BACKGROUND: Antisense-mediated exon skipping is a putative treatment for 
Duchenne muscular dystrophy (DMD). Using antisense oligonucleotides (AONs), the 
disrupted DMD reading frame is restored, allowing generation of partially 
functional dystrophin and conversion of a severe Duchenne into a milder Becker 
muscular dystrophy phenotype. In vivo studies are mainly performed using 
2'-O-methyl phosphorothioate (2OMePS) or morpholino (PMO) AONs. These compounds 
were never directly compared.
METHODS: mdx and humanized (h)DMD mice were injected intramuscularly and 
intravenously with short versus long 2OMePS and PMO for mouse exon 23 and human 
exons 44, 45, 46 and 51.
RESULTS: Intramuscular injection showed that increasing the length of 2OMePS 
AONs enhanced skipping efficiencies of human exon 45, but decreased efficiency 
for mouse exon 23. Although PMO induced more mouse exon 23 skipping, PMO and 
2OMePS were more comparable for human exons. After intravenous administration, 
exon skipping and novel protein was shown in the heart with both chemistries. 
Furthermore, PMO showed lower intramuscular concentrations with higher exon 23 
skipping levels compared to 2OMePS, which may be due to sequestration in the 
extracellular matrix. Finally, two mismatches rendered 2OMePS but not PMO AONs 
nearly ineffective.
CONCLUSIONS: The results obtained in the present study indicate that increasing 
AON length improves skipping efficiency in some but not all cases. It is 
feasible to induce exon skipping and dystrophin restoration in the heart after 
injection of 2OMePS and unconjugated PMO. Furthermore, differences in efficiency 
between PMO and 2OMePS appear to be sequence and not chemistry dependent. 
Finally, the results indicate that PMOs may be less sequence specific than 
2OMePS.