RTA 408 Capsules in Patients With Mitochondrial Myopathy - MOTOR

Inclusion Criteria: 1. Have mitochondrial myopathy as evidenced by the following 2 criteria (must meet both): 1. Have a history of exercise intolerance with or without weakness and/or progressive exercise intolerance (in which modest exercise typically provokes heaviness, weakness, aching of active muscles, or tachycardia) 2. Have a known primary mitochondrial DNA mutation or a nuclear DNA defect that is associated with reduced activity of at least 1 mitochondrially encoded respiratory chain complex 2. Be male or female and ≥18 years of age and ≤75 years of age 3. Have no changes to exercise regimen within 30 days prior to Study Day 1 and be willing to remain on the same exercise regimen during the 16-week study period 4. Have the ability to complete maximal exercise testing 5. Have a peak workload during maximal exercise testing of ≤ 1.5 W/kg 6. Be able to swallow capsules Exclusion Criteria: 1. Have uncontrolled diabetes (HbA1c \>11.0%) 2. Have B-type natriuretic peptide level \>200 pg/mL 3. Have a history of clinically significant left-sided heart disease and/or clinically significant cardiac disease 4. Have known active fungal, bacterial, and/or viral infection, including human immunodeficiency virus or hepatitis virus (B or C) 5. Have known or suspected active drug or alcohol abuse 6. Have clinically significant abnormalities of clinical hematology or biochemistry, including but not limited to elevations greater than 1.5 times the upper limit of normal of aspartate aminotransferase, alanine aminotransferase, or creatinine 7. Have any abnormal laboratory test value or serious pre-existing medical condition that, in the opinion of the investigator, would put the patient at risk by study enrollment 8. Have taken any of the following drugs within 7 days prior to Study Day 1 or plan to take any of these drugs during the time of study participation: 1. Sensitive substrates for cytochrome P450 2C8 or 3A4 (e.g., repaglinide, midazolam, sildenafil) 2. Substrates for p-glycoprotein transporter (e.g., ambrisentan, digoxin) 9. Have participated in any other interventional clinical study within 30 days prior to Study Day 1 10. Have a cognitive impairment that may preclude ability to comply with study procedures