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What is Alzheimer disease? Classified drugs used in Alzheimer disease explain mechanism of action therapeutic uses and adverse effect of cholinergic activators

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Alzheimer disease amyloid plaques neurofibrillary tangles pathology classification drugs

This medical infographic illustrates the pathology and future treatment strategies for Alzheimer's disease (AD). On the left, a sagittal view of a human brain labeled 'Brain of AD' highlights a central region in dark purple, indicating localized neurodegeneration or pathology. Arrows extend from this region to magnified cellular hallmarks: 'Neurofibrillary tangles' and 'Amyloid plaques.' The neurofibrillary tangles are depicted as purple, chain-like strands intertwined with a neuron's axonal and somatic structure. The amyloid plaque is shown as an extracellular deposit with a dark maroon core and a tan periphery. Below the anatomical illustrations, the graphic lists 'Three promising therapeutic strategies' for AD management: combination therapy, the use of multi-target drugs (MTD), and the exploration of new effective targets. This diagram serves as an educational summary of AD pathogenesis—specifically tau hyperphosphorylation and beta-amyloid accumulation—and the evolution of pharmacological approaches beyond traditional single-target inhibitors.

This medical infographic illustrates the pathology and future treatment strategies for Alzheimer's disease (AD). On the left, a sagittal view of a human brain labeled 'Brain of AD' highlights a central region in dark purple, indicating localized neurodegeneration or pathology. Arrows extend from this region to magnified cellular hallmarks: 'Neurofibrillary tangles' and 'Amyloid plaques.' The neurofibrillary tangles are depicted as purple, chain-like strands intertwined with a neuron's axonal and somatic structure. The amyloid plaque is shown as an extracellular deposit with a dark maroon core and a tan periphery. Below the anatomical illustrations, the graphic lists 'Three promising therapeutic strategies' for AD management: combination therapy, the use of multi-target drugs (MTD), and the exploration of new effective targets. This diagram serves as an educational summary of AD pathogenesis—specifically tau hyperphosphorylation and beta-amyloid accumulation—and the evolution of pharmacological approaches beyond traditional single-target inhibitors.

This medical schematic illustrates the neuroanatomical progression of Alzheimer’s disease pathology across different brain stages, focusing on amyloid-beta (Aβ) plaques and tau protein neurofibrillary tangles. The top row depicts Amyloid Plaque Stages A through C, showing a progression from localized mild density (blue) in the basal neocortex to moderate (green) and severe (red) densities as plaques spread throughout the isocortex. The bottom row displays the Tau Braak Stages (I-VI). Stages I-II show initial mild tau accumulation in the entorhinal region and hippocampus. Stages III-IV visualize moderate density spreading to the limbic system, while Stages V-VI demonstrate severe, widespread tau pathology throughout the isocortex. A color-coded legend indicates pathology density: blue for mild, green for moderate, and red for severe. An anatomical reference on the right identifies key regions like the Isocortex and Entorhinal Region/Hippocampus. Timelines below suggest a clinical progression spanning 30 to 48 years, highlighting the long-term nature of neurodegeneration.

This medical schematic illustrates the neuroanatomical progression of Alzheimer’s disease pathology across different brain stages, focusing on amyloid-beta (Aβ) plaques and tau protein neurofibrillary tangles. The top row depicts Amyloid Plaque Stages A through C, showing a progression from localized mild density (blue) in the basal neocortex to moderate (green) and severe (red) densities as plaques spread throughout the isocortex. The bottom row displays the Tau Braak Stages (I-VI). Stages I-II show initial mild tau accumulation in the entorhinal region and hippocampus. Stages III-IV visualize moderate density spreading to the limbic system, while Stages V-VI demonstrate severe, widespread tau pathology throughout the isocortex. A color-coded legend indicates pathology density: blue for mild, green for moderate, and red for severe. An anatomical reference on the right identifies key regions like the Isocortex and Entorhinal Region/Hippocampus. Timelines below suggest a clinical progression spanning 30 to 48 years, highlighting the long-term nature of neurodegeneration.

This medical illustration depicts the characteristic spatial progression of protein aggregation in Alzheimer's disease. The figure is divided into two primary rows: Row A focuses on ̢-amyloid (A̢) plaques, and Row B focuses on Tau-associated neurofibrillary tangles. Each row begins with a histological micrograph showing the morphology of the protein deposits, followed by a sequence of three sagittal brain diagrams illustrating staging. In Row A (orange shading), A̢ plaques are shown to initiate in the neocortex, subsequently spreading to the allocortex and deeper subcortical structures, following a top-down and outer-to-inner progression. In Row B (blue shading), neurofibrillary tangles are shown starting in the locus coeruleus and transentorhinal regions of the brainstem and medial temporal lobe, then progressing through the limbic system to the wider neocortex. White arrows indicate the direction of spread, emphasizing the stereotyped spatiotemporal evolution of AD pathology. This diagram serves as a pathophysiology resource for understanding Braak staging and the involvement of neuronal transport in the spread of proteopathic seeds.

