Polymethylated acylphloroglucinols from Rhodomyrtus tomentosa exert acetylcholinesterase inhibitory effects

Naturally Occurring Cholinesterase Inhibitors from Plants, Fungi, Algae, and Animals: A Review of the Most Effective Inhibitors Reported in 2012-2022

Since the development of the "cholinergic hypothesis" as an important therapeutic approach in the treatment of Alzheimer's disease (AD), the scientific community has made a remarkable effort to discover new and effective molecules with the ability to inhibit the enzyme acetylcholinesterase (AChE). The natural function of this enzyme is to catalyze the hydrolysis of the neurotransmitter acetylcholine in the brain. Thus, its inhibition increases the levels of this neurochemical and improves the cholinergic functions in patients with AD alleviating the symptoms of this neurological disorder. In recent years, attention has also been focused on the role of another enzyme, butyrylcholinesterase (BChE), mainly in the advanced stages of AD, transforming this enzyme into another target of interest in the search for new anticholinesterase agents. Over the past decades, Nature has proven to be a rich source of bioactive compounds relevant to the discovery of new molecules with potential applications in AD therapy. Bioprospecting of new cholinesterase inhibitors among natural products has led to the discovery of an important number of new AChE and BChE inhibitors that became potential lead compounds for the development of anti-AD drugs. This review summarizes a total of 260 active compounds from 142 studies which correspond to the most relevant (IC50 ≤ 15 μM) research work published during 2012-2022 on plant-derived anticholinesterase compounds, as well as several potent inhibitors obtained from other sources like fungi, algae, and animals.

Oligomeric phloroglucinols with hAChE inhibitory and antibacterial activities from tropic Rhodomyrtus tomentosa

Alzheimer's diseases (AD) and other infectious diseases caused by drug-resistance bacteria have posed a serious threat to human lives and global health. With the aim to search for human acetylcholinesterase (hAChE) inhibitors and antibacterial agents from medicinal plants, 16 phloroglucinol oligomers, including two new phloroglucinol monomers (1a and 1b), four new phloroglucinol dimers (3a, 3b, 4b, and 5a), six new phloroglucinol trimers (6a, 6b, 7a, 7b, 8a, and 8b), and two naturally occurring phloroglucinol monomers (2a and 2b), along with two known congeners (4a and 5b), were purified from the leaves of tropic Rhodomyrtus tomentosa. The structures and absolute configurations of these new isolates were unequivocally established by comprehensive analyses of their spectroscopic data (NMR and HRESIMS), ECD calculation, and single crystal X-ray diffraction. Structurally, 3a/3b shared a rare C-5' formyl group, whereas 6a/6b possessed a unique C-7' aromatic ring. In addition, 7a/7b and 8a/8b were rare phloroglucinol trimers with a bis-furan and a C-6' hemiketal group. Pharmacologically, the mixture of 3a and 3b showed the most potent human acetylcholinesterase (hAChE) inhibitory activity with an IC50 value of 1.21 ± 0.16 μM. The molecular docking studies of 3a and 3b in the hAChE binding sites were performed, displaying good agreement with the in vitro inhibitory effects. In addition, the mixture of 3a and 3b displayed the most significant anti-MRSA (methicillin-resistant Staphylococcus aureus) with MIC and MBC values of both 0.50 μg/mL, and scanning electron microscope (SEM) studies revealed that they could destroy the biofilm structures of MRSA. The findings provide potential candidates for the further development of anti-AD and anti-bacterial agents.

Phloroglucinols with hAChE and α-glucosidase inhibitory activities from the leaves of tropic Rhodomyrtus tomentosa

Four undescribed phloroglucinol meroterpenoids, rhodotomentodiones A-D, and one undescribed phloroglucinol dimer, rhodotomentodimer A, were obtained and structurally established from tropic Rhodomyrtus tomentosa leaves. Their structures were unambiguously elucidated based on the comprehensive analyses of the NMR and MS spectroscopic data, electronic circular dichroism (ECD) calculation, and single-crystal X-ray diffraction. In particular, rhodotomentodiones A and B represent the first examples of phloroglucinol meroterpenoids featuring a unique γ-pyranoid moiety. More importantly, rhodotomentodimer A exhibited the most potential human acetylcholinesterase (hAChE) and α-glucosidase inhibitory effects with IC₅₀ values of 7.5 μM and 5.6 μM, respectively. The possible interaction sites of the above potential hAChE and α-glucosidase inhibitor were achieved by molecular docking studies. These findings greatly enrich the diversity of natural products from Myrtaceae species, and provide potential candidates for the further development of anti-Alzheimer and antidiabetic diseases.

Two pairs of undescribed enantiomers isolated from the fungus Penicillium griseofulvum

Two pairs of undescribed enantiomers including a 2,5-diketopiperazine namely (±)-janthinolide G and a related analogue (±)-janthinolide H, were isolated from the crude extract of the fungus Penicillium griseofulvum together with five known compounds. Both two structures were determined by spectroscopic method and HRESIMS, whereas absolute stereochemistry was elucidated by using theoretical NMR calculation and ECD calculation. Janthinolide G is the first example of 2,5-diketopiperazine enantiomers with a cleavage diketopiperazine ring and comprises a terminal oxime group rarely seen in natural products. Biological screening of selected compounds found that 4 and 7 both exhibited weak α-glucosidase inhibitory effects, and a potential correlation was afforded by docking studies of α-glucosidase protein (PDB: 3TOP) and bioactive molecules. The plausible biosynthetic pathways of two unreported isolates are proposed here.

