Evaluation of the binding characteristics of [5-11C-methoxy]Donepezil in the rat brain for in vivo visualization of acetylcholinesterase

Donepezil, an acetylcholinesterase (AChE) inhibitor, has not been evaluated for its binding characteristics using a radioactive tracer, although its inhibitory action on AChE has been studied. The aim of this research is to examine whether AChE can be visualized in vivo and in vitro with [C-11]donepezil. [5-C-11-methoxy]Donepezil was synthesized by O-methylation using [C-11]methyl triflate. The binding of [C-11]donepezil to brain homogenates was higher in the brain stem and striatum, and it was lowest in the cerebellum. The in vitro autoradiographic study successfully demonstrated the specific binding of [C-11]donepezil to AChE in the rat brain. The IC50 value of binding was approximately 10 nM, which is comparable to the reported value for inhibiting enzyme activity (6 nM). Saturation experiments revealed that the B-max and K-d of [C-11]donepezil binding in vitro are 65 fmol/mg tissue and 39.8 nM, respectively. In accordance with the in vitro bindings, the in vivo distribution of [C-11]donepezil was heterogeneous in the rat brain. In the blocking experiments, the heterogeneous distribution disappeared in the presence of a large amount of unlabeled donepezil. These data suggest that [5-C-11-methoxy]donepezil can be potentially useful to image AChE non-invasively in the human brain by positron emission tomography.