Pharmacodynamics. Donepezil raises the concentration of acetylcholine (Ach) in brain synapses by preventing its breakdown, and this then stimulates the muscarinic receptors. In contrast to tacrine and physostigmine, which inhibit cholinesterase throughout the body, donepezil is specific for cholinesterase found in the brain and, therefore, is devoid of peripheral cholinomimetic adverse effects. Metadynamic simulation has shown that presence of donepezil near the active site of acetylcholinesterase (AChE) can inhibit its approach for acetylcholine hydrolysis and the docking study reveals that the drug molecule inside the active gorge of human AChE restricts the approach of ACh to Ser203 for the hydrolysis process, providing an insight into the mechanism of inhibition of AChE by donepezil (12).
Positron emission tomography has shown inhibition of cerebral acetylcholinesterase activity in Alzheimer disease patients following treatment with donepezil. Functional MRI has demonstrated that donepezil produces activation in the ventrolateral prefrontal cortex. In a randomized placebo-controlled study, donepezil treatment was shown to stabilize functional connectivity during resting state and brain activity during memory encoding in Alzheimer disease (29). Measurement of event-related potentials before and after 22 to 23 weeks of treatment with donepezil has revealed that prolonged P300 latency decreases as cognitive capability increases with improvement in recent memory (05).
Experimental studies in mice suggest that donepezil may improve cognitive function by increasing the hippocampal production of insulin-like growth factor I through sensory neuron stimulation, which may not be dependent on its acetylcholinesterase inhibitory activity. Donepezil treatment has been shown to rescue the cholinergic neurons in the medial septum from the neurodegeneration by olfactory bulbectomy in mice and significantly improve cognitive deficits (36). This is the first in vivo evidence of a neuroprotective effect of donepezil. However, in patients with mild Alzheimer disease, treatment with donepezil did not alter the progression of hippocampal deformation (34). MicroRNA-206 (miRNA-206 or miR-206) is now known to be involved in the pathogenesis of Alzheimer disease by suppressing the expression of brain-derived neurotrophic factor (BDNF) in the brain. A study has found that the expression of miR-206-3p is significantly upregulated in the hippocampus and cortex of Abeta precursor protein (APP)/presenilin-1 (PS1) transgenic mice, and donepezil administration significantly reversed this dysfunction (33). Furthermore, enhancement of the miR-206-3p level by AgomiR-206-3p significantly attenuated the antidementia effects of donepezil in APP/PS1 mice. The results of this study suggest that miR-206-3p is involved in the antidementia effects of donepezil.
Several disease modifying effects of donepezil, which include protection against amyloid β, ischemia, and glutamate toxicity; slowing of progression of hippocampal atrophy; and up-regulation of nicotinic acetylcholine receptors, are independent of cholinesterase inhibition support the potential of donepezil as a neuroprotective agent for Alzheimer disease rather than just symptomatic treatment (14). Results of a longitudinal study indicate significant improvement in rCBF in hypoprofused areas of the brain corresponding to improvement in ADAS-cog scores in patients with late-onset Alzheimer disease (28).
Pharmacokinetics. After a single oral dose of donepezil, peak plasma concentration is reached in 3 to 5 hours. It is well absorbed and has a bioavailability of 100%; this percentage is not affected by the time of day or food intake. It is extensively metabolized by the hepatic isoenzymes CYP2D6 and CYP3A4. Absorption and excretion rates of donepezil are slower in elderly than in young volunteers, but dose modification is not considered to be necessary in the elderly. According to the manufacturer, the pharmacokinetics of donepezil are minimally affected by hepatic or renal disease and no dose adjustment is necessary for these conditions.
Therapeutic drug monitoring. Recommended therapeutic range of donepezil is 30 to 75 ng/L. A novel, simple, specific, and sensitive high performance liquid chromatography assay for the detection and quantification of donepezil in serum of demented patients showed that concentrations suggested as therapeutic in the literature may only be reached either by high dosages or by using inhibitory metabolic effects of comedications (15).
Pharmacogenetics. The single nucleotide polymorphism rs1080985 in the CYP2D6 gene can influence the clinical efficacy of donepezil in patients with mild to moderate Alzheimer disease (22). Genotyping may be useful for predicting clinical responses to donepezil. Functional polymorphisms in the CYP2D6 gene can also influence the clinical efficacy of donepezil, and analysis of CYP2D6 genotypes is useful in identifying subgroups of Alzheimer patients with variable responses to donepezil treatment (27). Alzheimer patients with mutant allele *10 in the CYP2D6 gene may respond better to donepezil than those with wild allele *1 (37). Carriers of the ApoE E3 allele and the CYP2D6 rs1065852 polymorphism, which are related to Alzheimer disease, may provide clinically relevant information for predicting better therapeutic responses to donepezil therapy than noncarriers (17).
Delivery/formulations. Donepezil is also available in rapid disintegration tablet formulation for convenient administration and faster absorption. A higher-dose (23 mg/day) donepezil formulation provides more gradual systemic absorption, a longer time to maximum concentration versus the immediate-release formulation, and higher daily concentrations for patients with more advanced disease (25).