FDA Approves Fayuvi, First Gene Therapy for Sanfilippo Syndrome Type A, at $3.95 Million

Sanfilippo syndrome type A, also known as mucopolysaccharidosis type IIIA or MPS IIIA, is a rare inherited disease that progressively damages the brain and nervous system. Children born with the condition lose cognitive, language and other developmental abilities as the disease advances. It is caused by a deficiency of an enzyme called sulfamidase, which is encoded by the SGSH gene, leading to an abnormal accumulation of a complex sugar molecule known as heparan sulfate in the body and brain. That toxic buildup causes children to gradually lose cognitive, speech and motor skills.

For families, the diagnosis has long carried a devastating prognosis. Until this week, treatment was limited to managing symptoms, and there was no FDA-approved therapy designed to change the underlying course of the disease. The condition is estimated to affect approximately 3,000 to 5,000 patients in commercially accessible geographies, with a median life expectancy of 15 years.

That changed on September 17, 2026, when the U.S. Food and Drug Administration granted standard full approval to Fayuvi, also known as rebisufligene etisparvovec-hopf or UX111, for pediatric patients with MPS IIIA. It is the first-ever FDA-approved treatment for Sanfilippo syndrome Type A and the second gene therapy approval for Ultragenyx Pharmaceutical Inc. (NASDAQ: RARE), which also received a Priority Review Voucher.

Fayuvi is a one-time, intravenous gene therapy that uses a modified non-infectious adeno-associated virus serotype 9 (AAV9) to deliver a working copy of the SGSH gene into the patient's cells. Doing so enables production of sulfamidase, the enzyme missing or deficient in MPS IIIA, which allows heparan sulfate to be broken down in lysosomes and reduces its harmful buildup in the body and brain. Ultragenyx set a U.S. list price of $3.95 million for the one-time intravenous therapy, launching one of the world's most expensive drugs as a treatment for a rare, fatal childhood disease.

Key Facts

The FDA announced on September 17, 2026 that it approved Fayuvi, describing the decision as the first treatment for pediatric patients with mucopolysaccharidosis type IIIA. The agency said the approval marks a historic moment for children and families living with MPS IIIA. Ultragenyx said on September 17 that the FDA granted standard full approval of FAYUVI, calling it the first-ever approved treatment for the disease and the company's second gene therapy approval.

Safety and effectiveness were evaluated in an open-label, single-arm, multicenter study in pediatric patients aged 2 to 5 years. Fayuvi-treated patients maintained or improved cognitive function compared with an untreated historical control cohort. In the pivotal Transpher A trial, treated patients from the modified intention-to-treat population (N=17) were compared with untreated patients from an external natural-history cohort (N=27), and treated patients demonstrated a 23.5-point higher cognitive score over natural history during the study, measured as the mean change in Bayley-III Cognitive raw score from 24 to 60 months of age, with p below 0.0001. Clinical data now extend to nearly 8 years.

Reuters reported on September 17 that Ultragenyx set a U.S. list price of $3.95 million for its newly approved gene therapy, launching one of the world's most expensive drugs. Reuters reported on September 17 that Ultragenyx executives said on a conference call that the lifetime cost of caring for a child with the disease, who can spend years in a bedridden state, can exceed $8 million, and that J.P. Morgan analysts estimate Fayuvi could reach peak worldwide sales of $200 million to $250 million.

The most common adverse reactions, occurring in more than 5 percent of patients, were increases in liver enzymes (AST), nausea and vomiting, fever, decreased appetite, decreased white blood cell and platelet counts and increased amylase. Warnings include a risk of thrombotic microangiopathy (TMA) and, as with other AAV gene therapies, a potential long-term risk of insertional tumor development. Patients receive corticosteroids starting one day before infusion and for at least eight weeks after. Fayuvi received Orphan Drug, Fast Track and Breakthrough Therapy designations.

Ultragenyx said the product is expected to be available for shipment to qualified treatment centers within 30 to 60 days, supported by its UltraCare access program. FAYUVI is manufactured entirely in the U.S. at Ultragenyx's Gene Therapy Manufacturing Facility in Bedford, Massachusetts, and at Andelyn Biosciences in Columbus, Ohio. Stocktwits reported on September 17 that shares gained 13 percent in the regular session and added 3 percent after hours, after touching a fresh 52-week low of $12.73 the previous day, and that RARE stock has fallen 37 percent year to date. Reuters reported on September 17 that Ultragenyx shares closed 12.6 percent higher on Thursday.

