Viltepso fails main goal in confirmatory Phase 3 study

Company analyzing factors that may have influenced results

Margarida Maia, PhD avatar

by Margarida Maia, PhD |

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After nearly a year of treatment, Viltepso (viltolarsen) was well tolerated and tended to increase how fast boys with Duchenne muscular dystrophy (DMD) could stand from a lying position, but not significantly more than a placebo, according to a preliminary analysis of a Phase 3 study.

The main goal of that study, RACER53 (NCT04060199), was to confirm the clinical benefit of 80 mg/kg Viltepso over the placebo by showing that boys ages 4 to 7,who have a disease-causing mutation amenable to exon 53 skipping would stand faster after 48 weeks of treatment.

Given as a once-weekly infusion directly into the bloodstream, Viltepso was first approved by the U.S. Food and Drug Administration (FDA) in 2020 under an accelerated pathway, with confirmatory data a condition for gaining full approval.

“Considering the results of prior clinical studies, we have confidence that viltolarsen [Viltepso] can be a beneficial treatment for amenable patients with Duchenne,” Tsugio Tanaka, president of NS Pharma, which developed and markets Viltepso, said in a company press release.

The company is running further analyses to see if there are any factors that may have skewed the results, which don’t match those of earlier Phase 2 studies in which Viltepso was tested for up to four years.

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Age, treatment duration among factors for further analysis

“We are currently conducting further detailed data analyses and identifying factors that may have influenced the results,” Tanaka said. Factors to consider include age, treatment duration, and use of any other medications, such as corticosteroids.

DMD is caused by mutations in the DMD gene, which provides instructions for producing dystrophin, a protein found in muscles. When dystrophin is missing or doesn’t function properly, muscles become more sensitive to damage from wear and tear. As a result, they become gradually weaker over time.

Viltepso contains a short strip of genetic material, called an antisense oligonucleotide, that enables the protein-producing machinery in cells to move past a portion of the DMD gene called exon 53. The protein that’s produced is shorter but functional, which is expected to protect muscles from damage.

The treatment’s approval was based on the results of a Phase 2 study (NCT02740972) in which the once-weekly dose of 80 mg/kg Viltepso resulted in an increase in the amount of dystrophin present in skeletal muscles that was likely to predict clinical benefit.

Four-year data from an open-label extension study (NCT03167255) showed that Viltepso delayed motor function decline in boys with DMD, ages 4 to 9, compared with what would be seen without treatment in a historical control group matched for key factors.

In both Phase 2 and Phase 3 studies, side effects reported with Viltepso were primarily mild or moderate in severity. There were no side effects leading to treatment discontinuation over the course of the studies.