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

Machine learning accelerates pharmacophore-based virtual screening of MAO inhibitors

Nowadays, an efficient and robust virtual screening procedure is crucial in the drug discovery process, especially when performed on large and chemically diverse databases. Virtual screening methods, like molecular docking and classic QSAR models, are limited in their ability to handle vast numbers of compounds and to learn from scarce data, respectively. In this study, we introduce a universal methodology that uses a machine learning-based approach to predict docking scores without the need for time-consuming molecular docking procedures. The developed protocol yielded 1000 times faster binding energy predictions than classical docking-based screening. The proposed predictive model learns from docking results, allowing users to choose their preferred docking software without relying on insufficient and incoherent experimental activity data. The methodology described employs multiple types of molecular fingerprints and descriptors to construct an ensemble model that further reduces prediction errors and is capable of delivering highly precise docking score values for monoamine oxidase ligands, enabling faster identification of promising compounds. An extensive pharmacophore-constrained screening of the ZINC database resulted in a selection of 24 compounds that were synthesized and evaluated for their biological activity. A preliminary screen discovered weak inhibitors of MAO-A with a percentage efficiency index close to a known drug at the lowest tested concentration. The approach presented here can be successfully applied to other biological targets as target-specific knowledge is not incorporated at the screening phase.

Novel 6-hydroxybenzothiazol-2-carboxamides as potent and selective monoamine oxidase B inhibitors endowed with neuroprotective activity

In neurodegenerative diseases, using a single molecule that can exert multiple effects to modify the disease may have superior activity over the classical "one molecule-one target" approach. Herein, we describe the discovery of 6-hydroxybenzothiazol-2-carboxamides as highly potent and selective MAO-B inhibitors. Variation of the amide substituent led to several potent compounds having diverse side chains with cyclohexylamide 40 displaying the highest potency towards MAO-B (IC50 = 11 nM). To discover new compounds with extended efficacy against neurotoxic mechanisms in neurodegenerative diseases, MAO-B inhibitors were screened against PHF6, R3 tau, cellular tau and α-synuclein (α-syn) aggregation. We identified the phenethylamide 30 as a multipotent inhibitor of MAO-B (IC50 = 41 nM) and α-syn and tau aggregation. It showed no cytotoxic effects on SH-SY5Y neuroblastoma cells, while also providing neuroprotection against toxicities induced by α-syn and tau. The evaluation of key physicochemical and in vitro-ADME properties revealed a great potential as drug-like small molecules with multitarget neuroprotective activity.

The carboxy terminus causes interfacial assembly of oleate hydratase on a membrane bilayer

The soluble flavoprotein oleate hydratase (OhyA) hydrates the 9-cis double bond of unsaturated fatty acids. OhyA substrates are embedded in membrane bilayers; OhyA must remove the fatty acid from the bilayer and enclose it in the active site. Here, we show that the positively charged helix-turn-helix motif in the carboxy terminus (CTD) is responsible for interacting with the negatively charged phosphatidylglycerol (PG) bilayer. Super-resolution microscopy of Staphylococcus aureus cells expressing green fluorescent protein fused to OhyA or the CTD sequence shows subcellular localization along the cellular boundary, indicating OhyA is membrane-associated and the CTD sequence is sufficient for membrane recruitment. Using cryo-electron microscopy, we solved the OhyA dimer structure and conducted 3D variability analysis of the reconstructions to assess CTD flexibility. Our surface plasmon resonance experiments corroborated that OhyA binds the PG bilayer with nanomolar affinity and we found the CTD sequence has intrinsic PG binding properties. We determined that the nuclear magnetic resonance structure of a peptide containing the CTD sequence resembles the OhyA crystal structure. We observed intermolecular NOE from PG liposome protons next to the phosphate group to the CTD peptide. The addition of paramagnetic MnCl2 indicated the CTD peptide binds the PG surface but does not insert into the bilayer. Molecular dynamics simulations, supported by site-directed mutagenesis experiments, identify key residues in the helix-turn-helix that drive membrane association. The data show that the OhyA CTD binds the phosphate layer of the PG surface to obtain bilayer-embedded unsaturated fatty acids.

Molecular Mechanism and Structure-activity Relationship of the Inhibition Effect between Monoamine Oxidase and Selegiline Analogues

INTRODUCTION

To investigate the inhibition properties and structure-activity relationship between monoamine oxidase (MAO) and selected monoamine oxidase inhibitors (MAOIs, including selegiline, rasagiline and clorgiline).

METHODS

The inhibition effect and molecular mechanism between MAO and MAOIs were identified via the half maximal inhibitory concentration (IC50) and molecular docking technology.

RESULTS

It was indicated that selegiline and rasagiline were MAO B inhibitors, but clorgiline was MAO-A inhibitor based on the selectivity index (SI) of MAOIs (0.000264, 0.0197 and 14607.143 for selegiline, rasagiline and clorgiline, respectively). The high-frequency amino acid residues of the MAOIs and MAO were Ser24, Arg51, Tyr69 and Tyr407 for MAO-A and Arg42 and Tyr435 for MAO B. The MAOIs and MAO A/B pharmacophores included the aromatic core, hydrogen bond acceptor, hydrogen bond donor-acceptor and hydrophobic core.

CONCLUSION

This study shows the inhibition effect and molecular mechanism between MAO and MAOIs and provides valuable findings on the design and treatment of Alzheimer's and Parkinson's diseases.

A Comprehensive Review of the Docking Studies of Chalcone for the Development of Selective MAO-B Inhibitors

Monoamine oxidase B is a crucial therapeutic target for neurodegenerative disorders like Alzheimer's and Parkinson's since they assist in disintegrating neurotransmitters such as dopamine in the brain. Pursuing efficacious monoamine oxidase B inhibitors is a hot topic, as contemporary therapeutic interventions have many shortcomings. Currently available FDA-approved monoamine oxidase inhibitors like safinamide, selegiline and rasagiline also have a variety of side effects like depression and insomnia. In the quest for a potent monoamine oxidase B inhibitor, sizeable, diverse chemical entities have been uncovered, including chalcones. Chalcone is a renowned structural framework that has been intensively explored for its monoamine oxidase B inhibitory activity.The structural resemblance of chalcone (1,3-diphenyl-2-propen-1-one) based compounds and 1,4-diphenyl- 2-butene, a recognized MAO-B inhibitor, accounts for their MAO-B inhibitory activity. Therefore, multiple revisions to the chalcone scaffold have been attempted by the researchers to scrutinize the implications of substitutions onthe molecule's potency. In this work, we outline the docking investigation results of various chalcone analogues with monoamine oxidase B available in the literature until now to understand the interaction modes and influence of substituents. Here we focused on the interactions between reported chalcone derivatives and the active site of monoamine oxidase B and the influence of substitutions on those interactions. Detailed images illustrating the interactions and impact of the substituents or structural modifications on these interactions were used to support the docking results.

Isatin-based benzyloxybenzene derivatives as monoamine oxidase inhibitors with neuroprotective effect targeting neurogenerative disease treatment

Eighteen isatin-based benzyloxybenzaldehyde derivatives from three subseries, ISB, ISFB, and ISBB, were synthesized and their ability to inhibit monoamine oxidase (MAO) was evaluated. The inhibitory activity of all synthesized compounds was found to be more profound against MAO-B than MAO-A. Compound ISB1 most potently inhibited MAO-B with an IC50 of 0.124 ± 0.007 μM, ensued by ISFB1 (IC50 = 0.135 ± 0.002 μM). Compound ISFB1 most potently inhibited MAO-A with an IC50 of 0.678 ± 0.006 μM, ensued by ISBB3 (IC50 = 0.731 ± 0.028 μM), and had the highest selectivity index (SI) value (55.03). The three sub-parental compounds, ISB1, ISFB1, and ISBB1, had higher MAO-B inhibition than the other derivatives, indicating that the substitutions of the 5-H in the A-ring of isatin diminished the inhibition of MAO-A and MAO-B. Among these, ISB1 (para-benzyloxy group in the B-ring) displayed more significant MAO-B inhibition when compared to ISBB1 (meta-benzyloxy group in the B-ring). ISB1 and ISFB1 were identified to be competitive and reversible MAO-B inhibitors, having Ki values of 0.055 ± 0.010, and 0.069 ± 0.025 μM, respectively. Furthermore, in the parallel artificial membrane penetration assay, ISB1 and ISFB1 traversed the blood-brain barrier in the in vitro condition. Additionally, the current study found that ISB1 decreased rotenone-induced cell death in SH-SY5Y neuroblastoma cells. In docking and simulation studies, the hydrogen bonding formed by the imino nitrogen in ISB1 and the pi-pi stacking interaction of the phenyl ring in isatin significantly aided in the protein-ligand complex's stability, effectively inhibiting MAO-B. According to these observations, the MAO-B inhibitors ISB1 and ISFB1 were potent, selective, and reversible, making them conceivable therapies for neurological diseases.

