UX111 Is Approved as Fayuvi, the First Treatment for Sanfilippo Syndrome, and the Label Says Exactly Who It Is For

On September 17th, 2 days before its deadline, the FDA approved the first treatment in the history of Sanfilippo syndrome. The therapy families have known as UX111 for 4 years now has a brand name, Fayuvi, and a label. Families in this community have watched approval dates before and learned not to hold their breath, and the agency had already rejected this therapy once, in July 2025, for reasons that had nothing to do with whether it works. That history is why the second decision mattered so much, and why the details of what was approved matter more than the fact of it.

What Sanfilippo Syndrome Type A Takes From a Child

Sanfilippo syndrome type A, formally mucopolysaccharidosis type IIIA (MPS IIIA), is a genetic lysosomal storage disease that occurs in roughly 1 in 70,000 to 100,000 births. Children are born with 2 broken copies of the SGSH gene, which makes an enzyme responsible for breaking down a sugar molecule called heparan sulfate. Without the enzyme, heparan sulfate accumulates in cells, and the brain bears the worst of it.

The cruelty of the disease is its sequence. Children develop typically for their first year or two. Speech delays come first, then hyperactivity and sleep disturbance often severe enough to upend the whole household, and then the part parents describe as hardest: regression. Words already learned disappear. Skills already mastered unwind. The disease is sometimes called childhood dementia, and most children with the severe form do not survive their teens. There has never been an approved therapy, only symptom management. Families and researchers coordinate through groups like the Cure Sanfilippo Foundation, and our Sanfilippo syndrome page tracks every actively recruiting trial.

UX111 (rebisufligene etisparvovec), developed by Ultragenyx, is a one-time intravenous infusion. A self-complementary AAV9 vector, a hollowed-out virus shell chosen because it can cross into the brain, carries a working copy of the SGSH gene into the child's cells. Cells that receive the gene start producing the missing enzyme themselves, clearing heparan sulfate at its source rather than managing symptoms downstream (Ultragenyx, 2026).

Ultragenyx submitted the therapy for accelerated approval, with cerebrospinal fluid heparan sulfate as the biomarker meant to stand in for clinical benefit, and that is how most coverage described the review right up to the decision, and then the FDA did something else. The label and the approval letter contain no accelerated approval language and no confirmatory trial requirement. The agency granted a traditional approval on the clinical endpoint itself, the change in children's cognitive development over 3 years, which is the stronger form of approval and the one that does not leave a sword hanging over the therapy.

What the Transpher A Trial Showed, in the Label's Own Numbers

The efficacy population in the label is 17 children from the Transpher A study who received the approved dose and were either 2 years old or younger at treatment, or older than 2 with a developmental quotient of at least 60, meaning mild or moderate impairment rather than advanced disease. They were treated at a mean age of 21.8 months, the youngest at 3 months and the oldest at 33 months, and followed for a median of 4.2 years. The measure was the Bayley-III Cognitive raw score, a standard test of how a young child is developing, tracked between the ages of 24 and 60 months and compared against 27 untreated children from a natural history cohort (FDA, 2026).

Over that window, treated children gained a mean of 16.0 points while untreated children lost 7.6, a difference of 23.5 points with a confidence interval of 17.2 to 29.9. The direction is the whole story, because Sanfilippo type A takes skills away between ages 2 and 5 and these children were adding them. Cerebrospinal fluid heparan sulfate fell and stayed down as well, which is what the accelerated-approval case had rested on, but the FDA approved the drug on the developmental scores, not the biomarker.

“Treatment with UX111 AAV Gene Therapy Significantly Improved Clinical Function Across Multiple Developmental Domains in Children with Sanfilippo Syndrome Type A (MPS IIIA) Correlated with Sustained Reductions in CSF-HS”

Ultragenyx press release headline, February 2025

A fair caveat still belongs here. There was no placebo group, and a comparison against an external natural history cohort is weaker evidence than a randomized trial, since the two groups can differ in ways nobody measured. The untreated children were older at enrollment and started from higher baseline scores, which the analysis adjusts for by age but cannot fully erase. In an ultrarare, uniformly fatal childhood disease, randomizing children to placebo carries its own ethical weight, and the FDA has said in its newer rare disease frameworks that it will accept exactly this evidence pattern. It just did, and it went further than the sponsor asked.

