Lecanemab, a monoclonal antibody that clears amyloid protofibrils from the brain, slowed cognitive and functional decline by 27% over 18 months in adults with early Alzheimer’s disease, according to a phase 3 trial published in The New England Journal of Medicine. The finding, from the CLARITY AD study of 1,795 participants, provided the pivotal evidence for the drug’s FDA approval in 2023 and marked the first time removing amyloid from the brain was shown to alter the course of Alzheimer’s disease in a confirmatory trial.
How the trial was designed
CLARITY AD was an 18-month, multicenter, double-blind, phase 3 trial. Researchers enrolled 1,795 people aged 50 to 90 with early Alzheimer’s disease, defined as mild cognitive impairment or mild dementia due to Alzheimer’s, together with evidence of amyloid on positron emission tomography or in cerebrospinal fluid. Participants were randomly assigned in a 1:1 ratio to receive intravenous lecanemab at 10 mg per kilogram of body weight every two weeks, or placebo.
The primary endpoint was the change from baseline at 18 months in the Clinical Dementia Rating Sum of Boxes (CDR-SB), a standardized measure of cognition and daily function that ranges from 0 to 18. Key secondary endpoints included amyloid burden on brain imaging, cognitive testing on the Alzheimer’s Disease Assessment Scale, and daily function on the Alzheimer’s Disease Cooperative Study Activities of Daily Living Scale.
What the trial found
At 18 months, CDR-SB scores had increased by 1.21 points in the lecanemab group and 1.66 points in the placebo group, an adjusted mean difference of −0.45 points (P=0.00005). Higher scores indicate worse function, so the smaller increase in the treatment group represents slower disease progression, about 27% less decline than placebo over the same period. All key secondary endpoints, including amyloid clearance measured on PET imaging, also favored lecanemab.
The most notable safety findings involved amyloid-related imaging abnormalities (ARIA), a class of brain-imaging changes seen with all anti-amyloid antibodies. Any form of ARIA occurred in 21.3% of participants receiving lecanemab, compared with 9.3% receiving placebo. ARIA with edema (ARIA-E) occurred in 12.6% of the lecanemab group, and ARIA with microhemorrhages (ARIA-H) in 17.3%. Symptomatic ARIA was less common: 2.8% for ARIA-E and 0.7% for ARIA-H. Infusion reactions occurred in 26.4% of the treatment group, typically after the first dose.
Why the APOE ε4 gene matters
Risk of ARIA differed sharply by APOE ε4 status. Among APOE ε4 non-carriers, 5.4% developed ARIA-E; among heterozygous carriers, 10.9%; and among homozygous carriers, 32.6%. Symptomatic ARIA-E followed the same gradient, ranging from 1.4% in non-carriers to 9.2% in homozygotes. That difference has practical consequences. Genetic testing before treatment helps identify patients at higher risk, and current prescribing guidance recommends caution in APOE ε4 homozygotes and additional MRI monitoring during the early months of therapy.
How the effect size should be read
A 0.45-point difference on the CDR-SB is statistically significant and consistent across pre-specified subgroups, but no formal threshold defines a minimally clinically important difference on the scale. Independent commentary at the time described the effect as modest but real. In the trial itself, benefits appeared on multiple measures including cognition, function, and biomarkers of the disease, which supported the interpretation that the drug slows the underlying process rather than masking symptoms.
The trial did not answer several important questions. Whether the benefit persists or grows beyond 18 months is being examined in an open-label extension study. Whether patients who stop treatment continue to benefit, and whether lower doses or subcutaneous formulations could reduce the burden of biweekly infusions, are the subject of ongoing trials.
What the approval settled
Along with the subsequent trial of donanemab, CLARITY AD established that removing amyloid from the brain can alter the course of Alzheimer’s disease when treatment starts early. That result reopened investment in a field that had struggled with anti-amyloid failures for two decades, and it raised the practical stakes for earlier and more accurate diagnosis, since neither lecanemab nor donanemab is approved for later-stage disease. Ongoing trials are now examining anti-amyloid drugs in combination with therapies targeting tau or brain inflammation, on the hypothesis that the modest single-drug effect can be enlarged in combination.
The trial authors concluded that lecanemab “reduced markers of amyloid in early Alzheimer’s disease and resulted in moderately less decline on measures of cognition and function than placebo at 18 months but was associated with adverse events.” That framing, cautious about the size of the benefit but clear about its scientific significance, has held up in the years since. For patients and families weighing the treatment, the decision remains an individual one, best made with a specialist who can weigh disease stage, APOE genotype, other health conditions, and personal goals against the burdens of biweekly infusions and MRI monitoring. Cost and access remain real constraints in the United States and elsewhere.
References
- van Dyck CH, Swanson CJ, Aisen P, et al. Lecanemab in early Alzheimer’s disease. New England Journal of Medicine, 2023; 388: 9-21. DOI: 10.1056/NEJMoa2212948