The first Phase 2 clinical evidence that a tau Alzheimer’s drug can simultaneously reduce brain tangles and slow cognitive decline was presented at the Alzheimer’s Association International Conference (AAIC) 2026 in London on 14 July. Biogen’s diranersen, an antisense oligonucleotide targeting the genetic source of tau protein production, delivered what researchers and analysts described as the most significant data readout at this year’s conference, prompting the company to advance the therapy into Phase 3 development despite a missed primary endpoint in the Phase 2 CELIA study.
The results carry implications far beyond a single company’s pipeline. For three decades, the amyloid hypothesis has dominated Alzheimer’s drug development, shaping billions of pounds in research spending and producing the only two approved disease-modifying therapies. A Cochrane systematic review published in April 2026, pooling 17 randomised controlled trials of seven anti-amyloid antibodies and more than 20,000 patients, concluded that the drug class efficiently clears plaques from the brain but produces limited cognitive benefit for most patients. Against that backdrop, diranersen’s ability to target tau, the protein most closely correlated with cognitive decline, represents a potential turning point in how the field approaches treatment.
What the CELIA data showed
The Phase 2 CELIA study enrolled 416 participants with early Alzheimer’s disease across three dosing regimens of diranersen, administered intrathecally, and a placebo group. Participants had a mean age of 68 years, with 51 per cent female and approximately 69 per cent carrying at least one ApoE4 allele. The study population comprised 60 per cent with mild cognitive impairment and 40 per cent with mild Alzheimer’s disease dementia.
At 18 months, the 60 mg dose delivered the strongest clinical response. Compared with placebo, this dose demonstrated a 26 per cent slowing of clinical decline on the Clinical Dementia Rating Sum of Boxes (CDR-SB), a 42 per cent slowing on the ADAS-Cog13 cognitive assessment, and a 50 per cent slowing on the Mini-Mental State Examination (MMSE). Biogen reported that the majority of these endpoint differences achieved nominal statistical significance compared with placebo.
The biomarker data were equally striking. Diranersen produced mean reductions of 50 to 65 per cent in cerebrospinal fluid (CSF) total tau across all studied doses. In a tau PET imaging substudy of 131 participants, decreases from baseline were observed across all evaluated brain regions for every dose. Biogen stated that diranersen is the first tau-directed therapy to demonstrate reductions in both CSF total tau and brain tau pathology, as measured by PET, in a Phase 2 study.
The missed endpoint and the dosing paradox
The CELIA study did not meet its primary endpoint. The trial was designed to assess whether higher doses of diranersen produced greater clinical benefit on CDR-SB at 18 months, but this dose-response relationship was not observed. Higher doses drove greater tau reduction in the brain but did not translate into proportionally greater clinical improvement. The 60 mg dose, the lowest tested, showed the strongest clinical signal.
This discordance between biomarker and clinical outcomes has raised a central question for any tau Alzheimer’s drug entering late-stage development. Laura Nisenbaum, Interim Chief Science Officer at the Alzheimer’s Drug Discovery Foundation (ADDF), described the finding as raising important questions about optimal dosing that will be central to designing a robust Phase 3 programme.
Holly Kordasiewicz, Executive Vice President and Chief Development Officer at Ionis Pharmaceuticals, which discovered diranersen and licensed it exclusively to Biogen in December 2019, said the CELIA study was designed primarily as a dose-finding study to inform Phase 3 planning and to provide initial proof of efficacy. Biogen confirmed in May 2026 that it planned to advance diranersen into confirmatory Phase 3 development regardless of the missed primary endpoint, based on the growing body of evidence from Phase 1b and Phase 2 studies.
Why tau matters now
Tau tangles and amyloid plaques have been recognised as the twin pathological hallmarks of Alzheimer’s disease for more than three decades. The relationship between the two is becoming clearer. Amyloid accumulation appears to be an early event in the disease process, while tau pathology is more closely associated with the progression of cognitive decline. As Eisai Chief Clinical Officer Lynn Kramer explained, Alzheimer’s is a disease that accelerates over time, and once tau has taken over the process, anti-amyloid agents become less effective.
The clinical significance of this distinction is underscored by recent regulatory and research developments. The approved anti-amyloid therapies, Eisai and Biogen’s Leqembi (lecanemab) and Eli Lilly’s Kisunla (donanemab), have demonstrated the ability to clear amyloid plaques but have faced persistent questions about the degree of cognitive benefit they deliver and the risk of amyloid-related imaging abnormalities (ARIA), a potentially serious side effect involving brain swelling and microhaemorrhages. Diranersen, by contrast, targets tau through an entirely different mechanism and is not expected to cause ARIA based on its mode of action. The CELIA data were consistent with that expectation.
Professor Cath Mummery, Professor of Clinical Neurology at the UCL Queen Square Institute of Neurology and Consultant Neurologist at University College London Hospitals NHS Foundation Trust, said that the CELIA data provide some of the clearest evidence that reducing tau pathology can translate into clinically meaningful benefit. She described the magnitude of tau reduction and cognitive benefit observed in CELIA as among the most compelling reported to date in Alzheimer’s disease drug development.
