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Complete Response Letter (CRL) cited specific chemistry, manufacturing and controls (CMC) related observations that are resolvable FDA clinical reviews acknowledged that the clinical data are robust and biomarker data are supportive NOVATO, CA, USA I July 11, 2025 I Ultragenyx Pharmaceutical Inc. (NASDAQ: RARE), today announced that the U.S. Food and Drug Administration (FDA) has issued a Complete Response Letter (CRL) for its Biologics License Application (BLA) for UX111 (ABO-102) AAV gene therapy as a treatment for patients with Sanfilippo syndrome type A (MPS IIIA). “Our goal is to get UX111 to patients as quickly as possible knowing how critical this first therapy is to the Sanfilippo community. We have been diligently responding to the recent CMC observations and our priority is to resolve them so that we can resubmit the BLA as soon as possible,” said Emil D. Kakkis, M.D., Ph.D., chief executive officer and president of Ultragenyx. “We believe the CMC observations are readily addressable and many have already been addressed. While the CRL will delay the potential approval of UX111 to 2026, we are working with urgency to respond and resubmit.” In the CRL, the FDA requested that the company provide additional information and improvements related to specific aspects of CMC and observations from the recently completed manufacturing facility inspections. The company believes that these observations are readily addressable, related to facilities and processes, and are not directly related to the quality of the product. The company will be working with the FDA over the next few months to resolve the observations. Once resolution is achieved, the company expects to resubmit the BLA and anticipates up to a 6-month review period to follow the resubmission. Clinical review had been ongoing and the FDA has acknowledged that the neurodevelopmental outcome data provided to date are robust and the biomarker data provide additional supportive evidence. The CRL did not note any review issues related to the clinical data package nor clinical inspections, and asked that updated clinical data from current patients be included in the resubmission. About UX111 UX111 is a novel in vivo gene therapy in Phase 1/2/3 development for Sanfilippo syndrome type A (MPS IIIA), a rare fatal lysosomal storage disease with no approved treatment that primarily affects the brain. The therapy is designed to address the underlying SGSH enzyme deficiency responsible for abnormal accumulation of heparan sulfate, a glycosaminoglycan, in the brain that results in progressive cell damage and neurodegeneration. UX111 is dosed in a one-time intravenous infusion using a self-complementary AAV9 vector to deliver a functional copy of the SGSH gene to cells. These transduced cells then produce the enzyme and secrete it to be taken up by other brain cells, cross-correcting the enzyme deficiency. The product was originally developed by Abeona Therapeutics and transferred to Ultragenyx to complete development. The UX111 program has received Regenerative Medicine Advanced Therapy, Fast Track, Rare Pediatric Disease, and Orphan Drug designations in the U.S., and PRIME and Orphan medicinal product designations in the EU. If approved, the product will be commercialized with Ultragenyx’s existing metabolic disease team seeing the same biochemical genetics doctors. About Sanfilippo Syndrome Type A (MPS IIIA) Sanfilippo syndrome type A (MPS IIIA) is a rare, fatal lysosomal storage disease with no approved treatment that primarily affects the brain and is characterized by rapid neurodegeneration, with onset in early childhood. Children with MPS IIIA present with global developmental delay which eventually leads to progressive cognitive, language and motor decline, behavioral abnormalities and early death. MPS IIIA is estimated to affect approximately 3,000 to 5,000 patients in commercially accessible geographies with a median life expectancy of 15 years. MPS IIIA is caused by biallelic pathogenic variants in the SGSH gene that lead to a deficiency in the sulfamidase (SGSH) enzyme responsible for breaking down heparan sulfate, a glycosaminoglycans, which accumulate in cells throughout the body resulting in the observed rapid neurodegeneration that is associated with the disorder. About Ultragenyx Ultragenyx is a biopharmaceutical company committed to bringing novel therapies to patients for the treatment of serious rare and ultra-rare genetic diseases. The company has built a diverse portfolio of approved medicines and treatment candidates aimed at addressing diseases with high unmet medical need and clear biology, for which there are typically no approved therapies treating the underlying disease. The company is led by a management team experienced in the development and commercialization of rare disease therapeutics. Ultragenyx’s strategy is predicated upon time- and cost-efficient drug development, with the goal of delivering safe and effective therapies to patients with the utmost urgency. For more information on Ultragenyx, please visit the company’s website at: www.ultragenyx.com . SOURCE: Ultrgenyx Pharmaceutical