5-Vinylphosphonate improves tissue accumulation and efficacy of conjugated siRNAs in vivo

Abstract

5-Vinylphosphonate modification of siRNAs protects them from phosphatases, and improves silencing activity. Here, we show that 5-vinylphosphonate confers novel properties to siRNAs. Specifically, 5-vinylphosphonate (i) increases siRNA accumulation in tissues, (ii) extends duration of silencing in multiple organs and (iii) protects siRNAs from 5-to-3 exonucleases. Delivery of conjugated siRNAs requires extensive chemical modifications to achieve stability in vivo. Because chemically modified siRNAs are poor substrates for phosphorylation by kinases, and 5-phosphate is required for loading into RNA-induced silencing complex, the synthetic addition of a 5-phosphate on a fully modified siRNA guide strand is expected to be beneficial. Here, we show that synthetic phosphorylation of fully modified cholesterol-conjugated siRNAs increases their potency and efficacy in vitro, but when delivered systemically to mice, the 5-phosphate is removed within 2 hours. The 5-phosphate mimic 5-(E)-vinylphosphonate stabilizes the 5 end of the guide strand by protecting it from phosphatases and 5-to-3 exonucleases. The improved stability increases guide strand accumulation and retention in tissues, which significantly enhances the efficacy of cholesterol-conjugated siRNAs and the duration of silencing in vivo. Moreover, we show that 5-(E)-vinylphosphonate stabilizes 5 phosphate, thereby enabling systemic delivery to and silencing in kidney and heart.