FDA-Approved Oligonucleotide Therapies in 2017
Authors: Cy A. Stein, Daniela Castanotto
Journal: Molecular Therapy (2017)

Oligonucleotides (oligos) have been under clinical development for approximately the past 30 years, beginning with antisense oligonucleotides (ASOs) and apatmers and followed about 15 years ago by siRNAs. During that lengthy period of time, numerous clinical trials have been performed and thousands of trial participants accrued onto studies. Of all the molecules evaluated as of January 2017, the regulatory authorities assessed that six provided clear clinical benefit in rigorously controlled trials. The story of these six is given in this review. Oligonucleotides (oligos) have been under clinical development for approximately the past 30 years, beginning with antisense oligonucleotides (ASOs) and apatmers and followed about 15 years ago by siRNAs. During that lengthy period of time, numerous clinical trials have been performed and thousands of trial participants accrued onto studies. Of all the molecules evaluated as of January 2017, the regulatory authorities assessed that six provided clear clinical benefit in rigorously controlled trials. The story of these six is given in this review. The first antisense oligonucleotide approved for marketing by the Food and Drug Administration (FDA) was fomivirsen, a drug developed in a collaboration of Isis Pharmaecuticals with Novartis Ophthalmics. This 21-mer phosphorothioate oligodeoxynucleotide (which contains a CpG motif near its 5′ terminus) was intended for treatment of patients with cytomegalovirus (CMV) retinitis. The target was the mRNA that encoded the CMV immediate-early (IE)-2 protein, which is required for viral replication. In a study from the Vitravene Study Group,1Vitravene Study GroupA randomized controlled clinical trial of intravitreous fomivirsen for treatment of newly diagnosed peripheral cytomegalovirus retinitis in patients with AIDS.Am. J. Ophthalmol. 2002; 133: 467-474Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar fomivirsen was highly successful at ameliorating the signs and symptoms of CMV retinitis. Treatment consisted of weekly injections of 165 μg of fomivirsen injected directly into the vitreous humor, followed by maintenance injections every other week until evidence of retinitis progression. The median time to disease progression for the fomivirsen-treated group (n = 18) was 71 days versus 13 days for the untreated group (p = 0.0001). Overall during the course of the study, progression occurred in 44% of the treated versus 70% of the untreated patients. Vitravene was approved by the FDA in 1998 and by EMEA (European Agency for the Evaluation of Medicinal Products) in 1999. At the time, there was a high unmet need for an anti-cytomegalovirus retinitis drug. Subsequently, due to the development of high-activity anti-retroviral therapy (HAART), the number of CMV cases dramatically decreased. Novartis stopped marketing the drug in 2002 in Europe and in 2006 in the United States. Macugen (formerly pegaptanib) is an aptamer targeted to vascular endothelial growth factor (VEGF165). The molecule was first produced by the systematic evolution of ligands by exponential enrichment (SELEX)2Tuerk C. Gold L. Systematic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase.Science. 1990; 249: 505-510Crossref PubMed Scopus (7993) Google Scholar, 3Eaton B.E. The joys of in vitro selection: chemically dressing oligonucleotides to satiate protein targets.Curr. Opin. Chem. Biol. 1997; 1: 10-16Crossref PubMed Google Scholar strategy pioneered by Larry Gold (an excellent review by Ng et al.4Ng E.W. Shima D.T. Calias P. Cunningham Jr., E.T. Guyer D.R. Adamis A.P. Pegaptanib, a targeted anti-VEGF aptamer for ocular vascular disease.Nat. Rev. Drug Discov. 