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Estimate the value of in vivo plasma IC50 of donepezil, the concentration of donepezil in plasma that inhibits brain acetylcholinesterase (AChE) activity by 50% at the steady-state conditions of donepezil between the plasma and the brain.N-[11C] methylpiperidin-4-yl acetate ([11C]MP4A) positron emission tomography was performed in 16 patients with probable Alzheimer disease (AD) before and during the treatment of donepezil (5 mg/day) with a mean interval of 5.3 months. The plasma IC50 value of donepezil was estimated from plasma donepezil concentrations and cerebral cortical mean AChE inhibition rates measured by positron emission tomography, using one-parameter model.Donepezil reduced AChE activity uniformly in the cerebral cortex compared with the baseline in each AD patient, and the mean reduction rate in the cerebral cortex was 34.6%. The donepezil concentrations in the plasma ranged from 18.5 to 43.9 ng/mL with a mean of 28.9 ± 7.3 ng/mL. The plasma IC50 value was estimated to be 53.6 ± 4.0 ng/mL.Once the plasma IC50 of donepezil is determined, the brain AChE inhibition rate could be estimated from the plasma concentration of donepezil in each subject based on the plasma IC50. Now that the mean donepezil concentrations in the plasma, when the patients took 5 mg/day, remained 28.9 ng/mL, approximately half of the plasma IC50, higher dose of donepezil might provide further benefits for patients with AD. This technique can be also applied to measure the in vivo plasma IC50 of other cholinesterase inhibitors such as rivastigmine and galantamine.