Recent Advancements and SAR Studies of Synthetic Coumarins as MAO-B Inhibitors: An Updated Review

BACKGROUND

The oxidative deamination of a wide range of endogenous and exogenous amines is catalyzed by a family of enzymes known as monoamine oxidases (MAOs), which are reliant on flavin-adenine dinucleotides. Numerous neurological conditions, such as Parkinson's disease (PD) and Alzheimer's disease (AD), are significantly correlated with changes in the amounts of biogenic amines in the brain caused by MAO. Hydrogen peroxide, reactive oxygen species, and ammonia, among other toxic consequences of this oxidative breakdown, can harm brain cells' mitochondria and cause oxidative damage.

OBJECTIVE

The prime objective of this review article was to highlight and conclude the recent advancements in structure-activity relationships of synthetic derivatives of coumarins for MAO-B inhibition, published in the last five years' research articles.

METHODS

The literature (between 2019 and 2023) was searched from platforms like Science Direct, Google Scholar, PubMed, etc. After going through the literature, we have found a number of coumarin derivatives being synthesized by researchers for the inhibition of MAO-B for the management of diseases associated with the enzyme such as Alzheimer's Disease and Parkinson's Disease. The effect of these coumarin derivatives on the enzyme depends on the substitutions associated with the structure. The structure-activity relationships of the synthetic coumarin derivatives that are popular nowadays have been described and summarized in the current study.

RESULTS

The results revealed the updated review on SAR studies of synthetic coumarins as MAO-B inhibitors, specifically for Alzheimer's Disease and Parkinson's Disease. The patents reported on coumarin derivatives as MAO-B inhibitors were also highlighted.

CONCLUSION

Recently, coumarins, a large class of chemicals with both natural and synthetic sources, have drawn a lot of attention because of the vast range of biological actions they have that are linked to neurological problems. Numerous studies have demonstrated that chemically produced and naturally occurring coumarin analogs both exhibited strong MAO-B inhibitory action. Coumarins bind to MAO-B reversibly thereby preventing the breakdown of neurotransmitters like dopamine leading to the inhibition of the enzyme A number of MAO-B blockers have been proven to be efficient therapies for treating neurological diseases like Alzheimer's Disease and Parkinson's Disease. To combat these illnesses, there is still an urgent need to find effective treatment compounds.

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 Chromanone Derivatives as Multifunctional Agents for the Treatment of Alzheimer's Disease

Based on a multitarget strategy, a series of novel chromanone-1-benzyl-1,2,3,6-tetrahydropyridin hybrids were identified for the potential treatment of Alzheimer's disease (AD). Biological evaluation demonstrated that these hybrids exhibited significant inhibitory activities toward acetylcholinesterase (AChE) and monoamine oxidase B (MAO-B). The optimal compound C10 possessed excellent dual AChE/MAO-B inhibition both in terms of potency and equilibrium (AChE: IC₅₀ = 0.58 ± 0.05 μM; MAO-B: IC₅₀ = 0.41 ± 0.04 μM). Further molecular modeling and kinetic investigations revealed that compound C10 was a dual-binding inhibitor bound to both the catalytic anionic site and peripheral anionic site of AChE. In addition, compound C10 exhibited low neurotoxicity and potently inhibited AChE enzymatic activity. Furthermore, compound C10 more effectively protected against mitochondrial dysfunction and oxidation than donepezil, strongly inhibited AChE-induced amyloid aggregation, and moderately reduced glutaraldehyde-induced phosphorylation of tau protein in SH-SY5Y cells. Moreover, compound C10 displayed largely enhanced improvements in cognitive behaviors and spatial memory in a scopolamine-induced AD mice model with better efficacy than donepezil. Overall, the multifunctional profiles of compound C10 suggest that it deserves further investigation as a promising lead for the prospective treatment of AD.

