UX111 (Fayuvi, rebisufligene etisparvovec) for Sanfilippo syndrome type A (MPS IIIA)
What the FDA decided
The FDA approved Ultragenyx's BLA resubmission for UX111 (Fayuvi, rebisufligene etisparvovec) in Sanfilippo syndrome type A (MPS IIIA). Approved 2 days ahead of its September 19th action date as the first treatment in the history of Sanfilippo syndrome. A traditional approval on a clinical cognitive endpoint rather than the accelerated, biomarker-based approval many expected. The label covers pediatric patients with preserved neurodevelopmental function; the children in the efficacy analysis were treated at a mean age of 22 months.
Who this decision matters to
Sanfilippo syndrome, formally mucopolysaccharidosis type III (MPS III), is a lysosomal storage disorder in which the body cannot break down a sugar chain called heparan sulfate. It builds up mainly in the brain, causing a childhood dementia: behavioral changes, then loss of speech, thinking and movement. On September 17, 2026 the FDA approved Fayuvi, a one-time gene therapy for type A, the first treatment ever approved for any form of Sanfilippo syndrome.
Prevalence: Approximately 1 per 70,000 to 1 per 100,000 live births; highest prevalence in certain populations including Australia and Scandinavia. See our full Sanfilippo Syndrome page for current treatments, recruiting trials, and community resources.
About the drug
UX111: one-time intravenous AAV9 gene therapy that delivers a working copy of the SGSH gene so the child's cells produce sulfamidase, the enzyme missing in MPS IIIA, and can break down the heparan sulfate that accumulates in the brain. Our full UX111 profile covers how it works, side effects in plain language, and its trial history.
Where Sanfilippo Syndrome treatment stands today
Read our full Sanfilippo Syndrome treatment summary
Sanfilippo syndrome type A now has an approved treatment: Fayuvi (rebisufligene etisparvovec), a one-time AAV9 gene therapy approved by the FDA on September 17, 2026 for pediatric patients with preserved neurodevelopmental function. It is the first approved therapy for any MPS III subtype and it applies to type A only; types B, C and D involve different genes and still depend on clinical trials.
The children in the approval study were treated at a mean age of 22 months, so early diagnosis is now the practical determinant of who can benefit.
Research across all subtypes continues through enzyme replacement, substrate reduction, and gene therapy approaches, with trials often restricted to a single subtype because each has a distinct genetic basis. Gene therapy trials focus on vectors that cross the blood-brain barrier, since the disease is primarily neurological.
Beyond disease-modifying approaches, trials also address the behavioral problems, sleep disturbances, and neurodegeneration that affect quality of life. Heparan sulfate levels in urine and cerebrospinal fluid serve as biomarkers of disease activity and treatment response.
A metabolic genetics specialist can advise on whether a child is a candidate for Fayuvi and which trials fit a given subtype and stage.
The Sanfilippo Syndrome page has every treatment and trial →Meanwhile, 9 Sanfilippo Syndrome trials are recruiting
Whatever the FDA decides here, research on Sanfilippo Syndrome does not stop. A few currently enrolling studies, US sites first:
You'll hear about future FDA decisions and new recruiting trials for Sanfilippo Syndrome. Unsubscribe anytime.
Our coverage
Frequently asked questions
Has the FDA decided on UX111?
The FDA has already decided. It approved the application.
Details: the outcome and what it means are at the top of this page.
Source: FDA press release
What was UX111 approved for?
The FDA approved Ultragenyx's BLA resubmission for UX111 in Sanfilippo syndrome type A (MPS IIIA).
What to know: Approved 2 days ahead of its September 19th action date as the first treatment in the history of Sanfilippo syndrome. A traditional approval on a clinical cognitive endpoint rather than the accelerated, biomarker-based approval many expected. The label covers pediatric patients with preserved neurodevelopmental function; the children in the efficacy analysis were treated at a mean age of 22 months.
How it works: One-time intravenous AAV9 gene therapy that delivers a working copy of the SGSH gene so the child's cells produce sulfamidase, the enzyme missing in MPS IIIA, and can break down the heparan sulfate that accumulates in the brain.
More: the UX111 drug page explains it in plain language.
Is there already an FDA-approved treatment for Sanfilippo Syndrome?
Yes. Trial Friend's drug list includes 1 FDA-approved drug for Sanfilippo Syndrome.
Approved: Fayuvi (rebisufligene etisparvovec).
Check: the Sanfilippo Syndrome page also searches FDA drug labels for approved treatments.
What did the UX111 trials show?
Approval rested on Study 1, the open-label, single-arm, dose-escalation Transpher A trial (NCT02716246), with long-term follow-up in Study 2 (NCT04360265). There was no placebo group.
More: the UX111 drug page has the full trial history and sources.
What are the side effects of UX111?
The label gives 5 warnings their own sections. Liver toxicity: liver enzymes and bilirubin are monitored until 2 weeks after the corticosteroid taper ends, and longer if they rise.
Most common: raised liver enzymes, vomiting, behavior changes such as irritability, aggression or trouble sleeping, diarrhea, fever, low white blood cells.
More: the UX111 drug page lists the warnings in full.
How much will UX111 cost?
Trial Friend does not list a price for UX111. What you pay depends on your insurance and on help from the drugmaker or a charity fund.
Help paying: drugmaker programs and charity funds usually open at launch.
Find help: the patient assistance finder lists programs by insurance type.
What happens after the UX111 decision?
This page updates with the outcome the day it is announced.
If approved: getting the drug still takes time. Specialty pharmacy setup, insurance review and patient assistance often take weeks. Our guide covers what families can do on day 1.
If not approved: a complete response letter means the FDA declined the application in its current form. Many are about manufacturing or how data were presented, and companies often resubmit.
Where this date comes from
The FDA does not publish PDUFA dates; companies disclose them. This one comes from FDA press release. Dates can move, and the FDA can act early or late. This page rechecks against our calendar, which is re-verified daily.