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In the Pipeline-Spinal Muscular Atrophy: Gene Therapy Found Effective for Children with SMA

Talan, Jamie

doi: 10.1097/01.NT.0000527856.96641.4b
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Fifteen boys with spinal muscular atrophy 1 have reached developmental motor milestones, previously not achievable, with gene therapy to replace defective spinal motor neuron 1. Independent experts said the results are encouraging, but questions remain about how long the therapy would need to be administered and how long it would be effective.

Fifteen infants born with the most severe form of spinal muscular atrophy (SMA) received a single intravenous injection of an experimental gene therapy treatment designed to replaced the mutated gene encoding survival motor neuron 1 (SMN1), which causes SMA.

Now, almost three years later, the children who received the gene therapy have survived longer, had superior results on motor milestones, and achieved better motor function than historical controls, according to the study published in the November 2 issue of The New England Journal of Medicine.

Many of the children who received the gene therapy are reaching developmental milestones that were never expected. Some are sitting, standing, walking and talking, and everyone in the study is still alive.

By contrast, babies born with SMA1 mutations never learn to sit, walk, or talk — and the natural untreated history of the disease suggests that only 8 percent of babies without permanent ventilation support are still alive at 20 months.

“The findings put gene therapy in a special place in the history of medical interventions. We have changed the name of the game for children with SMA and possibly for similar types of genetic neuromuscular diseases,” said Jerry R. Mendell, MD, professor of neurology and pediatrics and head of the neuromuscular disease program at Nationwide Children's Hospital in Columbus, OH.

“We can assume now that all but one patient (the oldest) in the study has continued to improve,” added Dr. Mendell. “These children have to be making SMN protein. Watching these children inspires us to do more.”

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SMA is caused by homozygous deletions/mutations in the SMN1 gene. Humans have a backup copy, the SMN2 gene, but in the absence of the normal SMN1 gene, the babies are dependent on the SMN2 gene, which is not identical to SMN1. It differs in one critical place: a single nucleotide substitution that alters splicing and usually excludes exon 7 in the mRNA. The result is that there isn't enough normal SMN protein produced.

SMA1 infants can have the first clinical signs of hypotonia and severe weakness as newborns. They can never sit without support. The death of lower motor neurons leads to muscle weakness. They quickly develop respiratory and swallowing difficulties, and most will be reliant on feeding support by the time they are 1 years old.

The Ohio team has been developing gene therapy for SMA1 for almost a decade. They make adeno-associated virus serotype 9 (AAV-9) carrying self-complementary DNA encoding the missing SMN protein. The virus delivers the therapeutic transgene to the target cell and then enables the cell to pump out a healthy supply of SMN1 that is crucial for motor development.

In 2012, based on animal studies, Dr. Mendell asked the US Food and Drug Administration (FDA) for permission to conduct a phase I study. The federal agency wanted the Ohio scientists to start with older children, but Dr. Mendell and his team convinced them that they had a better chance for successful gene therapy in younger children, before there was too much motor neuron loss. The FDA agreed but said that the infants had to have clinical signs of disease at the start of the study.

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The study was designed to deliver a single dose of intravenous AAV-9 vector carrying the SMN1 gene. All 15 patients had a genetically confirmed diagnosis of SMA1, homozygous SMN1 exon 7 deletions, and two copies of SMN2.

The first three patients received a low dose (6.7x1013 vg per kilogram of body weight), and the other dozen received a high dose (2.0x1014 vg per kilogram). The primary outcome of the study was safety. Secondary outcomes included the time to death or the need for permanent ventilatory assistance.

The researchers also assessed the boys over the study period using the Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND) scale of motor function in the first three patients on the low dose and compared it to those on the high doses. Then, they compared developmental motor milestones of those in the high-dose group to babies in the natural history SMA1 studies.

The data in the NEJM study reflect outcomes through August 2017. But Dr. Mendell said that all babies in the trial are still alive and event-free. All patients in this study reached the 20-month timepoint when only 8 percent are predicted to survive. The CHOP INTEND scores increased by almost 10 points at one month post-gene delivery in the high-dose infants to 15.4 points at three months.

Traditionally, doctors see a decline on these tests of function, now showing these results: 11 of the 12 in the high-dose group could sit unassisted, nine rolled over, 11 were eating and could speak, and two were walking without assistance.

Dr. Mendell said four boys had elevated serum aminotransferase levels, but none had any clinical signs related to this increase. Steroids lowered the levels. (After the investigators observed the elevated levels in the very first patient on the low dose, they quickly redesigned the study to start all subsequent infants on oral prednisolone one day before the delivery of the gene therapy. They were given a dose of 1 mg per kilogram per day for a month.)

By the time of the NEJM study, the last pulmonary assessment was 30.8 months in the low-dose group and 25.7 months in those on the high dose. One patient in the low-dose group eventually needed permanent ventilation at 29 months. (Ligation of the salivary gland lowered the time spent on non-invasive ventilation by 25 percent to 15 hours a day.)

While patients in both low- and high-dose groups had increases on the CHOP INTEND measures, those in the high-dose group had significantly higher scores. “Patients in cohort 1 had a mean increase of 7.7 points from a mean baseline of 16.3 points, and those in cohort 2 had a mean increase of 24.6 points from a mean baseline of 28.2 points,” the researchers wrote in the paper.

