Cognitive Longevity · Prevention-Trial Frontier

Overview

Alzheimer’s-related amyloid changes can be detected years before a person develops measurable cognitive impairment. That has led to one of the field’s most important unanswered questions: could removing amyloid before symptoms begin delay or prevent the clinical disease?

As of July 10, 2026, the answer is not known. Lecanemab and donanemab are FDA-approved for selected patients with mild cognitive impairment or mild dementia due to Alzheimer’s disease, the stages in which they were studied. Starting either drug in a cognitively unimpaired person remains investigational. Two major Phase 3 programs—AHEAD 3-45 and TRAILBLAZER-ALZ 3—are testing presymptomatic treatment, but neither has posted efficacy results.¹²³⁵

The most important points are:

• Presymptomatic anti-amyloid treatment is not routine care. It should not be offered outside an appropriate clinical trial.
• These studies are secondary-prevention trials. Participants are cognitively unimpaired but already have biomarker evidence of Alzheimer’s-related amyloid pathology.
• The biological rationale is plausible but unproven. Removing a target earlier does not automatically produce a meaningful cognitive benefit.
• The A4 trial was negative. Solanezumab did not slow cognitive decline over 240 weeks in amyloid-positive, cognitively unimpaired adults.⁷
• Risk tolerance is different when a person feels well. ARIA, hemorrhage, MRI surveillance, infusion burden, and uncertain long-term benefit must be weighed against no current symptoms.
• Routine amyloid screening of healthy adults is not established. A biomarker can identify risk-associated biology without predicting an individual’s clinical future.

Evidence cutoff: This article reflects publicly available evidence through July 10, 2026. ClinicalTrials.gov listed AHEAD 3-45 and TRAILBLAZER-ALZ 3 as active, not recruiting, with no results posted. The main Alzheimer’s Association International Conference is scheduled for July 12–15, 2026; findings presented after the cutoff are not treated here as established results.³⁵¹³

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What Is “Preclinical” or Presymptomatic Alzheimer’s Disease?

In research, preclinical Alzheimer’s disease generally means that a person is cognitively unimpaired on clinical assessment but has biomarker evidence of Alzheimer’s-related pathology, usually amyloid. The person does not have mild cognitive impairment or dementia. The term describes a biological-risk stage, not a clinical diagnosis that predicts with certainty whether or when symptoms will occur.

It is also useful to distinguish:

• Primary prevention: attempting to prevent the disease process from developing in the first place.
• Secondary prevention: intervening after an underlying biological process is detectable but before clinical symptoms appear.

AHEAD 3-45 and TRAILBLAZER-ALZ 3 are secondary-prevention studies because participants already have evidence of amyloid pathology.

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What Is Approved Today?

Current U.S. labeling for Leqembi® (lecanemab) and Kisunla™ (donanemab) states that treatment should be initiated in patients with mild cognitive impairment or mild dementia stage of disease, the populations studied in the clinical trials. Amyloid pathology must be confirmed before treatment. The labels do not establish safety or effectiveness for starting treatment in cognitively unimpaired people.¹²

In early symptomatic disease, lecanemab and donanemab produced statistically significant but modest average slowing of decline in their pivotal trials. That finding provides the rationale for earlier intervention, but it does not prove that treatment before symptoms will prevent dementia.⁹¹⁰

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Why Treat Earlier?

Amyloid accumulation often begins many years before recognizable cognitive impairment. By the time symptoms emerge, tau pathology, synaptic injury, neurodegeneration, inflammation, and vascular or other co-pathologies may already be present. The hypothesis is that removing amyloid earlier might alter the downstream cascade more effectively.

There are at least three reasons the hypothesis could still fail:

• Amyloid may be necessary but not sufficient to drive later decline, and removing it may not reverse downstream processes once initiated.
• Some biomarker-positive people would never develop clinically significant dementia during their lifetime, exposing them to treatment without benefit.
• A biomarker effect can be large while the clinical effect is small or absent. Trials must therefore measure cognition and function, not plaque removal alone.

