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Delivering Exon-Skipping Therapy Into Muscle: FDA Grants Priority Review to Dyne’s New Duchenne Muscular Dystrophy Drug
Z-rostudirsen uses an antibody fragment to improve tissue delivery of an oligonucleotide, and the FDA will decide by January 2027 whether to grant accelerated approval; the real test is whether increased dystrophin can translate into lasting improvements in physical function.
Exon-skipping therapy for Duchenne muscular dystrophy is not a new concept; the longstanding challenge has been how to deliver enough drug to muscles throughout the body. Dyne Therapeutics is attempting to use an antibody fragment as a delivery vehicle. The U.S. Food and Drug Administration (FDA) has now accepted its Biologics License Application and granted it Priority Review, bringing the strategy to its first critical regulatory test.
The investigational zeleciment rostudirsen (z-rostudirsen; development code DYNE-251) is intended for patients whose genetic variants are amenable to exon 51 skipping. The FDA has set a Prescription Drug User Fee Act target action date of January 21, 2027. Acceptance means the data are sufficient to proceed to formal review; it does not mean the therapy has been approved. Dyne said that if the review is completed on schedule and approval is granted, the therapy could launch in the United States in the first quarter of 2027.
The therapy links a phosphorodiamidate morpholino oligomer (PMO) to an antigen-binding fragment that recognizes transferrin receptor 1. The PMO is designed to skip exon 51 during RNA splicing, restoring the disrupted reading frame in some patients and enabling production of a shorter but near-full-length dystrophin protein; the antibody fragment is intended to improve the drug’s efficiency in entering muscle cells. The proposed dosing regimen is an intravenous infusion once every four weeks.
The FDA is reviewing an accelerated approval application based primarily on dystrophin expression as a surrogate endpoint. Dyne said the registrational expansion cohort of the global Phase 1/2 DELIVER trial met its primary endpoint, with a significant increase in protein production, improvements in multiple functional measures, and an overall favorable safety profile; the patient organization Parent Project Muscular Dystrophy also reported results pointing in the same direction. However, this regulatory development itself does not provide complete numerical data, nor does it establish that functional benefit has been demonstrated.
Background
Duchenne muscular dystrophy is caused by variants in the DMD gene. Patients lack the dystrophin protein needed to maintain muscle-fiber stability, and the disease can progressively affect walking, upper-limb, respiratory, and cardiac function. Exon skipping applies only to specific variants; the exon 51 approach therefore serves a group of patients confirmed through genetic testing to have the corresponding splicing profile, rather than all patients with Duchenne muscular dystrophy.
If accelerated approval is granted, subsequent trials will still be required to verify clinical benefit. The global Phase 3 FORZETTO confirmatory trial and the DELIVER long-term extension study are underway and will examine whether increased protein levels can consistently slow functional decline, while adding long-term safety data. The FDA’s Priority Review shortens the decision timeline but does not lower this evidentiary threshold.