EDG-5506-203 MESA is an open-label extension study to assess the long-term effect of sevasemten (EDG-5506) on safety, biomarkers, and functional measures in adults and adolescents with Becker muscular dystrophy

This is an open-label, treatment extension study to evaluate the safety, tolerability, and durability of effect in long-term dosing of sevasemten. EDG-5506-203 MESA will provide continued access to sevasemten treatment to participants with Becker muscular dystrophy who were previously enrolled in EDG-5506-002 ARCH, completed EDG-5506-201 CANYON and GRAND CANYON, or completed EDG-5506-202 DUNE.

Key Inclusion Criteria:

1. Males with a diagnosis of BMD and participation in EDG-5506-002 ARCH, EDG-5506-201 CANYON and GRAND CANYON, or EDG-5506-202 DUNE. Participants are eligible if they complete the respective prior study visits as follows:

• EDG-5506-002 ARCH: Complete the final study Visit 27 [Month 24]; or, completion of the ET visit prior to Visit 27 [Month 24]
• EDG-5506-201 CANYON and GRAND CANYON: Complete the final study visit (Cohorts 1, 2, 4, and 5: Visit 12 [Month 12]; Cohort 6: Visit 11 [Month 18])
• EDG-5506-202 DUNE: Complete at least 36 weeks of open-label treatment (Visit 14 [Week 52])

Key Exclusion Criteria:

1. Any clinically significant changes during or following participation in EDG-5506-002, EDG-5506-201, or EDG-5506-202 that would affect the potential safety of the participant to receive sevasemten.
2. Receiving moderate or strong cytochrome P450 CYP3A4 inhibitors or inducers.
3. Receipt of an investigational drug other than sevasemten within 30 days or 5 half-lives (whichever is longer) of dosing in the present study.

3. Receipt of oral corticosteroids for the treatment of BMD in the previous 6 months.