Coumarin versus Chromone Monoamine Oxidase B Inhibitors: Quo Vadis?

Benzopyrone, a privileged scaffold in drug discovery: An overview of FDA-approved drugs and clinical candidates

Natural products have always served as an important source of drugs for treating various diseases. Among various privileged natural product scaffolds, the benzopyrone class of compounds has a substantial presence among biologically active compounds. One of the pioneering anticoagulant drugs, warfarin approved in 1954 bears a benzo-α-pyrone (coumarin) nucleus. The widely investigated psoriasis drugs, methoxsalen, and trioxsalen, also contain a benzo-α-pyrone nucleus. Benzo-γ-pyrone (chromone) containing drugs, cromoglic acid, and pranlukast were approved as treatments for asthma in 1982 and 2007, respectively. Numerous other small molecules with a benzopyrone core are under clinical investigation. The present review discusses the discovery, absorption, distribution, metabolism, excretion properties, and synthetic approaches for the Food and Drug Administration-approved and clinical-stage benzopyrone class of compounds. The role of the pyrone core in biological activity has also been discussed. The present review unravels the potential of benzopyrone core in medicinal chemistry and drug development.

Discovery of C-3 isoxazole substituted thiochromone S,S-dioxide derivatives as potent and selective inhibitors for monoamine oxidase B (MAO-B)

Developing new scaffolds for highly potent and selective inhibitors of human Monoamine Oxidase B (hMAO-B) is a crucial objective in enhancing the efficacy and safety in the clinical treatment of neurodegenerative diseases. In this study, we have identified a series of C-3 isoxazole-substituted thiochromone S,S-dioxide derivatives that exhibit strong inhibitory activity against hMAO-B. The strategy of oxidizing thiochromone to thiochromone S,S-dioxide solves the key defect of extreme insolubility observed for thiochromone analogues. In addition, the sulfone group contributes extra hydrogen(H)-bonding interactions with Tyr435, which significantly increases the activity of thiochromone S,S-dioxide derivatives against hMAO-B. Furthermore, the presence of isoxazole group provides potential H-bonding interaction and electrostatic interaction with the residue of Tyr326, while the rigid aryl ring introduces a potential steric conflict with Phe208 of hMAO-A to improve both potency and selectivity. In our investigations, several compounds (9c, 10c, 10e, 10g, 10l and 10m) demonstrate remarkable single-digit nanomolar potency. These compounds exhibit favorable cytotoxicity profiles in both differentiated SH-SY5Y and HVSMC cells, without apparent cardiotoxic effects. Moreover, compounds 10e and 10h do not lead to an increase in ROS levels in differentiated SH-SY5Y cells, further demonstrating their potential as safe and effective hMAO-B inhibitors. These findings indicate that the C-3 isoxazole substituted thiochromone S,S-dioxide analogues are potential leading compounds for the development of selective inhibitors with high potency.

Research progress of natural products and their derivatives against Alzheimer's disease

Alzheimer's disease (AD), a persistent neurological dysfunction, has an increasing prevalence with the aging of the world and seriously threatens the health of the elderly. Although there is currently no effective treatment for AD, researchers have not given up, and are committed to exploring the pathogenesis of AD and possible therapeutic drugs. Natural products have attracted considerable attention owing to their unique advantages. One molecule can interact with multiple AD-related targets, thus having the potential to be developed in a multi-target drug. In addition, they are amenable to structural modifications to increase interaction and decrease toxicity. Therefore, natural products and their derivatives that ameliorate pathological changes in AD should be intensively and extensively studied. This review mainly presents research on natural products and their derivatives for the treatment of AD.

Chromone-based monoamine oxidase B inhibitor with potential iron-chelating activity for the treatment of Alzheimer's disease

Based on the multitarget-directed ligands (MTDLs) strategy, a series of chromone-hydroxypyridinone hybrids were designed, synthesised, and evaluated as potential multimodal anti-AD ligands. Prospective iron-chelating effects and favourable monoamine oxidase B (MAO-B) inhibitory activities were observed for most of the compounds. Pharmacological assays led to the identification of compound 17d, which exhibited favourable iron-chelating potential (pFe³⁺ = 18.52) and selective hMAO-B inhibitory activity (IC₅₀ = 67.02 ± 4.3 nM, SI = 11). Docking simulation showed that 17d occupied both the substrate and the entrance cavity of MAO-B, and established several key interactions with the pocket residues. Moreover, 17d was determined to cross the blood-brain barrier (BBB), and can significantly ameliorate scopolamine-induced cognitive impairment in AD mice. Despite its undesired pharmacokinetic property, 17d remains a promising multifaceted agent that is worth further investigation.