The evaluation of isatin analogues as inhibitors of monoamine oxidase

The small molecule, isatin, is a well-known reversible inhibitor of the monoamine oxidase (MAO) enzymes with IC50 values of 12.3 and 4.86 μM for MAO-A and MAO-B, respectively. While the interaction of isatin with MAO-B has been characterized, only a few studies have explored structure-activity relationships (SARs) of MAO inhibition by isatin analogues. The current study therefore evaluated a series of 14 isatin analogues as in vitro inhibitors of human MAO-A and MAO-B. The results indicated good potency MAO inhibition for some isatin analogues with five compounds exhibiting IC50  < 1 μM. 4-Chloroisatin (1b) and 5-bromoisatin (1f) were the most potent inhibitors with IC50 values of 0.812 and 0.125 μM for MAO-A and MAO-B, respectively. These compounds were also found to be competitive inhibitors of MAO-A and MAO-B with Ki values of 0.311 and 0.033 μM, respectively. Among the SARs, it was interesting to note that C5-substitution was particularly beneficial for MAO-B inhibition. MAO inhibitors are established drugs for the treatment of neuropsychiatric and neurodegenerative disorders, while potential new roles in prostate cancer and cardiovascular disease are being investigated.

From Monoamine Oxidase Inhibition to Antiproliferative Activity: New Biological Perspectives for Polyamine Analogs

Monoamine oxidases (MAOs) are well-known pharmacological targets in neurological and neurodegenerative diseases. However, recent studies have revealed a new role for MAOs in certain types of cancer such as glioblastoma and prostate cancer, in which they have been found overexpressed. This finding is opening new frontiers for MAO inhibitors as potential antiproliferative agents. In light of our previous studies demonstrating how a polyamine scaffold can act as MAO inhibitor, our aim was to search for novel analogs with greater inhibitory potency for human MAOs and possibly with antiproliferative activity. A small in-house library of polyamine analogs (2-7) was selected to investigate the effect of constrained linkers between the inner amine functions of a polyamine backbone on the inhibitory potency. Compounds 4 and 5, characterized by a dianiline (4) or dianilide (5) moiety, emerged as the most potent, reversible, and mainly competitive MAO inhibitors (Ki < 1 μM). Additionally, they exhibited a high antiproliferative activity in the LN-229 human glioblastoma cell line (GI50 < 1 μM). The scaffold of compound 5 could represent a potential starting point for future development of anticancer agents endowed with MAO inhibitory activity.

Structure-Based Design of Novel MAO-B Inhibitors: A Review

With the significant growth of patients suffering from neurodegenerative diseases (NDs), novel classes of compounds targeting monoamine oxidase type B (MAO-B) are promptly emerging as distinguished structures for the treatment of the latter. As a promising function of computer-aided drug design (CADD), structure-based virtual screening (SBVS) is being heavily applied in processes of drug discovery and development. The utilization of molecular docking, as a helping tool for SBVS, is providing essential data about the poses and the occurring interactions between ligands and target molecules. The current work presents a brief discussion of the role of MAOs in the treatment of NDs, insight into the advantages and drawbacks of docking simulations and docking software, and a look into the active sites of MAO-A and MAO-B and their main characteristics. Thereafter, we report new chemical classes of MAO-B inhibitors and the essential fragments required for stable interactions focusing mainly on papers published in the last five years. The reviewed cases are separated into several chemically distinct groups. Moreover, a modest table for rapid revision of the revised works including the structures of the reported inhibitors together with the utilized docking software and the PDB codes of the crystal targets applied in each study is provided. Our work could be beneficial for further investigations in the search for novel, effective, and selective MAO-B inhibitors.

Purification of MAO A and MAO B from Mammalian Tissue Sources

Procedures are described for the purification of the mitochondrial-bound enzymes human and bovine monoamine oxidases A and B (MAO A and B) from placental and liver tissue sources, respectively. Enzyme purification follows isolation of the mitochondria and preparation of outer membrane particles. The membrane-bound enzymes are solubilized by treatment of membranes with phospholipases and detergent extraction. Functional bovine MAO B is purified by polymer fractionation and differential centrifugation. Functional human MAO A is purified by ion-exchange DEAE-Sepharose chromatography.

Monoamine oxidase inhibitors: A concise review with special emphasis on structure activity relationship studies

Monoamine oxidase enzyme is necessary for the management of brain functions. It oxidatively metabolizes monoamines and produces ammonia, aldehyde and hydrogen peroxide as by-products. Excessive production of by-products of monoamine metabolism generates free radicals which cause cellular apoptosis and several neurodegenerative disorders for example Alzheimer's disease, Parkinson's disease, depression and autism. The inhibition of MAOs is an attractive target for the treatment of neurological disorders. Clinically approved MAO inhibitors for example selegiline, rasagiline, clorgyline, pargyline etc. are irreversible in nature and cause some adverse effects while recently studied reversible MAO inhibitors are devoid of harmful effects of old monoamine oxidase inhibitors. In this review article we have listed various synthesized molecules containing different moieties like coumarin, chalcone, thiazole, thiourea, caffeine, pyrazole, chromone etc. along with their activity, mode of action, structure activity relationship and molecular docking studies.

5-Demethoxy-10'-ethoxyexotimarin F, a New Coumarin with MAO-B Inhibitory Potential from Murraya exotica L

Rutaceae plants are known for being a rich source of coumarins. Preliminary molecular docking showed that there was no significant difference for coumarins in Clausena and Murraya, both of which had high scoring values and showed good potential inhibitory activity to the MAO-B enzyme. Overall, 32 coumarins were isolated from Murraya exotica L., including a new coumarin 5-demethoxy-10′-ethoxyexotimarin F (1). Their structures were elucidated on the basis of a comprehensive analysis of 1D and 2D NMR and HRMS spectroscopic data, and the absolute configurations were assigned via a comparison of the specific rotations and the ECD exciton coupling method. The potential of new coumarin (1) as a selective inhibitor of MAO-B was initially evaluated through molecular docking and pharmacophore studies. Compound (1) showed selectivity for the MAO-B isoenzyme and inhibitory activity in the sub-micromolar range with an IC50 value of 153.25 ± 1.58 nM (MAO-B selectivity index > 172).

An updated patent review on monoamine oxidase (MAO) inhibitors

INTRODUCTION

: Monoamine oxidase (MAO) inhibitors are currently used as antidepressants (selective MAO-A inhibitors) or as co-adjuvants for neurodegenerative diseases (selective MAO-B inhibitors). The research within this field is attracting attention due to their crucial role in the modulation of brain functions, mood and cognitive activity, and monoamine catabolism.

AREAS COVERED

MAO inhibitors (2018-2021) are discussed according to their chemotypes. Structure-activity relationships are derived for each chemical scaffold (propargylamines, chalcones, indoles, benzimidazoles, (iso)coumarins, (iso)benzofurans, xanthones, and tetralones), while the chemical entities were divided into newly synthesized molecules and natural metabolites. The mechanism of action and type of inhibition are also considered. Lastly, new therapeutic applications are reported, which demonstrates the clinical potential of these inhibitors as well as the possibility of repurposing existing drugs for a variety of diseases.

