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December 13, 2016

Approving a Problematic Muscular Dystrophy DrugImplications for FDA Policy

Author Affiliations
  • 1Program on Regulation, Therapeutics, and Law (PORTAL), Division of Pharmacoepidemiology and Pharmacoeconomics, Department of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts
JAMA. 2016;316(22):2357-2358. doi:10.1001/jama.2016.16437

In September 2016, the US Food and Drug Administration (FDA) approved eteplirsen (Exondys 51), a new drug for Duchenne muscular dystrophy (DMD), overruling the recommendations of both its scientific staff and its external advisory committee. Duchenne muscular dystrophy is a progressive and usually fatal X-linked genetic disease caused by mutations in a gene that produces the protein dystrophin that helps stabilize muscle fibers. No disease-modifying treatments are available.

Eteplirsen was designed to offer a promising new therapeutic approach that would bypass a stop codon in a gene coding for dystrophin, allowing production of a truncated but functional version of the protein. In particular, eteplirsen targeted exon 51, the location of the stop codon in about 10% to 15% of patients with DMD (an estimated 2000-2500 cases in the United States). Despite this innovative mechanism, the development of eteplirsen was controversial, starting with its manufacturer-supported pivotal double-blind study, which involved only 12 patients: 8 were randomized to 2 different eteplirsen doses and 4 were randomized to placebo for 24 weeks. The latter were then switched to eteplirsen and all were to be followed for an additional 24 weeks. The sample size was substantially smaller than the study sample size in which a similar DMD drug, drisapersen, had been tested in 3 randomized trials that together enrolled 290 patients. The FDA declined to approve drisapersen in 2015 after these studies showed no clear benefit after 24 weeks in prespecified clinical end points, such as changes in a 6-minute walk test. Those trials also suggested the possibility of safety problems, including renal toxic effects and thrombocytopenia.

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