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Sims JR, et al. Donanemab in early symptomatic Alzheimer disease: the TRAILBLAZER-ALZ 2 randomized clinical trial. JAMA. 2023;330:512-527. PMID 37459141

One-paragraph summary

A multicentre (277 sites, 8 countries) 18-month phase 3 trial randomised 1,736 participants with early symptomatic Alzheimer's disease and amyloid plus low/medium or high tau on PET, 1:1, to intravenous donanemab or placebo every four weeks for 72 weeks, with blinded switching to placebo once dose-completion criteria were met. Mean age was 73.0 years, 57.4% were women, 68.1% had low/medium tau pathology, and 76% completed. The primary outcome, change in iADRS at 76 weeks, was −6.02 (95% CI −7.01 to −5.03) with donanemab versus −9.27 (−10.23 to −8.31) with placebo in the low/medium-tau population (difference 3.25; 95% CI 1.88–4.62; P<0.001), and −10.2 versus −13.1 in the combined population (difference 2.92; 1.51–4.33; P<0.001). CDR-SB change was 1.20 versus 1.88 (difference −0.67; 95% CI −0.95 to −0.40) in low/medium tau. Twenty-three of 24 gated outcomes were statistically significant. ARIA-E occurred in 205 participants (24.0%; 52 symptomatic) versus 18 (2.1%) on placebo; infusion-related reactions in 8.7% versus 0.5%; three deaths in the donanemab group and one in the placebo group were considered treatment related (NCT04437511) (PMID 37459141).

Key findings

  • iADRS difference 3.25 points (95% CI 1.88–4.62) in the low/medium-tau population; 2.92 (1.51–4.33) combined.
  • CDR-SB difference −0.67 (−0.95 to −0.40) low/medium tau; −0.70 (−0.95 to −0.45) combined.
  • 23 of 24 gated outcomes significant.
  • Baseline tau burden was used as a stratification variable, with the primary analysis population defined by low/medium tau — the first phase 3 AD trial to prespecify a tau-defined population.
  • ARIA-E 24.0% with 52 symptomatic cases; three treatment-related deaths.
  • Treatment was stopped once amyloid clearance criteria were met, establishing a finite-duration paradigm.

Limitations

  • The effect size, like lecanemab's, is below published MCIDs for the same instruments (Muir 2024, PMID 38561021).
  • The primary population is a tau-defined subgroup; the combined-population effect is smaller.
  • iADRS is a composite whose clinical anchoring is less established than CDR-SB's.
  • Three treatment-related deaths in 860 exposed participants over 76 weeks is a rate that longer or broader exposure could change in either direction.
  • 24% of participants did not complete.

Why it matters

TRAILBLAZER-ALZ 2 is the second positive phase 3 in Alzheimer's disease and the one that introduced two structural innovations: stratification by tau burden as a prespecified efficacy population, and treatment cessation on demonstrated amyloid clearance rather than indefinite dosing. Its ARIA-E rate of 24.0% with three treatment-related deaths also fixed the upper bound of the safety debate, and drove the donanemab appropriate-use recommendations to specify MRI exclusion criteria (>4 microbleeds, cortical superficial siderosis, major vascular contribution) and a defined surveillance schedule (Rabinovici 2025, PMID 40155270).

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