Recent developments in tacrine-based hybrids as a therapeutic option for Alzheimer's disease.

Alzheimer's disease (AD) is a multifactorial, irreversible, and age-related neurodegenerative disorder among the elderly. AD attracts attention due to its complex pathogenesis, morbidity and mortality rates, and the limitations of drugs used in the treatment of AD. Cholinesterase inhibitors and N-methyl-D-aspartate (NMDA) receptor antagonists are used in the clinic. While tacrine, donepezil, galantamine, and rivastigmine are cholinesterase inhibitors, memantine is a non-competitive NMDA receptor antagonist. However, these drugs could not delay the progress of AD. The traditional clinical approach, the one drug-one target concept, is not entirely effective in the treatment of AD. Also, it is urgent to develop potent and novel anti-AD drugs by the design concept of multi-target directed ligands (MTDLs) which combine pharmacophores interacting with different pathways in AD. This article provides an overview of the noteworthy structural modifications made to tacrine to develop novel candidates for anti-Alzheimer drugs. Due to the complex pathology of AD, multi-functional tacrine-based ligands targeting different hallmarks, β-amyloid, tau protein, N-methyl-D-aspartate receptor, cholinesterases, monoamine oxidases, secretases, have been studied. Here, tacrine-based derivatives including heterocyclic structures such as dihydroxypyridine, chromene, coumarin, pyrazole, triazole, tetrahydroquinolone, dipicolylamine, arylisoxazole were reported with promising anti-AD effects compared to tacrine. In vitro and in vivo assays showed that new tacrine-based hybrids, which are selective, neuroprotective, and non-hepatotoxic, might be considered as remarkable anti-AD drug candidates for further clinical studies.