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Page 38 Braun. J Transl Genet Genom. 2025;9:35-47 https://dx.doi.org/10.20517/jtgg.2024.79

DMD patients [17,18,19] .

Approved RNA-based therapies
Several antisense oligonucleotide (ASO) chemistries have been developed and approved by the FDA and by
the Japanese authority (vitolarsen). The current marketed products target mutations amenable to skipping
exons 51 (eteplirsen), 45 (casimersen), and 53 (golodirsen and vitolarsen). However, these therapies have
not yet been approved by the EMA. ASOs are small modified RNA sequences (20-30 nucleotides) that can
be systemically delivered and specifically bind to a target exon during pre-mRNA splicing, preventing the
inclusion of the exon into mRNA. Exon skipping can address certain deletions, duplications, and point
mutations. However, due to the turnover of both ASO and the transcript and protein, repeated treatments
are required [14,18] . More importantly, the ability of these therapies to produce truncated dystrophin remains
very limited, with levels typically ranging from 0.4% to 6% of normal dystrophin expression, which are not
sufficient for the biological protection of muscle fibers (estimated at 20% to 40%). These numbers are based
on numerous studies conducted in both animal models and DMD and BMD patients [20,21] . However, the
reliability of these figures is not guaranteed, as they may be influenced by factors such as the functionality of
the dystrophin produced, individual variations, and species-specific specificities.

Future directions
Efforts are increasingly focused on developing more efficient chemistries that enhance biodistribution,
extend pharmacokinetics (to reduce injection frequency), and target additional exons to expand the pool of
[18]
eligible patients . The first ASO exon-skipping drugs were limited by low myocardial efficacy and
suboptimal pharmacokinetics. To address these limitations, a new chemical approach based on
tricyclo-DNA antisense molecules with better bioavailability has been designed (NCT05753462) and is
[22]
currently being evaluated in DMD patients, with results pending. Additionally, a novel retargeted ASO,
AOC1044, a phosphorodiamidate morpholino oligomer (PMO) conjugated to a truncated anti-transferrin
antibody to enhance muscle cell uptake, has recently shown promising efficacy, with up to 25% restoration
of dystrophin production compared to normal levels and up to 80% reduction in Creatine Kinase (CK)
[23]
levels in a phase 1/2 clinical trial (NCT06244082).
Duplicated exons, including exon 2, one of the most commonly duplicated exons, are also eligible for exon
skipping. Skipping of duplicated exons potentially restores a full-length (and thus fully functional)
dystrophin. This approach is currently being investigated using an adeno-associated virus (AAV) vector
carrying four copies of a modified U7 small nuclear RNA that contains antisense sequences targeting the
splice donor (2 copies) and splice acceptor (2 copies) of the DMD exon 2 (NCT04240314).
[24]

However, the recruitment of patients with specific genetic subtypes within this already rare disease presents
a bottleneck for drug development, which is further complicated by the individual variability in clinical
scores over time. Additionally, to optimize statistical power and avoid bias, trial designs should prioritize
matching comparative groups based on reliable, data-driven prognostic factors. Recent studies suggest that
trial designs should avoid overemphasizing balance in genotype classification . This requires, for instance,
[25]
to include a natural history/baseline study to monitor clinical and biological evolution for at least 6 months
before enrolling DMD patients. Part of the genotype effect may already be captured through baseline
functional status, reducing the confounding influence of genotype. This is crucial for any type of therapeutic
approach to DMD.