The Food and Drug Administration has stopped a gene therapy study for Hunter syndrome after imaging turned up small nodules or fluid-filled masses in the spines of five children who received the treatment years ago.
REGENXBIO disclosed the clinical hold on its Hunter syndrome therapy on August 24, 2026. The therapy, RGX-121, is an investigational one-time treatment for the rare inherited disorder also known as mucopolysaccharidosis type II. The company said it no longer expects to resubmit its application for approval in the near term.
None of the five participants has symptoms. All continue to do well clinically and have shown overall stability to improvement on neurocognitive and neurobehavioral testing, according to the company. Trial investigators classified the findings as nonserious, and radiologists believe they are likely benign.
For the roughly 2,000 people worldwide with a diagnosed case of MPS II, this is not a story about an approved product being pulled from pharmacy shelves. Nothing anyone takes today changes. It is a story about a treatment that families had been told was close to a decision, and is now indefinitely further away.
The Findings Came From Monitoring That Would Not Normally Happen
The detail that makes this unusual is how the masses were found at all.
REGENXBIO put an expanded MRI monitoring plan in place a few months ago, covering both brain and spine, after a clinical hold involving RGX-111, its therapy for a related disorder called MPS I. That earlier hold followed the discovery of a tumor in a treated patient, and genetic analysis of a tumor sample later showed the delivery vector had integrated into the tumor genome, leading to a mutation in a known cancer-causing gene.
Enhanced imaging identified either a small nodule or a small cystic mass on spine MRIs in 5 of the 48 participants in the CAMPSIITE study who had received RGX-121 by injection into fluid-filled spaces around the brain roughly 3 to 6 years earlier. No nodules or masses were identified on any brain MRI.
The company was candid about what remains unknown. Because spine MRI is not routinely performed in MPS care or in MPS trials, there is no baseline data on how often findings like these occur in untreated patients. There is also no clinical or pathological evidence confirming the nature or the cause of what was seen. Investigators plan to continue to monitor these patients with periodic imaging and no other intervention at this time.
That uncertainty cuts in more than one direction. Roberto Giugliani, M.D., Ph.D., a professor of genetics at the Federal University of Rio Grande do Sul in Brazil, commented in the company's announcement that the findings may be part of the disease itself. Asymptomatic, likely benign findings like these "may be inherent to the impact of Hunter Syndrome throughout the body," he said, adding that he was pleased the patients were doing well. Readers should note that the comment was issued through the sponsor's own release.
A Setback That Follows a Year of Reversals
The hold lands on a program that has already moved back and forth. The FDA rejected the application in February, telling the company in a complete response letter to run a new study, treat more patients, and include a placebo group. By June, the agency had reversed course and agreed to review a filing based on existing studies under the accelerated approval pathway, which is why a resubmission had been expected this quarter. That plan is now off the table for the near term.
Curran Simpson, president and chief executive of REGENXBIO, said the company believes the findings are specific to this program. "We believe these findings are unique and limited to our Hunter Syndrome program," he said, adding that longer-term follow-up and further analysis are needed to assess the balance of benefit and risk, and noting that the company's Duchenne and retinal disease candidates use a different capsid and different routes of administration.
REGENXBIO and its partner NS Pharma said they will review additional imaging and longer-term follow-up data and wait for the FDA's full clinical hold letter before deciding next steps.
Who Carries the Weight of This Pause
Hunter syndrome is an X-linked recessive disease caused by a deficiency of the enzyme I2S, which allows sugar molecules called glycosaminoglycans to accumulate in tissues and damage cells and organs, including in the central nervous system. It primarily affects boys. More than 500 babies are born with it each year worldwide, and most have severe forms in which developmental delay becomes readily apparent by 18 to 24 months.
The families most affected by this decision fall into three groups. Parents of the five children with imaging findings now face periodic scans and an unresolved question. Families of other trial participants face the same monitoring without answers. And families who were waiting for a possible approval have lost a timeline they had been planning around, which, for a progressive neurological disease, is not an abstract loss.
Enzyme replacement therapy remains available and does not cross the blood-brain barrier well, which is precisely the gap gene therapy was meant to address. That gap is still there.
Reasonable Steps for Families and Clinicians
No action is required of the general public. Families with a child in the CAMPSIITE study should expect direct contact from their trial site and should direct questions there rather than to social media accounts discussing the hold. Families of children with MPS II who are not enrolled in a trial should keep scheduled specialist appointments and continue current treatment without change.
Anyone considering gene therapy outside a registered clinical trial should be cautious. Programs operating without regulatory oversight do not carry the safety monitoring that identified these findings in the first place, which is worth remembering: the imaging plan that produced this bad news is also the reason anyone knows about it.
The next milestones are the FDA's full clinical hold letter and the company's analysis of additional imaging. Neither has a published date. MedicalDaily will report the contents of the hold letter, any change in patient status, and whether the agency lifts or extends the pause.
The bottom line is narrow and worth stating without inflation. Five children have unexplained spine findings; none of them are sick from those findings, on a therapy that was never approved and is not available outside a study. The uncertainty is real, the harm is unproven, and the delay is the concrete cost.
Key Questions Answered
What is a clinical hold? It is an FDA order that pauses a study. It prevents new participants from receiving the investigational product and may require participants already receiving it to stop.
Are the five children sick? No. The company reports that all five are asymptomatic and clinically stable or improved on neurocognitive and neurobehavioral assessments. Investigators deemed the findings nonserious, and radiologists believe they are likely benign.
Does anyone know what caused the spine findings? No. REGENXBIO states there is no clinical or pathological evidence confirming the nature or causation of the findings and no comparison data showing how common such findings are in untreated MPS II patients.
Is RGX-121 available by prescription? No. It is investigational and has not been approved. Nothing about this hold changes any currently available treatment.
What is Hunter syndrome? A rare X-linked genetic disease in which a missing enzyme allows sugar molecules to build up in tissues, damaging cells and organs, including the brain. It primarily affects boys, and most cases are severe.
What should families of trial participants do? Contact the trial site directly. Sites are responsible for notifying participants and explaining what monitoring follows.
When will there be more information? After REGENXBIO receives the FDA's full clinical hold letter and completes further imaging analysis. No date has been announced.
