News|Articles|September 3, 2026

Ultragenyx's Apazunersen Fails Phase 3 Angelman Syndrome Trial

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Key Takeaways

Premarket equity erosion followed disclosure that apazunersen showed no efficacy versus placebo on Bayley-4 cognition or MDRI net response in a baseline-balanced pediatric ASPIRE population.
Mechanistic rationale centers on inhibiting the UBE3A antisense transcript to restore neuronal expression from the normally silenced paternal allele in a disorder with profound developmental disability and seizures.
Regulatory momentum had included BTD, orphan, rare pediatric disease, and fast track designations after FDA lifted an earlier clinical hold and phase 1/2 plus extension data suggested acceptable tolerability.
Corporate guidance emphasizes sustaining commercial execution after August 2026 approval of pariglasgene brecaparvovec-opnr for GSD Ia and pursuing UX111 in Sanfilippo syndrome, targeting 2027 profitability.
Interpretation remains constrained by topline reporting without detailed statistics, subgroup analyses, or a declared regulatory strategy, leaving uncertainty regarding additional trials versus discontinuation.

Apazunersen missed its primary endpoint in a phase 3 Angelman syndrome trial, meaning that patient population remains without an approved treatment option.

Shares of Ultragenyx Pharmaceutical fell more than 40% in premarket trading on September 3, 2026, after the company disclosed that apazunersen (GTX-102), an investigational antisense oligonucleotide therapy for Angelman syndrome, failed to meet its primary and secondary endpoints in the phase 3 ASPIRE trial, according to a report from Reuters.¹ Ultragenyx reported the topline results on September 2, saying the therapy did not achieve the primary endpoint of change in Bayley-4 cognitive raw score, nor the key secondary endpoint of net response on the Multidomain Responder Index (MDRI), though its safety profile remained consistent with earlier phase 1/2 data.²

Key facts

Drug: Apazunersen (GTX-102; Ultragenyx Pharmaceutical)

Class: Antisense oligonucleotide (intrathecal administration)

Indication studied: Angelman syndrome, pediatric patients

Trial: Phase 3 ASPIRE

Result: Failed primary (Bayley-4 cognitive score) and key secondary (MDRI) endpoints

Safety: Consistent with phase 1/2 profile; no new signals reported

Market reaction: Ultragenyx shares fell more than 40% in premarket trading

Regulatory history: FDA breakthrough therapy, orphan drug, fast track designations

Next steps: Ultragenyx evaluating program's future; deciding on expense reductions

"Based on everything we observed in the robust [p]hase 1/2 clinical development program and long-term extension study, we are disappointed by the Aspire result," said Emil Kakkis, MD, PhD, chief executive officer and president of Ultragenyx, in a company press release.² "Even more, we are disappointed for the global patient community who has invested so much in early-stage research, working to bring a first-ever treatment to their children."

What did the ASPIRE trial show?

ASPIRE compared apazunersen with placebo in children with Angelman syndrome. The randomized groups were comparable at baseline and consistent with the population studied in the earlier phase 2 trial. The company said there were no differences between the treated and control groups that could support efficacy, either in Bayley cognitive raw scores or in the MDRI's net response or the mean changes across its 5 individual endpoints.² Ultragenyx said it will evaluate the apazunersen program in light of the outcome and make a decision on its future. The company will also assess planned operations to implement expense reductions while supporting its commercial business.²

Why is a treatment for Angelman syndrome significant?

Angelman syndrome is a rare neurogenetic disorder caused by loss of function of the maternally inherited UBE3A gene, causing cognitive impairment, motor impairment, and debilitating seizures; most affected individuals cannot speak, and none can live independently.² There are currently no approved therapies for Angelman syndrome, which is estimated to affect approximately 60,000 people in commercially accessible geographies, according to the company.² Apazunersen is designed to target the UBE3A antisense transcript to reactivate expression of the paternal UBE3A allele, which is normally silenced in neurons.²

How did the apazunersen program reach this point?

In September 2021, FDA lifted a clinical hold on the phase 1/2 study and cleared GeneTx Biotherapeutics, Ultragenyx's development partner at the time, to begin dosing treatment-naive pediatric patients.³ The drug went on to receive breakthrough therapy designation, orphan drug designation, rare pediatric disease designation, and fast track designation from FDA, along with orphan and priority medicines designations from the European Medicines Agency, based on earlier phase 1/2 and long-term extension data.²

What's next for Ultragenyx?

Ultragenyx said it will continue to focus on its commercial business, including revenue from the August 2026 approval of pariglasgene brecaparvovec-opnr (Genglycos) for glycogen storage disease type Ia and the potential approval of UX111 for Sanfilippo syndrome. The company also reiterated its goal of reaching profitability in 2027.²^,⁴ It has not yet announced a decision on whether apazunersen will continue in development.²

What are the limitations?

These are topline results, and Ultragenyx has not yet released detailed statistical data from the trial or specified next steps for the apazunersen program. The company has not disclosed whether it plans to conduct additional analyses, pursue regulatory discussions, or discontinue development.

References

Ultragenyx shares crater after Angelman syndrome drug fails late-stage trial. Reuters. September 3, 2026. Accessed September 3, 2026. https://www.reuters.com/business/healthcare-pharmaceuticals/ultragenyx-shares-crater-after-angelman-syndrome-drug-fails-late-stage-trial-2026-09-03/
Ultragenyx Pharmaceutical. Ultragenyx announces Phase 3 Aspire results in Angelman syndrome. Press release. Published September 2, 2026. Accessed September 3, 2026. https://ir.ultragenyx.com/news-releases/news-release-details/ultragenyx-announces-phase-3-aspire-results-angelman-syndrome
Ultragenyx Pharmaceutical. GeneTx and Ultragenyx announce FDA has removed clinical hold on phase 1/2 clinical study of GTX-102 for the treatment of Angelman syndrome in the US. Published September 27, 2021. Accessed September 3, 2026. https://ir.ultragenyx.com/news-releases/news-release-details/genetx-and-ultragenyx-announce-fda-has-removed-clinical-hold
FDA. FDA approves first therapy for patients aged 8 years and older with glycogen storage disease type Ia. Press Release. Published August 19, 2026. Accessed September 3, 2026. https://www.fda.gov/news-events/press-announcements/fda-approves-first-therapy-patients-aged-8-years-and-older-glycogen-storage-disease-type-ia

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