Open-Label, Dose-Escalating Study Assessing Safety, Tolerability, Efficacy, of RP103 in Mitochondrial Disease

Inclusion Criteria: 1. Age ≥ 6 years and \< 18 years 2. Body weight ≥ 5 kg 3. Documented (genetically confirmed known mutation, i.e. no variants of uncertain significance) diagnosis of inherited mitochondrial disease other than Friedreich's ataxia (FRDA) 4. Moderate disease severity based on Newcastle Pediatric Mitochondrial Disease Scale (NPMDS) score, with a score between 15 to 45 inclusive \[Leber's Hereditary Optic Neuropathy (LHON) subjects are exempt of this inclusion criteria\], if approved by the sponsor. 5. For patients regularly taking dietary supplements such as creatinine, alpha-lipoic acid, coenzyme Q10 (CoQ10), vitamin B, carnitine, etc. they have to have been taking them for at least 3 months pre-study and will agree to keep these the same throughout the study (from the Screening Visit to Study Exit) 6. With respect to concomitant medications, the subject must: 1. Be willing to abstain from initiating dietary supplements and non-prescribed medications, except as allowed by the Investigator, throughout the study (from the Screening Visit to Study Exit); 2. Be on a stable dose of medications prescribed for seizure management and prevention. Stable dose in this context means unchanged for at least 30 days prior to the Screening Visit. 7. Willing and able to comply with study drug dosing requirements, i.e. ingest the RP103 capsules intact, or sprinkled in liquid or soft food, or using a g-tube 8. Sexually active female subjects of childbearing potential (i.e., not surgically sterile \[tubal ligation, hysterectomy, or bilateral oophorectomy\]) must agree to utilize two of the following acceptable forms of contraception throughout the study (from the Screening Visit to Study Exit): 1. Hormonal contraception: birth control pills, injection, patch, vaginal ring or implant; 2. Condom or diaphragm, with spermicide; 3. Intrauterine device (IUD) 4. Sterile male partner (vasectomy performed at least 6 months prior to the study). 9. Subjects's legally authorized representative must provide written informed consent; Subject must provide assent, if required by local/institutional requirements 10. Have mitochondrial myopathy as evidenced by one or more of the following criteria: 1. Weakness consistent with myopathy (e.g. accompanied by muscle wasting and/or absence of neuropathy) on physical exam 2. OR documented myopathy on the basis of muscle biopsy consistent with mitochondrial myopathy disease 3. OR weakness and/or progressive exercise intolerance (in which modest exercise typically provokes heaviness, weakness, aching of active muscles, or tachycardia). Weakness should be due to myopathy and not neuropathy or other causes as deemed by investigator Exclusion Criteria: 1. Documented diagnosis of concurrent inborn errors of metabolism 2. Non-elective hospitalization related to mitochondrial disease or direct complication of disease within 60 days prior to the Screening Visit. 3. Platelet count, lymphocyte count or hemoglobin below the lower limit of normal (LLN) at the Screening Visit 4. Hepatic insufficiency with liver enzyme tests (alkaline phosphatase, aspartate aminotransferase \[AST\] or alanine aminotransferase \[ALT\]) greater than 2.5 times the upper limit of normal (ULN) at the Screening Visit 5. Bilirubin \> 1.2 g/dL at the Screening Visit 6. Inability to complete the elements of the study, e.g., coma, hemodynamic instability or requiring continuous ventilator support. 7. Malabsorption requiring total parenteral nutrition (TPN), chronic diarrhea, bouts of pseudo obstruction 8. Severe end-organ hypo-perfusion syndrome secondary to cardiac failure resulting in lactic acidosis 9. Patients with suspected elevated intracranial pressure, pseudotumor cerebri (PTC) and/or papilledema 10. Severe gastrointestinal disease including gastroparesis 11. History of angina, myocardial infarction, or cardiac surgery within 2 years prior to the Screening Visit 12. Any clinically significant electrocardiogram (ECG), including dysrhythmia, or clinically significant abnormal laboratory finding not already listed above at the Screening Visit 13. History of drug or alcohol abuse 14. History of pancreatitis 15. Participated in an investigational drug trial within 30 days or, within 90 days for a biologic, device, or surgical treatment, for inherited mitochondrial diseases prior to the Screening Visit 16. Known or suspected hypersensitivity to cysteamine and penicillamine 17. Female subjects who are nursing, planning a pregnancy, known or suspected to be pregnant, or with a positive serum pregnancy test at the Screening Visit 18. Subject's who, in the opinion of the Investigator, are not able or willing to comply with the protocol.