This study is testing a medicine called surlorian in adults who have a genetic muscle condition known as autosomal dominant RYR1-related myopathy (RYR1-RM). The goal is to find out whether surlorian improves muscle weakness, and whether it is safe and well tolerated.

The study is taking place at several medical centers with doctors who specialize in treating people with RYR1-RM. Everyone in the study will receive both surlorian and a placebo (a "dummy" treatment) at different times, but neither the participants nor the study staff will know which one they are getting during each period.

During Treatment period 1, participants will be randomly assigned to receive either surlorian or a placebo, which will be followed up by a washout period. Following the washout, in Treatment period 2, participants will switch and receive the opposite treatment from what they received in the first period. This main part of the study lasts about 16 weeks.

After finishing the main, placebo-controlled part of the study participants may be able to join an open-label extension lasting approximately 12 months. In this extension, everyone receives surlorian.

Inclusion Criteria:

• Can understand the written informed consent, provides signed and witnessed written informed consent, and agrees to comply with all protocol requirements
• Confirmed genetic diagnosis of RYR1-RM with autosomal dominant mutation
• Clinical evidence of weakness affecting any proximal muscle group(s) as assessed by the Investigator
• Can walk 10 m with or without a cane (no other walking aid allowed)
• Is either a female of non-childbearing potential or male or female, who agrees to use highly effective contraception/preventive exposure measures from the time of first dose of IP (for a male participant) or the signing of the informed consent form (ICF) (for a female participant) during the trial, and until 7 days after the last dose of IP.

Exclusion Criteria:

• Unable or unwilling to understand and comply with protocol requirements or unlikely to complete the study as planned, as judged by the Investigator
• Any clinically significant medical condition that, in the opinion of the Investigator, would interfere with the study
• Females who are pregnant, breastfeeding or intend to become pregnant, or of childbearing potential not using adequate contraceptive methods
• Participants with severe pulmonary dysfunction at screening, or evidence of pulmonary exacerbation (defined as an acute worsening of respiratory symptoms that result from a decline in lung function)
• Cardiac disease by history or at screening that, in the Investigator's opinion, is likely to worsen overall performance of efficacy measures during the study
• History of seizure disorder, neurologic disease, or neuromuscular disease other than RYR1-RM
• History of chronic orthopedic issues, acute injury, or expected surgery during the study that may affect the ability to complete study assessments
• Positive test results at screening for human immunodeficiency virus (HIV), hepatitis B surface antigen (HBsAg), or hepatitis C virus (HCV)
• Participants with screening alanine aminotransferase (ALT) levels >3 × upper limit of normal (ULN) or screening aspartate aminotransferase (AST) levels >5 × ULN (isolated elevations of total bilirubin <2 × ULN with direct bilirubin below the ULN will be included)
• History within the past year of alcohol or other drug substance abuse
• Known hypersensitivity to the investigational product of related compounds
• Treatment with statins, proton pump inhibitors or H2 blockers within 7 days or 5 half-lives, whichever is longer, prior to the first dose of the study drug
• Treatment with sensitive or narrow therapeutic index CYP3A4 substrates within 7 days or 5 half-lives, whichever is longer, prior to first dose of the study drug
• Treatment with strong or moderate CYP2C8 inhibitor or inducers within 7 days or 5 half-lives, whichever is longer, prior to the first dose of the study drug
• Currently enrolled in another study or received treatment with any other investigational drug within 30 days or > 5 half-lives, whichever is longer, prior to screening
• Has reported any suicide ideation of Category 4 or 5 on the C-SSRS within 6 months prior to screening or any suicidal behavior in the last two years prior to screening as indicated by any 'yes' answers on the suicidal behavior section of the C-SSRS