Gene Replacement Improves Survival in Spinal Muscular Atrophy

A single IV infusion to replace the gene encoding survival motor neuron 1 (SMN1) increases survival among infants with spinal muscular atrophy type 1 (SMA1), according to research published in the November 2, 2017, issue of the New England Journal of Medicine. The treatment also improves motor function, and its effects are maintained for two years, the researchers said.

The loss or dysfunction of SMN1 causes SMA, a progressive disease characterized by the degeneration and loss of lower motor neurons. Onset of SMA1 typically occurs at one month of age. Children with the disease usually are weak, fail to achieve motor milestones, and have declines in respiration and swallowing. At a median age of 10.5 months, patients die or need permanent ventilatory assistance.

In December 2016, the FDA approved nusinersen for the treatment of SMA. A phase III study found that patients treated with nusinersen were more likely than controls to have improved motor function and event-free survival. The trial was stopped early because of the treatment’s efficacy.

A Small, Open-Label Trial

A mouse study indicated that IV administration of an adenoassociated viral vector (ie, AAV9) containing SMN1 reduced the effects of SMA and extended survival. Jerry R. Mendell, MD, Principal Investigator at Nationwide Children’s Hospital’s Center for Gene Therapy in Columbus, Ohio, and colleagues studied this therapeutic technique in humans.

Jerry R. Mendell, MD

They enrolled 15 patients with a genetically confirmed diagnosis of SMA1 into two cohorts. The first cohort received a low dose (6.7×1013 vg/kg) of treatment, and the second cohort received a high dose (2.0×1014 vg/kg). Because the first patient in cohort one had serum aminotransferase elevations, the investigators gave 1 mg/kg/day of oral prednisolone to all subsequent patients for 30 days, starting 24 hours before gene therapy.

The study’s primary outcome was treatment-related adverse events of grade 3 or higher. The secondary outcome was time until death or the need for permanent ventilatory assistance, which was defined as at least 16 hours/day of continuous respiratory assistance for at least 14 days. The achievement of motor milestones and Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND) scores were exploratory outcomes.

Motor Scores Improved

Three patients entered the low-dose cohort, and 12 were enrolled in the high-dose cohort. Patients’ mean age at treatment was 6.3 months in cohort 1 and 3.4 months in cohort 2. At the last follow-up, all patients had reached age 20 months, and none required permanent mechanical ventilation. Approximately 8% of patients in a historical cohort met these criteria.

All patients had increases from baseline in CHOP INTEND score and maintained these increases throughout the study. Patients in cohort 2 had mean increases of 9.8 points at one month and 15.4 points at three months. Eleven patients achieved and sustained scores greater than 40 points, which is considered clinically meaningful in SMA.

Of the patients in cohort 2, 11 sat unassisted, nine rolled over, 11 fed orally and could speak, and two walked independently. No patients in the historical cohorts achieved any of these milestones, and they rarely became able to speak.

Dr. Mendell and colleagues observed two treatment-related grade 4 adverse events. Both were elevations in serum aminotransferase levels that were attenuated after treatment with prednisolone. The researchers also noted three treatment-related nonserious adverse events (ie, asymptomatic elevations in serum aminotransferase levels that were resolved without additional prednisolone treatment).

The study results were consistent with those of the preclinical mouse study. During a follow-up period of as long as two years, Dr. Mendell and colleagues did not observe any decrease in treatment effect or regression in motor function among the study participants. The presence of antibodies to AAV9 could be a potential limitation of the therapy, however. Further research to assess the treatment’s safety and the durability of its effect are needed, according to the authors.

Comparing Two Treatments

It is difficult to compare the results of Dr. Mendell and colleagues with those of the phase III nusinersen study, because of the two trials’ different designs, said Ans T. van der Ploeg, MD, PhD, Chair of the Center for Lysosomal and Metabolic Diseases at the Erasmus MC University in Rotterdam, the Netherlands, in an accompanying editorial. One potential advantage of AAV9 gene therapy is that it might require a single IV infusion. Nusinersen, on the other hand, may require lifelong intrathecal treatment.

“The durability of the effects is uncertain for both treatments,” said Dr. van der Ploeg. “If the expression of the scAAV9 gene therapy declines over time, the same treatment may not be able to be repeated, because antibodies against AAV capsid proteins are anticipated to form.”

In addition, neither of the two therapies cures SMA type 1. Earlier treatment could be beneficial, as could a combination of both treatments, said Dr. van der Ploeg. But the high expected cost of nusinersen is “an important constraint,” she concluded.