Tofersen in SOD1-associated amyotrophic lateral sclerosis: From molecular mechanisms to regulatory milestones

Amyotrophic Lateral Sclerosis (ALS) is a progressive and ultimately fatal neurodegenerative disorder characterized by degeneration of upper and lower motor neurons. Mutations in the superoxide dismutase 1 (SOD1) gene account for approximately 2% of ALS cases and are associated with toxic protein misfolding and aggregation. Tofersen is an antisense oligonucleotide therapy designed to reduce the synthesis of mutant SOD1 protein through targeted mRNA degradation. While this strategy represents a gene-specific therapeutic approach for a subset of ALS patients, evidence regarding its efficacy, effectiveness and long-term outcomes continues to be evaluated in clinical trials and post-marketing studies.

First, to describe the molecular mechanisms underlying SOD1-associated ALS and second, to analyze the therapeutic development, clinical outcomes, and regulatory evolution of tofersen.

A narrative review was conducted in PubMed on preclinical and clinical studies published from 2016 through late 2025, complemented by an analysis of public registries and regulatory documentation. Clinical trials were identified through ClinicalTrials.gov and the Clinical Trials Information System (CTIS), and official reports from the Food and Drug Administration (FDA) and the European Medicines Agency (EMA) were reviewed to contextualize their development and regulatory evaluation.

Fifty-three publications were identified, of which 20 met predefined inclusion criteria after screening and full-text review. Preclinical studies showed reduced mutant SOD1 expression and prolonged survival in transgenic models. Phase I-II trials demonstrated safety, favorable pharmacokinetics, and dose-dependent reductions in SOD1 in the cerebrospinal fluid and plasma neurofilament light chain (NfL) levels. Although the phase III VALOR trial did not meet the primary ALSFRS-R endpoint (a validated questionnaire-based functional rating scale-revised for determining ALS disease progression) at 28 weeks, significant reductions in the surrogate biomarker NfL indicated target engagement and supported accelerated regulatory approval. Extension data suggested potential clinical benefit with early treatment. Ongoing studies, including ATLAS in presymptomatic carriers, and real-world European data support continued evaluation, alongside accelerated regulatory approvals by FDA and EMA.

Tofersen marks a paradigm shift in ALS management, establishing the foundation for precision medicine in neurodegenerative diseases. Its ongoing evaluation in the ATLAS trial will determine whether early intervention can prevent or delay disease onset in presymptomatic SOD1 mutation carriers.