniramine, and brompheniramine). Illicit drugs that stimulate the central nervous system such as ecstasy (MDMA, 3,4-methylenedioxymethamphetamine) act as serotonin releasers. Patient involvement is also crucial to ensure any interaction within the healthcare setting includes making other providers aware of a MAOI prescription as well as avoiding certain OTC medications that can interact adversely with MAOIs.

Fluorine-18: Radiochemistry and Target-Specific PET Molecular Probes Design

The positron emission tomography (PET) molecular imaging technology has gained universal value as a critical tool for assessing biological and biochemical processes in living subjects. The favorable chemical, physical, and nuclear characteristics of fluorine-18 (97% β+ decay, 109.8 min half-life, 635 keV positron energy) make it an attractive nuclide for labeling and molecular imaging. It stands that 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) is the most popular PET tracer. Besides that, a significantly abundant proportion of PET probes in clinical use or under development contain a fluorine or fluoroalkyl substituent group. For the reasons given above, 18F-labeled radiotracer design has become a hot topic in radiochemistry and radiopharmaceutics. Over the past decades, we have witnessed a rapid growth in 18F-labeling methods owing to the development of new reagents and catalysts. This review aims to provide an overview of strategies in radiosynthesis of [18F]fluorine-containing moieties with nucleophilic [18F]fluorides since 2015.

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.

Association between platelet MAO activity and lifetime drug use in a longitudinal birth cohort study

RATIONALE

Platelet monoamine oxidase (MAO) activity, a marker of central serotonergic capacity, has been associated with a variety of problem behaviours. However, studies on platelet MAO activity and addictive drugs have not consistently linked MAO activity with addiction or reported to predict illicit substance use initiation or frequency.

OBJECTIVES

Platelet MAO activity and illicit drug use was examined in a longitudinal birth cohort study.

METHODS

The sample included both birth cohorts (original n = 1238) of the Estonian Children Personality Behaviour and Health Study. Longitudinal association from age 15 to 25 years between platelet MAO activity and lifetime drug use was analysed by mixed-effects regression models. Differences at ages 15, 18 and 25 were analysed by t-test. Cox proportional hazard regression analysis was used to assess the association between platelet MAO activity and the age of drug use initiation.

RESULTS

Male subjects who reported at least one drug use event had lower platelet MAO activity compared to nonusers, both in cross-sectional and longitudinal analyses. Males with low platelet MAO activity had started to use drugs at a younger age. Moreover, in male subjects who had experimented with illicit drugs only once in lifetime, low platelet MAO activity was also associated with higher risk at a younger age. In females, platelet MAO activity was not associated with drug use.

CONCLUSION

In males, low platelet MAO activity is associated with drug abuse primarily owing to risk-taking at early age.

PET imaging of reactive astrocytes in neurological disorders

The reactive astrocytes manifest molecular, structural, and functional remodeling in injury, infection, or diseases of the CNS, which play a critical role in the pathological mechanism of neurological diseases. A growing need exists for dependable approach to better characterize the activation of astrocyte in vivo. As an advanced molecular imaging technology, positron emission tomography (PET) has the potential for visualizing biological activities at the cellular levels. In the review, we summarized the PET visualization strategies for reactive astrocytes and discussed the applications of astrocyte PET imaging in neurological diseases. Future studies are needed to pay more attention to the development of specific imaging agents for astrocytes and further improve our exploration of reactive astrocytes in various diseases.

Questions in the Chemical Enzymology of MAO

We have structure, a wealth of kinetic data, thousands of chemical ligands and clinical information for the effects of a range of drugs on monoamine oxidase activity in vivo. We have comparative information from various species and mutations on kinetics and effects of inhibition. Nevertheless, there are what seem like simple questions still to be answered. This article presents a brief summary of existing experimental evidence the background and poses questions that remain intriguing for chemists and biochemists researching the chemical enzymology of and drug design for monoamine oxidases (FAD-containing EC 4.1.3.4).

