UPDATE 2-Biogen gains overseas rights to Acorda's MS drug

* $110 mln up-front payment

* Biogen gains overseas rights to Fampridine-SR

* Acorda shares fall 6 percent; Biogen flat (Updates with details, analyst comment, share reaction; changes dateline, previously NEW YORK)

BOSTON, July 1 (Reuters) - Biogen Idec Inc (BIIB.O) has acquired rights outside the United States to Acorda Therapeutics Inc's (ACOR.O) experimental multiple sclerosis drug, the companies said on Wednesday.

Biogen, which makes the multiple sclerosis drugs Avonex and Tysabri, will pay Acorda $110 million up front for rights to Fampridine-SR. It could also pay up to $400 million more if the drug meets certain regulatory and sales milestones.

While some analysts believe the deal's terms are attractive to Acorda, the company's shares fell 6 percent to $26.49 in premarket trading as investors had hoped the company would be acquired.

"We believe that the news is a fundamental positive, though we would not be surprised to see the acquisition premium diminished," said Geoffrey Meacham, an analyst at J.P. Morgan. "We recognize that some investors were looking for a near-term acquisition and therefore would expect near-term trading to be choppy."

Cambridge, Massachusetts-based Biogen will make tiered, double-digit royalty payments to Acorda on overseas sales of Fampridine-SR, which is an oral, sustained release drug designed to improve walking ability in people with multiple sclerosis.

Acorda maintains rights to the drug in the United States, where the medicine is under review by regulators and a decision is expected by Oct. 22. The company has not yet submitted a European application for the drug.

Acorda will pay 7 percent of the upfront and milestone payments it receives from Biogen to Irish drugmaker Elan Corp (ELN.I), which manufactures Fampridine-SR.

Fampridine, if approved, would broaden Biogen's MS franchise as the company battles to boost sales of Tysabri, which have been curtailed amid concerns over its association with a potentially deadly brain infection.

Joel Sendek, an analyst at Lazard Capital Markets, estimates sales of Fampridine-SR outside the United States of $85 million in 2011 and $145 million in 2012.

Biogen shares were unchanged at $45.15 in premarket trading. (Reporting by Toni Clarke and Lewis Krauskopf in New York; Editing by Derek Caney and Maureen Bavdek)