EXPERT OPINION

MAO inhibitors here reported exhibit different potencies (from the micro- to nanomolar range) and isoform selectivity. These compounds are clinically licensed for multi-faceted neurodegenerative pathologies due to their ability to also act against other relevant targets (cholinesterases, inflammation, and oxidative stress). Moreover, the drug repurposing approach is an attractive strategy by which MAO inhibitors may be applied for the treatment of prostate cancer, inflammation, vertigo, and type 1 diabetes.

Development of novel monoamine oxidase B (MAO-B) inhibitors by combined application of docking-based alignment, 3D-QSAR, ADMET prediction, molecular dynamics simulation, and MM_GBSA binding free energy

Unsaturated ketone derivatives are known as inhibitors of monoamine oxidase B (MAO-B), a potential drug target of Parkinson's disease. Here, docking-based alignment, 3 D-QSAR (three-dimensional quantitative structure-activity relationship) studies, ADMET (absorption, distribution, metabolism, excretion, and toxicity) prediction, molecular dynamics (MD) simulation, and MM_GBSA binding free energy were performed on a novel series of MAO-B inhibitors. The objective is to predict new MAO-B inhibitors with high potency activity. The 3 D-QSAR models were created using comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA). Molecular docking findings indicated that compounds with strong inhibitory efficacy also had a high binding affinity. 3 D-QSAR studies showed the importance of steric, electrostatic, and H-bond acceptor fields on the inhibitory activity of MAO-B. Based on the appropriate 3 D-QSAR model, a new series of MAO-B inhibitors were predicted and their pharmacokinetic characteristics were evaluated using in silico ADMET prediction. All screened compounds show good oral bioavailability without any side effects. Moreover, the dynamic behavior and stability of the most active compounds were evaluated using MD simulations. The results showed that unsaturated ketone derivatives are stable and compact during the 100 ns of MD simulation. Finally, the binding free energy of complexes was determined using the MM_GBSA method; the findings indicated that the T1 compound is more stable (ΔG_binding = -409.506 KJ/mol) than the data set's highest active compound (ΔG_binding = -31.883 KJ/mol).Communicated by Ramaswamy H. Sarma.

Novel Series of Dual NRF2 Inducers and Selective MAO-B Inhibitors for the Treatment of Parkinson’s Disease

Parkinson’s disease (PD) is the second most prevalent neurodegenerative disease. It is characterized by a complex network of physiopathological events where oxidative stress plays a central role among other factors such as neuroinflammation and protein homeostasis. Nuclear factor-erythroid 2 p45-related factor 2 (NRF2) has a multitarget profile itself as it controls a plethora of cellular processes involved in the progression of the disease. In this line, we designed a novel family of 2-(1H-indol-3-yl)ethan-1-amine derivatives as NRF2 inducers with complementary activities. Novel compounds are based on melatonin scaffold and include, among other properties, selective monoamine oxidase B (MAO-B) inhibition activity. Novel multitarget compounds exhibited NRF2 induction activity and MAO-B selective inhibition, combined with anti-inflammatory, antioxidant, and blood–brain barrier permeation properties. Furthermore, they exert neuroprotective properties against oxidative stress toxicity in PD-related in vitro. Hit compound 14 reduced oxidative stress markers and exerted neuroprotection in rat striatal slices exposed to 6-hydroxydopamine or rotenone. In conclusion, we developed a promising family of dual NRF2 inducers and selective MAO-B inhibitors that could serve as a novel therapeutic strategy for PD treatment.

Novel Thiosemicarbazone Derivatives: In Vitro and In Silico Evaluation as Potential MAO-B Inhibitors

MAO-B inhibitors are frequently used in the treatment of neurodegenerative diseases such as Parkinson’s and Alzheimer’s. Due to the limited number of compounds available in this field, there is a need to develop new compounds. In the recent works, it was shown that various thiosemicarbazone derivatives show hMAO inhibitory activity in the range of micromolar concentration. It is thought that benzofuran and benzothiophene structures may mimic structures such as indane and indanone, which are frequently found in the structures of such inhibitors. Based on this view, new benzofuran/benzothiophene and thiosemicarbazone hybrid compounds were synthesized, characterized and screened for their hMAO-A and hMAO-B inhibitory activity by an in vitro fluorometric method. The compounds including methoxyethyl substituent (2b and 2h) were found to be the most effective agents in the series against MAO-B enzyme with the IC₅₀ value of 0.042 ± 0.002 µM and 0.056 ± 0.002 µM, respectively. The mechanism of hMAO-B inhibition of compounds 2b and 2h was investigated by Lineweaver–Burk graphics. Compounds 2b and 2h were reversible and non-competitive inhibitors with similar inhibition features as the substrates. The K_i values of compounds 2b and 2h were calculated as 0.035 µM and 0.046 µM, respectively, with the help of secondary plots. The docking study of compound 2b and 2h revealed that there is a strong interaction between the active sites of hMAO-B and analyzed compound.

Monoamine Oxidase (MAO) as a Potential Target for Anticancer Drug Design and Development

Monoamine oxidases (MAOs) are oxidative enzymes that catalyze the conversion of biogenic amines into their corresponding aldehydes and ketones through oxidative deamination. Owing to the crucial role of MAOs in maintaining functional levels of neurotransmitters, the implications of its distorted activity have been associated with numerous neurological diseases. Recently, an unanticipated role of MAOs in tumor progression and metastasis has been reported. The chemical inhibition of MAOs might be a valuable therapeutic approach for cancer treatment. In this review, we reported computational approaches exploited in the design and development of selective MAO inhibitors accompanied by their biological activities. Additionally, we generated a pharmacophore model for MAO-A active inhibitors to identify the structural motifs to invoke an activity.

Navigating into the Chemical Space of Monoamine Oxidase Inhibitors by Artificial Intelligence and Cheminformatics Approach

The monoamine oxidase (MAO) enzyme class is a prevalent target for many neurodegenerative and depressive disorders. Even though scrutinization of many promising drugs for the treatment of MAO inhibition has been carried out in recent times, a conclusive structural requirement for potent activity needs to be developed. Numerous approaches have been examined for the identification of structural features for potent MAO inhibitors (MAOIs) that mainly involve an array of computational studies, synthetic approaches, and biological evaluation. In this paper, we have analyzed ∼2200 well-known MAOIs to expand perceptions in the chemical space of MAOIs. The physicochemical properties of the MAOIs disclosed a discernible hydrophobic feature making a bunch discrete from the central nervous system (CNS) acting drugs, as exposed using the principal component analysis (PCA). The Murcko scaffold structure study revealed unfavorable and favorable scaffold structures, in both data sets, with the highest biological activity shown by the 3-phenyl-2H-chromen-2-one scaffold. This scaffold showed a polypharmacological effect. R-group disintegration and automatic structure-activity relationship (SAR) study resulted in identification of substructures responsible for the inhibitory bioactivity of the MAO-A and MAO-B enzymes. Moreover, with activity cliff analysis, significant biological activity was detected by simple molecular conversion in the chemical compound structure. In addition, we used the machine learning tool to generate a hypothesis wherein pyrazole, benzene ring, and amide containing structural functionalities can exhibit potential biological activities. This hypothesis revealed that CNS target drugs, C4155, C13390, C21265, C43862, C31524, C24810, C37100, C42075, and C43644, could be repurposed as valuable candidates for the MAO-B enzyme. For researchers, this study will bring new perceptions in the discovery and development of MAOIs and direct lead and hit optimization for the progress of small molecules beneficial for MAO-targeting associated diseases.

