RARE Stock Jumps 13% — Extends Gains After-Hours As FDA Clears First Sanfilippo Gene Therapy

• The FDA turned the therapy down in 2025 over manufacturing issues, forcing the company to resubmit this year.
• Thursday’s approval is the company’s second gene therapy nod and sixth FDA clearance overall.
• The company set the U.S. per-patient wholesale acquisition cost for Fayuvi at $3.95 million.

Shares of Ultragenyx Pharmaceutical Inc. (RARE) kept rising after the closing bell on Thursday after U.S. regulators approved Fayuvi, the first treatment for Sanfilippo syndrome type A.

The bounce followed a bruising stretch. A day earlier, the stock had touched a fresh 52-week low of $12.73, and it remains far below levels from early September, when a failed late-stage Angelman syndrome trial sent the shares down about 44% in a single session.

The stock gained 13% in the regular trading session on Thursday and added 3% after-hours.

UX111 Approval

The Food and Drug Administration (FDA) granted full approval of Fayuvi, also known as UX111, for pediatric patients. The rare inherited condition, sometimes called childhood Alzheimer’s, is caused by a missing enzyme that lets a sugar molecule called heparan sulfate build up in the brain. Children lose speech, thinking, and motor skills.

Fayuvi is given once, by vein. A harmless modified virus carries a working copy of the missing gene into cells so the body can make the enzyme and clear the toxic buildup. The agency said treated children maintained or improved cognitive function between ages 2 and 5, while untreated historical controls typically plateaued and then declined. Ultragenyx reported a 23.5-point advantage on a standard cognitive score versus natural history. Common side effects include elevated liver enzymes, vomiting, and fever.

Second Chance After A Manufacturing Rejection

The FDA turned the therapy down in 2025 over manufacturing issues, not the clinical data. Ultragenyx resubmitted the file this year; a decision had been due by September 19.

Thursday’s approval is the company’s second gene therapy nod and sixth FDA clearance overall. The product is expected to reach qualified U.S. treatment centers in 30 to 60 days.

The company set the U.S. per-patient wholesale acquisition cost for Fayuvi at $3.95 million.

How Did RARE Retail Traders React?

On Stocktwits, retail sentiment around RARE stock jumped from ‘bullish’ to ‘extremely bullish’ territory over the past 24 hours, while message volume rose from ‘normal’ to ‘extremely high’ levels.

A Stocktwits user expressed optimism about the priority review voucher the company received upon approval, noting it is valued at around $200 million on its own.

Another user expressed optimism that the stock could rally to $25, given the drug’s pricing.

RARE stock has fallen 37% year-to-date. 

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