Background: Duchenne muscular dystrophy (DMD) is caused by DMD frameshift mutations leading to a lack of dystrophin protein. Phosphorodiamidate morpholino oligomers (PMOs) restore the DMD reading frame, allowing for production of an internally truncated but functional dystrophin protein. Peptide-PMOs (PPMOs) are a next-generation platform in which a cell-penetrating peptide is conjugated to the PMO backbone to increase tissue penetration, exon skipping, and dystrophin production with less frequent dosing. Biological efficacy of SRP-5051, an investigational PPMO designed to target exon 51 skipping, was investigated in preclinical models.
Methods: Rapid, high-content, cell-based assays were developed using gymnotic delivery of PPMOs in DMD patient-derived myotubes, with the aim of taking a platform approach to quantifying PPMO pharmacodynamics and biological potency. In vivo systems included a disease-relevant, humanized DMD mouse model, hDMD del52/mdx, that contains a DMD transgene with exon 52 deleted on an mdx mouse background (lacking endogenous dystrophin), and a primate model using healthy male cynomolgus monkeys.
Results: Efficacy, measured by exon skipping and dystrophin production, was demonstrated in DMD patient myotubes in vitro at cellular SRP-5051 concentrations of 100−1000 nM. In hDMD del52/mdx mice, a single SRP-5051 injection resulted in dose-dependent increases in exon skipping and human dystrophin protein in skeletal muscle and heart. Repeated monthly dosing improved pharmacodynamic effects, with dystrophin protein accumulation through 20 weeks (5 doses); grip strength improvement was sustained through 20 weeks. In primates, a single SRP-5051 infusion resulted in dose-dependent increase in exon skipping and accumulation of drug in muscle for up to 28 days. Repeated monthly dosing over 12 weeks showed that exon skipping increased with each infusion, and SRP-5051 appeared to be well tolerated.
Conclusion: SRP-5051 showed dose-dependent efficacy in preclinical models. These data justify the monthly SRP-5051 dosing regimen used in ongoing clinical studies, and support further clinical investigation of this PPMO.