The FDA has approved Fayuvi (rebisufligene etisparvovec-hopf) from Ultragenyx — the first-ever treatment for Sanfilippo syndrome type A, a fatal childhood neurodegenerative disease that until now had no approved therapy.

The disease

Sanfilippo syndrome type A, also called MPS IIIA, is a rare inherited disorder in which children are missing a working copy of an enzyme called SGSH (sulfamidase). Without it, a complex sugar molecule called heparan sulfate accumulates to toxic levels throughout the body — and especially in the brain. The cruelty of the disease is its trajectory: affected children typically develop normally at first, then undergo progressive cognitive decline, losing speech, mobility and the skills they had gained, usually with a devastatingly short life expectancy. It belongs to the same family of “lysosomal storage disorders” as Tay-Sachs and Sanfilippo’s other subtypes.

How Fayuvi works

Fayuvi is a one-time intravenous gene therapy. It uses a modified, harmless virus — adeno-associated virus serotype 9 (AAV9), chosen for its ability to reach the nervous system — to deliver a functional copy of the SGSH gene into the patient’s cells. Once inside, the cells can produce the missing sulfamidase enzyme and begin breaking down the toxic heparan sulfate buildup, aiming to halt the relentless neurological damage at its source rather than merely managing symptoms.

The evidence

Approval, for pediatric patients ages 2 to 5, was based on an open-label, single-arm, multicenter study compared against untreated historical controls. The key result: treated children maintained or improved cognitive function — a striking departure from the disease’s usual pattern of plateau and decline. Because the natural history of MPS IIIA is so well documented and uniformly grim, holding cognition steady is itself a meaningful signal of benefit.

The risks

The therapy carries real safety considerations. Common adverse reactions (in more than 5% of patients) included elevated liver enzymes, nausea and vomiting, fever, reduced appetite, and low white-blood-cell and platelet counts. More seriously, the label carries a warning for thrombotic microangiopathy (a dangerous clotting condition), and — as with gene therapies generally — a noted theoretical long-term risk that the delivered genetic material could integrate into DNA and contribute to tumor development. These are weighed against a disease that is otherwise fatal.

Why it matters

“Today’s approval of Fayuvi is a meaningful step forward — not only for these children and their families, but for gene therapy to address rare diseases,” said Karim Mikhail of the FDA’s Center for Biologics Evaluation and Research. For families who previously had only supportive care to offer, a therapy aimed at the root cause is transformative. More broadly, it adds to a slow but growing list of gene therapies conquering ultra-rare, single-gene diseases — proof that the approach can reach even the brain, long considered a hard target. Long-term durability and safety will continue to be monitored. This is regulatory news, not medical advice.