2′-O,4′-C-ethylene-bridged nucleic acids (ENA): highly nuclease-resistant and thermodynamically stable oligonucleotides for antisense drug

To develop antisense oligonucleotides, novel nucleosides, 2′-O,4′-C-ethylene nucleosides and their corresponding phosphoramidites, were synthesized as building blocks. The 1H NMR analysis showed that the 2′-O,4′-C-ethylene linkage of these nucleosides restricts the sugar puckering to the N-conformation as well as the linkage of 2′-O,4′-C-methylene nucleosides which are known as bridged nucleic acids (BNA) or locked nucleic acids (LNA). The ethylene-bridged nucleic acids (ENA) showed a high binding affinity for the complementary RNA strand (ΔTm=+5.2 °C/modification) and were more nuclease-resistant than natural DNA and BNA/LNA. These results indicate that ENA have better properties as antisense oligonucleotides than BNA/LNA.