Evaluation of the binding characteristics of [5-¹¹C-methoxyl]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 [¹¹C]donepezil. [5-¹¹C-methoxy]Donepezil was synthesized by O-methylation using [¹¹C]methyl triflate. The binding of [¹¹C]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 [¹¹C]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 [¹¹C]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 [¹¹C]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 [5-¹¹C-methoxy]donepezil can be potentially useful to image AChE non-invasively in the human brain by positron emission tomography.