Efficacy of KL1333 in Adult Patients With Primary Mitochondrial Disease

The FALCON study is investigating whether the study medicine, KL1333, improves fatigue levels and physical abilities of people living with mitochondrial disease. The investigators are also evaluating the tolerability of the study medicine. For this study, the effects of KL1333 are compared with those from a placebo (a pill that looks like the study medicine but contains no active medicine). The study medicine (or placebo) is a tablet that is taken twice daily during the treatment period of 48 weeks. Participation in the FALCON study is divided into 3 parts: * Screening and baseline: 8-12 weeks * Treatment: 48 weeks * Safety follow-up: 5 weeks Total duration: 61 - 65 weeks Patients who complete the screening phase and are enrolled in the study are randomly assigned to receive either the study medicine (KL1333) or placebo (no active medication). Patients are more likely to receive the study medication than placebo (for every five people who take part, three receive KL1333 and two receive placebo). Neither the participants nor the study team know who is receiving the study medicine or placebo and participants are not able to change which treatment they are assigned.