FDA Approves Ultragenyx's Genglycos, First Gene Therapy for Ultra-Rare Glycogen Storage Disease Type Ia, at $2.7M List Price

Ultragenyx on Wednesday won Food and Drug Administration approval to sell the first treatment for patients with a rare genetic condition known as glycogen storage disease type Ia (GSD1a). The agency's decision was announced after the August 19 market close. Patients with GSD1a have a genetic mutation leaving them without a critical enzyme that helps the body stabilize blood sugar levels between meals. Ultragenyx's one-time therapy, known as Genglycos, is designed to deliver a functional gene to the liver, aiming to restore the enzyme and treat the potentially life-threatening metabolic imbalance.

The Trial Results

Genglycos was evaluated in a placebo-controlled Phase 3 study that enrolled 46 participants age 8 and older. At 48 weeks, results showed a reduction in the cornstarch requirements in the treatment cohort compared to the placebo arm. Genglycos-treated patients reduced their daily cornstarch intake by an average of 31% compared with placebo, and reduced their daily intake by an average of one dose per day. At week 96, the end of the crossover period, Ultragenyx reported that the Genglycos group showed an average 70% reduction in nighttime cornstarch consumption compared to baseline; for the crossover group, the average reduction was 75% compared to week 48.

Pricing and Reimbursement

Ultragenyx set Genglycos' per-patient wholesale acquisition cost at $2.7 million, company officials told investors and analysts on a Wednesday conference call. They plan to have the therapy available within 30 to 60 days at specialized treatment centers. Approval came with a priority review voucher; Ultragenyx plans to sell it. These regulatory fast passes are in great demand — this year, three vouchers sold for $180 million, $195 million and $200 million, respectively. Chief Financial Officer Howard Horn said the sale will support Ultragenyx's "path to profitability."

The Disease Burden

To prevent potentially life-threatening episodes of hypoglycemia, patients with GSD1a typically ingest a slurry of cornstarch every four hours, day or night. It is a demanding regimen that's combined with dietary restrictions and careful monitoring. "GSD1A is an urgent disease with round-the-clock demands on patients every day and night without holiday or break," Ultragenyx President and CEO Emil Kakkis said during a Wednesday evening conference call. "A gene therapy designed to deliver the missing enzyme is the ideal way to address this severe ultra-rare disease." William Blair models $362 million in peak sales for Genglycos.

Safety and Confirmatory Data

Serious adverse reactions reported in the study included anaphylaxis, adrenal insufficiency, high lactate levels and hypoglycemia. The most common adverse reactions included elevated levels of liver enzymes, nausea, headache, constipation and hyperglycemia; hypertriglyceridemia was more common with Genglycos than placebo (29% vs. 8%). The FDA decision was an accelerated approval based on data from the 48-week, placebo-controlled portion of the trial. Ultragenyx will need to provide two years of safety and efficacy clinical data from 50 patients who receive the therapy commercially and 20 control patients who sought Genglycos but were deemed ineligible due to antibodies against the engineered virus.

What to Watch Through Year-End

Three checkpoints follow. The sale of Ultragenyx's priority review voucher — likely within the next 90 days — will set the floor for the 2026 voucher market and provide non-dilutive funding for the UX111 Sanfilippo syndrome Type A gene therapy resubmission, which has a Sept. 19 FDA decision deadline. The pivotal GTX-102 Angelman syndrome trial readout in September or October is the next major Ultragenyx catalyst. And the Regeneron Pasatru approval for fibrodysplasia ossificans progressiva on August 20 — a monoclonal antibody that reduced new heterotopic bone formation by 90% in Phase 3 — is the second FDA action this week that validates the ultra-rare disease pricing model, putting pressure on payers to find reimbursement frameworks that can absorb $1.4M–$2.7M list prices.