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The Major Ongoing Trials

TRAILBLAZER-ALZ 3

TRAILBLAZER-ALZ 3 uses a decentralized design and plasma p-tau217 screening to identify potential participants before confirmatory procedures. A 2025 baseline report described 63,124 people screened, illustrating both the scale of the program and the difficulty of finding cognitively unimpaired adults who meet biomarker and safety criteria.⁴

The registry lists an estimated enrollment of 2,996 and follow-up of up to 332 weeks, with time to clinical progression as the primary outcome. An additional approximately 800-participant, 12-month addendum is assessing the safety of a different titration regimen. The study completed enrollment and remained active, not recruiting, with no posted results at the evidence cutoff.³

AHEAD 3-45

AHEAD 3-45 is one coordinated program with two biomarker-defined trials. A3 enrolls participants with intermediate amyloid and focuses primarily on whether lecanemab slows further amyloid accumulation. A45 enrolls participants with elevated amyloid and tests whether lecanemab slows cognitive change on the Preclinical Alzheimer Cognitive Composite 5 over 216 weeks. The program also evaluates safety, tau-related outcomes, and longer-term follow-up.⁵⁶

ClinicalTrials.gov listed AHEAD 3-45 as active, not recruiting, with estimated enrollment of 1,400, estimated primary completion in December 2028, and no posted results as of July 10, 2026.⁵

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The Negative Precedent: A4

The Anti-Amyloid Treatment in Asymptomatic Alzheimer’s Disease (A4) trial enrolled 1,169 cognitively unimpaired adults ages 65 to 85 with elevated amyloid on PET. Participants received solanezumab or placebo for 240 weeks. Solanezumab did not slow cognitive decline on the primary or secondary cognitive and functional outcomes.⁷

A4 does not settle the question for lecanemab or donanemab. Solanezumab targets soluble monomeric amyloid and did not substantially clear plaques, whereas the newer antibodies are effective plaque-clearing agents. But A4 is an important warning against assuming that an intuitive mechanism will translate into clinical prevention.

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A Provocative but Nonconfirmatory Signal: DIAN-TU

A 2025 open-label extension of the DIAN-TU gantenerumab program studied people from families with rare autosomal-dominant Alzheimer’s disease. Investigators reported amyloid removal and signals that longer treatment might delay clinical progression. However, the extension was not a continuously randomized placebo-controlled comparison: it was open label, used modeled or external comparisons, involved a rare genetic population, and had substantial attrition after the program was stopped early.⁸

Only 13 participants completed three years of open-label treatment, and estimates of clinical progression were imprecise. ARIA was also common. The report is scientifically important, but it is not proof that presymptomatic anti-amyloid treatment prevents sporadic Alzheimer’s dementia in the general population.

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Why the Risk-Benefit Balance Is Different Before Symptoms

Anti-amyloid antibodies can cause amyloid-related imaging abnormalities:

• ARIA-E: swelling or fluid-related MRI changes
• ARIA-H: microhemorrhages, superficial siderosis, or other bleeding-related changes

ARIA is often asymptomatic but can cause headache, confusion, visual symptoms, dizziness, gait difficulty, focal neurological deficits, seizures, or, rarely, serious or fatal hemorrhage. APOE ε4 carriers—especially homozygotes—have higher ARIA risk. Treatment requires baseline and surveillance MRIs, attention to antithrombotic therapy and bleeding risk, and the ability to recognize symptoms promptly.¹²

For a person already experiencing progressive cognitive impairment, a modest slowing of decline may justify these risks and burdens. For a person who functions normally and may never develop symptomatic dementia, the acceptable safety threshold is much higher. A successful prevention trial must show that clinical benefit is large and durable enough to justify treatment of many asymptomatic people.

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The Ethical and Psychological Questions

• Disclosure: Learning that amyloid is present can cause anxiety even though individual prognosis remains uncertain.
• Overdiagnosis: Some people may die of unrelated causes without ever developing clinically important cognitive impairment.
• Equity: PET, MRI, infusions, specialist care, and monitoring are resource-intensive and may widen access disparities.
• Insurance and planning: Biomarker knowledge can affect personal decisions and may have implications that differ by jurisdiction and insurance type.
• Long treatment horizons: Prevention could require years of therapy, monitoring, and cost before a benefit becomes measurable.

These issues are part of the reason presymptomatic biomarker disclosure and treatment belong in carefully designed research protocols rather than routine commercial screening.

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Should a Cognitively Healthy Person Get Tested or Treated Now?

Routine testing or anti-amyloid treatment is not currently recommended outside research. The 2025 Alzheimer’s Association blood-biomarker guideline addresses objective cognitive impairment in specialized care, not screening of asymptomatic adults. A positive amyloid biomarker cannot determine whether or when a cognitively healthy person will develop impairment.¹¹¹²

A clinical trial may be reasonable for a motivated person who meets eligibility criteria and understands the uncertainty, possibility of placebo, disclosure process, MRI and infusion burden, and potential adverse effects. Trial availability and enrollment status change, and the public registry should be checked rather than relying on promotional material.

People with a strong pattern of young-onset dementia in multiple generations should consider formal genetic counseling. Rare autosomal-dominant disease is different from having one older parent with late-onset Alzheimer’s disease.

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What Can Be Done Today?

