The Food and Drug Administration surprised many with the approval on Thursday of a drug for a rare muscular weakness disorder that it had initially rejected.

The agency announced the accelerated approval of Cambridge, Massachusetts-based Sarepta Therapeutics’ Vyondys 53 (golodirsen) for Duchenne muscular dystrophy, or DMD, in patients who carry a confirmed mutation of the dystrophin gene amenable to exon 53 skipping. Shares of the company were up about 28% on the Nasdaq when markets opened Friday.

The approval follows an earlier decision, in August, to reject the drug, with the FDA highlighting in its complete response letter – an official rejection notice – concerns about the risk of infections related to intravenous infusion ports and kidney toxicity in animal studies. The risk was observed at doses 10 times as high as the one used in patients, but the infection risk was not observed among patients in the pivotal study. Still, the agency urged that patients on the drug be monitored for renal function.

While the August rejection surprised many, some analysts saw it as a reaction to the agency’s highly controversial decision in late 2016 to give accelerated approval to an earlier DMD drug by Sarepta, Exondys 51 (eteplirsen), which uses exon 51 skipping.

At the time, the FDA’s Peripheral and Central Nervous System Drugs Advisory Committee had recommended against Exondys 51’s approval due to the pivotal study’s use of surrogate endpoint data for a small number of boys. Yet, FDA Center for Drug Evaluation and Research Director Janet Woodcock overruled reviewers’ objections and approved it anyway, advocating flexibility in the face of a life-threatening illness with no available treatments. Many have suspected that pressure from the DMD patient community may have played a role in the decision.

In a note to investors Friday, Baird analyst Brian Skorney wrote that the approval should help restore investor confidence in Sarepta. “We view rapid resolution of the Vyondys53 CRL as clear evidence that Sarepta continues to have a strong working relationship with the agency and believe the rapid overturn of this CRL should help restore investor confidence in Sarepta’s relationship with the FDA,” he wrote.

Skorney noted that before receiving the complete response letter, Sarepta had planned to file for approval of another DMD drug, casimersen, which works through exon 45 skipping, adding that he and colleagues are awaiting additional color with respect to that drug. Vyondys 53 and casimersen would increase the company’s share of the DMD market from the 13% of patients who are candidates for Exondys 51 to about 25%, he wrote. DMD is the most common form of childhood-onset muscular dystrophy, occurring in about 1-in-3,500 live male births, with muscular dystrophies generally affecting about 250,000 people in the U.S, according to the National Organization for Rare Diseases.

Oppenheimer analyst Hartaj Singh wrote that the news of Vyondys 53’s approval came unexpectedly, given that the company had not announced its resubmission after receiving the complete response letter and had not provided clarity about the path forward for the drug. Given the FDA’s cautioning about kidney toxicity in its announcement, Singh wrote that he and colleagues were awaiting further updates on the issues addressed in the letter and potential readthrough for a possible casimersen filing.

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