A Study to Evaluate Safety, Tolerability, and Efficacy of AB-1009 Gene Therapy (GAA Gene) in Adult Participants With Late Onset Pompe Disease (PROGRESS-GT LOPD)

Inclusion Criteria:

1. Participant must be ≥18 years of age at the time of signing the informed consentform.

2. Confirmed GAA enzyme deficiency from any tissue source and/or confirmed biallelicGAA gene mutations.

3. Undergone enzyme replacement treatment (ERT) (either alglucosidase alfa (Lumizyme®)or avalglucosidase alfa-ngpt (Nexviazyme®)), for at least 6 months (at least 10infusions) before signing the initial informed consent form. During the screeningprocess, participants need to remain on their current ERT until close to dosing;

4. FVC in the upright position ≥30% and ≤80% of predicted;

5. Capable of walking at least 100 meters in the 6MWT (use of a cane, quad cane, orstandard walker is permitted);

6. Contraceptive/barrier use by men and women requirements as per protocol.

7. Capable of giving informed consent and able to understand and comply with all studyprocedures.

Exclusion Criteria:

1. Severe cardiomyopathy, defined as left ventricular ejection fraction (LVEF) <40% orNew York Heart Association (NYHA) functional class 3 or above;

2. Require invasive mechanical ventilation, or rely on noninvasive ventilation duringthe day;

3. Intolerance to ERT or investigator-assessed intolerance to ERT, prior experience ofserious ERT-related infusion-associated reactions (IARs);

4. Have known intrinsic liver diseases, including hepatitis, HIV-related liver disease,prior diagnosis of portal hypertension, splenomegaly, hepatic encephalopathy, severefatty liver, cirrhosis or liver fibrosis ≥stage 2, ultrasound-identified liverneoplasms, or laboratory tests suggesting elevated alpha-fetoprotein. Patients withliver function tests including ALT or AST >3× upper limit of normal (ULN) or anytotal bilirubin above ULN during screening will also be excluded;

5. Prior or ongoing medical condition(s), physical finding(s), assessment findings, orlaboratory abnormality that, in the investigator's opinion, would impactparticipant's safety and compliance with the study procedures.

6. Have received gene therapy prior to screening;

7. Have received any systemic immunosuppressants (except inhalation or topical use)other than glucocorticoids or investigator-recommended immunosuppressants 30 daysprior to screening through completion of screening, and/or known intolerance toimmunosuppressants such as glucocorticoids;

8. Use of investigational drugs or drugs that could affect this study as evaluated bythe investigator within 30 days prior to screening through completion of Week 52 orwithin 5 half-lives of the investigational drug (whichever is longer);

9. Have received any vaccine within 30 days prior to dosing;

10. Other conditions that make the participant not eligible for the study according tothe investigator.