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.

A Concise Review of the Recent Structural Explorations of Chromones as MAO-B Inhibitors: Update from 2017 to 2023

Monoamine oxidases (MAOs) are a family of flavin adenine dinucleotide-dependent enzymes that catalyze the oxidative deamination of a wide range of endogenous and exogenous amines. Multiple neurological conditions, including Parkinson's disease (PD) and Alzheimer's disease (AD), are closely correlated with altered biogenic amine concentrations in the brain caused by MAO. Toxic byproducts of this oxidative breakdown, including hydrogen peroxide, reactive oxygen species, and ammonia, can cause oxidative damage and mitochondrial dysfunction in brain cells. Certain MAO-B blockers have been recognized as effective treatment options for managing neurological conditions, including AD and PD. There is still a pressing need to find potent therapeutic molecules to fight these disorders. However, the focus of neurodegeneration studies has recently increased, and certain compounds are now in clinical trials. Chromones are promising structures for developing therapeutic compounds, especially in neuronal degeneration. This review focuses on the MAO-B inhibitory potential of several synthesized chromones and their structural activity relationships. Concerning the discovery of a novel class of effective chromone-based selective MAO-B-inhibiting agents, this review offers readers a better understanding of the most recent additions to the literature.

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.

Repurposing of FDA-approved drugs as dual-acting MAO-B and AChE inhibitors against Alzheimer's disease: An in silico and in vitro study

An in silico consensus molecular docking approach and in vitro evaluations were adopted in the present study to explore a dataset of FDA-approved drugs as novel multitarget MAO-B/AChE agents in the treatment of Alzheimer's disease (AD). GOLD 5.3 and Glide were employed in the virtual assessments and consensus superimpositions of the obtained poses were applied to increase the reliability of the docking protocols. Furthermore, the top ranked molecules were subjected to binding free energy calculations using MM/GBSA, Induced fit docking (IFD) simulations, and a literature review. Consequently, the top four multitarget drugs were examined for their in vitro MAO-B and AChE inhibition effects. The consensus molecular docking identified Dolutegravir, Rebamipide, Loracarbef and Diflunisal as potential multitarget drugs. The biological data demonstrated that most of the docking scores were in good correlation with the in vitro experiments, however the theoretical simulations in the active site of MAO-B identified two false-positives - Rebamipide and Diflunisal. Dolutegravir and Loracarbef were accessed as active MAO-B inhibitors, while Dolutegravir, Rebamapide and Diflunisal as potential AChE inhibitors. The antiretroviral agent Dolutegravir exhibited the most potent multitarget activity - 41% inhibition of MAO-B (1 μM) and 68% inhibition of AChE (10 μM). Visualizations of the intermolecular interactions of Dolutegravir in the active sites of MAO-B and AChE revealed the formation of several stable hydrogen bonds. Overall, Dolutegravir was identified as a potential anti-AD drug, however further in vivo evaluations should be considered.

8-Amide and 8-carbamate substitution patterns as modulators of 7-hydroxy-4-methylcoumarin's antidepressant profile: Synthesis, biological evaluation and docking studies

Psychiatric and neurological disorders affect millions of people worldwide. Currently available treatments may help to improve symptoms, but they cannot cure the diseases. Therefore, there is an urgent need for potent and safe therapeutic solutions. 8-Amide and 8-carbamatecoumarins were synthetized and evaluated as human monoamine oxidase A and B (hMAO-A and hMAO-B) inhibitors. Comparison between both scaffolds has been established, and we hypothesized that the introduction of different substituents can modulate hMAO activity and selectivity. N-(7-Hydroxy-4-methylcoumarin-8-yl)-4-methylbenzamide (9) and ethyl N-(7-hydroxy-4-methylcoumarin-8-yl)carbamate (20) proved to be the most active and selective hMAO-A inhibitors (IC₅₀ = 15.0 nM and IC₅₀ = 22.0 nM, respectively), being compound 9 an irreversible hMAO-A inhibitor twenty-four times more active in vitro than moclobemide, a drug used in the treatment of depression and anxiety. Based on PAMPA assay results, both compounds proved to be good candidates to cross the blood-brain barrier. In addition, these compounds showed non-significant cytotoxicity on neuronal viability assays. Also, the best compound proved to have a t_1/2 of 6.84 min, an intrinsic clearance of 195.63 μL min^-1 mg^-1 protein, and to be chemically stable at pH 3.0, 7.4 and 10.0. Docking studies were performed to better understand the binding affinities and selectivity profiles for both hMAO isoforms. Finally, theoretical drug-like properties calculations corroborate the potential of both scaffolds on the search for new therapeutic solutions for psychiatric disorders as depression.

