Synthesis and Biochemical Evaluation of Multifunctional Acetylcholinesterase Inhibitor Hybrids for Treatment of Alzheimer's Disease.

Abstract

Alzheimer’s disease (AD) places a significant and dramatically increasing burden on society. While pathological hallmarks of this neurodegenerative disorder are known, the exact cause remains unclear. Acetylcholinesterase inhibitors (AChEis) may help with cognitive symptoms, but they are incapable of reversing disease progression. The multifunctional hybrid approach towards new AD treatments aims to combine two molecules, one of which is usually an AChEi, with beneficial properties for AD into a single chemical entity showing enhanced properties and capable of attacking multiple facets of the disease. Using this approach, several such hybrids were synthesized and characterized biochemically.

Tacrine was the first AChEi approved for the treatment of AD. Mefenamic acid is a non-steroidal anti-inflammatory drug capable of inactivating AChE through radicals generated in the presence of reactive oxygen species (ROS). Various tacrine- and 6-chlorotacrine-mefenamic acid hybrids were synthesized. Several were potent AChEis with low nanomolar IC50 values under standard conditions and in the presence ROS, the most potent being 13m (IC50 = 0.418 ± 0.025 nM, ROS IC50 = 0.009 ± 0.003 nM). Compared to tacrine, 13m exhibited >100-fold increase in potency under standard conditions and >20,000-fold increase in the presence of ROS.

Subsequent studies investigated the role of the linker and the need for covalent linkage of mefenamic acid in regards to AChE inhibition. Results suggest the mefenamic acid moiety in an amine-linked tacrine-mefenamic acid hybrid may not contribute to AChE inhibition under standard conditions, and the linker moiety alone may be responsible for the increase in potency. However, in the presence of ROS, there may be some advantage to amine-linked tacrine-mefenamic acid hybrids.

Additionally, 6-chlorotacrine-metal-amyloid-beta modulator hybrid 19 was synthesized. 19 displayed potent inhibition of AChE (IC50 = 2.37 ± 0.29 nM) and butyrylcholinesterase (IC50 = 2.01 ± 0.12 nM). Inhibition was largely retained in the presence of ROS, Cu2+/Zn2+, and amyloid-beta. 19 showed remarkable multifunctionality through interaction with Cu2+/Zn2+, control of metal-free and metal-induced amyloid-beta aggregate assembly, and disaggregation of preformed metal-free and metal-associated amyloid-beta aggregates. A particularly promising compound, 19 represents one of the few tacrine hybrids designed to specifically target the interplay of AChE/amyloid-beta/metals.