This double-blind, Phase 2, multiple-dose study will be conducted to evaluate the PK/PD, efficacy, safety, and tolerability of apitegromab in subjects \<2 years old with 5q autosomal recessive SMA who have delayed motor milestones for their age attributed to SMA at the discretion of the Investigator or a Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP-INTEND) score \<55.
Is <2 years old at the time of the informed consent
Had a gestational age of ≥35 weeks and gestational body weight ≥2.0 kg at birth
Has confirmed diagnosis of 5q autosomal recessive SMA
Has confirmed presence of SMN2 gene copy(ies)
Nutritional status that is not anticipated to be stable throughout the study or medical necessity for a gastric feeding tube, where most feeds are administered by this route
Major orthopedic issues such as severe scoliosis or severe contractures or interventional procedure, including spine or hip surgery, which is considered to have the potential to substantially limit the ability of the subject to be evaluated on any motor function outcome measures, within 6 months before Screening or anticipated during the study
Any other physical limitations (eg, the subject requires cast for contractures) that would prevent the subject from undergoing motor function outcome measures throughout the study.