Nitya Kodali

Background: Duchenne Muscular Dystrophy (DMD) is a severe, muscle-wasting X-linked recessive disease.¹ A lack of functional dystrophin in this disease results in muscle inflammation and necrosis, with eventual replacement by connective and adipose tissue.⁴ Most patients, even with treatment, pass away around age 30 due to cardiac and respiratory failure.⁴ As a way to lessen the DMD severity, some strategies like Antisense Oligonucleotide, or ASO therapy, focus on skipping the out-of-frame exon.¹ These “splice switching“ ASOs bind to pre-mRNA that is complementary, and thus selectively skip the problematic dmd exon, which results in a truncated form of dystrophin. This results in a milder phenotype and less severe dystrophinopathy, similar to Becker Muscular Dystrophy. Currently, there are 4 FDA-approved ASO drugs that are all phosphorodiamidate morpholino oligomers (PMOs).² Even though PMOs are very promising, issues include a lack of adequate distribution to the required tissues, degradation, and challenges in intracellular trafficking.² A way to address some of these challenges comes in the form of CPPs, which when conjugated to PMOs (PPMOs), have the ability to transport them across cell membranes.²

Objective: In this narrative review, we explore the potential usage of PPMO therapy to treat DMD.

 Methods: An online search was conducted in the PubMed database using the search terms “Cell-Penetrating Peptide”, “PMO”, and “Duchenne Muscular Dystrophy” with a timeframe of 2019 – 2026 to find primary research studies.

Results: A central study compared a PPMO, RC-1001, with the naked PMO, AVI-4225, in mdx mice, a mice model with a point mutation in the dmd gene. Their results showed significantly improved exon skipping, dystrophin levels, and muscle function with PPMO therapy.² A separate study utilized DG9, a CPP that has multiple entry pathways, conjugated to a PMO and compared it to the naked PMO using mdx mice. Results again showed significantly improved exon skipping and protein restoration, as well as improved cardiac function.⁵ Respiratory effects of RC-1001 therapy were also evaluated in mdx mice and showed dose-dependent improvement in function. A post-mortem study on the mice demonstrated significant exon skipping and dystrophin restoration as well as decreased inflammation and fibrosis in respiratory muscles.¹

Conclusion: PPMO therapy not only showed improved exon skipping, dystrophin protein restoration, and functional improvement, but also demonstrated a potential reduction in dosing frequency compared to the current weekly dosing schedule.² Further research could explore using different forms of CPPs in studies examining PPMO therapy. One study did exactly this and showed that D-form CPPs show nearly identical delivery of PMO therapy. Additionally, PMO-L-CPPs were also shown to be far less stable in human serum compared to PMO-D-CPPs.³ Another future step that could further improve PPMO therapy includes combining them with different technologies. This was shown in a study in which PPMO therapy was conjugated to a muscle homing peptide. Although there was no evidence of improved exon-skipping or dystrophin levels with CP10 conjugation, there was improved tissue concentration of PMO, showing synergistic potential.⁴

Works Cited:

1. Biswas DD, Rodriguez MYH, El Haddad L, et al. PPMO-based exon skipping therapy improves respiratory function in the mdx mouse model of Duchenne muscular dystrophy. Mol Ther Nucleic Acids. 2025;37(1):102810. Published 2025 Dec 26. doi:10.1016/j.omtn.2025.102810
2. Gan L, Wu LCL, Wood JA, et al. A cell-penetrating peptide enhances delivery and efficacy of phosphorodiamidate morpholino oligomers in mdx mice. Mol Ther Nucleic Acids. 2022;30:17-27. Published 2022 Aug 17. doi:10.1016/j.omtn.2022.08.019
3. Schissel CK, Farquhar CE, Malmberg AB, Loas A, Pentelute BL. Cell-Penetrating d-Peptides Retain Antisense Morpholino Oligomer Delivery Activity. ACS Bio Med Chem Au. 2022;2(2):150-160. Published 2022 Feb 16. doi:10.1021/acsbiomedchemau.1c00053
4. Schneider AE, Tanganyika-de Winter CL, Jirka SMG, et al. Using muscle homing peptide CyPep10 to deliver phosphorodiamidate morpholino oligomers in the mdx mouse. Mol Ther Nucleic Acids. 2025;36(3):102625. Published 2025 Jul 17. doi:10.1016/j.omtn.2025.102625
5. Shah MNA, Wilton-Clark H, Haque F, et al. DG9 boosts PMO nuclear uptake and exon skipping to restore dystrophic muscle and cardiac function. Nat Commun. 2025;16(1):4477. Published 2025 May 14. doi:10.1038/s41467-025-59494-8