2006; 5: 123-132Crossref PubMed Scopus (1125) Google Scholar provides a brief, concise description of the SELEX approach). Pegaptanib is 27 nucleobases in length. The molecule contains a phosphorothioate 3′-3′ deoxythymidine cap to promote nuclease stability; all of the purine ribose sugars are 2′-O-methylated and the pyrimidine ribose sugars all 2′-fluorinated.5Ruckman J. Green L.S. Beeson J. Waugh S. Gillette W.L. Henninger D.D. Claesson-Welsh L. Janjić N. 2′-Fluoropyrimidine RNA-based aptamers to the 165-amino acid form of vascular endothelial growth factor (VEGF165). Inhibition of receptor binding and VEGF-induced vascular permeability through interactions requiring the exon 7-encoded domain.J. Biol. Chem. 1998; 273: 20556-20567Crossref PubMed Scopus (647) Google Scholar A 40 kDa polyethylene glycol substituent was linked to the 5′ molecular terminus. Pegaptanib binds to the heparin binding site of VEGF1656Jellinek D. Green L.S. Bell C. Janjić N. Inhibition of receptor binding by high-affinity RNA ligands to vascular endothelial growth factor.Biochemistry. 1994; 33: 10450-10456Crossref PubMed Scopus (162) Google Scholar with an affinity in the picomolar range and inhibits its binding to both VEGFR1 and VEGFR2, though to a lesser extent for the latter. Macugen was approved by the FDA to treat age-related macular degeneration (AMD) of the retina. This disease is the leading cause of blindness in people over the age of 50. At least in part, it's caused by the VEGF165-stimulated growth of blood vessels (neovascularization) of the choroid (the vascular tissue beneath the retina) of the eye. Disease activity is especially severe in the macula of the retina and can result in central blindness. The safety and efficacy of pegaptanib was evaluated in two identical trials. These trials were prospective, randomized, double-masked, multi-centered, and dose finding and have been referred to as the VISION trials.7Gragoudas E.S. Adamis A.P. Cunningham Jr., E.T. Feinsod M. Guyer D.R. VEGF Inhibition Study in Ocular Neovascularization Clinical Trial GroupPegaptanib for neovascular age-related macular degeneration.N. Engl. J. Med. 2004; 351: 2805-2816Crossref PubMed Scopus (2103) Google Scholar, 8D'Amico D.J. VEGF inhibition study in ocular neovascularization (VISION): Second year efficacy data.Invest. Ophthalmol. Vis. Sci. 2005; 46: 2309Google Scholar There were 1,186 patients that received at least one treatment of pegatinib. The drug was injected into the ocular vitreous humor (after the application of the appropriate anesthesia) at doses of 0.3, 1.0, or 3.0 mg every 6 weeks for 48 weeks (nine treatments total). An equivalent number of patients received a sham treatment. Efficacy was determined by the ability of patients to visually read the letters on eye charts, a metric that is currently the standard in clinical ophthalmology trials. There were 70%, 71%, and 65% of patients receiving the 0.3, 1.0, and 3.0 mg doses of pegaptanib, respectively, compared to 55% of patients receiving the sham injections (p > 0.001, > 0.001, and > 0.03, respectively) that lost less than 15 letters of visual acuity, or about three lines on the study eye chart, in the 54 week duration of the trial. Because higher doses of pegaptanib did not lead to greater activity, the FDA approved the 0.3 mg dose in late 2004. All the doses of pegaptanib were safe. Side effects were mild to moderate in intensity and related to the injection process rather than the drug. These included five cases each of traumatic injury and lens detachment and 12 cases of endophthalmitis (most of these patients nevertheless remained in the trial). All these adverse events were seen in the pegaptanib-injected patients. As anticipated, there was no systemic toxicity. The results after 2 years of pegaptanib treatment were similar to those observed after 1 year, with a 45% mean relative benefit change in vision noted in drug versus sham injected patients (p < 0.01). However, since the FDA approval date in 2004, Macugen has led a peripatetic existence. Initially developed by NeXstar, Macugen was eventually acquired by Eyetech Pharmaceuticals, which in collaboration with Pfizer performed the randomized trials above. In 2005, Eyetech Pharmaceuticals was acquired by OSI Pharmaceuticals. However, in 2008, former Eyetech employees purchased the drug from OSI and established Eyetech, Inc., which in turn was acquired by Valeant Pharmaceuticals in 2010. The drug is currently marketed by Bausch and Lomb, which was acquired in 2013 by Valeant.9http://ophthalmologytimes.modernmedicine.com/ophthalmologytimes/news/modernmedicine/modern-medicine-news/valeant-acquires-eyetech.Google Scholar Sales of Macugen were satisfactory from 2004 through 2011. However, by 2010, sales