BIIB Stock On Track For Worst Day In 2 Years After RBC Questions If Alzheimer’s Drug Benefits Will Replicate In Further Study

• Biogen said the results support advancing diranersen into larger late-stage trials.
• Stifel expects the diranersen program to remain “highly risk-adjusted.”
• RBC Capital questioned how strong and replicable the mid-stage trial results are and whether the drug would achieve strong commercial uptake.

Shares of Biogen Inc (BIIB) fell 7% on Tuesday after the company released more detailed results from a mid-stage study of its experimental Alzheimer’s drug diranersen at a London conference, raising questions about dosing, consistency and long-term potential.

Biogen shared the detailed data at the Alzheimer’s Association International Conference in London. The stock is now on track for its worst day since July 2024, if losses hold.

BIIB's Study Overview

The mid-stage trial tested diranersen in 416 people with early Alzheimer’s disease, either mild cognitive impairment or mild dementia. The drug is designed to lower levels of tau, a protein that builds up in the brains of people with Alzheimer’s and is linked to nerve cell damage.

The study did not meet its main goal of showing a clear relationship between higher doses and better results on a key measure of thinking ability and daily function. However, researchers reported positive signals across all doses tested, with the lowest dose’s clinical benefits standing out.

The 60 mg dose given every six months produced the clearest effects. On standard tests of memory and thinking, it appeared to slow decline by roughly 42% on one cognitive measure and 50% on another compared with placebo. On the main overall measure of dementia severity and daily activities, it slowed worsening by about 26%.

Higher doses showed smaller or less consistent benefits on the same measures. Some tests of everyday functioning did not show a clear separation across dose groups from placebo at 18 months.

The drug, however, reduced tau levels in spinal fluid by 50–65% across doses. Further, the drug was generally well-tolerated, the company said. Most side effects were mild and related to the spinal injection procedure. No cases of brain swelling or bleeding — a concern with some other Alzheimer’s drugs — were reported.

Biogen said the results support advancing diranersen into larger late-stage trials. Longer-term data from an ongoing extension study are expected to provide more clarity on durability and dosing.

Analyst Views

Stifel said the 60 mg dose showed the strongest efficacy, but noted investors will likely question the safety profile and unusual dose response. The firm believes the results validate tau as a drug target "at the very least", though it expects the program to remain “highly risk-adjusted.” Stifel maintains a ‘Buy’ rating.

RBC Capital Markets analyst Brian Abrahams called the data “mostly in line with expectations, with some supportive signals.” While he described the tau-cognition link as “a scientific breakthrough,” he questioned how strong and replicable the results are and whether the drug would achieve strong commercial uptake. The firm is further "not convinced" a late-stage study will replicate the "best-case" 60mg activity, it noted. 
RBC still sees the stock pullback as an attractive entry point and kept its ‘Outperform’ rating with a $242 price target.

How Did BIIB Retail Traders React?

On Stocktwits, retail sentiment around BIIB stock stayed within the 'bullish' territory over the past 24 hours, while message volume remained at ‘normal' levels.

According to data from Koyfin, 22 of the 36 analysts covering BIIB stock rate it ‘Buy’ or higher, while 13 rate it ‘Hold’ and one rates it ‘Sell.’ The stock has a 12-month average price target of $224.45, representing a potential upside of about 7% from the stock’s last close.

BIIB stock has gained 11% year-to-date.

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