Dyne files z-rostudirsen for US approval in exon 51 DMD

Dyne Therapeutics (Nasdaq: DYN), based in Waltham, Massachusetts, has submitted a Biologics License Application (BLA) to the US FDA for zeleciment rostudirsen (z-rostudirsen, also known as DYNE-251) in patients with Duchenne muscular dystrophy (DMD) whose mutations are amenable to exon 51 skipping. The filing seeks accelerated approval based on dystrophin as a surrogate endpoint, placing z-rostudirsen in direct competition with Sarepta Therapeutics' eteplirsen (Exondys 51), the only currently approved exon 51–targeting therapy, which received its own accelerated approval in 2016.

The BLA proposes a dosing regimen of 20 mg/kg administered intravenously once every four weeks. Dyne has requested Priority Review, which, if granted by the FDA following a standard 60-day filing review period, would shorten the review timeline from 10 months to 6 months. The company said it expects a potential US launch of z-rostudirsen in Q1 2027, contingent on Priority Review being granted and approval received on the anticipated timeline.

Z-rostudirsen holds Breakthrough Therapy, Fast Track, and Rare Pediatric Disease designations from the FDA, as well as Orphan Drug designation from the FDA, the European Medicines Agency, and Japan's Ministry of Health, Labour and Welfare for the treatment of individuals with DMD amenable to exon 51 skipping.

The BLA is supported by data from the registrational expansion cohort of the DELIVER trial, a global Phase I/II study of z-rostudirsen. The company said the expansion cohort met its primary endpoint, with treatment resulting in a statistically significant increase in dystrophin production. Functional improvement was observed across multiple clinical endpoints, and the safety profile was described as favorable. Z-rostudirsen continues to be evaluated in a long-term extension of the DELIVER trial and in the global confirmatory Phase III FORZETTO trial, which is intended to verify clinical benefit under the accelerated approval framework.

DMD is a rare X-linked progressive neuromuscular disorder caused by mutations in the DMD gene that result in a near-complete absence of dystrophin, a protein critical for maintaining muscle structure and function. It is the most common form of childhood-onset muscular dystrophy, affecting approximately 12,000 individuals in the US and 16,000 in the EU. Symptoms typically emerge between ages three and five, beginning with proximal muscle weakness and progressing to loss of ambulation, respiratory compromise, and cardiomyopathy.

Exon-skipping antisense oligonucleotides work by redirecting pre-mRNA splicing to restore the reading frame of the dystrophin transcript, enabling production of a truncated but partially functional dystrophin protein. The exon 51–amenable subset represents approximately 13% of all DMD patients, making it the largest single exon-skipping target population.

Z-rostudirsen is a phosphorodiamidate morpholino oligomer (PMO) conjugated to an antigen-binding fragment (Fab) that binds to the transferrin receptor 1 (TfR1). This conjugation is designed to enhance delivery to muscle tissue and the central nervous system, potentially enabling higher dystrophin restoration than unconjugated PMO chemistries. Eteplirsen, the incumbent exon 51 therapy, uses unconjugated PMO chemistry and has been associated with dystrophin levels well below 1% of normal — a figure that has drawn sustained scrutiny from regulators and independent researchers regarding its clinical relevance.

Competitive and clinical context

The accelerated approval pathway for exon-skipping therapies in DMD has a well-established precedent. All four currently approved exon-skipping ASOs — eteplirsen (exon 51), golodirsen (Vyondys 53, exon 53), viltolarsen (Viltepso, exon 53), and casimersen (Amondys 45, exon 45) — received accelerated approval based on dystrophin as a surrogate endpoint. None has yet demonstrated verified clinical benefit through a completed confirmatory trial.

The FDA's willingness to grant accelerated approval based on dystrophin biomarker data establishes a regulatory framework directly applicable to z-rostudirsen's BLA. However, the agency has also faced criticism over the evidentiary standards applied to this drug class, and confirmatory trial obligations remain in force for all four approved agents. FORZETTO, Dyne's Phase III confirmatory study, will therefore carry significant weight for the long-term regulatory standing of z-rostudirsen if accelerated approval is granted.

Beyond the exon-skipping class, Sarepta Therapeutics' gene therapy delandistrogene moxeparvovec (Elevidys) received accelerated approval in 2023 and expanded approval in 2024 for ambulatory DMD patients aged four years and older. As a mutation-agnostic, one-time treatment, it occupies a distinct clinical niche but represents an alternative for the broader DMD population, including those with exon 51–amenable mutations.

Dyne is building a broader exon-skipping franchise beyond the exon 51 program. The company is advancing four additional development candidates — DYNE-253, DYNE-245, DYNE-244, and DYNE-255 — targeting exons 53, 45, 44, and 55, respectively. If z-rostudirsen's BLA proceeds to approval, it would serve as the first validated asset from Dyne's FORCE platform, which uses the TfR1-targeting conjugation strategy across its DMD programs.

The submission represents the first BLA filing for Dyne as a company, and the outcome of the FDA's review will be closely watched as a test of whether the conjugated PMO approach can demonstrate meaningfully higher dystrophin restoration than the existing standard of care in exon 51–amenable DMD.

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