Demonstration of Isoleucine 199 as a Structural Determinant for the Selective Inhibition of Human Monoamine Oxidase B by Specific Reversible Inhibitors

Privileged scaffolds as MAO inhibitors: Retrospect and prospects

This review aims to be a comprehensive, authoritative, critical, and readable review of general interest to the medicinal chemistry community because it focuses on the pharmacological, chemical, structural and computational aspects of diverse chemical categories as monoamine oxidase inhibitors (MAOIs). Monoamine oxidases (MAOs), namely MAO-A and MAO-B represent an enormously valuable class of neuronal enzymes embodying neurobiological origin and functions, serving as potential therapeutic target in neuronal pharmacotherapy, and hence we have coined the term "Neurozymes" which is being introduced for the first time ever. Nowadays, therapeutic attention on MAOIs engrosses two imperative categories; MAO-A inhibitors, in certain mental disorders such as depression and anxiety, and MAO-B inhibitors, in neurodegenerative disorders like Alzheimer's disease (AD) and Parkinson's disease (PD). The use of MAOIs declined due to some potential side effects, food and drug interactions, and introduction of other classes of drugs. However, curiosity in MAOIs is reviving and the recent developments of new generation of highly selective and reversible MAOIs, have renewed the therapeutic prospective of these compounds. The initial section of the review emphasizes on the detailed classification, structural and binding characteristics, therapeutic potential, current status and future challenges of the privileged pharmacophores. However, the chemical prospective of privileged scaffolds such as; aliphatic and aromatic amines, amides, hydrazines, azoles, diazoles, tetrazoles, indoles, azines, diazines, xanthenes, tricyclics, benzopyrones, and more interestingly natural products, along with their conclusive SARs have been discussed in the later segment of review. The last segment of the article encompasses some patents granted in the field of MAOIs, in a simplistic way.

Synthesis and Biological Evaluation of New Thiosemicarbazone Derivative Schiff Bases as Monoamine Oxidase Inhibitory Agents

Twenty-six novel thiosemicarbazone derivative B1–B26 were synthesized via condensation reactions between the corresponding thiosemicarbazides and aldehydes. The chemical characterization of the compounds was carried out by infrared (IR), mass (MS), proton and carbon nuclear magnetic resonance (¹H- and ¹³C-NMR) spectroscopic analyses. The compounds were investigated for their monoamine oxidase A (MAO-A) and monoamine oxidase B (MAO-B) inhibitory activity and most of them were more potent against MAO-A enzyme when compared with MAO-B enzyme. N-Cyclohexyl-2-[4-[(4-chlorophenyl)thio]benzylidene]hydrazine-1-carbothioamide (B24) was the most active compound against MAO-A. The enzyme kinetics study revealed that compound B24 has a reversible and competitive mode of binding. Interaction modes between compound B24 and MAO-A were clarified by docking studies. In addition, the favourable absorption, distribution, metabolism, and excretion (ADME) properties and non-toxic nature of compound B24 make this compound a promising MAO-A inhibitor.

Isatin inhibits SH-SY5Y neuroblastoma cell invasion and metastasis through MAO/HIF-1α/CXCR4 signaling

Isatin was reported to possess anticancer activities through its effect on tumor proliferation, apoptosis, and metastasis in vitro and in vivo. This study aimed to elucidate the underlying mechanism behind isatin's ability to inhibit neuroblastoma cell metastasis. Our results demonstrated that isatin could inhibit neuroblastoma cell proliferation, invasion, and migration in a dose-dependent manner. Moreover, isatin inhibited the expression level of monoamine oxidase A as well as that of its downstream protein hypoxia-inducible factor 1α. Further study indicated that isatin inhibited reactive oxygen species production, extracellular signal-regulated kinase activation, vascular endothelial growth factor receptor-1 phosphorylation, and chemokine receptor type 4 expression. All results support the potential antimetastatic effect of isatin in neuroblatoma cells.

