First FDA-Approved Gene Therapy for Glycogen Storage Disease Type Ia

GENGLYCOS (pariglasgene brecaparvovec-opnr) is an adeno-associated virus (AAV) vector-based gene therapy given as a single intravenous infusion. On the US Package Insert it is indicated to reduce daily cornstarch intake as an adjunct to nutritional management in adult and pediatric patients 8 years of age and older with glycogen storage disease type Ia (GSDIa).

This indication is approved under accelerated approval based on reduction in daily cornstarch intake. Continued approval may depend on confirmatory clinical benefit in confirmatory trial(s).

Limitation / selection (label): Select patients without detectable anti-AAV8 antibodies. Do not administer GENGLYCOS to patients with a detected result for antibodies to AAV8. An FDA-authorized companion antibody test is not currently available — contact Ultragenyx for testing information. Contraindicated in known severe hepatic fibrosis or cirrhosis.

Distinct path — not this label: Lifelong dietary cornstarch management and other metabolic supports are not this gene-therapy label. Other glycogen-storage diseases (non-Ia types) are not this labelled population. Today’s approved US gene-therapy pathway for GSD Ia ages 8+ is Ultragenyx GENGLYCOS STN 125858.

The FDA approved STN BL 125858/0 on 19 August 2026 under accelerated approval. Approval letter issues U.S. License Number 2390 to Ultragenyx Pharmaceutical, Inc. Letter attention: Karyn Packard. Associated NCT numbers on the letter include NCT03517085, NCT03970278, NCT04708015, NCT05139316, NCT06636383. RMAT, Fast Track, and Rare Pediatric Disease Priority Review Voucher noted on the FDA press announcement. Package Insert revised 8/2026.

Where this is taking place

Approved prescription route today: United States — FDA-labelled GENGLYCOS for GSD Ia ages 8+, administered at a Qualified Treatment Center with metabolic / genetics expertise, after anti-AAV8 antibody testing, baseline liver assessment, premedication, and planned corticosteroid prophylaxis. Company materials describe UltraCare Gene Therapy Guides (1-888-756-8657 / ultracaresupport.com) and a national QTC network. Company launch guidance at approval was commercial availability within roughly 30–60 days of 19 August 2026 — treat mid-September availability as a launch window, not a guarantee every centre is ready every day. This is a centre-administered gene therapy conversation — not a retail pharmacy pickup and not a DIY import.

Outside the United States: At the time of writing, no confirmed Health Canada Notice of Compliance, European Commission marketing authorisation, MHRA licence, TGA listing, Swissmedic authorisation, or PMDA/MHLW licence for GENGLYCOS / pariglasgene brecaparvovec. Do not import on your own.

Study 1 / GlucoGene — NCT05139316 — Multicenter, randomized, double-blind, placebo-controlled (Weeks 1–48) with crossover (Weeks 48–96). Lead sponsor: Ultragenyx Pharmaceutical Inc. CT.gov status at the time of writing: completed. Actual enrolment 49. Actual start 8 Nov 2021; actual primary completion 20 Feb 2024; actual study completion 20 Feb 2026. Study Director on the record: Medical Director, Ultragenyx Pharmaceutical Inc. Countries with listed sites include the United States, Brazil, Canada, Denmark, Germany, Italy, Japan, Netherlands, and Spain. A completed pivotal study is not open enrolment. Foreign trial sites are not a foreign marketing authorisation. Post-marketing Disease Monitoring Program / confirmatory obligations remain under accelerated approval.

The practical prescription route in the US is a metabolic genetics / GSD clinic working with a Qualified Treatment Center to confirm GSD Ia, document age ≥8, complete anti-AAV8 testing and liver evaluation, plan infusion + steroids, and arrange UltraCare / payer logistics — not treating cornstarch reduction as permission to ignore glucose monitoring.

Approval matrix

Access by country

• United States: Ask a metabolic genetics / GSD team about referral to an Ultragenyx Qualified Treatment Center for FDA-labelled GENGLYCOS. Labelled 19 August 2026 (accelerated) for adults and pediatrics ≥8 years with GSD Ia to reduce daily cornstarch intake as an adjunct to nutritional management. Recommended dose: 1.0×10¹³ gc/kg as a single IV infusion (kits of 3–15 vials by weight; ≥87 kg → 15 vials / 30 mL). Do not dose if anti-AAV8 antibodies are detected. Premedicate with acetaminophen + non-sedating antihistamine 30–60 minutes before infusion. Starting 2 weeks after infusion, prophylactic corticosteroids for a minimum of 8 weeks with taper (see PI Table 3). Monitor for infusion reactions, hepatotoxicity, and adrenal insufficiency during/after steroid taper. Vial NDC 69794-332-01; kit NDCs 69794-333-03 … 69794-345-15. Shipped frozen ≤ −60 °C. UltraCare Gene Therapy Guides: 1-888-756-8657 (Mon–Fri 9 a.m.–8 p.m. ET per company materials). Suspected adverse reactions: Ultragenyx 1-888-756-8657 or FDA MedWatch 1-800-FDA-1088. No list price or copay dollars in this article. Company/trade WAC figures exist in secondary coverage — omitted here. This is US labelled supply through QTCs, not a DIY import.

• Canada: GENGLYCOS authorisation not confirmed. Ask the Canadian metabolic clinic what legal paths exist in Canada when a Canadian GENGLYCOS label does not yet exist.

