UMMS Affiliation

RNA Therapeutics Institute; Program in Molecular Medicine; Graduate School of Biomedical Sciences

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Document Type



Biochemistry | Chemistry | Digestive System Diseases | Molecular Biology | Nucleic Acids, Nucleotides, and Nucleosides | Structural Biology


Small interfering RNAs (siRNAs) have revolutionized the treatment of liver diseases. However, robust siRNA delivery to other tissues represents a major technological need. Conjugating lipids (e.g. docosanoic acid, DCA) to siRNA supports extrahepatic delivery, but tissue accumulation and gene silencing efficacy are lower than that achieved in liver by clinical-stage compounds. The chemical structure of conjugated siRNA may significantly impact invivo efficacy, particularly in tissues with lower compound accumulation. Here, we report the first systematic evaluation of the impact of siRNA scaffold-i.e. structure, phosphorothioate (PS) content, linker composition-on DCA-conjugated siRNA delivery and efficacy in vivo. We found that structural asymmetry (e.g. 5- or 2-nt overhang) has no impact on accumulation, but is a principal factor for enhancing activity in extrahepatic tissues. Similarly, linker chemistry (cleavable versus stable) altered activity, but not accumulation. In contrast, increasing PS content enhanced accumulation of asymmetric compounds, but negatively impacted efficacy. Our findings suggest that siRNA tissue accumulation does not fully define efficacy, and that the impact of siRNA chemical structure on activity is driven by intracellular re-distribution and endosomal escape. Fine-tuning siRNA chemical structure for optimal extrahepatic efficacy is a critical next step for the progression of therapeutic RNAi applications beyond liver.


Small interfering RNAs, siRNAs, liver diseases, delivery, chemical structure

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© The Author(s) 2020. Published by Oxford University Press on behalf of Nucleic Acids Research. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

DOI of Published Version



Biscans A, Caiazzi J, Davis S, McHugh N, Sousa J, Khvorova A. The chemical structure and phosphorothioate content of hydrophobically modified siRNAs impact extrahepatic distribution and efficacy. Nucleic Acids Res. 2020 Aug 20;48(14):7665-7680. doi: 10.1093/nar/gkaa595. PMID: 32672813; PMCID: PMC7430635. Link to article on publisher's site

Journal/Book/Conference Title

Nucleic acids research

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Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.