Methylene blue and its analogues as antidepressant compounds

Methylene Blue (MB) is considered to have diverse medical applications and is a well-described treatment for methemoglobinemias and ifosfamide-induced encephalopathy. In recent years the focus has shifted to MB as an antimalarial agent and as a potential treatment for neurodegenerative disorders such as Alzheimer's disease. Of interest are reports that MB possesses antidepressant and anxiolytic activity in pre-clinical models and has shown promise in clinical trials for schizophrenia and bipolar disorder. MB is a noteworthy inhibitor of monoamine oxidase A (MAO-A), which is a well-established target for antidepressant action. MB is also recognized as a non-selective inhibitor of nitric oxide synthase (NOS) and guanylate cyclase. Dysfunction of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) cascade is strongly linked to the neurobiology of mood, anxiety and psychosis, while the inhibition of NOS and/or guanylate cyclase has been associated with an antidepressant response. This action of MB may contribute significantly to its psychotropic activity. However, these disorders are also characterised by mitochondrial dysfunction and redox imbalance. By acting as an alternative electron acceptor/donor MB restores mitochondrial function, improves neuronal energy production and inhibits the formation of superoxide, effects that also may contribute to its therapeutic activity. Using MB in depression co-morbid with neurodegenerative disorders, like Alzheimer's and Parkinson's disease, also represents a particularly relevant strategy. By considering their physicochemical and pharmacokinetic properties, analogues of MB may provide therapeutic potential as novel multi-target strategies in the treatment of depression. In addition, low MAO-A active analogues may provide equal or improved response with a lower risk of adverse effects.

Synthesis of New Hydrazone Derivatives for MAO Enzymes Inhibitory Activity

In the present work, 14 new 1-substituted-2-phenylhydrazone derivatives were synthesized to evaluate their inhibitory activity against hMAO enzymes. The structures of the newly synthesized hydrazones 2a–2n were characterized by IR, ¹H-NMR, ¹³C-NMR, HR-MS spectroscopic methods. The inhibitory activity of compounds 2a–2n against hMAO-A and hMAO-B enzymes was elucidated by using an in-vitro Amplex Red^® reagent assay based on fluorometric methods. According to the activity studies, 2a and 2b were found to be the most active compounds against hMAO-A enzyme, with IC₅₀ values of 0.342 µM and 0.028 µM, respectively. The most active compounds 2a–2b were evaluated by means of enzyme kinetics and docking studies. Moreover, these compounds were subjected to cytotoxicity and genotoxicity tests to establish their preliminary toxicological profiles and were found to be non-cytotoxic and non-genotoxic. Consequently, the findings of this study display the biological importance of compounds 2a, 2b as selective, irreversible and competitive inhibitors of hMAO-A. Docking studies revealed that there is a strong interaction between hMAO-A and the most active compound 2b.

What a Difference a Methyl Group Makes: The Selectivity of Monoamine Oxidase B Towards Histamine and N-Methylhistamine

Monoamine oxidase (MAO) enzymes catalyze the degradation of a very broad range of biogenic and dietary amines including many neurotransmitters in the brain, whose imbalance is extensively linked with the biochemical pathology of various neurological disorders. Although sharing around 70 % sequence identity, both MAO A and B isoforms differ in substrate affinities and inhibitor sensitivities. Inhibitors that act on MAO A are used to treat depression, due to their ability to raise serotonin concentrations, whereas MAO B inhibitors decrease dopamine degradation and improve motor control in patients with Parkinson disease. Despite this functional importance, the factors affecting MAO selectivity are poorly understood. Here, we used a combination of molecular dynamics (MD) simulations, molecular mechanics with Poisson-Boltzmann and surface area solvation (MM-PBSA) binding free energy evaluations, and quantum mechanical (QM) cluster calculations to address the unexpected, yet challenging MAO B selectivity for N-methylhistamine (NMH) over histamine (HIS), differing only in a single methyl group distant from the reactive ethylamino center. This study shows that a dominant selectivity contribution is offered by a lower activation free energy for NMH by 2.6 kcal mol^-1 , in excellent agreement with the experimental ΔΔG^≠_EXP =1.4 kcal mol^-1 , together with a more favorable reaction exergonicity and active-site binding. This study also confirms the hydrophobic nature of the MAO B active site and underlines the important role of Ile199, Leu171, and Leu328 in properly orienting substrates for the reaction.