The Chemotype of Chromanones as a Privileged Scaffold for Multineurotarget Anti-Alzheimer Agents

The multifactorial nature of Alzheimer's disease necessitates the development of agents able to interfere with different relevant targets. A series of 22 tailored chromanones was conceptualized, synthesized, and subjected to biological evaluation. We identified one representative bearing a linker-connected azepane moiety (compound 19) with balanced pharmacological properties. Compound 19 exhibited inhibitory activities against human acetyl-, butyrylcholinesterase and monoamine oxidase-B, as well as high affinity to both the σ1 and σ2 receptors. Our study provides a framework for the development of further chromanone-based multineurotarget agents.

Continuous Flow Biocatalysis: Synthesis of Coumarin Carboxamide Derivatives by Lipase TL IM from Thermomyces lanuginosus

Coumarin carboxamide derivatives are important building blocks for organic synthesis and chemical biology due to their excellent biopharmaceutical properties. In this paper, we demonstrate for the first time a two-step enzymatic synthesis of coumarin carboxamide derivatives. Salicylaldehyde and dimethyl malonate were reacted to obtain coumarin carboxylate methyl derivatives, which were then reacted with various amines under the catalysis of lipase TL IM from Thermomyces lanuginosus to obtain coumarin carboxamide derivatives in continuous flow reactors. We studied various reaction parameters on the yields. The important features of this method include mild reaction conditions, a short reaction time (40 min), reduced environmental pollution, higher productivity (STY = 31.2941 g L−1 h−1) and enzymes being relatively easy to obtain.

Coumarin-Resveratrol-Inspired Hybrids as Monoamine Oxidase B Inhibitors: 3-Phenylcoumarin versus trans-6-Styrylcoumarin

Monoamine oxidases (MAOs) are attractive targets in drug design. The inhibition of one of the isoforms (A or B) is responsible for modulating the levels of different neurotransmitters in the central nervous system, as well as the production of reactive oxygen species. Molecules that act selectively on one of the MAO isoforms have been studied deeply, and coumarin has been described as a promising scaffold. In the current manuscript we describe a comparative study between 3-phenylcoumarin (endo coumarin-resveratrol-inspired hybrid) and trans-6-styrylcoumarin (exo coumarin-resveratrol-inspired hybrid). Crystallographic structures of both compounds were obtained and analyzed. 3D-QSAR models, in particular CoMFA and CoMSIA, docking simulations and molecular dynamics simulations have been performed to support and better understand the interaction of these molecules with both MAO isoforms. Both molecules proved to inhibit MAO-B, with trans-6-styrylcoumarin being 107 times more active than 3-phenylcoumarin, and 267 times more active than trans-resveratrol.

Recent Developments of Coumarin-based Hybrids in Drug Discovery

Coumarin scaffold is a highly significant O-heterocycle, namely benzopyran-2-ones, form an elite class of naturally occurring compounds that possess promising therapeutic perspectives. Based on its broad spectrum of biological activities, the privileged coumarin scaffold is applied to medicinal and pharmacological treatments by several rational design strategies and approaches. Structure-activity relationships of the coumarin-based hybrids with various bioactivity fragments revealed significant information toward the further development of highly potent and selective disorder therapeutic agents. The molecular docking studies between coumarins and critical therapeutic enzymes demonstrated mode of action by forming noncovalent interactions with more than one receptor, further rationally confirm information about structure-activity relationships. This review summarizes recent developments relating to coumarin-based hybrids with other pharmacophores aiming to numerous feasible therapeutic enzymatic targets to combat various therapeutic fields, including anticancer, antimicrobic, anti-Alzheimer, anti-inflammatory activities.

