RARE Stock Hits Fresh 52-Week Low As Sanfilippo Drug Ruling Nears — Failed Angelman Syndrome Trial Weighs On Shares

• The FDA is expected to rule on UX111 by September 19.
• The agency rejected the original application in July 2025, citing manufacturing and quality-control problems rather than the clinical data.
• The September 2 readout from the late-stage Aspire trial of apazunersen (GTX-102) in Angelman syndrome continues to weigh on the stock.

Shares of Ultragenyx Pharmaceutical (RARE) slipped on Wednesday and hit a fresh 52-week low, as investors braced for a U.S. Food and Drug Administration (FDA) decision due this week on UX111, a one-time gene therapy for Sanfilippo syndrome type A.

The stock fell as much as 1.45% intraday. That modest drop comes after a much larger collapse earlier this month. On September 3, RARE plunged about 44% after a late-stage trial failure, wiping more than $1 billion off its market value. The shares have now lost roughly half their value in a month and more than 40% year to date.

Second Shot At Approval

UX111 (rebisufligene etisparvovec) is designed as the first treatment for Sanfilippo type A, a rare, fatal childhood disease that causes progressive brain damage. The FDA’s target decision date is September 19.

This is not the company’s first try. The agency rejected the original application in July 2025, citing manufacturing and quality-control problems rather than the clinical data. Ultragenyx resubmitted the file early this year. The FDA accepted it in April and set a new deadline. The company says longer-term study results, covering up to eight years in some children, show lasting drops in a key disease marker and better scores in younger patients. If approved, UX111 would be made in the United States.

Angelman Setback Still Hangs Over The Stock

The September 2 readout from the late-stage Aspire trial of apazunersen (GTX-102) in Angelman syndrome was a surprise miss. Angelman syndrome is a rare genetic disorder that affects the nervous system, causing severe developmental delays, balance problems, and little to no speech.

In the trial, the drug did not beat a sham treatment on thinking skills or a broader measure of daily function, despite encouraging earlier studies. Ultragenyx said it will review the program and cut costs in a “significant” way. Analysts, including Morgan Stanley and Barclays, subsequently slashed price targets.

What Else Is On The Books

The picture is not only setbacks. On August 19, the FDA approved Genglycos, Ultragenyx’s gene therapy for glycogen storage disease type Ia, and issued a rare-disease priority-review voucher.

The company’s second-quarter total revenue was $214 million, up 29% from a year earlier. The company still aims for profitability in 2027, now with a thinner pipeline and a leaner budget.

How Did RARE Retail Traders React?

On Stocktwits, retail sentiment around RARE stock stayed ‘bullish’ over the past 24 hours, while message volume was at ‘normal’ levels.

A Stocktwits user said the stock has hit its lows and will rise after the FDA decision for UX111. The user sees “high probability” of approval.

Another user noted that the previous rejection was due to manufacturing issues, not a clinical one.

RARE stock has fallen 44% year-to-date. 

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