The Food and Drug Administration has agreed to reverse its rejection of Regenxbio’s experimental gene therapy for a rare and fatal brain disease, the company said Monday, the latest about-face by the agency under new leadership.
Regenxbio plans to refile its application seeking approval of its Navsunli gene therapy in the third quarter, after it holds a formal meeting to resolve disputes with the FDA that is expected in July, Chief Executive Curran Simpson said.
The decision marks the third recent reversal by the FDA on a rare-disease drug. Replimune reached an agreement with the FDA last month to resubmit its twice-rejected melanoma therapy following pressure from the White House. Earlier in June, uniQure said the FDA had reversed course on its Huntington’s disease gene therapy, clearing a path to file for accelerated approval after previously requesting a new trial.
The U-turns represent a broader retreat from demands, pushed under former FDA vaccines and biotech drugs division leader Vinay Prasad, that rare-disease drugmakers test their experimental medicines against placebos. Companies developing such drugs said the requirement was impractical for diseases diagnosed in only a few dozen patients a year and prolonged development timelines.
Navsunli targets Hunter syndrome, a rare genetic disease that causes irreversible brain damage and cell death, usually leading to death in a patient’s midteens. About 2,000 people worldwide — nearly all boys — are affected, and roughly 50 people are diagnosed annually in the U.S. The therapy aims to replace a defective gene, enabling boys who receive the drug to produce a protein that prevents the buildup of harmful substances in the brain and tissues.
Before Regenxbio filed its first application in early 2025, the FDA had signed off on a plan to study the experimental therapy without giving a placebo to any subjects, Simpson said. The agency also discussed plans for a follow-up study confirming the treatment’s effectiveness. Regenxbio went ahead testing Navsunli in 13 subjects, all of whom received the one-time gene therapy.
In mid-2025, the FDA told Regenxbio that the study lacked a placebo arm and asked for more clinical data, Simpson said. In February, the agency rejected the application outright, citing uncertainty over study eligibility and requesting a new trial with subjects who got placebo. Regenxbio objected, arguing that such a new study would take years and pose an ethical problem, since untreated patients would almost certainly lose neurocognitive abilities.
Regenxbio executives met with then-FDA Commissioner Marty Makary in April to discuss the rejection. Makary encouraged the company to file a formal appeal, assuring Regenxbio that the application would be reviewed with “fresh eyes,” Simpson said.
Last week, after filing that appeal, Regenxbio executives met with FDA officials. Simpson said the agency indicated it would not require new subjects to be dosed for a confirmatory study, only that the original 13 be followed longer to monitor neurocognitive development.
“That was music to our ears, because that’s the original premise by which we started,” Simpson said. He said the FDA indicated it could review Navsunli in as little as two months.