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Sarepta stumbles on FDA rejection of a new drug to treat Duchenne muscular dystrophy

Sarepta Therapeutics was dealt a surprising setback Monday when the Food and Drug Administration rejected its marketing application for a second drug that aimed to treat children with Duchenne muscular dystrophy, a rare, inherited muscle-wasting disease.

In a statement, Sarepta said the FDA denied the approval of its drug, called Vyondys 53, due to the risk of infections related to intravenous infusion ports and kidney toxicity seen in animal experiments.

Sarepta shares fell 13% to $105 in after-hours trading. The stock finished the regular trading session down 4%.

The FDA did not issue its own statement explaining its negative decision, but the rejection of Vyondys 53 is an uncommon instance where the agency pushed back against a company seeking fast approval for a rare-disease drug based on an early biomarker — and before substantial evidence of patient benefit was obtained.

Fast approvals for rare-disease drugs have skyrocketed and become almost routine in recent years as the FDA established expansive rules meant to speed the development of medicines for unmet medical needs. The decision to reject Vyondys 53, therefore, might represent a new and more restrictive regulatory policy at FDA, which could affect all companies developing drugs for rare diseases.

But it might also only pertain to Sarepta, which has had a contentious relationship with the agency for many years.

“We are very surprised to have received the complete response letter this afternoon,” Sarepta CEO Doug Ingram said in a statement. “Over the entire course of its review, the agency did not raise any issues suggesting the non-approvability of golodirsen, including the issues that formed the basis of the complete response letter.” Golodirsen is the scientific name for Vyondys 53.

The company submitted data to the FDA showing Vyondys 53, administered via a weekly infusion, produced a small increase in an important muscle protein called dystrophin that is normally missing in children with Duchenne.

But not collected or submitted were any data showing Vyondys 53 capable of improving muscle function of Duchenne patients or slowing the progression of the disease. Sarepta was betting on a fast FDA approval first, before having to confirm treatment benefit in a later clinical trial.

Sarepta used this same strategy in 2016 to secure an accelerated approval for its first Duchenne drug, called Exondys 51. That decision was controversial. Outside advisors to the FDA — and even some of the agency’s own staff — concluded there was not sufficient evidence to demonstrate that Exondys 51 was effective in treating Duchenne.

Top officials at the FDA, led by Dr. Janet Woodcock, ignored the negative recommendations and approved the drug anyway, in part because of intense pressure from the families of children with Duchenne.

With the approval in 2016, Sarepta has been able to grow Exondys 51 into a product that generates $300 to $400 million in sales annually. However, the company has fallen years behind schedule completing the clinical trials required by the FDA to confirm the drug’s real benefit, as STAT reported last week.

Sarepta’s delinquency on Exondys 51 may have angered the FDA enough to play a role in the agency’s decision Monday to reject Vyondys 53.

Sarepta said it plans to meet with the FDA to address the issues that caused FDA to reject the drug. A confirmatory study of Vyondys 53 is currently in progress, but data on muscle function improvements are not expected for three or four years.

Vyondys 53 is designed to treat patients whose Duchenne is caused by an error in the DNA sequence known as exon 53. Approximately 8% of the 5,000 Duchenne patients in the U.S. carry the exon 53 error. Exondys 51 is used to treat the 13% of patients with disease caused by an error in the DNA sequence known as exon 51.


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