had declined to only $12 million/year in the face of stiff competition from Lucentis (ranibizumab; Novartis). This drug appears to be more effective than Macugen. Competition also came from the off-label use of the anti-VEGF mAb bevacizumab (Avastin; Genentech), which is less expensive. Nevertheless, as of 2017, Macugen still retains a relatively small, though somewhat precarious, market share for the treatment of AMD. Mipomersen is approved in the United States (but not in the European Union) for homozygous familial hypercholesterolemia (HoFH). This is a disease associated with very high plasma concentration of low-density lipoprotein (LDL). Molecularly, the disease is distinguished by loss of function mutations in both LDL-receptor genes. This results in the reduced liver uptake of plasma LDL cholesterol (LDL-C). Because of the lack of clearance of LDL-C, patients with this ailment develop cutaneous and tendinous xanthomatas (benign cholesterol-laden tumors), in addition to cardiovascular disease at a very young age. Patients not treated usually do not live beyond the age of about 30.10Raal F.J. Santos R.D. Blom D.J. Marais A.D. Charng M.J. Cromwell W.C. Lachmann R.H. Gaudet D. Tan J.L. Chasan-Taber S. et al.Mipomersen, an apolipoprotein B synthesis inhibitor, for lowering of LDL cholesterol concentrations in patients with homozygous familial hypercholesterolaemia: a randomised, double-blind, placebo-controlled trial.Lancet. 2010; 375: 998-1006Abstract Full Text Full Text PDF PubMed Scopus (745) Google Scholar LDL-C levels can be dramatically reduced by statins,11Besseling J. Sjouke B. Kastelein J.J.P. Screening and treatment of familial hypercholesterolemia - Lessons from the past and opportunities for the future (based on the Anitschkow Lecture 2014).Atherosclerosis. 2015; 241: 597-606Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar but even high dose intensity statins are often insufficient to reach therapeutic goals; i.e., elimination of the risk for coronary heart disease. The core protein of the LDL particle is apolipoprotein B (apoB). In an attempt to diminish circulating LDL-C, Crooke et al.,12Crooke R.M. Graham M.J. Lemonidis K.M. Whipple C.P. Koo S. Perera R.J. An apolipoprotein B antisense oligonucleotide lowers LDL cholesterol in hyperlipidemic mice without causing hepatic steatosis.J. Lipid Res. 2005; 46: 872-884Crossref PubMed Scopus (206) Google Scholar synthesized mipomersen (Isis 147764), an antisense 20-mer phosphorothioate 2′-methoxyethoxy (MOE) gapmer targeted to the coding region of the apoB mRNA (see below). The MOE groups were incorporated at positions 1–5 and 15–20 of the oligonucleotide chain, where they block exonuclease activity and increase the Tm of the apoB mRNA/ASO duplex by 1.1°C per MOE residue.13De Mesmaeker A. Haener R. Martin P. Moser H.E. Antisense oligonucleotides.Acc. Chem. Res. 1995; 28: 366-374Crossref Scopus (480) Google Scholar This stabilizes the duplex and helps to enable cleavage of the mRNA by RNase H1. apoB is the critical protein of atherogenic lipoprotein particles11Besseling J. Sjouke B. Kastelein J.J.P. Screening and treatment of familial hypercholesterolemia - Lessons from the past and opportunities for the future (based on the Anitschkow Lecture 2014).Atherosclerosis. 2015; 241: 597-606Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar and is central to the clearance of these particles through its binding to the LDL receptor on hepatocytes.14Davidson N.O. Shelness G.S. APOLIPOPROTEIN B: mRNA editing, lipoprotein assembly, and presecretory degradation.Annu. Rev. Nutr. 2000; 20: 169-193Crossref PubMed Scopus (234) Google Scholar This protein also plays a role in the production of very low-density lipoprotein (VLDL) particles by the liver and is associated with the VLDL particle. VLDL particles, in turn, are metabolic precursors of LDL particles.15Mahley, R. Biochemistry and Physiology of Lipid and Lipoprotein Metabolism. In Principles and Practice of Endocrinology and Metabolism, Third Edition, K.L. Becker, ed. (Lippincott Williams and Wilkins).Google Scholar After completion of studies in healthy volunteers and phase I and II clinical studies, four different phase III clinical trials were performed.10Raal F.J. Santos R.D. Blom D.J. Marais A.D. Charng M.J. Cromwell W.C. Lachmann R.H. Gaudet D. Tan J.L. Chasan-Taber S. et al.Mipomersen, an apolipoprotein B synthesis inhibitor, for lowering of LDL cholesterol concentrations in patients with homozygous familial hypercholesterolaemia: a randomised, double-blind, placebo-controlled trial.Lancet. 