In vitro Characterization of the Regional Binding Distribution of Amyloid PET Tracer Florbetaben and the Glia Tracers Deprenyl and PK11195 in Autopsy Alzheimer's Brain Tissue

BACKGROUND

Emerging evidence indicates a central role of gliosis in Alzheimer's disease (AD) pathophysiology. However, the regional distribution and interaction of astrogliosis and microgliosis in association with amyloid-β (Aβ) still remain uncertain.

OBJECTIVE

Here we studied the pathological profiles in autopsy AD brain by using specific imaging tracers.

METHODS

Autopsy brain tissues of AD (n = 15, age 70.4±8.5 years) and control cases (n = 12, age 76.6±10.9) were examined with homogenate binding assays, autoradiography for Aβ plaques (3H-florbetaben/3H-PIB), astrogliosis (3H-L-deprenyl), and microgliosis (3H-PK11195/3H-FEMPA), as well as immunoassays.

RESULTS

In vitro saturation analysis revealed high-affinity binding sites of 3H-florbetaben, 3H-L-deprenyl, and 3H-PK11195/3H-FEMPA in the frontal cortex of AD cases. In vitro3H-florbetaben binding increased across cortical and subcortical regions of AD compared to control with the highest binding in the frontal and parietal cortices. The in vitro3H-L-deprenyl binding showed highest binding in the hippocampus (dentate gyrus) followed by cortical and subcortical regions of AD while the GFAP expression was upregulated only in the hippocampus compared to control. The in vitro3H-PK11195 binding was solely increased in the parietal cortex and the hippocampus of AD compared to control. The 3H-florbetaben binding positively correlated with the 3H-L-deprenyl binding in the hippocampus and parietal cortex of AD and controls. Similarly, a positive correlation was observed between 3H-florbetaben binding and GFAP expression in hippocampus of AD and control.

CONCLUSION

The use of multi-imaging tracers revealed different regional pattern of changes in autopsy AD brain with respect to amyloid plaque pathology versus astrogliosis and microgliosis.

Evaluation of monoamine oxidase A and B type enzyme occupancy using non-radiolabelled tracers in rat brain

Monoamine oxidase (MAO) enzymes, type A and B metabolise the amine neurotransmitters of the body. Selective inhibition of either enzyme is an approach for treating neurodegenerative and stress-induced disorders, and inhibition of an enzyme is proportional to the binding of the MAO inhibitor. Conventionally, the binding of test compounds to enzymes is assessed by radiolabelled ligands in ex vivo and in vivo occupancy assays. Regulatory restrictions and turnaround time are the limitations of the methods that use radiolabelled ligands. But the use of non-radiolabelled tracers and sensitive mass spectrometry (LC-MS/MS) based assays accelerated the determination of target occupancy in pre-clinical species. A report on use of non-radiolabelled ligand in in vivo MAO occupancy assay is not available. The objectives of the present study were to optimise non-radiolabelled harmine and deprenyl as selective tracers in MAO-A and MAO-B occupancy assays and evaluate MAO occupancy of test compounds in rat brain. Tracer optimisation resulted in a detectable, stable, and low ratio (<3.0) of tracer concentrations between any two brain tissues. In occupancy assay, tracer was intravenously administered (10 μg/kg, harmine or 60 μg/kg, L-deprenyl) after the treatment with test compound (clorgyline or tranylcypromine or pargyline or phenelzine or thioperamide). Specific brain tissues were isolated at a defined interval and tracer concentrations were quantified using LC-MS/MS method. Pre-treatment with MAO inhibitors resulted in a decrease (maximum, 80-85%) in harmine or an increase (maximum, 85-300%) in L-deprenyl concentrations. But we considered the change in tracer concentration, relative to the vehicle and positive control groups to calculate MAO occupancy. The observed selectivity and ratio of occupancies (ED₅₀) of test compound towards MAO-A and MAO-B are comparable with the results from in vitro radiolabelled ligand-based inhibition assay. The results demonstrated the application of these non-radiolabelled tracers as suitable pre-clinical tools to determine MAO occupancy.