This medical illustration depicts the characteristic spatial progression of protein aggregation in Alzheimer's disease. The figure is divided into two primary rows: Row A focuses on ̢-amyloid (A̢) plaques, and Row B focuses on Tau-associated neurofibrillary tangles. Each row begins with a histological micrograph showing the morphology of the protein deposits, followed by a sequence of three sagittal brain diagrams illustrating staging. In Row A (orange shading), A̢ plaques are shown to initiate in the neocortex, subsequently spreading to the allocortex and deeper subcortical structures, following a top-down and outer-to-inner progression. In Row B (blue shading), neurofibrillary tangles are shown starting in the locus coeruleus and transentorhinal regions of the brainstem and medial temporal lobe, then progressing through the limbic system to the wider neocortex. White arrows indicate the direction of spread, emphasizing the stereotyped spatiotemporal evolution of AD pathology. This diagram serves as a pathophysiology resource for understanding Braak staging and the involvement of neuronal transport in the spread of proteopathic seeds.

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Alzheimer Disease (AD)

Definition

Alzheimer disease is a progressive, irreversible neurodegenerative disorder and the most common cause of dementia, accounting for approximately 70% of all dementias. It is characterized by a gradual decline in memory, cognition, language, and behavior, ultimately leading to complete functional dependence. The disease was first described in 1906 by Alois Alzheimer in a woman with paranoia, memory loss, and aphasia whose autopsy revealed amyloid plaques and neurofibrillary tangles (NFTs).
  • Bradley and Daroff's Neurology in Clinical Practice

Pathology

AD is defined by two hallmark pathological findings:
1. Amyloid Plaques (Neuritic/Senile Plaques)
  • Extracellular deposits of beta-amyloid (Aβ) protein derived from abnormal processing of amyloid precursor protein (APP)
  • Start in the neocortex, then spread to hippocampus, basal ganglia, thalamus, and brainstem
  • Neuritic plaques have a dense Aβ core surrounded by dystrophic neurites, microglia, and astrocytes
2. Neurofibrillary Tangles (NFTs)
  • Intraneuronal accumulations of hyperphosphorylated tau protein (normally a microtubule-stabilizing protein)
  • Follow the Braak staging pattern: earliest involvement in locus coeruleus and entorhinal cortex, spreading to hippocampus and ultimately the neocortex
  • When neurons die, tangles remain extracellularly as "ghost tangles"
Macroscopic findings: Diffuse cortical atrophy (especially multimodal association areas, limbic system, hippocampus). Primary motor, sensory, and visual cortices are relatively spared.
The progression of amyloid (Aβ) deposition and tau tangles in AD:
Alzheimer's disease pathology - amyloid plaque and tau tangle staging
AD pathology histology from Bielschowsky stain
Fig. Bielschowsky stain showing neurofibrillary tangles and amyloid plaques (CA1 hippocampus and temporal cortex) - Bradley and Daroff's Neurology

Cholinergic Deficit - The Basis of Drug Therapy

The cholinergic hypothesis is the cornerstone of AD pharmacotherapy: there is profound loss of cholinergic neurons in the nucleus basalis of Meynert and other basal forebrain regions, leading to reduced acetylcholine (ACh) in the hippocampus and cerebral cortex. These are regions critical for memory and cognition. Drugs that enhance cholinergic transmission aim to compensate for this deficit.

Classification of Drugs Used in Alzheimer Disease

ClassDrugs
Cholinesterase (AChE) InhibitorsDonepezil, Rivastigmine, Galantamine, Tacrine (withdrawn)
NMDA Receptor AntagonistMemantine
Anti-amyloid monoclonal antibodiesLecanemab, Aducanumab (newer agents)
Vitamin E / AntioxidantsAdjunctive use only
Note: Cholinesterase inhibitors are indicated for mild-to-moderate AD; memantine is indicated for moderate-to-severe AD. They are often combined for additive benefit.

