Material Event (Form 8-K)

Item 8.01 Other Events.

On February 3, 2026, Ultragenyx Pharmaceutical Inc. (the "Company") issued a press release announcing new long-term data from clinical studies evaluating UX111 (rebisufligene etisparvovec), an investigational AAV9 gene therapy for Sanfilippo syndrome type A (MPS IIIA), a fatal neurodegenerative lysosomal storage disorder. The results demonstrate substantial and durable biomarker improvements and meaningful functional benefits compared with natural history, with consistent and highly statistically significant results across age and disease severity. UX111 was well-tolerated and the safety profile remains favorable.

The data will be delivered in an oral presentation, Treatment with UX111 Reduced Cerebrospinal Fluid ("CSF") Heparan Sulfate ("HS") Exposure and Stabilized or Improved Functioning across Dose, Age, and Stage of MPS IIIA, at the WORLDSymposium™ 2026 on Friday, February 6 at 8 a.m. PST.

Clinical Improvements in Functional Abilities Compared to Natural History

Cognitive function, expressive and receptive communication, and fine and gross motor skills were measured using Bayley-III and compared to natural history data from untreated patients with reported rapid progressor phenotypes. Children under two years of age or with earlier stage disease at the time of treatment (n=17) demonstrated a +23.2 point (p<0.0001) treatment effect in the mean Bayley-III cognitive raw score compared to natural history data during 24-60 months of age.

In addition to cognitive function, clinical improvements were also observed across the other four subtests compared to natural history:

On separate caregiver-reported outcome utilizing Vineland 3, there were comparable improvements in the communication, motor, and personal subdomains.

Eight children reached a 36-month cognitive developmental age, enabling higher-level testing-none of the natural-history patients reached this milestone.

Functional Skill Retention in Later-Stage Children

Patients with older age or having more advanced disease at the time of treatment (n=10), showed retention of functional abilities in at least one of three areas at the time of last assessment that exceed typical decline patterns in untreated children with Sanfilippo syndrome type A. Specifically:

These findings are clinically relevant as these functions progressively worsen and are eventually lost in late childhood and early adolescence.

Significant and Durable Reduction in CSF Heparan Sulfate

Levels of CSF-HS decreased within the first month following treatment with UX111 (3x1013vg/kg) in the overall efficacy set (N=27), regardless of age or stage of disease progression at the time of treatment. As of the September 2025 cutoff date, the median

reduction in CSF-HS exposure was 63.98% (p<0.001). The majority of children treated (88.2% of younger patients and 81.5% of the overall efficacy set) achieved a 50% or greater reduction.

Safety profile remains favorable

UX111 was generally well tolerated across all doses (N=33), including the highest dose of 3x1013vg/kg, with a median follow-up 4.8 years (range 0.6-8.5). The most frequently reported treatment-emergent adverse events were elevations in liver enzymes. Treatment-related adverse events were mostly mild or moderate and resolved spontaneously. No participants experienced infusion-related hypersensitivity or anaphylaxis, and no incidences of thrombotic microangiopathy, myocarditis, dorsal root ganglion toxicity, or malignancy were associated with treatment.

BLA resubmitted to FDA with PDUFA date expected in third quarter 2026

These longer-term data were included in the resubmitted Biologics License Application (BLA) to the U.S. Food and Drug Administration (FDA) seeking accelerated approval for UX111. The Company anticipates up to a six-month review period from the date of resubmission per FDA regulations, with a PDUFA date expected in the third quarter of 2026.