My Alzheimer's Journey

Part 151 - Biogen Updates

May 15, 2026

On February 10, I wrote about Biogen’s Diranersen (BIIB080), an investigational drug designed to reduce the production of tau protein. Diranersen was developed by Ionis Pharmaceuticals. In late 2019, Biogen exercised a license option with Ionis and obtained a worldwide, exclusive, royalty-bearing license to develop and commercialize Diranersen. In April 2025, the FDA granted Fast Track designation to the Diranersen Phase 2 CELIA clinical trial.

Over the past month or two, I have heard rumblings that the Phase 2 CELIA clinical trial was not going well. Yesterday, Biogen announced that it will move ahead with registrational development of Diranersen. Registrational development is also known as a pivotal trial. These trials are generally Phase 3 studies.

What is interesting (and confusing) is that according to the Biogen press release, “the Phase 2 CELIA clinical trial had compelling topline results.” Priya Singhal, M.D., M.P.H., Executive Vice President and Head of Development at Biogen said, “We are excited by these Phase 2 data, which give us the confidence to advance Diranersen to registrational development. We look forward to engaging with regulators and the broader Alzheimer’s disease community on next steps. I would like to thank the patients, families, investigators, and study teams who participated in this pioneering study.” As I scrolled down through the rather lengthy press release, I found this statement: “CELIA did not meet its primary endpoint assessing dose response for change from baseline on the Clinical Dementia Rating–Sum of Boxes (CDR-SB) at Week 76.”

This is why I am confused. Why is Biogen pushing forward with the registrational development of Diaranersen? Maybe they have seen enough, and they are super confident, and they simply decided to get the Phase 3 trial going.

In plain English, the Phase 2 CELIA trial failed to meet its primary endpoint of showing a dose-response relationship in patients with early Alzheimer’s disease. The study did not demonstrate that higher doses were more effective than lower doses in reducing clinical decline.

These are the key results of the Phase 2 CELIA trial:

Failed Endpoint: The study missed the main goal of demonstrating that higher doses (115 mg) worked better than the lowest dose (60 mg).
Best Efficacy at Lowest Dose: Unexpectedly, the strongest evidence of slowed cognitive decline was seen at the lowest dose tested (60 mg every 24 weeks).
Target Reduction Achieved: Despite missing the primary endpoint, Diranersen showed “robust reductions” in tau protein in the cerebrospinal fluid (CSF) and brain pathology (tau PET) across all doses tested, supporting its mechanism as a tau-targeting antisense oligonucleotide.
Safety Profile: The safety profile was generally consistent with previous studies, though a higher incidence of serious adverse events was observed at the highest dose

Biogen promises to present the full Phase 2 CELIA data at the Alzheimer’s Association International Conference (AAIC) 2026. This conference is being held in London July 12 - 15.

In my opinion, there is a bit of spin going on at Biogen. Frankly, I understand why. There is a lot riding on the success of Diaranersen. Biogen’s shares sank in value by about 5% yesterday after the Diaranersen announcement.

As I stated when I first wrote about Diaranersen, I believe the drug has great potential. Diranersen is designed to reduce both extracellular and intracellular tau. Most other research efforts have focused only on extracellular tau. Pre-clinical studies were positive. Clearly, the Biogen team is confident that Diaranersen can achieve its objectives. This registrational development trial is extremely important.

Specific details on the registrational development trial have not been released.

In other Biogen news, they completed the purchase of Apellis Pharmaceuticals today. Apellis is a Massachusetts-based biotech company focused primarily on rare disease therapies. With the acquisitions, Biogen gains two commercial drugs, Empaveli and Syfovre.

Apellis was delisted from the NASDAQ exchange yesterday and the company is now a wholly owned subsidiary of Biogen. Each Apellis shareholder received $41 per share in cash. Based on future Syfovre sales, Apellis shareholders may receive additional cash somewhere down the road. Biogen borrowed roughly two billion dollars to close the deal.

Sources:

Biogen Press Release - May 14, 2026

Seeking Alpha - May 15, 2026

First Word Pharma - May 14, 2026

Medical Dialogues - May 15, 2026