Inclusion Criteria:

• Able to comprehend and willing to sign an Informed Consent Form (ICF) for patients 18 years of age and older. For patients less than 18 years of age, parent(s)/legal guardian(s) of patients must provide written informed consent prior to participation in the study and informed assent will be obtained from minors at least 12 years of age when required by regulation.
• Males or females with genetically confirmed diagnosis of SMA who are Type II, III or IV and at least 12 years of age
• Ambulatory patients, once having achieved a standing position independently, must be able to complete at least one lap in the 6-minute walk test (at least 50 meters) within 5 minutes without assistance.
• Non-ambulatory patients (defined as individuals who are unable to stay in a standing position independently and unable to take any steps with personal assistance or assistive devices; effectively requiring a wheelchair for all mobility needs) must be able to tolerate an upright sitting position, with support, continuously for 3 hours
• Hammersmith (HFMS-E) score ≥ 10 and ≤ 54
• Contracture of the elbow flexion and knee flexion ≤ 90 degrees
• Pre-study clinical laboratory findings within the normal range or, if outside the normal range, deemed not clinically significant by the Investigator
• Able to swallow an oral suspension and in the opinion of the Investigator, is expected to continue to be able to do so for the duration of the trial. Administration via a feeding tube is not allowed.
• Forced vital capacity (FVC) > 20% predicted and above this threshold for greater than 3 months
• Male patients who have reached puberty must agree to do either of the following from Screening until 10 weeks after the last dose of the investigational product unless they have had a vasectomy and confirmed sperm count is zero:
  • Abstain from sexual intercourse, OR
  • If having heterosexual intercourse, must use a condom and their female partners who are of childbearing potential must use a highly effective contraception method*
• Female patients who have had their first period will be considered of childbearing potential unless they are anatomically and physiologically incapable of becoming pregnant. If of childbearing potential, the female patients must:
  • Have a negative urine/serum pregnancy test at Screening AND
  • Abstain from heterosexual intercourse from Screening until 10 weeks after the last dose of investigational product OR
  • If having heterosexual intercourse, must use a highly effective contraception method* and require the male partners to use a condom from Screening until 10 weeks after the last dose of investigational product

    *Highly effective contraception methods include:

  • Established use of oral, injected or implanted hormonal methods of contraception
  • Placement of an intrauterine device (IUD) or intrauterine system (IUS)
• Male patients must agree to refrain from sperm donation from Screening until 10 weeks after the final study drug administration

Exclusion Criteria:

• History of significant hypersensitivity, intolerance, or allergy to any drug compound, food, or other substance, unless approved by the Investigator
• Hospitalization within 2 months of Screening
• History of stomach or intestinal surgery or resection that would potentially alter absorption and/or excretion of orally administered drugs (appendectomy, hernia repair, and/or cholecystectomy will be allowed)
• A clinically significant illness within 4 weeks of Screening
• History of alcoholism or drug addiction within 2 years prior to Screening
• History of smoking more than 10 cigarettes (or equivalent amount of tobacco) per day within 3 months prior to Screening
• Patient has used a strong CYP3A4 inhibitor within 7 days prior to first dose of study drug or a strong CYP3A4 inducer within 14 days prior to first dose of study drug
• Any other medical condition that would interfere with performance of testing including (but not limited to) significant joint pain or arthritis limiting mobility, and chronic neuromuscular pain sufficient to require ongoing analgesic medication
• Participation by two people at the same time that are living in the same household
• Participation in any other investigational study drug trial in which receipt of an investigational study drug occurred within 30 days or five half-lives of the other investigational study drug, whichever is greater, prior to Screening