The U.S. Food and Drug Administration approved Fayuvi (rebisufligene etisparvovec-hopf) on September 17, 2026, making it the first FDA-approved treatment for children with mucopolysaccharidosis type IIIA (MPS IIIA), also called Sanfilippo syndrome type A. The one-time intravenous gene therapy is approved for pediatric patients with preserved neurodevelopmental function and is designed to replace the missing or defective SGSH gene activity that drives the disease.
Sanfilippo syndrome type A is caused by loss of sulfamidase activity
MPS IIIA is a rare inherited lysosomal storage disorder caused by pathogenic variants in the SGSH gene. That gene encodes sulfamidase, an enzyme required to break down heparan sulfate inside lysosomes. When sulfamidase activity is severely reduced, partially degraded heparan sulfate accumulates in cells, including cells in the central nervous system.
The neurologic consequences are progressive. Children may initially develop relatively normally, then experience slowing or loss of language, cognition, behavior, and motor skills as the disease advances. Until this approval, care in the United States focused on symptoms and supportive management rather than an FDA-approved therapy intended to alter the underlying disease process.
Our rare-disease genetic medicine explainer describes why disorders caused by a single missing or dysfunctional gene are particularly attractive targets for gene-replacement strategies.
Fayuvi uses AAV9 to deliver a working copy of the SGSH gene
Fayuvi is a non-replicating recombinant adeno-associated virus serotype 9, or AAV9, vector carrying a functional copy of the human SGSH gene. The vector is administered once by intravenous infusion at a recommended dose of 3.0 × 1013 vector genomes per kilogram of body weight.
After entering cells, the vector provides genetic instructions for producing sulfamidase. Restoring enzyme activity is intended to improve breakdown of heparan sulfate and reduce its accumulation in lysosomes, including in the brain.
The approach is conceptually different from drugs that must be taken repeatedly. Fayuvi is intended as a one-time gene-transfer treatment, although patients require long-term follow-up because the biologic effects and potential risks of AAV gene therapy can persist for years.
The FDA efficacy analysis compared treated children with natural-history controls
The pivotal evidence did not come from a randomized placebo-controlled trial. Fayuvi was evaluated in open-label, single-arm studies, and the FDA compared treated children with an external natural-history cohort of untreated patients with MPS IIIA.
For the primary cognitive analysis, the modified intention-to-treat population included 17 Fayuvi-treated children and 27 untreated natural-history controls. Cognitive development was assessed using the Bayley Scales of Infant and Toddler Development, Third Edition, or Bayley-III.
From 24 to 60 months of age, the least-squares mean change in Bayley-III Cognitive raw score was +16.0 points in the Fayuvi group and -7.6 points in the natural-history group. The between-group difference was 23.5 points, with a 95% confidence interval from 17.2 to 29.9 and P<0.0001.
FDA interpreted this difference as evidence that treated children gained or maintained developmental skills while untreated children followed the expected pattern of developmental loss.
The approved population is narrower than “all children with Sanfilippo syndrome”
The FDA indication is specifically for pediatric patients with MPS IIIA who have preserved neurodevelopmental function. In the clinical program, eligibility for the efficacy population included children treated at age 2 years or younger, or children older than 2 years who had a Cognitive Developmental Quotient of at least 60 at treatment.
That matters because neurodegeneration in Sanfilippo syndrome progresses over time. A gene therapy that restores enzyme activity cannot necessarily reverse neuronal damage that has already occurred. The approval therefore does not establish the same benefit in children with advanced neurodevelopmental decline.
This timing issue resembles other neurogenetic diseases in which treatment may work best before irreversible injury accumulates. Our Zanvastro approval explainer covers a different rare neurologic disorder in which the treatment strategy also targets disease biology rather than symptoms alone.
The safety database is small, as expected for an ultra-rare disease
The FDA label reports safety data from 33 pediatric patients with MPS IIIA who received Fayuvi across three clinical studies. Twenty-seven received the recommended dose. Median follow-up after infusion was 4.1 years, with a range from 7 months to 7.8 years.
Among the 27 patients who received the recommended dose, increased liver enzymes occurred in 85%, vomiting in 67%, fever in 41%, decreased white blood cell counts in 30%, decreased appetite in 22%, and decreased platelet counts in 19%. Many reported events were mild or moderate, but the therapy carries clinically important monitoring requirements.
All patients receive corticosteroids beginning one day before infusion and continuing for at least eight weeks afterward. Liver function and platelet counts require repeated monitoring after treatment.
AAV gene therapy introduces risks that extend beyond the infusion day
The prescribing information warns about hepatotoxicity, thrombocytopenia, hypersensitivity and infusion reactions, thrombotic microangiopathy, and a potential risk of malignancy from vector DNA integration into the genome.
No thrombotic microangiopathy cases were reported in the Fayuvi clinical studies, but the complication has occurred with other AAV gene therapies, so FDA recommends clinical and laboratory monitoring after infusion.
The malignancy warning is also based on a theoretical and class-related concern rather than a demonstrated Fayuvi-associated cancer signal in the current clinical dataset. Because AAV vector DNA can potentially integrate into the genome, long-term surveillance remains important.
For a broader explanation of how gene-targeted treatments differ from conventional medicines, see our gene-editing explainer. Fayuvi is not CRISPR and does not edit the existing SGSH gene; it delivers an additional functional gene copy using an AAV vector.
The evidence has important limitations
The strongest limitation is the absence of a randomized concurrent control group. The pivotal comparison relied on an external natural-history cohort, which can introduce differences in patient selection, testing conditions, supportive care, and other factors that are harder to control than in a randomized trial.
The efficacy population was also very small: 17 treated children were compared with 27 untreated controls. That is understandable for an ultra-rare disease, but it limits precision for subgroup analyses and for estimating uncommon adverse events.
The clinical program was sponsored by the therapy’s developer, and the FDA approval relied heavily on company-submitted clinical and natural-history data. Longer independent follow-up will be important for confirming durability, neurodevelopmental outcomes, and long-term safety.
The approval changes Sanfilippo syndrome type A from symptom management to disease-targeted treatment
Fayuvi does not establish that every treated child will maintain normal development, and it does not erase damage that has already occurred. What the approval does establish is that replacing SGSH activity early enough can alter the expected developmental trajectory in a disease that previously had no FDA-approved disease-modifying treatment.
That makes the September 2026 approval important beyond one rare disorder. It is another example of gene therapy moving from experimental proof of concept toward one-time treatments aimed at the molecular cause of severe pediatric disease.
References
- U.S. Food and Drug Administration. FAYUVI (rebisufligene etisparvovec-hopf) prescribing information. Initial U.S. approval 2026. FDA prescribing information.
- U.S. Food and Drug Administration. FDA Approves First Gene Therapy for Pediatric Patients with Sanfilippo Syndrome Type A. September 17, 2026. FDA press release.