PET Radiotracers for CNS-Adrenergic Receptors: Developments and Perspectives

Epinephrine (E) and norepinephrine (NE) play diverse roles in our body’s physiology. In addition to their role in the peripheral nervous system (PNS), E/NE systems including their receptors are critical to the central nervous system (CNS) and to mental health. Various antipsychotics, antidepressants, and psychostimulants exert their influence partially through different subtypes of adrenergic receptors (ARs). Despite the potential of pharmacological applications and long history of research related to E/NE systems, research efforts to identify the roles of ARs in the human brain taking advantage of imaging have been limited by the lack of subtype specific ligands for ARs and brain penetrability issues. This review provides an overview of the development of positron emission tomography (PET) radiotracers for in vivo imaging of AR system in the brain.

18F-SMBT-1: A Selective and Reversible PET Tracer for Monoamine Oxidase-B Imaging

Reactive astrocytes play a key role in the pathogenesis of various neurodegenerative diseases. Monoamine oxidase-B (MAO-B) is one of the promising targets for the imaging of astrogliosis in the human brain. A novel selective and reversible MAO-B tracer, (S)-(2-methylpyrid-5-yl)-6-[(3-¹⁸F-fluoro-2-hydroxy)propoxy]quinoline (¹⁸F-SMBT-1), was successfully developed via lead optimization from the first-generation tau PET tracer ¹⁸F-THK-5351. Methods: SMBT-1 was radiolabeled with ¹⁸F using the corresponding precursor. The binding affinity of radiolabeled compounds to MAO-B was assessed using saturation and competitive binding assays. The binding selectivity of ¹⁸F-SMBT-1 to MAO-B was evaluated by autoradiography of frozen human brain tissues. The pharmacokinetics and metabolism were assessed in normal mice after intravenous administration of ¹⁸F-SMBT-1. A 14-d toxicity study after the intravenous administration of ¹⁸F-SMBT-1 was performed using rats and mice. Results: In vitro binding assays demonstrated a high binding affinity of ¹⁸F-SMBT-1 to MAO-B (dissociation constant, 3.7 nM). In contrast, it showed low binding affinity to MAO-A and protein aggregates such as amyloid-β and tau fibrils. Autoradiographic analysis showed higher amounts of ¹⁸F-SMBT-1 binding in the Alzheimer disease brain sections than in the control brain sections. ¹⁸F-SMBT-1 binding was completely displaced with the reversible MAO-B inhibitor lazabemide, demonstrating the high selectivity of ¹⁸F-SMBT-1 for MAO-B. Furthermore, ¹⁸F-SMBT-1 showed a high uptake by brain, rapid washout, and no radiolabeled metabolites in the brain of normal mice. ¹⁸F-SMBT-1 showed no significant binding to various receptors, ion channels, or transporters, and no toxic effects related to its administration were observed in mice and rats. Conclusion: ¹⁸F-SMBT-1 is a promising and selective MAO-B PET tracer candidate, which would be useful for quantitative monitoring of astrogliosis in the human brain.

The influence of genetic factors on personality and coping with stress among healthy late reproductive age women

Background

Psychological stress is a factor which predisposes people to many somatic and mental disorders. Women are at a significantly higher risk of stress than men, and their reactions to stress are stronger. Personality traits are thought to play a special role in the psychology of stress and may be crucial for the choice of a stress-coping strategy. Considering that stress is so common in everyday life, an attempt to understand how stress-coping styles are related to personality and genetic factors acquires special significance.

Purpose

The aim of this study was to analyze stress-coping styles and personality traits in healthy late reproductive age women with regard to genetic factors.

Patients and methods

The study involved 345 healthy late reproductive age women from northwest Poland, whose mean age was 42.3±4.5 years. The study was conducted using The Coping Inventory for Stressful Situations, The Neuroticism-Extraversion-Openness Five--Factor Inventory, and genetic testing.

Results

There were neither statistically significant relationships between personality traits and the genotype distribution of the 30-bp VNTR polymorphism in the MAO-A promoter region, nor between stress-coping styles and the genotype distribution of the 30-bp VNTR polymorphism in the MAO-A promoter region.

Conclusion

Based on the results, all personality traits statistically significantly correlated with the choice of task-oriented coping and emotion-oriented coping. Some of personality traits are genetically determined. The choice of a stress-coping style was significantly related to personality traits. A direct influence of genetic factors on the choice of a stress-coping style was not confirmed in our study.

