Inhibitory potential of nitrogen, oxygen and sulfur containing heterocyclic scaffolds against acetylcholinesterase and butyrylcholinesterase

Heterocycles are the key structures in organic chemistry owing to their immense applications in the biological, chemical, and pharmaceutical fields. Heterocyclic compounds perform various noteworthy functions in nature, medication, innovation etc. Most frequently, pure nitrogen heterocycles or various positional combinations of nitrogen, oxygen, and sulfur atoms in five or six-membered rings can be found. Inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes is a popular strategy for the management of numerous mental diseases. In this context, cholinesterase inhibitors are utilized to relieve the symptoms of neurological illnesses like dementia and Alzheimer's disease (AD). The present review focuses on various heterocyclic scaffolds and their role in designing and developing new potential AChE and BChE inhibitors to treat AD. Moreover, a detailed structure-activity relationship (SAR) has been established for the future discovery of novel drugs for the treatment of AD. Most of the heterocyclic motifs have been used in the design of new potent cholinesterase inhibitors. In this regard, this review is an endeavor to summarize the biological and chemical studies over the past decade (2010-2022) describing the pursuit of new N, O and S containing heterocycles which can offer a rich supply of promising AChE and BChE inhibitory activities.

Design, Synthesis, biological investigations and molecular interactions of triazole linked tacrine glycoconjugates as Acetylcholinesterase inhibitors with reduced hepatotoxicity

Tacrine is a known Acetylcholinesterase (AChE) inhibitors having hepatotoxicity as main liability associated with it. The present study aims to reduce its hepatotoxicity by synthesizing tacrine linked triazole glycoconjugates via Huisgen's [3 + 2] cycloaddition of anomeric azides and terminal acetylenes derived from tacrine. A series of triazole based glycoconjugates containing both acetylated (A-1 to A-7) and free sugar hydroxyl groups (A-8 to A-14) at the amino position of tacrine were synthesized in good yield taking aid from molecular docking studies and evaluated for their in vitro AChE inhibition activity as well as hepatotoxicity. All the hybrids were found to be non-toxic on HePG2 cell line at 200 μM (100 % cell viability) as compared to tacrine (35 % cell viability) after 24 h of incubation period. Enzyme kinetic studies carried out for one of the potent hybrids in the series A-1 (IC₅₀ 0.4 μM) revealed its mixed inhibition approach. Thus, compound A-1 can be used as principle template to further explore the mechanism of action of different targets involved in Alzheimer's disease (AD) which stands as an adequate chemical probe to be launched in an AD drug discovery program.

Biological evaluation and molecular docking of novel 1,3,4-thiadiazole-resorcinol conjugates as multifunctional cholinesterases inhibitors

Two series of novel 1,3,4-thiadiazole-resorcinol conjugates were efficiently synthesized and evaluated as cholinesterases inhibitors. N-Butyl- and N-chlorophenyl-5-amino-1,3,4-thiadiazol-2-yl)benzene-1,3-diols were identified as the most promising compounds of low nanomolar activity against AChE (IC₅₀ = 29-76 nM) and moderate activity against BuChE. The inhibition mechanism studies proved that the compounds are mixed type inhibitors. The docking simulations showed great affinity of the compounds for both enzymes. The modelled amine derivatives exhibited a similar arrangement in the catalytic anionic site of AChE similar to that of tacrine. The thiadiazole ring interacted with Trp84 and the phenyl groups created π-π stacking interactions with the residue - Phe330. The compounds showed better inhibition of the in vitro self-induced Aβ (1-42) aggregation than that compared with curcumin as well as antioxidant properties similar to those of quercetin. They exhibited metal ion chelating properties, acceptable cytotoxicity in vitro and favourable ADMET profile determined in silico.

Synthesis and AChE inhibitory activity of N-glycosyl benzofuran derivatives

Six N-glycosyl benzofuran derivatives were synthesized by the catalysis of organic bases and condensation agents. The benzofuran derivatives were obtained by the reaction of various salicylaldehydes in acetone, and then hydrolyzed to the corresponding carboxylic acids. Finally, the target compounds were synthesized by acylation and the reaction conditions were optimized. The acetylcholinesterase (AChE) inhibitory activity of the desired compounds was tested using Ellman’s method. Most of the compounds showed acetylcholinesterase-inhibition activity; N-(2,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-3-yl)benzofuran-2-carbxamide (5a) showed the best acetylcholinesterase inhibition, with an inhibitory rate of 84%.

Synthesis and bioactivity of novel C2-glycosyl oxadiazole derivatives as acetylcholinesterase inhibitors

A series of glycosyl-substituted 1,3,4-oxadiazoles were synthesized by cyclization of glycosyl-acylthiosemicarbazides via a base-catalyzed reaction. The starting glycosyl-acylthiosemicarbazide derivatives were obtained by the reaction of glycosyl isothiocyanate with various hydrazides. The acetylcholinesterase (AChE) inhibitory activities of the products were tested by Ellman’s method. The most active compounds were subsequently evaluated for the 50% inhibitory concentration (IC50) values. N-(1,3,4,6-tetra-O-benzyl-2-deoxy-β-D-glucopyranosyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole-2-amine (6i) possesses the best AChE -inhibition activity with an IC50 of 1.61±0.34 μm.

Synthesis and Evaluation of 5-Benzyl-1,3,4-Thiadiazole Derivatives as Acetylcholinesterase Inhibitors

Three novel 5-benzyl-1,3,4-thiadiazole derivatives were synthesised starting from phenylacetic acid derivatives. These compounds were characterised by NMR, HRMS and single-crystal X-ray diffraction analysis. 2-Pyrrolidyl-5-[2-(4-bromophenyl)methyl]-1,3,4-thiadiazole showed moderate acetylcholinesterase-inhibition activity with a 50% inhibitory concentration value of 33.16 μM. 2-Pyrrolidyl-5-[2-(4-bromophenyl)methyl]-1,3,4-thiadiazole and acetylcholinesterase docking was demonstrated using the Molecular Operating Environment program.

Synthesis and bioactivity of novel C2-glycosyl triazole derivatives as acetylcholinesterase inhibitors

New C2-glycosyl triazole derivatives 6a–l were synthesized by cyclization of glycosyl acylthiosemicarbazides 5 in refluxing 3 N sodium hydroxide aqueous solution. Substrates 5 were obtained by the reaction of glycosyl isothiocyanate 3 with various hydrazides. The acetylcholinesterase (AChE) inhibitory activities of compounds 6 were tested by Ellman’s method. Compounds that exhibited over 85% inhibition were subsequently evaluated for the IC50 values. Compound 6f possesses the best acetylcholinesterase-inhibition activity with IC50 of 1.46±0.25 μg/mL.

Synthesis and biological evaluation of novel C1-glycosyl thiazole derivatives as acetylcholinesterase inhibitors

A new series of C1-glycosyl thiazole derivatives was synthesised by the reaction of 1-(1,3,4,6-tetra- O-acetyl-2-deoxy-β-D-glucopyranos-2-yl)thiourea with 2-bromoacetophenone derivatives. Subsequent removal of the acetyl groups were conducted using NaOMe–MeOH. The structures of all new products were confirmed by IR, 1H NMR and HRMS (ESI). The acetylcholinesterase inhibitory activities of these new compounds were tested. Among them, N-(2-acetamido-3,4,6-tri- O-acetyl-2-deoxy-β-D-glucopyranosyl)-4-(4-nitrophenyl)-1,3-thiazole-2-amine showed the best activity with an inhibition rate of 43.21%.