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In an eagerly anticipated decision, the Food and Drug Administration Thursday approved the first gene therapy for muscular dystrophy.

"Today's approval addresses an urgent unmet medical need and is an important advancement in the treatment of Duchenne muscular dystrophy, a devastating condition with limited treatment options, that leads to a progressive deterioration of an individual's health over time," said Dr. Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, in a statement.

But the agency rejected a request to make the treatment available to all children with Duchenne muscular dystrophy, the most common form of the incurable muscle disease, who could still walk. Instead, the agency restricted access to patients ages four and five until more evidence is available that the therapy is safe and effective.

The decision elicited mixed reactions. Parents of children suffering from the genetic disorder, advocates and some doctors and researchers welcomed the limited approval. But some were disappointed the treatment isn't being made more widely available right away.

"Today is a very important day," Debra Miller, who leads CureDuchenne, an advocacy group, told NPR in an interview. "But every single day these boys are losing muscle cells. And so when you have a son with Duchenne and you see them getting weaker right before your eyes, you know we have to get therapies to patients sooner rather than later."

FDA's accelerated approval came with limits

Others, however praised the agency's restraint, though some argued even the limited approval was premature.

"This is a really critical decision for the FDA to get right," Dr. Caleb Alexander of Johns Hopkins University told NPR in an interview. Alexander voted against approval during a May meeting of an FDA advisory committee that narrowly recommended the agency grant approval.

"This has implications not only for those who may receive this product. But it also sends an important signal regarding what the FDA will require for future products to treat this and similarly devastating diseases," Alexander said.

The company that developed the treatment, Sarepta Therapeutics of Cambridge, Mass., said the therapy would be available as soon as possible. The treatment, called Elevidys, will cost $3.2 million for each patient, the company announced shortly after the approval.

Sarepta asked the FDA to approve the gene therapy under a program that allows the agency to provide access to treatments before direct evidence is available that they are effective.

But this accelerated approval process is controversial because some companies fail to follow through on their promises to confirm their treatments work. A drug approved this way to prevent premature birth was recently withdrawn after being found useless.

Sarepta's muscular dystrophy treatment is the first gene therapy approved under the program.

The disease, which almost exclusively affects boys, destroys muscles. Most boys end up in wheelchairs before they become teenagers. Eventually, their hearts and lungs give out. Most people with the disease die in their 30s or 40s.

The gene therapy works by infusing trillions of harmless viruses in single treatment that has been genetically modified to ferry a gene to patients' muscles.

Evidence for the gene therapy is indirect

The gene produces a miniature version of a protein called dystrophin, that boys with muscular dystrophy are missing or don't have enough of. The hope is this "micro-dystrophin" will at least help slow the progression of the disease.

But there's an intense debate about this. Sarepta based its request on how much micro-dystrophin it produces in patients' muscles — without direct evidence that's actually helping alleviate symptoms and prevent disease progression.

During the May advisory meeting, parents and doctors showed dramatic videos of children who could barely stand and walk, running, biking and and easily climbing stairs after the treatment.

But Alexander and other experts say it remains unclear the treatment is responsible and is safe.

"This product is not without risks. And I think the evidence is murky," Alexander says. "The evidence really doesn't meet the bar required to reach market."

And children who receive the treatment may then be ineligible to get other treatments in the pipeline that may be more effective.

"That's a really non-trivial concern," Alexander said.

But others said there is sufficient evidence to warrant broader approval, including preliminary evidence the treatment is improving boys' muscles, as well as animal data and clear evidence the therapy boosts micro-dystrophin in muscles.

"What's the old expression: 'Don't let perfect get in the way of good?' " said Jeffrey Chamberlain, who directs the Muscular Dystrophy Research Center at the University of Washington.

That said, Chamberlain was glad the FDA at least approved the treatment for younger children pending further data.

"You'd like to see approval for as broad a range of patients as possible. But we'll take what we can get at this point," Chamberlain said.

Michael Kelly, the chief scientific officer for CureDuchenne, says he hopes this will lead to other, even more effective gene therapies for the disease.

"This is a critical and really important step in treatment and this is going to lead the way and blaze a trail for the next round of better therapeutics," Kelly told NPR in an interview.