Evaluation of chromane derivatives: Promising privileged scaffolds for lead discovery within Alzheimer's disease

The chromane ring system is widely distributed in nature and has proven to be a highly potent pharmacophore in medicinal chemistry, which includes the area of Alzheimer's and Parkinson's diseases. We report on the development of a gem-dimethylchroman-4-ol family that was shown to give good inhibition of equine serum butyrylcholinesterase (eqBuChE) (in the range 2.9 - 7.3 μM) and in the same range of currently used drugs. We also synthesized a small library of gem-dimethylchroman-4-amine compounds, via a simple reductive amination of the corresponding chromanone precursor, that were also selective for eqBuChE presenting inhibitions in the range 7.6 - 67 μM. Kinetic studies revealed that they were mixed inhibitors. Insights into their mechanism of action were obtained through molecular docking and STD-NMR experiments, and the most active examples showed excellent drug-likeness and pharmacological properties predicted using Swiss-ADME. We also prepared a set of propargyl gem-dimethylchromanamines, for monoamine oxidase (MAO) inhibition but they were only moderately active (the best being 28% inhibition at 1 µM on MAO-B). Overall, our compounds were found to be best suited as inhibitors for BuChE.

Facile access to chiral chromanone-2-carboxylic acids enabled by Rhodium-catalyzed chemo- and enantioselective hydrogenation

Rh-catalyzed highly chemo- and enantioselective hydrogenation of chromone-2-carboxylic acids was first successfully established, providing a wide range of enantiopure chromanone-2-carboxylic acids with excellent results (up to 97% yield and 99%...

Chromanone-A Prerogative Therapeutic Scaffold: An Overview

Chromanone or Chroman-4-one is the most important and interesting heterobicyclic compound and acts as a building block in medicinal chemistry for isolation, designing and synthesis of novel lead compounds. Structurally, absence of a double bond in chromanone between C-2 and C-3 shows a minor difference from chromone but exhibits significant variations in biological activities. In the present review, various studies published on synthesis, pharmacological evaluation on chroman-4-one analogues are addressed to signify the importance of chromanone as a versatile scaffold exhibiting a wide range of pharmacological activities. But, due to poor yield in the case of chemical synthesis and expensive isolation procedure from natural compounds, more studies are required to provide the most effective and cost-effective methods to synthesize novel chromanone analogs to give leads to chemistry community. Considering the versatility of chromanone, this review is designed to impart comprehensive, critical and authoritative information about chromanone template in drug designing and development.

Synthesis of 3,3-Dihalogenated 2-Aminochromanones via Tandem Dihalogenation and Cyclization of o-Hydroxyarylenaminones with NXS (X = Cl or Br)

An unprecedented method for the synthesis of dichlorinated and dibrominated 2-amino-substituted chromanones is developed by employing enaminones and NCS/NBS as starting materials under microwave irradiation. The reactions proceed quickly to deliver products without using any catalyst or additive, thus providing practical access to 3,3-dihalogenated 2-aminochromanones.

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.

Exploration of 7-azaindole-coumaranone hybrids and their analogues as protein kinase inhibitors

7-Azaindole has been labelled a privileged scaffold for the design of new potent inhibitors of protein kinases. In this paper, we determined the inhibition profiles of novel mono- and disubstituted derivatives of 7-azaindole-coumaranone hybrids on various disease-related protein kinases. Eight hit compounds were identified, including a potent Haspin inhibitor with an IC₅₀ value of 0.15 μM. An interesting observation was that all active monosubstituted compounds displayed dual inhibition for Haspin and GSK-3β, while disubstituted derivatives inhibited GSK-3β and LmCK1 from Leishmania major parasite. Analyses of structure activity relationships (SARs) also revealed that mono-substitution with para-fluorobenzyloxy ring produced an equipotent inhibition of Haspin and GSK-3β. Haspin and GSK-3β are relevant targets for developing new anticancer agents while LmCK1 is an innovative target for leishmanicidal drugs. Novel compounds reported in this paper constitute promising starting points for the development of new anticancer and leishmanicidal drugs.

