The $3.95 Million Shot That Could Save a Child's Brain

The pivotal trial, called Transpher A, compared 17 treated kids to 27 untreated children from a natural history study.

The results were striking. Treated children scored 23.5 points higher on cognitive testing than their untreated counterparts, with a p-value below 0.0001. In plain English: the difference was enormous, and it almost certainly wasn't a fluke.

Even more encouraging, long-term follow-up data showed the benefit held up for nearly eight years. Kids maintained or improved their cognitive function, communication skills, and motor abilities. For a disease defined by relentless decline, stabilization alone would be remarkable. Improvement borders on extraordinary.

The safety profile included some expected issues: elevated liver enzymes, nausea, fever, and drops in white blood cell and platelet counts. The label also carries a warning about thrombotic microangiopathy (TMA), a serious blood vessel condition. These aren't trivial risks, but for a fatal disease with no alternatives, the calculus looks very different.

The Elephant in the Room Costs $3.95 Million

Let's talk about the number everyone will fixate on. At $3.95 million, Fayuvi is the second most expensive therapy in the United States, trailing only Lenmeldy (for metachromatic leukodystrophy) at $4.25 million. It leapfrogs Hemgenix for hemophilia B ($3.5 million), Casgevy for sickle cell disease ($2.2 million), and Zolgensma for spinal muscular atrophy ($2.125 million).

Is that price justified? The math depends on your perspective. Ultragenyx would argue (and they're not entirely wrong) that a one-time treatment for a fatal childhood disease, given to a tiny patient population, simply can't be priced like a cholesterol pill. The R&D costs get spread across hundreds of patients instead of hundreds of thousands. The economics of ultra-rare disease are genuinely brutal for drugmakers.

On the other hand, $3.95 million is still $3.95 million. Ultragenyx says it will distribute Fayuvi through Qualified Treatment Centers and offer its UltraCare program to help families navigate insurance coverage and logistics. That's corporate-speak for "we know this is going to be a nightmare to get covered, so we're hiring people to fight with insurers on your behalf."

The specialized distribution model could also slow access. Even when insurance agrees to pay, patients need to reach one of these designated centers. For a family in rural America with a sick child, that's another barrier.

Ultragenyx Needed This Win Badly

The Fayuvi approval didn't happen in a vacuum. Just two weeks earlier, on September 2, 2026, Ultragenyx's stock cratered roughly 45% in after-hours trading after its Phase 3 Angelman syndrome trial for GTX-102 (apazunersen) completely missed its primary endpoint. Analysts called it a "clean fail," which is Wall Street's polite way of saying "this drug doesn't work."

The fallout was swift. JPMorgan downgraded the stock to Neutral and slashed its price target from $80 to $36. Baird cut its target from $40 to $16. The stock eventually touched a 52-week low of about $13.78, reflecting a roughly 53% decline over the prior year.

So Fayuvi's approval isn't just a win for patients; it's a lifeline for Ultragenyx's credibility. The company has been positioning itself as a multi-product rare disease company, with gene therapies for glycogen storage disease (DTX401, with a PDUFA date expected in Q3 2026), Wilson disease (UX701), and OTC deficiency (DTX301) all moving through the pipeline.

The Angelman failure threatened to undermine that entire narrative. Fayuvi's approval, backed by real clinical data and a full (not accelerated) FDA nod, at least partially restores the story.

What This Means Going Forward

Fayuvi's approval is a genuine milestone for the Sanfilippo community. Families who had nothing now have something. The clinical data is real, the benefit is durable, and the FDA gave it a standard approval based on actual cognitive outcomes, not a surrogate biomarker.

But it also crystallizes the tension at the heart of gene therapy's promise. These treatments can be transformative for individual patients while being almost impossibly expensive for health systems. As more ultra-rare gene therapies reach the market, each one carrying a multi-million-dollar price tag, the question of who pays (and who gets left out) only gets louder.

For one small group of children and their families, though, September 17 was the day the impossible became possible. That's worth celebrating, even as we argue about the bill.

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