First gene therapy approved for glycogen storage disease type Ia

The FDA granted accelerated approval on 19 August 2026 to Genglycos (pariglasgene brecaparvovec-opnr) for adults and children aged 8 and over with glycogen storage disease type Ia. It is the first approved treatment for the condition, and the first gene therapy approval for Ultragenyx Pharmaceutical.

GSDIa is caused by a deficiency of glucose-6-phosphatase, the enzyme that releases free glucose from the liver and kidneys into the blood. Without it, blood sugar falls dangerously whenever a person goes too long without eating. Management is dietary and unrelenting: frequent meals and strict around-the-clock doses of uncooked cornstarch. Genglycos is a one-time AAV8 therapy delivering a functional G6PC gene to the liver.

In a randomised, double-blind, placebo-controlled study followed over 48 weeks after dosing, treated patients showed a statistically significant mean reduction from baseline in daily cornstarch intake of 31% compared with placebo — the primary endpoint — amounting to a mean of one dose per day fewer.

The endpoint is a surrogate, which is what makes this an accelerated approval: Ultragenyx must complete further trials to confirm clinical benefit. Treated patients also showed a mean 3% increase in glucose readings in the hypoglycaemic range versus placebo. Serious adverse reactions reported included anaphylaxis, adrenal insufficiency, high lactate and hypoglycaemia, and the label carries a tumorigenicity warning.