Biogen Inc. (Biogen (BIIB)) shares took a hit Thursday after the company released mixed Phase 2 results for its experimental Alzheimer's drug diranersen. The stock was down about 5% at $194.37 in afternoon trading. But if you look past the headline miss, there's actually a lot to like—and analysts are already pointing to the silver linings.
The drug, an antisense oligonucleotide that targets tau protein, failed to meet its primary endpoint in the CELIA study. That endpoint measured dose response on the Clinical Dementia Rating–Sum of Boxes scale at 76 weeks. But Biogen says the drug showed cognitive benefits and reductions in tau biomarkers across all tested doses. In fact, the company called it the first randomized Phase 2 trial of a tau-directed therapy to show both biomarker impact and signs of cognitive benefit in early Alzheimer's patients.
The cognitive decline slowing was most pronounced at the lowest dose—60 mg every 24 weeks. That's a good sign for a couple of reasons: lower doses usually mean fewer side effects and lower costs, and it suggests the drug has a wide therapeutic window. The safety profile was consistent with earlier studies, so no red flags there.
Biogen said it still plans to advance diranersen to registrational development. That's a vote of confidence, and analysts seem to agree it's the right call.
Analysts: The Primary Endpoint Wasn't Everything
William Blair wrote Thursday that meeting the primary endpoint wasn't a prerequisite for moving forward. "We did not view meeting the primary endpoint as a requisite for advancing development," the firm said. "Instead, we are cautiously optimistic to see that the slowing of cognitive decline was observed across all studied doses and in the lowest dose (diranersen 60 mg administered every 24 weeks), where the benefit was highest."
Analyst Myles Minter added that follow-up discussions with management suggested the result likely reached nominal statistical significance. He noted that efficacy in the range of anti-amyloid therapies—roughly 27% to low-30% slowing of cognitive decline versus placebo—seemed like a reasonable goal. The exact magnitude of the benefit won't be disclosed until the Alzheimer's Association International Conference, so we'll have to wait for the full picture.
Biogen originally licensed diranersen from Ionis Pharmaceuticals Inc. (Ionis (IONS)) in December 2019, getting worldwide exclusive rights to develop and commercialize it.
Apellis Deal Closes, Adding Two Revenue Generators
In a separate but related move, Biogen announced Thursday that it completed its acquisition of Apellis Pharmaceuticals. The deal adds two commercial drugs—EMPAVELI and SYFOVRE—to Biogen's portfolio, expanding its nephrology business. Those two drugs generated combined net product revenue of $689 million in 2025. Biogen expects the deal to boost revenue growth and become accretive to non-GAAP diluted EPS in 2027.
Under the terms, Apellis shareholders received $41 per share in cash plus a contingent value right tied to future SYFOVRE sales milestones. About 82.4% of Apellis shares were tendered, and the stock has stopped trading on Nasdaq.
The Bottom Line
Biogen's stock dip reflects the market's disappointment with a trial miss, but the underlying data suggest diranersen has real potential. The cognitive benefit, even at the lowest dose, is encouraging, and the biomarker reductions are hard to ignore. With the Apellis deal adding immediate revenue and a pipeline that still has promise, Biogen is playing a long game. Whether that game pays off depends on the full data set coming later this year—but for now, the story is more nuanced than a simple miss.
