Biogen (NASDAQ:BIIB) shares jumped 10% on Thursday after the company released encouraging Phase 2 data for diranersen, its experimental Alzheimer’s treatment targeting tau protein accumulation.
The results came from the company’s CELIA study, which evaluated the therapy in patients with early-stage Alzheimer’s disease.
Trial Shows Reductions in Tau Pathology
Biogen said diranersen produced significant reductions in tau pathology across all dose levels tested.
The findings were consistent with data previously observed in an earlier Phase 1b trial.
The treatment reduced both cerebrospinal fluid tau levels and tau pathology measured through positron emission tomography imaging, with reductions maintained throughout the dosing period.
Cognitive Decline Slowed Across Tested Doses
Pre-specified analyses of cognitive endpoints showed slowing of clinical decline across all studied doses.
According to the company, the most notable benefit was observed with the lowest dose of 60 mg administered every 24 weeks.
However, the study did not achieve its primary endpoint, which evaluated dose response for change from baseline on the Clinical Dementia Rating–Sum of Boxes at Week 76.
Study Included More Than 400 Participants
The 18-month CELIA study enrolled 416 participants diagnosed with either mild cognitive impairment related to Alzheimer’s disease or mild Alzheimer’s dementia.
Participants had not previously received anti-amyloid therapies.
Researchers evaluated three dosing regimens during the study:
- 60 mg every 24 weeks
- 115 mg every 24 weeks
- 115 mg every 12 weeks
Safety Profile Largely Consistent With Earlier Study
Biogen said the safety and tolerability profile of diranersen was generally in line with results from the earlier Phase 1b study.
The frequency of adverse events was similar across dose groups, although the highest-dose group experienced a greater number of serious adverse events.
Biogen Plans Registrational Development
The company said it intends to move diranersen into registrational development and plans to discuss the next regulatory steps with health authorities.
The U.S. Food and Drug Administration granted Fast Track designation to diranersen for Alzheimer’s disease treatment in 2025.
Diranersen is an antisense oligonucleotide designed to reduce production of tau protein.
Unlike therapies targeting only extracellular tau, diranersen aims to reduce both extracellular and intracellular tau.
Biogen said additional data from the study will be presented at the Alzheimer’s Association International Conference 2026.
Analysts View Advancement Decision as Positive Signal
Paul Matteis of Stifel said the company’s decision to continue development despite missing the primary endpoint reflects strong internal confidence in the program.
“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.”
Matteis also noted that under chief executive Chris Viehbacher, Biogen has aggressively reduced exposure to higher-risk research projects, suggesting management likely sees meaningful potential in the therapy.
Dose Response Raises Questions for Investors
One of the key discussion points for investors is the study’s non-linear dose response pattern.
Typically, higher doses are expected to deliver stronger efficacy, but the CELIA study showed the greatest cognitive benefit at the lowest 60 mg dose rather than at the higher 115 mg levels.
Matteis described the issue as an “open question,” suggesting it is theoretically possible that reducing tau levels too aggressively could create offsetting negative effects, although he stressed that conclusion remains speculative until more detailed data becomes available.