A Randomized, Blinded, Placebo-Controlled, Phase 1 Single Ascending Dose Study in Healthy Adult Male Volunteers and an Open-Label Multiple Ascending Dose Study in Pediatric SMA Participants Previously Treated with Onasemnogene Abeparvovec (Zolgensma*) to Evaluate the Safety, Tolerability, and Pharmacokinetics of BIIB115

Part A:
1. Ability to understand the purpose and risks of the study and provide signed
and dated informed consent and authorization to use confidential health
information in accordance with applicable privacy regulations.
2. Males aged 18 to 55 years, inclusive, at the time of informed consent.
3. Have a body mass index of 18 to 30 kg/m2, inclusive.
4. All participants must practice highly effective contraception as described
in Section 12.5 of the protocol.
5. Must be in good health as determined by the Investigator, based on medical
history and Screening evaluations.

Part B:
- Age 0.5 to 12 years old, inclusive, at the time of informed consent
- Weight =7 kg at the time of informed consent
- Genetic diagnosis of SMA (5q SMA homozygous survival motor neuron
1 (SMN1)gene deletion or mutation or compound heterozygous
mutation)- Survival motor neuron 2 (SMN2) copy number =1
- Must have received IV onasemnogene abeparvovec per the approved
label or per guidelines including the steroid regimen and monitoring
specified therein
- Treatment with onasemnogene abeparvovec =180 days prior to first
BIIB115 dose
- Potential for improvement due to suboptimal clinical status secondary
to SMA, as determined by the Investigator

