Chromenone: An emerging scaffold in anti-Alzheimer drug discovery

Alzheimer's disease (AD) presents a growing global health concern. In recent decades, natural and synthetic chromenone have emerged as promising drug candidates due to their multi-target potential. Natural chromenone, quercetin, scopoletin, esculetin, coumestrol, umbelliferone, bergapten, and methoxsalen (xanthotoxin), and synthetic chromenone hybrids comprising structures like acridine, 4-aminophenyl, 3-arylcoumarins, quinoline, 1,3,4-oxadiazole, 1,2,3-triazole, and tacrine, have been explored for their potential to combat AD. Key reactions used for synthesis of chromenone hybrids include Perkin and Pechmann condensation. The activity of chromenone hybrids has been reported against several drug targets, including AChE, BuChE, BACE-1, and MAO-A/B. This review comprehensively explores natural, semisynthetic, and synthetic chromenone, elucidating their synthetic routes, possible mode of action/drug targets and structure-activity relationships (SAR). The acquired knowledge provides valuable insights for the development of new chromenone hybrids against AD.

Propargylamine: an important moiety in drug discovery

Propargylamine is a chemical moiety whose properties have made it a widely distributed group within the fields of medicinal chemistry and chemical biology. Its particular reactivity has traditionally popularized the preparation of propargylamine derivatives using a large variety of synthetic strategies, which have facilitated the access to these compounds for the study of their biomedical potential. This review comprehensively covers and analyzes the applications that propargylamine-based derivatives have achieved in the drug discovery field, both from a medicinal chemistry perspective and from a chemical biology-oriented approach. The principal therapeutic fields where propargylamine-based compounds have made an impact are identified, and a discussion of their influence and growing potential is included.

Hydroxy-3-Phenylcoumarins as Multitarget Compounds for Skin Aging Diseases: Synthesis, Molecular Docking and Tyrosinase, Elastase, Collagenase and Hyaluronidase Inhibition, and Sun Protection Factor

Skin aging is a progressive biological process of the human body, and it is not only time-dependent. Differently substituted 3-phenylcoumarins proved to efficiently inhibit tyrosinase. In the current work, new substitution patterns have been explored, and the biological studies were extended to other important enzymes involved in the processes of skin aging, as elastase, collagenase and hyaluronidase. From the studied series, five compounds presented inhibitory activity against tyrosinase, one compound against elastase, eight compounds against collagenase and two compounds against hyaluronidase, being five compounds dual inhibitors. The 3-(4'-Bromophenyl)-5,7-dihydroxycoumarin (1) and 3-(3'-bromophenyl)-5,7-dihydroxycoumarin (2) presented the best profiles against tyrosinase (IC₅₀ = 1.05 µM and 7.03 µM) and collagenase (IC₅₀ = 123.4 µM and 110.4 µM); the 3-(4'-bromophenyl)-6,7-dihydroxycoumarin (4) presented a good inhibition against tyrosinase and hyaluronidase; the 3-(3'-bromophenyl)-6,7-dihydroxycoumarin (5) showed an effective tyrosinase and elastase inhibition; and 6,7-dihydroxy-3-(3'-hydroxyphenyl)coumarin (11) presented a dual profile inhibition against collagenase and hyaluronidase. Furthermore, considering the overall activities tested, compounds 1 and 2 proved to be the most promising anti-aging compounds. These compounds also showed to have a photo-protective effect, without being cytotoxic to human skin keratinocyte cells. To predict the binding site with the target enzymes, computational studies were also carried out.

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.

Multitarget therapeutic approaches for Alzheimer's and Parkinson's diseases: an opportunity or an illusion?

Alzheimer's and Parkinson's disease are the most prevalent neurodegenerative diseases and the leading causes of dementia worldwide. The etiology of these multifactorial pathologies is not completely known. The available therapeutic approaches can cause temporary relief of symptoms but cannot slow down their progression or cure them. Life-changing therapeutic solutions are urgently needed, as the number of people suffering from these pathologies has been increasing quickly over the last few decades. Several targets are being studied, and innovative approaches are being pursued to find new therapeutic options. This overview is focused on the most recent information regarding the paradigm of using multitarget compounds to treat both Alzheimer's and Parkinson's disease.

Highly Significant Scaffolds to Design and Synthesis Cholinesterase Inhibitors as Anti-Alzheimer Agents

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Alzheimer, a progressive disease, is a common term for memory loss which interferes with daily life through severe influence on cognitive abilities. Based on the cholinergic hypothesis, and Xray crystallographic determination of the structure of acetylcholinesterase (AChE) enzyme, the level of acetylcholine (ACh, an important neurotransmitter associated with memory) in the hippocampus and cortex area of the brain has a direct effect on Alzheimer. This fact encourages scientists to design and synthesize a wide range of acetylcholinesterase inhibitors (AChEIs) to control the level of ACh in the brain, keeping in view the crystallographic structure of AChE enzyme and drugs approved by the Food and Drug Administration (FDA).

