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Pompe disease is a rare glycogen storage disease caused by mutations in the enzyme acid α-glucosidase (GAA) resulting in pathological accumulation of glycogen in muscle tissues leading to progressive weakness and respiratory dysfunction. Enzyme replacement therapy (ERT) with GAA is currently the sole treatment option for patients with Pompe disease. ERT burdens patients with frequent intravenous infusions while insufficiently halting disease progression due to incomplete ERT skeletal muscle distribution. Glycogen synthase 1 (GYS1) has been proposed as a substrate reduction therapy (SRT) target for Pompe disease. Here, we report results from the first-in-human study of the orally available GYS1 inhibitor MZE001 in healthy subjects. In 88 participants, MZE001 was well-tolerated up to a single dose of 480 mg BID and multiple doses of 720 mg BID for 10 days. Noncompartmental analysis determined that the half-life and C trough concentrations of MZE001 could provide efficacious exposures with once or twice daily oral dosing. Change from baseline of peripheral blood mononuclear cell (PBMC) glycogen, which correlated with muscle glycogen levels in preclinical models, was significantly reduced dose-dependently following 10 days of MZE001 treatment in healthy subjects. A muscle biopsy sub-study demonstrated that 10 days of MZE001 (480 mg BID) dosing safely and substantially lowered muscle glycogen stores in healthy adults. This correlated with the PBMC exposure response and supports the use of PBMC glycogen reduction as a surrogate for muscle response, and MZE001 potential for development as the first oral substrate reduction therapy for patients with Pompe disease., (© 2024 Maze Therapeutics. Clinical Pharmacology & Therapeutics published by Wiley Periodicals LLC on behalf of American Society for Clinical Pharmacology and Therapeutics.)