Background: Approved therapies for Duchenne muscular dystrophy (DMD) use phosphorodiamidate morpholino oligomers (PMO) to induce exon skipping in the dystrophin pre-mRNA and enable translation of a shortened but functional dystrophin protein. Regretfully, PMOs display modest efficacy because of insufficient distribution to muscle. To overcome the limitations of current PMO-based DMD treatments, we developed the FORCETM platform, which harnesses the natural expression of transferrin receptor (TfR)1 on muscle cells for targeted delivery of oligonucleotide payloads with the goal of addressing the underlying cause of rare genetic muscle diseases. In previous work, we demonstrated that FORCE conjugates enhance PMO delivery to muscle, thereby achieving robust dystrophin expression and functional benefit in the BL10-mdx mouse model of DMD. Here, we leveraged D2-mdx mice to determine the therapeutic potential of the FORCE platform in muscle already damaged by fibrotic degeneration.
Objectives: FORCE-M23D is a mouse-specific FORCE conjugate engineered to deliver the M23D payload to muscle via TfR1-mediated uptake. M23D is an exon skipping PMO with demonstrated ability to restore the reading frame of the mutant Dmd RNA in mdx mice. D2-mdx mice are a severe model of DMD characterized by high levels of fibrosis. D2-mdx mice were administered monthly with FORCE-M23D or vehicle at 6 or 14 weeks of age. Dystrophin localization to the sarcolemma, muscle morphology, and fibrosis were assessed at 22 weeks of age.
Results: Repeat monthly dosing with FORCE-M23D achieved robust dystrophin restoration to the sarcolemma of cardiac and skeletal muscle and improved muscle morphology. Importantly, starting FORCE-M23D treatment at 6 weeks of age led to lower deposition of fibrotic tissue compared to initiation at 14 weeks of age.
Conclusions: We demonstrated that the FORCE platform improves the fibrotic phenotype of the D2-mdx mouse model of DMD. Our data suggest that earlier treatment with FORCE conjugates may lead to greater benefit.