Duchenne muscular dystrophy: FDA advisers narrowly vote in favor of experimental gene therapy

Most parents wouldn't be thrilled with the idea of their kids getting hooked up to an IV bag filled with trillions of viruses.

But for Melanie Hennick, whose son, Connor, has Duchenne muscular dystrophy, it was an opportunity she hoped would change his life.

"We knew this wasn't a cure," Hennick said. "But it was a chance."

Connor is one of just dozens of kids to have received SRP-9001, an experimental gene therapy that aims to slow or stop the progression of Duchenne muscular dystrophy, or DMD. Current treatments for the disease – which primarily affects boys because of the way it's inherited – include steroids and, later, heart drugs. But none stop it.

SRP-9001 uses viruses to ferry a copy of a gene to muscles to help make up for one that's causing the disease. Hennick and many other parents like her advocated for the treatment's accelerated approval Friday in a meeting of outside advisers to the US Food and Drug Administration.

The advisers voted 8-6 in favor of approving the treatment, and the FDA will now decide whether to follow their advice.

"The decision the FDA has to make doesn't just affect the patients in study 301 [an ongoing confirmatory trial that Sarepta is running]; it affects the entire field of drug development for Duchenne," said Dr. Caleb Alexander, a professor of epidemiology and medicine in the Bloomberg School of Public Health at Johns Hopkins University, who voted against recommending approval. "The totality of evidence … simply doesn't rise to the threshold that's required for accelerated approval."

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Dr. Raymond Roos, a neurology professor at the University of Chicago Medical Center, voted in favor. "The downside of the gene therapy here is relatively small compared to whether it really helps the patient, and for this reason, I voted yes," he said.

Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said his agency will take the recommendation and "do something that we have to do every day at FDA. … We have to manage through the uncertainty here."

The FDA's decision, expected by the end of the month, will have implications not just for families like Connor's but for how the agency regulates treatments like this one more broadly: It would be the first of its kind of medicine – one-time treatments delivering a gene to try to fix a disease – to get accelerated approval, a faster track through the regulatory process. Its approval would set a precedent for other drugs like this based on so-called surrogate endpoints, a measure of what the drug does in the body, before further clinical evidence is available.

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"Approval of a gene therapy for Duchenne muscular dystrophy will be huge," said Jeffrey Chamberlain, a professor at the University of Washington School of Medicine who helped pioneer gene therapy approaches for the disease. "This, I think, will spur further research and further development of gene therapies for other diseases."

DMD patients don't have a lot of time to wait. Kids with Duchenne typically lose the ability to walk before they’re teenagers and often don't live well into their 30s, Chamberlain said. He's not directly involved with SRP-9001, which is being developed by Sarepta Therapeutics, and is on the scientific advisory board for another company working on DMD gene therapies, Solid Biosciences.

"Gene therapy appears to be a really good approach to try to treat this disease, because it's a genetic disease," Chamberlain said. "The cause of the disease is a mutation in a single gene."

That gene is responsible for the production of dystrophin, a protein key to the structure of muscle cells.

"It's kind of like the two-by-fours that make up your house," Chamberlain said. "It's really important for just holding everything together."

SRP-9001, invented at Nationwide Children's Hospital in Columbus, Ohio, before being licensed for development by Sarepta, delivers a miniaturized version of the dystrophin gene to cells, aiming to help them make a version of the muscle-preserving protein.

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In a key clinical trial, the therapy appeared to do that. But it didn't meet another main goal: showing a benefit on a measure of muscle function, complicating SRP-9001's path through the FDA.

Sarepta blamed the outcome on an imbalance in how the trial separated patients into the placebo and treatment groups. But key FDA reviewers appear unconvinced.

"The clinical studies conducted to date do not provide unambiguous evidence that SRP-9001 is likely beneficial for ambulatory patients with DMD," agency reviewers wrote in briefing documents released ahead of Friday's meeting, referring to patients who can still walk – the group who will initially be eligible for the treatment if it gets approved.

Family after family who participated in Sarepta's trials, like the Hennicks, disagree with the reviewers. They say they believe that the treatment has helped keep their kids walking and running in ways they never would have without it.

"It's really miraculous," said Nate Plasman, whose son Andrew got SRP-9001 as part of the trial in January 2019, at age 4.

Andrew was away from school for more than two months when he got the experimental therapy, Plasman said, and when he returned, "his teachers at the preschool were blown away," he recalled. "They’re like, ‘Who is this kid?’ He's running. He's jumping. He's pedaling the tricycle. He's getting up and down off the ground" – all things he couldn't do as well before the therapy.

Marit Sivertson, mom to 9-year-old Brecken, agrees.

"We’ve seen the incredible changes with our son," she said. "He's not just walking around. He's running; he's swimming; he's diving. He's truly living the life that every 9-year-old boy ought to be living."

Sivertson and Plasman also spoke at Friday's meeting. Their goal isn't to secure the therapy for their own kids; because it's designed as a one-time treatment, they wouldn't take it again. They say they’re speaking up on behalf of children who are still waiting.

That wait is especially painful for Daniel and Lindsey Flessner, who have two sons with DMD. Their 5-year-old son, Mason, is in the SRP-9001 clinical trial. Their 2-year-old, Dawson, is still too young.

"With every trip, every fall, every time he stands up by walking up his legs using his hands to help stabilize him, it just keeps chipping away at us," Flessner said. "It's very painful as the parents watching your children struggling knowing all you can do is wait, when waiting is what you don't have time for."

In addition to questions about how well the treatment works, the FDA reviewers raised concerns about safety, particularly "related to the possibility of administering an ineffective gene therapy."

The reviewers focused on opportunity cost: Because of the viruses used to deliver the gene, patients can develop an immune response that could render future doses ineffective.

Chamberlain said work is underway to find ways to be able to administer more doses, if needed, but it's currently a one-and-done treatment.

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For now, he thinks this approach is the best hope for DMD patients.

"It's not perfect," he acknowledged. "It's not a complete cure, but from what I can gauge from the clinical results that have been released by Sarepta and some of the other companies, I think the micro-dystrophin gene therapy is working better than any other drug that's been tried for Duchenne muscular dystrophy."

It's unclear how long the effects will last; Sarepta is continuing trials, and a confirmatory study would be required as part of accelerated approval. Sarepta has proposed a trial that it's already running to satisfy that requirement, with results expected later this year.

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For families facing DMD, there's an opportunity cost to waiting, too. In its briefing documents for the FDA meeting, Sarepta estimated that accelerated approval would speed up broad access to SRP-9001 by at least a year, a time in which about 400 boys could lose the ability to walk and another 400, whose disease is more advanced, would die.

Melanie Hennick said Connor was admitted to the trial just weeks before he’d have aged out, at 8 years old. She said she believes the therapy is the reason Connor's doing so well.

"We had the opportunity to see Connor grow as an 8-, 9-, 10-, 11- and 12-year-old with more capacity than we ever dreamed," she said. "He climbs stairs unaided; he runs around; he plays football; he plays hockey; he plays on a baseball team. … Those are things that we never thought we would be able to see him do, especially at 12."