Indanone: a promising scaffold for new drug discovery against neurodegenerative disorders

Indanone is a versatile scaffold that has a number of pharmacological properties. The successful development and ensuing approval of indanone-derived donepezil as a drug of choice for Alzheimer's disease attracted significant scientific interest in this moiety. Indanones could act as small molecule chemical probes as they have strong affinity towards several critical enzymes associated with the pathophysiology of various neurological disorders. Inhibition of these enzymes elevates the levels of neuroprotective brain chemicals such as norepinephrine, serotonin and dopamine. Further, indanone derivatives are capable of modulating the activities of both monoamine oxidases (MAO-A and -B) and acetylcholinesterase (AChE), and thus could be useful in various neurodegenerative diseases. This review article presents a panoramic view of the research carried out on the indanone nucleus in the development of potential neuroprotective agents.

Screening of monoamine oxidase B inhibitors in Tibetan strawberry by ligand fishing based on enzyme functionalized cellulose filter paper

Tibetan strawberry (Fragaria nubicola) is a wild medicinal and edible plant in Tibet possessing various health benefits such as neuroprotection and anti-oxidation. However, there has been little study reported on its chemical constituents. To investigate the inhibitors of monoamine oxidase B (MAO-B) in Tibetan strawberry, we immobilized the enzyme onto cellulose filter paper for the first time to develop a new screening method. Two known glycosides (compounds 1 and 2) and one new iridoid glucoside (Compound 3) were fished out by this method, which was found to effectively inhibit MAO-B with IC50 values of 16.95 ± 0.93, 24.69 ± 0.20, and 46.77 ± 0.78 μM, respectively. Molecular docking and kinetic analysis were performed to reveal the inhibition mechanism of these compounds. Furthermore, compound 1 exhibited neuroprotective effects against 6-OHDA-induced injury on PC12 cells. The developed method exhibits the advantages of rapidness and effectiveness in screening of MAO-B inhibitors from complex herbal extracts.

Monoamine oxidase inhibition properties of 2,1-benzisoxazole derivatives

Monoamine oxidase (MAO) are flavoenzymes that metabolize neurotransmitter, dietary and xenobiotic amines to their corresponding aldehydes with the production of hydrogen peroxide. Two isoforms, MAO-A and MAO-B, are expressed in humans and mammals, and display different substrate and inhibitor specificities as well as different physiological roles. MAO inhibitors are of much therapeutic value and are used for the treatment of neuropsychiatric and neurodegenerative disorders such as depression, anxiety disorders, and Parkinson's disease. To discover MAO inhibitors with good potencies and interesting isoform specificities, the present study synthesized a series of 2,1-benzisoxazole (anthranil) derivatives and evaluated them as in vitro inhibitors of human MAO. The compounds were in most instances specific inhibitors of MAO-B with the most potent MAO-B inhibition observed for 7a (IC50 = 0.017 µM) and 7b (IC50 = 0.098 µM). The most potent MAO-A inhibition was observed for 3l (IC50 = 5.35 µM) and 5 (IC50 = 3.29 µM). It is interesting to note that 3-(2-aminoethoxy)-1,2-benzisoxazole derivatives, the 1,2-benzisoxazole, zonisamide, as well as the isoxazole compound, leflunomide, have been described as MAO inhibitors. This is however the first report of MAO inhibition by derivatives of the 2,1-benzisoxazole structural isomer.

Design, synthesis and biological evaluation of carbamate derivatives incorporating multifunctional carrier scaffolds as pseudo-irreversible cholinesterase inhibitors for the treatment of Alzheimer's disease

In this study, a series of carbamate derivatives incorporating multifunctional carrier scaffolds were designed, synthesized, and evaluated as potential therapeutic agents for Alzheimer's disease (AD). We used tacrine to modify the aliphatic substituent, and employed rivastigmine, indole and sibiriline fragments as carrier scaffolds. The majority of compounds exhibited good inhibitory activity for cholinesterase. Notably, compound C7 with sibiriline fragment exhibited potent inhibitory activities against human acetylcholinesterase (hAChE, IC50 = 30.35 ± 2.07 nM) and human butyrylcholinesterase (hBuChE, IC50 = 48.03 ± 6.41 nM) with minimal neurotoxicity. Further investigations have demonstrated that C7 exhibited a remarkable capacity to safeguard PC12 cells against H2O2-induced apoptosis and effectively suppressed the production of reactive oxygen species (ROS). Moreover, in an inflammation model of BV2 cells induced by lipopolysaccharide (LPS), C7 effectively attenuated the levels of pro-inflammatory cytokines. After 12 h of dialysis, C7 continued to exhibit an inhibitory effect on cholinesterase activity. An acute toxicity test in vivo demonstrated that C7 exhibited a superior safety profile and no hepatotoxicity compared to the parent nucleus tacrine. In the scopolamine-induced AD mouse model, C7 (20 mg/kg) significantly reduced cholinesterase activity in the brain of the mice. C7 was tested in a pharmacological AD mouse model induced by Aβ1-42 and attenuated memory deficits at doses as low as 5 mg/kg. The pseudo-irreversible cholinesterase inhibitory properties and multifunctional therapeutic attributes of C7 render it a promising candidate for further investigation in the treatment of AD.

