Biogen's (NASDAQ:BIIB) Tecfidera is one of the world's best-selling multiple sclerosis drugs, and on Thursday, a judge in the Northern District of West Virginia ruled that a key patent on it is invalid. The decision means that Mylan (NASDAQ:MYL), the plaintiff in the suit, will be able to launch a generic version of the drug, which last year accounted for $4.4 billion of Biogen's $14.4 billion in revenue.

A gavel resting on a desk.

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Tecfidera is a twice-daily oral drug prescribed to reduce the number and severity of MS relapses. Patent '514, which was just invalidated, would have protected it from generic competition until 2028.

When Mylan sued Biogen, it asserted that "a lack of written description" should result in the patent being rescinded. According to Reuters, Judge Irene Keeley agreed, finding that the biotech company didn't "adequately describe the claimed invention."

The ruling doesn't mean MS patients will be able to buy a generic formulation yet, though. The decision could be overturned in the appeal Biogen has already said it will file. The appeals process could take up to a year, and in the meantime, Biogen also plans to ask for an emergency order to prevent Mylan from launching a generic version of the drug while litigation continues.

Nevertheless, the West Virginia court's decision suggests that a similar challenge advancing in Delaware could also go against Biogen. If so, a generic version of Tecfidera could become available by the middle of next year, according to Umer Raffat, an analyst at Evercore ISI.