Mapping Chromone-3-Phenylcarboxamide Pharmacophore: Quid Est Veritas?

Chromone-3-phenylcarboxamides (Crom-1 and Crom-2) were identified as potent, selective, and reversible inhibitors of human monoamine oxidase B (hMAO-B). Since they exhibit some absorption, distribution, metabolism, and excretion (ADME)-toxicity liabilities, new derivatives were synthesized to map the chemical structural features that compose the pharmacophore, a process vital for lead optimization. Structure-activity relationship data, supported by molecular docking studies, provided a rationale for the contribution of the heterocycle's rigidity, the carbonyl group, and the benzopyran heteroatom for hMAO-B inhibitory activity. From the study, N-(3-chlorophenyl)-4H-thiochromone-3-carboxamide (31) (hMAO-B IC₅₀ = 1.52 ± 0.15 nM) emerged as a reversible tight binding inhibitor with an improved pharmacological profile. In in vitro ADME-toxicity studies, compound 31 showed a safe cytotoxicity profile in Caco-2, SH-SY5Y, HUVEC, HEK-293, and MCF-7 cells, did not present cardiotoxic effects, and did not affect P-gp transport activity. Compound 31 also protected SH-SY5Y cells from iron(III)-induced damage. Collectively, these studies highlighted compound 31 as the first-in-class and a suitable candidate for in vivo preclinical investigation.

Deciphering the detailed structure-activity relationship of coumarins as Monoamine oxidase enzyme inhibitors-An updated review

In the last few years, Monoamine oxidase (MAO) have emerged as a target for the treatment of many neurodegenerative diseases including anxiety, depression, Alzheimer's, and Parkinson's diseases. The MAO inhibitors especially selective and reversible inhibitors of either of the isoenzymes (MAO-A & MAO-B) have been given more attention as both the form have different therapeutic properties and hence can be used for different neurological disorders. The lack of selective and reversible inhibitors available for both the enzymes and severity of the neuronal disorder in society have opened a new door to the researchers to carry out large and dedicated researches in this field. Among the several classes of the molecule as the inhibitors, coumarins hold a rank as a potent scaffold with its ease of synthesis, high therapeutic potential, and reversibility in inhibiting MAOs. The current review is an update of the research in the field that covers the works during the last six years (2014-2020) with a major focus on the SAR of the coumarin derivatives including synthetic, natural, and hybrids of coumarins with FDA-approved drugs.

New Aspects of Monoamine Oxidase B Inhibitors: The Key Role of Halogens to Open the Golden Door

A large plethora of drugs and promising lead compounds contain halogens in their structures. The introduction of such moieties strongly modulates their physical-chemical features as well as pharmacokinetic and pharmacodynamic profile. The most important outcome was shown to be the ability of these halogens to favourably influence the drug-target interaction and energetic stability within the active site by the establishment of halogen bonds. This review attempted to demonstrate the key role exerted by these versatile moieties when correctly located in an organic scaffold to display Monoamine Oxidase (MAO) inhibition and selectivity towards the B isoform of this important enzyme. Human MAOs are well-recognized as therapeutic targets for mood disorders and neurodegenerative diseases and medicinal chemists were prompted to discover the structural requirements crucial to discriminate the slight differences between the active sits of the two isoforms (MAO-A and MAOB). The analysis of the structure-activity relationships of the most important scaffolds (hydrazothiazoles, coumarins, chromones, chalcones, pyrazolines) and the impact of halogen (F, Cl, Br and I) insertion on this biological activity and isozyme selectivity have been reported being a source of inspiration for the medicinal chemists.

4-Oxoquinolines and monoamine oxidase: When tautomerism matters

4-Oxoquinoline derivatives have been often used in drug discovery programs due to their pharmacological properties. Inspired on chromone and 4-oxoquinoline chemical structure similarity, a small series of quinoline-based compounds was obtained and screened, for the first time, toward human monoamine oxidases isoforms. The data showed the N-(3,4-dichlorophenyl)-1-methyl-4-oxo-1,4-dihydroquinoline-3-carboxamide 10 was the most potent and selective MAO-B inhibitor (IC₅₀ = 5.30 ± 0.74 nM and SI: ≥1887). The data analysis showed that prototropic tautomerism markedly influences the biological activity. The unequivocal characterisation of the quinoline tautomers was performed to understand the attained data. To our knowledge, there have been no prior reports on the characterisation of quinolone tautomers by 2D NMR techniques, namely by ¹H-¹⁵N HSQC and ¹H-¹⁵N HMBC, which are proposed as expedite tools for medicinal chemistry campaigns. Computational studies on enzyme-ligand complexes, obtained after MM-GBSA calculations and molecular dynamics simulations, supported the experimental data.

