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As Sarepta Therapeutics waits for the Food and Drug Administration to announce an approval decision on a new drug to treat Duchenne muscular dystrophy, the biotech is years behind schedule on a requirement to confirm the muscle-function benefit of its first and top-selling medicine Exondys 51.

The FDA has until Monday to decide if Sarepta can market a second drug called Vyondys 53 to treat patients, mostly young boys, with the inherited and fatal muscle-wasting disease. Approval would accelerate Sarepta’s revenue growth because it targets a different genetic form of Duchenne than Exondys 51.


But the long-term commercial success of both drugs is contingent on Sarepta producing definitive scientific proof of a clinical benefit for Duchenne patients. For Exondys 51, whose generic name is eteplirsen, that requirement is taking longer than expected — raising questions about Sarepta’s effectiveness and its commitment to fully meet FDA-mandated obligations.

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