The FDA is delaying a controversial Alzheimer's drug decision. Analysts say that's a good sign.

• The FDA is currently examining a Alzheimer's disease drug developed by Biogen and Eisai.
• Outside experts have said data from two halted trials don't prove the drug is effective.
• Regulators pushed back the decision deadline to June 7, at latest.

The Food and Drug Administration is delaying its decision deadline for Biogen and Eisai's hotly debated Alzheimer's disease drug, a move that analysts said bodes well for Biogen.

Cambridge, Massachusetts-based Biogen said Friday that the FDA's decision on its drug aducanumab has been pushed back by three months, moving the deadline from March 7 to June 7.

Biogen attributed the change to new data and analysis it had provided to regulators, which was considered a "major amendment" to its application.

Multiple analysts said the FDA's continued consideration of the drug, aducanumab, is a positive sign. "Based on our prior data analyses of FDA decisions, 'extensions' can commonly lead to FDA approval as they work through final details," Jefferies analyst Michael Yee wrote Friday. "FDA keeps working with (Biogen)... They could have rejected it here, but they did not."

Biogen's stock rose more than 6% on the news.

A turbulent road to the FDA

The drug, aducanumab, has been the subject of fierce debate over the last two years. It is one of few Alzheimer's treatments that's made it through the clinical trials and to the FDA for consideration.

But, the companies' two Phase 3 trials were temporarily shut down in 2019 after a futility analysis showed they were likely to fail. Biogen revived the drug after exploring data from one of the trials further and concluding that a subset of participants had benefited from aducanumab.

Aducanumab received a positive early review from the FDA last November, but a panel of outside advisors took issue with the trials' opposing outcomes and the data analysis presented by Biogen.

Read more: An influential group of health experts rejects Biogen's Alzheimer's drug

Baird analyst Brian Skorney said new data that caused the delay likely came from an ongoing extension trial, called the Embark trial. Biogen launched the study in March 2020 to continue testing subjects that had received aducanumab in either of the two Phase 3 trials that were shut down the year prior.

"It is implausible, in our view, that this uncontrolled study could address any of the concerns raised at the (advisory committee meeting). What it does do is buy Biogen additional time to step up advocacy group pressure on the agency and operate behind the scenes, as opposed to the visibility of a public advisory committee," he wrote in an analyst note.

Whether or not that's true, the extension is clearly supports the theory that aducanumab will be approved, Skorney said.