Study of SRP-1001 in Adult and Adolescent Participants With Facioscapulohumeral Muscular Dystrophy Type 1

The purpose of this study is to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics of SRP-1001 in participants with facioscapulohumeral muscular dystrophy Type 1 (FSHD1). In Part 1 of the study, participants will receive one dose of SRP-1001 or placebo. In Part 2 of the study, participants will receive 4 doses of SRP-1001 or placebo. Participants who complete Part 1 will have the option to re-screen and re-randomize into Part 2. All participants will undergo pre- and post-dose magnetic imaging resonance (MRI)-guided muscle biopsies (a total of 2 biopsies). Participants who complete Part 1 and enroll in Part 2 will be required to undergo an additional screening biopsy. Participants completing Part 1 or Part 2 may have the option to continue to receive drug in an open-label extension study or may be eligible to participate in later-stage clinical studies.

Key Inclusion Criteria: * Genetically confirmed FSHD1 based on screening evaluation or source verifiable medical record * Clinical severity score between 3 and 8 (scale, 0 to 10) * Must have an eligible lower extremity muscle for biopsy as determined from MRI by a central reader, with muscle fat fraction ≥10% and less than approximately 40% * Males or nonpregnant, nonlactating females ≥18 years of age who do not plan to become pregnant during the study, with an upper age limit of ≤70 years * Able and willing to provide written informed consent prior to the performance of any study specific procedures * Participants with a body mass index (BMI) between 18.0 and 35.0 kilograms/square meter, inclusive. A participant with FSHD1 and a BMI outside this range may be allowed into the study at the discretion of the principal investigator. * Must have eligible lower extremity muscle for biopsy as determined from MRI by a central reader * A 12-lead electrocardiogram at screening with no abnormalities that may compromise participant's safety in the study * Participants of childbearing potential and their partners must use highly effective contraception during the study and for at least 9 months following the end of study or last dose of study medication, whichever is later. Males must not donate sperm during the study from Day 1 until at least 9 months following the end of study or last dose of study medication, whichever is later. Key Exclusion Criteria: * Human immunodeficiency virus (HIV) infection as shown by presence of anti-HIV antibody (seropositive) at screening * Seropositive for hepatitis B or hepatitis C at screening * Uncontrolled hypertension * Severe cardiovascular disease * History of thrombolic events * Platelet count less that the lower limit of normal at screening * History or presence of: a hypercoagulable state, nephrotic range proteinuria, antiphospholipid antibody syndrome, myeloproliferative disease, inability to ambulate, use of hormone-based contraceptives. * Any contraindication to muscle biopsy or MRI Note: additional inclusion/exclusion criteria may apply per protocol