In a split vote, advisers to the Food and Drug Administration recommended that the agency approve the first gene therapy for Duchenne muscular dystrophy, the most common form of the genetic illness.

The vote, 8 to 6, came after a day of testimony from speakers for Sarepta Therapeutics, the maker of the gene therapy called SRP-9001, FDA scientists and families whose children have Duchenne muscular dystrophy. The question before the panel was whether the benefits for the treatment outweigh the risks.

While the FDA is not bound by the recommendations of its outside advisers, it usually follows them. The agency is expected to decide by the end of May.

Duchenne muscular dystrophy is the most common inherited neuromuscular disorder among children. It affects an estimated 10,000 to 12,000 children in the U.S. The genetic condition mainly afflicts boys and leads to progressive muscle damage, loss of ability to movement and eventually death.

Sarepta's treatment involves a single infusion of viruses that has been genetically modified to carry a gene to patients' muscles to produce a miniature version of a protein called dystrophin.

Patients with Duchenne muscular dystrophy are missing the muscle-protecting protein or don't make enough of it. While not a cure, Sarepta argues that its "micro-dystrophin" treatment can help slow the progression of the disease.

Mixed evidence divides experts

The company's request for approval rested mainly on how much micro-dystrophin the treatment produces in patients' muscles instead of waiting for clear, real-world evidence that it's actually helping patients.

Sarepta presented a complicated mix of results from animal and human studies in support of its application for an accelerated approval of SRP-9001. Dr. Craig McDonald from the University of California, Davis, who testified on behalf of the company, said, "We cannot afford to delay access to this transformational treatment." He said there's evidence micro-dystrophin levels after treatment are a good measure of its effectiveness, and that Sarepta does have preliminary evidence the treatment is, in fact, helpful.

During testimony from the public, Nathan Plasman described calls he'd get from his wife after their son was treated as part of a Sarepta study. "Sarah often opened with her excitedly exclaiming: 'You're never going to believe what Andrew just did today,' " he said. "Or we'd connect over FaceTime and she'd very discreetly whisper, 'Nate, Nate. Check this out,' showing me live footage of Andrew doing the unexpected. Racing up stairs, climbing indoor playground equipment, running, jumping. Hopping up off the ground after sitting or laying on the couch. We cried nearly a quadrillion tears of joy."

Tempering these stories, FDA scientists spent more than an hour raising questions about the evidence submitted by Sarepta for the gene therapy.

FDA scientist Dr. Mike Singer summarized some of the agency's concerns. "Some have to do with the manufacturing," he said. A process change led to reduced purity for the gene therapy in later production. "Others involve the nonclinical data, the results from animal studies," he added. "Additional uncertainties have to do with whether of expression of Sarepta's micro-dystrophin is suitable as a surrogate endpoint considered reasonably likely to predict clinical benefit. And how to know which patients it might help, and which it wouldn't."

There were also concerns about safety, including some cardiac and liver reactions.

Questions raised about confirmatory study completion

There was also discussion about an ongoing clinical study that is expected to reach a key milestone in September and that could provide confirmatory evidence on the effectiveness of the therapy. The FDA is evaluating the drug under an accelerated approval path that allows the company to provide preliminary data to start and then to follow up with more direct evidence. Some members raised concerns about whether the company would see the study through since it has not finished four previous confirmatory trials for different products.

Another issue is whether families would allow children receiving a placebo to stay in the confirmatory study if the FDA approves the drug in May, before the study is done. A company official said the risk of patients dropping out is low because the quickest route to treatment is to remain in the study. Also, most of the patients are far along in the study already.

Dr. Peter Marks, head of the FDA's Center for Biologics Evaluation and Research, weighed in shortly before the panel voted to say that the agency is "very serious" about making sure the confirmatory study gets completed. He said recent legislation gave the FDA more teeth to hold drugmakers accountable for following through with their obligations after receiving an accelerated approval.