DMD Gene Therapy Safe, Effective at 4 Years

PHOENIX — Children with Duchenne muscular dystrophy (DMD) treated with the only gene therapy to date to be approved for treatment of disease in the United States show sustained maintenance of motor function after 4 years, compared with untreated patients who showed significant decline over the same time period, new research shows.

“Functional assessments demonstrated long-term sustained stabilization of motor function that was clinically meaningful, at ages where functional decline would be expected based on natural history,” the investigators note in their abstract. Furthermore, the treatment, known as delandistrogene moxeparvovec-rokl (SRP-9001) was well-tolerated 4 years posttreatment, they add.

The study was presented here at the 2023 American Association of Neuromuscular Electrodiagnostic Medicine (AANEM) Annual Meeting.

Severe Type of DMD

Considered one of the most severe forms of muscular dystrophy, DMD causes progressive muscle wasting stemming from the root genetic cause of missing dystrophin in muscle cells. Often referred to as a molecular “shock absorber,” dystrophin stabilizes the sarcolemma during muscle contractions to prevent degeneration.

SRP-9001, a single-dose recombinant gene therapy administered as an intravenous infusion, was designed to deliver a trimmed down form of dystrophin to compensate for the deficit.

In July, the adeno-associated virus vector (AAV)-based SRP-9001 gene therapy was granted accelerated approval by the US Food and Drug Administration for the treatment of ambulatory pediatric patients aged 4-5 years with DMD with a confirmed mutation in the DMD gene.

The therapy is administered over 1-2 hours at a dose of 133 trillion vector genomes per kilogram of body weight.

For Study 101, one of several evaluating the novel therapy, a research team led by senior investigator Jerry Mendell, MD, an attending neurologist at Nationwide Children’s Hospital and professor of pediatrics and neurology at The Ohio State University, evaluated data on four ambulatory male patients aged 4-8 years who received a single IV infusion of the therapy.

All patients also received prednisone 1 mg/kg, 1 day pre-infusion and 30 days post-infusion.

At 4 years posttreatment, there were no new safety events. All treatment-related adverse events occurred mainly within the first 70 days, and all resolved.

The most commonly reported adverse reactions of the gene therapy include vomiting, nausea, increases in liver enzymes, pyrexia (fever), and thrombocytopenia, all of which occurred within 90 days of infusion and been manageable.

Risk mitigation strategies for hepatotoxicity or acute liver injury include pre- and post-infusion monitoring of liver enzymes, the authors note.

No serious abnormalities were observed in hematologic or chemistry panels, and while three patients had elevated gamma-glutamyl transpeptidase in the first 3 months post-treatment, those cases resolved with oral steroid treatment.

Significant improvements in function were observed, with a mean improvement in North Star Ambulatory Assessment (NSAA) scores from baseline of 7.0 points (range 4 to 11).

Exploratory analyses further showed that compared with a propensity-score–weighted external control cohort of 21 patients with DMD who did not receive the therapy, those receiving SRP-9001 had a statistically significant difference of 9.4 points in least-squares mean change from baseline to 4 years on the NSAA score (P = .0125).

Similar trends were observed in improvement from baseline in key measures of time to rise, 4-stair climb, and 10- and 100-meter walk/run function tests.

Other reported adverse events include acute serious liver injury, immune-mediated myositis, and myocarditis. Due to the latter risk, the therapy is contraindicated in patients with any deletion in exon 8 and/or exon 9 in the DMD gene.

The current 4-year update on SRP-9001 adds to clinical trial results that have been reported on more than 80 patients treated to date, with favorable results and consistent safety profiles reported at other time points.

Continued FDA approval for the therapy will be contingent upon verification of a clinical benefit in the confirmatory trials, including the EMBARK trial.

Increased Function, Long-Term Stability

Discussing the research at the meeting, Craig McDonald, MD, professor and chair of physical medicine & rehabilitation, a professor of pediatrics and study chair of the CINRG Duchenne Natural History Study at the University of California Davis Health, noted that topline results from the ongoing, confirmatory phase 3 EMBARK trial show functional benefits of SRP-9001 not only in 4- to 5-year-olds but also in other older age groups.

“What’s really striking, and in my mind the most impressive, is that when you follow these patients out 3 or 4 years…you see there is this bump in function followed by long-term stability, whereas the external control cohort predictably shows really quite significant declines in their [NSAA] functional values,” he said in his presentation.

“When you look at each individually treated patient vs their own predicted trajectory using their baseline values on the time function test, each of the patients actually has a really quite impressive stabilization of function over their predicted disease trajectory,” he added.

A caveat that SRP-9001 shares with other gene therapies is the issue of cost — reported in the range of $2-$3 million.

In the context of racial and socioeconomic disparities in access to diagnosis and care reported in DMD, Emma Ciafaloni, MD, a professor of neurology and pediatrics at the University of Rochester Medical Center, Rochester, New York, underscored the need to consider approval vs access to gene therapies and how to optimize access to the novel treatments. 

“We need to consider what the cost is, how it’s going to be accessed, and whether there is a sustainable model,” said Ciafaloni, who was not associated with the study. “There will need to be institutional readiness and support for specialized multidisciplinary clinics for gene therapy.”

She also noted “we need to consider how we can do better on a broader level, because this is not a provider problem or a manufacturer problem — it’s a society problem.”

The study was funded by Sarepta Therapeutics, Inc. McDonald reported consulting work for Sarepta Therapeutics and has been an investigator in SRP-9001 research. Ciafaloni reported serving on advisory boards or other relationships with Alexion, Argenx, Biogen, Amicus, Momenta, Medscape, Pfizer, Sanofi/Genzyme, Sarepta, Jansen, NS Pharma, CureSMA, Orphazyme, the PCORI, PPMD, PTC Therapeutics, and Santhera.