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.

Repurposing Pathogenic Variants of DMD Gene and its Isoforms for DMD Exon Skipping Intervention

“…In view of frequently encountered mutation of exon 49 and 50, Exon 51 skipping approach might benefit a large proportion [45/80 (56.25%)] of our study group. Before exon-skipping therapeutics took shape, Wilton in 1999 showed using mdx mouse model, that AONs can remove the mutation in the exon resulting in increased dystrophin production by its repetitive administration [35].…”

Peptide-conjugate antisense based splice-correction for Duchenne muscular dystrophy and other neuromuscular diseases

“…Two randomized placebo-controlled phase 2 clinical trials demonstrated improvement of the 6-minute walk test (6MWT) in children treated with 6 mg/kg/week drisapersen administered subcutaneously for a period of 24 weeks the effect which was maintained, albeit with reduced significance, 48 weeks after treatment [87]. However results were not reproducible in the phase III placebo-controlled trial that followed and no dystrophin production was detectable by western blotting in treated patient's muscle obtained through biopsy [88,89] although an increase in sarcolemmal dystrophin myofiber was observed by immunohistochemistry [[90], [91], [92]]. Adverse effects such as skin fragility at the site of injection, proteinuria and presence of alpha microalbumin in the urine occurred and became more prominent when dose was scaled up to 9 mg/kg/week, therefore, the US Food and Drug Administration (FDA) declined approval on the basis that ‘the standards of effectiveness have not been met’ [90].…”

“…However results were not reproducible in the phase III placebo-controlled trial that followed and no dystrophin production was detectable by western blotting in treated patient's muscle obtained through biopsy [88,89] although an increase in sarcolemmal dystrophin myofiber was observed by immunohistochemistry [[90], [91], [92]]. Adverse effects such as skin fragility at the site of injection, proteinuria and presence of alpha microalbumin in the urine occurred and became more prominent when dose was scaled up to 9 mg/kg/week, therefore, the US Food and Drug Administration (FDA) declined approval on the basis that ‘the standards of effectiveness have not been met’ [90]. However, intensified efforts to overcome such toxicity problems have led to the development of optimized stereopure ASOs.…”

“…The FDA recently approved eteplirsen (Exondys 51, AVI4658; Sarepta Therapeutics), targeting exon 51 in DMD patients, which achieved around 42% positive dystrophic fibers when administered intramuscularly to the extensor digitorum brevis in DMD patients in a single-blind placebo-controlled trial [[90], [91], [92]]. Therefore eteplirsen, like drisapersen could successfully mediate 51 exon skipping in dystrophic patients [86,92,95,96].…”

Targeted exon skipping rescues ciliary protein composition defects in Joubert syndrome patient fibroblasts

“…ASO-based therapeutic approaches to promote targeted exon skipping and restore transcript reading frame have been successfully tested in other genetic conditions. Eteplirsen, an ASO with a morpholino backbone, has recently received accelerated approval from the FDA for the treatment of Duchenne muscular dystrophy 38 .…”