Where previous tau drugs failed, diranersen offers a new signal
Diranersen’s significance as a tau Alzheimer’s drug is amplified by the long history of failed tau-targeting programmes that preceded it. The most recent was Johnson and Johnson’s posdinemab, an anti-tau antibody that failed its Phase 2b Autonomy trial in November 2025. The drug reduced phosphorylated tau in cerebrospinal fluid but failed to slow clinical decline. UCB’s bepranemab, another anti-tau antibody, also failed to demonstrate cognitive or functional benefit in Phase 2. Eli Lilly abandoned its antibody-based tau inhibitor zagotenemab in 2021, and its OGA inhibitor LY3372689 failed Phase 2 in 2024.
These prior failures were largely based on monoclonal antibodies targeting extracellular forms of tau. Biogen’s Priya Singhal, Executive Vice President and Head of Development, has drawn a distinction between that approach and diranersen’s mechanism of action. Unlike antibodies that attempt to clear tau from outside cells, diranersen targets MAPT messenger RNA to reduce the production of all tau isoforms, lowering both intracellular and extracellular tau protein. Andrew Tsai, analyst and managing director at Jefferies, observed that given the context of previous tau therapy failures, diranersen appears to be the first candidate that could succeed.
The broader tau landscape at AAIC 2026
Diranersen was not the only tau programme generating attention at the conference. Eisai presented data on etalanetug (E2814), its investigational anti-MTBR tau antibody, which demonstrated reductions in plasma eMTBR-tau243, a biomarker of tau tangle pathology. Etalanetug is currently being evaluated in the Tau NexGen Phase 2/3 trial in dominantly inherited Alzheimer’s disease, conducted under the Dominantly Inherited Alzheimer Network Trials Unit (DIAN-TU), and in a separate Phase 2 study in early sporadic Alzheimer’s disease. Both studies are assessing etalanetug added to lecanemab as standard of care, a combination therapy strategy that reflects the growing consensus that Alzheimer’s will ultimately require treatments targeting multiple pathologies.
Voyager Therapeutics presented preclinical data on VY1706, a gene therapy targeting tau via an intravenously delivered adeno-associated virus (AAV) capsid designed to cross the blood-brain barrier. In a six-month toxicology study in non-human primates, a single intravenous dose of VY1706 produced sustained tau protein reductions of up to 75 per cent in key brain regions and was well tolerated with no adverse findings in the central nervous system. The FDA cleared Voyager’s Investigational New Drug application for VY1706 in June 2026, and dosing in adults with early Alzheimer’s disease is expected in the second half of the year.
Al Sandrock, Chief Executive Officer of Voyager Therapeutics and formerly Head of Research and Development and Chief Medical Officer at Biogen, said he believes that within a decade, early detection and treatment with a combination of targeted therapies could lead to a world where Alzheimer’s disease is no longer a clinical inevitability.
The scale of the challenge ahead
The urgency of progress in Alzheimer’s therapeutics is driven by the scale of the disease burden. Global prevalence of Alzheimer’s disease and other dementias among adults aged 65 and older increased by 160 per cent between 1991 and 2021, rising from an estimated 18.7 million to 49 million prevalent cases, according to data from the Global Burden of Disease Study. Projections suggest the number of people living with dementia could reach 191 million by 2050. The Alzheimer’s Association has committed more than 490 million dollars in active and committed funding across more than 1,220 projects in 59 countries, with more than 30 per cent related to tau research.
Maria C. Carrillo, Chief Science Officer and Medical Affairs Lead at the Alzheimer’s Association, described the diranersen results as an encouraging step forward for people living with Alzheimer’s and their families. The Alzheimer’s Drug Discovery Foundation noted that the Alzheimer’s therapeutic pipeline continues to broaden, with 75 per cent of current clinical trials aimed at pathways beyond amyloid and tau, including inflammation, neurotransmitters, and metabolic dysfunction.
Biogen’s Priya Singhal stated that patients and families urgently need new approaches that address the complexity of Alzheimer’s disease, and that the company looks forward to working with health authorities and the broader Alzheimer’s community as diranersen advances to Phase 3. The FDA granted Fast Track designation to diranersen for the treatment of Alzheimer’s disease in 2025, a designation that may accelerate the regulatory review process.
What comes next
The central question for the field is whether a tau Alzheimer’s drug can deliver in Phase 3 what CELIA suggested in Phase 2. The dose-response paradox, in which the lowest dose produced the best clinical outcomes while higher doses drove greater tau clearance, will need to be resolved through careful trial design. Additional analyses from CELIA and the ongoing long-term extension study are expected at future scientific conferences.
For the Alzheimer’s community, the diranersen data mark a structural shift in how the disease can be approached. The amyloid-only era has not ended, but it is no longer the only viable path. As Isobel Coleman, Chief Executive Officer of the ADDF, observed, tau is one of the core pathologies the field will need to address if combination therapy is to become a reality. The data from AAIC 2026 suggest that goal is closer than it has ever been.
