Biogen Stock Slides After Trial Miss, But Analysts Stay Bullish

Key Points

• Biogen's diranersen missed its primary endpoint in the Phase 2 CELIA trial, sending BIIB shares down more than 10% on May 14.
• Despite the endpoint miss, diranersen showed robust tau reductions and slowed cognitive decline, and Biogen is advancing the drug into further development.
• Leqembi sales grew 74% year over year to $168 million, providing a stable commercial foundation while analysts maintain price targets above current levels.

U.S. Food & Drug Administration (FDA) events crystallize the risk/reward thesis for many biotechnology stocks, and Biogen Inc.'s NASDAQ: BIIB latest Alzheimer’s update shows why that trade can turn quickly.

Biogen entered May with momentum after a solid first-quarter earnings report and its planned acquisition of Apellis Pharmaceuticals NASDAQ: APLS for $41 per share, which would add two commercialized rare disease and immunology drugs and a nephrology infrastructure to anchor the felzartamab launch.

The bigger driver, however, was anticipation for Phase 2 data from CELIA, a study of diranersen, Biogen’s experimental tau-targeting treatment for Alzheimer’s disease.

That readout arrived on May 14 and produced a mixed reaction. Diranersen missed its primary endpoint, which measured dose response on the Clinical Dementia Rating-Sum of Boxes at Week 76, and BIIB fell more than 10%.

But the selloff may not tell the full story: Biogen still reported meaningful reductions in tau pathology and signs of slower clinical decline, giving investors a reason to keep the drug—and the stock’s long-term Alzheimer’s thesis—in focus.

Why Tau Matters for Biogen’s Alzheimer’s Pipeline

Diranersen is an antisense oligonucleotide (ASO) therapy that targets tau. Tau is a protein that stabilizes the internal transport system neurons use to carry nutrients and signals.

In Alzheimer’s disease, tau can become chemically altered, detach from its structural role and form tangles inside brain cells.

Along with amyloid plaques, those tangles are a defining feature of the disease, disrupting cellular communication, contributing to neuron death and tracking closely with the cognitive decline patients experience.

For investors, tau matters because it appears to be the more proximate driver of symptoms. A drug that reduces tau pathology, such as diranersen, opens the door to a potential amyloid-plus-tau combination therapy, which remains an important part of the long-term bull case for Biogen’s Alzheimer’s franchise.

What Biogen’s CELIA Trial Showed About Diranersen

CELIA was the first randomized Phase 2 study of a tau-directed therapy to show both biomarker impact and cognitive benefit in early Alzheimer’s disease.

Despite the primary endpoint miss, Biogen announced compelling topline results from the Phase 2 CELIA study of diranersen. The drug demonstrated robust reductions in both cerebrospinal fluid tau and tau pathology as measured by PET across all studied doses, with reductions maintained throughout the dosing period. Prespecified cognitive analyses also showed slowing of clinical decline across all studied doses, particularly at the lowest 60-milligram dose administered every 24 weeks.

Furthermore, the company is advancing diranersen. Biogen’s CEO, Chris Viehbacher, has been cutting risky research and development projects in recent years, so analysts are giving added weight to this “vote of confidence,” surmising that the internal data must be compelling.

But analysts have also treated the CELIA readout as more complicated than a simple failed trial.

The consensus price target is $215.62, representing a potential upside of about 14%. Guggenheim reiterated a Buy rating with a $260 price target, Oppenheimer raised its target to $300 and maintained an Outperform rating, and Mizuho maintained an Outperform at $236. 

Leqembi Sales and Apellis Deal Support Biogen’s Base Business

On a trailing 12-month basis, Biogen’s revenue is essentially flat but stable. The same story holds for the company’s adjusted earnings per share (EPS).

Biogen has 10 drugs in the market. The most recognizable one for investors is likely Leqembi, Biogen and Eisai’s approved treatment for early Alzheimer’s disease. Leqembi is the only drug with FDA-approved maintenance dosing options to keep slowing progression. Diranersen, by contrast, is still experimental and is being studied in CELIA, a Phase 2 trial focused on tau pathology.

Biogen's earnings report showed that Leqembi global in-market sales grew 74% year over year to $168 million. Plus, the FDA extended the review period for the Leqembi Iqlik subcutaneous starting dose Supplemental Biologics License Application (sBLA) by three months to a new Prescription Drug User Fee Act (PDUFA) date of Aug. 24, 2026.

Analysts generally viewed the delay as timing-related because Eisai said the extension followed submission of a major amendment and that the FDA had not raised approvability concerns to date.

Is BIIB a Buyable Dip After the Alzheimer’s Trial Selloff?

The chart tells a constructive longer-term story.

BIIB climbed steadily from the low $130s last summer to the high $180s today, with the 50-day SMA rising in lockstep and acting as reliable support throughout the uptrend. The stock is sitting just above that moving average right now, which is an encouraging sign given the post-CELIA selling pressure.

The MACD, however, flashes a near-term caution signal. The line has crossed below the signal line, and the histogram has turned negative. That suggests the pullback may not be fully exhausted. Volume on the May decline has been elevated, which is worth monitoring.

For patient investors, the 50-day SMA around $185 represents a logical, technically grounded entry zone. A hold at that level, particularly if the MACD begins to curl back upward, would suggest the broader uptrend remains intact ahead of Leqembi Iqlik’s Aug. 24 PDUFA date.