2010; 375: 998-1006Abstract Full Text Full Text PDF PubMed Scopus (745) Google Scholar, 16McGowan M.P. Tardif J.C. Ceska R. Burgess L.J. Soran H. Gouni-Berthold I. Wagener G. Chasan-Taber S. Randomized, placebo-controlled trial of mipomersen in patients with severe hypercholesterolemia receiving maximally tolerated lipid-lowering therapy.PLoS ONE. 2012; 7: e49006Crossref PubMed Scopus (184) Google Scholar, 17Stein E.A. Dufour R. Gagne C. Gaudet D. East C. Donovan J.M. Chin W. Tribble D.L. McGowan M. Apolipoprotein B synthesis inhibition with mipomersen in heterozygous familial hypercholesterolemia: results of a randomized, double-blind, placebo-controlled trial to assess efficacy and safety as add-on therapy in patients with coronary artery disease.Circulation. 2012; 126: 2283-2292Crossref PubMed Scopus (249) Google Scholar, 18Thomas G.S. Cromwell W.C. Ali S. Chin W. Flaim J.D. Davidson M. Mipomersen, an apolipoprotein B synthesis inhibitor, reduces atherogenic lipoproteins in patients with severe hypercholesterolemia at high cardiovascular risk: a randomized, double-blind, placebo-controlled trial.J. Am. Coll. Cardiol. 2013; 62: 2178-2184Crossref PubMed Scopus (181) Google Scholar Mipomersen was evaluated in distinct sets of patients: trial #1 required genetic confirmation of HoFH or untreated LDL-C >500 mg/dL with either xanthoma before 10 years of age or heterozygous FH (HeFH) in both parents;10Raal F.J. Santos R.D. Blom D.J. Marais A.D. Charng M.J. Cromwell W.C. Lachmann R.H. Gaudet D. Tan J.L. Chasan-Taber S. et al.Mipomersen, an apolipoprotein B synthesis inhibitor, for lowering of LDL cholesterol concentrations in patients with homozygous familial hypercholesterolaemia: a randomised, double-blind, placebo-controlled trial.Lancet. 2010; 375: 998-1006Abstract Full Text Full Text PDF PubMed Scopus (745) Google Scholar trial #2 required LDL-C >200 mg/dL;16McGowan M.P. Tardif J.C. Ceska R. Burgess L.J. Soran H. Gouni-Berthold I. Wagener G. Chasan-Taber S. Randomized, placebo-controlled trial of mipomersen in patients with severe hypercholesterolemia receiving maximally tolerated lipid-lowering therapy.PLoS ONE. 2012; 7: e49006Crossref PubMed Scopus (184) Google Scholar trial #3 required HeFH plus LDL-C >100 mg/dL + serum triglycerides <200 mg/dL and a diagnosis of coronary artery disease;17Stein E.A. Dufour R. Gagne C. Gaudet D. East C. Donovan J.M. Chin W. Tribble D.L. McGowan M. Apolipoprotein B synthesis inhibition with mipomersen in heterozygous familial hypercholesterolemia: results of a randomized, double-blind, placebo-controlled trial to assess efficacy and safety as add-on therapy in patients with coronary artery disease.Circulation. 2012; 126: 2283-2292Crossref PubMed Scopus (249) Google Scholar and trial #4 required LDL-C >100 mg/dL + serum triglycerides <200 mg/dL.18Thomas G.S. Cromwell W.C. Ali S. Chin W. Flaim J.D. Davidson M. Mipomersen, an apolipoprotein B synthesis inhibitor, reduces atherogenic lipoproteins in patients with severe hypercholesterolemia at high cardiovascular risk: a randomized, double-blind, placebo-controlled trial.J. Am. Coll. Cardiol. 2013; 62: 2178-2184Crossref PubMed Scopus (181) Google Scholar In every trial, patients were 2:1 randomized to 26 weekly, subcutaneous injections of either 200 mg mipomersen or placebo. All patients were maintained on maximally tolerated lipid lowering therapy. Most patients were middle aged, overweight, and about 50% had metabolic syndrome. Trial #1 (HoFH) assigned 34 patients to mipomersen and 17 to placebo. The mean percentage change from baseline in serum LDL-C levels was about 25% in the mipomersen group versus about 3% in the placebo group, p = 0.0003, as measured 28 weeks after the initiation of treatment. Similar changes in total cholesterol, non-HDL cholesterol, and, to a lesser extent triglycerides, were also seen. In trial #2, 39 patients were treated with mipomersen and 19 with placebo. In the mipomersen-treated patients (n = 27), LDL-C