Accelerating the identification of subtype selective inhibitors via Three-Dimensional Biologically Relevant Spectrum (BRS-3D): The monoamine oxidase subtypes as a case study

Subtype-selective drugs are of great therapeutic importance as they are expected to be more effective and with less side-effects. However, discovery of subtype selective inhibitors was hampered by the high similarity of the binding sites within subfamilies. In this study, we further evaluated the applicability of "Three-Dimensional Biologically Relevant Spectrum (BRS-3D)" for the identification of subtype-selective inhibitors. A case study was performed on monoamine oxidase, which has two subtypes related to distinct diseases. The inhibitory activity against MAO-A/B of 347 compounds experimentally tested in this research was reported. Compound M124 (5H-thiazolo[3,2-a]pyrimidin-5-one) with IC₅₀ less than 100 nM (SI = 23) was selected as a probe to investigate the structure selectivity relationship. Similarity search led to the identification of compound M229 and M249 with IC₅₀ values of 7.4 nM, 4 nM and acceptable selectivity index over MAO-A (M229 SI > 1351, M249 SI > 2500). The molecular basis for subtype selectivity was explored through docking study and attention based DNN model. Additionally, in silico ADME properties were characterized. Accordingly, it is found that BRS-3D is a robust method for subtype selectivity in the early stage of drug discovery and the compounds reported here can be promising leads for further experimental analysis.

Computational Fragment-Based Design Facilitates Discovery of Potent and Selective Monoamine Oxidase-B (MAO-B) Inhibitor

Parkinson's disease (PD) is one of the most common age-related neurodegenerative diseases. Inhibition of monoamine oxidase-B (MAO-B), which is mainly found in the glial cells of the brain, may lead to an elevated level of dopamine (DA) in patients. MAO-B inhibitors have been used extensively for patients with PD. However, the discovery of the selective MAO-B inhibitor is still a challenge. In this study, a computational strategy was designed for the rapid discovery of selective MAO-B inhibitors. A series of (S)-2-(benzylamino)propanamide derivatives were designed. In vitro biological evaluations revealed that (S)-1-(4-((3-fluorobenzyl)oxy)benzyl)azetidine-2-carboxamide (C3) was more potent and selective than safinamide, a promising drug for regulating MAO-B. Further studies revealed that the selectivity mechanism of C3 was due to the steric clash caused by the residue difference of Phe208 (MAO-A) and Ile199 (MAO-B). Animal studies showed that compound C3 could inhibit cerebral MAO-B activity and alleviate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic neuronal loss.

An epigenetic modifier induces production of 3-(4-oxopyrano)-chromen-2-ones in Aspergillus sp. AST0006, an endophytic fungus of Astragalus lentiginosus

Incorporation of the epigenetic modifier suberoylanilide hydroxamic acid (SAHA) into a potato dextrose broth culture of the endophytic fungus Aspergillus sp. AST0006 affected its polyketide biosynthetic pathway providing two new 3-(4-oxopyrano)-chromen-2-ones, aspyranochromenones A (1) and B (2), and the isocoumarin, (-)-6,7-dihydroxymellein (3). Eight additional metabolites (4-11) and two biotransformation products of SAHA (12-13) were also encountered. The planar structures and relative configurations of the new metabolites 1-2 were elucidated with the help of high-resolution mass, 1D and 2D NMR spectroscopic data and the absolute configurations of 1-3 were determined by comparison of experimental and calculated ECD data. Possible biosynthetic pathways to 1 and 2 are presented.

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.

