A Study to Learn About the Safety and Effects of Salanersen (BIIB115) When Given to Babies With Spinal Muscular Atrophy (SMA) Who Were Previously Treated With Onasemnogene Abeparvovec (STELLAR-2)

A Phase 3, Randomized, Double-Blinded Study to Evaluate the Safety and Efficacy of Salanersen (BIIB115) After Onasemnogene Abeparvovec Treatment in Infants With Genetically Diagnosed Spinal Muscular Atrophy

In this study, researchers will learn more about the safety and effects of BIIB115, also known as salanersen. Specifically, researchers will learn more about how salanersen works in babies who have already been treated with onasemnogene abeparvovec (OA) after being diagnosed with SMA.

Most people with SMA have changes in a gene called survival motor neuron 1, also known as SMN1. These changes lower the amount of SMN protein in their bodies. Without enough of this protein, motor neurons and muscles cannot work properly. A similar gene called SMN2 can help replace some of the lost SMN protein in the body. Salanersen works by helping the SMN2 gene to make more SMN protein.

OA works by replacing the missing or abnormal SMN1 gene. Sometimes, OA treatment may not work as well as expected. As a result, researchers are exploring whether giving another drug after OA could lead to better outcomes for people with SMA.

In this study, participants will have 2 SMN2 copies. The higher the copy number, the less severe the participant's SMA is. They will also have received treatment with OA by the time they were 42 days old and before showing any symptoms of SMA.

The main goal of the study is to learn more about the safety of giving salanersen to babies after OA treatment. Researchers will also learn more about whether salanersen can help make SMA symptoms less serious.

The main question researchers want to answer in this study is:

• How many participants have adverse events and serious adverse events after treatment?

Researchers will also learn more about:

• The effects on participants' motor symptoms and how many new movement milestones participants achieve.
• How many participants stay free of SMA symptoms.
• How much neurofilament protein is found in the blood after treatment.
• How much salanersen gets into the fluid surrounding the brain and spinal cord.
• How much salanersen gets into the blood. Researchers will use different tests to learn if motor symptoms are changing, including the World Health Organization (WHO) motor milestones and Hammersmith Infant Neurological Examination (HINE) Section 2 motor milestones.

The study will be done in 2 parts. Part A will last 1 year while Part B will last up to 4 years.

The study will be done as follows:

• First, participants will be screened to check if they can join the study. The screening period will be up to 6 months. Participants must have received OA treatment before the age of 42 days and started screening within 6 months of the OA dose.
• Participants will be assigned to 1 of 2 treatment groups by chance. This is a "double blind" study which means neither the participants, study doctor, nor site staff will know which treatment group the participants are assigned to.
• In this study, salanersen will be given as an intrathecal injection, which is an injection into the fluid surrounding the spine. This is done by a procedure called a lumbar puncture (LP) which involves inserting a needle into the lower back into the space around the spinal cord.
• During Part A, one group will receive 80 milligrams (mg) of salanersen while another group receives a sham (fake) procedure. This means that a small needle prick will be done, but no injection will be given.
• For each participant, the first visit of Part A will be 6 months after they receive OA treatment.
• Part A will have up to 6 clinic visits and 2 phone calls and last up to 1 year.
• During Part B, both groups of participants will receive 80 mg of salanersen once a year.
• Part B will have up to 12 clinic visits and 14 phone calls and last up to 4 years.
• In total, participants will be in the study for up to 5 and a half years.

Study Overview

• Status: Not yet recruiting
• Conditions: Muscular Atrophy, Spinal
• Intervention / Treatment: Drug: Salanersen; Procedure: Sham Procedure

Detailed Description

The primary objective of the study is to evaluate the safety and tolerability of adding salanersen 6 months after OA in participants with genetically diagnosed SMA who received presymptomatic treatment with OA.

The secondary objectives are to evaluate the efficacy and effect on biomarkers of salanersen after OA, and to evaluate the pharmacokinetics (PK) of salanersen in participants with genetically diagnosed SMA who have received presymptomatic treatment with OA.

