Investigating Dialkyl Aryl Phosphates as Selective Butyrylcholinesterase Inhibitors

Abstract

Cholinesterase inhibitors are currently the major drug type used to manage the progression of Alzheimer's disease. Numerous types of inhibitors including drugs such as tacrine and donepezil, and classes of compounds such as coumarins, organophosphates, and carbamates have been investigated for their ability to inhibit the two subfamilies of cholinesterases: acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). While the role of acetylcholinesterase‐mediated ester hydrolysis in termination of cholinergic transmission is well understood –and acetylcholinesterase inhibitors have played important roles in treatment of Alzehimer's disease and Huntington's disease— the biological role of butyrylcholinesterase is less well understood. However, recent studies have shown BChE activity is found to increase in patients with Alzheimer's disease, while AChE activity remains unchanged or declines. Thus, the use of molecules that selectively inhibit BChE has attracted attention as a potential therapeutic for patients with Alzheimer's disease. Previous studies have suggested dialkyl phenyl organophosphates may be effective and selective BChE inhibitors, but the effects of modifications on the aryl group were not reported. We investigated a series of dialkyl aryl organophosphates as selective inhibitors of BChE. The molecules provided an attractive scaffold to readily introduce substituents and develop more potent inhibitors. The inhibition constant (KI) and IC50 value were determined for each inhibitor using established kinetics assays. Our results show that incorporation of alkyl groups on the aryl moiety led to better inhibitors relative to the dibutyl phenyl phosphate for all inhibitors tested, but inhibition properties were dependent on the number and location of the methyl group(s). Increasing the size of aryl substituent to a naphthyl group led to a better inhibitor with the 2‐naphthyl analog showing a 10‐fold lower KI value compared to the 1‐naphthyl analog. Experiments to evaluate the connectivity of the alkyl substituents and the aryl group suggest the contribution of the aromatic moiety to inhibition is affected by the identity of the alkyl chains. To explore the structural basis for the differences in inhibitory properties, we performed computational modeling. Docking studies show the most potent inhibitors bind in single conformation making extensive van der Waals contacts with the alkyl and aromatic groups. These results identified molecules with improved inhibition properties compared to the previously reported dialkyl phenyl phosphate and begin to dissect the structural features responsible for inhibitor binding.Support or Funding InformationThis work is supported by CSULB startup funds and by the MARC program funded by the National Institute of General Medical Sciences, National Institutes of Health (T34GM008074). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.