Biogen Inc. (NASDAQ:BIIB) on Tuesday said it will advance investigational Alzheimer’s therapy diranersen into Phase 3 testing after new Phase 2 data showed the tau-targeting treatment slowed clinical decline while producing significant biomarker reductions.
Following the announcement, the stock fell nearly 9%. According to Bloomberg, RBC Capital Markets analyst Brian Abrahams said the findings “leave more questions than answers.”
While the data were largely in line with expectations, he said questions remain about their robustness and reproducibility. He also noted that the strongest clinical benefit was seen in the lowest-dose group despite greater reductions in tau at higher doses.
Separately, the company and Eisai also secured U.S. Food and Drug Administration (FDA) approval for a once-weekly subcutaneous initiation dose of LEQEMBI IQLIK, expanding treatment options for patients with early Alzheimer’s disease.
Phase 2 Data Support Next Step For Diranersen
The new findings from the Phase 2 CELIA study, presented at the Alzheimer’s Association International Conference 2026, build on previously reported topline results.
The company said diranersen demonstrated efficacy across all dose groups after 18 months, with the strongest results seen in patients receiving the 60 mg dose every six months.
Compared with placebo, the regimen slowed clinical decline by 26% on the Clinical Dementia Rating Sum of Boxes, while also showing improvements across cognitive and composite measures, including ADAS-Cog13, MMSE, modified iADRS, and ADCOMS.
Most of those differences reached nominal statistical significance.
Higher-dose regimens also slowed clinical decline across multiple endpoints, although the study did not meet its primary objective of demonstrating a dose-response relationship on CDR-SB at 18 months.
Tau Biomarker Reductions Back Mechanism
Beyond clinical outcomes, Biogen said diranersen achieved 50% to 65% reductions in cerebrospinal fluid total tau across all evaluated doses.
In a tau PET imaging substudy, decreases in brain tau pathology were observed across all assessed brain regions.
According to the company, diranersen is the first tau-directed therapy to demonstrate reductions in both cerebrospinal fluid total tau and brain tau pathology in a Phase 2 study.
The therapy was generally well tolerated, with most adverse events reported as mild or moderate.
The most common events included procedural pain, post-lumbar puncture syndrome, and confusional state, which generally resolved within a week.
More than 90% of participants completing the placebo-controlled portion elected to continue into the extension study.
FDA Approves Weekly LEQEMBI IQLIK Injection
On Monday, Biogen and Eisai Co., Ltd. said the FDA approved a supplemental Biologics License Application for once-weekly LEQEMBI IQLIK subcutaneous injections as an initiation treatment for early Alzheimer’s disease.
The approval allows patients to begin therapy using an autoinjector rather than intravenous infusions. The approved initiation regimen consists of 500 mg administered weekly as two 250 mg injections.
Patients can also receive LEQEMBI through either intravenous infusion or subcutaneous injection throughout treatment and switch between the two administration methods.
Phase 3 Clarity AD long-term extension data supported the approval, showing subcutaneous administration achieved exposure comparable to intravenous dosing, with similar expected efficacy, amyloid removal, and overall safety.
BIIB Stock Price Activity: Biogen shares were down 8.67% at $190.90 at the time of publication on Tuesday, according to Benzinga Pro data.
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