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AChEIs have slightly diverse pharmacological properties, but all of them work by inhibiting the segregation of ACh by blocking AChE. We reviewed significant scaffolds introduced as AChEIs. In some studies, the activity against butyrylcholinesterase (BuChE) has been evaluated as well because BuChE is a similar enzyme to neuronal acetylcholinesterase and is capable of hydrolyzing ACh. In order to study AChEIs effectively, we divided them structurally into 12 classes and briefly explained effective AChEIs and compared their activities against AChE enzyme.

Recent Advances in Multi-target Anti-Alzheimer Disease Compounds (2013 Up to the Present)

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Since the last century, when scientists proposed the lock-and-key model, the discovery of drugs has focused on the development of drugs acting on single target. However, single-target drug therapies are not effective to complex diseases with multi-factorial pathogenesis. Moreover, the combination of single-target drugs readily causes drug resistance and side effects. In recent years, multi-target drugs have increasingly been represented among FDA-approved drugs. Alzheimer’s Disease (AD) is a complex and multi-factorial disease for which the precise molecular mechanisms are still not fully understood. In recent years, rational multi-target drug design methods, which combine the pharmacophores of multiple drugs, have been increasingly applied in the development of anti-AD drugs. In this review, we give a brief description of the pathogenesis of AD and provide detailed discussions about the recent development of chemical structures of anti-AD agents (2013 up to present) that have multiple targets, such as amyloid-β peptide, Tau protein, cholinesterases, monoamine oxidase, β-site amyloid-precursor protein-cleaving enzyme 1, free radicals, metal ions (Fe2+, Cu2+, Zn2+) and so on. In this paper, we also added some novel targets or possible pathogenesis which have been reported in recent years for AD therapy. We hope that these findings may provide new perspectives for the pharmacological treatment of AD.

Emerging therapeutic potentials of dual-acting MAO and AChE inhibitors in Alzheimer's and Parkinson's diseases

No drug has been approved to prevent neuronal cell loss in patients suffering from Parkinson's disease (PD) or Alzheimer's disease (AD); despite increased comprehension of the underlying molecular causes, therapies target cognitive functional improvement and motor fluctuation control. Drug design strategies that adopt the "one protein, one target" philosophy fail to address the multifactorial aetiologies of neurodegenerative disorders such as AD and PD optimally. On the contrary, restoring neurotransmitter levels by combined combinatorial inhibition of cholinesterases, monoamine oxidases, and adenosine A_2A A receptors, in conjunction with strategies to counter oxidative stress and beta-amyloid plaque accumulation, would constitute a therapeutically robust, multitarget approach. This extensive review delineates the therapeutic advantages of combining dual-acting molecules that inhibit monoamine oxidases and cholinesterases and/or adenosine A_2A A receptors, and describes the structure-activity relationships of compound classes that include, but are not limited to, alkaloids, coumarins, chalcones, donepezil-propargylamine conjugates, homoisoflavonoids, resveratrol analogs, hydrazones, and pyrazolines. In the wake of recent advances in network biology, in silico approaches, and omics, this review emphasizes the need to consider conceptually informed research strategies for drug discovery, in the context of the mounting burden posed by chronic neurodegenerative diseases with complex aetiologies and pathophysiologies involving multiple signalling pathways and numerous drug targets.

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.

Synthesis and biological evaluation of 3-arylcoumarins as potential anti-Alzheimer's disease agents

Alzheimer's disease, a neurodegenerative illness, has the extremely complex pathogenesis. Accumulating evidence indicates there is a close relationship between several enzymes and Alzheimer's disease. Various substituted 3-arylcoumarin derivatives were synthesised, and their in vitro activity, including cholinesterase inhibitory activity, monoamine oxidase inhibitory activity, and antioxidant activity were investigated. Most of the compounds exhibited high activity; therefore 3-arylcoumarin compounds have the potential as drug candidates for the treatment of Alzheimer's disease.

