ZIP Study-OL Study of Safety, PK, Efficacy, PD, Immunogenicity of ATB200/AT2221 in Pediatrics Aged 0 to < 18 y.o. w/LOPD

Inclusion Criteria:

1. Male or female subjects (ERT-naïve [have never received a dose of rhGAA] orERT-experienced [have received rhGAA every 2 weeks for at least 6 months immediatelybefore enrollment, and if ERT dosage has been modified, must have been on themodified dosage for at least 3 months before enrollment]) diagnosed with LOPD whoare aged 12 to <18 years at screening (Cohort 1 only) or aged 0 months to < 12 yearsat 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 studyprotocol

4. If of reproductive potential and if sexually active, female and male subjects agreeto use a highly effective method of contraception throughout the duration of thestudy 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 forhealthy 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, orsubject (aged ≥ 5 to < 12 years; Cohort 2) performs one 6MWT (≥ 40 meters) atscreening that is valid, as determined by the clinical evaluator

Exclusion Criteria:

1. Subject has received any investigational/experimental drug, oral anabolic steroid orderivative, biologic, or device within 30 days or 5 half-lives of the therapy ortreatment, whichever is longer, before screening

2. Subject has received treatment with prohibited medications within 30 days ofscreening

3. Subject has received any gene therapy at any time

4. Subject has any intercurrent illness or condition at screening or baseline that maypreclude the subject from fulfilling the protocol requirements or suggests to theinvestigator and/or the medical monitor that the potential subject may have anunacceptable 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 withclassic IOPD

9. In the opinion of the investigator, the parent or legally authorized representativeis unlikely or unable to comply with the study requirements

10. Subject has any prior history of illness or condition known to affect motorfunction, such as, but not limited to, Guillain-Barre syndrome, cerebral palsy, etc

11. Subject who is diagnosed with Pompe disease via newborn screening and isasymptomatic (ie, showing no signs and symptoms of Pompe disease (Cohort 2 Only)