Ultragenyx's pivotal Angelman syndrome trial nears its only readout window
The fully enrolled Phase 3 Aspire study of GTX-102 is the sole registrational trial testing the UBE3A pathway, with data due in H2 2026.
Executive Summary
- Ultragenyx is heading into the readout for its pivotal Angelman syndrome trial, the only Phase 3 program anywhere testing a therapy aimed at the UBE3A gene defect that causes the disease.
- The trial finished enrollment on schedule, has kept its original completion timeline intact across repeated company updates, and carries a Breakthrough Therapy Designation from the FDA granted ahead of data.
- A single direct competitor exists in earlier-stage testing, and no trial in this indication or target has yet reported an efficacy result, so this readout will be the field's first efficacy signal for the mechanism.
- With no validated disease-modifying therapy for Angelman syndrome, a cognitive gain over sham that clinicians and regulators can call clinically meaningful is the threshold this readout needs to clear to change the trajectory for the mechanism.
The trial
The Aspire study (NCT06617429) is a randomized, sham-controlled Phase 3 trial testing GTX-102, an antisense oligonucleotide given intrathecally (into the spinal fluid), in 129 pediatric patients with Angelman syndrome caused by full maternal deletion of the UBE3A gene. The primary endpoint measures change from baseline in the Bayley-4 Cognitive Raw Score, assessed without caregiver input, at Day 338. Nine secondary endpoints span motor, communication, sleep and behavioral measures, alongside a multidomain responder index and standard safety reporting. Enrollment closed in July 2025 across 28 sites in the United States, Spain, Germany, Japan, Canada and Poland. NCT06617429Phase 3 Efficacy and Safety Study of GTX-102 in Pediatric Subjects With Angelman Syndrome (AS)NCT06617429
Probability of SuccessBased on the AppliedXL Probability of Success model. For more information about the methodology, read the research here.
The timeline
Ultragenyx has repeated the same second-half 2026 data guidance across four disclosures since August 2025, most recently in January 2026. The trial's registered primary completion date is July 1, 2026, and its status moved from Recruiting to Active, not recruiting in August 2025 once enrollment closed, a routine transition at planned enrollment end rather than a sign of trouble. The company's August 2025 earnings release confirmed the Phase 3 study was fully enrolled and disclosed that GTX-102 had received FDA Breakthrough Therapy Designation for Angelman syndrome the prior month. Enrollment rose from an anticipated 120 to an actual 129, a change the trial's own operational baseline classifies as within the routine band for this design. Ultragenyx+1Ultragenyx Reports Second Quarter 2025 Financial Results and Corporate UpdateAug 5, 2025Phase 3 Efficacy and Safety Study of GTX-102 in Pediatric Subjects With Angelman Syndrome (AS)NCT06617429
The competitive field
Angelman syndrome carries no approved disease-modifying therapy, and the field testing genetic causes of the disease remains sparse. Ionis Pharmaceuticals' ION582 (also known as obudanersen sodium), an antisense oligonucleotide targeting the paternal UBE3A-ATS transcript, is the closest comparator, running in an earlier-stage trial and a separate Phase 3 study (REVEAL, NCT06914609) with a 2027 completion date. No other program has reached Phase 3 testing a UBE3A-directed mechanism in this indication, making Aspire's result the first the field will have to benchmark against. GTX-102 itself has one completed Phase 1/2 study (NCT04259281), which reported no treatment-related serious adverse events and early, non-numeric signs of clinical activity in its first patients, alongside two other active company trials extending into later-stage and pediatric expansion testing.
The bar
Because no mechanism has yet established efficacy in this disease and Aspire is the only trial positioned to deliver a randomized, sham-controlled readout this year, the result that would change how the field reads UBE3A-directed therapy is a cognitive gain over sham that clears a threshold clinicians would call clinically meaningful, sustained enough to support a regulatory filing given the Breakthrough Therapy Designation already in hand.
This analysis was produced using AI-assisted reporting systems, AppliedXL data, and official public records. These systems undergo editorial review, quality checks, and regular audits by human experts. Errors may still occur, as with any automated system. Always consult the linked primary sources. Read our AI Editorial Policy.