7-Amidocoumarins as Multitarget Agents against Neurodegenerative Diseases: Substitution Pattern Modulation

This study explores the potential of 7-amidocoumarins as multitarget agents against Parkinson's and Alzheimer's diseases, by modulating the substitution patterns within the scaffold. Sixteen compounds were synthesized via 7-amino-4-methylcoumarin acylation, and in vitro evaluation of the molecules against hMAO-A, hMAO-B, hAChE, hBuChE and hBACE1 was performed. Five compounds turned out to be potent and selective hMAO-B inhibitors in the nanomolar range, six displayed inhibitory activity of hMAO-A in the low micromolar range, one showed hAChE inhibitory activity and another one hBACE1 inhibitory activity. MAO-B reversibility profile of 7-(4'-chlorobenzamido)-4-methylcoumarin (10) was investigated, with this compound being a reversible inhibitor. Neurotoxicity on motor cortex neurons and neuroprotection against H₂ O₂ were also studied, corroborating the safety profile of these molecules. Finally, theoretical ADME properties were also calculated, showing these molecules as good candidates for the optimization of a lead compound. Results suggest that by modulating the substitution pattern at position 7 of the scaffold, selective or multitarget molecules can be achieved.

A Primary Amide-Functionalized Heterogeneous Catalyst for the Synthesis of Coumarin-3-carboxylic Acids via a Tandem Reaction

A crystalline primary amide-based bifunctional heterogeneous catalyst, {[Cd₂(2-BPXG)(Fum)₂(H₂O)₂]·2H₂O}_n (1) (where, 2-BPXG = 2,2'-((1,4-phenylenebis(methylene))bis((pyridin-2-ylmethyl)azanediyl)) diacetamide and Fum = fumarate), has been developed for the one-pot synthesis of a series of potentially biologically active coumarin-3-carboxylic acids at room temperature via a Knoevenagel-intramolecular cyclization tandem reaction. Catalyst 1 is prepared at room temperature from a one-pot self-assembly process in 81% yield and high purity within a few hours and has a ladder-like polymeric architecture based on single-crystal X-ray diffraction. Additional characterization of 1 includes elemental analysis, infrared spectroscopy, thermogravimetric analysis, and powder X-ray diffraction. Based on the optimized conditions, it is determined that 1 is highly efficient (conditions: 2 mol % catalyst, 3 h, and 26-28 °C in methanol) for this reaction. Its recyclability up to five cycles without significant loss of activity and structural integrity is also demonstrated. Using both electron-donating and electron-withdrawing substituents on the salicylaldehyde substrate, seven different derivatives of coumarin-3-carboxylic acid were made. Additionally, the monoamine oxidase (MAO) inhibitor, coumarin-3-phenylcarboxamide, has also been synthesized from coumarin-3-carboxylic acid obtained in the catalysis process. A detailed mechanism of action is also provided.

Synthesis, in vitro enzyme activity and molecular docking studies of new benzylamine-sulfonamide derivatives as selective MAO-B inhibitors

Many studies have been conducted on the selective inhibition of human monoamine oxidase B (hMAO-B) enzyme using benzylamine-sulphonamide derivatives. Using various chemical modifications on BB-4h, which was reported previously by our team and showed a significant level of MAO-B inhibition, novel benzylamine-sulphonamide derivatives were designed, synthesised, and their MAO inhibition potentials were evaluated. Among the tested derivatives, compounds 4i and 4t achieved IC₅₀ values of 0.041 ± 0.001 µM and 0.065 ± 0.002 µM, respectively. The mechanism of hMAO-B inhibition by compounds 4i and 4t was studied using Lineweaver–Burk plot. The nature of inhibition was also determined to be non-competitive. Cytotoxicity tests were conducted and compounds 4i and 4t were found to be non-toxic. Molecular docking studies were also carried out for compound 4i, which was found as the most potent agent, within hMAO-B catalytic site.

Rutamarin: Efficient Liquid-Liquid Chromatographic Isolation from Ruta graveolens L. and Evaluation of Its In Vitro and In Silico MAO-B Inhibitory Activity

Naturally occurring coumarins are a group of compounds with many documented central nervous system (CNS) activities. However, dihydrofuranocoumarins have been infrequently investigated for their bioactivities at CNS level. Within the frame of this study, an efficient liquid–liquid chromatography method was developed to rapidly isolate rutamarin from Ruta graveolens L. (Rutaceae) dichloromethane extract (DCM). The crude DCM (9.78 mg/mL) and rutamarin (6.17 µM) were found to be effective inhibitors of human monoamine oxidase B (hMAO-B) with inhibition percentages of 89.98% and 95.26%, respectively. The inhibitory activity against human monoamine oxidase A (hMAO-A) for the DCM extract was almost the same (88.22%). However, for rutamarin, it significantly dropped to 25.15%. To examine the molecular interaction of rutamarin with hMAO- B, an in silico evaluation was implemented. A docking study was performed for the two enantiomers (R)-rutamarin and (S)-rutamarin. The (S)-rutamarin was found to bind stronger to the hMAO-B binging cavity.

