Studies on A group of novel tacrine-donepezil hybrids as anti-alzheimer compounds

Alzheimer's Disease (AD) is the most common form of dementia in elderly people. As the life expectancy has increased, the incidence of AD has risen dramatically. It is estimated that today, there are approximately 33 million people with AD. AD has a multifaceted pathology including decreased cholinergic transmission, amyloid β plaques, hyperphosphorolation of tau proteins and neurofibrillar tangles. As the disease progress, biometal dyshomeostasis, mitochondrial dysfunction, oxidative stress and neuroinflamation accompany to these pathologic pathways. Although extensive efforts has been made towards the understanding of the underlying molecular mechanisms in AD, exact patholgy of the disease has not been not fully understood yet. AChE inhibitors improve memory by increasing cholinergic transmission and 3 AChE inhibitors and 1 NMDA receptor agonist are approved by FDA in the treatment of AD. Peripheral anionic site of the AChE mediates non-classical functions of the enzyme, therefore development of inhibitors targetting dual site of the AChE is emerged as dual binding site inhibitor strategy in anti-Alzheimer drug discovery. Considering contribution of BChE to the hydrolysis of acethylcholine and disease pathology, selective and non-selective inhibitors for both enzymes have been designed. Nowadays, taking multifactorial AD pathology into account, Multi-Target Directed Ligands (MTDL) strategy is attracting more attention in the development of anti-Alzheimer drugs. Hybrid molecules combine two different pharmacophore groups in one molecule and an important drug development strategy in medicinal chemistry. In this dissertation, 26 novel tacrine-donepezil hybrids were designed, synthesized and biological activity molecular modelling studies were carried out on this compounds. Benzylpiperidine moiety of donepezil and tacrine were selected as pharmacophores and connected with hydrazone functional group to aim dual binding site inhibition of AChE. Previously, amyloid β aggregation inhibitory activity of the compounds bearing hydrazone functional group was reported. On the other hand, literature survey indicated that hydrazone containing compounds have a potential metal binding ability. Therefore, all the data were considered in the design of the tacrine-donepezil hybrids. In parallel with this, both AChE and BChE inhibitory activities of all compounds were evaluated then some compounds were tested for their inhibitory activity on amyloid β formations and finally metal complex formation ability of compound Non was examined. The synthesis of the designed compounds was realized in 5 steps. In the first step, 9-chloro-1,2,3,4-tetrahydroacrine was obtained then, nucleophilic substitution of this gave 9-hydrazino-1,2,3,4-tetrahydroacridine. On the other hand, various substituted benzylpiperidin derivatives were synthesized and reacted with hydrazino-compound to obtain hydrazone containing tacrine-donepezil hybrids. All of the final compounds were isolated and chracterized in their HCl salt forms. All of the title compounds were found active for both AChE and BChE and inhibited both enzymes at very low µM concentrations. Among the synthesized compounds, 3F was found to be the most active one towards AChE with an IC50 value of 0.105 µM while, 3D was the most active derivative towards BChE with an IC50 value of 0.028 µM. All of the tested compounds inhibited amyloid β aggregation at different ratios. Besides, spectrophotometric metal binding assay indicated that compound Non may have a potency to form complexes with Cu2+ and Fe2+. Apart from these, docking studies were performed with compound 3D and 3F on AChE and BChE enzymes. As predicted, the results suggested that the compounds interact with dual site of the AChE. Additionally, ADME properties of the compounds were evaluated and according to these predictions, the final compounds might express good bioavailability. Consequently, the title compounds showed promising multi-target profile both in in vitro and in silico assays as designed. All of the tacrine-donepezil hybrids were reported for the first time in this study.