Why Did RARE Stock Slump 46% After-Hours?

• Chief Executive Emil Kakkis said the company was disappointed, “even more” for families who had invested years in the research.
• Ultragenyx will now review the program and decide whether to keep it. It also said it will cut spending while supporting products already on the market.
• The miss comes after another late-stage failure in late 2025, when a brittle-bone drug, setrusumab (UX143), failed to meet its goal of reducing fractures.

Shares of Ultragenyx Pharmaceutical (RARE) plunged 46% after the close on Wednesday after the company said its experimental Angelman syndrome drug failed in a late-stage study.

The stock finished regular trading at $26.53, then slid into the mid-teens in after-hours trading.

What The Trial Showed

Ultragenyx said the late-stage Aspire study of apazunersen, also known as GTX-102, did not meet its primary goal or its key backup goal. Treated children did not improve beyond the control group on a standard cognitive test. They also showed no advantage on a broader score meant to capture several areas of daily functioning, including the five individual measures within that score. Safety looked similar to earlier studies.

Aspire enrolled 129 children ages 4 to 17 with the most common genetic form of the disease. They were assigned at random to the drug or a sham procedure for 48 weeks. The medicine is given by injection into the spinal fluid. Early studies had raised hopes. The larger, controlled trial did not confirm them.

Why Angelman Mattered

Angelman syndrome is a rare genetic brain disorder. Children typically have severe developmental delay, limited or no speech, movement problems, and seizures. There is no approved treatment that slows the disease itself. Ultragenyx estimates about 60,000 people live with it in markets the company could reach.

Chief Executive Emil Kakkis said the company was disappointed, “even more” for families who had invested years in the research. Ultragenyx will now review the program and decide whether to keep it. It also said it will cut spending while supporting products already on the market

What Is Left In The Pipeline

The miss comes after another late-stage failure in late 2025, when a brittle-bone drug, setrusumab (UX143), failed to meet its goal of reducing fractures, and the stock also collapsed.

Angelman became Ultragenyx's next major bet after the older trial failed.

The company is pointing instead to its commercial rare-disease franchise and two gene-therapy files. The FDA recently approved GENGLYCOS for glycogen storage disease type Ia. A decision on UX111, a gene therapy for Sanfilippo syndrome type A, is due later this month after the agency refused to approve it last year. Other programs, including a gene therapy for OTC deficiency, are still in testing, with results not due until 2027.

How Did RARE Retail Traders React?

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

A few Stocktwits users are now anticipating the FDA’s decision on RARE’s gene therapy for Sanfilippo syndrome type A

Another user said the company should release more data from the failed trial.

A third user opined that Ultragenyx should just buy Zevra Therapeutics to gain access to its drugs and boost revenue after the setback in its own pipeline.

RARE stock has fallen 15% year-to-date. 

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