INCLUSION CRITERIA

Age 18 to 65 years, inclusive, at the time of signing informed consent.

Confirmed GAA enzyme deficiency from any tissue source and/or confirmed biallelic GAA gene mutations.

Received enzyme replacement therapy (ERT) with alglucosidase alfa (Lumizyme), avalglucosidase alfa-ngpt (Nexviazyme), or cipaglucosidase alfa (Pombiliti) for at least 6 months (at least 10 infusions) before signing the initial informed consent form. Participants must remain on their current ERT during screening until close to dosing.

Upright forced vital capacity (FVC) of 30% to 80% of predicted, inclusive.

Able to walk at least 100 meters during the 6-minute walk test (6MWT). A cane, quad cane, or standard walker is permitted.

Male or female. Must comply with protocol requirements for contraception/barrier use.

Able to provide informed consent.

Able to understand and comply with all study procedures.

EXCLUSION CRITERIA

Severe cardiomyopathy, defined as left ventricular ejection fraction (LVEF) less than 40% or New York Heart Association (NYHA) functional class 3 or higher.

Requires invasive mechanical ventilation or relies on noninvasive ventilation during the day.

Intolerance to ERT, including investigator-assessed intolerance, or a history of serious ERT-related infusion-associated reactions (IARs).

Known intrinsic liver disease, including hepatitis, HIV-related liver disease, prior diagnosis of portal hypertension, splenomegaly, hepatic encephalopathy, severe fatty liver, cirrhosis, liver fibrosis stage 2 or higher, liver neoplasms identified by ultrasound, or laboratory findings suggesting elevated alpha-fetoprotein. Also excluded: ALT or AST greater than 3 times the upper limit of normal (ULN), or total bilirubin above ULN, during screening.

Prior or ongoing medical conditions, including active infection or malignancy within 5 years of screening (except basal or squamous cell skin cancer), physical findings, assessment findings, or laboratory abnormalities that, in the investigator's opinion, would affect participant safety and compliance with study procedures.

Received gene therapy before screening.

Received systemic immunosuppressants other than glucocorticoids from 30 days before screening through completion of screening (inhaled or topical use excepted), and/or known intolerance to immunosuppressants such as glucocorticoids. Other concomitant immunosuppression requires sponsor approval.

Use of investigational drugs or drugs that could affect this study, as evaluated by the investigator, within 30 days before screening through completion of Week 52, or within 5 half-lives of the investigational drug, whichever is longer.

Received any vaccine within 30 days before dosing.

Any other condition that, in the investigator's opinion, makes the participant ineligible.

Contraindication to MRI; hypersensitivity to contrast dyes, shellfish, or iodine; or implanted spinal rods, a cardiac pacemaker, or another implant that would distort cardiac MRI (cMRI) images.