Chemical Modification Strategies for Therapeutic Oligonucleotides: Mechanism Compatibility, Design Trade-Offs, and Translational Barriers.

Oligonucleotide therapeutics represent an expanding class of medicines that can regulate gene expression, RNA processing, protein translation, immune signaling, and biomolecular recognition through sequence-specific or structure-dependent mechanisms. Despite clinical progress, their application remains constrained by nuclease degradation, rapid clearance, inefficient tissue and cellular delivery, endosomal sequestration, off-target activity, immune recognition, and mechanism-specific requirements for target engagement. Chemical modification is central to oligonucleotide therapeutic development because it can mitigate some of these limitations while influencing target affinity, protein binding, pharmacokinetics, and intracellular activity. This review examines chemical modification strategies to improve the biological stability and functional performance of therapeutic oligonucleotides and is organized around major classes of chemical modification, including phosphate and backbone-linkage modifications, sugar and conformational modifications, backbone-replacement analogs, and conjugation-based approaches. Rather than presenting these chemistries as uniformly beneficial, this review emphasizes that the same modification can be enabling in one therapeutic mechanism and disruptive in another, so its value cannot be judged apart from the modality and molecular architecture in which it is placed. Clinically successful oligonucleotide designs are likely to rely on combinations of chemical features, including modified backbones, modified sugars, stereochemical control, terminal stabilization, and ligand- or formulation-based delivery strategies. Understanding how these features interact is essential to develop more predictable and mechanism-appropriate oligonucleotide therapeutics.