Donanemab, a monoclonal antibody that clears amyloid plaques from the brain, slowed cognitive and functional decline by 35% over 18 months in adults with early symptomatic Alzheimer’s disease and low-to-intermediate brain tau levels, according to a phase 3 trial published in JAMA. The finding, from the TRAILBLAZER-ALZ 2 study of 1,736 participants, supported the FDA’s full approval of donanemab (Kisunla) in 2024 and provided the second confirmatory demonstration that removing amyloid can alter the course of Alzheimer’s disease when treatment starts early.
Why targeting amyloid took two decades
The amyloid hypothesis, the idea that abnormal accumulation of amyloid-beta protein drives Alzheimer’s disease, has guided drug development for more than 25 years. Multiple anti-amyloid drug candidates failed in phase 3 trials during that period, either because they did not slow cognitive decline meaningfully, or because the safety signals outweighed the benefit. Two developments changed the trajectory: better patient selection using amyloid PET imaging and CSF biomarkers, and a shift toward drugs that target specific aggregated forms of amyloid rather than monomer. Donanemab is designed to bind a modified form of amyloid found in established plaques, and it clears those plaques in a matter of months.
How the trial was designed
TRAILBLAZER-ALZ 2 was an 18-month, multicenter, randomized, double-blind, placebo-controlled phase 3 trial conducted at 277 sites across eight countries. Investigators enrolled 1,736 people aged 60 to 85 with mild cognitive impairment or mild dementia due to Alzheimer’s disease, confirmed by amyloid PET imaging and stratified by tau PET into two populations. The low-to-intermediate tau group (n=1,182) was the pre-specified primary analysis population; the high tau group (n=552) was expected to progress faster and respond less well. Participants were randomized 1:1 to intravenous donanemab or placebo every four weeks for up to 72 weeks. Once amyloid clearance was confirmed on repeat PET imaging, donanemab-treated participants switched to placebo, a design that lets treatment stop when the biological target has been achieved.
The primary endpoint was change from baseline at 76 weeks in the Integrated Alzheimer’s Disease Rating Scale (iADRS), a combined measure of cognition and daily function. Key secondary endpoints included change in the Clinical Dementia Rating Sum of Boxes (CDR-SB), the ADAS-Cog13 cognitive scale, and activities-of-daily-living measures.
What the trial found
In the low-to-intermediate tau population, donanemab slowed decline by 35% on iADRS (P<0.001) and by 36% on CDR-SB (P<0.001) at 76 weeks compared with placebo. The between-group difference on CDR-SB was −0.67 points (95% CI, −0.95 to −0.40). Activities of daily living declined 40% less in the treatment group. Across statistical approaches, patients on donanemab were estimated to have gained 4.4 to 7.5 months of preserved function over the 18-month trial. In the combined population that included the high tau group, the effect was smaller but still significant: 22% slowing on iADRS and 29% on CDR-SB.
Amyloid PET scans confirmed the mechanism. By 76 weeks, most donanemab-treated participants had cleared amyloid to below the diagnostic threshold, and roughly half completed their course of infusions within 12 months, at which point they switched to placebo. Plasma biomarkers of Alzheimer’s pathology, including p-tau217, decreased in the treatment group and increased in the placebo group.
ARIA and other safety findings
Amyloid-related imaging abnormalities (ARIA), a class of brain-imaging changes seen with all anti-amyloid antibodies, were the most notable safety signal. ARIA with edema (ARIA-E) occurred in 24% of the donanemab group and ARIA with microhemorrhages (ARIA-H) in 31%. Most ARIA cases were mild-to-moderate on imaging and did not cause symptoms. Symptomatic ARIA-E occurred in 6% of treated patients. Two trial deaths were directly attributed to ARIA, and a third participant died following a serious ARIA event. Risk of ARIA differed sharply by APOE ε4 status, with homozygous carriers at substantially higher risk, a pattern also seen with lecanemab. Infusion reactions occurred in about 9% of treated patients.
Why disease stage matters more than tau alone
Pre-specified subgroup analyses in the low-to-intermediate tau population revealed a strong effect of disease stage. In the 214 participants with mild cognitive impairment (the earliest stage), donanemab slowed decline by 60% on iADRS and 46% on CDR-SB. In the 534 participants with mild dementia, the corresponding figures were 30% and 38%. Age also mattered: patients under 75 saw substantially larger benefit than those 75 or older. Both patterns point to the same conclusion, that intervention earlier in the disease course yields more benefit than intervention later.
How the effect size should be read
A 35% slowing of decline over 18 months translates to roughly four to seven months of preserved function, depending on the outcome measure. Whether that difference is noticeable in daily life depends on the specific patient. Independent commentary at the time described the effect as clinically meaningful but modest. The trial did not test whether the benefit persists or grows after donanemab is stopped, and long-term follow-up will be needed to answer that. What the trial clearly established is biological: donanemab clears amyloid, and clearing amyloid alters the trajectory of the disease when treatment starts before too much damage has accumulated.
What the evidence cannot yet answer
TRAILBLAZER-ALZ 2 was 18 months long. It cannot tell us whether the benefit widens with longer treatment, whether patients who cleared amyloid and stopped donanemab maintain their advantage over years, or whether repeat courses will be needed if amyloid re-accumulates. The trial’s ARIA rates likely reflect real-world risk imperfectly, since trial participants were younger and healthier than typical Alzheimer’s patients, and ARIA management in community practice differs from trial settings. Combination therapies pairing anti-amyloid antibodies with drugs targeting tau or brain inflammation are being tested on the hypothesis that the modest single-drug effect can be enlarged.
What the approval settled
Along with the earlier phase 3 trial of lecanemab, TRAILBLAZER-ALZ 2 provided the confirmatory evidence that the amyloid hypothesis holds up in clinical trials: removing amyloid can alter the course of Alzheimer’s disease when treatment begins early. That result reshaped the field. Investment has returned to Alzheimer’s drug development after two decades of setbacks, and the practical stakes for earlier and more accurate diagnosis are higher than ever, since neither anti-amyloid antibody helps patients with more advanced disease.
The trial authors concluded that donanemab “significantly slowed clinical progression at 76 weeks in participants with early symptomatic Alzheimer disease and amyloid and tau pathology.” Whether the trade-off between meaningful but modest benefit and the burden of infusions, MRI monitoring, and ARIA risk makes sense for any individual patient is a decision best made with a specialist, weighing disease stage, APOE genotype, other health conditions, and personal priorities against the reality that no anti-amyloid drug is a cure.
References
- Sims JR, Zimmer JA, Evans CD, et al. Donanemab in early symptomatic Alzheimer disease: the TRAILBLAZER-ALZ 2 randomized clinical trial. JAMA, 2023; 330: 512-527. DOI: 10.1001/jama.2023.13239