This is a Phase 3, open-label, multicenter study to evaluate the safety, PK, efficacy, PD, and immunogenicity of Cipaglucosidase Alfa/Miglustat treatment in enzyme replacement therapy (ERT)-experienced and ERT-naïve pediatric subjects with Pompe disease, aged 0 to < 18 years

Inclusion Criteria:

1. Male or female subjects (ERT-naïve [have never received a dose of rhGAA] or ERT-experienced [have received rhGAA every 2 weeks for at least 6 months immediately before enrollment, and if ERT dosage has been modified, must have been on the modified dosage for at least 3 months before enrollment]) diagnosed with LOPD who are aged 12 to <18 years at screening (Cohort 1 only) or aged 0 months to < 12 years at screening (Cohort 2 only)
2. Subject weighs ≤ 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 Cipaglucosidase Alfa/Miglustat
5. Subject has a sitting forced vital capacity (FVC) ≥ 30% of the predicted value for healthy Adolescents at screening (Cohort 1 only)
6. Subject (aged 12 to <18 years; Cohort 1) performs one 6-Minute Walk Test (6MWT) (≥ 75 meters) at screening that is valid, as determined by the clinical evaluator, or subject (aged ≥ 5 to < 12 years; Cohort 2) performs one 6MWT (≥ 40 meters) at screening that is valid, as determined by the clinical evaluator

Exclusion Criteria:

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)