First Sanfilippo Gene Therapy Fayuvi: How One Family's Viral Fundraiser Helped Deliver a $4 Million Treatment

WASHINGTON — The United States Food and Drug Administration (FDA) has approved Fayuvi, the first gene therapy for children with Sanfilippo syndrome type A, a rare inherited disorder that gradually damages the brain and usually claims lives before adulthood. The approval, granted on 17 September 2026, closes a journey that began in a research laboratory in the late 1990s, was kept alive by eight family foundations, and was pushed into its first human trial with the help of a viral fundraising video that raised $2 million in 2014.

Fayuvi, whose generic name is rebisufligene etisparvovec-hopf and which was formerly known as UX111, is a one-time intravenous infusion for paediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA) whose neurodevelopmental function is still preserved. It is the first FDA-approved treatment designed to address the underlying cause of the disease, rather than only managing its symptoms. Its maker, Ultragenyx Pharmaceutical, has set the price at nearly $4 million, tying it for the second-most expensive gene therapy ever, according to NPR's reporting.

Illustration: A child receiving a one-time gene therapy infusion in a hospital setting.

What Happened

Sanfilippo syndrome type A is caused by a faulty SGSH gene. Children with the condition cannot produce enough sulfamidase, an enzyme needed to break down heparan sulfate, a molecule the body needs to function. As heparan sulfate builds up, it progressively damages the brain and nervous system. Children lose the ability to speak and walk, develop seizures, and, according to the Cure Sanfilippo Foundation, usually do not live past their teens.

Fayuvi uses a modified adeno-associated virus serotype 9 (AAV9) vector to deliver a working copy of the SGSH gene into patients' cells, enabling them to produce sulfamidase. The therapy is given once, by intravenous infusion, in a healthcare setting equipped to manage infusion reactions. According to the FDA, all patients receive corticosteroid treatment beginning one day before the infusion and continuing for at least eight weeks afterwards.

The FDA assessed effectiveness in an open-label, single-arm, multicentre study of children with MPS IIIA, comparing changes in cognitive scores at ages two to five years with an untreated historical cohort. The agency said treated children maintained or improved cognitive function relative to untreated historical controls, against an expected natural course of plateau and decline. The comparison relied on historical untreated controls rather than randomised concurrent controls, and the FDA did not provide a numerical cognitive effect estimate.

The approval also came with safety warnings. The FDA identifies thrombotic microangiopathy — a condition affecting small blood vessels — as a risk, and the integration of delivered genetic material into the genome, with possible tumour development, as a potential long-term risk. Adverse reactions reported in more than 5% of patients included increased aspartate aminotransferase (AST), nausea and vomiting, fever, decreased appetite, reduced white blood cell and platelet counts, and increased amylase. Fayuvi received Orphan Drug, Fast Track and Breakthrough Therapy designations, and Ultragenyx also received a Rare Pediatric Disease Priority Review Voucher, which it has since agreed to sell for $203 million, according to Zacks.

Background

The therapy's roots go back to Nationwide Children's Hospital in Columbus, Ohio, where researcher Haiyan Fu had been working on a gene therapy for Sanfilippo in mice since the late 1990s. The work used a modified virus to deliver a working copy of the broken gene to patients' cells, but moving from mice to people needed money that traditional research funding had not supplied.

That changed through families. Eight family foundations, including Ben's Dream and Cure Kirby, kept Fu's work going until it was ready for a first human trial. Among them were Glenn and Cara O'Neill of Columbia, South Carolina. Cara O'Neill, a paediatrician, learned in 2013 that her daughter Eliza had Sanfilippo syndrome after a colleague suggested testing for a group of metabolic disorders just to rule them out. At the end of 2013, the family started the Cure Sanfilippo Foundation.

In the spring of 2014, the O'Neills reached out to strangers they found on the internet about making a viral video, and filmmaker Benjamin Von Wong made one for them over a few days. The "Saving Eliza" video broke GoFundMe's record at the time, raising $1 million in just a few weeks and soon reaching $2 million. That money helped fund the first human trial, which started in May 2016, with Eliza herself as the very first patient to receive the experimental therapy, at age six.

Eliza O'Neill is now 17. According to NPR's reporting, she did not regain her speech, but she continues to communicate using devices and picture cards. For her family, and for many families dealing with Sanfilippo syndrome, the approval of the first-ever treatment is, in NPR's words, a dream come true — but also bittersweet, because it arrives too late to reverse the damage already done in older children.

Illustration: Laboratory research behind a gene therapy for a rare inherited disorder.

