Antisense Oligonucleotides (ASOs)

Antisense oligonucleotides (ASOs) are short (usually 15–25 nucleotides, nt), single-stranded synthetic oligomers of nitrogenous bases that mimic DNA or RNA molecules. They can recognize specific messenger RNA (mRNA) molecules in a sequence-dependent manner. Their mechanism of action is by either steric blockage, interfering with the binding of other molecules to the target RNA, or by inducing target RNA degradation. The latter is usually the case for DNA-based ASOs (i.e., deoxynucleotide-based), which can form DNA/RNA hybrids. These hybrids are recognized by the ribonuclease RNase H1, which cleaves the target mRNA. Consequently, DNA-based ASOs are useful for downregulating the expression of a specific gene. Alternatively, ASOs can be used to activate gene expression. This is achieved by steric-only RNA-based ASOs, which can alter the mRNA maturation process (i.e., splicing), provoking the inclusion or the exclusion of a specific exon. Since single-stranded nucleic acids are prone to degradation by both extracellular and intracellular nucleases, biotherapeutic ASOs are usually heavily modified to increase their half-life in plasma, tissues, or both. This chapter will introduce the most common types of chemical modifications in currently clinically approved ASOs. Additionally, we will explore the interplay between chemical modifications and the pharmacokinetic and pharmacodynamic properties of different ASOs. Since 2016, the number of FDA-approved therapeutic ASOs has increased substantially. Some ASOs have proven to be extremely effective in clinical trials, while others have shown only limited efficacy. Adverse reactions will also be described, along with solutions derived from basic research on chemical modifications that can improve the therapeutic index of an ASO. The chapter includes several representative case studies, where all these factors are discussed.