The FDA approved the first treatment for Sanfilippo syndrome type A, a fatal childhood brain disease

Children with Sanfilippo syndrome type A have spent decades with nothing to slow the disease that is taking their abilities to speak, learn and walk. On Sept. 17, 2026, that changed: Ultragenyx Pharmaceutical’s Fayuvi, a one-time intravenous gene therapy, cleared the federal approval process, carrying a list price of $3.95 million and the hopes of families who have waited a long time.

The approval is small in patient numbers and large in meaning. Ultragenyx estimates that 3,000 to 5,000 people in the developed world live with the condition, which makes it one of the many rare diseases that rarely attract a commercial therapy. A treatment that targets the biology itself, rather than managing symptoms, is a new kind of option, and the first test of whether a single infusion can hold back a disease that has long been described as a childhood dementia. Parents, clinicians and patient groups will now watch how the label is used in practice, how many families can reach treatment centers, and how the early recipients fare over the coming years.

What the agency actually cleared

The FDA announced its approval of Fayuvi (rebisufligene etisparvovec-hopf) as the first treatment for pediatric patients with mucopolysaccharidosis type IIIA, the formal name for Sanfilippo syndrome type A. The agency describes a rare inherited disease that progressively damages the brain and nervous system, so that affected children lose cognitive, language and other developmental abilities over time, and the disorder is fatal. Until now, care meant managing symptoms, with no approved therapy designed to change the underlying course.

Fayuvi works by delivering a working copy of the SGSH gene using an AAV9 viral vector. The gene lets cells make sulfamidase, the enzyme that children with this disease lack or produce too little of. Without it, a sugar chain called heparan sulfate accumulates, and the buildup is what injures the brain. By restoring the enzyme, the therapy aims to let cells break the material down and reduce the accumulation. It is given once, as an infusion into a vein, in a healthcare setting equipped to manage infusion reactions.

Karim Mikhail, director of the agency’s Center for Biologics Evaluation and Research, called the decision “a meaningful step forward.” Acting Commissioner Kyle Diamantas said that “gene therapy holds tremendous promise for rare diseases like Sanfilippo syndrome type A,” and Megha Kaushal, acting deputy director of the Office of Therapeutic Products, tied the approval to the agency’s commitment to rigorous evidentiary standards.

The evidence behind the decision

The data come from an open-label, single-arm, multicenter study in children aged 2 to 5, which measured mean changes in cognitive scores. According to the FDA, treated children maintained or improved cognitive function compared with an untreated historical control cohort. The agency frames that result as a departure from the usual course for this age window, which is a plateau followed by decline.

The design deserves a careful reading. With no placebo group, the comparison rests on how untreated children are known to progress, and the trial was small. Fierce Pharma reported that the agency reviewed data from a small single-arm trial compared against natural history, with one subgroup of 17 children who were under 2 or at an earlier stage of disease and another of 10 older or later-stage children. For a disease this rare and this severe, regulators have often accepted such designs, but they leave open how durable the benefit will prove across the years when cognition matters most. Cognitive scores in young children also vary from visit to visit, which is one reason regulators lean on comparisons with carefully documented natural-history records and why longer follow-up of the treated group will carry so much weight.

Three programs helped move the application along. The therapy held orphan drug, fast track and breakthrough therapy designations, which exist to speed development of treatments for serious conditions with unmet need. Fierce Pharma also noted that the agency accepted a resubmission in April and granted it priority review.

Risks, steroids and a $3.95 million price

The label carries real warnings. The FDA lists a risk of thrombotic microangiopathy, a condition in which small blood clots form in tiny vessels, and a potential long-term risk, shared with other AAV-based gene therapies, that inserted genetic material could integrate into the genome and possibly lead to tumors. The most common adverse reactions, seen in more than 5 percent of patients, were increased liver enzyme levels, nausea and vomiting, fever, decreased appetite, lower white blood cell and platelet counts, and increased amylase.

Every patient receives corticosteroids beginning one day before the infusion and continuing for at least eight weeks afterward, a regimen meant to control the immune response to the viral vector. Families and clinicians will have to weigh that burden, and the monitoring that comes with it, against the possibility of preserving a child’s abilities.

Price will shape access. Ultragenyx chief commercial officer Erik Harris said the wholesale acquisition cost is $3.95 million, a list figure before discounts, rebates or outcomes-based reimbursement arrangements. Chief executive Emil Kakkis said the company recognizes “the profound urgency of making this therapy available to families,” and Fierce Pharma reported that no specific launch date was given, with manufacturing at facilities in Bedford, Massachusetts, and Columbus, Ohio. How insurers and state Medicaid programs respond will determine how quickly the first children actually receive treatment, and whether families outside the largest centers can travel for a one-time infusion that requires specialized monitoring afterward.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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