Novato, California, U.S., September 17, 2026
Ultragenyx Pharmaceutical Inc. announced that the U.S. Food and Drug Administration (FDA) has granted full approval to FAYUVI™ (rebisufligene etisparvovec-hopf), also known as UX111, for the treatment of neurologic manifestations of mucopolysaccharidosis type IIIA (MPS IIIA), or Sanfilippo syndrome Type A, in pediatric patients with preserved neurodevelopmental function. The approval marks the first FDA-approved treatment for Sanfilippo syndrome Type A, a rare inherited neurodegenerative disorder associated with progressive neurological decline. The FDA approval was issued on September 17, 2026, while Ultragenyx announced the milestone from its headquarters in Novato, California.
Ultragenyx FAYUVI Becomes First FDA-Approved T1A Therapy
FAYUVI is a one-time gene therapy designed to address an underlying genetic deficiency associated with MPS IIIA. The disease is caused by mutations affecting the SGSH gene, which results in deficiency of the sulfamidase enzyme. Without sufficient sulfamidase activity, heparan sulfate accumulates within lysosomes, including in the brain and other tissues, contributing to progressive neurological and developmental impairment. FAYUVI uses a modified, non-infectious adeno-associated virus serotype 9 (AAV9) vector to deliver a functional copy of the SGSH gene into cells. The intended result is production of the missing sulfamidase enzyme, allowing cells to break down accumulated heparan sulfate. The FDA labeling specifies that FAYUVI is administered as a single intravenous infusion and is indicated specifically for pediatric patients with MPS IIIA who have preserved neurodevelopmental function.
The FDA’s decision represents a significant regulatory milestone for the Sanfilippo syndrome Type A community because, before this approval, there was no FDA-approved therapy designed to alter the underlying disease course. The FDA described MPS IIIA as a rare inherited disease that progressively affects the brain and nervous system and can lead to loss of cognitive, language, and other developmental abilities.
Gene Therapy Targets the Underlying Genetic Defect
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The approval adds another gene therapy to Ultragenyx’s rare-disease portfolio and represents the company’s second gene therapy approval and sixth FDA approval overall, according to the company. Ultragenyx also received a Priority Review Voucher in connection with the FAYUVI approval. The regulatory review incorporated clinical studies identified by the FDA as NCT02716246, NCT04360265, and NCT04088734. The FDA granted FAYUVI Orphan Drug, Fast Track, and Breakthrough Therapy designations during its development, reflecting the serious nature of the disease and the need for therapeutic options.
The FDA prescribing information specifies a recommended dose of 3.0 × 10¹³ vector genomes per kilogram, administered through intravenous infusion. Patients require baseline assessments including liver function tests and evaluation of platelet counts and coagulation parameters before treatment. The safety profile includes potential adverse reactions such as increased liver enzymes, nausea and vomiting, fever, decreased appetite, reduced white blood cell and platelet counts, and increased amylase. The FDA prescribing information also contains an important warning concerning thrombotic microangiopathy (TMA). As with other AAV-based gene therapies, long-term monitoring is relevant because of the potential risk associated with integration of genetic material into the genome.
Ultragenyx Prepares FAYUVI for Patient Access
Following the approval, Ultragenyx said its UltraCare® program will support access to FAYUVI, while commercial product is expected to be available for shipment to qualified treatment centers within 30–60 days. The company said it is working with treatment centers and payers to support families through the gene therapy treatment process. FAYUVI’s approval expands the treatment landscape for Sanfilippo syndrome Type A, a rare genetic disorder that has historically lacked an FDA-approved disease-directed therapy. Its mechanism is based on delivering a functional SGSH gene using an AAV9 vector, with the goal of restoring production of sulfamidase and reducing the pathological accumulation of heparan sulfate.
The approval also reinforces the growing role of gene therapy in rare genetic diseases, where correcting or compensating for an underlying genetic defect can provide a therapeutic strategy distinct from conventional symptom management. For Ultragenyx, FAYUVI adds another approved genetic medicine to its rare-disease portfolio and establishes a new treatment option for eligible pediatric patients with MPS IIIA.
Source: Ultragenyx press release