Design, synthesis, and biological evaluation of novel donepezil-tacrine hybrids as multi-functional agents with low neurotoxicity against Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive memory loss and deficits in cognitive domains. Low choline levels, oxidative stress, and neuroinflammation are the primary mechanisms implicated in AD progression. Simultaneous inhibition of acetylcholinesterase (AChE) and reactive oxygen species (ROS) production by a single molecule may provide a new breath of hope for AD treatment. Here, we describe donepezil-tacrine hybrids as inhibitors of AChE and ROS. Four series of derivatives with a β-amino alcohol linker were designed and synthesized. In this study, the target compounds were evaluated for their ability to inhibit AChE and butyrylcholinesterase (BuChE) in vitro, using tacrine (hAChE, IC50 = 305.78 nM; hBuChE, IC50 = 56.72 nM) and donepezil (hAChE, IC50 = 89.32 nM; hBuChE, IC50 = 9137.16 nM) as positive controls. Compound B19 exhibited an excellent and balanced inhibitory potency against AChE (IC50 = 30.68 nM) and BuChE (IC50 = 124.57 nM). The cytotoxicity assays demonstrated that the PC12 cell viability rates of compound B19 (84.37 %) were close to that of tacrine (87.73 %) and donepezil (79.71 %). Potential therapeutic effects in AD were evaluated using the neuroprotective effect of compounds against H2O2-induced toxicity, and compound B19 (68.77 %) exhibited substantially neuroprotective activity at the concentration of 25 μM, compared with the model group (30.34 %). Furthermore, compound B19 protected PC12 cells from H2O2-induced apoptosis and ROS production. These properties of compound B19 suggested that it was a multi-functional agent with AChE inhibition, anti-oxidative, anti-inflammatory activities, and low toxicity and that it deserves further investigation as a promising agent for AD treatment.

Development of a New Class of Monoamine Oxidase-B Inhibitors by Fine-Tuning the Halogens on the Acylhydrazones

A total of 14 acyl hydrazine derivatives (ACH1-ACH14) were developed and examined for their ability to block monoamine oxidase (MAO). Thirteen analogues showed stronger inhibition potency against MAO-B than MAO-A. With a half-maximum inhibitory concentration of 0.14 μM, ACH10 demonstrated the strongest inhibitory activity against MAO-B, followed by ACH14, ACH13, ACH8, and ACH3 (IC50 = 0.15, 0.18, 0.20, and 0.22 μM, respectively). Structure-activity relationships suggested that the inhibition effect on MAO-B resulted from the combination of halogen substituents of the A- and/or B-rings. This series concluded that when -F was substituted to the B-ring, MAO-B inhibitory activities were high, except for ACH6. In the inhibition kinetics study, the compounds ACH10 and ACH14 were identified as competitive inhibitors, with Ki values of 0.097 ± 0.0021 and 0.10 ± 0.038 μM, respectively. In a reversibility experiment using the dialysis methods, ACH10 and ACH14 showed effective recoveries of MAO-B inhibition as much as lazabemide, a reversible reference. These experiments proposed that ACH10 and ACH14 were efficient, reversible competitive MAO-B inhibitors. In addition, the lead molecules showed good blood-brain barrier permeation with the PAMPA method. The molecular docking and molecular dynamics simulation study confirmed that the hit compound ACH10 can form a stable protein-ligand complex by forming a hydrogen bond with the NH atom in the hydrazide group of the compound.

A Multitarget Approach against Neuroinflammation: Alkyl Substituted Coumarins as Inhibitors of Enzymes Involved in Neurodegeneration

Neurodegenerative disorders (NDs) include a large range of diseases characterized by neural dysfunction with a multifactorial etiology. The most common NDs are Alzheimer's disease and Parkinson's disease, in which cholinergic and dopaminergic systems are impaired, respectively. Despite different brain regions being affected, oxidative stress and inflammation were found to be common triggers in the pathogenesis and progression of both diseases. By taking advantage of a multi-target approach, in this work we explored alkyl substituted coumarins as neuroprotective agents, capable to reduce oxidative stress and inflammation by inhibiting enzymes involved in neurodegeneration, among which are Carbonic Anhydrases (CAs), Monoamine Oxidases (MAOs), and Cholinesterases (ChEs). The compounds were synthesized and profiled against the three targeted enzymes. The binding mode of the most promising compounds (7 and 9) within MAO-A and -B was analyzed through molecular modeling studies, providing and explanation for the different selectivities observed for the MAO isoforms. In vitro biological studies using LPS-stimulated rat astrocytes showed that some compounds were able to counteract the oxidative stress-induced neuroinflammation and hamper interleukin-6 secretion, confirming the success of this multitarget approach.

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.

Monoamine oxidase B inhibitors based on natural privileged scaffolds: A review of systematically structural modification

Monoamine oxidase is a flavin enzyme that catalyzes the oxidation of monoamine neurotransmitters in the brain. Various toxic by-products, aldehydes and hydrogen peroxide produced during the catalytic process, can cause oxidative stress and neuronal cell death. Overexpression of MAO-B and insufficient dopamine concentration are recognized as pathological factors in neurodegenerative diseases (NDs) including Parkinson's disease (PD) and Alzheimer's disease (AD). Therefore, the inhibition of MAO-B is an attractive target for the treatment of NDs. Despite significant efforts, few selective and reversible MAO-B inhibitors have been clinically approved. Natural products have emerged as valuable sources of lead compounds in drug discovery. Compounds such as chromone, coumarin, chalcone, caffeine, and aurone, present in natural structures, are considered as privileged scaffolds in the synthesis of MAO-B inhibitors. In this review, we summarized the structure-activity relationship (SAR) of MAO-B inhibitors based on the naturally privileged scaffolds over the past 20 years. Additionally, we proposed a balanced discussion on the advantages and limitations of natural scaffold-based MAO-B inhibitors with providing a future perspective in drug development.

Design, synthesis and biological evaluation of novel indanones derivatives as potent acetylcholinesterase/monoamine oxidase B inhibitors

Aim: Based on a multitarget design strategy, a series of novel indanone-1-benzyl-1,2,3,6-tetrahydropyridin hybrids were identified for the potential treatment of Alzheimer's disease (AD). Results: These compounds exhibited significant inhibitory activities against acetylcholinesterase (AChE) and moderate inhibitory activities toward monoamine oxidase B (MAO-B). The optimal compound A1 possessed excellent dual AChE/MAO-B inhibition both in terms of potency (AChE: IC50 = 0.054 ± 0.004 μM; MAO-B: IC50 = 3.25 ± 0.20 μM), moderate inhibitory effects on self-mediated amyloid-β (Aβ) aggregation and antioxidant activity. In addition, compound A1 exhibited low neurotoxicity. More importantly, compound A1 showed significant cognitive and spatial memory improvements in the scopolamine-induced AD mouse model. Conclusion: All results suggest that compound A1 may become a promising lead of anti-AD drug for further development.

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.

