Conference Presentation

Biogen to detail salanersen Phase 1b data as Phase 3 program advances behind it

Salanersen already showed 75% NfL reductions and new motor milestones in gene-therapy-experienced children, and Biogen just won Breakthrough Therapy Designation on that basis.

By AppliedXLJuly 13, 2026Trial NCT05575011

Executive Summary

Biogen's pediatric salanersen cohort already produced a biomarker and functional signal substantial enough to support a Breakthrough Therapy Designation and a three-study global Phase 3 program, so the upcoming congress presentation restates evidence already in hand rather than a new readout.
The population studied so far was gene-therapy-experienced children with an incomplete response, not the treatment-naive and presymptomatic infants who make up most of the Phase 3 program, so the real test of durability sits ahead.
Salanersen is the only Phase 1-stage antisense candidate targeting SMN2 splicing in SMA, sitting alongside Biogen's own approved nusinersen and Roche's risdiplam as the direct comparators the once-yearly dosing regimen must differentiate against.
The trial's primary completion date has been pushed repeatedly, now landing more than five years past its original target, reflecting the long-term extension design rather than a threat to the Phase 3 timeline that already has its own trials underway.

What was shown

In the Phase 1b cohort of NCT05575011, 24 children ages 0.5 to 12 who had suboptimal clinical status despite prior treatment with onasemnogene abeparvovec received at least two doses of salanersen at 40 mg or 80 mg. Among those with elevated baseline neurofilament light chain, a marker of ongoing neurodegeneration, salanersen produced 75% reductions at six months that held through at least one year of follow-up. All 24 participants showed improvement on at least one endpoint from baseline, and 12 of the 24 achieved at least one new World Health Organization motor milestone while none lost milestones already reached. The most common adverse events were upper respiratory tract infection and vomiting at the 40 mg dose and pyrexia and upper respiratory tract infection at 80 mg, and Biogen described the drug as generally well-tolerated at both doses. Biogen

Probability of Success

Endpoint Met75%

Completes88%

Clinical Significance55%

Regulatory40%

How it fits the design

The trial's registered primary endpoint across its Parts A, B, and long-term extension is the number of participants with adverse events and serious adverse events, a safety and tolerability measure, not the biomarker or motor-milestone data Biogen highlighted. Nineteen secondary endpoints cover pharmacokinetics of BIIB115 and salanersen in serum and cerebrospinal fluid, delivered intrathecally, alongside the same safety measure. The 80 mg dose that showed the motor-milestone and biomarker signal is the dose being carried into the Phase 3 program. NCT05575011+1

Regulatory response

The FDA granted salanersen Breakthrough Therapy Designation for spinal muscular atrophy in June 2026, a decision Biogen said was based on the Phase 1b data. Breakthrough status brings more frequent FDA interaction and eligibility for rolling review but does not itself constitute an efficacy finding. Biogen has already initiated three global Phase 3 studies under the STELLAR program: an open-label study in treatment-naive, presymptomatic infants under six weeks old, and a randomized, double-blind, sham-controlled study testing salanersen roughly six months after gene therapy in infants who received it presymptomatically. Those populations differ from the suboptimal-responder cohort that generated the Phase 1b signal, so the Phase 3 program is testing whether the effect holds in an earlier-stage, treatment-naive setting. Biogen

Trial timeline

NCT05575011's primary completion date has moved four times since the trial started dosing in October 2022: from August 2024 to May 2027, then to September 2027, then pulled forward to November 2026, then pushed out again to November 2031 as of July 2025. Enrollment grew from a 38-participant target to 62, a change made in 2023 alongside the first completion-date extension, and the trial has held at 62 actual enrollees since. The trial moved to Active, not recruiting status in January 2025. The extended 2031 completion date reflects the long-term extension component of the study rather than a delay to the Phase 1b readout Biogen has already reported. NCT05575011

Competitive frame

Salanersen is the only Phase 1-stage asset registered against SMN2 splicing modulation in spinal muscular atrophy, making Biogen's own approved nusinersen (NCT05067790) and Roche's approved oral splicing modulator risdiplam (NCT05861999) the direct comparators salanersen's once-yearly intrathecal dosing must differentiate against on dosing frequency and durability. Biogen's own Phase 3 salanersen program (NCT07444450) is the nearest forward precedent for the mechanism. Gene therapy approaches from Novartis and other sponsors work through a different mechanism, restoring SMN1 function rather than modulating SMN2 splicing, and are not direct comparators to this class.

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