Open Label Extension to Assess Long Term Safety and Efficacy of KL1333 in Patients With Primary Mitochondrial Disease

Inclusion Criteria: * Completed the FALCON study (age 18 years or older), and in the opinion of the investigator and sponsor has been compliant with the study requirements * Willingness and ability to attend study appointments within the specified time windows * Willingness and ability to complete electronic patient-reported outcomes * Concomitant medications likely to remain stable throughout participation in the study where clinically possible * Willingness to suspend treatment with idebenone during the study Exclusion Criteria: * The subject is, in the investigator's opinion, unlikely to comply with the protocol, e.g., due to cognitive impairment, or is unsuitable for any reason. * Any medical, psychiatric, laboratory or other condition that may negatively affect the benefit-risk considerations of study participation or interfere with the interpretation of study results and, in the judgment of the investigator and/or the medical monitor, would make the subject inappropriate for entry into this study. Subjects not enrolling directly at the FALCON study completion visit (FALCON Week 48) or the safety follow-up visit (FALCON Week 53) study will be required to fulfill the additional exclusion criteria below during the screening visit: • General fatigue or muscle weakness due to causes other than mitochondrial disease, in the opinion of the investigator.