Discovery of coumarin derivatives as potent and selective cyclin-dependent kinase 9 (CDK9) inhibitors with high antitumour activity

Specific inhibition of CDK9 is considered a promising strategy for developing effective anticancer therapeutics. However, most of the reported CDK9 inhibitors are still at an early stage of development and lack selectivity against other CDKs. Herein, we discovered coumarin derivative 30i as a potent CDK9 inhibitor with high selectivity (8300-fold over CDK7). Binding mode analysis illustrated that the substituent coumarin moiety is a critical group for CDK9 selectivity by occupying a flexible hinge/αD region, which is sterically hindered in other CDKs. Compound 30i showed excellent cellular antiproliferative activity, moderate pharmacokinetic property and low hERG inhibition. Moreover, 30i significantly induced tumour growth inhibition in a dose-dependent manner without causing an obvious loss of body weight in an MV4-11 xenograft mice model. Altogether, these results suggest that 30i may serve as a potential acute myeloid leukaemia (AML) therapeutics by selectively targeting CDK9.

Rational design, synthesis and biological evaluation of novel multitargeting anti-AD iron chelators with potent MAO-B inhibitory and antioxidant activity

A series of (3-hydroxypyridin-4-one)-coumarin hybrids were developed and investigated as potential multitargeting candidates for the treatment of Alzheimer's disease (AD) through the incorporation of iron-chelating and monoamine oxidase B (MAO-B) inhibition. This combination endowed the hybrids with good capacity to inhibit MAO-B as well as excellent iron-chelating effects. The pFe³⁺ values of the compounds were ranging from 16.91 to 20.16, comparable to more potent than the reference drug deferiprone (DFP). Among them, compound 18d exhibited the most promising activity against MAO-B, with an IC₅₀ value of 87.9 nM. Moreover, compound 18d exerted favorable antioxidant activity, significantly reversed the amyloid-β_1-42 (Aβ_1-42) induced PC12 cell damage. More importantly, 18d remarkably ameliorated the cognitive dysfunction in a scopolamine-induced mice AD model. In brief, a series of hybrids with potential anti-AD effect were successfully obtained, indicating that the design of iron chelators with MAO-B inhibitory and antioxidant activities is an attractive strategy against AD progression.

A sustainable innovation for the tandem synthesis of sugar-containing coumarin derivatives catalyzed by lipozyme TL IM from Thermomyces lanuginosus in continuous-flow microreactors

We developed an efficient and environmentally friendly two-step tandem methodology for the synthesis of sugar-containing coumarin derivatives catalyzed by lipozyme TL IM from Thermomyces lanuginosus in continuous-flow microreactors. Compared to those observed for other methods, the salient features of this work including green reaction conditions, short residence time (50 min), and catalysts are more readily available and the biocatalysis reaction process is efficient and easy to control. This two-step tandem synthesis of coumarin derivatives using the continuous-flow technology is a proof of concept that opens the use of enzymatic microreactors in coumarin derivative biotransformations.

An effective and environmentally friendly two-step tandem protocol for the synthesis of sugar-containing coumarin derivatives catalyzed by lipozyme TL IM in continuous-flow microreactors has been developed.

Antiproliferative chromone derivatives induce K562 cell death through endogenous and exogenous pathways

A series of furoxan derivatives of chromone were prepared. The antiproliferative activities were tested against five cancer cell lines HepG2, MCF-7, HCT-116, B16, and K562, and two normal human cell lines L-02 and PBMCs. Among them, compound 15a exhibited the most potent antiproliferative activity. It was also found 15a produced more than 8 µM of NO at the peak time of 45 min by Griess assay. Generally, antiproliferative activity is positively related to NO release to some extent. Further in-depth studies on apoptosis-related mechanisms showed that 15a caused S-phase cell cycle arrest in a concentration-dependent manner and induced apoptosis significantly through mitochondria-related pathways. Human apoptosis protein array assay also demonstrated 15a increased the expression levels of pro-apoptotic Bax, Bad, HtrA2 and Trail R2/DR5. The expression of catalase and cell cycle blocker claspin were similarly up-regulated. In balance, 15a induced K562 cells death through both endogenous and exogenous pathways.

