A European regulatory panel voted against approval of Biogen's Alzheimer's disease drug, in a possible blow to the prospects of the drug already grappling with a slow rollout in the US.

Shares of the Massachusetts-based company were down 3.6% before the bell, after it announced the panel's negative vote to Aduhelm.

They had fallen roughly 25% in the past three months, up to yesterday's close.

The panel recommendation will be taken into consideration by the EMA, which is not bound to follow, but usually does.

The drug was approved by US regulators earlier this year despite mixed data, and after an independent panel to the US Food and Drug Administration voted against the approval.

Biogen said earlier this week that its research chief Alfred Sandrock, who led development of its Alzheimer's disease drug, was leaving the company after a 23-year stint, a move that analysts called abrupt.

Aduhelm, the first new treatment for the memory-robbing disease in nearly 20 years, has been battling slow uptake in the US as hospitals and health insurers await for a decision on US government coverage for the drug.

The EMA's advisory panel decides what the regulator's recommendation on treatments, vaccines or devices should be.

The panel, Committee for Medicinal Products for Human Use, will take a final opinion on Biogen's Aduhelm at a December meeting, the company said.

Once the EMA has endorsed or refused a product, the executive body of the European Union, the European Commission, makes the final and formal decision which typically matches what the EMA has said.