Huprine X is a novel high-affinity inhibitor of acetylcholinesterase that is of interest for treatment of Alzheimer's disease

Inhibitors of the enzyme acetylcholinesterase (AChE) slow and sometimes reverse the cognitive decline experienced by individuals with Alzheimer's disease. Huperzine A, a natural product used in traditional Chinese herbal medicine, and tacrine (Cognex) are among the potent AChE inhibitors used in this treatment, but the search for more selective inhibitors continues. We report herein the synthesis and characterization of (-)-12-amino-3-chloro-9- ethyl-6,7,10,11-tetrahydro-7,11-methanocycloocta[b]quinoline hydrochloride (huprine X), a hybrid that combines the carbobicyclic substructure of huperzine A with the 4-aminoquinoline substructure of tacrine. Huprine X inhibited human AChE with an inhibition constant K₁ of 26 pM, indicating that it binds to this enzyme with one of the highest affinities yet reported. Under equivalent assay conditions, this affinity was 180 times that of huperzine A, 1200 times that of tacrine, and 40 times that of E2020 (donepezil, Aricept), the most selective AChE inhibitor currently approved for therapeutic use. The association and dissociation rate constants for huprine X with AChE were determined, and the location of its binding site on the enzyme was probed in competition studies with the peripheral site inhibitor propidium and the acylation site inhibitor edrophonium. Huprine X showed no detectable affinity for the edrophonium-AChE complex. In contrast, huprine X did form a ternary complex with propidium and AChE, although its affinity for the free enzyme was found to be 17 times its affinity for the propidium-AChE complex. These data indicated that huprine X binds to the enzyme acylation site in the active site gorge but interferes slightly with the binding of peripheral site ligands.