Shares of Biogen (BIIB Quick QuoteBIIB - Free Report) were down 8.2% yesterday after the company presented data from the phase II CELIA study evaluating its experimental tau-targeting Alzheimer’s disease (AD) drug, diranersen, at the Alzheimer’s Association International Conference (AAIC).

The AAIC data confirmed that CELIA did not meet its primary endpoint, which assessed the dose-response for change from baseline at week 76 on the Clinical Dementia Rating–Sum of Boxes (CDR-SB) score, a widely used clinical scale that measures cognitive and functional decline in AD patients.

More on BIIB’s Data Presentation

While the investigational tau-targeting therapy demonstrated meaningful reductions in cognitive decline across multiple measures—including a 26% slowing on CDR-SB, 42% on ADAS-Cog13 and 50% on MMSE at the 60 mg dose — along with robust reductions in cerebrospinal fluid tau and brain tau pathology, the study failed to meet its primary endpoint of demonstrating a dose-response relationship because higher doses did not produce greater clinical benefit.

Importantly, in May 2026, Biogen had already confirmed that CELIA failed to meet its primary endpoint and the latest data presentation did not alter that outcome.

The lowest dose (60 mg every six months) generated the strongest clinical results, while the two higher-dose regimens showed progressively smaller benefits. In the press release, Biogen did confirm that “higher doses were not associated with greater slowing of decline.” The 60 mg regimen slowed CDR-SB decline by 26%, while the two 115 mg regimens, given every six months and every three months, showed only 14% and 9% slowing, respectively. Similar variability appeared across other cognitive measures.

Also, it can be inferred that investors seemed more worried about Biogen’s plans to advance diranersen into confirmatory phase III development, as the study failed to demonstrate the expected dose-dependent clinical benefit. The stronger efficacy observed with the lowest dose than with higher doses may have added to the uncertainty surrounding the drug's late-stage prospects.

Biogen has still not disclosed a timeline for initiating the late-stage development program for diranersen.

BIIB’s Stock Performance

Shares of Biogen have risen 9% year to date compared with the industry’s 2.9% growth.

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More on Biogen’s Development Activities With Diranersen

Diranersen remains among the more advanced tau-targeting therapies currently in development for AD, an area many researchers believe could complement existing amyloid-focused treatments or potentially provide improved disease-modifying benefits.

Biogen is developing diranersen in collaboration with Ionis Pharmaceuticals (IONS Quick QuoteIONS - Free Report) .

Biogen currently markets AD therapy Leqembi, which has been developed in collaboration with Japan-based Eisai, with the latter leading the clinical development and regulatory submissions.

Unlike Leqembi, which targets amyloid-beta plaques, diranersen is designed to reduce the production of tau, which is strongly linked to AD progression and cognitive decline.

Another marketed amyloid-targeting AD therapy is Eli Lilly’s (LLY Quick QuoteLLY - Free Report) Kisunla. Eli Lilly is also developing experimental candidates targeting tau as interest in alternative AD mechanisms continues to grow across the industry.

BIIB’s Zacks Rank

Biogen currently carries a Zacks Rank #3 (Hold). You can see the complete list of today’s Zacks #1 Rank (Strong Buy) stocks here.