Biphenylpiperazine Based MAO Inhibitors: Synthesis, Biological Evaluation, Reversibility and Molecular Modeling Studies

In this study, 21 new 1,4-biphenylpiperazine derivatives were designed, synthesized and evaluated as monoamine oxidase (MAO) inhibitors by in vitro fluorometric method. All these compounds exhibited inhibitory activity against hMAO enzymes, 17 analogues of them showed selectivity towards hMAO-B over hMAO-A enzyme. Compound 20 exhibited the best activity and selectivity towards hMAO-B with IC₅₀ value of 53 nM and selectivity index of 1122 folds over MAO-A, compared to the reference drugs rasagiline (IC₅₀ = 66 nM) and selegiline (IC₅₀ = 40 nM). Kinetic study and reversibility test of the most potent compound (20) revealed that it is reversible and mixed competitive inhibitor (K_i value is 17 nM for the inhibition of hMAO-B). Compound 20 was evaluated against normal NIH/3T3 mouse embryonic fibroblast cell lines and it was found that it is non-cytotoxic at its effective concentration against hMAO-B. Moreover, compound 20 and the most potent compounds have acceptable ADME properties and good pharmacokinetics profiles. Molecular docking simulations were performed for explanation and elucidation for the biological activity of compounds 19 and 20. Accordingly, 1,4- biphenylpiperazine derivatives can be considered as a promising lead to produce more potent and safer MAO inhibitors for management of various neurological disorders.

Promising Non-cytotoxic Monosubstituted Chalcones to Target Monoamine Oxidase-B

A library of monosubstituted chalcones (1-17) bearing electron-donating and electron-withdrawing groups on both aromatic rings were selected. The cell viability on human tumor cell lines was evaluated first. The compounds unable to induce detectable cytotoxicity (1, 13, and 14) were tested using the monoamine oxidase (MAO) activity assay. Interestingly, they inhibit MAO-B, acting as competitive inhibitors, with 13 and 14 showing the best profiles. In particular, 13 exhibited a potency higher than that of safinamide, taken as a reference. Docking studies and crystallographic analysis showed that in human MAO-B 13 binds with the halogen-substituted aromatic ring in the entrance cavity, similar to safinamide, whereas 14 is accommodated in the opposite way. The main conclusion of this cell biology, biochemistry, and structural study is to highlights 13 as a chalcone derivative that is worth consideration for the development of novel MAO-B-selective inhibitors for the treatment of neurodegenerative diseases.

Participation of glutamatergic and nitrergic systems in the striatal dopamine release induced by isatin, a MAO inhibitor

Isatin is a biofactor with different biochemical and pharmacological properties whose effects attract much attention because it is an endogenous inhibitor of the monoamine oxidase in the brain. When exogenously administrated, isatin increases dopamine levels in intact and denervated striatum of rats, an effect that could indicate its potential as a therapeutic agent in Parkinson disease. However, the neurochemical mechanisms by which isatin increases dopamine in the striatum are poorly understood. In the present study, we evaluate the role of the glutamatergic and nitrergic systems in the isatin-induced dopamine release from rat striatum. Our findings show that the intrastriatal administration of 10 mM isatin significantly increases the in vivo release of dopamine (1,104.7% ± 97.1%), and the amino acids glutamate (428.7% ± 127%) and taurine (221% ± 22%) from rat striatum measured by brain microdialysis. The pretreatment with MK-801 (500 µM) or AP5 (650 µM) (glutamatergic NMDA receptors antagonists) significantly reduces the effect of isatin on dopamine release by 52% and 70.5%, respectively. The administration of the nitric oxide synthase inhibitors, L-NAME (100 µM) or 7-NI (100 µM) also decreases the isatin-induced dopamine release by 77% and 42%, respectively. These results show that isatin, in addition to increasing dopamine release, also increases glutamate levels, and possibly activates NMDA receptors and nitric oxide production, which can promote a further increase in the dopamine release.

Structural exploration of multifunctional monoamine oxidase B inhibitors as potential drug candidates against Alzheimer's disease

AD is one of the most typical neurodegenerative disorders that suffer many seniors worldwide. Recently, MAO inhibitors have received increasing attention not only for their roles involved in monoamine neurotransmitters metabolism and oxidative stress but also for their additional neuroprotective and neurorescue effects against AD. The curiosity in MAO inhibitors is reviving, and novel MAO-B inhibitors recently developed with ancillary activities (e.g., Aβ aggregation and AChE inhibition, anti-ROS and chelating activities) have been proposed as multitarget drugs foreshadowing a positive outlook for the treatment of AD. The current review describes the recent development of the design, synthesis, and screening of multifunctional ligands based on MAO-B inhibition for AD therapy. Structure-activity relationships and rational design strategies of the synthetic or natural product derivatives (chalcones, coumarins, chromones, and homoisoflavonoids) are discussed.

Investigation into MAO B-Mediated Formation of CC112273, a Major Circulating Metabolite of Ozanimod, in Humans and Preclinical Species: Stereospecific Oxidative Deamination of (S)-Enantiomer of Indaneamine (RP101075) by MAO B

Ozanimod, recently approved for treating relapsing multiple sclerosis, produced a disproportionate, active, MAO B-catalyzed metabolite (CC112273) that showed remarkable interspecies differences and led to challenges in safety testing. This study explored the kinetics of CC112273 formation from its precursor RP101075. Incubations with human liver mitochondrial fractions revealed K _Mapp, V _max, and intrinsic clearance (Cl_int) for CC112273 formation to be 4.8 μM, 50.3 pmol/min/mg protein, and 12 μl/min/mg, respectively, whereas Michaelis-Menten constant (K _M) with human recombinant MAO B was 1.1 μM. Studies with liver mitochondrial fractions from preclinical species led to K _Mapp, V _max, and Cl_int estimates of 3.0, 35, and 33 μM, 80.6, 114, 37.3 pmol/min/mg, and 27.2, 3.25, and 1.14 μl/min/mg in monkey, rat, and mouse, respectively, and revealed marked differences between rodents and primates, primarily attributable to differences in the K _M Comparison of Cl_int estimates revealed monkey to be ∼2-fold more efficient and the mouse and rat to be 11- and 4-fold less efficient than humans in CC112273 formation. The influence of stereochemistry on MAO B-mediated oxidation was also investigated using the R-isomer of RP101075 (RP101074). This showed marked selectivity toward catalysis of the S-isomer (RP101075) only. Docking into MAO B crystal structure suggested that although both the isomers occupied its active site, only the orientation of RP101075 presented the C-H on the α-carbon that was ideal for the C-H bond cleavage, which is a requisite for oxidative deamination. These studies explain the basis for the observed interspecies differences in the metabolism of ozanimod as well as the substrate stereospecificity for formation of CC112273. SIGNIFICANCE STATEMENT: This study evaluates the enzymology and the species differences of the major circulating metabolite of ozanimod, CC112273. Additionally, the study also explores the influence of stereochemistry on MAO B-catalyzed reactions. The study is of significance to the DMD readers given that this oxidation is catalyzed by a non-cytochrome P450 enzyme, and that marked species difference and notable stereospecificity was observed in MAO B-catalyzed biotransformation when the indaneamine enantiomers were used as substrates.

Hydroxytyrosol as anti-parkinsonian molecule: Assessment using in-silico and MPTP-induced Parkinson's disease model

3-Hydroxytyrosol (HXT) is a natural polyphenol present in extra virgin olive oil. It is a key component of Mediterranean diet and is known for its strong antioxidant activity. The present study evaluated the potential of HXT as an anti-parkinsonian molecule in terms of its ability to inhibit MAO-B and thereby maintaining dopamine (DA) levels in Parkinson's disease (PD). In-silico molecular docking study followed by MMGBSA binding free energy calculation revealed that HXT has a strong binding affinity for MAO-B in comparison to MAO-A. Moreover, rasagiline and HXT interacted with the similar binding sites and modes of interactions. Additionally, molecular dynamics simulation studies revealed stable nature of HXT-MAO-B interaction and also provided information about the amino acid residues involved in binding. Moreover, in vitro studies revealed that HXT inhibited MAO-B in human platelets with IC₅₀ value of 7.78 μM. In vivo studies using MPTP-induced mouse model of PD revealed increase in DA levels with concomitant decrease in DA metabolites (DOPAC and HVA) on HXT treatment. Furthermore, MAO-B activity was also inhibited on HXT administration to PD mice. In addition, HXT treatment prevented MPTP-induced loss of DA neurons in substantia nigra and their nerve terminals in the striatum. HXT also attenuated motor impairments in PD mice assessed by catalepsy bar, narrow beam walk and open field tests. Thus, the present findings reveal HXT as a potential inhibitor of MAO-B, which may be used as a lead molecule for the development of therapeutics for PD.

