A Study to Evaluate ASP0367 in Participants With Primary Mitochondrial Myopathy

Inclusion Criteria: * Participant agrees and is able to adhere to the study requirements for the length of the study, including performing 6MWT. * Diagnosed with primary mitochondrial myopathy (PMM), consisting of the following: * Molecular genetic abnormality (i.e., nuclear or mitochondrial) known to be associated with mitochondrial dysfunction (such as, but not limited to, mitochondrial DNA (mtDNA) single deletions in chronic progressive external ophthalmoplegia (CPEO) and Kearns-Sayre syndrome (KSS); mtDNA m.3243 A \> G pathogenic nuclear or mitochondrial genome variants demonstrated to cause primary mitochondrial disease), and * Participant reported symptoms (i.e., muscle weakness, fatigue and exercise intolerance) or physical examination findings of myopathy that are the predominant symptoms of the participant's mitochondrial disorder. * Participant has been on stable dose regimen of coenzyme Q10 (CoQ10), carnitine, creatine, or other mitochondrial disease- focused vitamins or supplemental therapies for the treatment of symptoms of the mitochondrial disease for at least 3 months prior to randomization and intends to stay on a stable dose for duration of study period. * Participant has been on stable exercise regimen within 4 weeks prior to randomization and intends to stay on a stable regimen for duration of study period (for participants who participate in a regular exercise regimen). * Female participant is not pregnant and at least one of the following condition…