Acorda Therapeutics (ACOR - Get Report) lost an important legal challenge Friday to patents protecting its multiple sclerosis drug Ampyra. The news sent the stock plunging. 

A District Court judge found four Acorda patents on Ampyra were invalid due to obviousness. The ruling clears the way for an earlier-than-expected competition from generic versions of Acorda's drug, which is prescribed to multiple sclerosis patients to improve their walking ability. 

Ampyra sales were $493 million in 2016, or 95% of the Acorda's total revenue. The first generic version of the drug could launch in 2018, said Leerink analyst Paul Matteis.

"We are disappointed by the Court's decision and are preparing our appeal," said Acorda CEO Ron Cohen, in a statement. "Medical innovation depends on the recognition of valid intellectual property claims. We believe that we demonstrated novel and unexpected findings in our AMPYRA development program that led to the issuance of valid patents."

Acorda shares fell 21.5% to $21.00 Friday.

The company has been trying to expand the life of its Ampyra franchise. 

An attempt to move into new indications was stopped by a phase III failure in stroke rehabilitation last year the loss in the patent case means it will have to rely on its pipeline for vitality.

Ampyra had been expected to generate revenues of roughly $545 million this year.

Editors' pick: Originally published March 31.

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