Mesyl Phosphoramidate Oligonucleotides as Potential Splice-Switching Agents: Impact of Backbone Structure on Activity and Intracellular Localization

A series of 2'-deoxy and novel 2'-O-methyl and 2'-O-(2-methoxyethyl) (2'-MOE) oligonucleotides with internucleotide methanesulfonyl (mesyl, μ) or 1-butanesulfonyl (busyl, β) phosphoramidate groups has been synthesized for evaluation as potential splice-switching oligonucleotides. Evaluation of their splice-switching activity in spinal muscular atrophy patient-derived fibroblasts revealed no significant difference in splice-switching efficacy between 2'-MOE mesyl oligonucleotide and the corresponding phosphorothioate (nusinersen). Yet, a survival study with model neonatal mice has shown the antisense 2'-MOE mesyl oligonucleotide to be inferior to nusinersen at the highest dose of 40 mg/kg. A reason for their lower activity in vivo as ascertained by cellular uptake study by fluorescent confocal microscopy in HEK293 cell line could possibly be ascribed to compromised endosomal release and/or nuclear uptake of the 2'-OMe or 2'-MOE μ- and β-oligonucleotides compared to their phosphorothioate analog.