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Knocking down disease: a progress report on siRNA therapeutics

Key Points

  • The key bottlenecks for small RNA drugs that harness RNA interference (RNAi) for selective gene knockdown in vivo include their delivery across the plasma membrane and their release from endosomes into the cytosol.

  • Small interfering RNA (siRNA)-based drugs can now be delivered into the cytosol of hepatocytes to suppress gene expression in the liver.

  • Recent siRNA clinical trials show durable and potent gene silencing in the liver, with manageable toxicity for a handful of disease targets.

  • The most effective strategies for gene knockdown in the liver use second-generation lipid nanoparticles or GalNAc-conjugated siRNAs that are taken up by the asialoglycoprotein receptor, which is exclusively expressed by hepatocytes.

  • Achieving gene knockdown outside the liver is still clinically unproven. The most attractive strategies use topical administration of siRNAs to accessible tissue sites, such as the skin, eye or mucosa, or use siRNAs that are covalently linked to an RNA aptamer that binds with high affinity to a cell surface receptor selectively expressed on cells being targeted for gene knockdown.

  • Methods that are being developed to deliver siRNAs therapeutically may eventually prove to be useful for delivering other nucleic acid therapeutics, including antisense oligonucleotides, mRNAs for gene expression, or CRISPR–Cas9 (clustered regularly interspaced short palindromic repeats–CRISPR-associated 9) for gene editing.


Small interfering RNAs (siRNAs), which downregulate gene expression guided by sequence complementarity, can be used therapeutically to block the synthesis of disease-causing proteins. The main obstacle to siRNA drugs — their delivery into the target cell cytosol — has been overcome to allow suppression of liver gene expression. Here, we review the results of recent clinical trials of siRNA therapeutics, which show efficient and durable gene knockdown in the liver, with signs of promising clinical outcomes and little toxicity. We also discuss the barriers to more widespread applications that target tissues besides the liver and the most promising avenues to overcome them.

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Figure 1: Mechanism of gene knockdown by siRNAs.


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This work was supported by the Swedish Research Council (A.W.) and the US National Institutes of Health (NIH) grant CA139444 (to J.L.).

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Corresponding author

Correspondence to Judy Lieberman.

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Competing interests

J.L. is on the Scientific Advisory Board of Alnylam Pharmaceuticals. A.W. declares no competing interests.

PowerPoint slides


RNA interference

(RNAi). An endogenous gene silencing mechanism, present in virtually all eukaryotic cells, by which short double-stranded RNA molecules induce translational inhibition and/or degradation of mRNAs containing partially complementary sequences.

Gene knockdown

An experimental technique used to reduce gene expression using sequence-specific oligonucleotides, typically by RNA interference (RNAi) or antisense mechanisms.

RNA-induced silencing complex

(RISC). The catalytic effector complex of RNA interference (RNAi)-mediated gene silencing. The RISC is a multiprotein complex that incorporates one strand of a small interfering RNA (siRNA) or microRNA.


Oligonucleotides (DNA or RNA) selected to bind with high affinity to defined structures.


(miRNAs). Endogenous, ~21-nucleotide-long, imperfectly paired double-stranded RNA molecules present in both plants and animals that guide the silencing of a multitude of genes bearing partially complementary sequences.

Endosomal escape

The process of cytosolic entry of small interfering RNAs (siRNAs) from a vesicular compartment, following initial endocytosis of the siRNA (and delivery vehicle) into the target cell.

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Wittrup, A., Lieberman, J. Knocking down disease: a progress report on siRNA therapeutics. Nat Rev Genet 16, 543–552 (2015).

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