Why the FDA Rejected UX111 in 2025 and What Changed

On July 11, 2025, the FDA issued a complete response letter for UX111. Read past the headline and the details mattered enormously: the agency's requests concerned chemistry, manufacturing, and controls (CMC) and observations from manufacturing facility inspections. Not efficacy. Not safety. The science was never the objection; the factory paperwork was (Ultragenyx, 2025).

Ultragenyx resubmitted in early 2026, the FDA accepted the filing on April 2nd and set a September 19th action date, and the approval came on September 17th. The manufacturing argument had strengthened in the meantime: Ultragenyx had already won its first gene therapy approval in August 2026, Genglycos for glycogen storage disease type Ia, so the agency had recently cleared another gene therapy from the same company. The complete response letter turned out to be a 14-month delay rather than a verdict.

Who the Fayuvi Label Covers, and Who the Evidence Covers

The indication is a single sentence, and every clause in it matters: the treatment of neurologic manifestations of MPS IIIA in pediatric patients with preserved neurodevelopmental function. Type A only, because Fayuvi delivers the SGSH gene and types B, C and D are broken elsewhere. Neurologic manifestations, because that is what was measured, and preserved function, because the therapy gives the body an enzyme going forward rather than repairing what the disease has already taken. The label sets no age limit, and the evidence behind it is narrower than that in a way families deserve to see plainly.

None of that makes the approval wrong, and extrapolating from a well-run study to the edges of a label is standard in rare disease, where enrolling every group separately would mean no drug at all. What it changes is the conversation, because a 5-year-old with mild impairment is inside the indication and outside the trial, and the fair way to put it is early rather than excluded. It also turns the community's next fight into a plain one: the children who benefited most were diagnosed young, MPS IIIA is not on the federal newborn screening panel, and every month between symptom and diagnosis is now a month of treatable disease going untreated.

The 5 Warnings on the Fayuvi Label and What Each One Asks of You

Every child receives corticosteroids starting the day before the infusion and continuing for at least 8 weeks, which is the first practical fact of treatment: an 8-week course of steroids in a toddler, with the vaccination schedule possibly shifted around it. The label then carries 5 warnings, each with its own monitoring. Liver toxicity, with liver enzymes and bilirubin checked until 2 weeks after the steroid taper ends. Low platelets, checked weekly for the first month and then monthly for 6 months. Thrombotic microangiopathy, a rare condition in which small clots damage the kidneys and destroy red blood cells, which the label says should trigger immediate testing at the first sign. Hypersensitivity and infusion reactions, watched for during and after the hour-long infusion. The fifth is the theoretical risk shared by every AAV gene therapy, that the delivered DNA could integrate into the genome and contribute to a cancer, for which Ultragenyx asks families to report any malignancy at 1-888-756-8657.

The most common reactions in the trial were raised liver enzymes (85%), vomiting (67%), behavior changes such as irritability or trouble sleeping (56%), diarrhea (48%) and fever (41%), followed by low white blood cells, steroid-related Cushingoid features, reduced appetite, low platelets and anemia. Nothing on that list is unusual for an AAV9 gene therapy at this dose, and the monitoring schedule is the label's way of catching the serious ones early. The question to ask your center is not whether these risks exist but who is doing the weekly platelet count and the liver panel, where, and for how long.

How to Actually Get Fayuvi for Your Child

At approval, the things families most want to know, the price and the list of treatment centers, had not been published (Ultragenyx did name its UltraCare support program at approval), so what is known is structural. Fayuvi is a single infusion dosed by weight, shipped frozen as a patient-specific kit of 5 to 105 vials, given at a center equipped to manage infusion reactions, which in practice means a hospital with gene therapy experience rather than a community clinic. Before infusion a child needs liver, platelet and clotting tests and an anti-AAV9 antibody titer below 1:100.

The practical path starts with your metabolic genetics specialist, who will know whether your child's current developmental function fits the label and can begin the antibody testing now. Ultragenyx's medical information line is 1-888-756-8657. Gene therapies in this category have listed at several million dollars and are usually paid for through outcomes-based agreements between the manufacturer and insurers rather than a simple claim, so the prior authorization conversation will run through the treating center and Ultragenyx's UltraCare program, which the company announced with the approval. We will update this post and the Fayuvi drug page as those details are published, and our guide to what happens between approval and a first dose covers the gap that comes next.

Questions Sanfilippo Families Are Asking About Fayuvi