Biogen's tau drug slows Alzheimer's decline in first-of-its-kind trial

Overview

Updated Jul 14

For decades, nearly every Alzheimer's drug that reached patients aimed at one target: a sticky protein called amyloid. On July 14, 2026, Biogen showed the first clinical evidence that hitting a different protein, tau, can also slow the disease.

The drug, diranersen, missed the trial's main statistical goal. But it cut tau in the brain and slowed cognitive decline by about a quarter at its lowest dose. Biogen is taking it to late-stage trials anyway.

Why it matters

If tau-targeting holds up in larger trials, doctors could gain a second way to slow Alzheimer's, and possibly a drug to pair with today's amyloid treatments.

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Key Indicators

26%

Slowing of decline at low dose

Reduction in cognitive-decline rate on the CDR-SB scale at the 60 mg dose over 18 months.

50-65%

Tau cut in spinal fluid

Drop in total tau measured in cerebrospinal fluid across all doses tested.

416

Trial participants

People with early Alzheimer's enrolled in the Phase 2 CELIA study, down from a planned 735.

18 months

Study length

Length of the CELIA study before the readout.

Voices

Curated perspectives — historical figures and your fellow readers.

Ever wondered what historical figures would say about today's headlines?

People Involved

Szofia Bullain

Vice President, Alzheimer's clinical development, Biogen

Leading diranersen's move to late-stage trials

Holly Kordasiewicz

Head of Development, Ionis Pharmaceuticals

Co-developer of the drug's underlying technology

Laura Nisenbaum

Interim Chief Science Officer, Alzheimer's Drug Discovery Foundation

Independent voice assessing the field

Organizations Involved

Biogen

Biotechnology company

Developer of diranersen, advancing to registrational trials

A Cambridge, Massachusetts drugmaker that co-developed the amyloid drug Leqembi and is now testing a tau-targeting drug for Alzheimer's.

Ionis Pharmaceuticals

Biopharmaceutical company

Inventor of the antisense technology, Biogen's partner

A California biotech that pioneered antisense drugs and licensed the tau program to Biogen.

Timeline

April 2024 July 2026

3 events Latest: July 14th, 2026 · 2 months ago

July 2026
Full CELIA data presented at AAIC in London
Latest Presentation

Biogen showed the complete dataset, including a 26% slowing of decline at the 60 mg dose and 50-65% tau reductions in spinal fluid.
July 14th, 2026 • 2:00 PM BST
May 2026
Topline results: primary goal missed, but signals positive
Trial Result

Biogen and Ionis reported that CELIA missed its main statistical goal, yet cut tau and slowed decline across doses. Biogen said it would advance the drug.
May 14th, 2026
April 2024
Early trial shows the drug can lower tau
Research

A Phase 1 study of BIIB080 reported large drops in tau in spinal fluid, setting up the larger CELIA test.
April 1st, 2024

Scenarios

Biogen starts a Phase 3 diranersen trial

Likely Resolves by Jul 14, 2027

Discussed by: Biogen and Ionis statements; RBC Capital Markets and BMO Capital Markets notes

Biogen has said it plans to move diranersen into registrational development. The company would confirm a Phase 3 design, likely centered on the 60 mg dose, and register the trial. This is the path the company has already signaled after the CELIA readout.

Regulators wave the drug forward on the tau data

Uncertain Resolves by End of 2027

Discussed by: Neurology and biotech analysts tracking FDA precedent from earlier Biogen approvals

Biogen could seek an early regulatory path, arguing the tau reductions and cognitive signal justify it, echoing the accelerated approval of aducanumab. The Food and Drug Administration would need to grant a special designation or accept a biomarker-based filing. Skeptics note the missed primary endpoint makes this harder.

Larger trial fails to confirm the benefit

Possible Resolves by End of 2030

Discussed by: Braintrials analysis and independent researchers citing missing p-values and prior tau-drug failures

The cognitive signal came from pre-specified secondary analyses without disclosed confidence intervals, in a trial cut from 735 to 416 people. A properly powered Phase 3 against placebo could show no real benefit, as many earlier tau drugs did. That outcome would stall the program.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

2020-2021

Tau antibody drugs fail (2020-2021)

Genentech's semorinemab and Biogen's own gosuranemab, both antibodies aimed at tau floating between brain cells, failed to slow Alzheimer's in mid-stage trials despite hitting their targets in the body.

Then

Both programs were largely shelved for Alzheimer's.

Now

The failures suggested that mopping up tau outside cells was not enough, pushing researchers toward cutting tau production inside cells.

Why this matters now

Diranersen works differently, lowering how much tau the brain makes at all. Its signal is the first from any tau approach after those earlier defeats.

June 2021

Aducanumab's contested approval (2021)

The FDA approved Biogen's amyloid drug aducanumab, sold as Aduhelm, based on its ability to clear amyloid, even though two trials gave conflicting results on whether it helped patients. An advisory panel had voted against it, and several members resigned.

Then

Insurers and doctors largely refused to use it, and sales collapsed.

Now

Biogen pulled the drug in 2024, and the episode made regulators and analysts wary of biomarker-only approvals.

Why this matters now

Diranersen also missed its main clinical goal while moving a biomarker. That history shapes how regulators and investors will read Biogen's push to advance it.

November 2022

Lecanemab validates the amyloid target (2022)

Eisai and Biogen's lecanemab, later sold as Leqembi, slowed decline by about 27% in a large trial, giving the amyloid theory its clearest win after years of doubt. It won full FDA approval in 2023.

Then

Leqembi became a standard early-Alzheimer's treatment, though with safety monitoring for brain swelling.

Now

It set a benchmark, roughly a quarter slowing of decline, that new drugs are measured against.

Why this matters now

Diranersen's 26% slowing at its low dose lands near lecanemab's mark, which is why researchers call the tau result meaningful despite the missed goal.