Biogen Jumps as Alzheimer’s Tau Drug Shows Promise

Key Moments

Phase 2 Results Drive Biogen Stock Higher

Investing.com — Shares of Biogen Inc. (NASDAQ:BIIB) rose 10% on Thursday after the company released positive top-line results from its Phase 2 CELIA trial of diranersen, an investigational Alzheimer’s therapy that targets tau protein.

The company reported that diranersen produced strong reductions in tau pathology at every dose tested, and that these findings were in line with outcomes from an earlier Phase 1b study. Pre-specified analyses of cognitive measures indicated a slowing of clinical decline across all dose groups, with the greatest effect observed at the lowest dose of 60 mg given every 24 weeks.

Clinical Outcomes and Primary Endpoint

Across the dosing arms, diranersen was shown to lower both cerebrospinal fluid tau and tau pathology as measured by positron emission tomography. These reductions were maintained for the duration of the dosing period.

However, the study did not reach its primary endpoint, which evaluated a dose-response relationship for change from baseline on the Clinical Dementia Rating–Sum of Boxes at Week 76.

Study Design and Dosing Regimens

The CELIA trial ran for 18 months and included 416 patients with mild cognitive impairment attributed to Alzheimer’s disease or mild Alzheimer’s dementia who had not been treated previously with anti-amyloid therapies.

The study assessed three dosing regimens:

According to Biogen, the safety and tolerability findings were broadly similar to those seen in the Phase 1b program. The overall rate of adverse events was comparable among the dose groups, though serious adverse events occurred more frequently at the highest dose level.

Regulatory Path and Mechanism of Action

Biogen stated that it intends to advance diranersen into registrational-stage development and will begin discussions with regulators on the appropriate next steps. The company noted that the U.S. Food and Drug Administration has granted diranersen Fast Track designation for the treatment of Alzheimer’s disease in 2025.

Diranersen is described as an antisense oligonucleotide engineered to decrease production of tau protein. In contrast to therapies that focus solely on extracellular tau, diranersen is designed to reduce both extracellular and intracellular tau. Biogen said that detailed results from the CELIA study will be presented at the Alzheimer’s Association International Conference in 2026.

Analyst Reaction and Dose-Response Debate

Stifel analyst Paul Matteis commented that the failure to meet the primary endpoint warrants caution, but he also viewed the decision to move forward as a meaningful “vote of confidence” from Biogen’s leadership. He pointed out that under CEO Chris Viehbacher, the company has been proactive in discontinuing “risky” research and development initiatives, implying that internal data supporting diranersen must be strong.

“We would be surprised if they were advancing this on efficacy data that looks worse than the amyloid drugs,” Matteis wrote in a note to clients. “The ‘bar’ for advancing this shouldn’t necessarily be statistical significance, because there is still a lot we don’t know around the right population to test the tau thesis.”

One central issue for investors is the non-linear nature of the dose response. Rather than showing greater benefit at higher doses, the CELIA trial found the most pronounced cognitive effect at the 60 mg dose, not at either of the 115 mg regimens.

Matteis called this an “open question,” and suggested it is possible that reducing tau excessively could, in theory, lead to counterbalancing negative effects. He emphasized that this remains speculative until the complete data set is available.