• Study Type: Interventional
• Enrollment (Estimated): 42
• Phase: Phase 3

Contacts and Locations

• Study Contact: Name: US Biogen Clinical Trial Center; Phone Number: 866-633-4636; Email: clinicaltrials@biogen.com
• Study Contact Backup: Name: Global Biogen Clinical Trial Center; Email: clinicaltrials@biogen.com

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Child
• Accepts Healthy Volunteers: No

Description

Key Inclusion Criteria:

• Genetic documentation of 5q spinal muscular atrophy (SMA) homozygous gene deletion or mutation or compound heterozygous mutation.
• 2 copies of the survival motor neuron 2 (SMN2) gene.
• Onasemnogene Abeparvovec (OA) dose given at ≤ 42 days of age and screening initiated less than 6 months from OA dosing.

• OA dose given while participant was presymptomatic, per Investigator attestation. For this study, presymptomatic is defined as follows:

  • No clinical signs or symptoms at the time of OA dosing that are, in the opinion of the Investigator, strongly suggestive of SMA.
  • No absence of tendon reflexes (i.e., absence of all of biceps, knee and ankle tendons) at the time of OA dosing (e.g., Hammersmith Infant Neurological Examination (HINE) Section 1 or equivalent).
  • If Compound Muscle Action Potential (CMAP) data is available at the time of dosing, ulnar CMAP amplitude ≥ 2 millivolt (mV).

Key Exclusion Criteria:

• Any unresolved post-OA laboratory abnormalities defined as follows:

  • Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) must be less than 2 × Upper Limit of Normal (ULN) while not receiving corticosteroids within 30 days prior to dosing with salanersen or sham procedure (repeat testing may be performed if necessary).
  • Evidence of thrombocytopenia, indicated by the platelet count being lower than the normal range for the laboratory.
  • Evidence of elevated troponin-I levels, identified as elevated post-OA, and has not returned to the normal range.
• Confirmed demonstration of corrected QT interval, using Fridericia's correction method, of > 450 milliseconds (ms).
• Other than OA, any prior treatment with an approved SMA disease modifying therapy (e.g. nusinersen and/or risdiplam), a myostatin inhibitor therapy, or an investigational drug given for the treatment of SMA.
• Steroid treatment administered for the purpose of treating complications following OA within 14 days prior to dosing with salanersen or sham procedure on Day 1.

Note: Other protocol-defined inclusion/exclusion criteria will apply.

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: Quadruple

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Salanersen; Participants will receive a single dose of salanersen, 80 milligrams (mg) administered intrathecally on Day 1 in Part A. In Part B, participants will continue to receive four additional intrathecal doses of salanersen, 80 mg administered on Days 365, 730, 1095, and 1460.	Drug: Salanersen; Administered intrathecally; Other Names: BIIB115
Sham Comparator: Sham Procedure; Participants will undergo a sham procedure on Day 1 in Part A. In Part B, participants will receive four intrathecal doses of salanersen, 80 mg administered on Days 365, 730, 1095, and 1460.	Drug: Salanersen; Administered intrathecally; Other Names: BIIB115; Procedure: Sham Procedure; A sham lumbar puncture is a skin-only needle prick at the usual lumbar puncture site. The needle does not enter the spinal canal.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Time Frame
Parts A: Number of Participants with Adverse Events (AEs) and Serious Adverse Events (SAEs)	Part A: Up to Day 365