The monoamine oxidase inhibition properties of C6- and N1-substituted 3-methyl-3,4-dihydroquinazolin-2(1H)-one derivatives

Quinazolinone compounds are of interest in medicinal chemistry since they display a wide range of biological properties. In the present study, a series of C6- and N1-substituted 3-methyl-3,4-dihydroquinazolin-2(1H)-one derivatives were synthesised and evaluated as inhibitors of recombinant human monoamine oxidase (MAO). Some of these quinazolinones are structurally related to a series of 3,4-dihydro-2(1H)-quinolinone derivatives, which have previously been reported to act as specific inhibitors of MAO-B. The results document that, among 37 compounds synthesised, seven displayed IC₅₀ values < 1 µM for the inhibition of MAO-B. The most potent MAO-A inhibitor exhibits an IC₅₀ value of 7.43 µM while the most potent MAO-B inhibitor possesses an IC₅₀ value of 0.269 µM. Good-potency MAO inhibition was only observed among C6-substituted 3-methyl-3,4-dihydroquinazolin-2(1H)-one derivatives with N1-substitution yielding comparatively low-potency inhibition. MAO-B-specific inhibitors such as some of the quinazolinone compounds investigated here may act as leads for the design of therapies for neurodegenerative disorders such as Parkinson's disease.

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.

Ring-Closing Synthesis of Dibenzothiophene Sulfonium Salts and Their Use as Leaving Groups for Aromatic 18F-Fluorination

Herein, we report a novel intramolecular ring-closing reaction of biaryl thioethers that give access to highly functionalized dibenzothiophene sulfonium salts under mild conditions. The resulting precursors react regioselectively with [18F]fluoride to give [18F]fluoroarenes in predictable radiochemical yields. The strategy expands the available radiochemical space and provides superior labeling efficiency for clinically relevant PET tracers.

Neuroimaging-pathological correlations of [18F]THK5351 PET in progressive supranuclear palsy

Recent positron emission tomography (PET) studies have demonstrated the accumulation of tau PET tracer in the affected region of progressive supranuclear palsy (PSP) cases. To confirm the binding target of radiotracer in PSP, we performed an imaging-pathology correlation study in two autopsy-confirmed PSP patients who underwent [¹⁸F]THK5351 PET before death. One patient with PSP Richardson syndrome showed elevated tracer retention in the globus pallidus and midbrain. In a patient with PSP-progressive nonfluent aphasia, [¹⁸F]THK5351 retention also was observed in the cortical areas, particularly the temporal cortex. Neuropathological examination confirmed PSP in both patients. Regional [¹⁸F]THK5351 standardized uptake value ratio (SUVR) in antemortem PET was significantly correlated with monoamine oxidase-B (MAO-B) level, reactive astrocytes density, and tau pathology at postmortem examination. In in vitro autoradiography, specific THK5351 binding was detected in the area of antemortem [¹⁸F]THK5351 retention, and binding was blocked completely by a reversible selective MAO-B inhibitor, lazabemide, in brain samples from these patients. In conclusion, [¹⁸F]THK5351 PET signals reflect MAO-B expressing reactive astrocytes, which may be associated with tau accumulation in PSP.

An efficient preparation of labelling precursor of [11C]L-deprenyl-D2 and automated radiosynthesis

Background

The synthesis of [¹¹C]L-deprenyl-D₂ for imaging of astrocytosis with positron emission tomography (PET) in neurodegenerative diseases has been previously reported. [¹¹C]L-deprenyl-D₂ radiosynthesis requires a precursor, L-nordeprenyl-D₂, which has been previously synthesized from L-amphetamine as starting material with low overall yields. Here, we present an efficient synthesis of L-nordeprenyl-D₂ organic precursor as free base and automated radiosynthesis of [¹¹C]L-deprenyl-D₂ for PET imaging of astrocytosis. The L-nordeprenyl-D₂ precursor was synthesized from the easily commercial available and cheap reagent L-phenylalanine in five steps. Next, N-alkylation of L-nordeprenyl-D₂ free base with [¹¹C]MeOTf was optimized using the automated commercial platform GE TRACERlab® FX C Pro.