Design and synthesis of novel 3,4-dihydrocoumarins as potent and selective monoamine oxidase-B inhibitors with the neuroprotection against Parkinson's disease

The monoamine oxidase-B (MAO-B) inhibitors with neuroprotective effects are better for Parkinson's disease (PD) treatment, due to the complicated pathogenesis of PD. To develop new hMAO-B inhibitors with neuroprotection, a novel series of 3,4-dihydrocoumarins was designed as selective and reversible hMAO-B inhibitors to treat PD. Most compounds showed potent and selective inhibition for hMAO-B over hMAO-A with IC₅₀ values ranging from nanomolar to sub-nanomolar. Among them, compound 4d was the most potent hMAO-B inhibitor (IC₅₀ = 0.37 nM) being about 20783-fold more active than iproniazid, and exhibited the highest selectivity for hMAO-B (SI > 270,270). Kinetic studies revealed that compound 4d was a reversible and competitive inhibitor of hMAO-B. Neuroprotective studies indicated that compound 4d could protect PC12 cells from the damage induced by 6-OHDA and rotenone. Besides, compound 4d did not exhibit acute toxicity at a dose up to 2500 mg/kg (po), and could cross the BBB in parallel artificial membrane permeability assay. More importantly, compound 4d was able to significantly prevent the motor deficits in the MPTP-induced PD model. These results indicate that compound 4d is an effective and promising candidate against PD.

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.

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

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

A review on flavonoid-based scaffolds as multi-target-directed ligands (MTDLs) for Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is a multifactorial neurodegenerative disease. The target enzymes inhibition including cholinesterase, beta-secretase, monoamine oxidase and inhibition of amyloid-β aggregation as well as oxidative stress and metal chelation play an important role in the pathogenesis of AD. Chroman-4-one scaffold with benzo-γ-pyrone network is a privileged structure in organic synthesis and drug design. A large number of research has been carried out on modified naturally occurring chromanone scaffolds and/or synthesized new analogues, to obtain effective drugs for AD management. The present review summarizes aspects related to the multi-target-directed ligands (MTDLs) strategy in enzyme targets modulation performed with natural and synthesized chroman-4-one-based structures to look at their potential in the management of multifactorial Alzheimer's disease.

Synthesis and evaluation of small molecules bearing a benzyloxy substituent as novel and potent monoamine oxidase inhibitors

A new series of small molecules bearing a benzyloxy substituent have been designed, synthesized and evaluated for hMAO inhibitory activity in vitro. Most of the compounds were potent and selective MAO-B inhibitors, and were weak inhibitors of MAO-A. In particular, compounds 9e (IC₅₀ = 0.35 μM) and 10e (IC₅₀ = 0.19 μM) were the most potent MAO-B inhibitors, and exhibited the highest selectivity for MAO-B (9e, SI > 285.7-fold and 10e, SI = 146.8-fold). In addition, the structure-activity relationships for MAO-B inhibition indicated that electron-withdrawing groups in the open small molecules were more suitable for MAO-B inhibition, and substitutions at the benzyloxy of the open small molecules, particularly with the halogen substituted benzyloxy, were more favorable for MAO-B inhibition. Molecular docking studies have been done to explain the potent MAO-B inhibition of the open small molecules. Furthermore, the representative compounds 9e and 10e showed low neurotoxicity in SH-SY5Y cells in vitro. So the small molecules bearing the benzyloxy substituent could be used to develop promising drug candidates for the therapy of neurodegenerative diseases.

Synthesis and evaluation of 6-substituted 3-arylcoumarin derivatives as multifunctional acetylcholinesterase/monoamine oxidase B dual inhibitors for the treatment of Alzheimer’s disease

Compounds5oand5pwere both multifunctional hAChE/hMAO-B dual inhibitors for the treatment of AD.