Tofersen

Tofersen (BIIB067, marketed as Qalsody) is a 20-base 2'-O-methoxyethyl (MOE) gapmer antisense oligonucleotide that binds to and mediates RNase H-dependent degradation of superoxide dismutase 1 (SOD1) messenger RNA, reducing synthesis of both mutant and wild-type SOD1 protein in the central nervous system. Developed by Ionis Pharmaceuticals and licensed to Biogen, tofersen received accelerated approval from the United States Food and Drug Administration on April 25, 2023, for the treatment of amyotrophic lateral sclerosis (ALS) in adults with a confirmed SOD1 gene mutation, making it the first gene-targeted therapy approved for any form of ALS. Subsequent approvals followed from the European Medicines Agency (May 30, 2024), the Therapeutic Goods Administration of Australia (January 5, 2025), and Health Canada (March 3, 2025). SOD1 mutations, first identified in familial ALS by Rosen et al. in 1993, account for approximately 13 to 20 percent of familial ALS cases and 1 to 2 percent of sporadic ALS cases. Over 200 pathogenic SOD1 variants have been reported, producing a spectrum of disease phenotypes from rapidly progressive (p.A5V, median survival approximately 1 year from onset) to slowly progressive (p.D91A homozygous, median survival exceeding 10 years). The pathogenic mechanism is a toxic gain of function in which mutant SOD1 protein undergoes misfolding, aggregation, and formation of cytoplasmic inclusions in motor neurons and astrocytes, driving neurodegeneration through oxidative stress, mitochondrial dysfunction, endoplasmic reticulum stress, and impaired protein degradation. Tofersen addresses this mechanism directly by reducing the concentration of the causative toxic protein. The clinical development program comprises a Phase 1-2 ascending-dose trial (50 participants) establishing dose-dependent CSF SOD1 reduction of up to 36 percent at the 100 mg intrathecal dose; the Phase 3 VALOR trial (108 participants) that did not meet its primary efficacy endpoint of change in the ALS Functional Rating Scale-Revised (ALSFRS-R) at 28 weeks (difference 1.2 points, p = 0.97) but demonstrated robust reductions in CSF SOD1 protein (29 percent) and plasma neurofilament light chain (60 percent); and an open-label extension demonstrating that participants who initiated tofersen early had a statistically significant 3.5-point advantage on ALSFRS-R decline compared with those whose treatment was delayed (p = 0.03 at 52 weeks). Accelerated approval was based on the reduction in plasma neurofilament light chain as a surrogate endpoint reasonably likely to predict clinical benefit. The ongoing ATLAS trial (NCT04856982) evaluates tofersen initiation in presymptomatic SOD1 variant carriers using plasma neurofilament light chain as a biomarker trigger for randomization. Tofersen is administered intrathecally at a dose of 100 mg in 15 mL, with three loading doses at 14-day intervals followed by maintenance doses every 28 days. The estimated CSF half-life is approximately four weeks, and elimination occurs through exonuclease-mediated hydrolysis without cytochrome P450 involvement. The principal adverse events are procedure-related (pain, headache) and drug-related (CSF pleocytosis in 42 percent of treated participants, fatigue, arthralgia, myalgia). Serious neurologic adverse events including myelitis, radiculitis, papilledema with elevated intracranial pressure, and aseptic meningitis have been reported in approximately 7 percent of tofersen-treated participants. This monograph reviews the chemistry, development history, molecular mechanism, pharmacokinetics, preclinical and clinical evidence, sourcing and handling, adverse event profile, and a comparative assessment of five alternative ALS therapeutic agents against tofersen.