Long-Term Safety & Efficacy of Apitegromab in Patients With SMA Who Completed Previous Trials of Apitegromab

Inclusion Criteria: * Patients have completed the Phase 2 TOPAZ (Study SRK-015-002) trial or the Phase 3 SAPPHIRE (Study SRK-015-003) trial. (For TOPAZ, completed is defined as completion of Visit EC14 in Extension Period C or participating in TOPAZ at the time the trial is ended. For SAPPHIRE, completed is defined as completion of Visit 14 or participating in SAPPHIRE at the time the trial is ended) * Estimated life expectancy \>2 years from the Baseline Visit (Day 1) * Able to receive study drug infusions and provide blood samples through the use of a peripheral IV or a long-term IV access device that the patient has placed for reasons independent from the trial * Able to adhere to the requirements of the protocol, including travel to the trial site and completing all trial procedures and trial visits * Females of childbearing potential must have a negative pregnancy test at the Baseline Visit and agree to use at least 1 highly effective method of contraception throughout the trial and for 20 weeks after the last dose of apitegromab Exclusion Criteria: * Patient permanently discontinued study treatment during the feeder trial (i.e., TOPAZ or SAPPHIRE) * Nutritional status that was not stable over the past 6 months and is not anticipated to be stable throughout the trial or medical necessity for a gastric/nasogastric feeding tube, where the majority of feeds are given by this route, as assessed by the Investigator * Patient is currently enrolled in any investigational drug trial other than TOPAZ or SAPPHIRE * Prior history of severe hypersensitivity reaction or intolerance to SMN-targeted therapies * Prior history of severe hypersensitivity reaction or intolerance to apitegromab * Use of chronic daytime noninvasive ventilatory support for \>16 hours daily in the 2 weeks before dosing, or anticipated to regularly receive such daytime ventilator support chronically throughout the trial * Any acute or comorbid condition interfering with the well-being of the patient at the patient's last visit in TOPAZ or SAPPHIRE, (including active systemic infection, the need for acute treatment, or inpatient observation due to any reason). After resolution of the condition, the patient can be enrolled in the trial if they meet all the other eligibility criteria. * Pregnant or breastfeeding * Any other condition or clinically significant laboratory result or ECG value that, in the opinion of the Investigator, may compromise safety or compliance, would preclude the patient from successful completion of the trial, or interfere with the interpretation of the results