The Journey of Donepezil

Donepezil is completely absorbed after oral administration with
absolute bioavailability close to 100% — one of the few CNS drugs with essentially complete
systemic availability. This results from its relatively high aqueous solubility and lack of
significant first-pass metabolism. Peak plasma concentrations are reached in 3-4 hours after a
single dose, delayed to 5-7 hours after repeat dosing (reflecting enterohepatic recirculation).
Food does not affect the rate or extent of absorption; the drug may be taken with or without food.
The 23 mg formulation (higher-dose tablet for moderate-to-severe AD) uses a different
matrix delivery system. Donepezil orodispersible tablets (dispersed on the tongue) provide
an alternative for patients with dysphagia. Time of dosing (evening vs. morning) can affect
tolerability: evening dosing is associated with more vivid dreams and insomnia; morning dosing
reduces these adverse effects but may affect compliance.

Donepezil distributes extensively throughout the body with a
volume of distribution of approximately 12-16 L/kg, reflecting high tissue affinity including
robust CNS penetration. Plasma protein binding is approximately 96%, primarily to albumin (75%)
and alpha-1-acid glycoprotein (21%). High lipophilicity (logP ~4.0) enables efficient crossing
of the blood-brain barrier. Brain-to-plasma concentration ratios of approximately 3:1 are achieved
at steady state, indicating preferential CNS accumulation. Donepezil distributes to erythrocytes,
and red blood cell AChE inhibition correlates well with clinical AChE inhibitory activity. The
drug achieves steady-state plasma concentrations in approximately 15 days (reflecting the 70-hour
half-life). Distribution to peripheral tissues means peripheral AChE is also inhibited, contributing
to cholinergic adverse effects (GI, bradycardia).

Acetylcholinesterase (AChE) is the primary enzyme terminating
cholinergic neurotransmission — it hydrolyzes acetylcholine (ACh) at the synapse with extraordinary
efficiency (one of the fastest known enzymes: ~25,000 ACh molecules/AChE/second). In Alzheimer's
disease, degeneration of basal forebrain cholinergic neurons (in the nucleus basalis of Meynert)
reduces synaptic ACh in the hippocampus and cortex by approximately 60-90%, correlating with
cognitive decline. Donepezil binds the catalytic anionic site (CAS) and peripheral anionic site
(PAS) of AChE via dual simultaneous binding — a unique binding mode conferred by its piperidine-
benzylpiperidine scaffold connecting both binding sites. This bivalent binding produces slow-onset
competitive inhibition of AChE with high selectivity for AChE over butyrylcholinesterase (BuChE)
(AChE:BuChE ratio >1,200:1). Increased synaptic ACh activates M1 muscarinic receptors (memory
consolidation) and nicotinic receptors (cognitive performance) in limbic and cortical circuits.
The drug does not slow neurodegeneration.

Donepezil is metabolized by CYP2D6 (primary) and CYP3A4 (secondary)
to four identified metabolites: 6-O-desmethyl-donepezil (major active metabolite, pharmacologically
equivalent to parent via AChE inhibition), 5-O-desmethyl-donepezil, donepezil-N-oxide, and the
cis-N-oxide. 6-O-desmethyl-donepezil accounts for approximately 11% of the parent drug
concentrations at steady state and contributes meaningfully to overall cholinergic effect.
Glucuronidation of donepezil and metabolites also occurs. CYP2D6 poor metabolizers (approximately
7-10% of Caucasians, 1-3% of East Asians) have higher donepezil exposure (approximately 25-30%
increase in AUC). CYP2D6 inhibitors (paroxetine, fluoxetine, quinidine) increase donepezil exposure
and may exacerbate cholinergic adverse effects. CYP3A4 inhibitors (ketoconazole, erythromycin)
also modestly increase donepezil AUC.

Donepezil is excreted in urine (57%) and feces (15%) as unchanged
drug and metabolites. Approximately 17% of the dose is excreted as unchanged donepezil in urine.
The terminal elimination half-life is approximately 70 hours — the longest of any approved AChE
inhibitor — enabling once-daily dosing. This long half-life also means that AChE inhibition persists
for several weeks after discontinuation, and a gradual tapering approach is recommended when
stopping therapy to prevent cholinergic rebound. No dose adjustment is needed for renal or mild-
moderate hepatic impairment. Severe hepatic impairment reduces donepezil clearance. There are no
known dialytic effects on donepezil due to high protein binding.

Donepezil is approved for all stages of Alzheimer's disease (mild,
moderate, severe). Clinical benefit is modest but consistent: approximately 2-3 point improvement
on ADAS-cog scale (vs. 4-6 point natural decline). Compared to rivastigmine and galantamine, donepezil
has the most convenient once-daily dosing. Cholinergic adverse effects are the main limitation: nausea,
vomiting, diarrhea (managed by starting low at 5 mg/day for 4-6 weeks before increasing to 10 mg),
and bradycardia/AV block requiring caution in patients with sick sinus syndrome. Vivid dreams and
insomnia occur in up to 20% (managed by morning dosing or dose reduction). Combination with
memantine (NMDA antagonist) is common practice in moderate-severe AD. The drug is not disease-
modifying — amyloid beta and tau pathology continue unabated.