The muscular dystrophies are disorders of progressive muscular degeneration and weakness. As a group they display clinical heterogeneity that reflects the heterogeneity of molecular mechanisms responsible for them, and range from congenital to adulthood onset. Recent advances in the field include improved methods of diagnosis, continued identification of disease genes, and the development of a unified model of pathogenesis in facioscapulohumeral dystrophy. These advances are reflected in the development of new therapeutic approaches, some of which have already led to clinical trials in the dystrophinopathies and limb-girdle dystrophies.

The muscular dystrophies.

Since the publication of the Duchenne muscular dystrophy (DMD) care considerations in 2010, multidisciplinary care of this severe, progressive neuromuscular disease has evolved. In conjunction with improved patient survival, a shift to more anticipatory diagnostic and therapeutic strategies has occurred, with a renewed focus on patient quality of life. In 2014, a steering committee of experts from a wide range of disciplines was established to update the 2010 DMD care considerations, with the goal of improving patient care. The new care considerations aim to address the needs of patients with prolonged survival, to provide guidance on advances in assessments and interventions, and to consider the implications of emerging genetic and molecular therapies for DMD. The committee identified 11 topics to be included in the update, eight of which were addressed in the original care considerations. The three new topics are primary care and emergency management, endocrine management, and transitions of care across the lifespan. In part 1 of this three-part update, we present care considerations for diagnosis of DMD and neuromuscular, rehabilitation, endocrine (growth, puberty, and adrenal insufficiency), and gastrointestinal (including nutrition and dysphagia) management.

/pdf/diagnosis-and-management-of-duchenne-muscular-dystrophy-part-54w1we6s06.pdf

Diagnosis and management of Duchenne muscular dystrophy, part 1: diagnosis, and neuromuscular, rehabilitation, endocrine, and gastrointestinal and nutritional management

Objective
In prior open-label studies, eteplirsen, a phosphorodiamidate morpholino oligomer, enabled dystrophin production in Duchenne muscular dystrophy (DMD) with genetic mutations amenable to skipping exon 51. The present study used a double-blind placebo-controlled protocol to test eteplirsen's ability to induce dystrophin production and improve distance walked on the 6-minute walk test (6MWT).

Methods
DMD boys aged 7 to 13 years, with confirmed deletions correctable by skipping exon 51 and ability to walk 200 to 400 m on 6 MWT, were randomized to weekly intravenous infusions of 30 or 50 mg/kg/wk eteplirsen or placebo for 24 weeks (n = 4/group). Placebo patients switched to 30 or 50 mg/kg eteplirsen (n = 2/group) at week 25; treatment was open label thereafter. All patients had muscle biopsies at baseline and week 48. Efficacy included dystrophin-positive fibers and distance walked on the 6MWT.

Results
At week 24, the 30 mg/kg eteplirsen patients were biopsied, and percentage of dystrophin-positive fibers was increased to 23% of normal; no increases were detected in placebo-treated patients (p ≤ 0.002). Even greater increases occurred at week 48 (52% and 43% in the 30 and 50 mg/kg cohorts, respectively), suggesting that dystrophin increases with longer treatment. Restoration of functional dystrophin was confirmed by detection of sarcoglycans and neuronal nitric oxide synthase at the sarcolemma. Ambulation-evaluable eteplirsen-treated patients experienced a 67.3 m benefit compared to placebo/delayed patients (p ≤ 0.001).

Interpretation
Eteplirsen restored dystrophin in the 30 and 50 mg/kg/wk cohorts, and in subsequently treated, placebo-controlled subjects. Duration, more than dose, accounted for dystrophin production, also resulting in ambulation stability. No severe adverse events were encountered. Ann Neurol 2013;74:637–647

https://onlinelibrary.wiley.com/doi/epdf/10.1002/ana.23982

Eteplirsen for the treatment of Duchenne muscular dystrophy

Duchenne muscular dystrophy is a fatal neuromuscular disorder affecting around one in 3,500-5,000 male births that is characterized by progressive muscular deterioration. It is inherited in an X-linked recessive fashion and is caused by loss-of-function mutations in the DMD gene coding for dystrophin, a cytoskeletal protein that stabilizes the plasma membrane of muscle fibers. In September 2016, the US Food and Drug Administration granted accelerated approval for eteplirsen (or Exondys 51), a drug that acts to promote dystrophin production by restoring the translational reading frame of DMD through specific skipping of exon 51 in defective gene variants. Eteplirsen is applicable for approximately 14% of patients with DMD mutations. This article extensively reviews and discusses the available information on eteplirsen to date, focusing on pharmacological, efficacy, safety, and tolerability data from preclinical and clinical trials. Issues faced by eteplirsen, particularly those relating to its efficacy, will be identified. Finally, the place of eteplirsen and exon skipping as a general therapeutic strategy in Duchenne muscular dystrophy treatment will be discussed.

