Alzheimer’s disease
What is Alzheimer’s Disease?
Disease definition & pathophysiology
Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline, memory loss, behavioral changes, and impaired daily functioning.
Pathologically, it is defined by extracellular amyloid-beta plaques, intracellular neurofibrillary tangles (composed of hyperphosphorylated tau protein), and widespread neuronal loss, particularly in the brain’s memory region (hippocampus) and outer layer (cortex).
Inflammation, oxidative stress, and cholinergic deficits also significantly contribute to disease pathology.
Commonly used Pharmacodynamic (PD) Models
- Disease progression models describing cognitive decline (e.g., using Alzheimer’s Disease Assessment Scale-Cognitive Subscale (ADAS-Cog) scores).
- Indirect-response or turnover models describing amyloid-beta accumulation or clearance.
- Target engagement models for anti-amyloid antibodies or tau-targeted therapies.
Patient characteristics
Typical patient population
Primarily affects individuals aged 65 and older, with prevalence sharply rising with advancing age.
Early-onset Alzheimer’s (<65 years) accounts for less than 5% of all cases.
Incidence and prevalence slightly higher in females.
Risk factors & disease progression indicators
Risk factors include advanced age, genetic predisposition (ApoE4 allele), family history, cardiovascular disease, diabetes, traumatic brain injury, and lifestyle factors (low physical and cognitive activity).
Disease progression indicated by cognitive decline severity, impairment of daily activities, behavioral disturbances, and biomarker changes (amyloid and tau biomarkers in cerebrospinal fluid (CSF) or imaging).
Diagnosis & biomarkers
Key Clinical Biomarkers
Diagnosis combines clinical assessment (cognitive tests) with biomarker evaluation.
CSF biomarkers include decreased amyloid-beta (Aβ42), increased total tau (t-tau), and increased phosphorylated tau (p-tau).
Neuroimaging biomarkers (PET imaging) detect amyloid plaques, tau accumulation, and structural brain atrophy (MRI).
Disease Severity Classification
Severity commonly assessed using clinical scales like Mini-Mental State Examination (MMSE):
- Mild AD: MMSE ~20–26 (mild cognitive impairment, memory loss, preserved basic activities)
- Moderate AD: MMSE ~10–19 (significant cognitive deficits, daily function impaired)
- Severe AD: MMSE <10 (severe cognitive dysfunction, completely dependent)
Clinical Dementia Rating (CDR) also frequently used (scale from 0–3).
How can Alzheimer’s Disease be treated?
Treatment aim (PD-targets)
Current treatments primarily target cognitive symptoms through modulation of neurotransmitter systems, particularly cholinergic and glutamatergic pathways.
Emerging therapies target disease modification by reducing amyloid-beta plaques or tau protein accumulation.
Common Drug Classes & Regimens
Anti-dementia drugs are ATC N06D, within N: Nervous system.
Cholinesterase inhibitors (first-line treatment for mild-to-moderate AD)
- Donepezil (N06DA02), Rivastigmine (N06DA03), Galantamine (N06DA04):
- MoA: Enhance cholinergic neurotransmission by inhibiting acetylcholinesterase, temporarily stabilizing cognitive function.
NMDA receptor antagonists (moderate-to-severe AD)
- Memantine (N06DX01):
- MoA: Modulates glutamate activity, reducing excitotoxicity and neuronal damage; often combined with cholinesterase inhibitors in advanced disease.
Disease-modifying biologics (emerging treatments)
- Anti-amyloid antibodies: Aducanumab (N06DX03), Lecanemab (N06DX04), Donanemab (N06DX05)
- MoA: Target amyloid-beta, promoting clearance or reducing plaque formation.
Dose Adjustments
Dose titration required due to gastrointestinal or neurological side effects (cholinesterase inhibitors).
Renal impairment necessitates dose adjustment for memantine.
Elderly patients may require slower dose titration.
Commonly used PK-models
- Population PK models capturing variability due to age, renal function, genetic factors, or disease severity.
- One- or two-compartment models for cholinesterase inhibitors and memantine.
- PK-PD models linking drug exposure to cognitive outcomes (e.g., MMSE or ADAS-Cog scores) or biomarker responses (amyloid PET imaging).
Clinical studies
The European Medicines Agency (EMA) guideline, in effect since 2018, sets different efficacy requirements for each stage of the disease: before symptoms (preclinical AD), with mild symptoms before dementia (prodromal AD, or mild cognitive impairment due to AD), and dementia. In mild to moderate AD dementia, an effect on cognition should be confirmed by an effect on daily function or on a global clinical assessment, as co-primary endpoints. In prodromal AD, a single composite scale of cognition and daily function is also acceptable as the primary endpoint. For symptomatic treatments, the effect is measured as the change from baseline, and trials in mild to moderate AD have traditionally lasted 6 months, preferably with three arms: the new drug, an approved drug and placebo. For disease-modifying treatments in prodromal and mild to moderate AD, a trial duration of at least 18 months has been assumed sufficient. To show disease modification, the guideline favors comparing the rate of decline between groups over time (slope analysis), or else the time to a clinically important event such as dementia, rather than to a set decline on a rating scale such as the ADAS-Cog [1].
Clinical pharmacology studies
For disease-modifying treatments, evidence of a slower clinical decline should be accompanied by biomarker evidence that the degeneration of the brain (neurodegeneration) also progresses more slowly. The choice of biomarker is left open, with more weight given to biomarkers showing an effect on the downstream disease mechanisms, not only that the drug reaches its target (target engagement); where the biomarker changes are unclear, a change in the disease course shown with a suitable trial design may be accepted instead [1].