Early Intervention with Tofersen Sustains Functional Preservation and Survival in SOD1-ALS: A 3.5-Year Integrated Analysis
Highlights
- Early initiation of tofersen (100 mg intrathecal) led to a numerically slower decline in clinical function (ALSFRS-R), respiratory function (SVC), and muscle strength compared to delayed initiation.
- Tofersen treatment demonstrated a sustained reduction in neurofilament levels, a key biomarker of axonal injury and neurodegeneration.
- Long-term data suggest a survival benefit for patients treated with tofersen compared to the expected natural history of SOD1-associated Amyotrophic Lateral Sclerosis (ALS).
- The safety profile over 3.5 years remains consistent with previous reports, with most serious neurological adverse events being reversible.
Background: The Genetic Landscape of SOD1-ALS
Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder characterized by the progressive loss of motor neurons in the brain and spinal cord, typically leading to respiratory failure and death within three to five years of symptom onset. While the majority of ALS cases are sporadic, approximately 10% are familial. Pathogenic variants in the superoxide dismutase 1 (SOD1) gene account for about 2% of all ALS cases globally.
The pathophysiology of SOD1-ALS is primarily driven by a toxic gain-of-function of the mutant SOD1 protein, which leads to protein misfolding, oxidative stress, and mitochondrial dysfunction. Until recently, therapeutic options for ALS were limited to non-specific agents like riluzole and edaravone, which offer modest survival benefits but do not target the underlying genetic drivers. Tofersen represents a paradigm shift as the first and only approved antisense oligonucleotide (ASO) designed specifically to mediate the degradation of SOD1 messenger RNA (mRNA), thereby reducing the synthesis of the toxic SOD1 protein.
The VALOR Trial and OLE Study Design
The clinical development of tofersen culminated in the Phase 3 VALOR trial (NCT02623699) and its subsequent Open-Label Extension (OLE; NCT03070119). VALOR was a randomized, double-blind, placebo-controlled study involving 108 participants with SOD1-ALS across 32 sites in 10 countries. Participants were randomized in a 2:1 ratio to receive either tofersen 100 mg or a placebo administered intrathecally over 24 weeks.
Following the completion of the 28-week double-blind period, eligible participants could enroll in the OLE, where all patients received active tofersen treatment. This design allowed for a comparison between the “early-start” group (those who received tofersen from the beginning of VALOR) and the “delayed-start” group (those who transitioned from placebo to tofersen at the start of the OLE, approximately six months later). The integrated analysis presented here follows these cohorts for up to 148 weeks, with some participants reaching 3.5 years of total follow-up.
Key Findings: Efficacy Across Multi-Dimensional Endpoints
The integrated analysis focused on several critical efficacy domains, including biomarkers of neurodegeneration, clinical functionality, respiratory strength, and overall survival. The results underscore the importance of early therapeutic intervention.
Neurofilament Reduction and Biomarker Response
One of the most compelling findings from the tofersen program is the rapid and sustained reduction in plasma neurofilament light chain (NfL) levels. NfL is a marker of axonal damage; in SOD1-ALS, levels are typically highly elevated. In the VALOR trial, tofersen-treated patients saw a significant drop in NfL within weeks. The long-term data confirm that this reduction is maintained throughout the OLE. The delayed-start group achieved similar NfL reductions once they began tofersen, but they remained at a clinical disadvantage compared to the early-start group, suggesting that the axonal damage occurring during the six-month placebo period was not fully reversible.
Clinical Function and Respiratory Preservation
Functional decline was measured using the Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised (ALSFRS-R). Over the 148-week observation period, the early-start group showed a numerically smaller decline in ALSFRS-R scores compared to the delayed-start group (-9.9 points versus -13.5 points). This difference of 3.6 points is clinically meaningful, often representing the difference between independence and the need for assistance in activities of daily living.
Respiratory function, a primary determinant of mortality in ALS, was assessed via slow vital capacity (SVC). The early-start group experienced a decline of -13.8%, whereas the delayed-start group declined by -18.1%. This relative preservation of lung function aligns with the observed clinical stabilization in the early-intervention cohort.
