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Antisense Oligonucleotides for Duchenne Muscular Dystrophy: Why No Neurologist Should Skip This

Antisense Oligonucleotides for Duchenne Muscular Dystrophy: Why No Neurologist Should Skip This Duchenne muscular dystrophy (DMD) was described by the disease’s namesake in the mid-19th century. Over time, the phenotype, genetics, and pathogenesis of this fatal disorder of muscle degeneration have been well described. An understanding of DMD has become universally taught in medical schools and common knowledge among neurologists, physiatrists, and other physicians. Despite our now long-standing understanding of the disease, the development of new, meaningful treatments for these patients has been disappointing. The standard of care remains corticosteroids; however, this may evolve in the near future, as promising new therapies for DMD focusing on exon skipping have been described. Understanding these new therapies highlights both the exciting future of neuromuscular medicine yet also underscores significant challenges ahead. Moreover, the emergence of these treatments recently spurred the US Food and Drug Administration (FDA) to draft guidance for industry in the realm of developing drugs for treating dystrophinopathies.1 Exon skipping therapies work on the level of pre–messenger RNA. Duchenne muscular dystrophy and the milder Becker muscular dystrophy are both caused by mutations in the dystrophin gene on the X chromosome. Duchenne muscular dystrophy–causing mutations typically promote loss of the open reading frame and complete absence of the dystrophin protein. Becker-causing mutations, meanwhile, result in abnormal dystrophin protein that maintains some function. The goal of new exon skipping therapies is essentially to convert a DMD phenotype to a Becker phenotype. These drugs use an antisense oligonucleotide to “mask” mutated portions of DNA, thereby forcing spliceosomes into an alternate splicing pattern and restoring the open reading frame. While the science behind this technology may seem daunting to clinicians who are not frequently exposed to DMD, such therapies have been under development for many years. Initial data demonstrated that antisense oligonucleotides could restore the expression of dystrophin in muscle from the mdx mouse model of DMD.2 In 2001, von Deutekom et al3 showed that dystrophin expression could be restored in human muscle using an antisense oligonucleotide skipping exon 46. Briefly, muscle cells were isolated from 2 patients with DMD carrying deletions in exon 45. The most efficient antisense oligonucleotide used appeared to induce exon skipping in approximately 15% of reverse transcriptase–polymerase chain reaction product, and dystrophin could ultimately be localized in 75% of myotubes of DMD muscle. In the intervening years, we have seen much progress and multiple publications surrounding 2 promising drugs—eteplirsen and drisapersen—which are both designed to skip exon 51. Eteplirsen is a phosphorodiamidate morpholino oligomer that binds to exon 51. Most recently, Mendell et al4 published a double-blind placebo-controlled study of the drug’s ability to restore dystrophin production and improve function. In the first phase of the study, eteplirsen in 1 of 2 doses or placebo were given to 12 boys with DMD over 24 weeks. Subsequently, boys given placebo were randomized to 30 or 50 mg/kg/wk of eteplirsen, and all patients continued receiving eteplirsen for an additional 24 weeks. While no changes in the proportion of dystrophin-positive fibers were seen on muscle biopsy at 12 weeks, at 24 weeks, patients receiving the lower 30-mg/kg/wk dose had increases of dystrophin-positive fibers to 24% of normal. By 48 weeks, this increased to 52% of normal, and patients who received the 50-mg/kg/wk dose increased to 43% of normal. The functionality of dystrophin was confirmed by detection of neuronal nitric oxide synthase and sarcoglycans at the sarcolemma. At 48 weeks, there appeared to be a significant benefit on a 6-minute walk test (6MWT) measurement, evidenced by a 67.3-m improvement in patients who received either dose of eteplirsen. This, however, did exclude 2 subjects from the 30 mg/kg/wk group who experienced drastic decline in function soon after enrolling in the trial. Drisapersen is similarly an antisense oligonucleotide, with a 2′-O-methyl-phosphorothioate backbone. Thus far, however, its road to patients has been more complicated. A phase 1-2 study published in 2011 recruited 12 patients.5 Initially, 3 patients received each of 4 escalating doses for a 12-week period, after which a 12-week open-label period of the highest dose was continued. At 12 weeks, successful exon skipping and dystrophin production was found in patients receiving all but the lowest dose of drisapersen. After the 12 weeks of extended therapy, there appeared to be some benefit on the 6MWT measurement, with a mean improvement of 35.2 m. This result was certainly encouraging, but a subsequent phase 3 trial (clinicaltrials.gov NCT01254019) did not meet the primary end point of improvement of 6MWT compared with placebo. Subsequently, additional phase 2 data have been published: Voit et al6 randomized 53 patients to receive placebo, a continuous form of drisapersen, or an intermittent form. At 25 weeks, a statistically significant improvement in 6MWT was seen in the continuous drisapersen group, but not for the intermittent group, or either group