Results

A simple and efficient synthesis of L-nordeprenyl-D₂ precursor of [¹¹C]L-deprenyl-D₂ as free base has been developed in five synthetic steps with an overall yield of 33%. The precursor as free base has been stable for 9 months stored at low temperature (−20 °C). The labelled product was obtained with 44 ± 13% (n = 12) (end of synthesis, decay corrected) radiochemical yield from [¹¹C]MeI after 35 min synthesis time. The radiochemical purity was over 99% in all cases and specific activity was (170 ± 116) GBq/μmol.

Conclusions

A high-yield synthesis of [¹¹C]L-deprenyl-D₂ has been achieved with high purity and specific activity. L-nordeprenyl-D₂ precursor as free amine was applicable for automated production in a commercial synthesis module for preclinical and clinical application.

Assessment of Enzyme Inhibition: A Review with Examples from the Development of Monoamine Oxidase and Cholinesterase Inhibitory Drugs

The actions of many drugs involve enzyme inhibition. This is exemplified by the inhibitors of monoamine oxidases (MAO) and the cholinsterases (ChE) that have been used for several pharmacological purposes. This review describes key principles and approaches for the reliable determination of enzyme activities and inhibition as well as some of the methods that are in current use for such studies with these two enzymes. Their applicability and potential pitfalls arising from their inappropriate use are discussed. Since inhibitor potency is frequently assessed in terms of the quantity necessary to give 50% inhibition (the IC50 value), the relationships between this and the mode of inhibition is also considered, in terms of the misleading information that it may provide. Incorporation of more than one functionality into the same molecule to give a multi-target-directed ligands (MTDLs) requires careful assessment to ensure that the specific target effects are not significantly altered and that the kinetic behavior remains as favourable with the MTDL as it does with the individual components. Such factors will be considered in terms of recently developed MTDLs that combine MAO and ChE inhibitory functions.

Zinc in the Monoaminergic Theory of Depression: Its Relationship to Neural Plasticity

Preclinical and clinical studies have demonstrated that zinc possesses antidepressant properties and that it may augment the therapy with conventional, that is, monoamine-based, antidepressants. In this review we aim to discuss the role of zinc in the pathophysiology and treatment of depression with regard to the monoamine hypothesis of the disease. Particular attention will be paid to the recently described zinc-sensing GPR39 receptor as well as aspects of zinc deficiency. Furthermore, an attempt will be made to give a possible explanation of the mechanisms by which zinc interacts with the monoamine system in the context of depression and neural plasticity.

Neurochemical and metabolic effects of acute and chronic alcohol in the human brain: Studies with positron emission tomography

The use of Positron emission tomography (PET) to study the effects of acute and chronic alcohol on the human brain has enhanced our understanding of the mechanisms underlying alcohol's rewarding effects, the neuroadaptations from chronic exposure that contribute to tolerance and withdrawal, and the changes in fronto-striatal circuits that lead to loss of control and enhanced motivation to drink that characterize alcohol use disorders (AUD). These include studies showing that alcohol's reinforcing effects may result not only from its enhancement of dopaminergic, GABAergic and opioid signaling but also from its caloric properties. Studies in those suffering from an AUD have revealed significant alterations in dopamine (DA), GABA, cannabinoids, opioid and serotonin neurotransmission and in brain energy utilization (glucose and acetate metabolism) that are likely to contribute to compulsive alcohol taking, dysphoria/depression, and to alcohol-associated neurotoxicity. Studies have also evaluated the effects of abstinence on recovery of brain metabolism and neurotransmitter function and the potential value of some of these measures to predict clinical outcomes. Finally, PET studies have started to provide insights about the neuronal mechanisms by which certain genes contribute to the vulnerability to AUD. These findings have helped identify new strategies for prevention and treatment of AUD. This article is part of the Special Issue entitled "Alcoholism".

