A New Class of Medicine

For much of pharmaceutical history, medicines have been either small molecules — chemically synthesized compounds that interact with proteins — or biologics — large proteins like antibodies and hormones derived from living cells. RNA therapeutics represent a third, fundamentally different paradigm: medicines built from or targeting ribonucleic acid, the molecular intermediary between our DNA and the proteins that drive biology.

The mRNA vaccines developed during the COVID-19 pandemic introduced the general public to this concept, but RNA-based drugs had been in development for decades, and the platform now encompasses several distinct technology classes with broad therapeutic potential.

Messenger RNA (mRNA) Therapeutics

mRNA therapeutics work by delivering synthetic mRNA strands into cells. The cell's own ribosomes then read the mRNA and produce the protein it encodes. This allows doctors to temporarily instruct the body to make a specific protein — whether a viral antigen (as in vaccines), a missing enzyme, or a therapeutic antibody — without ever altering the patient's DNA.

Key advantages of mRNA medicines include:

  • Rapid design and manufacturing — sequences can be changed quickly to address new targets or variants
  • No risk of genomic integration (mRNA does not enter the cell nucleus)
  • Transient effect — the mRNA degrades naturally after a defined period
  • Broad applicability across infectious disease, cancer, rare genetic disorders, and more

Current applications beyond vaccines include mRNA therapies for personalized cancer neoantigen vaccines and protein replacement therapies for rare metabolic disorders.

Small Interfering RNA (siRNA)

While mRNA therapeutics add a protein-coding message, siRNA works in the opposite direction — silencing gene expression. siRNA molecules are short, double-stranded RNA sequences that are taken up by the cell's RNA interference (RNAi) machinery. They guide the RISC (RNA-Induced Silencing Complex) to a complementary mRNA target, which is then cleaved and degraded, preventing translation into protein.

This makes siRNA ideal for diseases driven by overexpression or gain-of-function mutations, where the goal is to reduce a pathological protein. Approved siRNA drugs are now available for conditions including hereditary transthyretin-mediated amyloidosis (hATTR), a serious progressive disease caused by misfolded protein deposits.

Antisense Oligonucleotides (ASOs)

ASOs are short, single-stranded synthetic nucleic acid sequences designed to bind to specific RNA targets. Depending on the design, they can:

  • Degrade the target mRNA (via RNase H recruitment)
  • Block translation of the mRNA into protein
  • Modify splicing of pre-mRNA to include or exclude specific exons

Splicing modulation is particularly powerful for certain genetic diseases. Nusinersen, an approved ASO therapy for spinal muscular atrophy (SMA), works by correcting abnormal splicing of the SMN2 gene — allowing production of functional survival motor neuron protein in patients who otherwise lack it.

Delivery: The Central Challenge

RNA molecules are inherently unstable in the body — nuclease enzymes in blood and tissues rapidly degrade them. Getting RNA therapeutics to the right cells in sufficient quantity is the field's central engineering problem.

Current delivery approaches include:

  • Lipid Nanoparticles (LNPs): The most clinically advanced system, used for mRNA vaccines and siRNA therapies. Lipid shells protect and deliver RNA to target cells, particularly liver hepatocytes.
  • GalNAc Conjugates: A sugar molecule (N-acetylgalactosamine) that specifically targets hepatocyte receptors, enabling subcutaneous delivery of siRNA and ASOs to the liver without a nanoparticle carrier.
  • Viral Vectors: Modified adeno-associated viruses can deliver RNA payloads, though concerns about immunogenicity limit use cases.

What's Coming Next

The RNA therapeutics pipeline is expanding rapidly into areas that were recently considered untreatable. Circular RNA, which resists degradation and can provide sustained protein expression, is entering early development. Self-amplifying RNA (saRNA) vaccines use the cell's own machinery to produce more copies of the therapeutic RNA, potentially allowing smaller doses. And combinatorial approaches pairing CRISPR with RNA delivery are enabling new forms of precision gene therapy.

The fundamental insight driving this field — that we can treat disease by directly controlling the flow of genetic information — represents one of the most consequential scientific shifts in modern medicine.