at 48 weeks. Hence, study of drisapersen continues. Of note, clinical neurologists must understand that these drugs are specific for certain disease-causing mutations. The application of such therapies will demand a thoughtful, personalized approach. While the anticipation surrounding eteplirsen and drisapersen has been great, skipping of exon 51 is applicable to only 13% of patients with DMD. In 2009, Aartsma-Rus et al7 described the theoretical applicability of exon skipping directed against 121 different mutated exons or combinations of exons. Ultimately, exon skipping could restore the open reading frame in 83% of all DMD mutations. The need for unique drugs for distinct mutations will be highly demanding on the drug development and clinical trial processes. It was likely with this significant task in mind that the FDA issued its June 2015 guidance for drug development in DMD, highlighting considerations for study design and end points that are most likely to move swiftly toward FDA approval.1 While the volume of possible mutations presents daunting challenges, so too does this highlight an exciting direction for neuromuscular care and neurology in general. Moving forward, increasing genetic insights will lend a focus to personalized medicine. Neurologists will tailor the choice of therapy according to each individual’s disease phenotype and genotype—a trend that will likely hold across many subspecialties in neurology. Additional challenges also exist. Thus far, all trials have been small. Trial end points have focused on increasing dystrophin expression in muscle or the clinical measure of performance on a 6MWT. To this point, exon skipping therapies have not been shown to extend life span or to improve quality of life in patients living with DMD. Cardiac and respiratory dysfunction are often fatal in this patient population; it is not at all clear that exon skipping will improve function in these vital areas. Nonetheless, antisense oligonucleotides offer tremendous promise, and their impact on patients with DMD may very well alter the practice of neuromuscular medicine. At this point, treatment of these patients often falls to pediatric subspecialists. Moving forward, as patients with DMD live longer into adulthood, adult neurologists will take on a more active role in their care. There may emerge a niche for transitioning or bridging these complex patients from multidisciplinary pediatric to adult clinics. Neurologists across all levels of training and subspecialties should take note of the exciting science behind antisense oligonucleotides and exon skipping. These therapies not only represent a potentially disease-modifying path for one of neuromuscular neurology’s most stubborn foes, but also foreshadow a future of personalized genetic therapies in neurology. Back to top Article Information Corresponding Author: Eva L. Feldman, MD, PhD, Department of Neurology, University of Michigan, 109 Zina Pitcher Pl, 5017 AAT-BSRB, Ann Arbor, MI 48109 (efeldman@umich.edu). Published Online: January 4, 2016. doi:10.1001/jamaneurol.2015.4011. Conflict of Interest Disclosures: None reported. Funding/Support: Support during the preparation of the manuscript was provided by the A. Alfred Taubman Medical Research Institute. Role of the Funder/Sponsor: The A. Alfred Taubman Medical Research Institute had no role in the preparation, review, or approval of the manuscript or decision to submit the manuscript for publication. Submissions: This Viewpoint is in a projected series of Next Generation Neurology Viewpoints emphasizing innovative original concepts and approaches to understanding the causation of neurological disease and providing new and effective therapies. We welcome the submission of Viewpoints that incorporate these objectives. References 1. US Department of Health and Human Services Food and Drug Administration. Duchenne Muscular Dystrophy and Related Dystrophinopathies: Developing Drugs for Treatment. Silver Spring, MD: Food and Drug Administration Center for Drug Evaluation and Research; 2015. 2. Dunckley MG, Manoharan M, Villiet P, Eperon IC, Dickson G. Modification of splicing in the dystrophin gene in cultured Mdx muscle cells by antisense oligoribonucleotides. Hum Mol Genet. 1998;7(7):1083-1090.PubMedGoogle ScholarCrossref 3. van Deutekom JC, Bremmer-Bout M, Janson AA, et al. Antisense-induced exon skipping restores dystrophin expression in DMD patient derived muscle cells. Hum Mol Genet. 2001;10(15):1547-1554.PubMedGoogle ScholarCrossref 4. Mendell JR, Rodino-Klapac LR, Sahenk Z, et al; Eteplirsen Study Group. Eteplirsen for the treatment of Duchenne muscular dystrophy. Ann Neurol. 2013;74(5):637-647.PubMedGoogle ScholarCrossref 5. Goemans NM, Tulinius M, van den Akker JT, et al. Systemic administration of PRO051 in Duchenne’s muscular dystrophy. N Engl J Med. 2011;364(16):1513-1522.PubMedGoogle ScholarCrossref 6. Voit T, Topaloglu H, Straub V, et al. Safety and efficacy of drisapersen for the treatment of Duchenne muscular dystrophy (DEMAND II): an exploratory, randomised, placebo-controlled phase 2 study. Lancet Neurol. 2014;13(10):987-996.PubMedGoogle ScholarCrossref 7. Aartsma-Rus A, Fokkema I, Verschuuren J, et al. Theoretic applicability of antisense-mediated exon skipping for Duchenne muscular dystrophy mutations. Hum Mutat. 2009;30(3):293-299.PubMedGoogle ScholarCrossref http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png JAMA Neurology American Medical Association