Crystal structures, Hirsfeld surface analysis and a computational study of four ethyl 2-oxo-2H-chromene-3-carboxylate derivatives: a survey of organyl 2-oxo-2H-chromene-3-carboxylate structures

Crystal structures, Hirshfeld surface analysis and a computational study have been carried out on 2-oxo-2H-chromene-3-carboxylates. Crystal structures are reported for ethyl R-2-oxo-2H-chromene-3-carboxylate derivatives, 2a: R=6-Me, 2b: 7-Me, 2c: 7-Me, 2d: R=7-MeO. In contrast to 2-oxo-2H-chromene-3-carboxamides, 1, in which classical intramolecular N–H···O hydrogen bonds stabilize planar structures and hinder rotation of the amido group out of the coumarin plane in 2, without an equivalent hydrogen bond, there is a greater rotational freedom of the carboxylate group. The interplanar angles between the coumarin core and its attached –C(O)–R substituent in crystalline 2a, 2b, 2c and 2d are 10.41(6), 36.65(6), 10.4(2) and 5.64(6)°, respectively, with distances between the carbonyl oxygen atoms of 2.8255(16), 2.9278(16), 4.226(2) and 2.8328(14) Å, respectively. A theoretical study of molecular conformations was carried out at the M06-2X density level with the 6-31+G(d) and aug-cc-pVTZ basis sets, in methanol solution modeled by PCM, indicated that the most stable conformations had the carbonyl group of the ester in the plane of the coumarin core: the s-cis arrangement of the ester carbonyl and the 2-oxo moieties being the slightly more stable than the s-trans form by less than 0.5 kcal/mol. The experimental conformations of 2a and 2d match well the low energy s-cis arrangement, and 2c matches the slightly lesser stable s-trans arrangement found in the theoretical study. A survey of the molecular conformations of more than 50 2H-chromene-3-carboxylates derivatives in the CCDC data base indicated two distinct groupings of conformations, s-cis and s-trans, each with interplanar angles <30°.

MAO inhibitory activity of bromo-2-phenylbenzofurans: synthesis, in vitro study, and docking calculations

Monoamine oxidase (MAO) is an enzyme responsible for metabolism of monoamine neurotransmitters which play an important role in brain development and function. This enzyme exists in two isoforms, and it has been demonstrated that MAO-B activity, but not MAO-A activity, increases with aging. MAO inhibitors show clinical value because besides the monoamine level regulation they reduce the formation of by-products of the MAO catalytic cycle, which are toxic to the brain. A series of 2-phenylbenzofuran derivatives was designed, synthesized and evaluated against hMAO-A and hMAO-B enzymes. A bromine substituent was introduced in the 2-phenyl ring, whereas position 5 or 7 of the benzofuran moiety was substituted with a methyl group. Most of the tested compounds inhibited preferentially MAO-B in a reversible manner, with IC50 values in the low micro or nanomolar range. The 2-(2'-bromophenyl)-5-methylbenzofuran (5) was the most active compound identified (IC50 = 0.20 μM). In addition, none of the studied compounds showed cytotoxic activity against the human neuroblastoma cell line SH-SY5Y. Molecular docking simulations were used to explain the observed hMAO-B structure-activity relationship for this type of compounds.

Synthesis and structure-activity relationship study of novel 3-heteroarylcoumarins based on pyridazine scaffold as selective MAO-B inhibitors

Compounds of hybrid structure pyridazine-coumarin were discovered as potent, selective and reversible inhibitors of monoamine oxidase B (MAO-B). These compounds were synthesized in good yield following a multistep approach based on Knoevenagel reaction and using as key intermediate pyridazinone 16, which was obtained from maleic anhydride and furan. Compounds 9b and 9d are the most active compounds of these series, with IC₅₀ values in the sub-micromolar range, and lack of cytotoxic effects. Theoretical calculation of ADME properties also suggested a good pharmacokinetic profile for both compounds. Docking simulations provided insights into enzyme inhibitor interactions and allowed us to rationalize the observed structure-activity relationships (SARs).

Dual inhibitors of cholinesterases and monoamine oxidases for Alzheimer’s disease

Accumulating evidence indicates a solid relationship between several enzymes and Alzheimer’s disease. Cholinesterases and monoamine oxidases are closely associated with the disease symptomatology and progression and have been tackled simultaneously using several multifunctional ligands. This design strategy offers great chances to alter the course of Alzheimer’s disease, in addition to alleviation of the symptoms. More than 15 years of research has led to the identification of various dual cholinesterase/monoamine oxidase inhibitors, while some showing positive outcomes in clinical trials, thus giving rise to additional research efforts in the field. The aim of this review is to provide an update on the novel dual inhibitors identified recently and to shed light on their therapeutic potential.

