The stock fell $3.46, or 12.3 percent, to $24.73 in midday trading. Shares have traded between $14.42 and $35.65 over the last 52 weeks.

Biogen is paying Acorda $110 million upfront for overseas rights to the drug and Acorda could receive an additional $400 million based on regulatory approvals and sales. Biogen already sells the multiple sclerosis drugs Tysabri and Avonex.

Acorda will keep U.S. rights to Fampridine-SR, which is still being reviewed by the Food and Drug Administration and has yet to be reviewed by European regulators.

Lazard Capital Markets analyst Joel Sendek reaffirmed a "Buy" rating and $35 price target, saying the upfront payment is higher than anticipated. Previously, he expected a partnership outside the U.S. to bring the company $85 million upfront.

JPMorgan analyst Geoffrey Meacham also called the news positive for the company, but said some investors had been hoping that a company would acquire Acorda and the deal could make that more complicated.

"Strategic flexibility is reduced, but not eliminated," he said, adding that Biogen seemed the natural longer-term partner for the drug.

"Further, the bulk of Fampridine economics are in the U.S., so we believe that the U.S. opportunity alone, plus outstanding terms on ex-U.S. economics, could still be attractive to other parties," he said. "That said, we recognize that some investors were looking for a near-term acquisition and therefore would expect near-term trading to be choppy."

He reaffirmed a "Overweight" rating on Acorda.

A service of YellowBrix, Inc.