Study to Evaluate the Safety of AT2220 (Duvoglustat) in Pompe Disease

Inclusion Criteria: * Male or female, 18 to 74 years of age inclusive * Diagnosis of Pompe disease based on clinical assessment, enzyme assay, and/or genotyping. Confirmatory genotyping will be performed on all participants who are screened for the study * Naïve to enzyme replacement therapy (ERT) or has not received ERT in the 3 months prior to screening * Willing not to initiate ERT or other prohibited treatment during study participation * Functional grade for arms and/or legs ≥2 OR sitting forced vital capacity ≥30% and \<80% of predicted value, reproducible between screening and baseline (±15%) * Participants of reproductive potential agree to use reliable methods of contraception during the study * Participant or legal representative is willing and able to provide written informed consent Exclusion Criteria: * Any intercurrent condition that may preclude accurate interpretation of study data * Obstructive pulmonary disease * Invasive ventilatory support * Use of noninvasive ventilatory support \>8 hours/day while awake * History of QTc prolongation \>450 milliseconds (msec) for males and \>470 msec for females * History of allergy or sensitivity to the study drug, including any prior serious adverse reaction to iminosugars (such as miglustat or miglitol) * Pregnancy or breast-feeding * Current or recent drug or alcohol abuse * Treatment with another investigational drug within 30 days of study start * Use of prohibited medications ≤3 months prior to screening * Otherwise unsuitable for the study in the opinion of the Investigator