Effective dystrophin restoration by a novel muscle-homing peptide-morpholino conjugate in dystrophin-deficient mdx mice

Antisense oligonucleotide (AO) - mediated splice correction therapy for Duchenne muscular dystrophy (DMD) has shown huge promise from recent phase IIb clinical trials, however high doses and costs are required and targeted delivery can lower both of these. We have previously demonstrated the feasibility of targeted delivery of AOs by conjugating a chimeric peptide, consisting of a muscle-specific peptide (MSP) and a cell-penetrating peptide, to AOs in mdx mice. Although increased uptake in muscle was observed, the majority of peptide-AO conjugate was found in the liver. To search for more effective muscle-homing peptides, we carried out in vitro biopanning in myoblasts and identified a novel 12-mer peptide (M12) showing preferential binding to skeletal muscle compared to the liver. When conjugated to morpholino oligomers (PMOs), approximately 25% of normal level of dystrophin expression was achieved in body-wide skeletal muscles in mdx mice with significant recovery in grip strength, whereas less than 2% in corresponding tissues treated with either MSP-PMO or unmodified PMO under identical conditions. Our data provide evidences for the first time that a muscle-homing peptide alone can enhance AO delivery to muscle without appreciable toxicity at 75 mg/kg, suggesting M12-PMO can be an alternative option to current AOs.