Biogen Inc. (Biogen (BIIB)) had a rough Tuesday. The company announced it would push its investigational Alzheimer's therapy diranersen into Phase 3 testing, based on new Phase 2 data that showed the tau-targeting treatment slowed clinical decline and reduced key biomarkers. But investors weren't impressed — the stock fell nearly 9%.
Why the sell-off? 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 noted that questions remain about their robustness and reproducibility. The most puzzling part: the strongest clinical benefit was seen in the lowest-dose group, even though higher doses produced greater reductions in tau protein.
It's a bit like giving someone a small dose of painkiller and seeing better results than a larger dose — it makes you wonder what's really going on.
On the brighter side, Biogen and its partner Eisai also secured FDA approval for a once-weekly subcutaneous initiation dose of LEQEMBI IQLIK, expanding treatment options for early Alzheimer's patients. That's good news, but it wasn't enough to offset the diranersen concerns.
Phase 2 Data: Encouraging but Confusing
The new data from the Phase 2 CELIA study were presented at the Alzheimer's Association International Conference 2026. After 18 months, diranersen showed efficacy across all dose groups, with the strongest results in patients receiving 60 mg every six months. Compared with placebo, that regimen slowed clinical decline by 26% on the Clinical Dementia Rating Sum of Boxes (CDR-SB). Improvements were also seen on other cognitive measures like ADAS-Cog13, MMSE, and ADCOMS, with most differences reaching nominal statistical significance.
But here's the catch: the study did not meet its primary objective of demonstrating a dose-response relationship on CDR-SB at 18 months. In other words, higher doses didn't consistently lead to better outcomes. That's a red flag for analysts who want to see a clear cause-and-effect relationship.
Tau Biomarker Reductions: A First
On the biomarker front, diranersen achieved 50% to 65% reductions in cerebrospinal fluid total tau across all doses. In a tau PET imaging substudy, decreases in brain tau pathology were observed across all brain regions. Biogen claims diranersen is the first tau-directed therapy to show reductions in both fluid and imaging tau biomarkers in a Phase 2 study.
The therapy was generally well tolerated, with most adverse events being mild or moderate — things like procedural pain, post-lumbar puncture syndrome, and confusional state, which usually resolved within a week. More than 90% of participants who completed the placebo-controlled portion chose to continue into the extension study, which suggests patients were willing to keep taking it.
FDA Approves Weekly LEQEMBI Injection
On Monday, Biogen and Eisai announced that the FDA approved a supplemental Biologics License Application for once-weekly LEQEMBI IQLIK subcutaneous injections as an initiation treatment for early Alzheimer's disease. This means patients can start therapy with an autoinjector instead of intravenous infusions. The approved regimen is 500 mg weekly, given as two 250 mg injections. Patients can also switch between IV and subcutaneous administration throughout treatment.
The approval was supported by Phase 3 Clarity AD long-term extension data, which showed that subcutaneous administration achieved exposure comparable to IV dosing, with similar efficacy, amyloid removal, and safety.
At the time of publication Tuesday, Biogen shares were down 8.67% at $190.90.
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