#082 · Drug of the Day Piperidine AChE inhibitor Alzheimer's (Aricept) Rx · not a disease-modifier 2026-09-08

Donepezil

Raise the acetylcholine that is left

(±)-2-[(1-benzylpiperidin-4-yl)methyl]-5,6-dimethoxy-2,3-dihydroinden-1-one · C24H29NO3 · MW 379.49 g/mol · CAS 120014-06-4 · Aricept · ChEMBL CHEMBL502

Donepezil (Aricept). A reversible, mixed competitive/noncompetitive acetylcholinesterase inhibitor that raises synaptic ACh in whatever cholinergic terminals Alzheimer's has not yet destroyed. Modest cognitive benefit, dose-limited by nausea, diarrhea, vivid dreams, and vagal bradycardia / syncope. It is the pharmacological opposite of scopolamine #066: one blocks muscarinic receptors, the other floods them with leftover transmitter. PDB 6O4W is a real human AChE–donepezil crystal.

Primary targetAChE
MechanismReversible inhibitor
AChE IC50~6 – 20 nM
PDB6O4W (real donepezil)
T½~70 h
Cholinergic AEsGI · bradycardia
Vs scopolamineOpposite direction
01 · Mechanism of Action

Inhibit the Esterase, Hope There Are Still Synapses

Donepezil spans AChE's gorge from the catalytic anionic site to the peripheral site — the classic dual-binding pose in 6O4W (human AChE, Gerlits et al.). ACh lingers; nicotinic and muscarinic receptors see more agonist. Benefit is symptomatic and small on average; it does not rebuild cortex. Memantine (#043) is often combined: different receptor, different kinetics.

Because you are globally boosting ACh, you buy the cholinergic toxidrome-lite: nausea, diarrhea, cramps, insomnia or vivid dreams (especially if dosed at night), bradycardia, heart block, syncope, and peptic-ulcer risk with NSAIDs. Anticholinergics (including many bladder and allergy drugs) pharmacologically cancel it.

① AChE gorge block

Benzylpiperidine in the CAS, dimethoxyindanone toward the PAS. Real crystal, 6O4W.

② More synaptic ACh

Whatever nucleus basalis terminals remain. No new neurons.

③ Cardiac vagal

Bradycardia, AV block, syncope — check pulse, caution with β-blockers and sick sinus.

④ GI cholinergic

Nausea, diarrhea, anorexia, weight loss. The usual titration-limiter.

⑤ CYP2D6 / 3A4

Both contribute; inhibitors can raise levels. t½ ~70 h means slow approach to steady state (~2 weeks).

⑥ Anticholinergic collision

Diphenhydramine, oxybutynin, scopolamine, many TCAs — opposite pharmacology, worse cognition.

Donepezil → AChE gorge occupancy → ↑ ACh · modest cognition · cholinergic AEs
+ anticholinergic → cancel the signal → lost benefit, stacked delirium risk
02 · Pharmacokinetics

A 70-Hour Half-Life Is Not a Gentle Titration Accident

Well absorbed; Tmax ~3–4 h; t½ ~70 h; steady state ~15 days. CYP2D6 and CYP3A4; glucuronides. 5 mg → 10 mg (and 23 mg ER) titration is GI-limited.

Tmax~3 – 4 h
T½~70 h
Steady state~2 weeks
CYP2D6 + 3A4
Bioavailabilityhigh
Foodminimal effect
Renalnot the main off-switch
TitrationGI / heart-rate limited
03 · Opposite of #066, Neighbor of #043

Don't Prescribe an Anticholinergic on Top and Call It Care

Scopolamine and diphenhydramine are delirium drugs in the elderly; donepezil is trying to do the reverse. Memantine is an NMDA uncompetitive blocker with different kinetics — combination is common and mechanistically coherent. Donepezil is not a stimulant, not a cure, and overnight 'Aricept dreams' are a cholinergic REM story, not a haunting.

04 · FlexAIDΔS · Shannon Entropy Analysis

A Rigid Gorge Plug With Almost No Rotors to Pay For

FlexAIDΔS · Entropy Commentary

Donepezil is a relatively rigid piperidine–indanone. It pays little ΔS_conf to occupy a pre-organized AChE gorge that evolved to swallow ACh. The crystal (6O4W) shows the Shannon collapse: gorge waters out, aromatic lining packed against the benzyl and indanone. High affinity from shape complementarity more than a single covalent warhead — reversible, mixed kinetics.

05 · Harm Reduction

Cholinergic Means Gut and Vagus

Overdose looks like a cholinergic toxidrome. Everyday harm is bradycardia, falls, and GI loss.

Bradycardia / syncope / falls

  • Check HR; caution with β-blockers, digoxin, sick sinus, late-degree block
  • Syncope → fracture in exactly the population prescribed this

GI / ulcer / NSAIDs

  • Nausea, diarrhea, anorexia, weight loss
  • Peptic ulcer risk rises with NSAIDs

Anticholinergic cancellation

Practice

  • Titrate slowly; 23 mg is not a prize
  • Combine with memantine on purpose, not by accident
SERIOUS: Symptomatic bradycardia, heart block, syncope-related trauma. Cholinergic crisis in overdose (SLUDGE + weakness). Not a respiratory-depressant opioid, but it will not protect anyone from one.
3D · Human AChE + donepezil (real co-crystal) PDB: 6O4W
Loading structure from RCSB…
Receptor (refined cartoon)
Contact residues
Ligand (ball-and-stick)
Structure: 6O4W — native human acetylcholinesterase with donepezil (Gerlits et al.). A genuine co-crystal spanning the catalytic gorge — not a surrogate. Rotate · scroll to zoom · right-drag to translate.
View on RCSB →

Receptor Binding Affinities

donepezil · AChE
Target Affinity Rel. Action
AChE
acetylcholinesterase
IC50 ≈ 6 – 20 nM
reversible gorge inhibitor
Inhibitor
BChE
butyrylcholinesterase
weaker
less selective than AChE
Weaker
nAChR / mAChR
direct
no (indirect via ACh)
not scopolamine
Indirect
NMDA
memantine site
no
combination is two drugs
None
Donepezil AChE IC50 low-nM (ChEMBL CHEMBL502). PDB 6O4W is a genuine human AChE–donepezil complex.