• European Union / United Kingdom / Australia / Japan / other countries: Regulator authorisation not confirmed on the sources checked. Open or completed trial sites are not a commercial foreign label.

• If your regulator has not authorised it: do not import on your own. Ask the local clinician about documented special-access / named-patient rules, referral to a centre in a labelled country, or waiting — without treating a US kit as a foreign pack.

Who is eligible (from the US label)

This is a GSD Ia ages 8+ conversation on the Package Insert — one-time IV gene therapy as an adjunct to nutritional management — not a cornstarch-free promise and not for antibody-positive patients on the selection rule.

United States (Package Insert, revised August 2026):

• Indication: Reduce daily cornstarch intake as an adjunct to nutritional management in adult and pediatric patients 8 years of age and older with GSD Ia.
• Accelerated approval: Based on cornstarch reduction; confirmatory benefit still required.
• Patient selection: Without detectable anti-AAV8 antibodies; baseline liver tests; do not give if AAV8 antibodies detected.
• Contraindication: Known severe hepatic fibrosis or cirrhosis.
• Recommended dosage: 1.0×10¹³ gc/kg single IV infusion; follow PI infusion rates; concentration 3.0×10¹³ gc/mL; 2 mL extractable volume per vial.
• Warnings: Hypersensitivity / infusion reactions (including anaphylaxis); hepatotoxicity; adrenal insufficiency during/after corticosteroids; theoretical AAV integration / tumorigenicity — report tumors to Ultragenyx; do not use in pregnancy; contraception guidance on PI (females ≥12 months; males semen donation / pregnancy prevention 6 months).
• Most common adverse reactions (≥10%): increased transaminases, nausea.
• Vaccines: Adjust schedule for immunosuppression; avoid vaccines 1 month prior to GENGLYCOS per the PI drug-interactions section.
• Pediatrics: Established in ≥8 years (Study 1 included patients 8–17); not established below age 8.
• Geriatrics: Discuss with clinician; adequate numbers ≥65 not established on sources used here.

Not labelled on the sources checked for this article:

• Children under 8.
• Patients with detectable anti-AAV8 antibodies (selection rule: do not administer).
• Patients with known severe hepatic fibrosis or cirrhosis.
• Canadian, EU, UK, Japanese, Australian, or Swiss GENGLYCOS marketing authorisations (not confirmed here).
• Treating completed NCT05139316 as open commercial enrolment.

What the pivotal study showed (Package Insert)

Use the Package Insert for a treatment-centre conversation. Company decks are supportive reading.

Study 1 — NCT05139316 (GlucoGene):

• Multicenter, randomized, double-blind, placebo-controlled Weeks 1–48; crossover thereafter.
• PEAP safety set described on PI: 21 GENGLYCOS / 25 placebo at recommended dose during Weeks 1–48.
• Efficacy analysis N=46 in Table 5 on PI.

USPI Table 5 — Week 48 efficacy (Study 1):

Must-disclose: Cornstarch went down on average, but the share of glucose readings in the hypoglycemic range went up more on GENGLYCOS than placebo at Week 48. This stays in the clinician conversation as a labelled safety finding.

Week-96 observational follow-up among GENGLYCOS-treated patients with available data (PI): mean percent change in daily cornstarch −60.8% (SD 19.7) in 15/21; mean change in daily cornstarch doses −2.0 (SD 1.3) in 15/21; mean change in % glucose <70 mg/dL +5.2% (SD 6.3) in 19/21.

The CT.gov status is completed. Confirmatory post-marketing / Disease Monitoring Program obligations remain under accelerated approval (approval letter PMRs). Per company 8-K, Ab+ patients who cannot receive commercial GENGLYCOS may serve as controls in the enhanced DMP. That is not a treatment route for Ab+ patients.

How it is taken (US label)

Research team (from primary sources only)

• Applicant / manufacturer: Ultragenyx Pharmaceutical, Inc. — U.S. License 2390 (approval letter).
• Approval letter attention: Karyn Packard (Ultragenyx).
• Pivotal study sponsor: Ultragenyx Pharmaceutical Inc. (NCT05139316).
• Study Director (CT.gov): Medical Director, Ultragenyx Pharmaceutical Inc.
• Patient support (company): UltraCare Gene Therapy Guides — 1-888-756-8657 / ultracaresupport.com.
• Names and phones below are limited to what appears on the Package Insert, approval letter, FDA press announcement, ClinicalTrials.gov, and clearly labelled company support materials.

Primary sources

1. FDA Package Insert — GENGLYCOS (pariglasgene brecaparvovec-opnr), Revised 8/2026 — https://www.fda.gov/media/194364/download
2. FDA Accelerated BLA Approval Letter, STN BL 125858/0, 19 August 2026 — https://www.fda.gov/media/194371/download
3. FDA press announcement — First therapy for GSD Ia ages 8+ (19 August 2026) — https://www.fda.gov/news-events/press-announcements/fda-approves-first-therapy-patients-aged-8-years-and-older-glycogen-storage-disease-type-ia
4. FDA GENGLYCOS product page — https://www.fda.gov/vaccines-blood-biologics/genglycos
5. ClinicalTrials.gov NCT05139316 (Study 1 / GlucoGene)
6. Ultragenyx company announcement / UltraCare QTC language (access logistics; not the label) — ir.ultragenyx.com