Overview

siRNA or small interfering RNA is a short double-stranded RNA molecule that silences a specific gene by degrading its matching messenger RNA (mRNA). In therapeutics and research, siRNA works through RNA interference (RNAi), a natural cellular process that blocks production of a disease-related or experimentally targeted protein.

Small (or short) interfering RNA (siRNA) is a double-stranded RNA molecule that affects gene expression by silencing genes. siRNA plays a major role in post-transcriptional gene silencing (PTGS) by binding to a homologous messenger RNA (mRNA) and triggering its degradation, thereby preventing translation into protein.

This mechanism, referred to as RNA interference (RNAi), has therapeutic applications for slowing or preventing disease development and progression. In addition to gene silencing, siRNAs are known to play a role in chromosome organization and antiviral defense.

What is RNA interference (RNAi)?

RNA interference (RNAi) is a natural gene-silencing process that cells use to regulate gene expression. In the siRNA pathway, a short double-stranded RNA guides the RNA-induced silencing complex (RISC) to a complementary mRNA, which is then cleaved and prevented from producing protein.

How does siRNA work?

siRNA works by entering the cell, loading into the RNA-induced silencing complex (RISC) and using its guide strand to find a matching messenger RNA (mRNA). RISC then cleaves the target mRNA, reducing expression of the encoded protein. This is why siRNA is widely used for targeted gene silencing in both research and medicine.

How are siRNA therapeutics optimized for stability and efficacy?

siRNA therapeutics are optimized through chemical modifications and delivery technologies that improve stability, reduce immune activation and increase target-cell uptake. Common strategies include modified nucleotides, GalNAc conjugation for liver targeting and lipid nanoparticles (LNPs) for protected systemic delivery.

Which delivery and conjugation strategies improve siRNA performance?

Small molecules can be conjugated to siRNAs to improve their pharmacological properties. Below are a few examples.

1. GalNAc: GalNAc (N-acetylgalactosamine) has been shown to enhance the cellular uptake and intracellular localization of siRNA, leading to increased efficacy in RNA interference-based therapies. The targeted delivery of GalNAc-siRNA conjugates to specific cells or tissues can minimize off-target effects and reduce the risk of toxicity.
2. PEGylation: Polyethylene glycol (PEG) is the most widely used small molecule for siRNA conjugation. PEGylation can increase the stability and circulation time of siRNA in the bloodstream, thereby reducing the required frequency of administration and increasing its therapeutic efficacy.
3. Cholesterol: Cholesterol can be conjugated to siRNA to form lipoplexes, improving the stability and uptake of siRNA into cells.
4. Antibodies:  Antibodies  specific to a target cell type can be conjugated to siRNA to increase the specificity and efficacy of RNAi-based therapies.
5. Peptides: Peptides, such as cell-penetrating peptides (CPPs) and tumor-homing peptides, can be conjugated to siRNA to enhance the cellular uptake and delivery of siRNA to specific tissues.
6. Lipid nanoparticles: Lipid nanoparticles can be used to encapsulate siRNA to improve its stability, increase its circulation time and enhance its delivery to target cells.

These small-molecule conjugates can significantly enhance the pharmacological properties of siRNA, making it a more promising therapeutic strategy for various diseases.

siRNA Applications

What is siRNA used for in research and medicine?

siRNA is used in research to study gene function, validate drug targets and model disease pathways and in medicine to silence disease-causing genes. The most advanced clinical applications today are siRNA therapeutics for liver-directed and cardiometabolic diseases, where targeted delivery is more established.

• Gene knockdown experiments: Introduction of siRNA molecules that target specific genes to reduce their expression and study the resulting  phenotypic  changes in cells or organisms. This technique is used to study the function of genes in many biological processes, such as cellular signaling pathways, disease development and cell differentiation.
• Functional genomics: Compares the phenotype of cells treated with different siRNAs to determine which genes are involved in specific cellular processes and pathways.

This approach can help identify new drug targets and the molecular mechanisms underlying complex biological processes.

• Disease modeling: Reduction of specific gene expression in cells or animal models allows the study of pathogenesis and the testing of new therapeutic strategies. Disease-specific screens using a siRNA library can identify the genes involved in the development of a particular disease.
• Toxicity testing: siRNA can be used to test the toxicity of small-molecule compounds and biologics in cells. By reducing the expression of genes involved in cellular stress responses, researchers can determine the impact of a potential drug on cellular health and viability.

