Ultragenyx Pharmaceutical (NASDAQ: RARE) saw its shares climb 12.6% on Thursday after the U.S. Food and Drug Administration (FDA) granted full approval to FAYUVI (rebisufligene etisparvovec-hopf), making it the first approved treatment for children with Sanfilippo syndrome type A. The approval removes a major regulatory overhang, though the company will not begin commercial shipments immediately. Ultragenyx expects to start shipping the one-time gene therapy to qualified treatment centers within 30 to 60 days.
Shares closed the regular session at $14.50, up $1.62 from Wednesday's close of $12.88. The stock traded as high as $14.57 and finished near that level, indicating sustained buying interest rather than a fleeting spike. Trading volume reached 12.13 million shares, more than 3.6 times the average volume of 3.35 million shares over the prior 62 sessions.
The approval is a full approval, not a conditional or accelerated one. Ultragenyx had originally sought accelerated approval, but the FDA granted standard approval based on a pivotal analysis comparing 17 treated children with 27 untreated patients from an external natural-history cohort. Treated patients showed a 23.5-point higher Bayley-III cognitive raw score over the study period, with a p-value below 0.0001. Follow-up data extends to nearly eight years, according to the company.
FAYUVI, previously known as UX111, is an AAV9 gene therapy designed for pediatric patients with preserved neurodevelopmental function. Sanfilippo syndrome type A is a rare inherited disorder where heparan sulfate accumulates and progressively damages the brain. Until now, treatment was limited to managing symptoms. The FDA noted that FAYUVI is the first therapy intended to alter the disease's course.
While the approval is a significant milestone, the commercial launch will be complex. The therapy is administered as a single intravenous infusion, and treatment centers must be trained to manage corticosteroids and monitor liver function. Ultragenyx is limiting distribution to U.S. institutions that are qualified to administer gene therapy. The company must also secure reimbursement and identify eligible children early, which will be critical to converting regulatory success into sales.
The addressable patient population is small. In its June-quarter filing, Ultragenyx estimated that 3,000 to 5,000 people live with MPS IIIA in the developed world. However, the approved population is limited to pediatric patients with preserved neurodevelopmental function, and the actual number of treatment candidates will be further narrowed by diagnosis and access. The company has not yet disclosed a U.S. list price or provided sales guidance for FAYUVI.
Ultragenyx also received a Priority Review Voucher (PRV) from the FDA, a transferable right that can expedite review of a future drug application. The company's Sept. 17 Form 8-K records the voucher but does not assign it a value or indicate whether it will be sold. A sale could provide non-dilutive cash, while internal use could accelerate another program. Either way, the voucher is a one-time benefit and does not reflect ongoing demand for FAYUVI.
The company's balance sheet remains a concern. As of June 30, Ultragenyx had $436 million in cash, cash equivalents, and marketable securities, but it burned $97 million in operating cash during the second quarter. The company reported $214 million in quarterly revenue and a net loss of $92 million. Its 2026 guidance calls for $730 million to $760 million in total revenue, excluding potential new product launches. FAYUVI could provide upside to that guidance, but the company has not yet provided enough detail on pricing or patient starts to quantify it.
At Thursday's close, Ultragenyx's market capitalization was approximately $1.43 billion, based on 98.6 million shares outstanding as of July 31. The stock remains about 64% below its 52-week high of $39.89 and only 14% above its low of $12.73. The approval addresses one part of the investment thesis, but launch execution, cash burn, and pipeline setbacks remain significant risks.
Investors will be watching for concrete next steps: the announced price, the number and location of qualified treatment centers, the first shipment, payer coverage, and initial patient starts. Management's decision on the PRV and updates on its path to profitability in 2027 will also be key. If launch indicators lag, the small eligible population and specialized delivery system could limit sales. If access forms quickly, Thursday's 12.6% gain may be just the beginning of a re-rating driven by an approval that came with more regulatory certainty than the market expected.