Computational Drug Target Screening through Protein Interaction Profiles

The development of computational methods to discover novel drug-target interactions on a large scale is of great interest. We propose a new method for virtual screening based on protein interaction profile similarity to discover new targets for molecules, including existing drugs. We calculated Target Interaction Profile Fingerprints (TIPFs) based on ChEMBL database to evaluate drug similarity and generated new putative compound-target candidates from the non-intersecting targets in each pair of compounds. A set of drugs was further studied in monoamine oxidase B (MAO-B) and cyclooxygenase-1 (COX-1) enzyme through molecular docking and experimental assays. The drug ethoxzolamide and the natural compound piperlongumine, present in Piper longum L, showed hMAO-B activity with IC₅₀ values of 25 and 65 μM respectively. Five candidates, including lapatinib, SB-202190, RO-316233, GW786460X and indirubin-3′-monoxime were tested against human COX-1. Compounds SB-202190 and RO-316233 showed a IC₅₀ in hCOX-1 of 24 and 25 μM respectively (similar range as potent inhibitors such as diclofenac and indomethacin in the same experimental conditions). Lapatinib and indirubin-3′-monoxime showed moderate hCOX-1 activity (19.5% and 28% of enzyme inhibition at 25 μM respectively). Our modeling constitutes a multi-target predictor for large scale virtual screening with potential in lead discovery, repositioning and drug safety.

A review on tacrine-based scaffolds as multi-target drugs (MTDLs) for Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial neurological disorder among elderly people and combinatorial factors such as genetic, lifestyle, and environmental are involved in onset and disease progression. It has been demonstrated that loss of cholinergic transmission is one of the most significant causes of AD. One strategy currently being investigated for the development of new therapeutics relates to the enhancement of cholinergic system through several ways. At this juncture, anticholinesterase inhibitors have absorbed lots of attention and different marketed drugs such as donepezil, rivastigmine, tacrine, and galantamine have been developed. 9-Amino-1,2,3,4-tetrahydroacridine known as tacrine was introduced in 1945 as an efficient anticholinesterase agent. The mechanism of action of tacrine was proved to inhibit the metabolism of acetylcholine and therefore extending its activity and raising levels in the cerebral cortex. However, extensive use of tacrine was limited since it showed various side effects and toxicity. Thus, lots of efforts were carried out to prepare tacrine analogues to overcome the related adverse effects. This review describes differently synthesized tacrine-based scaffolds as cholinesterase inhibitors to manage Alzheimer's disease (AD).

Crystal structures of three 6-substituted coumarin-3-carboxamide derivatives

Three coumarin derivatives, viz. 6-methyl-N-(3-methyl-phen-yl)-2-oxo-2H-chromene-3-carboxamide, C18H15NO3 (1), N-(3-meth-oxy-phen-yl)-6-methyl-2-oxo-2H-chromene-3-carboxamide, C18H15NO4 (2), and 6-meth-oxy-N-(3-meth-oxy-phen-yl)-2-oxo-2H-chromene-3-carboxamide, C18H15NO5 (3), were synthesized and structurally characterized. The mol-ecules display intra-molecular N-H⋯O and weak C-H⋯O hydrogen bonds, which probably contribute to the approximate planarity of the mol-ecules. The supra-molecular structures feature C-H⋯O hydrogen bonds and π-π inter-actions, as confirmed by Hirshfeld surface analyses.

Synthesis, biological evaluation and molecular modeling studies of phthalazin-1(2H)-one derivatives as novel cholinesterase inhibitors

A series of donepezil analogues based on phthalazin-1(2H)-one scaffold was studied as hChEIs. The biological results revealed that the structural modifications proposed significantly affected ChE inhibitory potency as well as selectivity AChE/BuChE.

Exploring coumarin potentialities: development of new enzymatic inhibitors based on the 6-methyl-3-carboxamidocoumarin scaffold

Novel 6-methyl-3-carboxamidocoumarins were synthesized by an effective three step synthetic strategy and screened towards MAO, AChE and BuChE enzymes.

Implication of coumarins towards central nervous system disorders

Coumarins are widely distributed, plant-derived, 2H-1-benzopyran-2-one derivatives which have attracted intense interest in recent years as a result of their diverse and potent pharmacological properties. Particularly, their effects on the central nervous system (CNS) have been established. The present review discusses the most important pharmacological effects of natural and synthetic coumarins on the CNS, including their interactions with benzodiazepine receptors, their dopaminergic and serotonergic affinity, and their ability to inhibit cholinesterases and monoamine oxidases. The structure-activity relationships pertaining to these effects are also discussed. This review posits that natural or synthetic coumarins have the potential for development in the therapy of psychiatric and neurodegenerative disorders, including Alzheimer's and Parkinson's diseases, schizophrenia, anxiety, epilepsy, and depression.