Mesyl phosphoramidate backbone modified antisense oligonucleotides targeting miR-21 with enhanced in vivo therapeutic potency

The design of modified oligonucleotides that combine in one molecule several therapeutically beneficial properties still poses a major challenge. Recently a new type of modified mesyl phosphoramidate (or μ-) oligonucleotide was described that demonstrates high affinity to RNA, exceptional nuclease resistance, efficient recruitment of RNase H, and potent inhibition of key carcinogenesis processes in vitro. Herein, using a xenograft mouse tumor model, it was demonstrated that microRNA miR-21–targeted μ-oligonucleotides administered in complex with folate-containing liposomes dramatically inhibit primary tumor growth via long-term down-regulation of miR-21 in tumors and increase in biosynthesis of miR-21–regulated tumor suppressor proteins. This antitumoral effect is superior to the effect of the corresponding phosphorothioate. Peritumoral administration of μ-oligonucleotide results in its rapid distribution and efficient accumulation in the tumor. Blood biochemistry and morphometric studies of internal organs revealed no pronounced toxicity of μ-oligonucleotides. This new oligonucleotide class provides a powerful tool for antisense technology.