New Insights on the Activity and Selectivity of MAO-B Inhibitors through In Silico Methods

To facilitate the identification of novel MAO-B inhibitors, we elaborated a consolidated computational approach, including a pharmacophoric atom-based 3D quantitative structure-activity relationship (QSAR) model, activity cliffs, fingerprint, and molecular docking analysis on a dataset of 126 molecules. An AAHR.2 hypothesis with two hydrogen bond acceptors (A), one hydrophobic (H), and one aromatic ring (R) supplied a statistically significant 3D QSAR model reflected by the parameters: R² = 0.900 (training set); Q² = 0.774 and Pearson's R = 0.884 (test set), stability s = 0.736. Hydrophobic and electron-withdrawing fields portrayed the relationships between structural characteristics and inhibitory activity. The quinolin-2-one scaffold has a key role in selectivity towards MAO-B with an AUC of 0.962, as retrieved by ECFP4 analysis. Two activity cliffs showing meaningful potency variation in the MAO-B chemical space were observed. The docking study revealed interactions with crucial residues TYR:435, TYR:326, CYS:172, and GLN:206 responsible for MAO-B activity. Molecular docking is in consensus with and complementary to pharmacophoric 3D QSAR, ECFP4, and MM-GBSA analysis. The computational scenario provided here will assist chemists in quickly designing and predicting new potent and selective candidates as MAO-B inhibitors for MAO-B-driven diseases. This approach can also be used to identify MAO-B inhibitors from other libraries or screen top molecules for other targets involved in suitable diseases.

Discovery of novel 2-hydroxyl-4-benzyloxybenzyl aniline derivatives as potential multifunctional agents for the treatment of Parkinson's disease

To discover novel multifunctional agents for the treatment of Parkinson's disease, a series of 2-hydroxyl-4-benzyloxybenzyl aniline derivatives was designed, synthesized and evaluated. The biological screening indicated that representative compound 6h possessed excellent MAO-B inhibition (IC₅₀ = 0.014 μM), high antioxidant activity (ORAC = 2.14 Trolox equivalent), good metal chelating ability, appropriate BBB permeability and significant neuroprotective effect. Additionally, 6h exhibited great ability to alleviate the neuroinflammtion by suppressing the activation of NF-κB pathway in vitro. Furthermore, 6h can also ameliorate MPTP induced Parkinson's disease symptoms in mice by improving the dopamine level and repressing oxidative damage. These results indicated that compound 6h was a promising candidate for further development against PD.

Investigation of the monoamine oxidase inhibition properties of benzoxathiolone derivatives

The treatment of neuropsychiatric and neurodegenerative disorders such as depression and Parkinson’s disease represent significant challenges in healthcare. Enzymes that metabolise neurotransmitter amines are important drug targets for these disorders and inhibitors of these enzymes have played key roles as therapeutic agents. For example, inhibitors of monoamine oxidase (MAO) A have been used for decades as antidepressant agents and act by inhibiting the central metabolism of serotonin and noradrenaline, while MAO-B inhibitors conserve central dopamine supply and have been used to treat Parkinson’s disease. Literature reports that benzoxathiolone derivatives act as potent MAO inhibitors with specificity for the MAO-B isoform. To expand on these findings, the present study synthesised series of benzoxathiolone derivatives and investigated their human MAO inhibition properties. The results showed that the benzoxathiolone derivatives were potent MAO inhibitors, with the most potent compounds exhibiting IC50 values of 0.083 and 0.086 µM (4d and 5e) and 0.0069 and 0.0066 µM (3a and 3b) for MAO-A and MAO-B, respectively. Compounds 4d and 5e are significantly more potent MAO-A inhibitors compared to those reported previously. It may be concluded that benzoxathiolone derived compounds may act as future leads for the development of new treatments for depression and Parkinson’s disease.

Graphical Abstract

Design, synthesis, and evaluation of N-methyl-propargylamine derivates as isoform-selective monoamine oxidases inhibitors for the treatment of nervous system diseases

A novel series of N-methyl-propargylamine derivates were designed, synthesized, and evaluated as isoform-selective monoamine oxidases (MAO) inhibitors for the treatment of nervous system diseases. The in vitro studies showed some of the compounds exhibited considerable MAO-A selective inhibitory activity (IC₅₀ of 14.86-17.16 nM), while some of the others exhibited great MAO-B selective inhibitory activity (IC₅₀ of 4.37-17.00 nM). Further studies revealed that compounds A2 （IC₅₀ against MAO-A: 17.16 ± 1.17 nM） and A5 (IC₅₀ against MAO-B: 17.00 ± 1.10 nM) had significant abilities to protect PC12 cells from H₂O₂-induced apoptosis and reactive oxygen species (ROS) production. The parallel artificial membrane permeability assay showed A2 and A5 would be potent to cross the blood-brain barrier. The results indicated that A2 showed potential use in the therapy of MAO-A related diseases, such as depression and anxiety; while A5 exhibited promising ability in the treatment of MAO-B related diseases, such as Alzheimer's disease and Parkinson's disease.

3-(3,4-Dichlorophenyl)-5-(1H-indol-5-yl)-1,2,4-oxadiazole

3-(3,4-Dichlorophenyl)-5-(1H-indol-5-yl)-1,2,4-oxadiazole was synthesized via the condensation of 3,4-dichlorobenzamidoxime and methyl 1H-indole-5-carboxylate using a superbasic medium (NaOH/DMSO). The compound was tested as a potential inhibitor of human monoamine oxidase (MAO) A and B. It demonstrated a notable inhibition with an IC50 value of 0.036 μM for the MAO-B and isoform specificity. The product was characterized by 1H-NMR, 13C-NMR, and HRMS. In conclusion, the new active MAO-B inhibitor may serve as a candidate for the future discovery of therapeutic agents for neurodegenerative disorders such as Parkinson’s disease.

Ligand fishing of monoamine oxidase B inhibitors from Platycodon grandiflorus (Jacq.) A.DC. roots by the enzyme functionalised magnetic nanoparticles

INTRODUCTION

As a famous traditional Chinese medicine, roots of Platycodon grandiflorus (Jacq.) A.DC. have shown multiple effects against neurodegenerative diseases. To investigate the components against Parkinson's disease (PD), the roots of P. grandiflora were selected as the research subject.