/pdf/eteplirsen-in-the-treatment-of-duchenne-muscular-dystrophy-1osef3orey.pdf

Eteplirsen in the treatment of Duchenne muscular dystrophy

Objective: Phosphorodiamidate morpholino oligomers (PMOs) are synthetic nucleic acid analogs that can be designed to sequence-specifically block spliceosomes from binding to dystrophin pre-mRNA, resulting in omission of the targeted exon from the transcript and restoration of the reading frame with the goal of enabling synthesis of internally-shortened dystrophin. Background: DMD, a rare, X-linked genetic disease results in progressive muscle degeneration and premature death. DMD is primarily caused by whole exon deletions in the dystrophin gene resulting in a shift of the mRNA reading frame that prevents production of functional dystrophin protein. Design/Methods: As of June 3, 2016, 81 of 150 treated patients had received weekly eteplirsen for ≥1 year. Results: PMO eteplirsen received accelerated approval in the US for patients with a dystrophin gene mutation amenable to exon 51 skipping based on an increase in dystrophin in skeletal muscle in some patients. Mean dystrophin increases as measured by Western blot were observed following 180 weeks of treatment in the pivotal Phase II Studies 201/202 when compared to untreated DMD controls (N=11; +0.85%, p=0.007) and at Week 48 in Phase III Study PROMOVI when compared to baseline (N=12; +0.28%, p=0.008). Immunohistochemistry analysis at Week 180 in Study 201/202 also showed mean increases in dystrophin as measured by % dystrophin-positive fibers (N=11; +16.27%, p 4.5 years of treatment. Conclusions: Eteplirsen is the first exon skipping therapy approved for the treatment of Duchenne muscular dystrophy amenable to exon 51 skipping. Lessons learned from the eteplirsen clinical development program can aid in development of PMO therapies targeting additional exons. Study Supported by: Sarepta Therapeutics, Inc. Disclosure: Dr. Charleston has received personal compensation for activities with Sarepta Therapeutics as an employee. Dr. Schnell has received personal compensation for activities with Sarepta Therapeutics as a full time employee. Dr. Dworzak has received personal compensation for activities with Sarepta Therapeutics as an employee. Dr. Donoghue has received personal compensation for activities with Sarepta Therapeutics as an employee. Dr. Lynch has received personal compensation for activities with Sarepta Therapeutics, Inc. as an employee. Dr. Lewis has nothing to disclose. Dr. Chen has nothing to disclose. Dr. Rodino-Klapac has nothing to disclose. Dr. Sahenk has nothing to disclose. Dr. Voss has received personal compensation for activities with Sarepta Therapeutics as an employee. Dr. DeAlwis has received personal compensation for activities with Sarepta Therapeutics as an employee. Dr. Frank has received personal compensation for activities with Sarepta Therapeutics, Inc. as an employee. Dr. Eliopoulos has received personal compensation for activities with Sarepta Therapeutics as an employee. Dr. Mendell has received personal compensation for activities with Sarepta Therapeutics, Inc.

Eteplirsen for the Treatment of Duchenne Muscular Dystrophy (DMD) (S42.001)

Objective: Creatine kinase (CK) levels are increased on dried blood spots in newborns related to the birthing process. As a marker for newborn screening, CK in Duchenne muscular dystrophy (DMD) results in false-positive testing. In this report, we introduce a 2-tier system using the dried blood spot to first assess CK with follow-up DMD gene testing. Methods: A fluorometric assay based upon the enzymatic transphosphorylation of adenosine diphosphate to adenosine triphosphate was used to measure CK activity. Preliminary studies established a population-based range of CK in newborns using 30,547 deidentified anonymous dried blood spot samples. Mutation analysis used genomic DNA extracted from the dried blood spot followed by whole genome amplification with assessment of single-/multiexon deletions/duplications in the DMD gene using multiplex ligation-dependent probe amplification. Results: DMD gene mutations (all exonic deletions) were found in 6 of 37,649 newborn male subjects, all of whom had CK levels >2,000U/l. In 3 newborns with CK >2,000U/l in whom DMD gene abnormalities were not found, we identified limb-girdle muscular dystrophy gene mutations affecting DYSF, SGCB, and FKRP. Interpretation: A 2-tier system of analysis for newborn screening for DMD has been established. This path for newborn screening fits our health care system, minimizes false-positive testing, and uses predetermined levels of CK on dried blood spots to predict DMD gene mutations. ANN NEUROL 2012;71:304–313

http://depts.washington.edu/neurolog/images/journalclub/Evidence-based_path_MD.Mendell.2012.6.20.pdf

Maha Mahmoud

Papers

Evidence-based path to newborn screening for Duchenne muscular dystrophy.