Our Take
Breakthrough designation is a regulatory signal, not a clinical victory; the real test is whether Phase III data in infants and adults matches the Phase Ib promise in a small, pre-selected cohort.
Why it matters
SMA patients currently cycle through multiple treatments with diminishing returns. Once-yearly dosing could reduce treatment burden and open a pathway for patients who've exhausted gene therapy options. Biogen's three-arm Phase III programme will determine if this extends beyond the 24-person Phase Ib cohort.
Do this week
Clinicians: audit your current SMA patient roster for prior gene therapy exposure and neurofilament trajectory to identify candidates for STELLAR-II screening when enrollment opens June 2026.
Biogen's salanersen clears FDA breakthrough hurdle
The FDA has granted breakthrough therapy designation to salanersen (BIIB115), Biogen's antisense oligonucleotide for spinal muscular atrophy (SMA). The designation follows Phase Ib data presented at the 2026 Muscular Dystrophy Association Clinical and Scientific Conference and the fifth International Scientific Congress on SMA Europe.
The Phase Ib trial enrolled 24 participants aged 0.5 to 12 years. All showed improvements in at least one motor endpoint. Twelve of 24 achieved a new WHO motor milestone. Neurofilament light chain levels dropped meaningfully at six months and remained suppressed through follow-up. Adverse events were mild to moderate (company-reported).
Salanersen works as an intrathecal injection that corrects SMN2 pre-mRNA splicing to increase survival motor neuron (SMN) protein production. Unlike earlier antisense therapies, the new chemistry allows once-yearly administration rather than monthly or biannual dosing.
The breakthrough designation triggers a three-arm Phase III programme now recruiting:
- STELLAR-I: infants under six weeks old, treatment-naïve or presymptomatic, with genetic SMA diagnosis.
- SOLAR: teenagers and adults aged 15–60, either untreated or previously treated with risdiplam.
- STELLAR-II: infants previously treated with gene therapy (onasemnogene abeparvovec-xioi) before six weeks of age, planned to start June 2026.
Breakthrough designation accelerates FDA review timelines and signals that preliminary evidence supports the potential for substantial improvement over existing therapies. It does not guarantee approval.
The post-gene-therapy question
SMA is a rare neuromuscular disorder caused by loss of motor neurons. Gene therapy (onasemnogene) became standard of care for infants, but efficacy varies and retreatment is not yet standard. Some patients show suboptimal response or plateau. Salanersen targets this population directly.
The Phase Ib cohort was small and retrospectively selected for prior gene therapy failure. STELLAR-II will test whether once-yearly salanersen works in a prospectively enrolled, larger population. If motor function gains hold in Phase III and durability extends beyond 12 months, salanersen could become a bridge or follow-on therapy.
Dosing frequency matters clinically. Once yearly means fewer intrathecal procedures, lower infection risk, and better adherence. For pediatric patients and their families, reduced treatment burden translates to more stable development windows.
What to watch in the Phase III read-outs
Breakthrough designation does not resolve the open questions: efficacy in untreated infants (STELLAR-I), durability in older populations (SOLAR), and comparative benefit in gene-therapy-exposed cohorts (STELLAR-II). Each trial targets a different patient population and baseline motor function, so heterogeneous outcomes are likely.
The FDA will scrutinize neurofilament levels as a proxy for neurodegeneration, but motor milestones remain the primary clinical endpoint. Watch for durability data beyond 12 months and long-term safety signals in the intrathecal space (dorsal root ganglion toxicity has been an antisense concern).
Enrollment timelines matter. STELLAR-II starts June 2026. If recruitment lags in the gene-therapy-exposed arm, regulatory confidence in that subgroup may soften. Approval timing and label breadth will hinge on Phase III consistency and the FDA's appetite for subgroup analysis.