PDUFA Date

Ultragenyx's DTX401 gene therapy faces FDA priority review verdict August 23

The FDA's Priority Review decision on DTX401 would deliver the first therapy addressing GSDIa's underlying enzyme deficiency, resting on a 49-patient Phase 3 that already met its primary endpoint.

By AppliedXLJuly 15, 2026Trial NCT05139316

Executive Summary

The FDA is due to rule on Ultragenyx's biologics license application for a Phase 3 gene therapy in a rare metabolic disease that today has no approved drug treatment, under a Priority Review clock that compresses the standard review timeline.
The application rests on a completed, placebo-controlled Phase 3 trial that already reported a met primary endpoint, shifting the decision's uncertainty toward regulatory execution rather than open clinical questions.
Approval would establish the first therapy to address the disease at its enzymatic root cause rather than through dietary cornstarch management, the current standard of care.
The therapy is the only Phase 3 asset targeting this gene in this indication, with the nearest peer still in earlier-phase testing, so this single decision effectively sets the pace for the field.

The catalyst

The FDA accepted Ultragenyx's biologics license application for DTX401 and granted it Priority Review, assigning a PDUFA action date of August 23, 2026, the company said. The application covers Glycogen Storage Disease Type Ia (GSDIa), a rare inherited disorder in which mutations in the G6PC gene impair the enzyme G6Pase, causing severe fasting hypoglycemia and excess liver glycogen storage. Patients currently manage the disease with frequent cornstarch dosing around the clock; there is no approved pharmacologic therapy. "If approved, DTX401 would be the first treatment to address the disease at its root cause," said Eric Crombez, Ultragenyx's chief medical officer. Ultragenyx

Probability of Success

Endpoint Met100%

Completes88%

Clinical Significance76%

Regulatory57%

The trial behind the filing

The BLA is built on the Phase 3 GlucoGene study (NCT05139316), a randomized, quadruple-masked, placebo-controlled trial that enrolled 49 patients across the United States, Japan, Italy, Brazil, Canada, Denmark, Germany, the Netherlands, and Spain. The trial's primary endpoint was percent change from baseline to Week 48 in daily cornstarch intake, and the study reported that DTX401 patients achieved reductions in both the quantity and frequency of cornstarch dosing while maintaining low hypoglycemia and improved fasting tolerance. Secondary measures, including the Patient Global Impression of Change scale, also showed improvement, and the therapy was reported as well tolerated with an acceptable safety profile. The submission draws on a broader development program spanning 52 treated patients with up to six years of follow-up. The trial completed in February 2026, with its primary completion date landing in February 2024, roughly ten months after the date first set when the study began. NCT05139316+1

Regulatory pathway

DTX401 carries Rare Pediatric Disease, orphan drug, Fast Track, and Regenerative Medicine Advanced Therapy (RMAT, an FDA designation for cell and gene therapies) designations from the FDA, alongside orphan drug and PRIME designations from the European Medicines Agency. Ultragenyx opened a rolling BLA submission in August 2025 and the FDA accepted the completed application for Priority Review in February 2026, setting the six-month review clock toward the August 23 action date. None of these designations guarantee a favorable outcome; they signal the FDA's engagement with a rare disease that has no approved gene therapy, not a predetermined result. Ultragenyx

The competitive field

Gene replacement approaches for GSDIa remain sparse. DTX401 is the only Phase 3 asset built around the G6PC target in this indication, with Beam Therapeutics' BEAM-301, a gene-editing candidate rather than gene replacement, still in Phase 1/2 testing for the same disease. Moderna's mRNA-3745, an mRNA therapy rather than a one-time AAV vector, targets the same G6PC pathway but sits in Phase 1/2 with a different modality and delivery approach, limiting it as a direct comparator. Beyond the target itself, AAV gene therapies for other monogenic diseases, such as Janssen's RPGR programs in retinitis pigmentosa and Prevail Therapeutics' PR001 in Gaucher disease, share the modality but not the target, underscoring that DTX401's specific mechanism has no other Phase 3 rival. The historical readout base for this exact target-indication pairing is thin: two prior completed trials in this pairing are recorded, both without termination, though the small count limits any base-rate inference.

What the decision must resolve

With the primary endpoint already reported as met, the FDA's binary decision now turns less on whether the biology works and more on whether the review clears program-specific execution questions: manufacturing consistency at the new Bedford, Massachusetts facility where DTX401 would be produced entirely in the U.S., the durability of effect beyond Week 48, and the adequacy of the safety database drawn from a 49-patient trial. Given the absence of any approved comparator in this indication, the bar the FDA and the field will apply is not superiority to a standard drug treatment, since none exists, but whether the totality of the safety and efficacy record supports approving a one-time gene therapy for a disease currently managed only through diet. Ultragenyx

This analysis was produced using AI-assisted reporting systems, AppliedXL data, and official public records. These systems undergo editorial review, quality checks, and regular audits by human experts. Errors may still occur, as with any automated system. Always consult the linked primary sources. Read our AI Editorial Policy.