A scientific approach to improve Morpholino-based Exon-Skipping therapy for Duchenne Muscular Dystrophy

In late 2016, a 30-nucleotide PMO for exon 51 skipping, called Eteplirsen was conditionally approved by the FDA. Its efficacy has remained debatable. Systemic delivery of Eteplirsen to DMD patients had shown a dose-dependent response but with high variability in dystrophin expression (Cirak et al., 2011). We investigated the influence of myofibre regeneration on exon skipping by treating dystrophin-null mdx mice with labelled-PMOs together with timed pulses of BrdU. This enabled us to define the stage of regeneration, relative to systemic drug delivery, that coincides with optimal PMO uptake. PMO accumulation was exclusive to inflamed regions where it entered inflammatory cells and newly forming myotubes. We concluded much of the variability in PMO-induced dystrophin expression reflects the favourable PMO uptake into inflamed and regenerating muscle regions. The uptake mechanism is based on the fusion of PMO-loaded myoblasts into the newly repairing segments of muscle fibres (Novak et al., 2017).

Next, we designed shorter 25-mer PMOs directed to the same Eteplirsen-targeted pre-mRNA region and compared their efficacies in vitro and in vivo in the mdx52 murine model. Our results showed that skipped-dystrophin induction was comparable between the 30-mer PMO sequence of Eteplirsen and one of the shorter PMOs, while the other 25-mer PMOs showed lower exon-skipping efficacies. Shorter PMOs would make higher doses economically feasible, and high dosing would result in better drug uptake into muscle, induce higher levels of dystrophin restoration in DMD muscle, and, increase the clinical efficacy (Akpulat et al., 2018).

Literature:

Cirak et al., 2011, The Lancet; Novak et al., 2017 Nature Communication; Akpulat et al., 2018 Molecular Therapy Nucleic Acids