Clinical trials of exon skipping in Duchenne muscular dystrophy

ABSTRACT

Introduction: DMD is most common form of severe childhood muscular dystrophy. The large size of the DMD gene challenges DNA repair or replacement. However, experimental in vitro and in vivo studies using antisense oligonucleotides (AONs) showed that exon skipping could result in production of a truncated dystrophin protein with potential to modify the dystrophic phenotype. This provided the impetus for clinical testing as discussed in this review.

Area covered: Clinical trials were begun using alternative strategies employing antisense oligonucleotides to correct the reading frame of the mutant DMD gene. Exon 51 was target for skipping because it potentially captures the largest subgroup of DMD patients estimated to be 13% of the affected patients. One strategy used drisapersen, a 2′-O-methyl-phosphorothioate oligonucleotide (2OMePS) while the other employed a phosphorodiamidate morpholino oligomer (PMO) called eteplirsen.

Expert opinion: Drisapersen initially introduced by intramuscular injection appeared to favorably skip exon 51 but follow up subcutaneous injections, attempting to produce more widespread skipping resulted in unexpected adverse events and failure to show clinical improvement. In contrast clinical experience using eteplirsen not only precisely skipped exon 51 but also resulted in clinical benefit showing a change in the rate of clinical decline and preservation of ambulation compared to natural history controls with minimal drug-related adverse events.

KEYWORDS:

• Duchenne muscular dystrophy
• dystrophin
• exon skipping
• drisapersen
• eteplirsen

Article highlights

• Duchenne muscular dystrophy (DMD) is the most common severe childhood muscular dystrophy caused by a mutant DMD gene resulting in dystrophin deficiency leading to muscle membrane fragility and muscle fiber breakdown

• Exon skipping is an evolving strategy that holds promise but is still in the early stages of clinical development.

• the exon skipping trials described in this report employed antisense oligonucleotides designed to skip exon 51 of the DMD gene, targeted because it captures the largest subgroup of DMD patients with potential for clinical improvment

• Drisapersen is a 2′-O-methyl-phosphorothioate oligonucleotide (2OMePS) administered by subcutaneous injections. At the site of site of injection patients experienced inflammatory skin reactions. Drug reaching the kidney caused proteinuria because of a negatively charged phosphorothioate linkage binding to renal epithelium. Initially IM injections appeared to show promise but when given systemically clinical improvement was minimal. FDA approval was denied for this exon skipping agent.

• The oligonucleotide, eteplirsen has a morpholino backbone and a charge-neutral phosphorodiamidate linker. The side effect profile is minimal. DMD treatment results in small levels of dystrophin measured by western blots and an increased number of dystrophin positive fibers. Clinically skipping exon 51 preserves distance in the six-minute-walk test compared to an age- and mutation-matched control group. In addition, eteplirsen prolongs ambulation in DMD boys. The drug has been given accelerated by the FDA.

• Future revision in the chemical backbone using a peptide-conjugated morpholino is anticipated to promote dystrophin expression in the heart increase dystrophin in skeletal muscle.

This box summarizes key points contained in the article.

Acknowledgments

All clinical trials referred to in this review were approved by the Investigational Research Board at Nationwide Children’s Hospital, Columbus, OH. Studies were funded by Sarepta Therapeutics and registered on ClinicalTrials.gov under identification numbers NCT01396239 and NCT01540409. Eugenio Mercuri received a grant from the Italian Telethon to support the work described in this review. We are grateful to the patients and families making these observations possible. In addition, the Eteplirsen Study Group and their supporting staff were instrumental in making this possible. Upon conclusion of the first 25 weeks, eteplirsen was administered closer to home on a weekly basis. The DMD boys returned to Nationwide Children’s Hospital for muscle biopsies and clinical evaluations.

Declaration of interest

JR Mendell was the PI on Clinical Trial performed at Nationwide Children’s Hospital. He serves as consultant to Sarepta Therapeutics and serves on their Advisory Board. LR Rodino-Klapac and Zarife Sahenk were part of the investigative team and were co-investigators on the exon skipping trial at Nationwide Children’s Hospital. None of the authors has a financial interest in eteplirsen (Exondys51). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Additional information

Funding

This paper was funded by Sarepta Therapeutics.