Monoamine Oxidase Inhibitors: From Classic to New Clinical Approaches

Monoamine oxidases (MAOs) are involved in the oxidative deamination of different amines and neurotransmitters. This pointed them as potential targets for several disorders and along the last 70 years a wide variety of MAO inhibitors have been developed as successful drugs for the treatment of complex diseases, being the first drugs approved for depression in the late 1950s. The discovery of two MAO isozymes (MAO-A and B) with different substrate selectivity and tissue expression patterns led to novel therapeutic approaches and to the development of new classes of inhibitors, such as selective irreversible and reversible MAO-B inhibitors and reversible MAO-A inhibitors. Significantly, MAO-B inhibitors constitute a widely studied group of compounds, some of them approved for the treatment of Parkinson's disease. Further applications are under development for the treatment of Alzheimer's disease, amyotrophic lateral sclerosis, and cardiovascular diseases, among others. This review summarizes the most important aspects regarding the development and clinical use of MAO inhibitors, going through mechanistic and structural details, new indications, and future perspectives. Monoamine oxidases (MAOs) catalyze the oxidative deamination of different amines and neurotransmitters. The two different isozymes, MAO-A and MAO-B, are located at the outer mitochondrial membrane in different tissues. The enzymatic reaction involves formation of the corresponding aldehyde and releasing hydrogen peroxide (H₂O₂) and ammonia or a substituted amine depending on the substrate. MAO's role in neurotransmitter metabolism made them targets for major depression and Parkinson's disease, among other neurodegenerative diseases. Currently, these compounds are being studied for other diseases such as cardiovascular ones.

Natural based piperine derivatives as potent monoamine oxidase inhibitors: an in silico ADMET analysis and molecular docking studies

Neurodegenerative disorders follow numerous pathological ways concerning overexpression of monoamine oxidase and formation of reactive oxygen species. The computational design of the piperine derivatives has given the significant MAO inhibitors with considerable antioxidant potential. Molecular docking provided the mechanistic insight of the compounds within the hMAO active site. In the current study we have prepared a series of compounds related to piperine and investigated them through monoamine oxidase A and B assay and evaluated the free radical scavenging activity. The synthesized compounds were analyzed by using in silico techniques within the active site of MAO and the ADMET properties were also calculated. The results obtained in this study indicated the interesting therapeutic potential of some compounds such as 7and 17c as most promising hMAO-A inhibitors whereas compounds 15, 5 and 17b were found as hMAO-B inhibitors. Moreover, we assessed the antioxidant potential of the piperine analogues and compounds 5, 17b, and 7 showed very modest antioxidant activity against DPPH and H2O2 radicals. The outcome of the study indicating that the piperine related derivatives are found as considerable MAO inhibitors and antioxidants. Moreover, the SAR structure activity relationships are depicting the structural features required for the MAO inhibition. In case of MAO activity, good correlations were found among the calculated and experimental results.

Marine natural products with monoamine oxidase (MAO) inhibitory activity

CONTEXT

Research interest in monoamine oxidase (MAO) as a promising drug target for neurodegenerative diseases has a long history. However, efforts to develop MAO inhibitors (MAOIs) from marine sources have been limited, despite the increasing number of interesting marine natural products.

OBJECTIVE

To review the potential of marine natural products as MAOIs source, including their activities and selectivity on MAO.

METHODS

Public databases such as SciFinder, MarinLit and PubMed were systematically searched from 1991 until Dec 2019. MAO and MAOI were the key terms searched combined with marine natural products and marine.

RESULTS

Six classes of marine natural products with good selectivity between the two MAO subtypes were organized with their selectivity and sources.

CONCLUSIONS

This is the first review to investigate the potential of marine natural products as MAOIs source. Despite the small number of known MAOIs from marine sources, marine natural products are potential leads for the further development of MAOI drugs with novel chemical frames and good selectivity.

Virtual screening and drug repurposing experiments to identify potential novel selective MAO-B inhibitors for Parkinson's disease treatment

The main study's purpose is to detect novel natural products (NPs) that are potentially selective MAO-B inhibitors and, additionally, to computationally reposition the marketed drugs with a new therapeutic role for Parkinson's disease. To reach the goals, 3D similarity search, docking, ADMETox, and drug repurposing approaches were employed. Thus, an unbiased benchmarking dataset was built including selective and nonselective inhibitors for MAO-B compliant with both ligand- and structure-based virtual screening approaches. A retrospective and prospective mining scenario was applied to SPECS NP and DrugBank databases to detect novel scaffolds with potential benefits for Parkinson's disease patients. Out of the three best selected natural products, cardamomin showed excellently predicted drug-like properties, superior pharmacological profile, and specific interactions with MAO-B active site, indicating a potential selectivity over MAO-B. Two marketed drugs, fenamisal and monobenzone, were proposed as promising candidates repurposed for Parkinson's disease. The application of shape, physicochemical, and electrostatic similarity searches protocol emerged as a plausible solution to explore MAO-B inhibitors selectivity. This protocol might serve as a rewarding tool in early drug discovery and can be extended to other protein targets.

A comprehensive review of monoamine oxidase inhibitors as Anti-Alzheimer's disease agents: A review

Monoamine oxidases (MAO-A and MAO-B) are mammalian flavoenzyme, which catalyze the oxidative deamination of several neurotransmitters like norepinephrine, dopamine, tyramine, serotonin, and some other amines. The oxidative deamination produces several harmful side products like ammonia, peroxides, and aldehydes during the biochemical reaction. The concentration of biochemical neurotransmitter alteration in the brain by MAO is directly related with several neurological disorders like Alzheimer's disease and Parkinson's disease (PD). Activated MAO also contributes to the amyloid beta (Aβ) aggregation by two successive cleft β-secretase and γ-secretase of amyloid precursor protein (APP). Additionally, activated MAO is also involved in aggregation of neurofibrillary tangles and cognitive destruction through the cholinergic neuronal damage and disorder of the cholinergic system. MAO inhibition has general anti-Alzheimer's disease effect as a consequence of oxidative stress reduction prompted by MAO enzymes. In this review, we outlined and addressed recent understanding on MAO enzymes such as their structure, physiological function, catalytic mechanism, and possible therapeutic goals in AD. In addition, it also highlights the current development and discovery of potential MAO inhibitors (MAOIs) from various chemical scaffolds.

Integrated Binary QSAR-Driven Virtual Screening and In Vitro Studies for Finding Novel hMAO-B-Selective Inhibitors

The increased activity of monoamine oxidase (MAO) enzymes may lead to serious consequences since they reduce the level of neurotransmitters and are associated with severe neurodegenerative diseases. The inhibition of this enzyme, especially the B isoform, plays a vital role in the treatment of Parkinson's disease (PD). This study is aimed to find novel human MAO-B (hMAO-B) selective inhibitors. A total of 256.750 compounds from the Otava small molecules database were virtually screened gradually by employing several screening techniques for this purpose. Initially, a high-throughput virtual screening (HTVS) method was employed, and 10% of the molecules having high docking scores were subjected to binary QSAR models for further screening of their therapeutic activities against PD, Alzheimer's disease (AD), and depression as well as for their toxicity and pharmacokinetic properties. Then, enzyme selectivity of the ligands towards the A and B forms that passed through all the filters were studied using the induced-fit docking method and molecular dynamics simulations. At the end of this exhaustive research, we identified two hit molecules ligand 3 (Otava ID: 7131545) and ligand 4 (Otava ID: 7566820). Based on the in vitro results, these two compounds (ligands 3 and 4) together with ligands 1 and 2 found in our previous study showed activity at the nanomolar (nM) level, and the results indicated that these four ligands inhibit hMAO-B better than the FDA-approved drug selegiline.