was reduced by 36%, as compared with an increase of about 13% in n = 12 placebo patients (p < 0.001). Changes in serum apoB, non-HDL-C, and Lp(a) (see below) were similar, though serum triglyceride levels were reduced to a much lesser extent. A >15% decrease in LDL-C was seen in almost 80% versus 17% of the mipomersen versus placebo-treated patients and about 25% of the mipomersen patients achieved a >50% reduction in LDL-C. In trial #3 (HeFH), 83 of the randomized patients received mipomersen and 41 received placebo. LDL-C was decreased by a mean of 28% in the 73 mipomersen patients who completed the study, compared to an increase of 5% in those treated with placebo. Total cholesterol, apoB, non-HDL-C, and Lp(a) levels were also reduced (all p < 0.001), though HDL-C levels were unaltered. As in the other trials above, triglyceride levels were reduced to a lesser extent. Finally, in trial #4, in patients with severe hypercholesterolemia at high cardiovascular risk, 60 mipomersen patients and 44 placebo (out of 158 randomized) patients completed treatment. Mipomersen reduced LDL-C by 36% versus 4.5% (p < 0.001) and a targeted LDL-C level of <100 mg/dL was achieved in 76% versus 38% of placebo treated patients. Reductions in apoB and non-HDL-C were similar to that observed for LDL-C, but reductions in triglycerides and total cholesterol were somewhat smaller. Despite the wealth of efficacy data, initial regulatory opinion of mipomersen was not favorable. On December 13, 2012, the European Medicines Agency (EMA) refused marketing authorization for the drug.15Mahley, R. Biochemistry and Physiology of Lipid and Lipoprotein Metabolism. In Principles and Practice of Endocrinology and Metabolism, Third Edition, K.L. Becker, ed. (Lippincott Williams and Wilkins).Google Scholar The EMA acknowledged that the drug reduced average LDL-C in patients with HoFH and in severe heterozygous hypercholesterolemia by 25%–36%. However, the agency also observed that a "high proportion" of patients discontinued the drug by 2 years, mostly due to adverse events such as injection site reactions and liver toxicity. The EMA was also concerned about the increasing possibility of developing hepatic steatosis (fatty liver), and that the probability of developing serious cardiovascular events after mipomersen treatment was in fact greater than placebo. The EMA therefore concluded that the mipomersen induced reduction of cholesterol levels did not outweigh its cardiovascular risk. However, on January 13, 2013 the U.S. FDA granted approval for the marketing of Kynamro. Nevertheless, the drug failed to generate much in the way of sales. This was probably due to competition from a small molecule, which seems to have garnered most of the tiny group of patients with HoFH. By the beginning of 2016, the collaboration of Isis (now renamed Ionis) and Genzyme to market the drug had dissolved. By May, 2016, Kynamro's rights were sold by Ionis to Kastle Therapeutics. At the time of this writing (January, 2017), the clinical fate of mipomersen is not clear. Eteplirsen (Sarepta) is a 30-mer phosphomorpholidate oligonucleotide.19Excerpted from: Stein, C.A. (2016). Eteplirsen approved for Duchenne muscular dystrophy: The FDA faces a difficult choice. Mol. Ther. 24, 1884–1885.Google Scholar Unlike charged phosphorothioate oligonucleotides, uncharged phosphomorpholidates, which were first introduced more than 35 years ago, are not substrates for nucleases and cannot elicit RNase H activity. These properties are important for oligonucleotide-induced splice-switching activity. Oligonucleotide-directed splice switching is used to manipulate alternative pre-mRNA splicing. Splice-switching oligos (SSOs) can be employed to correct an irregular splicing event or to induce the expression of a new splicing variant, which may have a therapeutic function. The SSOs are generally targeted to bind one of the key splicing sequences (usually the donor or the acceptor splice site) that define the exons' boundaries within the gene pre-mRNA. When the exon boundary is masked, the splicing machinery searches downstream for another suitable site, thus skipping the problematic exon. Eteplirsen is designed to skip exon 51 of the dystrophin protein.19Excerpted from: Stein, C.A. (2016). Eteplirsen approved for Duchenne muscular dystrophy: The FDA faces a difficult choice. Mol. Ther. 24, 1884–1885.Google Scholar Eteplirsen has been evaluated in young male patients with the devastating disease known as Duchenne muscular dystrophy (DMD). This X-linked disease is found in approximately 1/3,500–1/5,000 live male births20Robinson-Hamm J.N. Gersbach C.A. Gene therapies that restore dystrophin expression for the treatment of Duchenne muscular dystrophy.Hum. Genet. 