FDA Places New Clinical Hold on Regenxbio's Hunter Syndrome Gene Therapy
Updated: 1 day ago
On August 24, 2026, Regenxbio said the FDA had placed a clinical hold on RGX-121, its gene therapy for Hunter syndrome (MPS II). MRI scans had found "small nodules or cystic masses" in the spines of 5 of 48 patients in the Phase 1/2/3 CAMPSIITE study. The patients had no symptoms from the findings. Still, the company said it no longer plans to resubmit the therapy for approval in the near term.
It's a sharp turn for a program that, just two months earlier, seemed close to the finish line.
A lysosomal storage disease
Hunter syndrome is a rare inherited disorder that mostly affects boys, because the gene involved, IDS, sits on the X chromosome. IDS makes an enzyme called iduronate-2-sulfatase. Its job is to break down large sugar molecules called glycosaminoglycans (GAGs), including heparan sulfate.
Without the enzyme, GAGs build up inside cells throughout the body. Children develop coarse facial features, joint stiffness, enlarged organs, and heart and airway problems. In the severe, neuronopathic form, the buildup also damages the brain, causing developmental regression and early death.
Weekly enzyme replacement therapy can ease some body symptoms. But the enzyme is a large protein that doesn't cross the blood-brain barrier, so it can't protect the brain. That gap is what RGX-121 was designed to fill.
How RGX-121 works
RGX-121, also known as clemidsogene lanparvovec or NAVSUNLI, uses an AAV9 vector, which can transduce cells of the central nervous system, to deliver a working copy of IDS directly into the fluid around the brain. Brain cells that take up the gene begin making the enzyme themselves. Some of that enzyme is released and can be taken up by neighboring cells, so a treated cell can help clear GAGs from untreated cells nearby, a process called cross-correction.
This lets brain cells produce the enzyme locally, behind the blood-brain barrier that blocks intravenous enzyme replacement.
Regenxbio's application relied on levels of heparan sulfate breakdown products in cerebrospinal fluid as a biomarker, a measurable sign that the enzyme is working in the brain.
A year of whiplash
RGX-121's regulatory path in 2026 has been anything but smooth:
January: The FDA halted Regenxbio's gene therapy trials in Hunter and Hurler syndromes after a brain tumor was found in a trial participant.
February: The FDA issued a Complete Response Letter, declining to approve RGX-121. The agency initially recommended a new study with more patients and a placebo arm.
June: After leadership changes at the FDA, the agency told Regenxbio that "no additional studies or patients are needed." The company planned to resubmit in the third quarter.
August: The new hold, prompted by the spinal MRI findings, put that resubmission on ice.
That sequence reflects two forces at once: an FDA that has become more willing to accept limited data for rare diseases, and safety signals that no amount of regulatory flexibility can ignore.
What do the MRI findings mean?
At this point, nobody knows. The nodules and cysts were found on imaging, not because of symptoms, and Regenxbio said patients remain clinically stable. Possible explanations range from harmless findings related to the underlying disease to effects of the treatment itself.
That uncertainty is exactly why the hold matters. AAV gene therapies deliver genes that can stay active for years. When something unexpected shows up, regulators and companies need time to understand it before more patients are treated. Regenxbio said longer-term follow-up and more data analysis are needed to assess the therapy's benefit-risk profile.
The bigger picture for AAV
This hold is part of a broader conversation about AAV safety at high doses and in the central nervous system. Over the past few years, the field has seen liver toxicity, immune reactions, and other serious events with some AAV therapies. Each case teaches researchers more about dose, delivery route, immune suppression, and long-term monitoring.
It's also a reminder of what long-term follow-up is for. MRI monitoring in CAMPSIITE caught these findings early, in patients without symptoms. That's the system working as designed, even when the news is unwelcome.
What it means for families
For families of boys with neuronopathic Hunter syndrome, the setback is painful. There's still no approved treatment that protects the brain, and time matters in a progressive disease. The hope now is that the findings can be explained quickly and that a safe path forward emerges, whether for RGX-121 or for other brain-directed approaches in development.
The bottom line
In June, RGX-121 looked like it was about to be resubmitted. In August, a safety signal put it back on hold. This story shows how fast things move in gene therapy, and why careful long-term monitoring remains essential even as regulators speed up approvals.
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— T.N., Gene Tech Times
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