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, over manufacturing issues that Ultragenyx said did not involve the clinical data. 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. All 4 types of Sanfilippo syndrome together affect about 1 in 70,000 newborns, according to MedlinePlus, and type A is one of the 2 most common. 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. Until Fayuvi, there was no 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 does not carry the confirmatory trial requirement attached to accelerated approvals.

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.

“Significantly Improved Clinical Function Across Multiple Developmental Domains”

Ultragenyx press release headline describing its own data, 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. The FDA accepted this evidence and went further than the sponsor asked, granting a traditional approval.

Why the FDA Rejected UX111 in 2025 and What Changed

On July 11, 2025, the FDA issued a complete response letter for UX111. Ultragenyx said the letter asked for more chemistry, manufacturing, and controls (CMC) information and for changes tied to observations from manufacturing facility inspections, and that it raised no review issues with the clinical data package (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. In the meantime, Ultragenyx won its first gene therapy approval in August 2026, Genglycos for glycogen storage disease type Ia, STAT reported on August 20, 2026. 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, which matches the children studied, who were under 2 or had only mild or moderate impairment at treatment. 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 common in rare disease, where studying every group separately is often not possible. What it changes is the conversation, because a 5-year-old with mild impairment may be inside the indication but is older than any child studied, and the fair way to put it is early rather than excluded. It also makes the community's next push a plain one: the children studied were all treated before age 3, MPS IIIA is not on the federal newborn screening panel, and a late diagnosis can now cost a child the chance to be treated while function is preserved.

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 across the 27 children given the approved dose in 3 studies 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. The label's monitoring schedule is meant to catch 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

Ultragenyx set a wholesale price of $3.95 million before any rebates or discounts, MedCity News and BioPharma Dive reported on September 18, 2026. It did not publish a list of treatment centers at approval, so the rest of 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. 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 when the treatment center list is 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