Synthesis and bioactivity studies on potential anti-alzheimer heteroaromatic hydrazide-hydrazone derivatives

Alzheimer's disease (AD) is a progressive and fatal neurodegenerative disease characterized clinically by memory and cognitive deficits. It is primarily seen in the elderly population and is the most common cause of dementia. The most important pathological features of Alzheimer's disease are β-amyloid (Aβ) extracellular plaques, intracellular neurofibrillary tangles (NFT) resulting from excessive phosphorylation of tau protein, and loss of cholinergic neurons in the basal forebrain. Inadequate cholinergic transmission plays an important role in the development of cognitive, functional and behavioral symptoms in Alzheimer's disease. Therefore, treatments are generally planned to increase the function of the cholinergic system either with receptor agonist or acetylcholinesterase inhibitors (AChEI). Based on the cholinergic hypothesis, the most successful approach to treat Alzheimer's disease is to restore the level of acetylcholine (ACh). ACh is metabolized by two types of cholinesterases (ChE), namely acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). However, Alzheimer's therapy is based primarily on AChEI, which can increase ACh levels in cholinergic synapses. The cholinergic hypothesis is the only theory that explains the nature of Alzheimer's disease and is still accepted. In addition to this, almost all medicines currently used to relieve the symptomatic effects of moderate to severe Alzheimer species are based on this hypothesis. The main goal of the cholinergic hypothesis is to improve the activity of AChE. Studies have shown that AChE is responsible for a variety of non-catalytic pathways, including pro-aggregation activity of Aβ. In Alzheimer's disease, the activity of acetylcholinesterase is increased, which increases the breakdown of acetylcholine and leads to a decrease in the amount of acetylcholine in the brain. The other relationship between the enzyme and Alzheimer's disease is the partial effect of the enzyme on the formation of neurofibrillary tangles and amyloid plaques. It has been shown that acetylcholinesterase complexes with growing fibrils, leading to aggregation of Aβ peptide fragments. It has been understood that the structural region that triggers the formation of Aβ peptide fibrils of the AChE enzyme is the peripheral anionic region (PAS) of the enzyme. Especially, molecules interacting with PAS, or both the enzymatic region and the PAS region, inhibit pro-aggregation activity of AChE for Aβ. Besides, some AChE inhibitors not only promote cholinergic transmission but also prevent toxic Aβ synthesis, accumulation and aggregation. In summary, AChE inhibition has been shown to be a critical strategy in the effective treatment of Alzheimer's disease. Many aromatic structures have potential charge-transfer komplex, hydrophobic, cation- π and π-π interactions or hydrogen bonding with aromatic amino acid residues at both the AChE entrance and the bottom of the gorge. In the search for new potent acetylcholinesterase inhibitors, many compounds, including Alzheimer's drugs such as tacrine, donepezil, and galantamine, have many heterocyclic rings with significant inhibitory activity. Many heterocyclic systems have been used as a skeleton to design new compounds with AChE inhibitory activity. On the other hand, it has been reported that many compounds carrying the hydrazide-hydrazone functional group have cholinesterase inhibitor activity. Hydrazones have hydrogen donor and acceptor nitrogen atoms and can generate hydrogen bond with amino acids in the enzyme gorge. For this reason, it is one of the preferred pharmacophore for molecular design. In our study, 39 compounds were designed and synthesized 4-(1H-benzimidazole/benzoxazole/benzothiazol-2-yl) benzohydrazide-hydrazone derivatives including bothbenzimidazole, benzoxazole or benzothiazole skeleton and hydrazone structures. Synthesis of the compounds was carried out in three steps. Firstly, 4-formylbenzoate is reacted with o-phenylenediamine, 2-aminophenol and 2-aminothiophenol to give methyl 4-(1H-benzimidazol-2-yl)benzoate, 4-(1H-benzoxazole-2-yl)benzoate and 4-(1H-benzothiazol-2-yl)benzoate derivatives, respectively. Then, the products of all the ester structures obtained at the end of the first step are reacted with hydrazine hydrate to yield hydrazide intermediates. Finally, all of the hydrazide derivatives were condensed with various aromatic aldehydes to give hydrazide-hydrazone final compounds. The structures of the synthesized and purified compounds were clarified by interpretation of IR, 1H NMR, mass spectra and elemental analysis results. The inhibitor effects on AChE/BuChE were determined by the modified Ellman test. All synthesized compounds showed from good to moderate inhibition of the AChE enzyme with an IC50 value of 1.0-10.7 μM. Compound AH-9 was found to be the most effective compound against AChE (IC50 = 1.0 μM), indicating a similar inhibition with the reference compound galantamine. Besides, compound CH-6 exhibited good AChE inhibition activity as well as good BuChE inhibition (IC50 = 1.83 μM M AChE; 1.81 μM BuChE). Compounds AH-9, AH-5 were examined spectroscopically metal complexing capacity and the shifts observed in the UV spectrum were reported as preliminary data on the complexing potential of the compounds. Log P value, one of the important physicochemical parameters, was calculated by MOE 2011.10 (Molecular Operating Environment) program for the final compounds and the relation with activity was evaluated. All compounds are predicted to cross the blood-brain barrier. 30 of the 39 final compounds obtained within this thesis are not registered in the literature but reported for the first time in this study.