Ultragenyx Phase 3 Angelman Study Fails, Shares Fall 44%

What happened

Ultragenyx Pharmaceutical Inc. (Nasdaq: RARE) said its Phase 3 Aspire study of apazunersen (GTX-102) in Angelman syndrome did not meet its primary or key secondary endpoints. The company announced the results on September 2, 2026, according to a press release on its investor relations site and reports from Reuters, Endpoints News, Fierce Biotech, and other outlets.

The trial's primary endpoint was the change from baseline in the Bayley-4 cognitive raw score. The key secondary endpoint was a multidomain responder measure. Both were missed, per the company's release. Shares fell about 44% following the news, with the stock closing at $15.30 on the event date, according to price data.

Angelman syndrome is a rare genetic neurodevelopmental disorder. Apazunersen is an investigational antisense oligonucleotide designed to target the underlying cause of the disease.

Company response and context

Fierce Biotech reported that Ultragenyx is planning "significant expense reductions" following the failure. The company has not yet detailed what those reductions will involve.

Reuters noted the setback affects a key late-stage asset for the mid-cap rare-disease company. The company has not commented on the future of apazunersen in Angelman syndrome beyond the trial results.

Ultragenyx has other programs, including UX111, for which a U.S. Food and Drug Administration decision is expected around September 19, according to posts on X. That claim has not been independently verified by the company's press releases or major news outlets in the provided sources.

What this means

A Phase 3 trial is a large, late-stage study that tests whether a drug is effective and safe enough for regulatory approval. When a Phase 3 trial fails its primary endpoint — the main measure of success — it often means the drug may not work as intended, and the company may not be able to file for approval for that indication.

The Bayley-4 cognitive raw score is a standard measure of cognitive development in young children. In this case, the trial measured whether apazunersen improved cognitive function in children with Angelman syndrome compared to placebo.

The stock drop reflects investors' reassessment of the company's value without a successful Angelman program. With the failure, Ultragenyx loses a potentially major revenue opportunity, and the company says it will cut expenses to preserve cash.

The upcoming FDA decision on UX111, a gene therapy for another rare disease, could be a near-term catalyst, but no verified details are available. Investors should watch for official company announcements.