FDA Approves Fayuvi: First Gene Therapy for Sanfilippo Syndrome Type A
The U.S. Food and Drug Administration (FDA) has approved Fayuvi (rebisufligene etisparvovec-hopf), also known as UX111, for pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA), commonly known as Sanfilippo syndrome type A.
The approval, announced on September 17, 2026, marks the first FDA-approved treatment specifically for Sanfilippo syndrome type A. Fayuvi is a single-dose gene therapy designed to address the underlying genetic cause of the disease and represents a significant development for families affected by this rare and progressive neurological disorder.
What Is Sanfilippo Syndrome Type A?
Sanfilippo syndrome type A is a rare inherited lysosomal storage disorder that primarily affects the brain and nervous system. The disease is caused by changes in the SGSH gene, which results in a deficiency of the sulfamidase enzyme.
When this enzyme is deficient, the body cannot properly break down heparan sulfate. The resulting accumulation can contribute to progressive damage to cells, particularly within the central nervous system.
Children with MPS IIIA can experience developmental delays and progressive loss of cognitive, language, behavioral and motor abilities. The disease is progressive and life-limiting, making the development of treatments that address its underlying cause an important area of research.
According to the FDA, before Fayuvi's approval, treatment for MPS IIIA was focused on managing symptoms rather than an FDA-approved therapy designed to alter the underlying course of the disease.
How Does Fayuvi Work?
Fayuvi is an adeno-associated virus serotype 9, or AAV9, vector-based gene therapy. It is administered as a single intravenous infusion.
The therapy is designed to deliver a functional copy of the gene responsible for producing sulfamidase. By providing genetic instructions for the missing enzyme, the treatment is intended to help cells produce sulfamidase and improve the breakdown of accumulated heparan sulfate.
The FDA-approved indication is specifically for the treatment of neurological manifestations of MPS IIIA in pediatric patients with preserved neurodevelopmental function.
The FDA prescribing information states that Fayuvi is administered as a single dose. The recommended dose is 3.0 × 10¹³ vector genomes per kilogram of body weight.
Clinical Evidence Behind the Approval
The approval was supported by clinical data from the Transpher A study and associated long-term follow-up. According to Ultragenyx, the available data include follow-up extending to nearly eight years.
One measure examined in the clinical development program was cerebral spinal fluid heparan sulfate. Treatment was associated with reductions in this biomarker, providing evidence that the therapy was producing the intended biochemical effect.
Clinical outcomes were also evaluated using the Bayley-III Cognitive Scale. Ultragenyx reported that patients who received Fayuvi demonstrated a higher mean cognitive raw score compared with an external natural-history cohort during the period evaluated.
In the modified intention-to-treat population, 17 treated patients were compared with 27 untreated patients from a comparable natural-history cohort. Ultragenyx reported a 23.5-point difference in cognitive score over natural history, with a reported p-value of less than 0.0001. The company identified these results as part of the efficacy basis supporting the FDA's standard approval.
As with any clinical study involving a rare disease, the relatively small patient population is an important consideration when interpreting the evidence. The FDA approval provides regulatory confirmation of the therapy's indicated use, while continued monitoring will help expand understanding of its long-term benefits and risks.
Safety and Important Warnings
Fayuvi is a gene therapy, and treatment requires medical evaluation and monitoring before and after administration.
The FDA prescribing information includes several important warnings and precautions. These include hepatotoxicity, thrombocytopenia, thrombotic microangiopathy, hypersensitivity and infusion reactions, and a potential risk of malignancy associated with possible integration of AAV vector DNA into the genome.
Liver enzyme elevations were observed in clinical studies. Patients require assessment of liver function before treatment and monitoring afterward. Corticosteroids are administered before and after the infusion.
Decreases in platelet counts have also been observed. The prescribing information recommends platelet monitoring weekly for the first four weeks and then monthly for six months following treatment, with additional monitoring when clinically appropriate.
Although thrombotic microangiopathy was not observed in Fayuvi clinical studies, the FDA recommends monitoring because TMA has been reported with other AAV gene therapies.
Infusion reactions, including hypersensitivity and anaphylaxis, are another potential risk. Patients are monitored during and after administration for signs of these reactions.
The most common adverse reactions reported in the prescribing information include increased liver enzymes, vomiting, abnormal behavior, diarrhea, fever, decreased white blood cell counts, decreased appetite and reduced platelet counts, among others.
Access to Fayuvi
Ultragenyx has announced that its UltraCare program will provide support for eligible patients and caregivers navigating the treatment process. The company also plans to make Fayuvi available through a network of Qualified Treatment Centers, which are healthcare institutions trained and equipped to administer gene therapies.
Ultragenyx said commercial product is expected to be available for shipment to Qualified Treatment Centers within 30 to 60 days following the approval. Actual access can depend on factors including patient eligibility, treatment-center availability, insurance coverage and reimbursement arrangements.
The FDA approval letter confirms that the agency approved the biologics license application for rebisufligene etisparvovec-hopf on September 17, 2026, and authorized the product under the proprietary name FAYUVI.
A Milestone for Rare Disease Research
Fayuvi's approval is also notable because the therapy's development involved years of research and collaboration among scientists, clinicians, patient organizations and biotechnology companies.
Ultragenyx acquired rights to the program after earlier development by researchers associated with Ohio State University and Nationwide Children's Hospital and subsequent work by Abeona Therapeutics. The company's announcement credited researchers, clinicians, patient advocates and families with helping advance the program through development.
For the Sanfilippo community, the FDA decision establishes a new treatment option where previously there was no FDA-approved therapy specifically intended to modify the underlying disease process.
The approval does not mean that Fayuvi is a cure or that every child with MPS IIIA will experience the same outcome. Rather, it represents the introduction of an FDA-approved gene therapy for an extremely rare neurological disease and provides an additional option for eligible pediatric patients.
As treatment centers begin administering Fayuvi, longer-term follow-up will be important for understanding its durability, safety and effects across patients with MPS IIIA.
Conclusion
The FDA's approval of Fayuvi represents a significant development in the treatment landscape for Sanfilippo syndrome type A. The therapy uses gene transfer technology to provide genetic instructions intended to restore production of the sulfamidase enzyme, addressing an underlying biological problem associated with MPS IIIA.
For eligible children and their families, the approval creates a new treatment pathway. At the same time, gene therapy involves specialized administration, extensive monitoring and important safety considerations.
The next phase will focus on making treatment accessible to eligible patients, supporting families through the treatment process and continuing to collect long-term clinical information.
Sources:
- U.S. Food and Drug Administration, “FDA Approves First Gene Therapy for Pediatric Patients with Sanfilippo Syndrome Type A,” September 17, 2026.
- U.S. Food and Drug Administration, FAYUVI product information and prescribing information.
- U.S. Food and Drug Administration, FAYUVI approval letter, September 17, 2026.
- Ultragenyx Pharmaceutical Inc., “Ultragenyx Announces Approval of FAYUVI Gene Therapy,” September 17, 2026.
Medical Disclaimer:
This article is provided for general informational and educational purposes only. It is not medical advice, a diagnosis, or a recommendation for treatment. Fayuvi is a prescription gene therapy with important risks, eligibility requirements and monitoring requirements. Patients and caregivers should consult a qualified healthcare professional and review the FDA-approved prescribing information before making decisions about treatment. Medical information can change as additional evidence becomes available.