Inclusion Criteria: * Diagnosis of Pompe disease by blood or skin fibroblast GAA assay and two pathogenic variants in the GAA gene, * Age: Greater than or equal to 18 years at enrollment. * Subjects are capable of giving written informed consent. * Able to walk at least 100 meters on the 6MWT (with assistive devices permitted). * FVC within the range of 30% to less 90% (inclusive) of predicted in the upright position. * Subjects with a confirmed diagnosis of LOPD who have been treated with ERT for at least 104 weeks (inclusive) immediately preceding screening and receiving a stable dose of ERT for the 52-week period immediately preceding dosing. Exclusion Criteria: * Invasive ventilation required or noninvasive ventilation required while awake and upright. * FVC \<20% of predicted (supine). * Received any live vaccination 2 months prior to study Day 1. * Pregnant or nursing mothers. * Serology consistent with exposure to HIV, or serology consistent with active hepatitis A, B or C infection. Any active liver disease. * Active infection based upon clinical symptoms. * Having started respiratory muscle strength training in the last 6 months prior to study day 1 or having discontinued respiratory muscle strength training in the 6-month period preceding study day 1, or having started respiratory strength training greater than 6 months prior to study day 1 and unwilling to continue for the first year of study participation. * Received an investigational drug or participated in another interventional study within 90 days prior to Study Day 1. Additionally, subjects cannot participate in any other interventional clinical trial throughout the first 78 weeks after receiving ACTUS-101.