Muscle Strength and Quality of Life
Muscle strength was quantified using a handheld dynamometry (HHD) megascore. While both groups experienced progressive weakness, the early-start group exhibited less severe decline (-0.38 vs -0.43 points). Furthermore, quality of life measures, including the ALSAQ-5 and EQ-5D-5L, favored the early-start group. Specifically, the ALSAQ-5 score increased (indicating worsening) by 17.0 points in the early-start group compared to 22.5 points in the delayed-start group.
Survival Outcomes
Survival analysis remains the gold standard in ALS research. The integrated data indicated that tofersen treatment prolonged survival relative to the expected natural history of the disease. While a direct comparison to a concurrent placebo group over 3.5 years is not possible due to ethical crossover designs, the divergence in survival curves between the early-start and delayed-start groups highlights that every month of delayed treatment carries a risk of irreversible motor neuron loss.
Safety and Tolerability Profile
The safety profile of tofersen over the long term was generally manageable. Most adverse events (AEs) were related to the disease progression of ALS or the procedural aspects of repeated lumbar punctures (e.g., procedural headache, back pain). However, serious neurological adverse events were reported, including myelitis, chemical meningitis, and increased intracranial pressure. Importantly, these events were rare, and in the cases where they occurred, they were largely reversible with appropriate medical management or temporary discontinuation of the drug. Very few participants discontinued the OLE due to drug-related AEs, suggesting a favorable benefit-risk ratio for this patient population.
Expert Commentary: Mechanistic Insights and Clinical Implications
The success of tofersen validates the “toxic gain-of-function” hypothesis in SOD1-ALS and proves that ASOs can effectively modulate disease pathways in the central nervous system. From a clinical perspective, the VALOR and OLE data reinforce the “time is motor neuron” mantra. The significant gap in outcomes between those who started treatment six months earlier versus those who started later emphasizes that once motor neurons are lost, they cannot be recovered.
Clinicians should prioritize early genetic testing for all patients presenting with ALS symptoms. Identifying a SOD1 mutation early allows for the initiation of tofersen before extensive axonal damage—reflected by peak NfL levels—occurs. Furthermore, the use of NfL as a surrogate biomarker has been pivotal. The FDA’s accelerated approval of tofersen was based largely on the reduction of plasma NfL, and these long-term results provide the clinical evidence required to confirm that biomarker changes do indeed translate into functional and survival benefits.
Limitations of the study include the small sample size, which is inherent to orphan diseases like SOD1-ALS, and the lack of a long-term randomized placebo control. However, the use of the OLE and comparison to natural history cohorts provides a robust framework for understanding the drug’s impact.
Conclusion
The final data from the VALOR trial and its Open-Label Extension provide definitive evidence that tofersen is a transformative therapy for SOD1-ALS. By targeting the root cause of the disease at the genetic level, tofersen not only slows the progression of functional and respiratory decline but also offers the hope of extended survival. For the medical community, these findings underscore the necessity of early diagnosis through genetic screening and the immediate initiation of targeted therapy to optimize long-term patient outcomes.
Funding and Trial Registration
The VALOR and OLE studies were funded by Biogen.
ClinicalTrials.gov Identifiers: VALOR (NCT02623699); OLE (NCT03070119).
References
1. Miller TM, Cudkowicz ME, Shaw PJ, et al. Long-Term Tofersen in SOD1 Amyotrophic Lateral Sclerosis. JAMA Neurol. 2026;83(2):115-125. doi:10.1001/jamaneurol.2025.4946.
2. Miller TM, Cudkowicz ME, Genge A, et al. Trial of Tofersen for SOD1-ALS. N Engl J Med. 2022;387(12):1099-1110. doi:10.1056/NEJMoa2204705.
3. Benatar M, Wuu J, Andersen PM, et al. Design of a Randomized, Phase 3 Trial of Tofersen in People with Presymptomatic SOD1 Mutations at Risk for ALS (ATLAS). Muscle Nerve. 2022;66(5):531-539. doi:10.1002/mus.27690.
