New azepino[4,3-b]indole derivatives, as selective butyrylcholinesterase inhibitors with potential for neurodegenerative disorders

Starting from the cholinergic hypothesis, which originally implied a major role of acetylcholinesterase (AChE) in the cognitive impairment of the Alzheimer’s disease (AD), the role of butyrylcholinesterase (BChE) has progressively become more crucial in the AD development and progression. Indeed, it has been shown that the levels of AChE in the AD brain decrease by as much as 90%, whilst the levels of BChE, mainly in the G1 form (i.e., globular form of monomer structure), increase, suggesting that inhibition of BChE may represent a privileged target to develop new drugs for treating neurodegenerative diseases. As a matter of fact, in the last years several efforts have been made to identify selective BChE inhibitors, such as tricyclic cymserine analogs, which proved beneficial in vivo in animal models, most likely by recovering cholinergic activity and/or by restoring AChE:BChE activity ratios to the levels observed in the healthy brain. Previously, we reported the ChE inhibition activity of novel medium-sized nitrogen-containing heterocycles (e.g., tetrahydroazocines) fused on indole, which showed ChE inhibition activity. Herein, we explore the ChE inhibition activity of a series of 3,4,5,6-tetrahydroazepino[4,3-b]indole derivatives, some of which proved to be highly potent and selective BChE inhibitors, with low toxicity as assessed in vitro on neuroblastoma cell cultures. Among the newly synthesized compounds, the lactam derivative 2 showed the highest BChE inhibition potency (IC50 = 1.5 nM), whereas further investigation showed that the above tricyclic system could provide a promising scaffold for new multimodal derivatives with potential in the treatment of neurodegenerative disorders.