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Page 40 Braun. J Transl Genet Genom. 2025;9:35-47 https://dx.doi.org/10.20517/jtgg.2024.79

AAVs come in the form of various serotypes that differ in tissue tropism and transduction efficiency that
[36]
may also be species-dependent . AAV8, AAV9, AAVrh74 (highly similar to AAV8), and Myo-AAV are
currently used in clinical trials for DMD. The sarcolemmal dystrophin is organized into highly focal and
[37]
nonmobile nuclear domains of ~80 µm in size , although we cannot exclude the existence of a diffusible
cytoplasmic fraction that could be below the level of detection, as well as the contribution of mobile nuclei.
Therefore, any AAV gene therapy should transduce as many myonuclei as possible to avoid an ineffective
mosaic microdystrophin expression.

As of June 2024, the FDA has approved one product, Elevidys (AAVRh74-hMD1), developed by Sarepta/
Roche, for patients aged 4 years and older . The EMA is currently reviewing the application for patients
[38]
aged 4 to 7 years only. Long-term and extension studies are ongoing (NCT05881408, NCT04626674,
NCT05096221, NCT06128564, NCT0596735). Another product developed by Pfizer was recently dropped
after missing the primary endpoint in a phase 3 trial (NCT05689164, NCT02907619, NCT05429372). The
death of two patients during this study certainly influenced the sponsor’s decision. This product uses an
AAV9 vector carrying a different version of microdystrophin . Based on public releases and
[12]
communications at congresses, we know that Solid Bioscience has also decided to stop the development of
its AAV9-microdystrophin (the third version of truncated dystrophin) and switched to a second-generation
vector, SGT-003 (NCT06138639) based on Myo-AAV vector with expected reduced immunogenicity and
improved efficiency. Additionally, two other sponsors are pursuing the clinical development of two
AAV8-microdystrophin vectors. Genethon has completed the dose-escalation phase of its seamless clinical
program with an AAV8-hMD1 (the same microdystrophin version as Sarepta) (Eudra-CT2020-002093-27)
and Regenxbio is completing the dose-escalation phase of a very similar vector that includes a small
sequence added to the C-terminal part of the microdystrophin to improve part of its function
(NCT05693142, NCT06491927). Both of these programs have now entered into the pivotal phase based on
promising functional data.

The degree of functionality and therapeutic impact of microdystrophins in humans remains unclear. The
retained portions of the protein and the level of expression may influence its function. Chamberlain et al.
[39]
[40]
and Boehler et al. provide a detailed discussion of critical microdystrophin elements. Additionally, the
[39]
quantity of microdystrophin expressed may contribute to some aspects of clinical benefit . All sponsors are
using different tissue-specific promoters (MHCK7, MHC, K8, spc5.12, and spc2.12) . Therapeutic doses
[12]
generally exceed 10 vg/kg except for Genethon (3 × 10 vg/kg). However, uniform and head-to-head
14
13
titrations should be performed to accurately assess the differences in viral loads injected. Large-scale
production methods also vary. Some use plasmid-transfected HEK293 cells of different strains, while others
(such as Solid Biosciences) use an Herpes Simplex (HSV)-based process. Purification methods may also
differ . Overall, these variations may result in differences in transduction efficiency, functional benefit, and
[41]
safety.

AAVs transduce differentiated fibers but not satellite cells as initially reported. Kwon et al., however,
reported that satellite cells can be transduced with AAV vectors and can undergo gene editing to restore the
dystrophin reading frame in a sensitive Cre/lox-based dual-reporter mdx mouse model. Interestingly, higher
AAV transduction of satellite cells was seen in mdx mice compared to Wild Type (WT) mice, supposedly
[42]
because of the constant need for regeneration and a higher number of activated satellite cells in mdx mice .

Safety and durability of AAV-based gene therapy
AAV integration into the host genome