Dopaminergic system contribution to the antidepressant-like effect of 3-phenyl-4-(phenylseleno) isoquinoline in mice

Depression is a serious disorder characterized by imbalance of mood and emotions, which is accompanied by the reduction in the monoaminergic signaling. The monoamine oxidase inhibition could lead to an increase in monoaminergic neurotransmitter levels in the brain. According to our previous study, 3-phenyl-4-(phenylseleno) isoquinoline (PSI) is a selective and reversible MAO-B inhibitor in vitro. The present study investigated the putative ex vivo inhibitory effect of a single PSI dose on the cerebral MAO activity and its antidepressant-like action in the mouse forced swimming test (FST). Additionally, the dopaminergic system contribution to the antidepressant-like effect of PSI was also evaluated. For this, PSI was dissolved in canola oil to determine time-course (0.5-24 h) and dose-response (25-100 mg/kg, 10 ml/kg, intragastrically) curves of MAO activity inhibition using adult C57Bl/6 male mice. A single PSI dose of 100 mg/kg inhibited the MAO-B activity in the whole brain 8 h after administration to mice, while it did not alter the MAO-A activity. The FST was carried out 0.5, 8, and 24 h after the PSI administration (100 mg/kg) or vehicle, but its antidepressant-like effect was demonstrated only at 0.5 and 8 h after treatment. Lastly, the contribution of dopaminergic system in the PSI antidepressant-like effect was demonstrated by using dopamine receptors antagonists, SCH23390, haloperidol and sulpiride. Thus, a single PSI dose of 100 mg/kg had an antidepressant-like effect in mice subjected to the FST 0.5 and 8 h after its administration. Moreover, the inhibition of cerebral MAO-B activity and modulation of dopamine receptors contributed to the antidepressant-like effect of PSI in mice.

Hepatic monoamine oxidase B is involved in endogenous geranylgeranoic acid synthesis in mammalian liver cells

Geranylgeranoic acid (GGA) originally was identified in some animals and has been developed as an agent for preventing second primary hepatoma. We previously have also identified GGA as an acyclic diterpenoid in some medicinal herbs. Recently, we reported that in human hepatoma-derived HuH-7 cells, GGA is metabolically labeled from ¹³C-mevalonate. Several cell-free experiments have demonstrated that GGA is synthesized through geranylgeranial by oxygen-dependent oxidation of geranylgeraniol (GGOH), but the exact biochemical events giving rise to GGA in hepatoma cells remain unclear. Monoamine oxidase B (MOAB) has been suggested to be involved in GGOH oxidation. Here, using two human hepatoma cell lines, we investigated whether MAOB contributes to GGA biosynthesis. Using either HuH-7 cell lysates or recombinant human MAOB, we found that: 1) the MAO inhibitor tranylcypromine dose-dependently downregulates endogenous GGA levels in HuH-7 cells; and 2) siRNA-mediated MAOB silencing reduces intracellular GGA levels in HuH-7 and Hep3B cells. Unexpectedly, however, CRISPR/Cas9-generated MAOB-KO human hepatoma Hep3B cells had GGA levels similar to those in MAOB-WT cells. A sensitivity of GGA levels to siRNA-mediated MAOB downregulation was recovered when the MAOB-KO cells were transfected with a MAOB-expression plasmid, suggesting that MAOB is the enzyme primarily responsible for GGOH oxidation and that some other latent metabolic pathways may maintain endogenous GGA levels in the MAOB-KO hepatoma cells. Along with the previous findings, these results provide critical insights into the biological roles of human MAOB and provide evidence that hepatic MAOB is involved in endogenous GGA biosynthesis via GGOH oxidation.

New role for crinamine as a potent, safe and selective inhibitor of human monoamine oxidase B: In vitro and in silico pharmacology and modeling

ETHNOPHARMACOLOGICAL RELEVANCE

The development of selective inhibitors of monoamine oxidase B (MAO-B) has been essential in treating Parkinson's disease. However, the apparent hepatotoxicity and drug-drug interactions of current inhibitors accentuate the need for the development of novel pharmacotherapies. Crossyne guttata (L.) D. & U. Müll-Doblies is used frequently by Rastafarian bush doctors to treat alcoholism, a disorder which is also accentuated by MAO.

OBJECTIVE

The study sought to isolate, identify and characterise the biologically active constituents of C. guttata based on their ability to inhibit the MAO enzymes.

MATERIALS AND METHODS

Column chromatography was used to isolate the biologically active alkaloids of C. guttata. The ability of the alkaloids to inhibit the biotransformation of 4-aminoantipyrine by the MAO enzymes was evaluated in vitro. In silico docking was conducted using AutoDock Vina server while the pharmacokinetic properties of the compounds were evaluated using SwissADME.

RESULTS

Chromatographic separation of an ethanolic fraction of C. guttata yielded the alkaloids crinamine 1 and epibuphanisine 2. 1 and 2 along with structurally related alkaloids haemanthamine 3 and haemanthidine 4 were evaluated for their ability to inhibit the action of isozymes of MAO in vitro. Alkaloids effected submicromolar IC₅₀ values against MAO-B, the most potent of which being crinamine 1 (0.014 μM) > haemanthidine 4 (0.017 μM) > epibuphanisine 2 (0.039 μM) > haemanthamine 3 (0.112 μM). Binding energies of the alkaloids correlated well with their inhibitory potential with crinamine displaying the best binding efficacy and binding energy score with MAO-B.

DISCUSSION AND CONCLUSION

Crinamine and epibuphanisine exhibited potent and selective inhibitory activity towards MAO-B. After comprehensive in silico investigations encompassing robust molecular docking analysis, the drug-like attributes and safety of the alkaloids suggest the crinamine is a potentially safe drug for human application.

Flurbiprofen-chalcone hybrid Mannich base derivatives as balanced multifunctional agents against Alzheimer's disease: Design, synthesis and biological evaluation

The complex pathogenesis of Alzheimer's disease (AD) calls for multitarget approach for disease management. Herein, a series of novel flurbiprofen-chalcone hybrid Mannich base derivatives were designed and synthesized. The biological screening results indicated that most of the derivatives exhibited potent multi-target effects involved in AD. In particular, compound 6c bearing a pyrrolidine group showed the highest activities against self- and Cu²⁺-induced Aβ_1-42 aggregation (70.65% and 54.89% at 25.0 µM, respectively), highly selective inhibition towards AChE and MAO-B (IC₅₀ = 7.15 μM and 0.43 μM respectively), good antioxidant ability and metal-chelating property. Moreover, 6c displayed excellent anti-neuroinflammatory activity and appropriate BBB permeability in vitro. These outstanding results qualified compound 6c as a promising multifunctional agent for further development of disease-modifying treatment of AD.

1,3,4-Oxadiazol-2-ylbenzenesulfonamides as privileged structures for the inhibition of monoamine oxidase B

The present study investigates the monoamine oxidase (MAO) inhibition properties of a series of ten 5-aryl-1,3,4-oxadiazol-2-ylbenzenesulfonamides. The target compounds were synthesized by dehydration of the corresponding N,N'-diacylhydrazines with phosphorus oxychloride to yield the 1,3,4-oxadiazole cycle with concomitant transformation of the sulfonamide to the sulfonyl chloride group. Treatment with aqueous ammonia in acetonitrile regenerated the target sulfonamides. The results of the enzymology document that these compounds are potent and specific MAO-B inhibitors with the most potent compound exhibiting an IC50 value of 0.0027 µM. An analysis of the structure-activity relationships shows that the 4-benzenesulfonamides are significantly more potent MAO-B inhibitors than the corresponding 3-benzenesulfonamides, and that the corresponding N,N'-diacylhydrazine synthetic precursors are weak MAO inhibitors. Although MAO inhibition by oxadiazole compounds are known, this is the first report of nanomolar MAO inhibition potencies recorded for sulfonamide derivatives. MAO-B specific inhibitors such as those discovered here may be of interest in the treatment of neurodegenerative disorders such as Parkinson's disease.

