Development of Antisense Oligonucleotide Gapmers for Amyotrophic Lateral Sclerosis.

Advances in genetics and RNA biology have transformed the therapeutic landscape for neurodegenerative disease. Antisense oligonucleotide (ASO) therapies-particularly RNase H-activating "gapmers"-now enable sequence-specific suppression of pathogenic transcripts in the central nervous system. In amyotrophic lateral sclerosis (ALS), tofersen, a 2'-MOE-modified phosphorothioate ASO targeting SOD1 mRNA, has entered clinical use. By promoting RNase H-mediated degradation of SOD1 transcripts after intrathecal delivery, tofersen lowers SOD1 protein in cerebrospinal fluid and plasma neurofilament light chain, demonstrating robust target engagement and neurodegeneration biomarker improvement. Although pivotal trials did not meet primary functional endpoints (ALSFRS-R) at 28 weeks, longer-term and real-world observations suggest potential clinical benefit with earlier initiation, a hypothesis now being tested in presymptomatic SOD1 variant carriers. This chapter reviews the development of gapmer ASOs for ALS with a focus on tofersen-covering chemistry and mechanism, preclinical validation, clinical efficacy and safety, biomarker readouts, and ongoing trials-while outlining key challenges (CNS delivery, adverse event monitoring, endpoint sensitivity) and future directions, including earlier intervention guided by biomarkers and extension to additional genetic forms of ALS.