Key Targets for Multi-Target Ligands Designed to Combat Neurodegeneration

HIGHLIGHTS Compounds that interact with multiple targets but minimally with the cytochrome P450 system (CYP) address the many factors leading to neurodegeneration.Acetyl- and Butyryl-cholineEsterases (AChE, BChE) and Monoamine Oxidases A/B (MAO A, MAO B) are targets for Multi-Target Designed Ligands (MTDL).ASS234 is an irreversible inhibitor of MAO A >MAO B and has micromolar potency against the cholinesterases.ASS234 is a poor CYP substrate in human liver, yielding the depropargylated metabolite.SMe1EC2, a stobadine derivative, showed high radical scavenging property, in vitro and in vivo giving protection in head trauma and diabetic damage of endothelium.Control of mitochondrial function and morphology by manipulating fission and fusion is emerging as a target area for therapeutic strategies to decrease the pathological outcome of neurodegenerative diseases. Growing evidence supports the view that neurodegenerative diseases have multiple and common mechanisms in their aetiologies. These multifactorial aspects have changed the broadly common assumption that selective drugs are superior to "dirty drugs" for use in therapy. This drives the research in studies of novel compounds that might have multiple action mechanisms. In neurodegeneration, loss of neuronal signaling is a major cause of the symptoms, so preservation of neurotransmitters by inhibiting the breakdown enzymes is a first approach. Acetylcholinesterase (AChE) inhibitors are the drugs preferentially used in AD and that one of these, rivastigmine, is licensed also for PD. Several studies have shown that monoamine oxidase (MAO) B, located mainly in glial cells, increases with age and is elevated in Alzheimer (AD) and Parkinson's Disease's (PD). Deprenyl, a MAO B inhibitor, significantly delays the initiation of levodopa treatment in PD patients. These indications underline that AChE and MAO are considered a necessary part of multi-target designed ligands (MTDL). However, both of these targets are simply symptomatic treatment so if new drugs are to prevent degeneration rather than compensate for loss of neurotransmitters, then oxidative stress and mitochondrial events must also be targeted. MAO inhibitors can protect neurons from apoptosis by mechanisms unrelated to enzyme inhibition. Understanding the involvement of MAO and other proteins in the induction and regulation of the apoptosis in mitochondria will aid progress toward strategies to prevent the loss of neurons. In general, the oxidative stress observed both in PD and AD indicate that antioxidant properties are a desirable part of MTDL molecules. After two or more properties are incorporated into one molecule, the passage from a lead compound to a therapeutic tool is strictly linked to its pharmacokinetic and toxicity. In this context the interaction of any new molecules with cytochrome P450 and other xenobiotic metabolic processes is a crucial point. The present review covers the biochemistry of enzymes targeted in the design of drugs against neurodegeneration and the cytochrome P450-dependent metabolism of MTDLs.

Molecular aspects of monoamine oxidase B

Monoamine oxidases (MAO) influence the monoamine levels in brain by virtue of their role in neurotransmitter breakdown. MAO B is the predominant form in glial cells and in platelets. MAO B structure, function and kinetics are described as a background for the effect of alterations in its activity on behavior. The need to inhibit MAO B to combat decreased brain amines continues to drive the search for new drugs. Reversible and irreversible inhibitors are now designed using data-mining, computational screening, docking and molecular dynamics. Multi-target ligands designed to combat the elevated activity of MAO B in Alzheimer's and Parkinson's Diseases incorporate MAO inhibition (usually irreversible) as well as iron chelation, antioxidant or neuroprotective properties. The main focus of drug design is the catalytic activity of MAO, but the imidazoline I2 site in the entrance cavity of MAO B is also a pharmacological target. Endogenous regulation of MAO B expression is discussed briefly in light of new studies measuring mRNA, protein, or activity in healthy and degenerative samples, including the effect of DNA methylation on the expression. Overall, this review focuses on examples of recent research on the molecular aspects of the expression, activity, and inhibition of MAO B.