Computational Insight into the Mechanism of the Irreversible Inhibition of Monoamine Oxidase Enzymes by the Antiparkinsonian Propargylamine Inhibitors Rasagiline and Selegiline

Monoamine oxidases (MAOs) are flavin adenine dinucleotide containing flavoenzymes that catalyze the degradation of a range of brain neurotransmitters, whose imbalance is extensively linked with the pathology of various neurological disorders. This is why MAOs have been the central pharmacological targets in treating neurodegeneration for more than 60 years. Still, despite this practical importance, the precise chemical mechanisms underlying the irreversible inhibition of the MAO B isoform with clinical drugs rasagiline (RAS) and selegiline (SEL) remained unknown. Here we employed a combination of MD simulations, MM-GBSA binding free energy evaluations, and QM cluster calculations to show the MAO inactivation proceeds in three steps, where, in the rate-limiting first step, FAD utilizes its N5 atom to abstracts a hydride anion from the inhibitor α-CH₂ group to ultimately give the final inhibitor-FAD adduct matching crystallographic data. The obtained free energy profiles reveal a lower activation energy for SEL by 1.2 kcal mol^-1 and a higher reaction exergonicity by 0.8 kcal mol^-1, with the former being in excellent agreement with experimental ΔΔG^‡_EXP = 1.7 kcal mol^-1, thus rationalizing its higher in vivo reactivity over RAS. The calculated ΔG_BIND energies confirm SEL binds better due to its bigger size and flexibility allowing it to optimize hydrophobic C-H···π and π···π interactions with residues throughout both of enzyme's cavities, particularly with FAD, Gln206 and four active site tyrosines, thus overcoming a larger ability of RAS to form hydrogen bonds that only position it in less reactive orientations for the hydride abstraction. Offered results elucidate structural determinants affecting the affinity and rates of the inhibition reaction that should be considered to cooperate when designing more effective compounds devoid of untoward effects, which are of utmost significance and urgency with the growing prevalence of brain diseases.

2-Phenyloxazole-4-carboxamide as a Scaffold for Selective Inhibition of Human Monoamine Oxidase B

A series of 2-phenyloxazoles bearing an amide group at position 4 were designed and synthesized for evaluation as potential inhibitors of human recombinant monoamine oxidases (hrMAOs). Results of kinetics experiments demonstrated that all compounds behave as competitive MAO inhibitors, with good selectivity toward the MAO-B isoform. The most potent and selective derivatives are characterized by inhibition constant (K_i ) values in the sub-micromolar range and a good selectivity index (K_i MAO-A /K_i MAO-B >50). Some derivatives were also found to be able to inhibit MAO activity in nerve growth factor (NGF)-differentiated PC12 cells, taken as a model of neuronal cells. In particular, 2-(2-hydroxyphenyl)-N-phenyloxazole-4-carboxamide (compound 4 a) may be a promising new scaffold, exerting the highest selectivity and inhibitory effect toward MAOs in NGF-differentiated PC12 cell lysates, without compromising cell viability. Molecular docking analysis allowed a rationalization of the experimentally observed binding affinity and selectivity.

Benextramine and derivatives as novel human monoamine oxidases inhibitors: an integrated approach

The two human monoamine oxidase isoforms (namely MAO A and MAO B) are enzymes involved in the catabolism of monoamines, including neurotransmitters, and for this reason are well-known and attractive pharmacological targets in neuropsychiatric and neurodegenerative diseases, for which novel pharmacological approaches are necessary. Benextramine is a tetraamine disulfide mainly known as irreversible α-adrenergic antagonist, but able to hit additional targets involved in neurodegeneration. As the molecular structures of monoamine oxidases contain nine cysteine residues, the aim of this study was to evaluate benextramine and eleven structurally related polyamine disulfides as potential MAO inhibitors. Most of the compounds were found to induce irreversible inactivation of MAOs with inactivation potency depending on both the polyamine structure and the enzyme isoform. The more effective compounds generally showed preference for MAO B. Structure-activity relationships studies revealed the key role played by the disulfide core of these molecules in the inactivation mechanism. Docking experiments pointed to Cys323, in MAO A, and Cys172, in MAO B, as target of this type of inhibitors thus suggesting that their covalent binding inside the MAO active site sterically impedes the entrance of substrate towards the FAD cofactor. The effectiveness of benextramine in inactivating MAOs was demonstrated in SH-SY5Y neuroblastoma cell line. These results demonstrated for the first time that benextramine and its derivatives can inactivate human MAOs exploiting a mechanism different from that of the classical MAO inhibitors and could be a starting point for the development of pharmacological tools in neurodegenerative diseases.

SAR and molecular mechanism studies of monoamine oxidase inhibition by selected chalcone analogs

The present study describes the synthesis of a series of 22 chalcone analogs. These compounds were evaluated as potential human MAO-A and MAO-B inhibitors. The compounds showed varied selectivity against the two isoforms. The IC₅₀ values were found to be in the micromolar to submicromolar range. The K_i values of compound 16 were determined to be 0.047 and 0.020 μM for the inhibition of MAO-A and MAO-B, respectively. Dialysis of enzyme-inhibitor mixtures indicated a reversible competitive mode of inhibition. Most of the synthesized chalcone analogs showed a better selectivity toward MAO-B. However, introducing of 2,4,6-trimethoxy substituents on ring B shifted the selectivity toward MAO-A. In addition, we investigated the molecular mechanism of MAO-B inhibition by selected chalcone analogs. Our results revealed that these selected chalcone analogs increased dopamine levels in the rat hepatoma (H4IIE) cells and decreased the relative mRNA expression of the MAO-B enzyme.

Selective Inhibition of Human Monoamine Oxidase B by Acacetin 7-Methyl Ether Isolated from Turnera diffusa (Damiana)

The investigation of the constituents that were isolated from Turnera diffusa (damiana) for their inhibitory activities against recombinant human monoamine oxidases (MAO-A and MAO-B) in vitro identified acacetin 7-methyl ether as a potent selective inhibitor of MAO-B (IC₅₀ = 198 nM). Acacetin 7-methyl ether (also known as 5-hydroxy-4′, 7-dimethoxyflavone) is a naturally occurring flavone that is present in many plants and vegetables. Acacetin 7-methyl ether was four-fold less potent as an inhibitor of MAO-B when compared to acacetin (IC₅₀ = 50 nM). However, acacetin 7-methyl ether was >500-fold selective against MAO-B over MAO-A as compared to only two-fold selectivity shown by acacetin. Even though the IC₅₀ for inhibition of MAO-B by acacetin 7-methyl ether was ~four-fold higher than that of the standard drug deprenyl (i.e., Selegiline^TM or Zelapar^TM, a selective MAO-B inhibitor), acacetin 7-methyl ether’s selectivity for MAO-B over MAO-A inhibition was greater than that of deprenyl (>500- vs. 450-fold). The binding of acacetin 7-methyl ether to MAO-B was reversible and time-independent, as revealed by enzyme-inhibitor complex equilibrium dialysis assays. The investigation on the enzyme inhibition-kinetics analysis with varying concentrations of acacetin 7-methyl ether and the substrate (kynuramine) suggested a competitive mechanism of inhibition of MAO-B by acacetin 7-methyl ether with Ki value of 45 nM. The docking scores and binding-free energies of acacetin 7-methyl ether to the X-ray crystal structures of MAO-A and MAO-B confirmed the selectivity of binding of this molecule to MAO-B over MAO-A. In addition, molecular dynamics results also revealed that acacetin 7-methyl ether formed a stable and strong complex with MAO-B. The selective inhibition of MAO-B suggests further investigations on acacetin 7-methyl as a potential new drug lead for the treatment of neurodegenerative disorders, including Parkinson’s disease.

