FDA Approves Fayuvi, the First Treatment for Sanfilippo Syndrome Type A

On September 17, 2026, the FDA approved Fayuvi (UX111) from Ultragenyx for children with Sanfilippo syndrome type A, also known as mucopolysaccharidosis type IIIA (MPS IIIA). It's the first treatment of any kind for the disease. The approval came through the accelerated pathway, just over a year after an earlier rejection.

Cara O'Neill, chief science officer of the Cure Sanfilippo Foundation, put it plainly. Until now, she said, families were told "to take their kids home and love them." With Fayuvi, they "would be given hope and an action plan."

Why it's called childhood Alzheimer's

Sanfilippo syndrome type A is caused by mutations in both copies of the SGSH gene. That gene makes an enzyme called sulfamidase, which helps break down a complex sugar called heparan sulfate inside lysosomes, the cell's degradative organelles.

Without the enzyme, heparan sulfate piles up, especially in the brain. Children often seem healthy at first. Then, typically in early childhood, development slows, then stops, then reverses. Children lose speech, the ability to walk, and the ability to feed themselves. Severe behavioral changes and sleep problems are common. Many don't survive past their teens.

The pattern of losing skills already learned is why families and advocates call it "childhood Alzheimer's."

How Fayuvi works

Fayuvi is a gene replacement therapy. It delivers a working copy of SGSH using an AAV9 vector, one of the few capsids that can cross the blood-brain barrier after intravenous delivery. It's given as a single intravenous infusion.

Once inside cells, the gene lets them make sulfamidase, which clears heparan sulfate from the lysosomes. Reducing heparan sulfate storage in neurons is expected to slow or halt further neurodegeneration.

Patients also receive immune-modulating medicine, starting the day before infusion and continuing for at least eight weeks, to reduce the immune response to the vector.

What the evidence showed

The approval rests on Ultragenyx's Transpher A clinical program and its long-term follow-up study. Given how rare and fast-moving the disease is, the FDA accepted data without a randomized placebo group, relying on a biomarker and comparisons with natural history.

The biomarker: heparan sulfate in spinal fluid. In 27 patients who received the highest dose:

• Heparan sulfate in cerebrospinal fluid dropped by a median of about 64% from baseline.

• 81.5% of patients achieved at least a 50% reduction.

• The drop began within the first month and has been sustained through as long as 8.5 years of follow-up.

Development. In younger patients (17 children), cognitive scores on the Bayley-III scale were substantially better than in untreated natural-history comparisons. Older or more advanced patients (10 children) kept abilities such as communicating, walking independently, and feeding themselves beyond the ages when those skills are usually lost.

Safety. The most common side effect was raised liver enzymes, a known effect of AAV therapy. Most cases were mild to moderate, and one more serious elevation resolved. No infusion reactions, blood clotting disorders, heart inflammation, nerve toxicity, or cancers were seen during follow-up.

The road to approval

Fayuvi's path wasn't straightforward. The FDA first accepted the application for priority review in February 2025. In July 2025, it issued a Complete Response Letter, citing manufacturing observations and asking for more long-term data. Ultragenyx resubmitted in January 2026 with an extra year of follow-up. The FDA accepted it in April and approved it two days ahead of its September 19 deadline.

The story shows a pattern seen across 2026: the FDA holding firm on manufacturing quality and long-term data, while being flexible about trial design in ultra-rare disease.

What it means

For families: For the first time, a Sanfilippo type A diagnosis comes with a treatment option. Because the disease destroys brain tissue over time, the data suggest the earlier a child is treated, the more can be preserved. That makes fast diagnosis, and possibly newborn screening, far more important.

For the field: Fayuvi joins a small but growing list of AAV therapies that reach the brain through the bloodstream. It also strengthens the case for using biomarkers like cerebrospinal fluid heparan sulfate in lysosomal diseases, where running large placebo-controlled trials is often impossible.

Open questions: Ultragenyx didn't announce a price at approval. Access will depend on cost, insurance coverage, and how quickly children can be diagnosed and reach treatment centers.

The bottom line

For decades, Sanfilippo syndrome type A meant watching a child lose everything they'd learned. Fayuvi restores sulfamidase production in the brain, and gives families a treatment option for the first time.

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Sources: STAT · NeurologyLive · ASGCT news