• Address blood pressure, LDL cholesterol, diabetes, smoking, inactivity, hearing loss, vision loss, depression, excessive alcohol use, social isolation, and head-injury risk.
• Seek a clinical evaluation when there is progressive cognitive or functional change rather than ordering biomarkers in isolation.
• Consider research participation through reputable academic or registered clinical-trial programs.
• Avoid clinics offering off-label anti-amyloid treatment to cognitively unimpaired people as established prevention.
• Revisit the evidence when the major trials report clinical outcomes, not merely amyloid reduction.

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What Would a Positive Trial Change?

A convincingly positive prevention trial could reshape care: define a population in whom biomarker screening has actionable value, establish how early to treat, clarify the magnitude and duration of benefit, and set safety and monitoring standards. It would also create difficult questions about cost, access, overdiagnosis, treatment duration, and whether blood tests are accurate enough for population-scale pathways.

A negative result would be equally informative. It could show that amyloid removal alone is insufficient before symptoms, that the selected stage or dose was wrong, or that treatment must target tau, inflammation, vascular injury, or combinations of mechanisms. Either outcome should be judged by cognition and function, not by a biomarker headline alone.

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The Bottom Line

Treating Alzheimer’s disease before symptoms begin is one of the most consequential experiments in neurology. The hypothesis is plausible, the trials are large, and the results could change screening and prevention. But as of July 10, 2026, efficacy results are not available, and no anti-amyloid drug is approved for cognitively unimpaired people.

At Los Altos Neurology, presymptomatic anti-amyloid treatment is regarded as a research question, not established care. We can help patients and families distinguish late-onset family history from rare inherited disease, evaluate actual cognitive symptoms, interpret biomarkers in appropriate clinical context, and understand what participation in a registered prevention trial would involve. We do not present an investigational intervention as proven prevention.

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References

1. U.S. Food and Drug Administration. LEQEMBI (lecanemab-irmb) Prescribing Information. 2025.
2. U.S. Food and Drug Administration. KISUNLA (donanemab-azbt) Prescribing Information. 2025.
3. ClinicalTrials.gov. TRAILBLAZER-ALZ 3: Donanemab in Participants With Preclinical Alzheimer’s Disease (NCT05026866). Last update posted January 21, 2026.
4. Yaari R, Holdridge KC, Williamson M, et al. Donanemab in preclinical Alzheimer’s disease: Screening and baseline data from TRAILBLAZER-ALZ 3. Alzheimer’s & Dementia. 2025;21(9):e70662. doi:10.1002/alz.70662.
5. ClinicalTrials.gov. AHEAD 3-45: Lecanemab in Preclinical and Early Preclinical Alzheimer’s Disease (NCT04468659). Last update posted June 4, 2026.
6. Rafii MS, Sperling RA, Donohue MC, et al. The AHEAD 3-45 Study: Design of a prevention trial for Alzheimer’s disease. Alzheimer’s & Dementia. 2023;19(4):1227–1233. doi:10.1002/alz.12748.
7. Sperling RA, Donohue MC, Raman R, et al. Trial of Solanezumab in Preclinical Alzheimer’s Disease. N Engl J Med. 2023;389:1096–1107. doi:10.1056/NEJMoa2305032.
8. Bateman RJ, Li Y, McDade EM, et al. Safety and efficacy of long-term gantenerumab treatment in dominantly inherited Alzheimer’s disease: an open-label extension of the DIAN-TU trial. Lancet Neurol. 2025;24(4):316–330. doi:10.1016/S1474-4422(25)00024-9.
9. van Dyck CH, Swanson CJ, Aisen P, et al. Lecanemab in Early Alzheimer’s Disease. N Engl J Med. 2023;388:9–21. doi:10.1056/NEJMoa2212948.
10. Sims JR, Zimmer JA, Evans CD, et al. Donanemab in Early Symptomatic Alzheimer Disease: The TRAILBLAZER-ALZ 2 Randomized Clinical Trial. JAMA. 2023;330(6):512–527. doi:10.1001/jama.2023.13239.
11. Palmqvist S, Whitson HE, Allen LA, et al. Alzheimer’s Association clinical practice guideline on the use of blood-based biomarkers in specialized care. Alzheimer’s & Dementia. 2025;21:e70535. doi:10.1002/alz.70535.
12. Jansen WJ, Ossenkoppele R, Knol DL, et al. Prevalence of Cerebral Amyloid Pathology in Persons Without Dementia: A Meta-analysis. JAMA. 2015;313(19):1924–1938. doi:10.1001/jama.2015.4668.
13. Alzheimer’s Association. Alzheimer’s Association International Conference 2026. July 12–15, 2026, London and online.

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