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

The purpose of this study is to investigate if the study medicine, KL1333, is safe, well-tolerated and effective long-term in improving the symptoms of fatigue and impacts on daily living and functional capacity (physical abilities) in people with PMD.

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.