Blood SOD1 Activity in ALS Patients Receiving Tofersen Treatment.

OBJECTIVE: The antisense oligonucleotide tofersen is the first disease-modifying drug for SOD1-related amyotrophic lateral sclerosis (ALS) and was approved because of its ability to reduce SOD1 protein and neurofilament levels. The effect of tofersen on SOD1 activity is unclear but of clinical relevance because homozygous SOD1 mutations, linked to reduced SOD1 activity, cause severe motor neuron impairment and tofersen-induced reduction could be deleterious. Therefore, monitoring of SOD1 activity is urgently needed. METHODS: SOD1 activity was analyzed in blood samples from a discovery (n = 120) and a validation cohort (n = 208), including controls, patients with sporadic ALS (sALS), C9orf72 mutation carriers (c9ALS), asymptomatic (SOD1-asym), and symptomatic SOD1 mutation carriers (SOD1-ALS). SOD1 activity was characterized in 18 patients receiving tofersen treatment. RESULTS: SOD1 activity was significantly lower in both SOD1-asym (median = 7.5 U/mg, interquartile range [IQR] = 7.0-9.0 U/mg) and SOD1-ALS (median = 7.9 U/mg, IQR = 7.1-9.1 U/mg) relative to controls (median = 9.2 U/mg, IQR = 8.9-10.0 U/mg, p < 0.0001), c9ALS (median = 9.4 U/mg, IQR = 8.9-10.2 U/mg, p < 0.0001) and sALS (median 9.5 U/mg, IQR 8.8-9.9 U/mg, p < 0.0001). SOD1 activity was significantly lower in individuals with deleterious SOD1 variants than in individuals with neutral variants. During tofersen treatment, neurofilament and SOD1 protein levels decreased, whereas SOD1 activity remained stable. INTERPRETATION: The data indicate that the positive tofersen treatment effect is independent of SOD1 activity. Lower SOD1 activity is specific to SOD1-ALS, already present in the asymptomatic phase and depends on the mutation type. Future studies should determine whether tofersen treatment affects cerebrospinal fluid (CSF) SOD1 activity. ANN NEUROL 2026.