Estimation of Plasma IC50 of Donepezil Hydrochloride for Brain Acetylcholinesterase Inhibition in Monkey Using N-[11C]methylpiperidin-4-yl Acetate ([11C]MP4A) and PET

Abstract

Donepezil hydrochloride is a potent and selective inhibitor for brain acetylcholinesterase (AChE) and is currently used worldwide for the treatment of Alzheimer's disease. Until now, there is no in vivo study on the relation between the plasma concentration and the brain AChE inhibition. The purpose of this study was to estimate in vivo plasma IC 50 of donepezil in living monkeys by measuring plasma donepezil concentration (LC/MS/MS) and brain AChE activity with positron emission tomography (PET) and N-[ 11 C]methylpiperidin-4-yl acetate, which is an acetylcholine analog recently developed by us for quantifying in vivo brain AChE activity. PET scans with donepezil at two doses, 100 mg/kg (donepezil-1; N ¼ 5) or 250 mg/kg (donepezil-2; N ¼ 5), were performed using the same monkeys at 4-week intervals. Before each PET scan, baseline PET scans (N ¼ 10 in total) were performed without donepezil. The plasma donepezil concentrations 14 min after intravenous injection were proportional to the doses, 17.272.9 ng/ml (donepezil-1) and 44.075.0 ng/ml (donepezil-2), and the mean AChE inhibitions in four neocortical regions as evaluated by PET were also dose-dependent, 27% (donepezil-1) and 53% (donepezil-2). In IC 50 estimation, measured plasma donepezil concentrations were corrected for the change during PET scan. The IC 50 values (estimate7SE) were 4279.0 (ng/ml; donepezil-1), 3473.2 (donepezil-2), and 3774.1 (combined data). The present method may be useful for in vivo evaluation of other AChE inhibitors and novel drugs.

Use of a novel radiometric method to assess the inhibitory effect of donepezil on acetylcholinesterase activity in minimally diluted tissue samples

“…In particular, pre-clinical assessment of AChE inhibition in the minimally diluted brain tissue by a centrally acting cholinesterase inhibitor is important to predict therapeutic effects and to optimize the dose, because, as in the blood, the inhibitors non-specifically bind to tissue constituents. Indeed, it was estimated that 99% of donepezil binding was non-specific in monkey brain Novel method for monitoring anti-cholinesterases T Kikuchi et al (Shiraishi et al, 2005). A similar result was observed in this study.…”

“…In 0.65 mg·mL -1 (1500-fold dilution) of monkey brain homogenate, in which tissue binding is considered negligible, the estimated IC50 for donepezil (15 nM) was almost the same as that for human AChE (12 nM) (Camps et al, 2008). The IC50 value, 390 nM, in 200 mg·mL -1 (fivefold dilution) of monkey brain homogenate is close to the value of 500-700 nM estimated in living monkeys (Shiraishi et al, 2005). From these IC50 values for donepezil in minimally diluted and highly diluted brain tissue, non-specific tissue binding of donepezil was estimated to be 96% [(1-15 nM/390 nM) ¥ 100%], which is consistent with the previously reported value estimated with positron emission tomography (Shiraishi et al, 2005).…”

“…To monitor cholinesterase inhibition in the tissue, minimal dilution of the tissue samples is preferred to avoid reactivation of carbamylated cholinesterases and dilution of the reversible inhibitors (Winteringham and Fowler, 1966;Williams and Casterline, 1969;Nostrandt et al, 1993;Hunter et al, 1997;Padilla et al, 2007). In addition, the IC 50 values of inhibitors in highly diluted tissue would be different from those in minimally diluted tissue, because an inhibitor such as donepezil shows non-specific binding to tissue components with a high fraction, and as a result, a small fraction of an inhibitor could interact with cholinesterases in the intact tissue (Shiraishi et al, 2005). Consequently, evaluation of cholinesterases in minimally diluted tissue samples is important in pre-clinical studies.…”

“…11 C]methylpiperidin-4-yl propionate, have been used to evaluate the pharmacological effect of the AChE specific inhibitor, donepezil, used for the treatment of Alzheimer's disease (Kuhl et al, 2000;Kaasinen et al, 2002;Bohnen et al, 2005;Shiraishi et al, 2005). Cerebral cholinesterase activity has been measured in vivo by positron emission tomography, and in vitro by evaluation of choline ester derivatives, and the metabolism of the compounds in the blood and brain was measured with TLC (Irie et al, 1996;Iyo et al, 1997;Kikuchi et al, 2001;Snyder et al, 2001;Shao et al, 2003;Roivainen et al, 2004).…”

Antagonism of morphine-induced central respiratory depression by donepezil in the anesthetized rabbit

“…However, there is at least one previous study investigating quantitative dose/response relationship between ACh inhibition and intravenously administered donepezil. This study suggests that donepezil at 100 μg/kg inhibits about 30% of the acetylcholinesterase activity and donepezil at 250 μg/kg inhibits about 55% of the activity in living monkeys (Shiraishi et al, 2005). In order to induce even greater inhibitory effects with donepezil, we applied 400 μg/kg donepezil in the current study.…”