Tofersen treatment in SOD1 p.Leu145Phe ALS: real-world outcomes in a genetically homogeneous Croatian cohort

Background: Antisense oligonucleotide tofersen targets SOD1 mRNA and reduces production of misfolded SOD1 protein, with demonstrated biomarker and functional signals in clinical trials and open-label extensions. Real-world reports from genetically heterogeneous SOD1 ALS cohorts describe variable functional trajectories. Data from genetically homogeneous founder populations remain limited. We investigated clinical trajectories in a cohort carrying the same pathogenic SOD1 variant to better characterize mutation-specific patterns in a real-world setting. Methods: We conducted a single-center observational study at the National Referral Center for Neuromuscular Diseases and Clinical Electromyoneurography (UHC Zagreb, Croatia). Eight adults with genetically confirmed SOD1 p.Leu145Phe ALS received intrathecal tofersen according to the approved regimen. ALS Functional Rating Scale-Revised (ALSFRS-R) scores were recorded at each dosing visit, and longitudinal slopes were calculated using linear regression. Safety and tolerability were evaluated descriptively. Biomarker and formal respiratory measurements were not routinely available. Results: All patients exhibited lower limb-onset, predominantly lower motor neuron phenotypes consistent with a slow-progressing founder variant. Median age at symptom onset was 60 years, and median therapeutic delay was 48 months. Median on-treatment ALSFRS-R slope was -0.28 points/month (range +0.04 to -0.57). Two patients demonstrated stable trajectories, while the remainder showed gradual decline. These patterns fall within the slower range reported in heterogeneous real-world SOD1 cohorts and are consistent with the known natural history of this mutation. Tofersen was well tolerated, with no serious treatment-related adverse events. Conclusions: In this genetically homogeneous SOD1 p.Leu145Phe cohort, functional trajectories during tofersen therapy reflected the mutation's slow-progressing phenotype. These findings provide real-world clinical context but do not permit conclusions regarding treatment efficacy. Further mutation-specific studies incorporating prospective baseline assessment and biomarker monitoring are needed to clarify therapeutic impact.