The approval covers both adult and paediatric patients aged eight years and above.
GSDIa is a rare genetic metabolic disorder caused by a deficiency in the enzyme required for releasing glucose from the liver into the bloodstream.
This deficiency impairs the liver’s ability to manage glucose levels, leading to a risk of severe hypoglycaemia and requiring continuous nutritional monitoring, often involving frequent intake of raw cornstarch as oral glucose replacement.
The FDA granted accelerated approval for Genglycos, also known as DTX401, based on data from a 48-week double-blind, randomised, placebo-controlled Phase III study involving 46 participants who were treated with either DTX401 or a placebo.
The GlucoGene study demonstrated a statistically significant reduction in cornstarch requirements among those who received the gene therapy.
According to the trial’s protocol, participants who completed 48 weeks were able to cross over to the alternate treatment, and follow-up analyses are planned at weeks 96 and 144.
As part of the approval, Ultragenyx will provide two years of efficacy and safety data from an open-label commercial treatment group comprising 50 patients.
This will be conducted alongside 20 control patients who are unable to receive Genglycos due to anti–adeno-associated virus serotype 8 (AAV8) antibodies.
Data collection will occur via the GSDIa Disease Monitoring Program, which will track both new commercial patients and previous trial participants for up to ten years.
Ultragenyx chief medical officer Dr Eric Crombez said: “The approval of Genglycos fulfills our commitment to provide the first therapy that directly targets the root cause of GSDIa.
“The reduced reliance on cornstarch, experienced by patients in our clinical studies, demonstrates this gene therapy’s ability to establish the normal breakdown of glycogen to produce glucose during fasting or episodes of metabolic stress.
“This ability to regulate glucose has alleviated the disease burden and has the potential to mitigate the risk of severe or life-threatening hypoglycaemia for these patients.”
Genglycos is set to be distributed through a network of Qualified Treatment Centers and manufactured at the company’s facility in Bedford, Massachusetts.
In July 2025, Ultragenyx received a complete response letter from the FDA concerning its biologics licence application for UX111 (ABO-102), an adeno-associated virus gene therapy aimed at treating Sanfilippo syndrome type A (MPS IIIA).