Eteplirsen treatment for Duchenne muscular dystrophy: Exon skipping and dystrophin production

Charleston JS, Schnell FJ, Dworzak J, et al. Eteplirsen treatment for Duchenne muscular dystrophy: Exon skipping and dystrophin production. Neurology 2018;90:e2146-e2154.

Charleston_2018_Neurology. Jun 12;90(24);e2146-e2154

Twelve patients included in the placebo-controlled trial for eteplirsen*1, where followed in an open-label extension study (the patients originally receiving placebo, where as well treated with eteplirsen in the extension study). During evaluation after 36 months a slower functional decline compared to natural history data was observed; however the differences were small.*2 In this paper, dystrophin restoration after 180 weeks of treatment was assessed by different methods for 11 of the patients. In nine out of 11 patients an increase in dystrophin expression compared to non-treated patients and a proper localisation of dystrophin was seen. Dystrophin levels are, however, still very low and below the level that is considered to be the threshold for a clinically meaningful improvement.

*1 Mendell JR, Rodino-Klapac LR, Sahenk Z, et al. Eteplirsen for the treatment of Duchenne muscular dystrophy. Annals of neurology 2013;74:637-47.

Mendell_2013_Ann Neurol. 2013 Nov;74(5);637-47

*2 Mendell JR, Goemans N, Lowes LP, et al. Longitudinal effect of eteplirsen versus historical control on ambulation in Duchenne muscular dystrophy. Annals of neurology 2016;79:257-71.

Mendell_2016_Ann Neurol. Feb;79(2);257-71

Abstract

OBJECTIVE: To describe the quantification of novel dystrophin production in patients with Duchenne muscular dystrophy (DMD) after long-term treatment with eteplirsen.

METHODS: Clinical study 202 was an observational, open-label extension of the randomized, controlled study 201 assessing the safety and efficacy of eteplirsen in patients with DMD with a confirmed mutation in the DMD gene amenable to correction by skipping of exon 51. Patients received once-weekly IV doses of eteplirsen 30 or 50 mg/kg. Upper extremity muscle biopsy samples were collected at combined study week 180, blinded, and assessed for dystrophin-related content by Western blot, Bioquant software measurement of dystrophin-associated immunofluorescence intensity, and percent dystrophin-positive fibers (PDPF). Results were contrasted with matched untreated biopsies from patients with DMD. Reverse transcription PCR followed by Sanger sequencing of newly formed slice junctions was used to confirm the mechanism of action of eteplirsen.

RESULTS: Reverse transcription PCR analysis and sequencing of the newly formed splice junction confirmed that 100% of treated patients displayed the expected skipped exon 51 sequence. In treated patients vs untreated controls, Western blot analysis of dystrophin content demonstrated an 11.6-fold increase (p = 0.007), and PDPF analysis demonstrated a 7.4-fold increase (p < 0.001). The PDPF findings were confirmed in a re-examination of the sample (15.5-fold increase, p < 0.001). Dystrophin immunofluorescence intensity was 2.4-fold greater in treated patients than in untreated controls (p < 0.001).

CONCLUSION: Taken together, the 4 assays, each based on unique evaluation mechanisms, provided evidence of eteplirsen muscle cell penetration, exon skipping, and induction of novel dystrophin expression.

CLASSIFICATION OF EVIDENCE: This study provides Class II evidence of the muscle cell penetration, exon skipping, and induction of novel dystrophin expression by eteplirsen, as confirmed by 4 assays.

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