Monoamine oxidase-B inhibitors as potential neurotherapeutic agents: An overview and update

Monoamine oxidase (MAO) inhibitors have made significant contributions and remain an indispensable approach of molecular and mechanistic diversity for the discovery of antineurodegenerative drugs. However, their usage has been hampered by nonselective and/or irreversible action which resulted in drawbacks like liver toxicity, cheese effect, and so forth. Hence, the search for selective MAO inhibitors (MAOIs) has become a substantial focus in current drug discovery. This review summarizes our current understanding on MAO-A/MAO-B including their structure, catalytic mechanism, and biological functions with emphases on the role of MAO-B as a potential therapeutic target for the development of medications treating neurodegenerative disorders. It also highlights the recent developments in the discovery of potential MAO-B inhibitors (MAO-BIs) belonging to diverse chemical scaffolds, arising from intensive chemical-mechanistic and computational studies documented during past 3 years (2015-2018), with emphases on their potency and selectivity. Importantly, readers will gain knowledge of various newly established MAO-BI scaffolds and their development potentials. The comprehensive information provided herein will hopefully accelerate ideas for designing novel selective MAO-BIs with superior activity profiles and critical discussions will inflict more caution in the decision-making process in the MAOIs discovery.

Discovery of monoamine oxidase inhibitors by medicinal chemistry approaches

Neuropsychiatric disorders, such as Alzheimer's disease (AD), Parkinson's disease (PD) and depression, have seriously inconvenienced the lives of patients. Growing evidence indicates that these diseases are closely related to the monoamine oxidase (MAO) enzyme, making it an attractive target for the exploitation of potent MAO inhibitors (MAOIs) with high selectivity and low side effects. Although various MAOIs have been discovered, the discovery of an ideal MAOI is not an easy task. In this review, we discuss the currently available rational design strategies for obtaining ideal MAOIs, including ligand-based and receptor-based design strategies, and these strategies were further illustrated with the aid of specific examples from the recent literature. To better understanding the biological activity of MAO, we also highlight the binding modes of typical inhibitors against MAO. Besides, advanced strategies for finding upcoming potent MAOIs were prospected.

Novel monoamine oxidase inhibitors based on the privileged 2-imidazoline molecular framework

Series of structurally diverse 2-imidazoline derivatives have been synthesized by condensation of substituted aldehydes with ethylenediamine, Pd-catalyzed N-arylation of 2-imidazolines and by the formation of 1,2,4-oxadiazoles and benzoxazepines from 2-imidazoline-containing precursors. The 2-imidazoline derivatives were evaluated as potential inhibitors of human monoamine oxidase (MAO) A and B. Among the 2-imidazolines, good potency inhibitors were discovered with compound 9p (IC₅₀ = 0.012 µM) being the most potent MAO-B inhibitor, while compound 9d (IC₅₀ = 0.751 µM) was the most potent MAO-A inhibitor of the series. These potencies are in the same range as those of reference MAO inhibitors used in the clinic. Among 33 compounds evaluated, 13 exhibited IC₅₀ values in the submicromolar range for the inhibition of an MAO isoform. It is postulated that the imidazoline moieties of some of these inhibitors may be recognized by the imidazoline I₂-binding site of MAO. Good potency MAO inhibitors may be useful for the treatment of neuropsychiatric and neurodegenerative disorders such as depression and Parkinson's disease, and future application for the treatment of prostate cancer, congestive heart failure and Alzheimer's disease. In addition, high potency 2-imidazoline-derived MAO inhibitors may be used as potential probes for the imidazoline binding sites of the MAOs, as well as to determine alternative binding regions of imidazoline within the MAO active site.

Membrane Reconstitution of Monoamine Oxidase Enzymes on Supported Lipid Bilayers

Monoamine oxidase A and B (MAO-A and B) are mitochondrial outer membrane enzymes that are implicated in a number of human diseases, and the pharmacological inhibition of these enzymes is a promising therapeutic strategy to alleviate disease symptoms. It has been suggested that optimal levels of enzymatic activity occur in the membrane-associated state, although details of the membrane association process remain to be understood. Herein, we have developed a supported lipid bilayer platform to study MAO-A and B binding and evaluate the effects of known pharmacological inhibitors on the membrane association process. By utilizing the quartz crystal microbalance-dissipation (QCM-D) technique, it was determined that both MAOs exhibit tight binding to negatively and positively charged bilayers with distinct concentration-dependent binding profiles while only transiently binding to neutral bilayers. Importantly, in the presence of known inhibitors, the MAOs showed increased binding to negatively charged bilayers, although there was no effect of inhibitor treatment on binding to positively charged bilayers. Taken together, our findings establish that the membrane association of MAOs is highly dependent on membrane surface charge, and we outline an experimental platform to support the in vitro reconstitution of monoamine oxidases on synthetic membranes, including the evaluation of pharmacological drug candidates.

Monoamine Oxidases

Monoamine oxidases A and B (MAO A and B) are mammalian flavoenzymes bound to the outer mitochondrial membrane. They were discovered almost a century ago and they have been the subject of many biochemical, structural and pharmacological investigations due to their central role in neurotransmitter metabolism. Currently, the treatment of Parkinson's disease involves the use of selective MAO B inhibitors such as rasagiline and safinamide. MAO inhibition was shown to exert a general neuroprotective effect as a result of the reduction of oxidative stress produced by these enzymes, which seems to be relevant also in non-neuronal contexts. MAOs were successfully expressed as recombinant proteins in Pichia pastoris, which allowed a thorough biochemical and structural characterization. These enzymes are characterized by a globular water-soluble main body that is anchored to the mitochondrial membrane through a C-terminal α-helix, similar to other bitopic membrane proteins. In both MAO A and MAO B the enzyme active site consists of a hydrophobic cavity lined by residues that are conserved in the two isozymes, except for few details that determine substrate and inhibitor specificity. In particular, human MAO B features a dual-cavity active site whose conformation depends on the size of the bound ligand. This article provides a comprehensive and historical review of MAOs and the state-of-the-art of these enzymes as membrane drug targets.

Proposing Novel MAO-B Hit Inhibitors Using Multidimensional Molecular Modeling Approaches and Application of Binary QSAR Models for Prediction of Their Therapeutic Activity, Pharmacokinetic and Toxicity Properties

Monoamine oxidase (MAO) enzymes MAO-A and MAO-B play a critical role in the metabolism of monoamine neurotransmitters. Hence, MAO inhibitors are very important for the treatment of several neurodegenerative diseases such as Parkinson's disease (PD), Alzheimer's disease (AD), and amyotrophic lateral sclerosis (ALS). In this study, 256 750 molecules from Otava Green Chemical Collection were virtually screened for their binding activities as MAO-B inhibitors. Two hit molecules were identified after applying different filters such as high docking scores and selectivity to MAO-B, desired pharmacokinetic profile predictions with binary quantitative structure-activity relationship (QSAR) models. Therapeutic activity prediction as well as pharmacokinetic and toxicity profiles were investigated using MetaCore/MetaDrug platform which is based on a manually curated database of molecular interactions, molecular pathways, gene-disease associations, chemical metabolism, and toxicity information. Particular therapeutic activity and toxic effect predictions are based on the ChemTree ability to correlate structural descriptors to that property using recursive partitioning algorithm. Molecular dynamics (MD) simulations were also performed to make more detailed assessments beyond docking studies. All these calculations were made not only to determine if studied molecules possess the potential to be a MAO-B inhibitor but also to find out whether they carry MAO-B selectivity versus MAO-A. The evaluation of docking results and pharmacokinetic profile predictions together with the MD simulations enabled us to identify one hit molecule (ligand 1, Otava ID: 3463218) which displayed higher selectivity toward MAO-B than a positive control selegiline which is a commercially used drug for PD therapeutic purposes.