Biogen’s Salanersen Earns FDA Breakthrough Therapy Designation for SMA

The FDA has granted Breakthrough Therapy Designation to salanersen (BIIB115), an investigational antisense oligonucleotide (ASO) developed by Biogen for the treatment of spinal muscular atrophy (SMA), the company announced on June 4, 2026. The designation is supported by exploratory Phase 1b findings indicating that some children with SMA who had previously received gene therapy and remained clinically suboptimal experienced improvements in motor function and evidence of slowed neurodegeneration following treatment with salanersen.¹

Salanersen is designed to increase production of survival motor neuron (SMN) protein by correcting SMN2 pre-mRNA splicing. The investigational therapy is administered intrathecally and is being developed as a once-yearly treatment option for individuals living with SMA.¹

What data supported the FDA’s decision?

According to Biogen, the FDA's decision was based on results from an ongoing phase 1b study involving 24 pediatric participants aged six months to 12 years who received at least two doses of salanersen.¹

Investigators reported clinically meaningful improvements in motor function among children who had previously received gene therapy but continued to experience disease burden. Biomarker analysis also demonstrated reductions in neurofilament light chain (NfL), a marker associated with ongoing neurodegeneration.¹

Among participants with elevated baseline NfL levels, investigators observed approximately 75% reductions after six months of treatment, with those reductions maintained during follow-up. All study participants showed improvement from baseline on at least one efficacy endpoint, while half achieved at least one new World Health Organization motor milestone. Importantly, all participants maintained previously attained motor milestones throughout the study period.¹

Salanersen was generally well tolerated at both evaluated dose levels. Most adverse events were reported as mild to moderate in severity.¹

“This designation reflects the FDA’s determination that salanersen has the potential to demonstrate substantial improvement over available therapies.” — Stephanie Fradette, Pharm.D., Head of the Rare Neurology Development Unit at Biogen.¹

How could salanersen fit into the SMA treatment paradigm?

Although disease-modifying therapies have significantly improved outcomes for individuals with SMA, treatment gaps remain, particularly among patients who continue to experience disease progression despite receiving currently available therapies.

"The FDA’s designation of salanersen as a breakthrough therapy recognizes that there is continued unmet need in spinal muscular atrophy, and there is more that can be done for people impacted by the disease," Diana Castro, MD, of the Neurology Rare Disease Center in Flower Mound, Texas, stated in the announcement.¹

Biogen noted that some children enrolled in the phase 1b study gained important motor functions, including sitting and walking, after initiating salanersen treatment. These findings have generated interest in evaluating whether the therapy could provide additional benefit following gene therapy administration.¹

What studies are planned next?

Biogen is advancing salanersen through a global phase 3 development program consisting of three clinical studies.¹

The STELLAR-1 study is currently recruiting clinically presymptomatic infants younger than six weeks of age who have a genetic diagnosis of SMA. The SOLAR study is recruiting adolescents and adults aged 15 to 60 years who are either treatment-naïve or previously treated with risdiplam. Meanwhile, STELLAR-2 is expected to begin enrollment in June 2026 and will evaluate salanersen in infants approximately six months after treatment with onasemnogene abeparvovec-xioi gene therapy.¹

The studies are designed to assess the safety and efficacy of annual 80 mg dosing across a broad range of SMA patient populations.¹

Why does breakthrough therapy designation matter?

Breakthrough Therapy Designation is intended to accelerate development and regulatory review for therapies that treat serious conditions and demonstrate preliminary evidence of substantial improvement over available treatments.²

For salanersen, the designation may facilitate more frequent interactions with the FDA and potentially streamline development as Biogen seeks to determine the therapy's role in the evolving SMA treatment landscape.¹

SMA remains one of the leading genetic causes of infant mortality and affects approximately one in 10,000 live births worldwide. While recent therapeutic advances have transformed patient outcomes, continued efforts are focused on improving motor function, preserving neurological health, and addressing residual unmet needs across the disease spectrum^.1,3

References

1. Biogen's salanersen receives FDA Breakthrough Therapy Designation for spinal muscular atrophy. (2026 Jun 4). Biogen. https://investors.biogen.com/news-releases/news-release-details/biogens-salanersen-receives-fda-breakthrough-therapy-designation
2. U.S. Food and Drug Administration. Breakthrough Therapy designation. FDA. Accessed June 4, 2026. https://www.fda.gov/drugs/nda-and-bla-approvals/breakthrough-therapy-approvals
3. Spinal muscular atrophy fact sheet. (2026 Feb). National Institute of Neurological Disorders and Stroke. chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.mda.org/sites/default/files/2026/02/SMA-Fact-Sheet.pdf#:~:text=SMA%20is%20classified%20as%20a,SMA%2C%20motor%20neurons%20in%20the