Antisense Oligonucleotides for Duchenne Muscular Dystrophy: Why No Neurologist Should Skip This

JAMA Neurology , Volume 73 (3) – Mar 1, 2016

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American Medical Association
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Copyright © 2016 American Medical Association. All Rights Reserved.
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2168-6149
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2168-6157
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10.1001/jamaneurol.2015.4011
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Abstract

Duchenne muscular dystrophy (DMD) was described by the disease’s namesake in the mid-19th century. Over time, the phenotype, genetics, and pathogenesis of this fatal disorder of muscle degeneration have been well described. An understanding of DMD has become universally taught in medical schools and common knowledge among neurologists, physiatrists, and other physicians. Despite our now long-standing understanding of the disease, the development of new, meaningful treatments for these patients has been disappointing. The standard of care remains corticosteroids; however, this may evolve in the near future, as promising new therapies for DMD focusing on exon skipping have been described. Understanding these new therapies highlights both the exciting future of neuromuscular medicine yet also underscores significant challenges ahead. Moreover, the emergence of these treatments recently spurred the US Food and Drug Administration (FDA) to draft guidance for industry in the realm of developing drugs for treating dystrophinopathies.1 Exon skipping therapies work on the level of pre–messenger RNA. Duchenne muscular dystrophy and the milder Becker muscular dystrophy are both caused by mutations in the dystrophin gene on the X chromosome. Duchenne muscular dystrophy–causing mutations typically promote loss of the open reading frame and complete absence of the dystrophin protein. Becker-causing mutations, meanwhile, result in abnormal dystrophin protein that maintains some function. The goal of new exon skipping therapies is essentially to convert a DMD phenotype to a Becker phenotype. These drugs use an antisense oligonucleotide to “mask” mutated portions of DNA, thereby forcing spliceosomes into an alternate splicing pattern and restoring the open reading frame. While the science behind this technology may seem daunting to clinicians who are not frequently exposed to DMD, such therapies have been under development for many years. Initial data demonstrated that antisense oligonucleotides could restore the expression of dystrophin in muscle from the mdx mouse model of DMD.2 In 2001, von Deutekom et al3 showed that dystrophin expression could be restored in human muscle using an antisense oligonucleotide skipping exon 46. Briefly, muscle cells were isolated from 2 patients with DMD carrying deletions in exon 45. The most efficient antisense oligonucleotide used appeared to induce exon skipping in approximately 15% of reverse transcriptase–polymerase chain reaction product, and dystrophin could ultimately be localized in 75% of myotubes of DMD muscle. In the intervening years, we have seen much progress and multiple publications surrounding 2 promising drugs—eteplirsen and drisapersen—which are both designed to skip exon 51. Eteplirsen is a phosphorodiamidate morpholino oligomer that binds to exon 51. Most recently, Mendell et al4 published a double-blind placebo-controlled study of the drug’s ability to restore dystrophin production and improve function. In the first phase of the study, eteplirsen in 1 of 2 doses or placebo were given to 12 boys with DMD over 24 weeks. Subsequently, boys given placebo were randomized to 30 or 50 mg/kg/wk of eteplirsen, and