The U.S. Food and Drug Administration announced on September 17, 2026 that it has approved Fayuvi (rebisufligene etisparvovec-hopf), the first treatment for pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA), also known as Sanfilippo syndrome type A, according to the official release. The agency said the approval gives families of children with the rare, progressively degenerative disease an option for the first time to alter its underlying course rather than only manage symptoms.

What the Approval Covers

Fayuvi is a one-time therapy given by intravenous infusion, using a modified, non-infectious adeno-associated virus serotype 9 (AAV9) to carry a working copy of the SGSH gene into a patient's cells, the FDA said. The goal is to enable the body to produce sulfamidase, the enzyme that is missing or deficient in people with MPS IIIA, so that heparan sulfate can be properly broken down in lysosomes instead of building up harmfully throughout the body and brain.

MPS IIIA is described by the FDA as a rare inherited disease that progressively damages the brain and nervous system, causing children to lose cognitive, language and other developmental abilities over time. Until this approval, the agency said, treatment for the disease was limited to managing symptoms, with no FDA-approved therapy designed to change its underlying course.

"The Trump Administration is committed to bringing safe and effective treatments to patients with the most urgent and unmet needs. The approval of Fayuvi marks a historic moment for children and families living with MPS IIIA, which is a disease that has, until now, offered no approved treatment to alter its devastating course," said Acting FDA Commissioner Kyle Diamantas, J.D., adding that gene therapy "holds tremendous promise for rare diseases like Sanfilippo syndrome type A."

Clinical Evidence Behind the Decision

The FDA said the safety and effectiveness of Fayuvi were evaluated in an open-label, single-arm, multicenter clinical study in pediatric patients with MPS IIIA. The study measured mean changes in cognitive scores in patients between the ages of 2 and 5 years, a window the agency describes as critical to the disease's developmental trajectory.

According to the release, Fayuvi-treated patients maintained or improved cognitive function compared with an untreated historical control cohort, which the FDA characterized as a meaningful divergence from the expected natural disease course of plateau and decline during this period.

"Achieving meaningful neurodevelopmental benefit through a single intravenous administration represents a significant scientific milestone — demonstrating that systemic AAV9-mediated gene delivery can reach the central nervous system at therapeutically relevant levels in pediatric patients," said Megha Kaushal, M.D., M.Sc., Acting Deputy Director of the Office of Therapeutic Products. Karim Mikhail, B. Pharm., M.S., Director of the Center for Biologics Evaluation and Research, called the approval "a meaningful step forward — not only for these children and their families, but for the promise of gene therapy to address rare and devastating diseases where the need for safe and effective treatment is the most urgent."

Approval Snapshot

Safety Profile and How It Is Administered

The FDA said the most commonly reported adverse reactions, occurring in more than 5% of patients across clinical studies, included increases in liver enzymes (AST), nausea and vomiting, fever, decreased appetite, decreased white blood cell and platelet counts, and increased amylase. The agency flagged thrombotic microangiopathy (TMA) as an important safety warning, and noted that, as with other AAV-based gene therapies, there is a potential long-term risk that the inserted genetic material could integrate into the genome and potentially lead to tumor development.

• Setting: Fayuvi must be given in a healthcare setting equipped to manage infusion reactions
• Pre-treatment: All patients receive corticosteroid treatment beginning one day before infusion
• Post-treatment: Corticosteroid treatment continues for a minimum of eight weeks afterward

The FDA noted that Fayuvi had previously received Orphan Drug, Fast Track, and Breakthrough Therapy designations, regulatory pathways the agency uses to speed the development and review of treatments for serious or rare conditions. Approval was granted to Ultragenyx Pharmaceutical, Inc.

How the FDA Reviews Gene Therapies

The FDA is the federal agency within the U.S. Department of Health and Human Services responsible for assuring the safety, effectiveness and security of human and veterinary drugs, vaccines and other biological products, as well as medical devices, food, cosmetics and tobacco products, according to its standard agency description. Gene therapies like Fayuvi are regulated as biologics and reviewed by the agency's Center for Biologics Evaluation and Research and its Office of Therapeutic Products, the units whose leaders are quoted in this approval announcement.

For ultra-rare pediatric diseases such as MPS IIIA, where large randomized trials are often impractical because of small patient populations, the FDA has increasingly relied on single-arm studies compared against historical control cohorts, as it did in evaluating Fayuvi. This approval follows a broader pattern of recent agency decisions on first-of-their-kind therapies, including the FDA's first-of-its-kind treatment for chronic hepatitis B and its accelerated approval of Jemperli for rectal cancer patients, both of which relied on expedited regulatory pathways for conditions with significant unmet medical need.

Frequently asked questions

Who is eligible to receive Fayuvi?

The FDA approved Fayuvi for pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA), also known as Sanfilippo syndrome type A. The pivotal study that supported approval measured cognitive outcomes in children between the ages of 2 and 5 years.

How is Fayuvi given to patients?

Fayuvi is administered as a single, one-time intravenous infusion in a healthcare setting equipped to manage infusion reactions, according to the FDA. Patients also receive corticosteroid treatment starting one day before the infusion and continuing for a minimum of eight weeks afterward.

What are the main safety risks associated with Fayuvi?

The FDA listed thrombotic microangiopathy (TMA) as an important safety warning, along with a potential long-term risk that the therapy's inserted genetic material could integrate into the genome and lead to tumor development. The most common adverse reactions, reported in more than 5% of patients, included liver enzyme increases, nausea, vomiting, fever, decreased appetite, and decreased blood cell counts.

Who manufactures Fayuvi?

The FDA granted approval of Fayuvi to Ultragenyx Pharmaceutical, Inc. The therapy had previously received Orphan Drug, Fast Track, and Breakthrough Therapy designations from the agency.