What diseases can siRNA help treat?

siRNA can help treat diseases driven by harmful or overexpressed genes by reducing the production of the target protein at the mRNA level. Current and emerging areas include hereditary diseases, cardiometabolic disorders, liver disease, selected infectious diseases, oncology and inflammatory conditions.

1. Cancer: siRNA can be used to silence oncogenes that drive the growth and spread of cancer cells. By reducing the expression of these genes, siRNA can inhibit the growth and progression of cancer cells.
2. Viral infections: siRNA can be used to target specific viral genes and reduce their expression during viral infections.
3. Genetic disorders: siRNA can be used to treat genetic disorders, including inherited diseases, by targeting specific disease-causing genes and reducing their expression.

In addition to these diseases, siRNA is being explored as a potential therapeutic approach for other diseases, including neurodegenerative, cardiovascular and autoimmune diseases.

Beyond the examples above, siRNA is being applied in real-world settings across both discovery and clinical development. In early research, scientists use siRNA screens to identify gene function, validate drug targets and map signaling pathways involved in disease.

In translational programs, siRNA helps connect target biology to phenotype, providing evidence for whether suppressing a gene could produce a meaningful therapeutic effect. In medicine, approved siRNA drugs have already demonstrated that gene silencing can deliver durable benefit in patients, particularly for liver-associated conditions where targeted delivery is more mature.

Emerging areas of application include cardiometabolic disease, oncology, inflammatory disorders and selected infectious diseases. These opportunities are especially important for targets that are difficult to address with conventional small molecules or antibodies.

At the same time, the field is actively working to expand delivery beyond the liver so that siRNA can reach tissues such as tumors, the lung, the central nervous system and immune-relevant sites with greater precision.

What siRNA drugs are approved?

Approved siRNA drugs include patisiran, givosiran, lumasiran, inclisiran, vutrisiran and nedosiran. These siRNA therapeutics show that RNA interference can deliver clinically meaningful gene silencing, especially in liver-targeted and cardiometabolic indications.

Examples of approved siRNA therapeutics:

1. Patisiran is a siRNA-based drug that targets the transthyretin (TTR) gene, which is associated with hereditary transthyretin-mediated amyloidosis (hATTR), a rare genetic disease. Patisiran was approved by the US Food and Drug Administration (FDA) in 2018 and is the first siRNA therapeutic approved.
2. Inclisiran is a siRNA-based drug that targets proprotein convertase subtilisin/kexin type 9 (PCSK9), a protein involved in regulating cholesterol levels in the blood. Inclisiran was approved by the FDA in 2021 and is used to lower cholesterol levels in patients with cardiovascular disease.
3. Lumasiran is a siRNA-based drug that targets the hydroxyacid oxidase 1 (HAO1) gene, which is associated with primary hyperoxaluria type 1 (PH1), a rare genetic disease. Lumasiran was approved by the FDA in 2020 and is the first siRNA therapeutic approved for the treatment of PH1.

What are the limitations of siRNA therapeutics?

The main limitations of siRNA therapeutics are delivery, off-target effects, immune activation and manufacturing complexity. Although approved siRNA drugs have validated the modality, broader use depends on improving tissue-specific delivery beyond the liver and maintaining consistent safety, durability and scalability.

Delivery constraints: Efficient, tissue-specific delivery outside the liver remains difficult and is a major barrier to broader therapeutic use

• Off-target effects: Partial sequence matching can unintentionally suppress non-target genes, complicating safety and interpretation of results
• Immune activation: Some siRNA constructs or formulations can trigger innate immune responses if not carefully optimized
• Manufacturing and formulation complexity: Scalable production, reproducibility and quality control are essential for clinical translation
• Durability and dosing tradeoffs: Long-lasting silencing can be beneficial, but dose selection and retreatment strategies must be carefully managed·

Despite these limitations, the field continues to advance through improved chemical modifications, ligand-conjugate strategies, more refined nanoparticle platforms and more refined target selection. The next phase of siRNA innovation will likely depend on solving the challenge of extrahepatic delivery, minimizing unintended effects and demonstrating consistent benefit in more common and complex diseases.