Can Fayuvi FDA approval lift RARE stock further?

Ultragenyx Pharmaceutical shares jumped 12.6% to $14.50 on September 17 after the FDA approved Fayuvi, a gene therapy for children with Sanfilippo syndrome Type A. (Source: Yahoo Finance, Barron's, Reuters)

The FDA granted full approval to Fayuvi (UX111) for pediatric patients with Sanfilippo syndrome Type A, a rare inherited disorder that causes progressive neurological decline. The approval makes Fayuvi the first treatment targeting the underlying cause of the disease.

Fayuvi is administered as a single intravenous infusion. It uses a modified AAV9 virus to deliver a working copy of the missing SGSH gene, enabling cells to produce the enzyme needed to break down toxic heparan sulfate buildup in the brain.

Ultragenyx reported that treated children maintained or improved cognitive function between ages 2 and 5. The company reported a 23.5-point advantage on a standard cognitive score compared with an external natural-history group.

The FDA previously declined approval in 2025 because of manufacturing-related issues, rather than concerns over the clinical data. Ultragenyx resubmitted the application this year, eventually securing approval on September 17.

RARE closed at $14.50, up 12.58%, on September 17, with more than 12.4 million shares changing hands. The stock remained about 37% lower year-to-date after a steep decline earlier in September following a failed late-stage Angelman syndrome trial.