Ultragenyx Pharmaceutical Inc. announced that the first patient has been dosed in its Phase 1/2 study of UX053, an investigational messenger RNA (mRNA) therapy in development for the treatment of Glycogen Storage Disease Type III (GSDIII). UX053 is Ultragenyx?s most advanced investigational mRNA therapy. The company is developing a preclinical pipeline of mRNA therapies through its long-term collaboration with Arcturus Therapeutics Inc., a clinical-stage messenger RNA medicines company. Phase 1/2 Clinical Trial Design: The two-part Phase 1/2 clinical trial will evaluate the safety, tolerability and efficacy of UX053 in adults age 18 and older with GSDIII. Part 1 is open label and will enroll up to 10 patients who will receive a single ascending dose of UX053 administered via intravenous infusion. Part 2 is double-blind and will evaluate five repeat doses at escalating dose levels in up to 16 patients across four cohorts randomized 3:1 to UX053 or placebo. The primary endpoints are treatment-emergent adverse events (TEAEs), serious TEAEs and related TEAEs in both parts of the study. Secondary endpoints include pharmacokinetic parameters. Exploratory endpoints include clinician- and patient-reported outcomes, muscle strength, blood sugar, and biomarkers of liver, cardiac and muscle health. About Glycogen Storage Disease Type III: GSDIII is caused by deficiency in the glycogen debranching enzyme, which results in glycogen accumulation in the liver, heart, and muscle and the inability to release glucose as needed to provide energy and regulate blood glucose levels. Patients with GSDIII experience hepatomegaly (enlarged liver), hypoglycemia, hyperlipidemia, progressive liver fibrosis (scarring), and skeletal and cardiac muscle disease. There are no approved medicines for GSDIII and the current standard of care is a strict diet, including frequent doses of cornstarch to reduce the risk of hypoglycemia, high protein intake, avoidance of fasting, and avoidance of simple sugars. GSDIII affects more than 10,000 patients worldwide. About UX053: UX053 is an investigational mRNA therapy encoding full-length, glycogen debranching enzyme that is encapsulated in a lipid nanoparticle. It is designed to provide the deficient protein in patients with GSDIII. Preclinical studies of UX053 demonstrated reductions in glycogen content in the liver in multiple models of the disease. The U.S. Food and Drug Administration (FDA) and the European Commission have granted Orphan Drug Designation for UX053 for the treatment of GSDIII.