Chromenones as Multineurotargeting Inhibitors of Human Enzymes

The complex nature of multifactorial diseases, such as Morbus Alzheimer, has produced a strong need to design multitarget-directed ligands to address the involved complementary pathways. We performed a purposive structural modification of a tetratarget small-molecule, that is contilisant, and generated a combinatorial library of 28 substituted chromen-4-ones. The compounds comprise a basic moiety which is linker-connected to the 6-position of the heterocyclic chromenone core. The syntheses were accomplished by Mitsunobu- or Williamson-type ether formations. The resulting library members were evaluated at a panel of seven human enzymes, all of which being involved in the pathophysiology of neurodegeneration. A concomitant inhibition of human acetylcholinesterase and human monoamine oxidase B, with IC₅₀ values of 5.58 and 7.20 μM, respectively, was achieved with the dual-target 6-(4-(piperidin-1-yl)butoxy)-4H-chromen-4-one (7).

Novel C7-Substituted Coumarins as Selective Monoamine Oxidase Inhibitors: Discovery, Synthesis and Theoretical Simulation

There is a continued need to develop new selective human monoamine oxidase (hMAO) inhibitors that could be beneficial for the treatment of neurological diseases. However, hMAOs are closely related with high sequence identity and structural similarity, which hinders the development of selective MAO inhibitors. “Three-Dimensional Biologically Relevant Spectrum (BRS-3D)” method developed by our group has demonstrated its effectiveness in subtype selectivity studies of receptor and enzyme ligands. Here, we report a series of novel C7-substituted coumarins, either synthesized or commercially purchased, which were identified as selective hMAO inhibitors. Most of the compounds demonstrated strong activities with IC₅₀ values (half-inhibitory concentration) ranging from sub-micromolar to nanomolar. Compounds, FR1 and SP1, were identified as the most selective hMAO-A inhibitors, with IC₅₀ values of 1.5 nM (selectivity index (SI) < −2.82) and 19 nM (SI < −2.42), respectively. FR4 and FR5 showed the most potent hMAO-B inhibitory activity, with IC₅₀ of 18 nM and 15 nM (SI > 2.74 and SI > 2.82). Docking calculations and molecular dynamic simulations were performed to elucidate the selectivity preference and SAR profiles.

Critical evaluation of current Alzheimer's drug discovery (2018-19) & futuristic Alzheimer drug model approach

Alzheimer's disease (AD), a neurodegenerative disease responsible for death of millions of people worldwide is a progressive clinical disorder which causes neurons to degenerate and ultimately die. It is one of the common causes of dementia wherein a person's incapability to independently think, behave and decline in social skills can be quoted as major symptoms. However the early signs include the simple non-clinical symptoms such as forgetting recent events and conversations. Onset of these symptoms leads to worsened conditions wherein the AD patient suffers severe memory impairment and eventually becomes unable to work out everyday tasks. Even though there is no complete cure for AD, rigorous research has been going on to reduce the progress of AD. Currently, a very few clinical drugs are prevailing for AD treatment. So this is the need of hour to design, develop and discovery of novel anti-AD drugs. The main factors for the cause of AD according to scientific research reveals structural changes in brain proteins such as beta amyloid, tau proteins into plaques and tangles respectively. The abnormal proteins distort the neurons. Despite the high potencies of the synthesized molecules; they could not get on the clinical tests up to human usage. In this review article, the recent research carried out with respect to inhibition of AChE, BuChE, NO, BACE1, MAOs, Aβ, H3R, DAPK, CSF1R, 5-HT4R, PDE, σ₁R and GSK-3β is compiled and organized. The summary is focused mainly on cholinesterases, Aβ, BACE1 and MAOs classes of potential inhibitors. The review also covers structure activity relationship of most potent compounds of each class of inhibitors alongside redesign and remodeling of the most significant inhibitors in order to expect cutting edge inhibitory properties towards AD. Alongside the molecular docking studies of the some final compounds are discussed.

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.