Why p-OMe- and p-Cl-β-Methylphenethylamines Display Distinct Activities upon MAO-B Binding

Despite their structural and chemical commonalities, p-chloro-β-methylphenethylamine and p-methoxy-β-methylphenethylamine display distinct inhibitory and substrate activities upon MAO-B binding. Density Functional Theory (DFT) quantum chemical calculations reveal that β-methylation and para-substitution underpin the observed activities sustained by calculated transition state energy barriers, attained conformations and key differences in their interactions in the enzyme’s substrate binding site. Although both compounds meet substrate requirements, it is clear that β-methylation along with the physicochemical features of the para-substituents on the aromatic ring determine the activity of these compounds upon binding to the MAO B-isoform. While data for a larger set of compounds might lend generality to our conclusions, our experimental and theoretical results strongly suggest that the contrasting activities displayed depend on the conformations adopted by these compounds when they bind to the enzyme.

Evaluation of Natural and Synthetic 1,4-naphthoquinones as Inhibitors of Monoamine Oxidase

Previous reports have documented that 1,4-naphthoquinones act as inhibitors of the monoamine oxidase (MAO) enzymes. In particular, fractionation of the extracts of cured tobacco leafs has led to the characterization of 2,3,6-trimethyl-1,4-naphthoquinone, a non-selective MAO inhibitor. To derive structure-activity relationships for MAO inhibition by the 1,4-naphthoquinone class of compounds, this study investigates the human MAO inhibitory activities of fourteen structurally diverse 1,4-naphthoquinones of natural and synthetic origin. Of these, 5,8-dihydroxy-1,4-naphthoquinone was found to be the most potent inhibitor with an IC50 value of 0.860 μm for the inhibition of MAO-B. A related compound, shikonin, inhibits both the MAO-A and MAO-B isoforms with IC50 values of 1.50 and 1.01 μm, respectively. It is further shown that MAO-A and MAO-B inhibition by these compounds is reversible by dialysis. In this respect, kinetic analysis suggests that the modes of MAO inhibition are competitive. This study contributes to the discovery of novel MAO inhibitors, which may be useful in the treatment for disorders such as Parkinson's disease, depressive illness, congestive heart failure and cancer.

Recent developments on the structure–activity relationship studies of MAO inhibitors and their role in different neurological disorders

Development of MAO inhibitors as effective drug candidates for the management and/or treatment of different neurological disorders.

Design, synthesis, biological evaluation and docking studies of sulfonyl isatin derivatives as monoamine oxidase and caspase-3 inhibitors

Sulfonyl isatin derivatives as multifunctional agents showing monoamine oxidase and caspase-3 inhibitory activities in the low micromolar range.

A Strategy to Employ Clitoria ternatea as a Prospective Brain Drug Confronting Monoamine Oxidase (MAO) Against Neurodegenerative Diseases and Depression

Ayurveda is a renowned traditional medicine practiced in India from ancient times and Clitoria ternatea is one such prospective medicinal herb incorporated as an essential constituent in a brain tonic called as medhya rasayan for treating neurological disorders. This work emphasises the significance of the plant as a brain drug there by upholding Indian medicine. The phytochemicals from the root extract were extricated using gas chromatography-mass spectrometry assay and molecular docking against the protein Monoamine oxidase was performed with four potential compounds along with four reference compounds of the plant. This persuades the prospect of C. ternatea as a remedy for neurodegenerative diseases and depression. The in silico assay enumerates that a major compound (Z)-9,17-octadecadienal obtained from the chromatogram with a elevated retention time of 32.99 furnished a minimum binding affinity energy value of -6.5 kcal/mol against monoamine oxidase (MAO-A). The interactions with the amino acid residues ALA 68, TYR 60 and TYR 69 were analogous to the reference compound kaempferol-3-monoglucoside with a least score of -13.90/-12.95 kcal/mol against the isoforms (MAO) A and B. This study fortifies the phytocompounds of C. ternatea as MAO-inhibitors and to acquire a pharmaceutical approach in rejuvenating Ayurvedic medicine.

