Biogen announced that the U.S. Food and Drug Administration (FDA) has granted salanersen Breakthrough Therapy Designation for the treatment of spinal muscular atrophy (SMA). Breakthrough Therapy designation is a process designed to expedite the development and review of drugs that are intended to treat a serious condition, and preliminary clinical evidence indicates that the drug may demonstrate substantial improvement over available therapy on a clinically significant endpoint(s). Salanersen is an investigational novel antisense oligonucleotide (ASO) and has the potential to offer high efficacy in SMA with once-yearly dosing.

The FDA?s decision is based on data from the Phase 1b study of salanersen, which were recently presented at the 2026 Muscular Dystrophy Association (MDA) Clinical & Scientific Conference and the 5th International Scientific Congress on SMA (SMA Europe 2026). After initiation of once-yearly salanersen in children with SMA who had a suboptimal response to prior gene therapy, clinically meaningful improvements in motor function were observed as well as slowing of neurodegeneration, as measured by reduced neurofilament levels. Salanersen was generally well-tolerated in the study.

The salanersen Phase 3 program consists of three global studies: STELLAR-1 (recruiting), an open-label study, will evaluate the effects of salanersen in young (under 6 weeks old), treatment-naïve and clinically presymptomatic infants with a genetic diagnosis of SMA; SOLAR (recruiting), an open-label study, will evaluate the effects of salanersen in teens and adults (aged 15?60 years) with SMA who are either treatment-naïve or previously treated with risdiplam; STELLAR-2 (recruitment expected to begin in June 2026), a randomized, double-blind, sham-controlled study, will evaluate the effects of salanersen when initiated ~6 months after onasemnogene abeparvovec-xioi in infants with SMA who received presymptomatic treatment with gene therapy at 6 weeks of age or younger. Salanersen (BIIB115) is a novel, intrathecally administered antisense oligonucleotide (ASO) in development for SMA. Salanersen is designed to correct splicing of SMN2 pre-mRNA to increase production of SMN protein.

It has a new chemistry that leads to high potency, enabling the potential for high efficacy with once-yearly dosing. Salanersen is being evaluated in three global Phase 3 studies designed to evaluate safety and efficacy of 80 mg administered once-yearly in a broad spectrum of individuals living with SMA. Biogen licensed the global development, manufacturing and commercialization rights for salanersen from Ionis Pharmaceuticals Inc. Salanersen was discovered by Ionis.

The Phase 1b study included participants (n=24, aged 0.5-12 years), who received at least 2 doses of salanersen (40 mg or 80 mg). The 80 mg dose will be further evaluated in the Phase 3 studies. In participants who received salanersen 40 mg and 80 mg and had elevated baseline concentrations of neurofilament light chain (NfL), a potential marker of ongoing neurodegeneration, meaningful reductions (75%) in NfL levels were observed at six months; these reductions were sustained throughout the follow-up period.

All 24 participants treated with salanersen experienced increases from baseline on one or more endpoints. Notably, 12 of the 24 achieved at least one new WHO motor milestone, and all participants maintained the motor milestones documented at their baseline. Salanersen has been generally well-tolerated at both 40 and 80 mg doses in the ongoing Phase 1 study, and most adverse events (AEs) have been mild to moderate in severity.

As of the analysis, the most common AEs in the 40 mg group were upper respiratory tract infection and vomiting, and the most common AEs in the 80 mg group were pyrexia and upper respiratory tract infection.