Abstract
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.
Highlights
Small interfering RNA-based therapeutics are revolutionizing human medicine, for liver indications
Our findings demonstrate that improved extrahepatic efficacy of Small interfering RNA (siRNA) requires combinatorial optimization of conjugate identity, siRNA chemical structure, PS content and linker chemistry
Conjugation of oligonucleotides to a variety of chemical entities allows for modulation of bioavailability, tissues exposure and, in some cases, cell-type specific delivery
Summary
Small interfering RNA (siRNA)-based therapeutics are revolutionizing human medicine, for liver indications. The effect of chemical structure on siRNA distribution and efficacy in extrahepatic tissues has never been systematically evaluated; and is not fully understood. We methodically evaluated how altering chemical structure, PS content and linker chemistry of conjugated siRNAs affects extrahepatic tissue distribution and activity in vivo (Figure 1). To this end, we synthesized and delivered a panel of diverse DCA-conjugated siRNAs with asymmetric (5-nucleotide (nt) overhang), conventional (2nt overhang) or blunt (no overhang) ends, varying numbers of PS modifications, and differing linker chemistries to mice. Our findings demonstrate that improved extrahepatic efficacy of siRNAs requires combinatorial optimization of conjugate identity, siRNA chemical structure, PS content and linker chemistry
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