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van Dyck CH, et al. Lecanemab in early Alzheimer's disease. N Engl J Med. 2023;388:9-21. PMID 36449413

One-paragraph summary

An 18-month multicentre phase 3 trial randomised 1,795 participants aged 50–90 with early Alzheimer's disease (MCI or mild dementia due to AD) and amyloid confirmed by PET or CSF, 1:1, to intravenous lecanemab 10 mg/kg every two weeks or placebo. Mean baseline CDR-SB was approximately 3.2 in both groups. The adjusted least-squares mean change from baseline at 18 months was 1.21 with lecanemab and 1.66 with placebo (difference −0.45; 95% CI −0.67 to −0.23; P<0.001). In a 698-participant substudy, brain amyloid fell by 59.1 Centiloids more than placebo (95% CI −62.6 to −55.6). Secondary outcomes favoured lecanemab: ADAS-cog14 −1.44 (95% CI −2.27 to −0.61), ADCOMS −0.050 (−0.074 to −0.027), ADCS-MCI-ADL +2.0 (1.2–2.8). Infusion-related reactions occurred in 26.4% and amyloid-related imaging abnormalities with oedema or effusions in 12.6% (NCT03887455) (PMID 36449413).

Key findings

  • Primary endpoint met: CDR-SB difference −0.45 points (95% CI −0.67 to −0.23) at 18 months.
  • Amyloid removal was large: −59.1 Centiloids versus placebo.
  • All four key secondary endpoints favoured lecanemab.
  • ARIA-E 12.6%; infusion reactions 26.4%.
  • The authors' own conclusion is deliberately measured: "moderately less decline… but was associated with adverse events. Longer trials are warranted."

Limitations

  • The effect size sits below published minimal clinically important differences for CDR-SB (+1 in MCI, +2 in mild AD) (Muir 2024, PMID 38561021).
  • Eighteen months; nothing is established about accumulation, plateau or reversal beyond that, or about stopping rules.
  • Enrolment required biomarker confirmation and excluded high-microbleed and anticoagulated participants, so the trial population is less mixed-pathology than the treated population will be (Boyle 2018, PMID 29244218).
  • Functional unblinding is plausible given a 26.4% infusion-reaction rate and visible ARIA on monitoring MRI.
  • Real-world ARIA rates and their stage dependence emerged only later: 22% ARIA overall and 27% symptomatic ARIA in mild dementia versus 1.8% in MCI (Paczynski 2025, PMID 40354064).

Why it matters

CLARITY AD is the first trial in Alzheimer's disease to meet a prespecified primary clinical endpoint with a mechanism-based therapy, and it converted the amyloid hypothesis from a contested theory into a partially validated therapeutic target. It is simultaneously the origin of the field's central practical dispute — a statistically robust effect smaller than any published threshold for individual clinical importance — which is why the FDA granted traditional approval, the EMA authorised the drug only for ApoE ε4 non-carriers and heterozygotes, and NICE concluded in final draft guidance that the benefit does not justify the cost.

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