By Carl O’Donnell

(Reuters) – Shares of Sarepta Therapeutics Inc were down around 4% Wednesday after the U.S. Food and Drug Administration revealed documents detailing concerns it had about the safety and efficacy of one of the company’s recently approved Duchenne muscular dystrophy (DMD) drugs.

The drug, Vyondys 53, also known as golodirsen, was originally rejected by the FDA last year before the agency reversed that decision in December and approved it.

It was the second controversial drug approval for Sarepta. The company saw its first drug, Exondys 51, which also treats DMD, approved in 2016 despite concerns raised by experts about its effectiveness.

The newly released documents outlined concerns ranging from kidney toxicity, to infections, to low levels of effectiveness as reasons for the FDA’s original rejection of the drug. The FDA cited some of these concerns at the time of its original rejection of Vyondys 53 but the documents provide further detail.

The documents added that Vyondys 53 comes with greater risk of kidney toxicity than Exondys 51 and that the risks of infection, which exist for both drugs, were not as well understood at the time Exondys was approved. Therefore, while Exondys was approved based on assumptions that it had “essentially no risk,” Vyondys 53 has a less favorable risk profile.

“I have reached the conclusion that this small benefit, which has not yet been verified, does not outweigh its risks,” Ellis Unger, a director at the Center for Drug Evaluation and Research, wrote in his letter.

Sarepta successfully appealed the FDA’s decision in 2019. The company said it addressed the kidney toxicity issue in the drug’s label, which recommends monthly monitoring of patients.

The FDA said in a statement to Reuters that its eventual approval of Vyondys 53 demonstrates its flexibility in approving new drugs for serious diseases that have no satisfactory treatments available. It added that it stands by its eventual decision to approve the drug.

Both of Sarepta’s drug approvals are contingent on it conducting additional post-marketing studies to show their efficacy and submitting them to the FDA.

(This story corrects typographical errors in paragraphs 5 and 8)

(Reporting by Carl O’Donnell; Editing by David Gregorio)