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Part B: Number of Participants with Adverse Events (AEs) and Serious Adverse Events (SAEs)		Part B: Up to Day 1825
Parts A and B: Change From Baseline in Plasma Levels of Neurofilament Light Chain (NfL)	Blood will be collected to characterize changes in plasma NfL following treatment with salanersen. NfL is a protein released from damaged neurons and is a biomarker of neurodegeneration.	Part A: At Days 180 and 365; Part B: Up to Day 1825
Parts A and B: Change from Baseline in Compound Muscle Action Potential (CMAP) Amplitudes	CMAP is a well-validated method for tracking disease progression in neuromuscular disorders such as SMA and amyotrophic lateral sclerosis and has been proposed as a potential biomarker of a therapeutic effect in SMA. CMAPs will be performed for the following nerve-muscle pairs: ulnar-abductor digiti minimi and peroneal-tibialis anterior.	Part A: At Day 365 and Part B: Up to Day 1825
Parts A and B: Percentage of Participants Attaining World Health Organization (WHO) Motor Milestones	The WHO motor milestones will include six key developmental milestones: sitting without support, standing with assistance, hands-and-knees crawling, walking with assistance, standing alone, and walking alone.	Part A: At Day 365 and Part B: Up to Day 1825
Parts A and B: Percentage of Participants Attaining Hammersmith Infant Neurological Examination Section 2 (HINE-2) Motor Milestones	Section 2 of the HINE is used to assess motor milestones and includes 8 motor milestone categories: voluntary grasp (0 to 3), ability to kick in supine position (0 to 4), head control (0 to 2), rolling (0 to 3), sitting (0 to 4), crawling (0 to 4), standing (0 to 3), and walking (0 to 3). Total HINE-2 score is the sum of points from each item and can range from 0 to 26, with higher scores depicting a better level of ability.	Part A: At Day 365 and Part B: Up to Day 1825
Parts A and B: Change From Baseline in Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND) Motor Function Scale	The CHOP INTEND test is designed to evaluate the motor skills of participants with significant motor weakness. It includes 16 items (capturing neck, trunk, and proximal and distal limb strength) structured to move from easiest to hardest with the grading including gravity eliminated (lower scores) to antigravity movements (higher scores). All item scores range from 0-4. The total score ranges from 0-64, with higher scores depicting better motor function.	Part A: At Day 365 and Part B: Up to Day 1825
Parts A and B: Percentage of Participants who Remain Free of Clinically Manifested Spinal Muscular Atrophy (SMA)		Part A: At Day 365 and Part B: At Day 545
Part B: Percentage of Participants who Develop Spinal Muscular Atrophy (SMA) Subtypes (Non-Sitters, Sitters and Walkers) as Assessed by the Investigator		Part B: Up to Day 1825
Part B: Hammersmith Functional Motor Scale Expanded (HFMSE) Total Score	The HFMSE is a tool used to assess motor function in individuals with SMA. Participants will be asked to complete a specific movement and are then graded on the quality and execution of that movement. Higher scores indicate higher levels of motor ability. The overall score is the sum of the scores for all 33 items, with a maximum score of 66, with higher scores depicting better ability to perform activities.	Part B: Up to Day 1825
Part B: Revised Upper Limb Module (RULM) Total Score	The RULM is developed to assess upper limb functional abilities of participants with SMA. This test consists of a total of 20 upper limb performance items that are reflective of activities of daily living. The RULM is scored from 0 to 37 points, with higher scores indicating better function.	Part B: Up to Day 1825
Parts A and B: Time to Death (Overall Survival)		Part A: Up to Day 365 and Part B: Up to Day 1825
Parts A and B: Time to Death or Permanent Ventilation	Permanent ventilation is defined as tracheostomy or ≥16 hours ventilation/day continuously for >21 days in the absence of an acute reversible event.	Part A: Up to Day 365 and Part B: Up to Day 1825
Parts A and B: Concentration of Salanersen in Cerebrospinal Fluid (CSF)		Part A: Up to Day 365 and Part B: Up to Day 1460
Parts A and B: Concentration of Salanersen in Serum		Part A: Up to Day 365 and Part B: Up to Day 1825

Collaborators and Investigators

• Sponsor: Biogen
• Investigators: Study Director: Medical Director, Biogen

Study record dates

Study Major Dates

• Study Start (Estimated): September 4, 2026
• Primary Completion (Estimated): July 27, 2029
• Study Completion (Estimated): July 26, 2033

Study Registration Dates

• First Submitted: February 26, 2026
• First Submitted That Met QC Criteria: February 26, 2026
• First Posted (Actual): March 3, 2026

Study Record Updates

• Last Update Posted (Actual): March 3, 2026
• Last Update Submitted That Met QC Criteria: February 26, 2026
• Last Verified: February 1, 2026

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Central Nervous System Diseases; Nervous System Diseases; Neuromuscular Diseases; Neurodegenerative Diseases; Spinal Cord Diseases; Motor Neuron Disease; Muscular Atrophy, Spinal
• Other Study ID Numbers: 277SM301; 2025-523857-32 (Other Identifier: EU Trial (CTIS) number)

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: YES
• IPD Plan Description: In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on https://www.biogentrialtransparency.com/

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: Yes
• Studies a U.S. FDA-regulated device product: No