An Open-label Long-term Follow-up Study of SAT-3247 for Participants With Duchenne Muscular Dystrophy Including Those Who Participated in SAT-3247-CL-101

• Previously participated in the SAT-3247-CL-101 parent clinical trials (Group A only).
• No previous treatment with SAT-3247 (Group B only)
• Continued status of stable glucocorticosteroid dose or no glucocorticosteroid dose for the duration of the trial.
• Continued stable doses of prescription medicines (excluding glucocorticosteroids) and over-the-counter medicines and/or herbal supplements for supportive care for the duration of the trial.
• Previously received delandistrogene moxeparvovec (brand name Elevidys) either in a prior clinical trial or in the commercial setting > 18 months prior to screening whose muscle function tests have stabilized or demonstrated decline ≥ 3 months prior to Screening (Group B only)
• Receiving a stable dose of givinostat (brand name Duvyzat) for at least 18 months or longer prior to the Screening (Group B only)
• Have previously received an exon skipper ≥ 6 months prior to Screening whose muscle function tests have stabilized or demonstrated decline ≥ 3 months prior to Screening (Group B only)
• Have received any medication indicated for DMD (other than corticosteroids, including vamorolone, gene therapy, givinostat, or an exon-skipping therapy) whose last dose was ≥ 6 months prior to Screening (Group B only)
• have received prior treatment with an investigational gene therapy product (other than delandistrogene moxeparvovec) ≥ 24 months prior to Screening
• Ejection fraction ≥ 40% at Screening (Group B only)
• Ability to understand the nature of the trial and any hazards of participating.
• Ability to communicate satisfactorily with the investigator and physiotherapist and to participate in and comply with the requirements of the entire trial including scheduled visits, procedures, laboratory tests, questionnaires, wearable devices, and study restrictions.
• Willingness to give written consent or assent (if not of cognitive capacity of consent in the jurisdiction where the study is being conducted) and parent/legal guardian willing to give written consent to participate (if participant is not of cognitive capacity to consent) after reading the information and consent form, and after having the opportunity to discuss the trial with the investigator or their delegate.
• All participants, if sexually active, agree to follow the contraception requirements and sperm donation limitations of the trial as described in the protocol.

Exclusion Criteria:

• Presence of acute medical condition, chronic illness or history of chronic illness (other than DMD) sufficient to invalidate the participant's participation in the trial or make it unnecessarily hazardous in the judgment of the investigator.
• Evidence of significant hepatic dysfunction, defined as GLDH > 2X upper limit of normal (ULN) at Screening (Group B only)
• Entry item score on the Performance of Upper Limb (PUL2.0) assessment > 5 (Group B only)
• Requirement for daytime ventilator assistance (Group B only)
• Participants expected to require spine surgeries or hospitalizations for non-acute health needs within 12 months.
• Participants with acute gastrointestinal symptoms (e.g., nausea, vomiting, diarrhea, heartburn) or acute infection (such as influenza) or a significant infection or known inflammatory process at Screening.
• Severe behavioral or cognitive problems that preclude participation in the study, in the opinion of the investigator.
• Development of symptomatic cardiomyopathy since completion of the parent trial.

• Inability to swallow tablets.

  a. Tablets can be split or crushed and stirred into flavored beverages or food (e.g., apple sauce, yogurt) followed by immediate administration.

• Receipt of an investigational product (including prescription medicines and investigational devices) as part of another clinical trial since completion of the parent trial or in the follow-up period of another clinical trial at the time of Screening for this study.

  a. Use of deflazacort or vamorolone in jurisdictions where these are considered investigational as they have not received health authority marketing authorization will not be exclusionary.

• Current or expected use during the study of any medications that are known strong or moderate inhibitors of CYP3A4 (Group B only)
• Consumption of grapefruit, grapefruit juice, and grapefruit-containing products within 30 days of the first dose of investigational product and for the duration of the study (Group B only)
• Possibility that the participant will not cooperate with the requirements of the protocol or is unable or unwilling to comply with the study requirements according to investigator's decision.
• Employee, contractors, or consultants of the Sponsor, the CRO, and/or study site or their relatives.