all patients continued receiving eteplirsen for an additional 24 weeks. While no changes in the proportion of dystrophin-positive fibers were seen on muscle biopsy at 12 weeks, at 24 weeks, patients receiving the lower 30-mg/kg/wk dose had increases of dystrophin-positive fibers to 24% of normal. By 48 weeks, this increased to 52% of normal, and patients who received the 50-mg/kg/wk dose increased to 43% of normal. The functionality of dystrophin was confirmed by detection of neuronal nitric oxide synthase and sarcoglycans at the sarcolemma. At 48 weeks, there appeared to be a significant benefit on a 6-minute walk test (6MWT) measurement, evidenced by a 67.3-m improvement in patients who received either dose of eteplirsen. This, however, did exclude 2 subjects from the 30 mg/kg/wk group who experienced drastic decline in function soon after enrolling in the trial. Drisapersen is similarly an antisense oligonucleotide, with a 2′-O-methyl-phosphorothioate backbone. Thus far, however, its road to patients has been more complicated. A phase 1-2 study published in 2011 recruited 12 patients.5 Initially, 3 patients received each of 4 escalating doses for a 12-week period, after which a 12-week open-label period of the highest dose was continued. At 12 weeks, successful exon skipping and dystrophin production was found in patients receiving all but the lowest dose of drisapersen. After the 12 weeks of extended therapy, there appeared to be some benefit on the 6MWT measurement, with a mean improvement of 35.2 m. This result was certainly encouraging, but a subsequent phase 3 trial (clinicaltrials.gov NCT01254019) did not meet the primary end point of improvement of 6MWT compared with placebo. Subsequently, additional phase 2 data have been published: Voit et al6 randomized 53 patients to receive placebo, a continuous form of drisapersen, or an intermittent form. At 25 weeks, a statistically significant improvement in 6MWT was seen in the continuous drisapersen group, but not for the intermittent group, or either group at 48 weeks. Hence, study of drisapersen continues. Of note, clinical neurologists must understand that these drugs are specific for certain disease-causing mutations. The application of such therapies will demand a thoughtful, personalized approach. While the anticipation surrounding eteplirsen and drisapersen has been great, skipping of exon 51 is applicable to only 13% of patients with DMD. In 2009, Aartsma-Rus et al7 described the theoretical applicability of exon skipping directed against 121 different mutated exons or combinations of exons. Ultimately, exon skipping could restore the open reading frame in 83% of all DMD mutations. The need for unique drugs for distinct mutations will be highly demanding on the drug development and clinical trial processes. It was likely with this significant task in mind that the FDA issued its June 2015 guidance for drug development in DMD, highlighting considerations for study design and end points that are most likely to move swiftly toward FDA approval.1 While the volume of possible mutations presents daunting challenges, so too does this highlight an exciting direction for neuromuscular care and neurology in general. Moving forward, increasing genetic insights will lend a focus to personalized medicine. Neurologists will tailor the choice of therapy according to each individual’s disease phenotype and genotype—a trend that will likely hold across many subspecialties in neurology. Additional challenges also exist. Thus far, all trials have been small. Trial end points have focused on increasing dystrophin expression in muscle or the clinical measure of performance on a 6MWT. To this point, exon skipping therapies have not been shown to extend life span or to improve quality of life in patients living with