The FDA approval of lecanemab (Leqembi, Eisai/Biogen) in January 2023 and donanemab (Kisunla, Eli Lilly) in July 2024 represent the first disease-modifying therapies for Alzheimer's disease to achieve full regulatory approval — ending a 20-year streak of high-profile clinical failures and fundamentally validating the amyloid hypothesis of Alzheimer's pathogenesis. While the magnitude of benefit is modest and safety concerns are real, the treatments mark a turning point in the management of early-stage Alzheimer's disease that will reshape neurology practice over the next decade.
How Anti-Amyloid Antibodies Work
Both lecanemab and donanemab are monoclonal antibodies targeting amyloid beta — the misfolded protein that aggregates into plaques in the brain of Alzheimer's patients, accumulating years to decades before clinical symptoms appear. They work by binding to amyloid fibrils and protofibrils and triggering microglial phagocytosis (immune clearance) of amyloid deposits. Both have demonstrated near-complete amyloid clearance on amyloid PET imaging in treated patients — providing biological proof of mechanism that earlier anti-amyloid antibodies (bapineuzumab, solanezumab) failed to achieve convincingly.
Clinical Trial Results
The Phase 3 CLARITY AD trial for lecanemab enrolled 1,795 patients with early Alzheimer's disease (MCI or mild dementia) and confirmed amyloid pathology. At 18 months, lecanemab reduced the rate of clinical decline on the CDR-SB (Clinical Dementia Rating Sum of Boxes) by 27% compared to placebo — a statistically significant difference. The TRAILBLAZER-ALZ 2 trial for donanemab showed a 35% slowing of clinical decline on the iADRS (Integrated Alzheimer's Disease Rating Scale) in the full study population, reaching 60% in participants with low-to-intermediate tau pathology. Critically, both trials showed disease slowing — not reversal — and average absolute clinical differences are small (0.45 CDR-SB points for lecanemab), raising important questions about clinical meaningfulness for individual patients.
ARIA: The Key Safety Concern
Amyloid-related imaging abnormalities (ARIA) — consisting of brain microhemorrhages (ARIA-H) and edema (ARIA-E) — are the defining safety concern of anti-amyloid therapy, occurring in 21% (ARIA-E) and 36% (ARIA-H) of lecanemab patients versus 9% and 17% of placebo patients. Most ARIA is asymptomatic and resolves spontaneously on MRI surveillance; symptomatic ARIA occurred in 3% of patients. However, three fatal ARIA cases during lecanemab trials raised regulatory and clinical attention, particularly in APOE ε4 homozygotes who have substantially higher ARIA rates and severity. Mandatory MRI monitoring protocols require significant imaging infrastructure, and APOE genotyping before treatment initiation is now widely recommended.
Who Is a Candidate?
Based on trial eligibility criteria and prescribing information, appropriate candidates for anti-amyloid therapy are: (1) age 50–85, (2) diagnosis of MCI or mild Alzheimer's dementia, (3) confirmed amyloid pathology by PET scan or CSF biomarkers (Aβ42/40 ratio, p-tau), (4) no contraindications including anticoagulation therapy, history of multiple microhemorrhages, or certain MRI findings. Moderate-to-severe Alzheimer's patients and those with amyloid-negative biomarkers are not candidates. The requirement for amyloid PET confirmation has dramatically increased demand for nuclear medicine imaging capacity.
The Diagnostic Revolution Required
Anti-amyloid therapy is accelerating a broader shift toward biomarker-based Alzheimer's diagnosis years before clinical symptoms. Plasma p-tau217 tests (achieving >90% sensitivity/specificity for amyloid pathology) from companies including ALZpath, Fujirebio, and Quanterix are now FDA-cleared as screening tools, enabling blood-based identification of candidates for confirmatory PET — creating a clinically viable, scalable diagnostic pathway. Memory care clinics and neurology practices building dementia diagnostic infrastructure should ensure adequate supplies of laboratory consumables for blood draw protocols supporting biomarker testing programs.