Multifunctional 3-Schiff base-4-hydroxycoumarin derivatives with monoamine oxidase inhibition, anti-β-amyloid aggregation, metal chelation, antioxidant and neuroprotection properties against Alzheimer's disease

These 3-Schiff base-4-hydroxycoumarin derivatives were multifunctional agents with monoamine oxidase inhibition, anti-β-amyloid aggregation, metal chelation, antioxidant and neuroprotection properties against Alzheimer's disease.

Flavins and flavoproteins: applications in medicine

The potential of flavoproteins as targets of pharmacological treatments is immense. In this review we present an overview of the current research progress on medical interventions based on flavoproteins with a special emphasis on cancer, infectious diseases, and neurological disorders.

Inhibition of monoamine oxidase by selected phenylalkylcaffeine analogues

Objectives

Caffeine represents a useful scaffold for the design of monoamine oxidase (MAO) type B inhibitors. Specifically, substitution on the C8 position yields structures which are high-potency MAO-B inhibitors. To explore the structure–activity relationships of MAO-B inhibition by caffeine-derived compounds, this study examines the MAO inhibitory properties of a series of phenylalkylcaffeine analogues.

Methods

Employing the recombinant human enzymes, the potencies (IC50 values) by which the caffeine analogues inhibit MAO-A and MAO-B were measured. The reversibility of inhibition of a selected inhibitor was determined by measuring the recovery of enzyme activity after dilution and dialysis of enzyme-inhibitor mixtures.

Key findings

The results document that the phenylalkylcaffeine analogues are reversible and selective MAO-B inhibitors with a competitive mode of inhibition. The most potent analogue, 8-(7-phenylheptyl)caffeine, exhibits IC50 values for the inhibition of MAO-A and MAO-B of 3.01 μm and 0.086 μm, respectively. Increasing the length of the alkyl side chain leads to enhanced MAO-A and MAO-B inhibitory potency while introduction of a carbonyl group reduces MAO-B inhibitory potency.

Conclusions

Phenylalkylcaffeines represent a new class of high-potency MAO-B inhibitors with the longer alkyl side chains yielding enhanced inhibitory activity. Such compounds may represent useful leads for the development of anti-parkinsonian therapies.

Monoamine oxidases in development

Monoamine oxidases (MAOs) are flavoproteins of the outer mitochondrial membrane that catalyze the oxidative deamination of biogenic and xenobiotic amines. In mammals there are two isoforms (MAO-A and MAO-B) that can be distinguished on the basis of their substrate specificity and their sensitivity towards specific inhibitors. Both isoforms are expressed in most tissues, but their expression in the central nervous system and their ability to metabolize monoaminergic neurotransmitters have focused MAO research on the functionality of the mature brain. MAO activities have been related to neurodegenerative diseases as well as to neurological and psychiatric disorders. More recently evidence has been accumulating indicating that MAO isoforms are expressed not only in adult mammals, but also before birth, and that defective MAO expression induces developmental abnormalities in particular of the brain. This review is aimed at summarizing and critically evaluating the new findings on the developmental functions of MAO isoforms during embryogenesis.

Activity-based probes for studying the activity of flavin-dependent oxidases and for the protein target profiling of monoamine oxidase inhibitors

High profile: new activity-based protein profiling (ABPP) probes have been designed that target exclusively monoamine oxidases A and B within living cells (see picture; FAD=flavin adenine dinucleotide, FMN=flavin monodinucleotide). With these probes it could be shown that the MAO inhibitor deprenyl, which is in clinical use against Parkinson's disease, shows unique protein specificity despite its covalent mechanism of action.