First, the good news.

Federal regulators have given the OK for the first drug to treat a form of muscular dystrophy.

The bad news.

Researchers aren’t entirely sure yet if it will be effective.

Amid controversy, the U.S. Food and Drug Administration (FDA) this week approved the first drug to treat patients with Duchenne muscular dystrophy (DMD).

The decision comes after months of debate into the efficacy of the drug Exondys 51.

In April, an advisory committee for the FDA concluded there was not enough persuasive evidence to prove the drug was effective in treating DMD.

The committee members had previously argued pharmaceutical company Sarepta Therapeutics had failed to provide enough evidence in their clinical trial of just 12 patients.

Now, Exondys 51 has been granted accelerated approval by the FDA, under the condition Massachusetts-based Sarepta conducts a broader confirmatory clinical trial.

“In rare diseases, new drug development is especially challenging due to the small numbers of people affected by each disease and the lack of medical understanding of many disorders,” said Dr. Janet Woodcock, director of the FDA’s Center for Drug Evaluation and Research, in a statement.

“Accelerated approval makes this drug available to patients based on initial data, but we eagerly await learning more about the efficacy of this drug through a confirmatory clinical trial that the company must conduct after approval,” she said.

Read more: Get the facts on muscular dystrophies »

Deadly disease mostly affects boys

DMD is a genetic disorder characterised by muscle weakness and degeneration.

It is the most common childhood form of muscular dystrophy, and symptoms usually show up between the ages of 3 and 5.

The disease is caused by an absence of dystrophin, a protein that helps keep muscles intact. DMD primarily affects boys but in rare cases girls may also be affected. The disease affects one in every 3,600 boys worldwide.

Over time, people with DMD suffer muscle weakness, beginning in their hips, pelvic area, thighs, and shoulders. As the disease progresses the muscles in the arms, legs, and trunk are also affected.

Many patients require a wheelchair by their early teens. As the disease worsens, life-threatening heart and respiratory conditions can occur.

Life expectancy for DMD varies, as does the severity of the disease. But most patients die in their 20s or 30s.

Read more: Get the facts on Duchenne muscular dystrophy »

Two decades of research

Exondys 51 is the first drug on the market in the Unites States to treat DMD. It comes after decades of research.

“This is the outcome the Muscular Dystrophy Association dreamed of 25 years ago when it was the first to invest in the breakthrough research that led to development of eteplirsen (Exondys 51),” said Steven M. Derks, Muscular Dystrophy Association (MDA) president and chief executive officer, in a statement.

Two decades ago, the MDA began funding work by researcher Steve Wilton, Ph.D., foundation chair in molecular therapy at Murdoch University in Perth, Australia.

Wilton’s work focuses on exon skipping, which led to the development of Exondys 51.

“There was a great deal of skepticism when I started this work,” Wilton told Healthline. “I was not certain how it would work out and it was important to do the right experiments to show that exon skipping was a viable therapy.”

Wilton explained that our genes consist of coding blocks (exons) separated by noncoding blocks (introns).

During gene expression, the introns are removed and the exons are spliced together to make a gene message that is translated into a protein. This protein acts as a molecular shock absorber, giving strength and stability to muscle fibers.

For those with DMD, a “spelling error” occurs during this process that creates a nonfunctional protein. The drug Wilton created acts like a “white out” that covers up this spelling error.

“Exondys 51 acts like a genetic patch to mask an exon associated with the spelling error so that it is skipped and can be translated into a shorter but functional shock absorber,” Wilton explained.

He hopes this exon-skipping therapy will also pave the way for the treatment of other gene disorders.

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Criticism and praise

The approval brought cheers from parents of children with DMD, but it also has sparked a feud within the FDA and the scientific community, according to an article in Forbes magazine.

Several members of the FDA advisory committee are upset their 7-to-6 vote in April recommending the drug not be approved was overturned by higher ranking officials, the Forbes article reports.

Some committee members appealed the approval, but the ruling was upheld.

Some scientists who spoke to Forbes also criticized the clinical trial that involved 12 boys. They said the sample size was too small, and there was no placebo-based control group.

In addition, the article noted Sarepta reportedly plans to charge $300,000 per patient per year for the new drug.

Nonetheless, the approval of the drug is being lauded as a watershed moment for those affected by DMD.

“Exondys 51 represents the culmination of many years of work across our entire organization and the Duchenne community to address a critical unmet need by bringing this novel medicine to patients,” said Dr. Edward Kaye, Sarepta’s chief medical officer, in a press release.

Although a clinical benefit (how a patient feels or functions or whether they survive) for people with DMD on Exondys 51 is yet to be established, the FDA granted Exondys 51 fast track designation.

This expedites the review of drugs that are intended to treat serious conditions and demonstrate the potential to address an unmet medical need.

In a statement, the FDA said “in making this decision, the FDA considered the potential risks associated with the drug, the life-threatening and debilitating nature of the disease for these children, and the lack of available therapy.”

“For our families, therapy options can’t come soon enough,” said Dr. Valerie Cwik, the MDA’s chief medical and scientific officer, in a statement.

“MDA is eager for this treatment to get into the hands of those whom it can help,” she added. “We fully expect this accelerated approval will be an inspiration and a catalyst to more innovation and follow-on funding for drug development across the board.”