An Open-Label, Dose-Escalating Study to Assess the Safety, Tolerability, Efficacy, Pharmacokinetics and Pharmacodynamics of Cysteamine Bitartrate Delayed-release Capsules (RP103) for Treatment of Children With Inherited Mitochondrial Disease

• Age ≥ 6 years and \< 18 years

• Body weight ≥ 5 kg

• Documented (genetically confirmed known mutation, i.e. no variants of uncertain significance) diagnosis of inherited mitochondrial disease other than Friedreich's ataxia (FRDA)

• 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.

• 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)

• With respect to concomitant medications, the subject must:

∙ 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);

‣ 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.

• 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

• 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):

∙ Hormonal contraception: birth control pills, injection, patch, vaginal ring or implant;

‣ Condom or diaphragm, with spermicide;

‣ Intrauterine device (IUD)

‣ Sterile male partner (vasectomy performed at least 6 months prior to the study).

• Subjects's legally authorized representative must provide written informed consent; Subject must provide assent, if required by local/institutional requirements

⁃ Have mitochondrial myopathy as evidenced by one or more of the following criteria:

• Weakness consistent with myopathy (e.g. accompanied by muscle wasting and/or absence of neuropathy) on physical exam

∙ OR documented myopathy on the basis of muscle biopsy consistent with mitochondrial myopathy disease

∙ 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