Conjugated Antisense Oligonucleotides for Skipping of Duchenne Muscular Dystrophy Exon 53: A Cautionary Study

Exon skipping antisense oligonucleotides (AONs) have been extensively studied as a promising method of treating Duchenne muscular dystrophy (DMD), yet the clinical efficacy of the conditionally approved AONs still remains low. Using phosphorothioated locked nucleic acid/2'-fluoro-RNA AONs, we aimed to increase AON efficiency by employing skeletal muscle-targeting conjugate molecules, cholesterol, and docosanoic acid to improve the biodistribution of the therapeutic. While conjugate molecules were able to induce high levels of skipping in an <i>in vitro</i> model, <i>in vivo</i> studies in the hDMDdel52/<i>mdx</i> mouse model caused adverse symptomatic and systemic immune reactions, up to and including death, with little to no appreciable increase in exon skipping. Our study cautions against using these AON conjugates in an animal model due to severe toxicity.