Analysis

What this really means is that gene therapy has crossed another threshold, moving from a handful of ultra-rare metabolic and neuromuscular conditions into a disease that attacks the developing brain. The FDA noted that achieving meaningful neurodevelopmental benefit through a single intravenous administration represents a significant scientific milestone. The clinical dataset is small, but the comparison against a natural-history cohort rather than a placebo arm reflects how difficult it would be to randomize families facing a fatal pediatric diagnosis.

The approval also carries a reminder that the path to market is rarely smooth. Stocktwits reported on September 17 that the FDA had turned the therapy down in 2025 over manufacturing issues, not clinical data, and that Ultragenyx resubmitted this year, with a decision due by September 19. The company subsequently secured clearance, and the regulator granted a Priority Review Voucher that a Stocktwits user noted is valued at around $200 million. The bigger picture here is that manufacturing readiness and regulatory strategy now matter as much as the underlying biology in gene therapy.

The price of $3.95 million will dominate public discussion. Reuters reported on September 17 that Ultragenyx executives framed the figure against a lifetime cost of care that can exceed $8 million for a child who can spend years in a bedridden state. That comparison is the industry's standard argument for one-time therapies, and it is a real consideration for payers, but the arithmetic is far from settled for every family or every health system. The J.P. Morgan estimate of $200 million to $250 million in peak worldwide sales suggests Ultragenyx expects a narrowly defined patient population rather than a blockbuster.

It is also worth noting that Ultragenyx is not a company arriving at this moment from a position of strength. Stocktwits reported on September 17 that the stock has fallen 37 percent year to date, weighed down earlier by a failed late-stage Angelman syndrome trial that sent shares down about 44 percent in a single session. Reuters reported on September 17 that shares closed 12.6 percent higher on Thursday. A single approval does not erase a difficult year, but it does give the company a commercial product with no direct competitor.

Why It Matters

For families, the significance is not abstract. Glenn O'Neill, president and co-founder of the nonprofit Cure Sanfilippo Foundation, whose daughter Eliza was diagnosed in 2013 and treated with the therapy in 2016 at age 6, said the approval means real hope and a real life-changing chance to have a different outcome than the traditional course of this disease. Eliza's parents told Reuters ahead of the decision that she has never had a seizure and is not on a feeding tube or in a wheelchair, and that she attends school and swims.

The approval also matters because it validates a long research arc. Ultragenyx said the vector was first developed at Nationwide Children's Hospital more than a decade ago by Haiyan Fu, Ph.D., and Doug McCarty, Ph.D., and licensed to Abeona before Abeona out-licensed it to Ultragenyx. Kevin M. Flanigan, M.D., director of the Center for Gene Therapy at Nationwide Children's Hospital, was principal investigator. The FDA's Karim Mikhail, Director of the Center for Biologics Evaluation and Research, said parents and clinicians have been waiting far too long for an option.

Finally, the approval matters for the broader rare disease field. Sanfilippo syndrome Type A has been described as a progressive and fatal neurodegenerative disease, and the FDA said that until this approval, treatment was limited to managing symptoms. A single intravenous administration that changes the trajectory of a brain disease in children offers a template that other programs, in other lysosomal storage disorders and beyond, will point to when they seek approval.

Next Up

The immediate next step is commercial delivery. Ultragenyx said the product is expected to be available for shipment to Qualified Treatment Centers within 30 to 60 days, with the UltraCare program supporting access. The company also said clinical data now extend to nearly 8 years, and long-term follow-up will continue as patients age, particularly given warnings about thrombotic microangiopathy and a potential long-term risk of insertional tumor development.

Beyond that, the attention will turn to payer negotiations, real-world uptake and whether the $3.95 million price survives scrutiny from insurers and governments. Ultragenyx chief executive Emil D. Kakkis, M.D., Ph.D., called the approval a historic milestone for a community that has waited far too long. Whether the health system treats it as one will be determined in the months after the first infusions.