Overview of novel multifunctional agents based on conjugates of γ-carbolines, carbazoles, tetrahydrocarbazoles, phenothiazines, and aminoadamantanes for treatment of Alzheimer's disease

Alzheimer's disease (AD) is a multifactorial neurodegenerative process whose effective treatment will require drugs that can act simultaneously on multiple pathogenic targets. Here, we present an overview of our previous multitarget studies of five groups of novel hybrid structures that combine, through spacers, five pharmacophores that have been found promising for AD treatment: γ-carbolines, carbazoles, tetrahydrocarbazoles, phenothiazines, and aminoadamantanes. Biological activity of the compounds was assessed by a battery of assays. These included inhibitory potency against acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) as indicators of potential for cognition enhancement and against carboxylesterase (CaE) to exclude unwanted inhibition of this biotransformation pathway. Displacement of propidium from the peripheral anionic site of AChE was determined as a predictor of anti-aggregation activity. Binding to the two sites of the NMDA subtype of the glutamate receptor was conducted as an additional indicator of potential cognition enhancement and neuroprotection. Propensity to protect against mitochondrial triggers of cell death was evaluated by tests of mitochondrial potential and calcium-induced swelling as indicators of mitochondrial permeability transition. Antioxidant potential was measured to evaluate the tendency to prevent oxidative stress. Potential for disease modification was gauged by the ability to stimulate microtubule assembly. Finally, binding modes of conjugates to AChE and BChE were studied using quantum mechanical-assisted molecular docking. We found selective BChE inhibitors (conjugates of γ-carbolines and phenothiazine I, γ-carbolines and carbazoles II, and aminoadamantanes and carbazoles III) as well as inhibitors of both cholinesterases (conjugates of γ-carbolines and methylene blue IV and bis-γ-carbolines with ditriazole-containing spacers V). These compounds combined potentials for cognition enhancement, neuroprotection, and disease modification. None of the conjugates exhibited high potency against CaE, thereby precluding potential drug-drug interactions from CaE inhibition. Thus, the studied compounds exhibited positive characteristics of multitarget drugs, indicating their potential for the next generation of AD therapeutics.