OBJECTIVE

Screening and identifying of monoamine oxidase B (MAO-B) inhibitors from the roots of P. grandiflorum via enzyme functionalised magnetic nanoparticles (MNPs)-based ligand fishing combined with high-performance liquid chromatography-mass spectrometry (HPLC-MS) analysis.

METHOD

MAO-B functionalised MNPs have been synthesised for screening MAO-B inhibitors from the roots of P. grandiflorum. The ligands were identified by HPLC-MS and nuclear magnetic resonance (NMR) analysis, and their anti-PD activity was evaluated via MAO-B inhibition assay and cell viability assay in vitro.

RESULTS

Two MAO-B inhibitors were fished out and identified by HPLC-MS as protocatechuic aldehyde (1) and coumarin (2), with the half maximal inhibitory concentrations of 28.54 ± 0.39 and 25.39 ± 0.29 μM, respectively. Among them, 1 could also significantly increase the viability of 6-hydroxydopamine-damaged PC12 cells.

CONCLUSION

The results are helpful to elucidate the anti-PD activity of the plant, and the ligand fishing method has shown good potential in discovery of MAO-B inhibitors.

In silico studies of natural product-like caffeine derivatives as potential MAO-B inhibitors/AA2AR antagonists for the treatment of Parkinson's disease

Parkinson's disease is considered the second most frequent neurodegenerative disease. It is described by the loss of dopaminergic neurons in the mid-brain. For many decades, L-DOPA has been considered as the gold standard for treating Parkinson's disease motor symptoms, however, due to the decrease of efficacy, in the long run, there is an urgent need for novel antiparkinsonian drugs. Caffeine derivatives have been reported several times for their neuroprotective properties and dual blockade of monoamine oxidase (MAO) and adenosine A2A receptors (AA2AR). Natural products are currently attracting more focus due to structural diversity and safety in contrast to synthetic drugs. In the present work, computational studies were conducted on natural product-like caffeine derivatives to search for novel potent candidates acting as dual MAO-B inhibitors/AA2AR antagonists for Parkinson's disease. Our findings revealed two natural products among the top hits: CNP0202316 and CNP0365210 fulfill the requirements of drugs acting on the brain. The selected lead compounds were further studied using molecular dynamics simulation to assess their stability with MAO-B. Current findings might shift the interest towards natural-based compounds and could be exploited to further optimize caffeine derivatives into a successful dual-target-directed drug for managing and halting the neuronal damage in Parkinson's disease patients.

Coumarin-Based Dual Inhibitors of Human Carbonic Anhydrases and Monoamine Oxidases Featuring Amino Acyl and (Pseudo)-Dipeptidyl Appendages: In Vitro and Computational Studies

The involvement of human carbonic anhydrase (hCA) IX/XII in the pathogenesis and progression of many types of cancer is well acknowledged, and more recently human monoamine oxidases (hMAOs) A and B have been found important contributors to tumor development and aggressiveness. With a view of an enzymatic dual-blockade approach, in this investigation, new coumarin-based amino acyl and (pseudo)-dipeptidyl derivatives were synthesized and firstly evaluated in vitro for inhibitory activity and selectivity against membrane-bound and cytosolic hCAs (hCA IX/XII over hCA I/II), as well as the hMAOs, to estimate their potential as anticancer agents. De novo design of peptide-coumarin conjugates was subsequently carried out and involved the combination of the widely explored coumarin nucleus with the unique biophysical and structural properties of native or modified peptides. All compounds displayed nanomolar inhibitory activities towards membrane-anchored hCAs, whilst they were unable to block the ubiquitous CA I and II isoforms. Structural features pertinent to potent and selective CA inhibitory activity are discussed, and modeling studies were found to support the biological data. Lower potency inhibition of the hMAOs was observed, with most compounds showing preferential inhibition of hMAO-A. The binding of the most potent ligands (6 and 16) to the hydrophobic active site of hMAO-A was investigated in an attempt to explain selectivity on the molecular level. Calculated Ligand Efficiency values indicate that compound 6 has the potential to serve as a lead compound for developing innovative anticancer agents based on the dual inhibition strategy. This information may help design new coumarin-based peptide molecules with diverse bioactivities.

Resveratrol Analogues as Dual Inhibitors of Monoamine Oxidase B and Carbonic Anhydrase VII: A New Multi-Target Combination for Neurodegenerative Diseases?

Neurodegenerative diseases (NDs) are described as multifactorial and progressive syndromes with compromised cognitive and behavioral functions. The multi-target-directed ligand (MTDL) strategy is a promising paradigm in drug discovery, potentially leading to new opportunities to manage such complex diseases. Here, we studied the dual ability of a set of resveratrol (RSV) analogs to inhibit two important targets involved in neurodegeneration. The stilbenols 1−9 were tested as inhibitors of the human monoamine oxidases (MAOs) and carbonic anhydrases (CAs). The studied compounds displayed moderate to excellent in vitro enzyme inhibitory activity against both enzymes at micromolar/nanomolar concentrations. Among them, the best compound 4 displayed potent and selective inhibition against the MAO-B isoform (IC50 MAO-A 0.43 µM vs. IC50 MAO-B 0.01 µM) with respect to the parent compound resveratrol (IC50 MAO-A 13.5 µM vs. IC50 MAO-B > 100 µM). It also demonstrated a selective inhibition activity against hCA VII (KI 0.7 µM vs. KI 4.3 µM for RSV). To evaluate the plausible binding mode of 1−9 within the two enzymes, molecular docking and dynamics studies were performed, revealing specific and significant interactions in the active sites of both targets. The new compounds are of pharmacological interest in view of their considerably reduced toxicity previously observed, their physicochemical and pharmacokinetic profiles, and their dual inhibitory ability. Compound 4 is noteworthy as a promising lead in the development of MAO and CA inhibitors with therapeutic potential in neuroprotection.