Cholinergic Activators - Cholinesterase Inhibitors

Mechanism of Action

Cholinesterase inhibitors are reversible, nonacylating inhibitors of acetylcholinesterase (AChE) and/or butyrylcholinesterase (BuChE) - the enzymes that catabolize acetylcholine in the synapse. By blocking enzymatic breakdown, they increase synaptic concentrations of acetylcholine, particularly in the hippocampus and cerebral cortex, thereby potentiating cholinergic neurotransmission and producing modest improvements in memory and goal-directed thought.
DrugSelectivityReversibility
DonepezilAChE-selective; CNS-selective (minimal peripheral)Reversible, non-competitive
RivastigmineBoth AChE + BuChE (pseudoirreversible)Pseudoirreversible (slow dissociation)
GalantamineAChE inhibitor + allosteric nicotinic receptor modulatorReversible, competitive
TacrineNon-selective AChE (all forms); significant peripheral activityReversible
  • Kaplan and Sadock's Synopsis of Psychiatry; Kaplan & Sadock's Comprehensive Textbook of Psychiatry

Pharmacokinetics (Key Points)

DrugHalf-lifeDosingNotes
Donepezil70 hrsOnce daily100% oral bioavailability; metabolized by CYP2D6, 3A4
Rivastigmine~1 hr (plasma), 10 hrs (effect)Twice daily (oral); Patch once dailyDegraded by esterases; patch reduces GI side effects
Galantamine~6 hrsTwice dailyAlkaloid from Galanthus nivalis (daffodil); food reduces peak by 25%
Tacrine2-4 hrsFour times dailyLargely withdrawn due to hepatotoxicity

Therapeutic Uses

  1. Mild to moderate Alzheimer dementia - all three marketed agents (donepezil, rivastigmine, galantamine) are FDA-approved; donepezil is also approved for severe AD
  2. Slowing disease progression - slow progression of memory loss and reduce behavioral symptoms (apathy, depression, hallucinations, anxiety, purposeless motor behaviors)
  3. Lewy body dementia and Parkinson disease dementia - donepezil and rivastigmine are beneficial
  4. Vascular dementia - may respond to AChE inhibitors
  5. Traumatic brain injury - cognitive deficits may benefit from donepezil/rivastigmine
  6. Practical benefit: delay or reduction of need for nursing home placement
Donepezil is the only cholinesterase inhibitor approved for severe AD.

Adverse Effects

All cholinergic side effects result from excess peripheral muscarinic/nicotinic stimulation (SLUDGE: Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis).

Donepezil

  • Generally best tolerated
  • Nausea, diarrhea, vomiting (<3% at 5 mg; higher at 10 mg) - typically resolve after 3 weeks
  • Bradycardia (especially with underlying cardiac disease); syncope (rare)
  • Weight loss
  • Insomnia, muscle cramps

Rivastigmine

  • More peripheral activity than donepezil → higher GI side effects
  • Nausea, vomiting, dizziness, headache, diarrhea, abdominal pain, anorexia, fatigue, somnolence
  • Adverse effects more common at doses >6 mg/day; reduced with transdermal patch
  • Does not cause hepatic, renal, or hematologic abnormalities

Galantamine

  • Dizziness, headache, nausea, vomiting, diarrhea, anorexia - generally mild and transient
  • GI effects less severe than rivastigmine

Tacrine (withdrawn/historical)

  • Hepatotoxicity (most significant): ALT/AST elevation in 25-30% of patients; requires weekly LFT monitoring for 18 weeks
  • Nausea, vomiting, myalgia, anorexia, rash
  • Requires four-times-daily dosing - impractical
  • Contraindicated in jaundice; discontinued clinically due to these issues
Adverse Effect Incidence Table (from clinical trials):
DrugDose (mg/day)NauseaVomitingDiarrheaDizziness
Donepezil54%3%9%15%
Donepezil1017%10%17%13%
Rivastigmine1-414%7%10%15%
Rivastigmine6-1248%27%17%24%
Galantamine1613%6%12%-
  • Kaplan and Sadock's Synopsis of Psychiatry, Table 21-56

Contraindications and Precautions

  • Peptic ulcer disease - increased gastric acid secretion via vagal cholinergic pathways
  • Sick sinus syndrome / cardiac conduction defects - risk of bradycardia/heart block
  • Asthma / COPD - bronchoconstriction risk
  • Seizure disorders - cholinergic activation can lower seizure threshold
  • Idiosyncratic reaction: rare catastrophic reaction with grief and agitation (self-limited on discontinuation)

Quick Summary Flowchart

Alzheimer Disease
    ↓
Cholinergic deficit (↓ACh in hippocampus/cortex)
    ↓
Cholinesterase Inhibitors block AChE → ↑ACh in synapses
    ↓
Agents: Donepezil → Rivastigmine → Galantamine (Tacrine withdrawn)
    ↓
Indications: Mild-Moderate AD (all); Severe AD (donepezil only)
    ↓
ADRs: GI (nausea/vomiting/diarrhea), Bradycardia, Hepatotoxicity (tacrine only)
Sources: Kaplan and Sadock's Synopsis of Psychiatry; Kaplan & Sadock's Comprehensive Textbook of Psychiatry; Bradley and Daroff's Neurology in Clinical Practice
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