Reduced systemic exposure and brain uptake of donepezil in rats with scopolamine-induced cognitive impairment

1. Donepezil (DPZ) is an acetylcholinesterase (AchE) inhibitor used in the mild to moderately severe Alzheimer's disease. Among its major metabolites, 6-O-desmethyl DPZ (6-DDPZ), 5-O-desmethyl DPZ (5-DDPZ) and DPZ N-oxide, the anti-AchE activities of 5-DDPZ and DPZ N-oxide have never been clearly identified before. Besides, there is no report on simultaneous determination of DPZ and its three metabolites in the brain, thus their uptake in hippocampus and cortex are unknown. Therefore, the current studies are proposed aiming to: (1) investigate the anti-AchE activities and brain uptake of DPZ and its three metabolites and (2) compare their pharmacokinetics and brain uptake between normal and scopolamine-induced rats.2. DPZ and its three metabolites demonstrated anti-AchE activities with the IC₅₀ in the order of DPZ (7.20 × 10^-2μM), 6-DDPZ (1.14 × 10^-1μM), 5-DDPZ (4.03 × 10^-1μM) and DPZ N-oxide (1.61 μM). They were also evenly distributed in the brain and retained much longer in the brain than that in plasma in normal rats.3. Compared to normal rats, C_max, AUC_0→24h and AUC_0→∞ of DPZ were reduced by 52.0%, 31.2% and 30.1%, respectively; T_max of DPZ and its three metabolites were prolonged and their brain uptake were decreased in scopolamine-induced rats, suggesting the potential reduced absorption of DPZ.