Isatoic anhydrides as novel inhibitors of monoamine oxidase

The monoamine oxidase (MAO) enzymes metabolise neurotransmitter amines in the central and peripheral tissues, and thereby contribute to the regulation of neurotransmission. Inhibitors of MAO modulate the levels of neurotransmitters in the central nervous system, and have been used for several decades for the treatment of depression and Parkinson's disease, while potential new therapeutic applications in other diseases such as prostate cancer and heart failure may exist. In the interest of discovering new classes of chemical compounds that potently inhibit the MAOs, the present study synthesises a series of ten isatoic anhydrides and evaluates their potential as in vitro inhibitors of human MAO-A and MAO-B. The isatoic anhydrides bear structural similarity to a series of 3,4-dihydro-2(1H)-quinolinones as well as to series of isatins and phthalimides that have been reported to act as potent MAO-B inhibitors. The results document that the isatoic anhydrides inhibit both MAO isoforms with the most potent inhibitors exhibiting IC₅₀ values of 0.010 µM (1b and 1h) and 0.0047 µM (1j) for MAO-A and MAO-B, respectively. Molecular docking suggests that isatoic anhydrides exhibit similar binding modes and interactions with MAO-A and MAO-B, which may explain their potent inhibition of both isoforms. It may be concluded that the isatoic anhydrides represent a new class of MAO inhibitors, while it is interesting to note that very few studies on the pharmacological actions of isatoic anhydrides have been reported. As a secondary aim, the isatoic anhydrides were also evaluated as potential inhibitors of d-amino acid oxidase (DAAO), acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE).

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, "general chemicals," natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10-15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

The evaluation of N-propargylamine-2-aminotetralin as an inhibitor of monoamine oxidase

Monoamine oxidase B (MAO-B) inhibitors are established therapy for Parkinson's disease and act, in part, by blocking the MAO-catalysed metabolism of dopamine in the brain. Two propargylamine-containing MAO-B inhibitors, selegiline [(R)-deprenyl] and rasagiline, are currently used in the clinic for this purpose. These compounds are mechanism-based inactivators and, after oxidative activation, form covalent adducts with the FAD co-factor. An important consideration is that selegiline and rasagiline display specificity for MAO-B over the MAO-A isoform thus reducing the risk of tyramine-induced changes in blood-pressure. In the interest of discovering new propargylamine MAO inhibitors, the present study synthesises racemic N-propargylamine-2-aminotetralin (2-PAT), a compound that may be considered as both a six-membered ring analogue of rasagiline and a semi-rigid N-desmethyl ring-closed analogue of selegiline. The in vitro human MAO inhibition properties of this compound were measured and the results showed that 2-PAT is a 20-fold more potent inhibitor of MAO-A (IC₅₀ = 0.721 µM) compared to MAO-B (IC₅₀ = 14.6 µM). Interestingly, dialysis studies found that 2-PAT is a reversible MAO-A inhibitor, while acting as an inactivator of MAO-B. Since reversible MAO-A inhibitors are much less liable to potentiate tyramine-induced side effects than MAO-A inactivators, it is reasonable to suggest that 2-PAT could be a useful and safe therapeutic agent for disorders such as Parkinson's disease and depression.

Ligand fishing based on tubular microchannel modified with monoamine oxidase B for screening of the enzyme's inhibitors from Crocus sativus and Edgeworthia gardneri

A novel strategy of performing ligand fishing with enzyme-modified open tubular microchannel was proposed for screening bioactive components present in medicinal plants. Monoamine oxidase B was immobilized onto the surface of the microchannel for the first time to specifically extract its ligands when the plant's extracts solution flows through the channel. The thermal and the storage stability of immobilized monoamine oxidase B were significantly enhanced after immobilization. Crocin I and Ⅱ were extracted from Crocus sativus, and tiliroside was extracted from Edgeworthia gardneri. All the three compounds were inhibitors of the enzyme with the half-maximal inhibitory concentration values of 26.70 ± 0.91, 19.88 ± 2.78, and 15.65 ± 0.85 μM, respectively. The enzyme inhibition kinetics and molecular docking were investigated. This is the first report on the inhibitory effects of tiliroside and crocin Ⅱ. The novel ligand fishing method proposed in this work possesses advantages of rapidness, high efficiency, and tiny sample consumption compared to routine ligand fishing, with promising potential for screening active natural products in complex mixtures.

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.

Structural exploration of selected C6 and C7-substituted coumarin isomers as selective MAO-B inhibitors

Monoamine Oxidase B is considered a successful target for developing antiparkinsonian drugs. Due to the side effects of current MAO-B inhibitors, there's an urgent need for novel potent and highly selective MAO-B inhibitors. A recent study has shown that coumarins tend to be more selective towards MAO-B than MAO-A when connected to a hex-5-ynyloxy chain at position 6 in contrast to their C7-isomers. The present study describes the mode of interaction of the C6 and C7-substituted coumarin isomers characterized by their difference in selectivity towards MAO-B through molecular docking and molecular dynamics simulations in an effort to elucidate the structural components and molecular interactions that may be responsible for MAO-B selectivity. Three isomeric coumarin pairs connected to ether chain at position 6 or 7 were taken from the literature and modelled according to their IUPAC nomenclature. Molecular docking study revealed one π- π stacking interaction with Tyr-326 in common between the selective coumarin C6-isomers. Resulting complexes of one isomeric coumarin pair that displayed the highest selectivity shift towards MAO-B were subject to 100 ns molecular dynamics simulations study to analyze the stability of the docked complexes. Molecular dynamics revealed that the C7-isomer is relatively stable in both MAO isoforms through the simulation duration, whereas the C6-isomer deemed unstable for MAO-A which may be due to the bulky Phe-208 residue in MAO-A. Our results might be applied for further development and optimization of coumarin derivatives into a successful drug against Parkinson's disease.Communicated by Ramaswamy H. Sarma.

