FDA Decision Expected by August 18, 2025

NOVATO, Calif., Feb. 18, 2025 – Ultragenyx Pharmaceutical Inc. (NASDAQ: RARE) has announced that the U.S. Food and Drug Administration (FDA) has accepted for review the Biologics License Application (BLA) seeking accelerated approval for UX111 (ABO-102), an AAV gene therapy developed to treat Sanfilippo syndrome type A (MPS IIIA). The FDA has granted the application Priority Review and set a Prescription Drug User Fee Act (PDUFA) action date of August 18, 2025. Additionally, the agency has indicated that it does not currently plan to hold an advisory committee meeting for the application (GlobeNewswire, 2025).

Significance of FDA Acceptance

Dr. Emil D. Kakkis, CEO and President of Ultragenyx, emphasized the importance of this milestone, stating: “Acceptance of the UX111 BLA brings us closer to being able to provide a first-ever treatment for Sanfilippo syndrome type A. If our application is successful, we are prepared to launch this therapy for patients and their families. By reaching alignment with the Agency on a path forward for accelerated approval in neuronopathic MPS diseases, our UX111 program could serve as a step towards advancing drug development across multiple metabolic diseases of the brain” (GlobeNewswire, 2025).

Clinical Evidence Supporting UX111

The BLA submission is supported by clinical data, including findings from the pivotal Transpher A study and long-term follow-up studies presented at WORLDSymposium™ 2025. These studies demonstrated that UX111 treatment led to a rapid and sustained decrease in cerebrospinal fluid (CSF) heparan sulfate (HS) levels in patients with Sanfilippo syndrome type A, regardless of age or disease progression at the time of treatment. The data also revealed statistically significant improvements in Bayley-III raw scores for cognition, receptive communication, and expressive communication in the modified intent-to-treat (mITT) group compared to natural history data from untreated patients. These clinical improvements were correlated with substantial and sustained reductions in CSF-HS levels (GlobeNewswire, 2025).

Regarding safety, the most frequently observed treatment-related adverse events included elevated liver enzymes, which were generally mild (Grade 1) or moderate (Grade 2) in severity, with all cases resolving over time (GlobeNewswire, 2025).

About UX111 Gene Therapy

UX111 is an in vivo gene therapy currently in Phase 1/2/3 development for Sanfilippo syndrome type A, a rare lysosomal storage disorder with no approved treatment. The therapy is designed to address the SGSH enzyme deficiency responsible for the abnormal accumulation of heparan sulfate in the brain, which leads to progressive neurodegeneration. UX111 is administered as a one-time intravenous infusion using a self-complementary AAV9 vector to deliver a functional copy of the SGSH gene to cells. Once transduced, these cells produce the enzyme and secrete it for uptake by other brain cells, addressing the enzyme deficiency. Originally developed by Abeona Therapeutics, the program was transferred to Ultragenyx for final development and commercialization (GlobeNewswire, 2025).

UX111 has received multiple regulatory designations, including Regenerative Medicine Advanced Therapy, Fast Track, Rare Pediatric Disease, and Orphan Drug designations in the U.S., along with PRIME and Orphan medicinal product designations in the EU. If approved, the therapy will be commercialized by Ultragenyx’s metabolic disease team, targeting the same network of biochemical genetics specialists (GlobeNewswire, 2025).

Understanding Sanfilippo Syndrome Type A

Sanfilippo syndrome type A (MPS IIIA) is a rare, fatal lysosomal storage disorder that primarily affects the brain and is characterized by rapid neurodegeneration starting in early childhood. Patients experience global developmental delays, followed by progressive cognitive, language, and motor decline, along with behavioral abnormalities, ultimately leading to early mortality. MPS IIIA is caused by mutations in the SGSH gene, which result in a deficiency of the sulfamidase (SGSH) enzyme. This deficiency leads to the accumulation of heparan sulfate in cells, driving the neurodegeneration associated with the disease. The disorder affects an estimated 3,000 to 5,000 individuals in commercially accessible regions, with a median life expectancy of approximately 15 years (GlobeNewswire, 2025).

About Ultragenyx

Ultragenyx Pharmaceutical Inc. is a biopharmaceutical company dedicated to developing novel therapies for rare and ultra-rare genetic diseases. The company has built a diverse portfolio of approved medicines and investigational therapies designed to address high unmet medical needs. Ultragenyx is led by a management team with extensive experience in rare disease therapeutic development and commercialization. The company prioritizes time- and cost-efficient drug development strategies to deliver safe and effective treatments with urgency (Ultragenyx, 2025).

For further details, visit Ultragenyx’s website.

Forward-Looking Statements

This announcement contains forward-looking statements regarding Ultragenyx’s expectations for UX111 and its regulatory approval process. These statements are subject to risks and uncertainties, including the unpredictability of clinical drug development, regulatory review timelines, potential safety concerns, market viability, and reliance on third-party partners. More details on these risks are available in Ultragenyx’s quarterly reports filed with the Securities and Exchange Commission (SEC) (Ultragenyx, 2025).

Ultragenyx regularly updates investors through its Investor Relations website and LinkedIn page (Ultragenyx LinkedIn).

Sources:

• GlobeNewswire. (2025). Ultragenyx Announces FDA Acceptance and Priority Review of the Biologics License Application (BLA) for UX111 AAV Gene Therapy to Treat Sanfilippo Syndrome Type A (MPS IIIA). Retrieved from GlobeNewswire

• Ultragenyx. (2025). Company Website and Investor Relations. Retrieved from Ultragenyx.