FDA approval puts a $4 million gene therapy for rare brain disorder within reach

Children diagnosed with Sanfilippo syndrome now have access to a new gene therapy that aims to halt their condition's progression. The U.S. Food and Drug Administration approved the treatment in September 2026. Called Fayuvi, the therapy is administered as a single infusion and costs nearly $4 million per patient. This price point ties it for the second most expensive gene therapy ever sold, according to the drug price research firm 46brooklyn.

Sanfilippo syndrome is a rare inherited disorder caused by a faulty gene. The defect prevents the body from breaking down heparin sulfate, a molecule essential for normal cell function. As the substance accumulates, it causes severe neurological complications. Affected children typically lose the ability to speak and walk and suffer from seizures. The Cure Sanfilippo Foundation reports that most patients do not survive past their teenage years.

The path to approval began in 2013, when Eliza O'Neill, then three years old, was diagnosed with the disease in Columbia, South Carolina. Her mother, Cara O'Neill, is a pediatrician who recognized the symptoms after a colleague suggested testing for metabolic disorders. Eliza's father, Glenn O'Neill, recalled the immediate urgency of the situation. 'It wasn't really until I got home that I realized how quickly the degeneration happened,' he said. 'And that was really the point where we said, "Gosh. Whatever we need to do, we need to do it now."'

Research into a cure had been stalled for years due to funding gaps. Haiyan Fu, a researcher at Nationwide Children's Hospital in Columbus, Ohio, had been developing a gene therapy for Sanfilippo in mice since the late 1990s. The approach uses a modified virus to deliver a working copy of the broken gene to cells. By 2009, Fu considered abandoning the work due to lack of resources. Families of affected children stepped in to provide financial support. In total, eight patient foundations, including Ben's Dream and a Cure for Kirby, sustained the research until it was ready for human trials.

To fund the first clinical trial, Glenn O'Neill launched a crowdfunding campaign in 2014. He partnered with filmmaker Benjamin Von Wong to create a viral video titled 'Saving Eliza.' The campaign raised $1 million in a few weeks, breaking the then-current record for the platform GoFundMe. The total eventually reached $2 million. These funds helped launch the first human trial in May 2016, where Eliza became the first patient to receive the treatment.

The promising results from the trials attracted the attention of the biotech industry. In 2022, the company Ultragenyx acquired the rights to the gene therapy and continued its development. The company later submitted data to the FDA for review, leading to the September approval. Emil Kakkis, CEO of Ultragenyx, stated that the treatment is most effective when administered to young children. 'The treatment will work best the younger the child is because the more brain is remaining,' Kakkis said.

Kakkis noted that children treated at six months of age have shown positive outcomes. He added that the company aims to remove logistical barriers to access. 'There should be no logistical location, site or state or other kind of rule that stops us from getting the patient treated,' he said. 'We understand what [we've] got to do.'

Fu learned of the approval shortly after it was announced, receiving calls and messages from families who had supported her early research. Some of these families had lost children during the wait. Fu described her feelings as mixed, noting, 'It shouldn't take this long.'

Eliza O'Neill is now nearly 17 years old. She received the therapy at age six, a time when she had already lost the ability to speak. The speech did not return, but she now communicates using picture cards and assistive devices. She attends school and, this summer, learned to float on a pool noodle. Her father, Glenn O'Neill, described the journey as difficult but rewarding. 'I just feel proud of my girl,' he said. 'It's been a rough road for her, but we also know she's very lucky.'