Phenothiazine, anthraquinone and related tricyclic derivatives as inhibitors of monoamine oxidase

Inhibitors of the monoamine oxidase (MAO) enzymes are important agents for the treatment of central nervous system disorders and have established roles in the therapy of neuropsychiatric diseases such as depression and in the neurodegenerative disorder, Parkinson's disease. A number of good potency MAO inhibitors consist of tricyclic ring systems as exemplified by the structures of harmine and the phenothiazine compound methylene blue. In an attempt to discover novel MAO inhibitors, 30 phenothiazine, anthraquinone and related tricyclic derivatives were selected and evaluated as potential inhibitors of human MAO-A and MAO-B. The results show that, in general, the tricyclic compounds are specific inhibitors of MAO-A over the MAO-B isoform. Quinizarin (IC₅₀ = 0.065 µM), 2-chloro-7-methoxy-10H-phenothiazine (IC₅₀ = 0.576 µM) and xanthone (IC₅₀ = 0.623 µM) proved to be the most potent MAO-A inhibitors, while the most potent MAO-B inhibition was recorded with 2-chloro-7-methoxy-10H-phenothiazine (IC₅₀ = 1.34 µM), 1,2-diaminoanthraquinone (IC₅₀ = 2.41 µM) and emodin (IC₅₀ = 3.24 µM). These compounds may undergo further preclinical evaluation and development, and may also serve as potential lead compounds for the future design of MAO inhibitors.

In vitro and in vivo evaluation of fluorinated indanone derivatives as potential positron emission tomography agents for the imaging of monoamine oxidase B in the brain

Monoamine oxidases (MAOs) play a key role in the metabolism of major monoamine neurotransmitters. In particular, the upregulation of MAO-B in Parkinson's disease, Alzheimer's disease and cancer augmented the development of selective MAO-B inhibitors for diagnostic and therapeutic purposes, such as the anti-parkinsonian MAO-B irreversible binder l-deprenyl (Selegiline®). Herein we report on the synthesis of novel fluorinated indanone derivatives for PET imaging of MAO-B in the brain. Out of our series, the derivatives 6, 8, 9 and 13 are amongst the most affine and selective ligands for MAO-B reported so far. For the derivative 6-((3-fluorobenzyl)oxy)-2,3-dihydro-1H-inden-1-one (6) exhibiting an outstanding affinity (K_iMAO-B = 6 nM), an automated copper-mediated radiofluorination starting from the pinacol boronic ester 17 is described. An in vitro screening in different species revealed a MAO-B region-specific accumulation of [¹⁸F]6 in rats and piglets in comparison to L-[³H]deprenyl. The pre-clinical in vivo assessment of [¹⁸F]6 in mice demonstrated the potential of indanones to readily cross the blood-brain barrier. Nonetheless, parallel in vivo metabolism studies indicated the presence of blood-brain barrier metabolites, thus arguing for further structural modifications. With the matching analytical profiles of the radiometabolite analysis from the in vitro liver microsome studies and the in vivo evaluation, the structure's elucidation of the blood-brain barrier penetrant radiometabolites is possible and will serve as basis for the development of new indanone derivatives suitable for the PET imaging of MAO-B.

The inhibition of catechol O-methyltransferase and monoamine oxidase by tetralone and indanone derivatives substituted with the nitrocatechol moiety

The enzymes, catechol O-methyltransferase (COMT) and monoamine oxidase (MAO) are important drug targets, and inhibitors of these enzymes are established therapy for symptomatic Parkinson's disease (PD). COMT inhibitors enhance the bioavailability of levodopa to the brain, and therefore are combined with levodopa for the treatment of motor fluctuations in PD. Inhibitors of the MAO-B isoform, in turn, are used as monotherapy or in conjunction with levodopa in PD, and function by reducing the central degradation of dopamine. It has been reported that 1-tetralone and 1-indanone derivatives are potent and specific inhibitors of MAO-B, while compounds containing the nitrocatechol moiety (e.g. tolcapone and entacapone) are often potent COMT inhibitors. The present study attempted to discover compounds that exhibit dual COMT and MAO-B inhibition by synthesizing series of 1-tetralone, 1-indanone and related derivatives substituted with the nitrocatechol moiety. These compounds are structurally related to series of nitrocatechol derivatives of chalcone that have recently been investigated as potential dual COMT/MAO inhibitors. The results show that 4-chromanone derivative (7) is the most promising dual inhibitor with IC₅₀ values of 0.57 and 7.26 μM for COMT and MAO-B, respectively, followed by 1-tetralone derivative (4d) with IC₅₀ values of 0.42 and 7.83 μM for COMT and MAO-B, respectively. Based on their potent inhibition of COMT, it may be concluded that nitrocatechol compounds investigated in this study are appropriate for peripheral COMT inhibition, which represents an important strategy in the treatment of PD.

Design, synthesis and evaluation of 3-hydroxypyridin-4-ones as inhibitors of catechol-O-methyltransferase

The most effective treatment of Parkinson's disease is restoring central dopamine levels with levodopa, the metabolic precursor of dopamine. However, due to extensive peripheral metabolism by aromatic L-amino acid decarboxylase and catechol-O-methyltransferase (COMT), only a fraction of the levodopa dose reaches the brain unchanged. Thus, by preventing levodopa metabolism and increasing the availability of levodopa for uptake into the brain, the inhibition of COMT would be beneficial in Parkinson's disease. Although nitrocatechol COMT inhibitors have been used in the treatment of Parkinson's disease, efforts have been made to discover non-nitrocatechol inhibitors. In the present study, the 3-hydroxypyridin-4-one scaffold was selected for the design and synthesis of non-nitrocatechol COMT inhibitors since the COMT inhibitory potential of this class has been illustrated. Using COMT obtained from porcine liver, it was shown that a synthetic series of ten 3-hydroxypyridin-4-ones are in vitro inhibitors with IC50 values ranging from 4.55 to 19.8 µM. Although these compounds are not highly potent inhibitors, they may act as leads for the development of non-nitrocatechol COMT inhibitors. Such compounds would be appropriate for the treatment of Parkinson's disease. 3-Hydroxypyridin-4-ones have been synthesised and evaluated as non-nitrocatechol COMT inhibitors. In vitro, the IC50 values ranged from 4.55 to 19.8 μM.

