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

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.