In Pompe disease, acid alpha-glucosidase (GAA) deficiency leads to lysosomal glycogen accumulation. FORTIS (NCT04174105) is an ongoing multicenter, open-label, ascending-dose phase 1/2 trial evaluating the safety/tolerability/preliminary efficacy of AT845, a gene replacement therapy that expresses human GAA in muscles, in adults with late-onset Pompe disease (LOPD). Eleven participants received intravenous AT845 with subsequent monitoring at a single dose of 3×10^13 vg/kg (n=2, cohort 1) or 6×10^13 vg/kg (n=9, cohort 2), with 24 weeks–4 years follow-up at data cutoff (July 22, 2025). At baseline, participants were aged 24–70 years (median 52), with median 11 years enzyme replacement therapy (ERT; range: 2–17). All participants had increases in vector copy number in muscle within 3 months; GAA protein and enzyme activity levels were variable. At data cutoff, 9/11 participants had discontinued ERT 10–48 weeks (median 17) after AT845 dosing, with 5 remaining off ERT between 1 and >3.5 years; 2 never discontinued. Liver enzyme elevations (4–8 weeks onset) were the most common AE, reported in 9/11 participants including 3 SAEs (grade 3 ALT/AST elevations, n=2; grade 4 GGT, n=1). All were asymptomatic and managed with corticosteroids. One participant in cohort 2 experienced a medically significant grade 2 peripheral sensory neuropathy. At 48 weeks, median change from baseline in % predicted upright forced vital capacity was 2.4 (range: −1.0–5.7; n=2) for cohort 1 and –3.0 (−13.0–7.3; n=7) for cohort 2; median change from baseline in % predicted 6-minute walk test was −0.19 (–0.9–0.5; n=2) for cohort 1 and –1.03 (–13.5–4.3; n=6) for cohort 2. These outcomes remained mainly stable ≤4 years post-dosing. Treatment with AT845 was generally safe and demonstrated stabilized muscle and respiratory function in a small cohort of LOPD patients who discontinued ERT. These findings were previously disclosed at WORLDSymposium 2026.