Introduction
Amyotrophic lateral sclerosis (ALS) remains a devastating neurodegenerative disorder, with limited disease-modifying therapies available. Among genetic subtypes, mutations in the superoxide dismutase 1 (SOD1) gene represent a significant cause of inherited ALS. Tofersen, an antisense oligonucleotide (ASO), targets SOD1 mRNA to reduce its expression, offering a precision medicine avenue for SOD1-ALS. While clinical trials have demonstrated tofersen’s safety and effects on cerebrospinal fluid biomarkers, direct evidence of its central nervous system (CNS) distribution and resultant molecular effects in human motor tissue has been lacking. This landmark autopsy tissue study provides critical insights into tofersen’s CNS penetrance and impact on SOD1 protein levels post intrathecal administration in individuals who participated in tofersen trials or expanded access programs.
Clinical Context and Unmet Need
ALS affects motor neurons critically, leading to progressive paralysis and death, usually within a few years of symptom onset. SOD1 mutations account for approximately 10-20% of familial ALS cases. Prior to the development of ASOs like tofersen, no treatments addressed the underlying genetic pathology. Given the challenges of delivering therapeutics efficiently across the blood-brain barrier and achieving sufficient motor neuron target engagement, confirming tofersen’s tissue distribution and SOD1 suppression in human CNS tissues is essential to validate its mechanism and optimize clinical use.
Study Design and Methods
This cross-sectional case series involved autopsy tissue from eight individuals with genetically confirmed SOD1-ALS who had received multiple intrathecal doses of tofersen (20 to 100 mg), enrolled in NIH-registered clinical trials or the Expanded Access Program between 2018 and 2026. Their families consented to postmortem tissue donation. Autopsies were performed at three prominent U.S. academic medical centers, incorporating comprehensive neuropathological evaluations. Tofersen concentrations in spinal cord and motor cortex tissues were quantified using hybridization ELISA. SOD1 mRNA and protein levels were assessed comparatively against a cohort of tofersen-naive SOD1-ALS controls using quantitative RT-PCR and ELISA assays. Histological analyses included immunohistochemistry and in situ hybridization to localize tofersen presence and SOD1 transcript abundance, alongside investigations of pathological SOD1 aggregates and immune cell infiltration.
Key Findings
Tissue concentration data demonstrated a strong correlation between tofersen levels in motor cortex and spinal cord with expected pharmacokinetic models based on individual dose histories, confirming reliable CNS penetration following intrathecal administration. Among three patients with recent dosing, SOD1 mRNA and protein reductions in lumbar spinal cord tissues ranged markedly from 45% to 84%. Remarkably, these reductions persisted despite missed final doses before death, denoting durable pharmacodynamic effects. Residual somatic motor neurons showed direct evidence of tofersen uptake and diminished SOD1 mRNA signals. Both treated and untreated SOD1-ALS tissues revealed misfolded SOD1 protein inclusions, consistent with disease pathology, indicating that tofersen reduces but does not completely eliminate pathogenic aggregates. Importantly, localized meningeal and perivascular lymphocytic responses were observed in five recently dosed individuals, absent in those whose last dosing had occurred remotely, suggesting transient immunologic effects potentially related to repeated intrathecal dosing.
Interpretation and Biological Plausibility
These autopsy findings validate preclinical and cerebrospinal fluid biomarker data, confirming that therapeutic intrathecal doses of tofersen achieve biologically meaningful CNS distribution and robust suppression of pathogenic SOD1 expression in critical motor neuron populations. The observed SOD1 protein reduction supports the molecular mechanism underlying tofersen’s disease-modifying potential. Persistent misfolded protein inclusions alongside low SOD1 mRNA levels imply residual pathology and the complex dynamics of protein aggregation and clearance. The transient lymphocytic infiltrates might reflect pharmacologic immune modulation or response to foreign nucleic acids, warranting further investigation into long-term safety.
Strengths and Limitations
This study leverages rare autopsy materials from well-characterized clinical trial participants, providing direct tissue evidence impossible to obtain otherwise. The robust correlation with dosing history and preclinical models enhances confidence in the ASO’s CNS kinetics. However, the modest sample size and heterogeneity in dosing intervals before death limit broad generalizability. Postmortem interval, patient variability, and disease stage might influence molecular measurements. Moreover, absence of longitudinal in vivo imaging or functional correlation limits integration with clinical outcomes.
Clinical and Research Implications
By elucidating tofersen’s CNS distribution and confirming its target engagement at the molecular level in human motor system tissues, this study strengthens the therapeutic rationale for its use in SOD1-ALS and informs dosing strategies to maximize efficacy while monitoring safety. The observations of immune responses could guide monitoring protocols in clinical care. Future research should explore longitudinal clinical-pathological correlations, refine ASO delivery platforms, and investigate combinatorial strategies addressing residual protein aggregates.
Conclusion
The present autopsy tissue analysis from tofersen-treated SOD1-ALS patients affirms precise CNS delivery and strong SOD1 suppression within relevant somatic motor neurons. This translational evidence substantiates antisense oligonucleotide therapy as a viable disease-modifying approach for genetic ALS. Continued integration of clinical trial data with neuropathological findings will be critical to optimize patient outcomes and expand therapeutic frontiers in neurodegeneration.
Funding and Trial Registration
The study was supported by institutional research funds and aligned with clinical trials registered under NCT02623699 and NCT03070119, as well as the Expanded Access Program.
References
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