Effective gene silencing activity of prodrug-type 2′-O-methyldithiomethyl siRNA compared with non-prodrug-type 2′-O-methyl siRNA

Abstract Small interfering RNAs (siRNAs) are an active agent to induce gene silencing and they have been studied for becoming a biological and therapeutic tool. Various 2′- O -modified RNAs have been extensively studied to improve the nuclease resistance. However, the 2′- O -modified siRNA activities were often decreased by modification, since the bulky 2′- O -modifications inhibit to form a RNA-induced silencing complex (RISC). We developed novel prodrug-type 2′- O -methyldithiomethyl (MDTM) siRNA, which is converted into natural siRNA in an intracellular reducing environment. Prodrug-type 2′- O -MDTM siRNAs modified at the 5′-end side including 5′-end nucleotide and the seed region of the antisense strand exhibited much stronger gene silencing effect than non-prodrug-type 2′- O -methyl (2′- O -Me) siRNAs. Furthermore, the resistances for nuclease digestion of siRNAs were actually enhanced by 2′- O -MDTM modifications. Our results indicate that 2′- O -MDTM modifications improve the stability of siRNA in serum and they are able to be introduced at any positions of siRNA.