RARE Rises as FDA Accepts Resubmitted BLA for Sanfilippo Syndrome

Shares of Ultragenyx Pharmaceutical RARE rose 5% after the company announced that the FDA has accepted its resubmitted biologics license application (BLA) seeking accelerated approval for AAV gene therapy candidate, UX111, which is being developed for the treatment of Sanfilippo syndrome type A (MPS IIIA). The regulatory body has set a target action date of Sept. 19, 2026.

If approved, UX111 would be the first approved therapy for the treatment of MPS IIIA and would be manufactured entirely within the United States.

MPS IIIA is a rare, fatal lysosomal storage disorder with no approved treatment, primarily affecting the central nervous system. Approximately 3,000 to 5,000 patients in commercially accessible geographies are affected by this disease, with a median life expectancy of 15 years.

Year to date, RARE’s shares have declined 2.4% against the industry’s 5.7% growth.

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RARE’s BLA Resubmission Addresses Issues in CRL

Ultragenyx Pharmaceuticals initially submitted the BLA in December 2024 under the accelerated approval pathway, and the FDA granted priority review in February 2025.

However, the FDA issued a Complete Response Letter (CRL) for the UX111 BLA in July 2025, requesting additional information and improvements regarding certain chemistry, manufacturing, and controls (“CMC”) elements, along with findings from the manufacturing facility inspections. According to Ultragenyx Pharmaceuticals, the issues raised were facility- and process-related, which were readily addressable and not tied to product quality.

The resubmitted BLA addresses all CMC observations cited in the CRL. It also includes longer-term data across multiple measures of neurological benefit to support an intermediate clinical endpoint for accelerated approval, further reinforced by cerebrospinal fluid (CSF) heparan sulfate (HS) and other biomarker data, in line with agreements reached with the FDA during the clinical review.

Rationale Behind RARE’s UX111 BLA Submission

The original BLA submission was backed by data, including findings from the phase I/II/III Transpher A study. The study demonstrated that treatment with UX111 led to a rapid and lasting reduction in HS levels in the CSF of patients with MPS IIIA. Maintaining lower CSF HS levels over time was linked to improved long-term cognitive development, contrasting with the decline observed in natural history data. The investigational gene therapy was overall well-tolerated in the study. Adverse events related to treatment with the candidate were mostly mild to moderate in severity.

The CRL did not raise concerns about the clinical data or inspections but requested the inclusion of updated clinical data from current patients in the resubmission.

More on RARE’s UX111

UX111 is a novel in vivo gene therapy to treat the underlying sulfamidase enzyme deficiency, which is responsible for the abnormal accumulation of HS in the brain that leads to progressive cell damage and neurodegeneration in children with MPS IIIA.

UX111 enjoys the Regenerative Medicine Advanced Therapy, Fast Track, Rare Pediatric Disease and Orphan Drug designations in the United States. In the European Union, the candidate has been granted the PRIME and Orphan Medicinal Product designations.

RARE’ Zacks Rank & Stocks to Consider

Ultragenyx Pharmaceutical currently carries a Zacks Rank #3 (Hold).

Some better-ranked stocks in the biotech sector are Catalyst Pharmaceuticals CPRX and Indivior Pharmaceuticals INDV, each currently sporting a Zacks Rank #1 (Strong Buy), and ANI Pharmaceuticals ANIP, which carries a Zacks Rank #2 (Buy). You can see the complete list of today’s Zacks #1 Rank stocks here.

Over the past 60 days, estimates for Catalyst Pharmaceuticals’ 2026 earnings per share have risen from $2.55 to $2.87. CPRX shares have gained 5.8% year to date.

Catalyst Pharmaceuticals’ earnings beat estimates in each of the trailing four quarters, with the average surprise being 35.19%.

Over the past 60 days, estimates for Indivior Pharmaceuticals’ 2026 earnings per share have risen from $2.89 to $3.08. INDV shares have lost 15.1% year to date.

Indivior Pharmaceuticals’ earnings beat estimates in each of the trailing four quarters, with the average surprise being 74.53%.

Over the past 60 days, estimates for ANI Pharmaceuticals’ earnings per share have increased from $8.14 to $8.99 for 2026. Year to date, shares of ANIP have declined 6%.

ANI Pharmaceuticals' earnings beat estimates in each of the trailing four quarters, with the average surprise being 22.21%.

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