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# Why the FDA Granted Full Approval to Fayuvi on Seventeen Patients
- URL: https://bioweek.com/why-the-fda-granted-full-approval-to-fayuvi-on-seventeen-patients/
- Published: 2026-09-18T05:02:56.000Z
- Updated: 2026-09-18T05:02:56.000Z
- Description: Ultragenyx's AAV9 gene therapy is the first treatment for Sanfilippo syndrome Type A. Full approval on 17 treated children against a natural history control now collides with a diagnostic window most families still miss.
- Author: BioWeek
- Tags: Pipeline, Science, Policy

On September 17, 2026, the FDA [approved](https://content.govdelivery.com/accounts/USFDA/bulletins/42b4481?ref=bioweek.com) Fayuvi (rebisufligene etisparvovec-hopf), an AAV9 gene therapy from Ultragenyx Pharmaceutical (NASDAQ: RARE), as the first treatment for pediatric Sanfilippo syndrome Type A (mucopolysaccharidosis IIIA). The agency issued a standard full approval, not an accelerated one. Ultragenyx also received a Rare Pediatric Disease Priority Review Voucher.

The company [announced](https://www.biospace.com/press-releases/ultragenyx-announces-approval-of-fayuvi-gene-therapy-the-first-ever-fda-approved-treatment-for-sanfilippo-syndrome-type-a-mps-iiia?ref=bioweek.com) the decision the same day. The commercial clock is short: distribution to Qualified Treatment Centers is slated to start in 30 to 60 days. The scientific clock is shorter. Fayuvi can replace missing sulfamidase. It cannot bring back neurons that have already died.

### Seventeen patients, twenty-seven controls

Full approval rests on an open-label, single-arm cohort of 17 children in the modified intention-to-treat population of Transpher A ([NCT02716246](https://clinicaltrials.gov/study/NCT02716246?ref=bioweek.com)), compared with an external natural history cohort of 27 untreated children. The efficacy endpoint was Bayley-III (BSITD-III) cognitive raw scores from 24 to 60 months of age. Treated children showed a 23.5-point higher cognitive score over natural history during the study period (p < 0.0001).

That is a large effect on a small, nonrandomized sample. There is no contemporaneous placebo arm. The control is historical. For a uniformly fatal neurodegenerative disease with no prior therapy, regulators accepted the comparison. Investors and clinicians still have to live with the residual uncertainty: how much of the 23.5-point gap is gene therapy, and how much is who got treated, and when.

Ultragenyx also [reported](https://ir.ultragenyx.com/news-releases/news-release-details/ultragenyx-announces-data-demonstrating-treatment-ux111-results?ref=bioweek.com) a sustained drop in cerebrospinal fluid heparan sulfate, the glycosaminoglycan that piles up when SGSH is missing. Time-normalized AUC showed a 63% to 65% reduction in CSF-HS exposure (p < 0.0001), with durability out to nearly eight years of follow-up. The cognitive endpoint is what the label has to carry. The biomarker is what argues the biology is still on.

### A systemic AAV9, not a brain catheter

Fayuvi is a single intravenous infusion of a self-complementary AAV9 vector carrying a functional human SGSH gene, dosed at 3 x 10^13 vector genomes per kilogram. The vector is designed to cross the blood-brain barrier from the bloodstream, avoiding surgical intracerebroventricular or intrathecal catheters. For a preschool population already facing anesthesia risk and behavioral crises, that delivery choice is part of the product.

The same systemic exposure drives the toxicity. Elevated liver transaminases occurred in 85% of patients, vomiting in 67%, abnormal behavior in 56%, diarrhea in 48%, pyrexia in 41%, leukopenia in 30%, Cushingoid features in 30%, and thrombocytopenia in 19%. Prophylactic corticosteroids start one day before infusion and continue for at least eight weeks. The label also warns of thrombotic microangiopathy and the theoretical risk of AAV genomic integration and oncogenesis. The 23.5-point cognitive gain is not a free lunch. It is purchased with months of steroids and a high-dose AAV load in a child's liver.

### The window the screen does not catch

The FDA label restricts Fayuvi to pediatric patients with preserved neurodevelopmental function. That clause is the real commercial and clinical bottleneck. Sulfamidase can stop further substrate accumulation. It cannot resurrect dead neurons. Most children with Sanfilippo Type A are diagnosed between ages 2 and 4, when speech delays and behavior changes appear, right at the edge of the window where neurodegeneration becomes irreversible.

Type A is not on the federal Recommended Uniform Screening Panel. Without universal newborn screening, the typical path to a QTC still runs through delayed speech, a metabolic workup, and a molecular confirmation that often arrives after the label's "preserved function" language has already started to close. Full approval without a screening mandate creates a catch-22: the therapy is licensed for children who still have cognition to save, and the health system finds most of them only after that cognition has begun to slip.

### From academic vector to commercial product

The construct was engineered at Nationwide Children's Hospital and Ohio State University by Haiyan Fu and Doug McCarty, licensed to Abeona Therapeutics, then stalled under clinical-stage capital pressure before Ultragenyx acquired the program and pushed it through BLA submission. Manufacturing sits at Ultragenyx's Bedford, Massachusetts site and Andelyn Biosciences in Columbus, Ohio. The science is two decades old. The commercial product is days old.

Fayuvi is now a fully approved gene therapy with a 17-patient efficacy core, a 27-patient external control, a 63% to 65% CSF-HS reduction, and a safety profile that demands steroids and TMA vigilance. The next test is not another p-value. It is whether children still inside the therapeutic window can be found before the neurons the vector cannot replace are already gone.