On August 6, Biogen (NASDAQ:BIIB) completed its acquisition of RayThera, a privately held biotech developing small-molecule immunology therapies whose lead program entered Phase 1 trials in July 2026. The deal lands just over a week after the company posted a second-quarter beat that sent shares up nearly 5% on July 29. Together, the two moments capture where Biogen stands right now: a company reshaping itself around newer growth engines while still carrying the weight of its aging multiple sclerosis business.
A Pipeline Getting Real Legs
The RayThera deal gives Biogen fresh assets aimed at immune-mediated conditions, and unlike many early-stage acquisitions, the lead program is no longer just a slide in a pipeline deck. It entered Phase 1 development in July 2026, months before the deal even closed. Priya Singhal, Biogen’s Executive Vice President and Head of Development, called the acquisition proof of “strong execution on our open innovation model in sourcing external opportunities” and pointed to immunology as a strategic growth area for the company.
That framing lines up with what showed up in the second-quarter numbers. Leqembi, Biogen’s Alzheimer’s drug developed with Eisai, grew sales 15% year over year to $184 million despite a slow launch shaped by cost and efficacy concerns. The rare-disease portfolio, including Skyclarys and Spinraza, grew 11% to $602 million. CEO Chris Viehbacher told analysts those marketed products were “already in themselves enough to help Biogen get back to a growth story,” with a late-stage pipeline that includes lupus drug litifilimab and kidney-disease drug felzartamab arriving “on top of a growing basis instead of a stable basis.” Biogen also raised its full-year revenue outlook to mid-single-digit percentage growth, reversing an earlier forecast for a mid-single-digit decline.
The Price Tag Keeps Climbing
Biogen’s older business is still shrinking. Multiple sclerosis drug revenue fell 13% year over year to $963 million in the second quarter, a reminder that the franchise that built the company is fading faster than the newer products are scaling. And the RayThera purchase does not stand alone. It follows the $5.6 billion acquisition of Apellis Pharmaceuticals earlier in 2026, which gave Biogen its foothold in kidney disease and two approved rare-disease drugs. Layering one acquisition on top of another raises the bill for integration and execution risk at a time when management already needs the newer drugs to outrun the MS decline.
The cost of that dealmaking already shows up in guidance. Biogen cut its 2026 adjusted earnings-per-share forecast to a range of $12 to $13, down from $14.25 to $15.25, citing a $3.85 per share impact tied to acquisition-related charges. A revenue outlook that improved and an earnings outlook that got cut in the same report is the kind of split that leaves investors debating which number matters more.
Where Investors Are Placing Bets
Hedge fund ownership of Biogen slipped from 64 funds in the prior quarter to 59 in the most recent one, a modest pullback rather than a rush for the exits. Short interest sits at just 3.92% of float, which suggests little organized skepticism is built into the stock right now. As of September 1, at a forward price-to-earnings ratio of 18.05, shares are priced for the growth story management is now selling, not for a business still working through an MS decline and acquisition charges.
What Happens Next
Biogen has spent 2026 trying to prove its growth story runs deeper than one drug or one deal, and the RayThera and Apellis acquisitions are the clearest evidence of that effort. The bull case rests on Leqembi, the rare-disease portfolio, and an immunology pipeline actually advancing into human trials rather than sitting on paper. The bear case rests on a legacy MS business still declining by double digits and an earnings forecast that just absorbed a real cut. For the growth story to hold, the newer assets need to keep outpacing what is being lost in multiple sclerosis.
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