Synthesis of modified double stranded RNAs containing duplex regions between amide-linked RNA and RNA at both ends and enhanced nuclease resistance.

Synthetic short-interfering RNA (siRNA) has been a powerful tool to control gene expression. Chemical modification of the 3'-overhang regions of siRNA with amide-linked RNAs improved the exonuclease resistance and was compatible with RNAi function in cultured cells. To improve endonuclease resistance of the siRNA having amide-linked RNA modification at the 3'-terminal regions, we designed and synthesized new modified double stranded RNAs (dsRNAs) the 3'-terminal regions of which were modified with amide-linked RNA moieties and formed duplex structures with the complementary unmodified RNA moieties. The modified dsRNAs showed remarkable increase of stability in cell culture medium containing 10% serum in comparison with the modified siRNAs having 3'-overhang regions of amide-linked RNAs. The result suggests that the resistance of siRNA against endonuclease as well as 3'-exonuclease in serum can be enhanced by the introduction of the structures of amide-linked RNA:unmodified RNA duplexes at both ends of siRNA.