Gene Therapy for Hunter Syndrome Hits Regulatory Wall: What the RGX-121 Rejection Means for Rare Disease Treatments
**Regenxbio's RGX-121 showed remarkable biomarker reduction in clinical trials, but the FDA isn't convinced that's enough** --- In November 2025, a three-year-old boy named Ollie made headlines when doctors at Manchester Children's Hospital described his response to an experimental gene therapy...
Regenxbio's RGX-121 showed remarkable biomarker reduction in clinical trials, but the FDA isn't convinced that's enough
In November 2025, a three-year-old boy named Ollie made headlines when doctors at Manchester Children's Hospital described his response to an experimental gene therapy as "exciting" — a potential turning point for children with Hunter syndrome, a rare genetic disorder that progressively damages the brain and body.
But across the Atlantic, the company developing the most advanced gene therapy for the same condition just hit a wall.
In February 2026, the FDA rejected Regenxbio's Biologics License Application for RGX-121, an adeno-associated virus (AAV) vector therapy designed to deliver a functional copy of the iduronate-2-sulfatase gene directly to the brains of boys with mucopolysaccharidosis type II, also known as Hunter syndrome.
The rejection surprised some investors but not everyone in the gene therapy field. The agency cited concerns about the trial design — specifically, the use of a surrogate endpoint and the lack of an untreated control group.
"The FDA agreed in principle to the trial design, but multiple issues persisted," Curran Simpson, Regenxbio's CEO, said in a statement. "We are concerned about FDA's feedback regarding the overall development path and evaluation of the data in the context of the urgent need for this irreversible ultra-rare disease."
The promise
The data wasn't trivial. In the CAMPSIITE pivotal trial, 13 patients treated with RGX-121 showed an 82% median reduction in CSF HS D2S6 — a biomarker believed to correlate with cognitive outcomes — through one year post-treatment. Earlier dose-finding cohorts showed 85% reduction sustained through two years. Neurodevelopmental outcomes appeared positive, with children acquiring new skills rather than losing them.
For an ultra-rare disease affecting roughly one in 100,000 male births — almost exclusively boys — the therapy represented a potential first-of-its-kind treatment targeting the underlying genetic cause, not just managing symptoms.
"We have been waiting 20 years to see a boy like Ollie doing as well as he is," Professor Simon Jones, a co-leader of the Manchester trial, told the BBC in November 2025.
The problem
But the FDA wasn't convinced. The agency accepted the biomarker as "reasonably likely to predict clinical benefit" — a standard for accelerated approval — but ultimately questioned whether the data robustly demonstrated benefit. The agency recommended running a new trial with more patients, longer follow-up, or an untreated control arm.
All those options are "challenging" in an ultra-rare condition where patients are few and recruitment is difficult.
The rejection came on the heels of a clinical hold placed on RGX-121 in January 2026, after a patient in a related trial for a different MPS disorder (RGX-111 for MPS I) developed a brain tumor. The FDA extended the hold to RGX-121 due to platform and population similarities, though Regenxbio noted no causal link was established and no tumors appeared in any of the 32 RGX-121 trial participants.
The bigger picture
The Regenxbio case reflects a broader tension in gene therapy development for rare diseases. The FDA under Commissioner Martin Makary has signaled flexibility for genetic medicines targeting unmet needs, outlining a new "plausible mechanism" pathway. Yet the agency's decision-making hasn't always matched that messaging.
Regenxbio is seeking a meeting with the FDA to discuss next steps. In the meantime, the company's shares have fallen significantly, and the path to approval — if it comes — will take years and additional trials.
For families like Ollie's, the regulatory setback is a reminder that scientific promise and commercial availability are different things. The boy in Manchester received a different therapy (ex vivo stem cell gene therapy, not AAV), and his long-term outcomes are still being monitored.
The question now is whether the biomarker reductions seen in the RGX-121 trial will translate to real-world cognitive benefits — and whether Regenxbio can design a trial that satisfies the FDA's concerns.
Editor: This story is developing. A previous version mischaracterized the UK trial as involving the same therapy; the Manchester patient received a different gene therapy approach.
What we don't know:
Related: FDA rejection letters for rare disease gene therapies have also affected UniQure (Huntington's) and Capricor (Duchenne cardiomyopathy) in recent months.
