Biogen's salanersen wins FDA Breakthrough Therapy tag for SMA
Biogen has secured FDA Breakthrough Therapy Designation for salanersen, its investigational antisense oligonucleotide (ASO) in development for spinal muscular atrophy (SMA). The agency's decision is supported by exploratory data from a Phase 1b study suggesting the drug may offer meaningful benefit to patients who responded poorly to earlier gene therapy treatment.
The designation is designed to expedite the development and review of therapies for serious conditions where preliminary evidence indicates potential substantial improvement over available treatment. For Biogen, it represents a notable regulatory endorsement at a time when the SMA treatment landscape is becoming increasingly crowded with approved options.
Phase 1b findings
The Phase 1b study enrolled 24 participants aged between six months and 12 years, all of whom received at least two doses of salanersen at either 40 mg or 80 mg. Among those with elevated baseline levels of neurofilament light chain (NfL) — a potential biomarker of ongoing neurodegeneration — the data showed a 75% mean reduction in NfL by six months, sustained throughout the follow-up period. Twelve of the 24 participants achieved at least one new WHO motor milestone, and no participant lost a milestone recorded at baseline.
Diana Castro, MD, of the Neurology Rare Disease Center in Flower Mound, Texas, who participated in the study, said the data showed "unexpected improvements" on exploratory endpoints in children previously dosed with gene therapy, with some gaining functional abilities such as sitting and walking. The Phase 1b data were presented at the 2026 MDA Clinical and Scientific Conference and at SMA Europe 2026.
Salanersen is administered intrathecally and works by correcting splicing of SMN2 pre-mRNA to increase production of survival motor neuron (SMN) protein. The once-yearly dosing interval — enabled by the compound's high-potency chemistry, according to Biogen — would differentiate it from the monthly or more frequent schedules associated with some existing SMA treatments. Biogen licensed global development, manufacturing and commercialisation rights from Ionis Pharmaceuticals, which discovered the compound.
Phase 3 programme and market context
The Phase 3 programme comprises three global studies. STELLAR-1 is recruiting treatment-naïve presymptomatic infants under six weeks of age. SOLAR is recruiting teenagers and adults aged 15 to 60 who are treatment-naïve or previously treated with risdiplam. STELLAR-2, a randomised double-blind sham-controlled study evaluating salanersen initiated approximately six months after onasemnogene abeparvovec-xioi, is expected to begin recruiting in June 2026.
The SMA treatment landscape has shifted substantially since the approval of nusinersen, risdiplam and onasemnogene abeparvovec-xioi, and the key commercial question for salanersen is where it fits alongside — or displaces — those established agents. The STELLAR-2 design in particular addresses a well-recognised clinical gap: infants who receive gene therapy early but fail to achieve optimal motor outcomes. That population has no approved second-line option, and Phase 1b data suggesting neurodegeneration can be slowed in this cohort will be closely scrutinised when full results are published.
Stephanie Fradette, PharmD, Head of the Rare Neurology Development Unit at Biogen, described the designation as "a significant milestone for our SMA portfolio," while noting that the Phase 3 studies are designed to establish the drug's definitive role in the treatment landscape. Investors will look to enrolment pace across STELLAR-1, STELLAR-2 and SOLAR, and to the timing of interim readouts, as the next material signals for the programme.