siRNA

How has siRNA transformed gene silencing?

Small interfering RNA (siRNA) has unlocked a new era of precise, clinically validated gene silencing. The field reached a major milestone in 2018 with the FDA approval of Patisiran (Alnylam Pharmaceuticals), establishing lipid nanoparticles as safe and efficient carriers for hepatic RNA delivery. This was followed by the success of N-acetylgalactosamine (GalNAc)–siRNA conjugates, which rely on polymer-based architectures and receptor-mediated uptake in hepatocytes, reinforcing siRNA as a robust therapeutic platform.

What is siRNA and how does it mediate gene silencing?

siRNA overview

siRNA is a short, double-stranded RNA molecule, typically 20–25 nucleotides in length, with 2-nucleotide 3′ overhangs, that harness the cell’s natural RNA interference (RNAi) pathway.

RNAi pathway & siRNA function

Inside the cell, long double-stranded RNAs (dsRNAs) are processed by the ribonuclease Dicer into siRNAs, which are then loaded into the RNA-induced silencing complex (RISC). Within RISC, the passenger strand is released while the guide strand directs the complex to a complementary mRNA sequence through Watson-Crick base pairing. The target mRNA is subsequently cleaved by the Ago2 endoribonuclease, preventing mRNA translation into proteins. By exploiting this endogenous mechanism, siRNA enables highly specific and potent gene knockdown without altering the genome, offering a versatile platform to silence virtually any disease-relevant gene.

Where is siRNA applied?

The precision and potency of siRNA make it a valuable modality across both therapeutic and research applications:

Genetic and rare diseases: siRNA enables selective silencing of disease-causing genes, with strong clinical validation in liver-targeted indications.
Cancer therapy: siRNA can be used for targeted knockdown of oncogenes and pathways involved in tumor growth and drug resistance.
Functional genomics: In research settings, siRNA provides a rapid and reversible method for transient gene silencing, making it a powerful tool to study gene function and validate therapeutic targets.

How to overcome the challenges in siRNA therapeutics?

After intravenous administration, siRNA faces multiple biological barriers, including enzymatic degradation by nucleases, renal clearance, immune uptake, crossing the cell plasma membrane, and endosomal entrapment. Two main strategies have been developed to overcome these challenges.

Chemical modifications to enhance siRNA performance

The first is chemical modification of the siRNA molecule. Modifications to the ribose, phosphate backbone, or bases (e.g., phosphorothioate linkages, 2′-O-methyl substitutions) could improve siRNA stability by enhancing nuclease resistance and reducing immune recognition.

siRNA carriers to address delivery barriers

The second strategy involves the use of nanoparticle-based delivery systems, including lipid- and polymer-based nanocarriers, which shield siRNA from degradation, facilitate cellular internalization, and enable efficient delivery to target tissues. Lipid nanoparticles (LNPs), in particular, have proven highly effective in delivering siRNA, ensuring potent gene silencing.

How to optimize siRNA delivery into cells?

Free siRNA is negatively charged and highly susceptible to nuclease degradation, making efficient cellular delivery challenging. To address this, researchers have developed multiple delivery strategies, including physical methods (e.g., electroporation), chemical conjugation approaches (e.g., most notably N-acetylgalactosamine (GalNAc) conjugation), and non-viral nanocarriers like lipid- and polymer-based nanoparticles.

Lipid Nanoparticles (LNPs): The clinically proven siRNA delivery platform

With their modular design, scalable manufacturing, and established regulatory framework, LNPs have emerged as the preferred platform for RNA therapeutics. Their tunable composition allows different RNA cargos to be efficiently encapsulated, while standardized production protocols and quality control measures support reproducible performance and accelerate clinical translation. LNPs are compatible with both in vivo and ex vivo applications, enabling a wide range of therapeutic strategies.

The clinical success of LNP-mediated siRNA delivery was demonstrated by Patisiran (Alnylam Pharmaceuticals) in 2018, the first FDA-approved siRNA therapeutic, which treats polyneuropathy caused by hereditary transthyretin amyloidosis (hATTR) by silencing hepatic transthyretin expression, establishing siRNA as a validated therapeutic modality.

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