2016; 135: 1029-1040Crossref PubMed Scopus (50) Google Scholar and is characterized by mutations (such as intragenic deletions, duplications, and point mutations) in the dystrophin gene. These mutations disrupt the reading frame of the dystrophin mRNA and cause the introduction of premature stop codons,20Robinson-Hamm J.N. Gersbach C.A. Gene therapies that restore dystrophin expression for the treatment of Duchenne muscular dystrophy.Hum. Genet. 2016; 135: 1029-1040Crossref PubMed Scopus (50) Google Scholar leading to mRNA degradation and the loss of protein synthesis21Mendell J.R. Rodino-Klapac L.R. Sahenk Z. Roush K. Bird L. Lowes L.P. Alfano L. Gomez A.M. Lewis S. Kota J. et al.Eteplirsen Study GroupEteplirsen for the treatment of Duchenne muscular dystrophy.Ann. Neurol. 2013; 74: 637-647Crossref PubMed Scopus (538) Google Scholar, 22van Deutekom J.C. de Kimpe S.J. Campion G.V. Antisense oligonucleotides as personalized medicine for Duchenne muscular dystrophy.Drug Discov. Today Ther. Strateg. 2013; 10: e149-e156Crossref Scopus (7) Google Scholar in striated muscle. The clinical consequences of these mutations are catastrophic for the patient. The dystrophin protein is located beneath the sarcolemma (the muscle cell membrane). Via the binding of its N-terminal to F-actin and its C-terminal to β-dystroglycan, it connects the cellular cytoskeleton to the muscle cell membrane, the sarcolemma. Histologically, the loss of dystrophin results in inflammation, muscle degeneration, and its replacement with fibrotic tissue and fat.23Cirak S. Arechavala-Gomeza V. Guglieri M. Feng L. Torelli S. Anthony K. Abbs S. Garralda M.E. Bourke J. Wells D.J. et al.Exon skipping and dystrophin restoration in patients with Duchenne muscular dystrophy after systemic phosphorodiamidate morpholino oligomer treatment: an open-label, phase 2, dose-escalation study.Lancet. 2011; 378: 595-605Abstract Full Text Full Text PDF PubMed Scopus (697) Google Scholar Children and young adults who suffer from DMD progressively lose neuromuscular function as they age. This often culminates in their inability to walk by their mid-teens. Sadly, muscle deterioration is progressive: The inevitable result is cardiomyopathy, respiratory failure, and death by the time DMD patients are in their twenties.24Mendell J.R. Goemans N. Lowes L.P. Alfano L.N. Berry K. Shao J. Kaye E.M. Mercuri E. Eteplirsen Study Group and Telethon Foundation DMD Italian NetworkLongitudinal effect of eteplirsen versus historical control on ambulation in Duchenne muscular dystrophy.Ann. Neurol. 2016; 79: 257-271Crossref PubMed Scopus (363) Google Scholar It is not possible to express in words the anguish of the parents of DMD patients nor the suffering those who are afflicted by this terrible ailment must stoically endure. The molecular lesion, in about 13% of the patients with DMD, is an exon deletion that more commonly occurs in exons 47–63.25Kinali M. Arechavala-Gomeza V. Feng L. Cirak S. Hunt D. Adkin C. Guglieri M. Ashton E. Abbs S. Nihoyannopoulos P. et al.Local restoration of dystrophin expression with the morpholino oligomer AVI-4658 in Duchenne muscular dystrophy: a single-blind, placebo-controlled, dose-escalation, proof-of-concept study.Lancet Neurol. 