Investigation of pyrazolo[1,5-a]quinoxalin-4-ones as novel monoamine oxidase inhibitors

The monoamine oxidase (MAO) enzymes are key metabolic enzymes of neurotransmitter and other bioactive amines, and represent important drug targets for the treatment of neuropsychiatric and neurodegenerative disorders. Inhibitors of MAO are established medications for the treatment of depression and Parkinson's disease, and may have future roles in other disease states such as the therapy of prostate cancer, cardiovascular disease and inflammatory diseases. Based on these considerations, the present study synthesizes a series of 22 pyrazolo[1,5-a]quinoxalin-4-one derivatives and evaluated them as potential inhibitors of human MAO-A and MAO-B. The results show that 8 derivatives inhibit MAO-A, and 3 derivatives inhibit MAO-B with IC₅₀ values in the submicromolar range (<1 µM). The most potent MAO-A inhibitor, N-[5-(acetyloxy)-2-(4-chlorophenyl)-4-oxo-4,5-dihydropyrazolo[1,5-a]quinoxalin-7-yl]acetamide (7c), exhibit an IC₅₀ value of 0.028 µM and displays 50-fold selectivity for MAO-A over MAO-B. The most potent MAO-B inhibitor, 2-(4-methylphenyl)-4-oxo-4,5-dihydropyrazolo[1,5-a]quinoxaline-7-carbonitrile (4f), exhibit an IC₅₀ value of 0.617 µM and displays 8-fold selectivity for MAO-B. This is the first report of MAO inhibition by pyrazolo[1,5-a]quinoxalin-4-one derivatives, and this study concludes that these compounds are suitable leads for the future development of MAO inhibitors, particularly of the MAO-A isoform.

The evaluation of 1-tetralone and 4-chromanone derivatives as inhibitors of monoamine oxidase

Abstract

Monoamine oxidase (MAO) is of much clinical relevance, and inhibitors of this enzyme are used in the treatment for neuropsychiatric and neurodegenerative disorders such as depression and Parkinson’s disease. The present study synthesises and evaluates the MAO inhibition properties of a series of 33 1-tetralone and 4-chromanone derivatives in an attempt to discover high-potency compounds and to expand on the structure–activity relationships of MAO inhibition by these classes. Among these series, eight submicromolar MAO-A inhibitors and 28 submicromolar MAO-B inhibitors are reported, with all compounds acting as specific inhibitors of the MAO-B isoform. The most potent inhibitor was a 1-tetralone derivative (1h) with IC₅₀ values of 0.036 and 0.0011 µM for MAO-A and MAO-B, respectively. Interestingly, with the reduction of 1-tetralones to the corresponding alcohols, a decrease in MAO inhibition potency is observed. Among these 1-tetralol derivatives, 1p (IC₅₀ = 0.785 μM) and 1o (IC₅₀ = 0.0075 μM) were identified as particularly potent inhibitors of MAO-A and MAO-B, respectively. Potent compounds such as those reported here may act as leads for the future development of MAO-B specific inhibitors.

Graphic abstract

The present study describes the MAO inhibitory activities of a series of 1-tetralone and 4-chromanone derivatives. Numerous high-potency MAO-B specific inhibitors were identified.

Electronic supplementary material

The online version of this article (10.1007/s11030-020-10143-w) contains supplementary material, which is available to authorised users.

Evaluation of nitrocatechol chalcone and pyrazoline derivatives as inhibitors of catechol-O-methyltransferase and monoamine oxidase

Literature reports that chalcones inhibit the monoamine oxidase (MAO) enzymes, mostly with specificity for the MAO-B isoform, while nitrocatechol compounds are established inhibitors of catechol-O-methyltransferase (COMT). Based on this, nitrocatechol derivatives of chalcone have been proposed to represent dual-target-directed compounds that may inhibit both MAO-B and COMT. Both these enzymes play key roles in the metabolism of dopamine and levodopa, and inhibitors are thus relevant to the treatment of Parkinson's disease. The present study expands on the discovery of dual MAO-B/COMT inhibitors by synthesising additional nitrocatechol derivatives of chalcones which include heterocyclic derivatives, and converting them to the corresponding pyrazoline derivatives. The newly synthesised chalcone and pyrazoline compounds were evaluated as inhibitors of human MAO and rat COMT, and the inhibition potencies were expressed as IC₅₀ values. A pyrazoline derivative, compound 8b, was the most potent COMT inhibitor with an IC₅₀ value of 0.048 μM. This is more potent than the reference COMT inhibitor, entacapone, which has an IC₅₀ value of 0.23 μM. The results indicated that the pyrazoline derivatives (IC₅₀ = 0.048-0.21 µM) are more potent COMT inhibitors than the chalcones (IC₅₀ = 0.14-0.29 µM). Unfortunately, the chalcone and pyrazoline derivatives were weak MAO inhibitors with IC₅₀ values > 41.4 µM. This study concludes that the nitrocatechol derivatives investigated here are promising COMT inhibitors, while not being suitable as MAO inhibitors. Using molecular docking, potential binding modes and interactions of selected inhibitors with COMT are proposed.

Monoamine oxidase inhibition by selected dye compounds

Monoamine oxidase (MAO) is an important drug target as the MAO isoforms play key roles in neurodegenerative disorders such as Alzheimer's disease and Parkinson's disease, as well as in neuropsychiatric diseases such as depression. Methylene blue is an inhibitor of MAO-A, while azure B, the major metabolite of methylene blue, and various other structural analogues retain the ability to inhibit MAO-A. Based on this, the present study evaluated 22 dyes, many of which are structurally related to methylene blue, as potential inhibitors of human MAO-A and MAO-B. The results highlighted three dye compounds as good potency competitive and reversible MAO inhibitors, and which exhibit higher MAO inhibition than methylene blue: acridine orange, oxazine 170 and Darrow red. Acridine orange was found to be a MAO-A specific inhibitor (IC50  = 0.017 μM), whereas oxazine 170 is a MAO-B specific inhibitor (IC50  = 0.0065 μM). Darrow red was found to be a non-specific MAO inhibitor (MAO-A, IC50  = 0.059 μM; MAO-B, IC50  = 0.065 μM). These compounds may be advanced for further testing and preclinical development, or be used as possible lead compounds for the future design of MAO inhibitors.