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By August 2017, 56 serious adverse events were observed in 13 of the 15 patients. The investigators concluded that two of these events were related to treatment — the elevations in the serum aminotransferase levels were 31 times the upper limit of the normal range for alanine aminotransferase and 14 times the upper limit for aspartate aminotransferase, but they observed no other liver-function abnormalities or clinical signs. The boys were successfully treated with prednisolone. There were also 241 non-serious adverse events, and three were treatment related, also pertaining to the high serum aminotransferase levels, which were less than 10 times the upper limit of the normal range.

The patients who were youngest at the time of the gene therapy infusion did better on all counts, said Dr. Mendell. He added that the children in this study will continue to be followed until they are around 15 years old.

The study was supported by the Chicago-based AveXis, Sophia's Cure Foundation, and the Research Institute at Nationwide Children's Hospital. Dr. Mendell has no investment ties to the gene therapy. He sits on the scientific advisory board of AveXis and has received a consulting fee but his group does not get paid by the company to run the clinical trial. Dr. Mendell is advising AveXis on the design of a multicenter study.

There are other molecular-based trials for SMA. In December 2016, the FDA approved the first treatment for SMA — an antisense oligonucleotide called nusinersen (Spinraza) that also works to increase SMN2 protein. Infants and young children in the study also reached many developmental milestones, including assisted sitting up, standing and walking. The FDA fast-tracked the drug, and the company, Ionis Pharmaceuticals, is still sponsoring ongoing clinical trials. The treatment requires multiple and ongoing intrathecal doses, with an estimated cost of $750,000 for the first year of treatment, with subsequent annual treatments dropping to $375,000 per year per patient.

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Commenting on the study, R. Jude Samulski, PhD, former director of University of North Carolina (UNC) Gene Therapy Center and professor in the department of pharmacology at the University of North Carolina in Chapel Hill, said: “These are very compelling data. It is the beginning of what the field can expect going forward.”

Dr. Samulski, who cloned the original AAV vector, has been working on AAV for almost 40 years. AAV-9, the vector used in this gene therapy trial for SMA1, is unique in that it crosses the blood-brain barrier, and he said it is hoped that more of the gene-packed viruses can deliver a working therapeutic gene to neurons. Different routes of administration are being considered, which ultimately could cut down on the dose and production of vector, and potentially be less expensive, he said.

Dr. Samulski added that the field will be watching to see whether the genes continue to work throughout childhood into adulthood when neuronal pruning and other normal and important developmental processes take place.

“Will the therapy given at birth be sufficient to do the job for life?” he asked. “No one knows. The data are very compelling that the AAV-9 vector got into neurons. Current understanding is we keep these neurons for life so the therapy should persist based on our understanding of the vector biology in animal studies. The only other variables that could impact long-term expression are related to a natural process such as neuronal turnover or age-specific neuronal pruning.”

Other human and animal studies suggest that the vector persists if the cells are there. “In humans, depending on the cell population, vectors have been detected ten years after the infusion,” Dr. Samulski said. “And because neurons are non-dividing cells, the hope is that the vector DNA will be long-lasting.”

The other future unknown is whether it would work to re-administer another round of vectors. “When you deliver a high dose of vector it generates a classic antibody immune response. These immune responses would prevent the vector working a second time due to neutralization,” Dr. Samulski said.

He said that researchers are working on ways to bypass this potential scenario, adding, “There may be ways to avoid the patient making an antibody response.”

Dr. Samulski disclosed that he is founder of AskBio Pharmaceuticals, a gene therapy company. He invented AAV technology, including the self-complementary AAV vectors used in the gene therapy study.

Jeffrey S. Chamberlain, PhD, the McCaw Endowed Chair in Muscular Dystrophy and director of the Sen. Paul D. Wellstone Muscular Dystrophy Research Center at the University of Washington, agreed that the gene therapy is clearly working in these boys. “It is an exciting advance,” he said. “But it is not a complete cure. There are still some neurological challenges and several unknowns.” Among unanswered questions: “What is the optimal age [for the therapy]?”

“It is clear that the earlier the better but now there should be an increased emphasis on prenatal carrier testing,” he said. “And how long will the treatment last? And how much is due to motor neuron targeting versus muscle targeting? If we can understand more about how the vector is working it could enable further advances in the approach.”

“In 2016, we saw new clinical treatments that modulate gene expression, and now in 2017 we are seeing gene replacement therapy. It is really exciting,” said Brent L. Fogel, MD, PhD, FAAN, associate professor of neurology and human genetics and director of the neurogenetics clinic and the Clinical Neurogenomics Research Center at the David Geffen School of Medicine at University of California, Los Angeles. He said that a key advantage of the gene therapy is that it is delivered intravenously and only one time.

“However,” he added, “we don't yet know how good the long-term outcome will be from this single dose. Another big question is whether there will be any long-term safety concerns. We won't know this until there are more numbers over a longer time.”

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•. Mendell JR, Al-Zaidy S, Shell R, et al Single-dose gene-replacement therapy for spinal muscular atrophy http:// N Engl J Med 2017; 377: 1713–1722.
    © 2017 American Academy of Neurology