2009; 8: 918-928Abstract Full Text Full Text PDF PubMed Scopus (571) Google Scholar Eteplirsen, targeted to the splice-donor region of exon 51, induces skipping of exon 51.26Aartsma-Rus A. Ferlini A. Goemans N. Pasmooij A.M. Wells D.J. Bushby K. Vroom E. Balabanov P. Translational and regulatory challenges for exon skipping therapies.Hum. Gene Ther. 2014; 25: 885-892Crossref PubMed Scopus (38) Google Scholar This produces a truncated, though in-frame, partially functional dystrophin protein, similar to what is found in Becker's muscular dystrophy. This variant leads to a disease in which the clinical symptoms can be much less severe than those of patients with Duchenne's. The age of onset of Becker's can be variable and many sufferers retain both their ability to ambulate and their active life style for many decades. As of 2013, numerous in vitro and in vivo studies had demonstrated restoration of dystrophin protein via an exon-skipping strategy.21Mendell J.R. Rodino-Klapac L.R. Sahenk Z. Roush K. Bird L. Lowes L.P. Alfano L. Gomez A.M. Lewis S. Kota J. et al.Eteplirsen Study GroupEteplirsen for the treatment of Duchenne muscular dystrophy.Ann. Neurol. 2013; 74: 637-647Crossref PubMed Scopus (538) Google Scholar, 22van Deutekom J.C. de Kimpe S.J. Campion G.V. Antisense oligonucleotides as personalized medicine for Duchenne muscular dystrophy.Drug Discov. Today Ther. Strateg. 2013; 10: e149-e156Crossref Scopus (7) Google Scholar, 23Cirak S. Arechavala-Gomeza V. Guglieri M. Feng L. Torelli S. Anthony K. Abbs S. Garralda M.E. Bourke J. Wells D.J. et al.Exon skipping and dystrophin restoration in patients with Duchenne muscular dystrophy after systemic phosphorodiamidate morpholino oligomer treatment: an open-label, phase 2, dose-escalation study.Lancet. 2011; 378: 595-605Abstract Full Text Full Text PDF PubMed Scopus (697) Google Scholar, 24Mendell J.R. Goemans N. Lowes L.P. Alfano L.N. Berry K. Shao J. Kaye E.M. Mercuri E. Eteplirsen Study Group and Telethon Foundation DMD Italian NetworkLongitudinal effect of eteplirsen versus historical control on ambulation in Duchenne muscular dystrophy.Ann. Neurol. 2016; 79: 257-271Crossref PubMed Scopus (363) Google Scholar, 25Kinali M. Arechavala-Gomeza V. Feng L. Cirak S. Hunt D. Adkin C. Guglieri M. Ashton E. Abbs S. Nihoyannopoulos P. et al.Local restoration of dystrophin expression with the morpholino oligomer AVI-4658 in Duchenne muscular dystrophy: a single-blind, placebo-controlled, dose-escalation, proof-of-concept study.Lancet Neurol. 2009; 8: 918-928Abstract Full Text Full Text PDF PubMed Scopus (571) Google Scholar In 2016, a phase III study24Mendell J.R. Goemans N. Lowes L.P. Alfano L.N. Berry K. Shao J. Kaye E.M. Mercuri E. Eteplirsen Study Group and Telethon Foundation DMD Italian NetworkLongitudinal effect of eteplirsen versus historical control on ambulation in Duchenne muscular dystrophy.Ann. Neurol. 2016; 79: 257-271Crossref PubMed Scopus (363) Google Scholar was published in which patients with DMD were treated with eteplirsen. The primary functional assessment examined in this trial was the 6-minute walk test (6MWT). This metric is currently the sole primary validated efficacy endpoint accepted by regulatory authorities.22van Deutekom J.C. de Kimpe S.J. Campion G.V. Antisense oligonucleotides as personalized medicine for Duchenne muscular dystrophy.Drug Discov. Today Ther. Strateg. 2013; 10: e149-e156Crossref Scopus (7) Google Scholar Treatments were double blinded; patients were randomly assigned to three cohorts (n = 4 each): (1) placebo; (2) eteplirsen 30 mg/kg; or (3) 50 mg/kg every week for 24 weeks. Patients then received open-label drug, while placebo patients also received open-label drug (30 or 50 mg/kg) for over 3 years. Because all placebo patients eventually received active drug treatment, pooled historical controls were used as the basis of comparison. The extraordinary and provocative results of this trial were that the distance lost on the 6MWT test at 36 months was 151 m less than in the historical controls (p < 0.01; from about 350 to 250 m in the eteplirsen-treated patients to about 350 to 100 m in the historical controls). After 3 years, only 16.7% of the eteplirsen-treated patients lost ambulation, compared with 46.2% of the historical control patients. Several treating physicians have privately voiced their opinion that eteplirsen substantially altered the natural history of the disease in responding patients. In addition, there were no reports of treatment-related serious adverse events. None of the observed adverse events led to treatment interruptions or dose adjustments. However, despite the chorus of numerous eteplersen advocates, including treating physicians, parents, and over 100 members of Congress, an FDA advisory panel, by a 6–7 vote, wasn't convinced: Sarepta had disregarded r