In the last two decades, antisense oligonucleotides (AONs) that induce corrective exon skipping have matured as promising therapies aimed at tackling the dystrophin deficiency that underlies the severe and progressive muscle fiber degeneration in Duchenne muscular dystrophy (DMD) patients. Pioneering first generation exon 51 skipping AONs like drisapersen and eteplirsen have more recently been followed up by AONs for exons 53 and 45, with, to date, a total of four exon skipping AON drugs having reached (conditional) regulatory US Food and Drug Administration (FDA) approval for DMD. Nonetheless, considering the limited efficacy of these drugs, there is room for improvement. The aim of this study was to develop more efficient [2'-O-methyl-modified phosphorothioate (2'OMePS) RNA] AONs for DMD exon 51 skipping by implementing precision chemistry as well as identifying a more potent target binding site. More than a hundred AONs were screened in muscle cell cultures, followed by a selective comparison in the hDMD and hDMDdel52/mdx mouse models. Incorporation of 5-methylcytosine and position-specific locked nucleic acids in AONs targeting the drisapersen/eteplirsen binding site resulted in 15-fold higher exon 51 skipping levels compared to drisapersen in hDMDdel52/mdx mice. However, with similarly modified AONs targeting an alternative site in exon 51, 65-fold higher skipping levels were obtained, restoring dystrophin up to 30% of healthy control. Targeting both sites in exon 51 with a single AON further increased exon skipping (100-fold over drisapersen) and dystrophin (up to 40%) levels. These dystrophin levels allowed for normalization of creatine kinase (CK) and lactate dehydrogenase (LDH) levels, and improved motor function in hDMDdel52/mdx mice. As no major safety observation was obtained, the improved therapeutic index of these next generation AONs is encouraging for further (pre)clinical development.

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