FDA halts Regenxbio Hunter syndrome gene therapy trial again after spinal masses in five children

The FDA placed a clinical hold on Regenxbio's gene therapy for Hunter syndrome on August 24, 2026, after small masses were found on the spines of five children who had been dosed with the treatment. None of the five had symptoms tied to the masses, the company said in a regulatory update.

The pause is the second the FDA has placed on a Regenxbio rare-disease gene therapy in roughly seven months, and it lands on a program the company had been building toward a regulatory filing. The earlier hold, in January, was on a different Regenxbio treatment for a related disorder, MPS I (Hurler syndrome), after a single treated boy developed an intraventricular brain tumor. That case is the one reshaping how regulators look at the rest of the pipeline.

The two programs share a delivery vehicle: an adeno-associated virus, or AAV. AAV has been the workhorse of in-vivo gene therapy for two decades because it slips into cells efficiently and, in early studies, was thought to integrate into the host genome only rarely and at apparently random sites. The January tumor changed that assumption. Regenxbio's own analysis of the boy's tumor found that the AAV vector had integrated near a proto-oncogene called PLAG1, a gene that, when overexpressed, drives cell growth. STAT, reporting on the August 24 hold, characterized the January case as the first tumor conclusively linked to AAV gene therapy.

That is what turns an isolated event into a delivery-vehicle question. If a single AAV dose can land near a growth-driving gene and produce a tumor roughly seven months after treatment, every program that uses AAV to deliver a corrective gene to dividing cells faces a version of the same calculation.

The August 24 hold is the FDA's response to that calculation. Spinal masses in five dosed children, none of them symptomatic, would not, on the older read of AAV safety, have triggered a clinical hold on their own. The agency is acting on the integration concern, not on the patients.

Regenxbio has pushed back on the framing. In a statement on its investor-relations site, the company said the safety profile of RGX-121 in more than 30 treated patients, including some dosed nearly seven years ago, is unchanged, and that the pivotal trial had demonstrated a meaningful efficacy profile. CEO Curran Simpson said he was surprised by the hold, citing the distinct therapies and the unchanged safety database. A same-day press release on PR Newswire carried the same language.

For families, the pause extends an already long wait. Hunter syndrome is a rare, progressive lysosomal storage disorder, the class of inherited conditions in which the body cannot break down certain sugars and fats; children with the severe form typically present in the first years of life and face a steady loss of cognitive and physical function. RGX-121 is designed to deliver a working copy of the missing gene, with the aim of slowing that decline. The five children in the cohort had been dosed and followed; none had reported symptoms from the masses, but the trial is on hold while the FDA reviews the data and the company responds.

The next moves are concrete. Regenxbio has to show the FDA what the masses are made of, whether they show the same PLAG1 integration signature as the January tumor, and whether the pattern holds across other dosed patients. The same questions now apply to every other AAV gene-therapy sponsor with active pediatric programs in dividing tissue.