Biogen Stock Surges 10% as Diranersen Delivers Positive Phase 2 Alzheimer’s Data
Biogen Inc. (NASDAQ: BIIB) shares jumped 10% Thursday after the company unveiled positive results from its Phase 2 CELIA study of diranersen, an experimental Alzheimer’s treatment that targets tau protein.
The study delivered robust reductions in tau pathology across every dose tested, mirroring the results of an earlier Phase 1b trial. Furthermore, pre-specified analyses of cognitive endpoints showed slowing of clinical decline across all doses studied — most notably at the lowest dose of 60 mg administered every 24 weeks.
Diranersen drove reductions in both cerebrospinal fluid tau and tau pathology measured by positron emission tomography across all dose groups, and those reductions held steady throughout the dosing period. However, the study did not hit its primary endpoint, which assessed dose response for change from baseline on the Clinical Dementia Rating–Sum of Boxes at Week 76.
Study Design and Safety Profile
The 18-month trial enrolled 416 participants with mild cognitive impairment due to Alzheimer’s disease or mild Alzheimer’s disease dementia, none of whom had previously received anti-amyloid therapy. Researchers evaluated three doses: 60 mg every 24 weeks, 115 mg every 24 weeks, and 115 mg every 12 weeks.
On the safety front, diranersen’s tolerability profile largely tracked with the Phase 1b study. Adverse event rates remained comparable across dose groups, although the highest dose produced a higher incidence of serious adverse events.
Biogen Advances Toward Registration
Despite the primary endpoint miss, Biogen plans to push diranersen into registrational development and will engage with regulators on next steps. The U.S. Food and Drug Administration previously granted Fast Track designation to diranersen for the treatment of Alzheimer’s disease in 2025.
Diranersen, an antisense oligonucleotide, works by reducing tau protein production. Unlike therapies that target only extracellular tau, diranersen aims to lower both extracellular and intracellular tau. Biogen will present the full data at the Alzheimer’s Association International Conference in 2026.
Wall Street Weighs In
Stifel analyst Paul Matteis acknowledged that the primary endpoint miss warrants caution, but he framed Biogen’s decision to advance the program as a meaningful “vote of confidence” from management. Under CEO Chris Viehbacher, Biogen has aggressively cut “risky” R&D projects — which suggests the internal data must be compelling.
“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.”
The Non-Linear Dose Response Puzzle
Meanwhile, one detail has caught Wall Street’s attention: the non-linear dose response. Typically, researchers expect higher doses to deliver greater efficacy. In the CELIA study, however, the strongest cognitive benefit emerged at the 60 mg dose rather than the higher 115 mg doses.
Matteis flagged this as an “open question,” speculating that lowering tau too aggressively could theoretically produce a counter-weighing negative effect — though he stressed this remains purely theoretical until Biogen presents the full data.
