This a randomized placebo controlled, double-blind phase II study to explore the pharmacokinetics, safety and efficacy of sonlicromanol in children (from birth to 17 years) with genetically confirmed mitochondrial disease of which the gene defect is known to decrease one or more oxidative phosphorylation system enzymes and who suffer from motor symptoms ("KHENERGYC").

Mitochondrial Diseases (MD) are rare progressive, multi-system, often early onset and fatal disorders affecting both children and adults. Despite advances in the understanding of mitochondrial disorders, treatment options are extremely limited and, to date, largely supportive. Therefore, there is an urgent need for novel treatments. Sonlicromanol (KH176) is an orally bio-available small molecule under development for the treatment of these disorders. The current study will explore the pharmacokinetics, safety and efficacy of sonlicromanol in children (from birth to 17 years) with genetically confirmed mitochondrial disease of which the gene defect is known to decrease one or more oxidative phosphorylation system enzymes and who suffer from motor symptoms. The primary objective of the study is to evaluate the effect of sonlicromanol on motor symptom severity in children with genetically confirmed mitochondrial disease affecting oxidative phosphorylation during a 6 month treatment period (GMFM). The trial consists of 2 phases, with the main phase being a randomized placebo controlled, double-blind, phase II parallel group study to explore the efficacy and safety of sonlicromanol in twenty-four (24) children with mitochondrial disease and motor symptoms,. The first phase is an adaptive PharmacoKinetics (PK) study with 4 days treatment (to expected steady state in most subjects) in the following age-groups: birth - 1 year, 1-2 years, 2- 6 years, 6-12 years, and 12 - 17 years. An age group should have at least 3 subjects before being analysed. Subjects will take 4 days of open-label sonlicromanol orally at the anticipated adult-equivalent dose. After completion of enrolment in an age group, the PK data from that age group will be analysed to confirm the adult-equivalent dose that will be used thereafter in the second phase of the trial. Older age groups will be studied before younger age groups. In the second phase subjects will be randomized (by age group) over 2 groups. Group 1 will receive an adult-equivalent dose of sonlicromanol twice daily orally for 26 weeks. Group 2 will receive matching placebo twice daily for 26 weeks. A final follow-up visit is scheduled 2 weeks after the intake of the last dose of the treatment period. Duration of Subject Participation: The overall study duration of the trial for a eligible subject is estimated to be approximately 7 months, consisting of up to 4 weeks screening, 26 weeks (6 months) of treatment and 2 week post-treatment follow-up. At the end of the study treatment all participants will be offered to continue treatment with sonlicromanol during a open label extension (OLE) trial for 12 months.