Ultragenyx Phase 3 Aspire trial fails in Angelman syndrome
Ultragenyx Pharmaceutical has reported that its Phase 3 Aspire study of apazunersen (GTX-102) in Angelman syndrome failed to meet its primary endpoint, dealing a significant setback to one of the rare-disease field's most watched development programmes. The Novato, California-based company said the antisense oligonucleotide (ASO) therapy did not show a statistically meaningful change from baseline in Bayley-4 cognitive raw score, and also missed the key secondary endpoint of net response on the Multidomain Responder Index (MDRI).
The company noted that randomised groups were comparable at baseline and consistent with patients enrolled in the earlier Phase 1/2 programme. No differences between treated and control groups could support an efficacy signal across Bayley cognition scores or across any of the five individual endpoints comprising the MDRI. The safety profile observed in Aspire was described as consistent with Phase 1/2, meaning no new safety signals emerged, though that finding offers limited commercial comfort given the efficacy result.
Programme in limbo
Chief executive Emil Kakkis said the company was "disappointed by the Aspire result" and extended that sentiment to the patient community, which he said had invested heavily in early-stage research in the hope of a first-ever treatment. Ultragenyx said it will now evaluate the apazunersen programme and decide on its "disposition," a phrase that typically signals a company is weighing whether to continue, out-license, or shelve an asset. The company also said it will implement "significant expense reductions" in response to the outcome, though no specific headcount or operational targets were named.
Kakkis sought to reassure investors by pointing to commercial momentum elsewhere in the portfolio. He highlighted the recent approval of GENGLYCOS for glycogen storage disease type Ia, the potential approval of UX111 for Sanfilippo syndrome, and geographic expansion of existing products as near-term revenue drivers. The company reiterated its aim to reach profitability in 2027.
A crowded and frustrated field
The Aspire failure underscores the difficulty of translating promising early signals in neurodevelopmental rare diseases into Phase 3 success. Angelman syndrome, which affects an estimated 60,000 people across commercially accessible markets, is caused by loss of function of the maternally inherited UBE3A allele and has no approved disease-modifying therapy. The condition causes cognitive and motor impairment, seizures, and near-total loss of speech; affected individuals require lifelong care.
Apazunersen was designed to inhibit the UBE3A antisense transcript, which silences the paternal copy of UBE3A in neurons, thereby reactivating the deficient protein. The mechanism drew considerable regulatory interest: the therapy held Breakthrough Therapy Designation, Orphan Drug Designation, Rare Pediatric Disease Designation, and Fast Track Designation from the FDA, as well as Orphan and PRIME designations from the EMA. Those designations reflect the agency's view of early data quality and unmet need rather than a predetermined approval outcome.
Several other groups are pursuing gene therapy and RNA-based approaches in Angelman syndrome, and the Aspire failure will prompt renewed scrutiny of endpoint selection and patient stratification across the field. The MDRI, a composite responder tool designed to capture multi-domain functional change, had been positioned as a regulatory-aligned endpoint; its failure alongside the Bayley cognitive scale suggests the signal observed in Phase 1/2 did not hold at scale. Regulators and rivals alike will study the full dataset, expected to be presented at a future medical conference, for clues about subgroup behaviour and whether any patient population might still benefit.