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(,)-0-modified RNAs have been extensively studied to improve the nuclease resistance. However, the 2(,)-0-modified siRNA activities were often decreased by modification, since the bulky 2(,)-0-modifications inhibit to form a RNA-induced silencing complex (RISC). We developed novel prodrug-type 2(,)-0-methyldithiomethyl (MDTM) siRNA, which is converted into natural siRNA in an intracellular reducing environment. Prodrug-type 2(,)-0-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(,)-0-methyl (2(,)-0-Me) siRNAs. Furthermore, the resistances for nuclease digestion of siRNAs were actually enhanced by 2(,)-0-MDTM modifications. Our results indicate that 2(,)-0-MDTM modifications improve the stability of siRNA in serum and they are able to be introduced at any positions of siRNA.