FDA Approves First Gene Therapy for Sanfilippo Syndrome Type A: What the Approval Actually Shows

The Food and Drug Administration approved Fayuvi on September 17, the first therapy cleared to treat the underlying cause of Sanfilippo syndrome type A. Here is what the approval package actually shows — including the parts that warrant caution.

For families whose children have mucopolysaccharidosis type IIIA (MPS IIIA), better known as Sanfilippo syndrome type A, there has never been an approved treatment that changes the course of the disease. Care has meant managing symptoms while watching a child who developed normally lose language, cognition and motor skills.

That changed on September 17, when the FDA approved Fayuvi (rebisufligene etisparvovec-hopf), a one-time gene therapy from Ultragenyx Pharmaceutical. It is the first treatment of any kind approved for MPS IIIA.

What the therapy does

Sanfilippo type A is an inherited disease caused by a faulty SGSH gene. Without a working copy, the body cannot make enough sulfamidase, an enzyme that breaks down a long sugar molecule called heparan sulfate. The molecule accumulates inside lysosomes — the cell’s recycling compartments — and the damage falls hardest on the brain and nervous system.

Fayuvi is delivered as a single intravenous infusion. It uses a modified, non-infectious virus known as adeno-associated virus serotype 9 (AAV9) to carry a working copy of SGSH into the patient’s cells. Those cells can then produce sulfamidase and clear the buildup.

The choice of AAV9 matters beyond this one disease. Reaching the central nervous system from a vein, rather than injecting directly into the brain or spinal fluid, has been one of the hardest problems in gene therapy. Megha Kaushal, acting deputy director of the FDA’s Office of Therapeutic Products, called the result a demonstration that “systemic AAV9-mediated gene delivery can reach the central nervous system at therapeutically relevant levels in pediatric patients.”

What the trial showed — and how it was designed

This is where the detail deserves attention, because the study design is unusual by the standards most people associate with drug approvals.

The FDA evaluated Fayuvi in an open-label, single-arm, multicenter study in children with MPS IIIA. The main measure was the mean change in cognitive scores among patients aged 2 to 5 — the window in which untreated children typically plateau and then decline.

Treated children maintained or improved cognitive function. But there was no randomized control group receiving a placebo. Instead, their results were compared against an untreated historical control cohort — records of children with the same disease, observed previously.

That is a legitimate and common approach for ultra-rare pediatric diseases, where randomizing a dying child to placebo raises serious ethical problems and where there may not be enough patients to fill two arms. It is also a genuine limitation: historical comparisons cannot fully rule out differences in how patients were selected, diagnosed or cared for across time. Readers should understand the evidence for what it is — persuasive in context, but not equivalent to a large randomized trial.

The risks the label carries

The FDA listed adverse reactions reported in more than 5% of patients: elevated liver enzymes (AST), nausea and vomiting, fever, reduced appetite, lowered white blood cell and platelet counts, and increased amylase.

Two warnings stand out:

• Thrombotic microangiopathy (TMA) — a serious condition involving clotting in small blood vessels — is flagged as an important safety risk.
• Potential long-term cancer risk. As with other AAV-based gene therapies, the inserted genetic material could in principle integrate into the genome and contribute to tumor development. This is a theoretical risk shared across the class, not an observed outcome in this trial, but it is why recipients of gene therapies are followed for years.

Treatment is not a simple infusion appointment. It must be given in a healthcare setting equipped to manage infusion reactions, and every patient receives corticosteroids starting the day before the infusion and continuing for at least eight weeks afterward.

What is still unknown

Three questions the approval does not answer:

Price. The FDA does not set or announce prices. Ultragenyx has not been required to disclose one as part of the approval, and one-time gene therapies for rare diseases have historically launched at prices in the seven figures. Coverage decisions by Medicaid — which insures a large share of children with rare genetic conditions — will follow separately.

Durability. A single infusion is designed to work for life, but the follow-up period in an approval of this kind is measured in years, not decades. Whether cognitive benefit holds into adolescence and adulthood is not yet established.

Who qualifies. The efficacy data centered on children aged 2 to 5. Because Sanfilippo damage accumulates before diagnosis, timing is likely to matter — which puts weight on newborn screening programs that do not currently include MPS IIIA in most states.

Why this approval is being watched

Fayuvi received Orphan Drug, Fast Track and Breakthrough Therapy designations — the three pathways the agency uses to speed treatments for serious conditions with unmet need.

For the families affected, the practical significance is immediate: a disease with no disease-modifying option now has one. For gene therapy more broadly, the significance is the delivery route: if an intravenous AAV9 infusion can reach the brain at therapeutic levels in children, the same approach becomes more plausible for other neurological conditions that have resisted treatment.

Source: U.S. Food and Drug Administration news release, September 17, 2026.

This article is for general information and is not medical advice. Families considering gene therapy should discuss eligibility, risks and monitoring with their child’s specialist.