Determination of Anti-Alzheimer's Disease Activity of Selected Plant Ingredients

Neurodegenerative diseases, among which one of the more common is Alzheimer’s disease, are the one of the biggest global public health challenges facing our generation because of the increasing elderly population in most countries. With the growing burden of these diseases, it is essential to discover and develop new treatment options capable of preventing and treating them. Neurodegenerative diseases, among which one of the most common is Alzheimer’s disease, are a multifactorial disease and therefore demand multiple therapeutic approaches. One of the most important therapeutic strategies is controlling the level of acetylcholine—a neurotransmitter in cholinergic synapses—by blocking the degradation of acetylcholine using acetylcholinesterase inhibitors such as tacrine, galantamine, donepezil and rivastigmine. However, these drugs can cause some adverse side effects, such as hepatotoxicity and gastrointestinal disorder. Thus, the search for new, more effective drugs is very important. In the last few years, different active constituents from plants have been tested as potential drugs in neurodegenerative disease therapy. The availability, lower price and less toxic effects of herbal medicines compared with synthetic agents make them a simple and excellent choice in the treatment of neurodegenerative diseases. The empirical approach to discovering new drugs from the systematic screening of plant extracts or plant-derived compounds is still an important strategy when it comes to finding new biologically active substances. The aim of this review is to identify new, safe and effective compounds that are potential candidates for further in vivo and clinical tests from which more effective drugs for the treatment of Alzheimer’s disease could be selected. We reviewed the methods used to determine anti-Alzheimer’s disease activity. Here, we have discussed the relevance of plant-derived compounds with in vitro activity. Various plants and phytochemical compounds have shown different activity that could be beneficial in the treatment of Alzheimer’s disorders. Most often, medicinal plants and their active components have been investigated as acetylcholinesterase and/or butyrylcholinesterase activity inhibitors, modifiers of β-amyloid processing and antioxidant agents. This study also aims to highlight species with assessed efficacy, usable plant parts and the most active plant components in order to identify species and compounds of interest for further study. Future research directions are suggested and recommendations made to expand the use of medicinal plants, their formulations and plant-derived active compounds to prevent, mitigate and treat Alzheimer’s disease.

Cassane diterpenoids from the seeds of Caesalpinia bonduc and their nitric oxide production and α-glucosidase inhibitory activities

Ten previously undescribed cassane diterpenoids, cassabonducins A-J, and eleven known compounds were isolated from the seeds of Caesalpinia bonduc. The structures of the undescribed compounds were elucidated by extensive analysis of spectroscopic data (IR, HRESIMS, and ¹H, ¹³C and 2D NMR) and their absolute configurations were determined by the ECD data and single-crystal X-ray diffraction analysis. ε-Caesalpin-Ⅶ was obtained from natural resources for the first time. Cassabonducin A possessed noteworthy inhibitory activity against LPS-induced nitric oxide (NO) production in RAW 264.7 macrophages with IC₅₀ value of 6.12 μM. Cassabonducin D and neocaesalpin N showed moderate α-glucosidase inhibition at the concentration of 50 μM with inhibitory capacities of 47.17% and 43.83%, respectively.

Thirteen cyathane diterpenoids with acetylcholinesterase inhibitory effects from the fungus Cyathus africanus

Eight undescribed cyathane diterpenoids, representative specialised metabolites of the genus Cyathus, named cyathins Q-X, along with five known congeners, were isolated from the liquid fermentation of Cyathus africanus. Their structures and absolute configurations were elucidated by integrating NMR spectroscopic analyses, electronic circular dichroism (ECD) calculations, and X-ray diffraction. Reasonable correction to the C-12 configuration of cyathin I was corroborated by the crystal data. The structural identification in this research expanded the number of candidates to allow for more bioactivity-screening options. Among them, (12S)-11α,14α-epoxy-13α,14β,15-trihydroxycyath-3-ene displayed significant acetylcholinesterase (AChE) inhibitory effect with an IC₅₀ value of 4.60 ± 0.85 μM. Molecular docking studies were also performed to unravel the underlying modes of interactions with the active sites of AChE for active compounds.

Acylphloroglucinol trimers from Callistemon salignus seeds: Isolation, configurational assignment, hAChE inhibitory effects, and molecular docking studies

Alzheimer's disease (AD) diagnoses are greatly increasing in frequency as the global population ages, highlighting an urgent need for new anti-AD strategies. With the aim to search for human acetylcholinesterase (hAChE) inhibitors from the species of Myrtaceae family, ten acylphloroglucinol trimers (APTs), including eight new APTs, callistemontrimers A-H (1a, 1b, 2a, 2b, 3a, 3b, 4b, and 5b), and two naturally occurring ones (4a and 5a), along with one reported triketone-acylphloroglucinol-monoterpene adduct (6), were obtained and structurally characterized from the hAChE inhibitory acetone extract of Callistemon salignus seeds. The structures and their absolute configurations for new APTs were unequivocally established via the detailed interpretation of extensive spectroscopic data (HRESIMS and NMR), ECD calculations, and single crystal X-ray diffraction, whereas the absolute configurations of known APTs were determined by further chiral separation, and calculated ECD calculations. The results of hAChE inhibitory assay revealed that an enantiomeric mixture of 2a/2b, 2a, and 2b are good hAChE inhibitors with IC₅₀ values of 1.22 ± 0.23, 2.28 ± 0.19, and 4.96 ± 0.39 μM, respectively. Molecular docking was used to uncover the modes of interactions for bioactive compounds with the active site of hAChE. In addition, 2 and 6 displayed moderate neurite outgrowth-promoting effects with differentiation rates of 6.16% and 6.19% at a concentration of 1.0 μM, respectively.