Part A: 1. Any reason, anatomical or otherwise (including abnormal
hematology/coagulation), that presents increase of risk of complication from
multiple LP procedures required for dosing and CSF collection, per the
investigator discretion. 2. History of any clinically significant cardiac,
endocrine, gastrointestinal, hematologic, hepatic, immunologic, metabolic,
urologic, pulmonary, neurologic, dermatologic, psychiatric, or renal disease,
or other major disease, as determined by the Investigator. 3. History of severe
allergic or anaphylactic reactions, or of any allergic reactions that in the
opinion of the Investigator are likely to be exacerbated by any component of
the study treatment, including LP procedures. 4. History of, or ongoing
malignancy, carcinoma in situ, or high-grade dysplasia (with the exception of
no more than 1 basal cell carcinoma or squamous cell carcinoma that was
completely excised and cured at least 12 months prior to randomization).
Participants with cancers in remission for greater than 5 years prior to Day -1
may be included after discussion with the Sponsor. 5. Systolic blood pressure
> 150 mmHg or < 90 mmHg after resting in a sitting position for at least
5 minutes at screening or prior to dosing. If out of range, testing may be
repeated once at screening and once prior to dosing. 6. Clinically significant
(as determined by the Investigator) 12-lead ECG abnormalities. 7. Confirmed
demonstration of corrected QT interval, using Fridericia*s correction method,
of > 450 ms. 8. Plans to undergo elective procedures or surgeries at any
time after signing the ICF through the follow-up visit. 9. History of a suicide
attempt within 5 years prior to screening or suicidal ideation in the past 6
months as indicated by a positive response (*Yes*) to either Question 4 or
Question 5 of the Screening/Baseline version of the C-SSRS at screening.
Participants with a history of a suicide attempt spanning more than 5 years
should be evaluated by a mental health care practitioner before enrollment in
the study. 10. History of, or positive test result at Screening for, HIV. 11.
History of hepatitis C infection or positive test result at Screening for HCV
antibody. 12. Current hepatitis B infection (defined as positive for hepatitis
B surface antigen [HBsAg] and/or total hepatitis B core antibody [anti-HBc]).
Participants with immunity to hepatitis B from previous natural infection
(defined as negative HBsAg, positive anti-HBc, and positive hepatitis B surface
antibody [anti-HBs]) or vaccination (defined as negative HBsAg, negative
anti-HBc, and positive anti-HBs) are eligible to participate in the study. 13.
Chronic, recurrent, or serious infection (e.g., pneumonia, septicemia), as
determined by the Investigator, within 90 days prior to Screening or between
Screening and Day -1 14. For the following parameters, if a participant has an
out-of-range result that is not clinically significant (as determined by the
Investigator), the test may be repeated once during the Screening period. The
participant may be enrolled if the repeated result is within the reference
range. • Any value for ALT, AST, bilirubin, or serum creatinine that is above
the ULN at Screening or Baseline. • Any value of hemoglobin that is < 7.45
mmol/L (approx. < 12 g/dL) at Screening or Baseline. • Any value for
platelets that is below the lower limit of normal at Screening or Baseline. •
Any value out of normal range for absolute or differential WBC counts at
screening or Baseline. Participants with absolute WBC counts within the normal
range but with variations from normal ranges for differential WBC counts that
are not clinically significant (as determined by the Investigator) can be
included in the study. 15. Current enrollment or a plan to enroll in any
interventional clinical study of a drug, biologic, or device, in which an
investigational treatment or approved therapy for investigational use is
administered within 3 months (or 5 half-lives of the agent, whichever is
longer) prior to randomization. 16. Use of any prescription medication,
over-the-counter oral medication that is known to alter hepatic or renal
clearance (excluding acetaminophen/paracetamol), or nutraceutical (e.g., St.
John*s wort, ginseng, ginkgo biloba) within 28 days prior to Day -1; use of
other over-the-counter oral medication, vitamins, dietary supplements, or
antacids within 14 days prior to Day -1; and an unwillingness or inability to
refrain from this use during study participation, unless specifically permitted
elsewhere within this protocol. 17. History of alcohol or substance abuse (as
determined by the Investigator), a positive drug/alcohol test at Screening or
Day 1, or alcohol use within 48 hours prior to randomization, or an
unwillingness to refrain from alcohol 3 months after dosing, and then for the
rest of the study with a limit of 6 units per week with no more than 2 units a
day. An unwillingness to refrain from illicit or recreational drugs during the
study. Participants who test positive for cannabinoids due to occasional
marijuana use, as determined by the Investigator, and who agree to refrain from
using marijuana for the duration of the study may be enrolled at the
Investigator*s discretion, after consultation with Biogen. 18. History or
evidence of habitual use of tobacco- or nicotine-containing products within 90
days of Screening and unwillingness to abstain for 3 months after dosing. 19.
Out of the ordinary, excessive exercise (as determined by the Investigator)
within 48 hours of Day -1, per Investigator discretion. 20. Three or more
copies of either SMN1 or SMN2. Participants with zero SMN2 copies may be
enrolled at the Sponsor*s discretion. 21. Blood donation (1 unit or more)
within 90 days prior to Screening, plasma donation from 1 week prior to
Screening, and platelet donation from 6 weeks prior to Screening. Part B: -
Severe or serious AEs related to onasemnogene abeparvovec therapy that are
ongoing during Screening - Interval of <180 days between onasemnogene
abeparvovec therapy and first BIIB115 dose - Ongoing steroid treatment
following onasemnogene abeparvovec at time of screening - History of drug
induced liver injury or liver failure per Hy's law definition - History of
thrombotic micrangiopathy - Treatment with any SMN2-splicing modifier
(nusinersen or risdiplam) after receiving onasemnogene abeparvovec. Treatment
with nusinersen <12 months from the first dose of BIIB115. - Any reason,
anatomical or otherwise (including abnormal hematology/coagulation),that
presents increase of risk of complication from the LP procedures, CSF
circulation, or safety assessments, including a history of hydrocephalus or
implanted shunt for CSF drainage - Permanent ventilation, defined as
tracheostomy or =16 hours ventilation/day continuously for =21 days in the
absence of an acute reversible event NOTE: Other protocol defined
Inclusion/Exclusion criteria may apply.