The drug company Biogen Inc. says it will seek federal approval for a medicine to treat early Alzheimer's disease, a landmark step toward finding a treatment that can alter the course of the most common form of dementia.

The announcement Tuesday is a surprise because the company earlier this year stopped two studies of the drug when partial results suggested it was not working. The company now says a new analysis of more results suggest that the drug helped reduce a decline of thinking skills at the highest dose.

The drug, called aducanumab, aims to help the body clear harmful plaques from the brain. Cambridge, Massachusetts-based Biogen is developing it with a Japanese company, Eisai Co. Ltd.

Shares of Biogen soared 36% to $305 in pre-market trading Tuesday.

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