Why It Matters

Until Fayuvi, care for Sanfilippo syndrome type A was limited to managing symptoms. There was no approved treatment aimed at the disease itself, and no approved vaccine or therapy that could change its course. The FDA approval therefore matters in two ways: it gives eligible young children a disease-directed option for the first time, and it shows that a treatment for an ultra-rare disease can reach approval at all.

The price matters just as much. At nearly $4 million for a single infusion, Fayuvi ties for the second-most expensive gene therapy ever, according to drug-price research firm 46brooklyn, cited by NPR. Ultragenyx chief executive Emil Kakkis has said he is committed to making sure every patient is treated regardless of price, but access will in practice depend on insurers, health systems and treatment centres. The company is limiting distribution to United States institutions trained to administer gene therapy, and it expects to ship the therapy to qualified treatment centres within 30 to 60 days of approval.

The approval also matters for how rare-disease research gets funded. The Sanfilippo story is unusual: a laboratory programme sustained for years by family foundations, then accelerated by a viral video made by a family with no background in drug development. Ultragenyx acquired the rights to the therapy in 2022, continued studying it, and submitted it for FDA review. Without the families' early money, the first human trial in 2016 might never have happened — a point NPR's October 2026 feature, republished on 1 November 2026, puts at the centre of the story.

At the same time, the evidence base has limits that families and clinicians should understand. The pivotal comparison was against a historical cohort, not a randomised trial, and overall survival data and long-term safety — including the potential tumour risk from genome integration — will take years to clarify. Fayuvi is approved only for paediatric patients whose neurodevelopmental function is preserved, so it is not a treatment for every child or adult living with the disease.

Illustration: A family recording a fundraising video to support medical research.

What Happens Next

In the short term, the test is delivery. Ultragenyx must get a complex, one-time gene therapy — with corticosteroid cover, infusion-reaction readiness and liver-function monitoring — into qualified centres, and families of newly diagnosed children must learn about it early enough for the preserved-function criterion to be met. The earlier a child is diagnosed, the more the therapy can plausibly offer, which puts a premium on newborn and early-childhood screening and on rapid referral.

Regulators and researchers will also be watching long-term follow-up. The FDA's approval rests on cognitive outcomes measured in early childhood against historical controls; confirming that benefit holds, and that no late safety signal emerges, will require years of monitoring. The company's sale of its priority review voucher for $203 million adds capital, but it does not change the clinical questions that follow-up must answer.

For other rare diseases, the Sanfilippo story is likely to be studied as a funding model as much as a medical one. Family foundations carried a laboratory idea through its unfunded years, a viral campaign supplied the bridge to a human trial, and a commercial sponsor carried it through approval. Whether that model can be repeated — and whether health systems will pay nearly $4 million per child when it is — will shape the next decade of gene therapy for conditions too rare to attract conventional investment. This article is general information only, not medical advice; families should discuss treatment options with their child's clinicians.

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FAQ

What is Sanfilippo syndrome type A?

Sanfilippo syndrome type A, or MPS IIIA, is a rare inherited metabolic disorder in which a faulty SGSH gene leaves children unable to produce enough sulfamidase. Heparan sulfate builds up and progressively damages the brain, causing loss of speech and walking ability, seizures, and usually death before adulthood.

What is Fayuvi and how is it given?

Fayuvi (rebisufligene etisparvovec-hopf, formerly UX111) is a one-time intravenous gene therapy that uses an AAV9 vector to deliver a working SGSH gene into cells. Patients receive corticosteroids from one day before infusion for at least eight weeks afterwards, in a setting equipped to manage infusion reactions.

Who is eligible for Fayuvi?

The FDA approved Fayuvi for paediatric patients with MPS IIIA whose neurodevelopmental function is still preserved. It is not approved for every child or adult with the disease, and eligibility is assessed by specialist treatment centres.

How much does Fayuvi cost?

Ultragenyx set the price at nearly $4 million for the one-time infusion, tying it for the second-most expensive gene therapy ever, according to NPR. The company's chief executive has said he is committed to ensuring every patient is treated regardless of price, but actual access depends on insurers and health systems.

How did a family's fundraiser help create Fayuvi?

The O'Neill family's 2014 "Saving Eliza" video raised $1 million in a few weeks and soon reached $2 million, helping fund the first human trial, which began in May 2016 with Eliza as its first patient. Eight family foundations in total kept the underlying research going; Ultragenyx acquired the therapy in 2022 and took it through FDA approval in September 2026.

Sources

NPR, "A family's fundraising for gene therapy research pays off," 22 October 2026 (republished 1 November 2026); FDA approval coverage via ReachMD, "FDA Approves First Gene Therapy for Pediatric MPS IIIA," 9 October 2026; Zacks, "RARE's Fayuvi Approval Spurs Second PRV Sale Worth $203M in a Week," October 2026; Ultragenyx / FDA approval reporting, September 2026.