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.

Benzo[b]tiophen-3-ol derivatives as effective inhibitors of human monoamine oxidase: design, synthesis, and biological activity

A series of benzo[b]thiophen-3-ols were synthesised and investigated as potential human monoamine oxidase (hMAO) inhibitors in vitro as well as ex vivo in rat cortex synaptosomes by means of evaluation of 3,4-dihydroxyphenylacetic acid/dopamine (DOPAC/DA) ratio and lactate dehydrogenase (LDH) activity. Most of these compounds possessed high selectivity for the MAO-B isoform and a discrete antioxidant and chelating potential. Molecular docking studies of all the compounds underscored potential binding site interactions suitable for MAO inhibition activity, and suggested structural requirements to further improve the activity of this scaffold by chemical modification of the aryl substituents. Starting from this heterocyclic nucleus, novel lead compounds for the treatment of neurodegenerative disease could be developed.

Evaluation of Selected Natural Compounds as Dual Inhibitors of Catechol-O-Methyltransferase and Monoamine Oxidase

Background:

The most effective symptomatic treatment of Parkinson’s disease remains the metabolic precursor of dopamine, L-dopa. To enhance the efficacy of L-dopa, it is often combined with inhibitors of the enzymes, catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) B, key metabolic enzymes of L-dopa and dopamine.

Objective:

This study attempted to discover compounds that exhibit dual inhibition of COMT and MAO-B among a library of 40 structurally diverse natural compounds. Such dual acting inhibitors may be effective as adjuncts to L-dopa and offer enhanced value in the management of Parkinson’s disease.

Methods:

Selected natural compounds were evaluated as in vitro inhibitors of rat liver COMT and recombinant human MAO. Reversibility of MAO inhibition was investigated by dialysis.

Results:

Among the natural compounds morin (IC50 = 1.32 µM), chlorogenic acid (IC50 = 6.17 µM), (+)-catechin (IC50 = 0.86 µM), alizarin (IC50 = 0.88 µM), fisetin (IC50 = 5.78 µM) and rutin (IC50 = 25.3 µM) exhibited COMT inhibition. Among these active COMT inhibitors only morin (IC50 = 16.2 µM), alizarin (IC50 = 8.16 µM) and fisetin (IC50 = 7.33 µM) were noteworthy MAO inhibitors, with specificity for MAO-A.

Conclusion:

None of the natural products investigated here are dual COMT/MAO-B inhibitors. However, good potency COMT inhibitors have been identified, which may serve as leads for future development of COMT inhibitors.

Optimization of pyrrolo[3,4-f]indole-5,7-dione and indole-5,6-dicarbonitrile derivatives as inhibitors of monoamine oxidase

In recent studies, we have investigated the monoamine oxidase (MAO) inhibition properties of pyrrolo[3,4-f]indole-5,7-dione and indole-5,6-dicarbonitrile derivatives. Since numerous high potency MAO inhibitors are present among these chemical classes, the present study synthesizes 44 additional derivatives in an attempt to further derive structure-activity relationships (SARs) and to establish optimal substitution patterns for MAO inhibition. The results show that, with the exception of one compound, all indole-5,6-dicarbonitrile derivatives (10) exhibit submicromolar IC₅₀ values for the inhibition of MAO, with the most potent MAO-A inhibitor exhibiting an IC₅₀ value of 0.006 μM while the most potent MAO-B inhibitor exhibits an IC₅₀ value of 0.058 μM. Interestingly, an N-oxide derivative (4c) also proved to be a potent and nonspecific MAO inhibitor. With the exception of one compound, all of the pyrrolo[3,4-f]indole-5,7-diones (28) also exhibit submicromolar IC₅₀ values for the inhibition of an MAO isoform. The most potent inhibitor exhibit an IC₅₀ value of 0.011 μM for MAO-A. This study proposes that high potency MAO inhibitors such as those investigated here, may act as lead compounds for the development of treatments for neurodegenerative and neuropsychiatric disorders such as Parkinson's disease and depression.

Novel approaches to the discovery of selective human monoamine oxidase-B inhibitors: is there room for improvement?

Introduction: Selective monoamine oxidase-B (MAO-B) inhibitors are currently used as coadjuvants for the treatment of early motor symptoms in Parkinson's disease. They can, based on their chemical structure and mechanism of inhibition, be categorized into reversible and irreversible agents. Areas covered: This review provides a comprehensive update on the development state of selective MAO-B inhibitors describing the results, structures, structure-activity relationships (SARs) and Medicinal chemistry strategies as well as the related shortcomings over the past five years. Expert opinion: Researchers have explored and implemented new and old chemical scaffolds achieving high inhibitory potencies and isoform selectivity. Most of them were characterized and proposed as multitarget agents able to act at different levels (including AChE inhibition, H3R or A_2AR antagonism, antioxidant and chelating properties, Aβ_1-42 aggregation reduction) in the network of aetiologies of neurodegenerative disorders. These results can also be used to avoid 'cheese-reaction' effects and the occurrence of serotonergic syndrome in patients.

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.

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.

4-(3-Nitrophenyl)thiazol-2-ylhydrazone derivatives as antioxidants and selective hMAO-B inhibitors: synthesis, biological activity and computational analysis

A new series of 4-(3-nitrophenyl)thiazol-2-ylhydrazone derivatives were designed, synthesised, and evaluated to assess their inhibitory effect on the human monoamine oxidase (hMAO) A and B isoforms. Different (un)substituted (hetero)aromatic substituents were linked to N1 of the hydrazone in order to establish robust structure–activity relationships. The results of the biological testing demonstrated that the presence of the hydrazothiazole nucleus bearing at C4 a phenyl ring functionalised at the meta position with a nitro group represents an important pharmacophoric feature to obtain selective and reversible human MAO-B inhibition for the treatment of neurodegenerative disorders. In addition, the most potent and selective MAO-B inhibitors were evaluated in silico as potential cholinesterase (AChE/BuChE) inhibitors and in vitro for antioxidant activities. The results obtained from molecular modelling studies provided insight into the multiple interactions and structural requirements for the reported MAO inhibitory properties.