ZIP Study - A Study of the Safety, Pharmacokinetics, Efficacy, Pharmacodynamics, and Immunogenicity of ATB200/AT2221 in Pediatric Subjects Aged 0 to < 18 Years With Pompe Disease

Open to people ages up to 17 years

1. Male or female subjects (ERT-naïve or ERT-experienced diagnosed with LOPD
2. Subject weighs ≥ 25 kg and ≤ 115 kg. (Cohort 1 Only)
3. Subject must have a diagnosis of LOPD based on documentation as defined in study protocol
4. If of reproductive potential and if sexually active, female and male subjects agree to use a highly effective method of contraception throughout the duration of the study and for up to 90 days after their last dose of ATB200/AT2221
5. Subject has a sitting forced vital capacity (FVC) ≥ 30% of the predicted value for healthy Adolescents at screening
6. Subject performs one 6-Minute Walk Test (6MWT) (≥ 75 meters) (Cohort 1), Subjects aged ≥ 5 to < 12 years who perform one 6MWT (≥ 40 meters) (Cohort 2) at screening that is valid, as determined by the clinical evaluator

1. Subject has received any investigational/experimental drug, oral anabolic steroid or derivative, biologic, or device within 30 days or 5 half-lives of the therapy or treatment, whichever is longer, before screening
2. Subject has received treatment with prohibited medications within 30 days of screening
3. Subject has received any gene therapy at any time
4. Subject has any intercurrent illness or condition at screening or baseline that may preclude the subject from fulfilling the protocol requirements or suggests to the investigator and/or the medical monitor that the potential subject may have an unacceptable risk by participating in this study
5. Subject has a hypersensitivity to any of the excipients in ATB200, approved rhGAA, or AT2221
6. Female subject is pregnant or breast-feeding at screening
7. Subject requires the use of ventilation support for > 6 hours per day while awake
8. Subject has evidence of moderate to severe hypertrophic cardiomyopathy aligning with classic IOPD
9. In the opinion of the investigator, the parent or legally authorized representative is unlikely or unable to comply with the study requirements
10. . Subject has any prior history of illness or condition known to affect motor function, such as, but not limited to, Guillain-Barre syndrome, cerebral palsy, etc
11. . Subject who is diagnosed with Pompe disease via newborn screening and is asymptomatic (ie, showing no signs and symptoms of Pompe disease (Cohort 2 Only)