Isatin derivatives as potent cholinesterase inhibitors: recent developments and future challenges

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline, memory impairment, and neuronal loss, primarily associated with β-amyloid plaque deposition, tau hyperphosphorylation, and cholinergic dysfunction. Since the disruption of cholinergic neurotransmission is a key pathological feature of AD, the cholinesterase (ChE) inhibitors targeting acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) remain the mainstay of symptomatic treatment by enhancing synaptic acetylcholine levels. However, the currently approved small molecules, including tacrine, donepezil, rivastigmine, and galantamine, are limited by their modest efficacy, poor selectivity, and adverse effects, necessitating the development of improved therapeutic agents. Among emerging scaffolds, isatin (1H-indole-2,3-dione) has attracted considerable research attention owing to its structural versatility, synthetic accessibility, and favorable pharmacological properties. Recent studies have reported diverse isatin-based derivatives, including hydrazones, Schiff bases, thiosemicarbazones, spirooxindoles, and molecular hybrids, exhibiting potent AChE and BChE inhibitory activities, often in the submicromolar or nanomolar ranges. Structure-activity relationship studies have revealed the critical influence of electronic effects, linker optimization, N-substitution, and molecular hybridization on inhibitory potency and selectivity. This review summarizes the most promising isatin-based cholinesterase inhibitors reported from 2020 to the present, highlighting their biological activities, structure-activity relationships, and potential for the development of next-generation anti-Alzheimer therapeutics.