Antisense and RNA-Based Mechanisms

## Introduction

RNA-based therapeutics intervene at the genetic information level, modulating gene expression before proteins are synthesized. This approach enables targeting of previously undruggable proteins and addresses genetic diseases at their root cause. The field accelerated dramatically after the success of COVID-19 mRNA vaccines demonstrated the platform's clinical scalability.

## Antisense Oligonucleotides (ASOs)

ASOs are short (15–25 nucleotide) single-stranded synthetic DNA or RNA molecules that bind complementary mRNA through Watson-Crick base pairing. They act through two distinct mechanisms:

**RNase H-dependent degradation**: DNA-like ASOs (gapmer design) form DNA:RNA duplexes that are recognized and cleaved by endogenous RNase H1, destroying the target mRNA and reducing protein production. Inotersen degrades transthyretin (TTR) mRNA for hereditary ATTR amyloidosis.

**Splice-switching (steric blocking)**: Fully modified ASOs sterically block splice sites or regulatory elements on pre-mRNA without recruiting RNase H, redirecting splicing. Nusinersen forces inclusion of exon 7 in SMN2 pre-mRNA, producing functional survival motor neuron protein for spinal muscular atrophy—a transformative therapy for an otherwise fatal disease.

## Small Interfering RNA (siRNA)

siRNAs are double-stranded RNA molecules (~21 nucleotides) that harness the endogenous RNA interference (RNAi) pathway. After cellular uptake, the guide (antisense) strand loads into the RISC (RNA-induced silencing complex), which scans cytoplasmic mRNAs for complementary sequences and cleaves them catalytically. One RISC complex can destroy hundreds of mRNA copies, providing amplified silencing.

**Patisiran** was the first FDA-approved siRNA (2018), delivered via lipid nanoparticles to silence hepatic TTR. **Inclisiran** silences PCSK9 mRNA in hepatocytes via GalNAc conjugation, lowering LDL cholesterol with twice-yearly subcutaneous dosing.

## mRNA Therapeutics

Synthetic mRNA directs ribosomes to produce therapeutic proteins transiently. Lipid nanoparticle (LNP) delivery protects mRNA from extracellular nucleases and mediates endosomal escape. N1-methylpseudouridine modification reduces innate immune activation (TLR recognition) while maintaining translational efficiency.

## Key Chemical Modifications

- **Phosphorothioate backbone**: Resists nuclease degradation and enhances plasma protein binding
- **2'-O-methyl / 2'-MOE sugar**: Increase binding affinity and metabolic stability
- **GalNAc conjugation**: Targets asialoglycoprotein receptor on hepatocytes for liver-directed delivery

## Key Takeaways

- ASOs silence genes via RNase H cleavage or redirect pre-mRNA splicing
- siRNA leverages the RISC complex for catalytic, amplified mRNA destruction
- Chemical modifications and GalNAc conjugation solve the stability and delivery challenges
- RNA therapeutics unlock previously undruggable protein targets