DMD. Cardiac and respiratory dysfunction are often fatal in this patient population; it is not at all clear that exon skipping will improve function in these vital areas. Nonetheless, antisense oligonucleotides offer tremendous promise, and their impact on patients with DMD may very well alter the practice of neuromuscular medicine. At this point, treatment of these patients often falls to pediatric subspecialists. Moving forward, as patients with DMD live longer into adulthood, adult neurologists will take on a more active role in their care. There may emerge a niche for transitioning or bridging these complex patients from multidisciplinary pediatric to adult clinics. Neurologists across all levels of training and subspecialties should take note of the exciting science behind antisense oligonucleotides and exon skipping. These therapies not only represent a potentially disease-modifying path for one of neuromuscular neurology’s most stubborn foes, but also foreshadow a future of personalized genetic therapies in neurology. Back to top Article Information Corresponding Author: Eva L. Feldman, MD, PhD, Department of Neurology, University of Michigan, 109 Zina Pitcher Pl, 5017 AAT-BSRB, Ann Arbor, MI 48109 (efeldman@umich.edu). Published Online: January 4, 2016. doi:10.1001/jamaneurol.2015.4011. Conflict of Interest Disclosures: None reported. Funding/Support: Support during the preparation of the manuscript was provided by the A. Alfred Taubman Medical Research Institute. Role of the Funder/Sponsor: The A. Alfred Taubman Medical Research Institute had no role in the preparation, review, or approval of the manuscript or decision to submit the manuscript for publication. Submissions: This Viewpoint is in a projected series of Next Generation Neurology Viewpoints emphasizing innovative original concepts and approaches to understanding the causation of neurological disease and providing new and effective therapies. We welcome the submission of Viewpoints that incorporate these objectives. References 1. US Department of Health and Human Services Food and Drug Administration. Duchenne Muscular Dystrophy and Related Dystrophinopathies: Developing Drugs for Treatment. Silver Spring, MD: Food and Drug Administration Center for Drug Evaluation and Research; 2015. 2. Dunckley MG, Manoharan M, Villiet P, Eperon IC, Dickson G. Modification of splicing in the dystrophin gene in cultured Mdx muscle cells by antisense oligoribonucleotides. Hum Mol Genet. 1998;7(7):1083-1090.PubMedGoogle ScholarCrossref 3. van Deutekom JC, Bremmer-Bout M, Janson AA, et al. Antisense-induced exon skipping restores dystrophin expression in DMD patient derived muscle cells. Hum Mol Genet. 2001;10(15):1547-1554.PubMedGoogle ScholarCrossref 4. Mendell JR, Rodino-Klapac LR, Sahenk Z, et al; Eteplirsen Study Group. Eteplirsen for the treatment of Duchenne muscular dystrophy. Ann Neurol. 2013;74(5):637-647.PubMedGoogle ScholarCrossref 5. Goemans NM, Tulinius M, van den Akker JT, et al. Systemic administration of PRO051 in Duchenne’s muscular dystrophy. N Engl J Med. 2011;364(16):1513-1522.PubMedGoogle ScholarCrossref 6. Voit T, Topaloglu H, Straub V, et al. Safety and efficacy of drisapersen for the treatment of Duchenne muscular dystrophy (DEMAND II): an exploratory, randomised, placebo-controlled phase 2 study. Lancet Neurol. 2014;13(10):987-996.PubMedGoogle ScholarCrossref 7. Aartsma-Rus A, Fokkema I, Verschuuren J, et al. Theoretic applicability of antisense-mediated exon skipping for Duchenne muscular dystrophy mutations. Hum Mutat. 2009;30(3):293-299.PubMedGoogle ScholarCrossref

Journal

JAMA NeurologyAmerican Medical Association

Published: Mar 1, 2016

Keywords: mutation,antisense oligonucleotides,duchenne's muscular dystrophy,neuroscience,x chromosomes,neurologists,exons

References