The role of MAO in personality and drug use

Monoamine oxidases, both MAO-A and MAO-B, have been implicated in personality traits and complex behaviour, including drug use. Findings supporting the involvement of MAO-A and MAO-B in shaping personality and in the development of strategies of making behavioural choices come from a variety of studies that have examined either prevalence of gene variants in clinical groups or population-derived samples, estimates of enzyme activity in blood or, by positron emission tomography, in the brain and, most recently, measurement of methylation of the gene. Most of the studies converge in associating MAO-A and MAO-B with impulsive, aggressive or antisocial personality traits or behaviours, including alcohol-related problems, and for MAO-A available evidence strongly supports interaction with adverse environmental exposures in childhood. What is known about genotype effects, and on expression and activity of the enzyme in the brain and in blood has not yet been possible to unite into a mechanistic model of the role of monoamine systems, but the reason for this low degree of generalization is likely caused by the cross-sectional nature of investigation that has not incorporated the developmental effects of MAO-s in critical time windows, including the foetal period. The "risk variants" of both MAO-s appear to increase behavioural plasticity, as supportive environments may particularly well enhance the hidden potential of their carriers. Importantly, male and female brain and behaviours have been found very different with regard to MAO×life events interaction. Future studies need to take into consideration these developmental aspects and sex/gender, as well as to specify the role of different types of environmental factors.

Monoamine oxidase and agitation in psychiatric patients

Subjects with schizophrenia or conduct disorder display a lifelong pattern of antisocial, aggressive and violent behavior and agitation. Monoamine oxidase (MAO) is an enzyme involved in the degradation of various monoamine neurotransmitters and neuromodulators and therefore has a role in various psychiatric and neurodegenerative disorders and pathological behaviors. Platelet MAO-B activity has been associated with psychopathy- and aggression-related personality traits, while variants of the MAOA and MAOB genes have been associated with diverse clinical phenotypes, including aggressiveness, antisocial problems and violent delinquency. The aim of the study was to evaluate the association of platelet MAO-B activity, MAOB rs1799836 polymorphism and MAOA uVNTR polymorphism with severe agitation in 363 subjects with schizophrenia and conduct disorder. The results demonstrated significant association of severe agitation and smoking, but not diagnosis or age, with platelet MAO-B activity. Higher platelet MAO-B activity was found in subjects with severe agitation compared to non-agitated subjects. Platelet MAO-B activity was not associated with MAOB rs1799836 polymorphism. These results suggested the association between increased platelet MAO-B activity and severe agitation. No significant association was found between severe agitation and MAOA uVNTR or MAOB rs1799836 polymorphism, revealing that these individual polymorphisms in MAO genes are not related to severe agitation in subjects with schizophrenia and conduct disorder. As our study included 363 homogenous Caucasian male subjects, our data showing this negative genetic association will be a useful addition to future meta-analyses.

Drug Development in Alzheimer's Disease: The Contribution of PET and SPECT

Clinical trials aiming to develop disease-altering drugs for Alzheimer’s disease (AD), a neurodegenerative disorder with devastating consequences, are failing at an alarming rate. Poorly defined inclusion-and outcome criteria, due to a limited amount of objective biomarkers, is one of the major concerns. Non-invasive molecular imaging techniques, positron emission tomography and single photon emission (computed) tomography (PET and SPE(C)T), allow visualization and quantification of a wide variety of (patho)physiological processes and allow early (differential) diagnosis in many disorders. PET and SPECT have the ability to provide biomarkers that permit spatial assessment of pathophysiological molecular changes and therefore objectively evaluate and follow up therapeutic response, especially in the brain. A number of specific PET/SPECT biomarkers used in support of emerging clinical therapies in AD are discussed in this review.

In Vivo Metabolic Trapping Radiotracers for Imaging Monoamine Oxidase-A and -B Enzymatic Activity

The isozymes of monoamine oxidase (MAO-A and MAO-B) are important enzymes involved in the metabolism of numerous biogenic amines, including the neurotransmitters serotonin, dopamine, and norepinephrine. Recently, changes in concentrations of MAO-B have been proposed to be an in vivo marker of neuroinflammation associated with Alzheimer's disease. Previous developments of in vivo radiotracers for imaging changes in MAO enzyme expression or activity have utilized the irreversible propargylamine-based suicide inhibitors or high-affinity reversibly binding inhibitors. As an alternative approach, we have investigated 1-[(11)C]methyl-4-aryloxy-1,2,3,6-tetrahydropyridines as metabolic trapping agents for the monoamine oxidases. MAO-mediated oxidation and spontaneous hydrolysis yield 1-[(11)C]methyl-2,3-dihydro-4-pyridinone as a hydrophilic metabolite that is trapped within brain tissues. Radiotracers with phenyl, biphenyl, and 7-coumarinyl ethers were evaluated using microPET imaging in rat and primate brains. No isozyme selectivity for radiotracer trapping was observed in the rat brain for any compound, but in the monkey brain, the phenyl ether demonstrated MAO-A selectivity and the coumarinyl ether showed MAO-B selectivity. These are lead compounds for further development of 1-[(11)C]methyl-4-aryloxy-1,2,3,6-tetrahydropyridines with optimized brain pharmacokinetics and isozyme selectivity.

