Onasemnogene Abeparvovec in Type I Spinal Muscular Atrophy: 24-Month Follow-Up From the Italian Registry

Objective Onasemnogene abeparvovec (OA) is an AAV9-based gene therapy for spinal muscular atrophy type I (SMA I). Real-world outcomes show increased response variability compared to clinical trials, and follow-up data beyond 12-18 months are limited. The aim of this 24-month prospective observational study is to comprehensively describe the clinical outcomes of an Italian cohort of SMA I patients treated with OA. Methods Based on recent literature, patients' treatment status was categorized as: monotherapy (OA only), bridge therapy (transition to OA within 3 months of starting nusinersen or risdiplam), or switch therapy (transition to OA after > 3 months of 1st treatment). Linear mixed-effects models examined predictors of improvement (CHOP-INTEND), adjusting for baseline motor function, SMN2 copy number, age, and prior treatment. Descriptive analyses were used to show changes in motor, respiratory, and nutritional milestones. Results The cohort included 64 patients: 27 monotherapy, 9 bridge, and 28 switch. All patients showed significant improvement over 24 months (beta = 20.40 points/year, p < 0.001). Patients who switched showed slower improvement (beta = -3.76, p = 0.038) compared to monotherapy, while those who bridged showed no difference. Older age at treatment was associated with slower improvement (beta = -1.48 points/year per month, p = 0.002). Of 49 non-sitters at baseline, 39 (80%) achieved sitting and 5 (10%) achieved walking. No new safety signals emerged in the second year of follow-up. Interpretation Age and baseline motor functional status significantly influence outcomes; however, substantial confounding, particularly the initial treatment, limits the ability to isolate individual effects. Longer follow-up is essential for evaluating therapeutic responses in heterogeneous SMA I populations.