FDA Clears Ultragenyx Gene Therapy for Glycogen Storage Disease Type Ia

The U.S. Food and Drug Administration has granted accelerated approval to Genglycos, a one-time gene therapy developed by Ultragenyx Pharmaceutical for adults and children aged eight years and older with glycogen storage disease type Ia.

The approval makes Genglycos the first FDA-authorized treatment for this rare inherited metabolic disorder. It is also designed to address the genetic cause of the disease rather than focusing only on managing its symptoms.

What causes glycogen storage disease type Ia?

Glycogen storage disease type Ia occurs when the body lacks glucose-6-phosphatase, an enzyme needed to convert stored glycogen into glucose.

Without sufficient enzyme activity, the liver cannot release glucose effectively into the bloodstream. Patients may therefore develop dangerous episodes of low blood sugar, particularly during periods without food.

The condition also requires demanding nutritional management. Many patients rely on frequent doses of uncooked cornstarch as an oral source of glucose, including during overnight hours. Although this approach helps prevent hypoglycemia, it can lead to substantial fluctuations in blood sugar.

Genglycos uses an adeno-associated virus serotype 8 delivery system to transport a functional G6PC gene to liver cells. The goal is to help the liver produce the missing enzyme and improve its ability to maintain glucose levels during fasting.

Phase 3 Data of Ultragenyx

The FDA decision was supported by results from the 48-week GlucoGene Phase 3 trial.

The randomized, double-blind, placebo-controlled study enrolled 46 participants aged eight years and older. Participants received either the gene therapy or placebo.

Among the 44 patients included in the modified intention-to-treat analysis, those treated with Genglycos required significantly less daily cornstarch than the placebo group.

After the initial 48-week period, eligible participants switched to the other treatment. Follow-up assessments were conducted at Weeks 96 and 144 to examine longer-term outcomes.

The FDA authorization is based on the reduction in daily cornstarch intake as an additional part of nutritional management. Because the approval uses the accelerated pathway, continued authorization may depend on further evidence confirming clinical benefit.

Long-Term Monitoring Planned

Ultragenyx has committed to expanding its existing GSDIa Disease Monitoring Program as part of the post-approval requirements.

The company plans to collect two years of safety and effectiveness data from 50 patients receiving commercial treatment and 20 control participants. The control group will include people who sought treatment but cannot receive Genglycos because they have antibodies against the AAV8 delivery vehicle.

Researchers will assess changes in cornstarch use, fasting tolerance, glucose control, and other clinical measures in routine treatment settings.

The monitoring program is expected to follow previously studied patients and newly treated individuals for up to 10 years.

Access Will Use Specialist Centers

Genglycos will be administered through a nationwide network of qualified treatment centers. These sites will receive specialized training in the preparation and delivery of gene therapy.

Ultragenyx also plans to support patients and caregivers through its UltraCare program. Specially trained gene therapy guides will help families understand insurance coverage, coordinate treatment support, and navigate the administration process.

A New Option for an Ultra-Rare Disorder

Glycogen storage disease type Ia affects an estimated 1,500 to 2,500 people in the United States and approximately 6,000 to 8,000 people worldwide in commercially accessible markets.

The approval gives eligible patients an option intended to reduce dependence on continuous cornstarch supplementation. However, Genglycos is approved as an addition to nutritional management, not as an immediate replacement for all dietary measures.

Patients and families will need to work with specialist treatment teams to determine whether the therapy is appropriate. Long-term monitoring will also be important because the accelerated approval requires additional evidence confirming its clinical benefits.