Imaging of neuroinflammation in Alzheimer's disease, multiple sclerosis and stroke: Recent developments in positron emission tomography

Neuroinflammation is thought to play a pivotal role in many diseases affecting the brain, including Alzheimer's disease, multiple sclerosis and stroke. Neuroinflammation is characterised predominantly by microglial activation, which can be visualised using positron emission tomography (PET). Traditionally, translocator protein 18kDa (TSPO) is the target for imaging of neuroinflammation using PET. In this review, recent preclinical and clinical research using PET in Alzheimer's disease, multiple sclerosis and stroke is summarised. In addition, new molecular targets for imaging of neuroinflammation, such as monoamine oxidases, adenosine receptors and cannabinoid receptor type 2, are discussed. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

PET imaging for addiction medicine: From neural mechanisms to clinical considerations

Positron emission tomography (PET) has been shown to be an effective imaging technique to study neurometabolic and neurochemical processes involved in addiction. That is, PET has been used to research neurobiological differences in substance abusers versus healthy controls and the pharmacokinetics and pharmacodynamics of abused drugs. Over the past years, the research scope has shifted to investigating neurobiological effects of abstinence and treatment, and their predictive power for relapse and other clinical outcomes. This chapter provides an overview of PET methodology, recent human PET studies on drug addiction and their implications for clinical treatment.

Specific binding of [(18)F]fluoroethyl-harmol to monoamine oxidase A in rat brain cryostat sections, and compartmental analysis of binding in living brain

We investigated [(18)F]fluoroethyl-harmol ([(18)F]FEH) as a reversible and selective ligand for positron emission tomography (PET) studies of monoamine oxidase A (MAO-A). Binding of [(18)F]FEH in rat brain cryostat sections indicated high affinity (KD = 3 nM), and density (Bmax; 600 pmol/g). The plasma free fraction was 45%, and untransformed parent constituted only 13% of plasma radioactivity at 10 min after injection. Compartmental analysis of PET recordings in pargyline-treated rats showed high permeability to brain (K1; 0.32 mL/g/min) and slow washout (k2; 0.024/min), resulting in a uniformly high equilibrium distribution volume (VD; 20 mL/g). Using this VD to estimate unbound ligand in brain of untreated rats, the binding potential ranged from 4.2 in cerebellum to 7.2 in thalamus. We also calculated maps of rats receiving [(18)F]FEH at a range of specific activities, and then estimated saturation binding parameters in the living brain. In thalamus, striatum and frontal cortex KD was globally close to 300 nM and Bmax was close to 1600 pmol/g; the 100-fold discrepancy in affinity suggests a very low free fraction for [(18)F]FEH in the living brain. Based on a synthesis of findings, we calculate the endogenous dopamine concentration to be 0.4 μM in the striatal compartment containing MAO-A, thus unlikely to exert competition against [(18)F]FEH binding in vivo. In summary, [(18)F]FEH has good properties for the detection of MAO-A in the rat brain by PET, and may present logistic advantages for clinical research at centers lacking a medical cyclotron. We made a compartmental analysis of [(18)F]fluoroethylharmol ([(18)F]FEH) binding to monoamine oxidase A (MAO-A) in living rat brain and estimated the saturation binding parameters from the binding potential (BPND). The Bmax was of comparable magnitude to that in vitro, but with apparent affinity (300 nM), it was 100-fold lower in vivo. PET imaging with [(18) F]FEH is well suited for quantitation of MAO-A in living brain.