Biogen Inc. stock climbed about 10% on Thursday following publication of results from the Phase 2 CELIA study of diranersen, an investigational treatment designed to reduce tau protein in Alzheimer’s disease. The company said the trial produced strong reductions in measures of tau pathology across every dose tested, with findings that were consistent with an earlier Phase 1b study.
The CELIA trial enrolled 416 participants over an 18-month period. Patients had either mild cognitive impairment due to Alzheimer’s disease or mild Alzheimer’s disease dementia and had not previously received anti-amyloid therapy. Three dosing regimens were evaluated: 60 mg administered every 24 weeks, 115 mg every 24 weeks, and 115 mg every 12 weeks.
Biogen reported that diranersen reduced cerebrospinal fluid tau levels and lowered tau pathology as assessed by positron emission tomography across all dose groups. Those reductions were reported to be maintained throughout the dosing interval. In pre-specified analyses of cognitive endpoints, the company said it observed slowing of clinical decline across all doses, with the clearest effect seen at the lowest dose of 60 mg every 24 weeks.
At the same time, the study did not achieve its primary endpoint, which evaluated dose response for change from baseline on the Clinical Dementia Rating - Sum of Boxes (CDR-SB) at Week 76. Biogen noted the failure to meet that pre-specified dose-response endpoint while highlighting the consistency of biomarker and cognitive findings across dosing arms.
Safety and tolerability in CELIA were described as generally consistent with observations from the Phase 1b trial. Adverse event rates were broadly comparable across the three dose groups; however, Biogen reported a higher incidence of serious adverse events at the highest dose tested.
Following these results, Biogen said it plans to advance diranersen into registrational development and will engage with regulatory authorities to determine next steps. The U.S. Food and Drug Administration previously granted diranersen Fast Track designation for the treatment of Alzheimer’s disease in 2025.
Diranersen is an antisense oligonucleotide engineered to reduce production of tau protein. Unlike approaches that primarily target extracellular tau, diranersen is designed to reduce both extracellular and intracellular tau. Biogen indicated that the CELIA data will be presented at the Alzheimer’s Association International Conference in 2026.
Summary
Biogen reported Phase 2 findings for diranersen showing sustained reductions in tau biomarkers and evidence of slowed clinical decline at multiple doses in an 18-month study of 416 participants. The trial missed its primary dose-response endpoint on the CDR-SB at Week 76. Safety matched earlier data overall, with more serious adverse events observed at the highest dose. Biogen intends to pursue registrational development and will consult with regulators; the drug holds FDA Fast Track designation.
Key points
- Positive biomarker and cognitive signals across all doses in the Phase 2 CELIA study - relevant to biotech and healthcare sectors.
- The trial enrolled 416 participants with early-stage Alzheimer's who had not received anti-amyloid therapy, evaluating three dosing regimens over 18 months - important for clinical development and investor assessment.
- Biogen plans to advance diranersen to registrational development and will engage regulators; the drug has FDA Fast Track status from 2025 - consequential for regulatory and capital markets activity.
Risks and uncertainties
- The study did not meet its primary endpoint assessing dose response on the CDR-SB at Week 76 - a clear clinical risk for regulatory evaluation and market expectations.
- A higher incidence of serious adverse events was reported at the highest dose - a safety consideration that affects clinical development decisions and prescribing risk assessments.
- Advancement to registrational development and regulatory discussions are planned but not guaranteed to produce approval - an uncertainty for investors and the broader biotech and healthcare markets.