A Study to Evaluate Vitamin B3 Derivative to Treat Mitochondrial Myopathy

Inclusion Criteria: * Biochemically and/or genetically confirmed or confirmed primary mitochondrial myopathy based on published diagnostic criteria. * Biochemically confirmed mitochondrial disorder would mean that the patient meets clinical criteria and has either biopsy or biochemical testing that supports the diagnosis. * Confirmed mitochondrial disorder means that the patient meets published clinical criteria for the diagnosis and has also had confirmatory genetic testing for the disorder type. * Agreed to avoid vitamin supplementation or nutritional products with vitamin B3 forms 14 days prior to the enrollment and during the study in order to not exceed 200 mg/day of vitamin B3 derivatives intake. * Female of childbearing potential agreed to use effective contraception throughout the study. * Written, informed consent to participate in the study. Exclusion Criteria: * Unwilling to comply with the follow-up schedule. * Inability or refusal to give informed consent by the patient or a legally authorized representative. * Known pregnancy or breastfeeding. * Concurrent participation in another investigational drug study or within washout period of treatment. * Presence of other medical symptoms or condition, which may interfere with interpretation of outcome measures as determined by the study PI. Clinical / Laboratory Exclusion Criteria: * Estimated Glomerular Filtration Rate (eGFR) \< 30 mL/min. * Patients in permanent Renal Replacement Therapy. * Serum alkaline phosphatase 50% above normal limit. * Serum aspartate transaminase 50% above normal limit. * Serum Thyroxine (T4) 50% above or below normal limit. * Serum Thyroid Stimulating Hormone (TSH) 50% above or below normal limit. * Severe anemia with Hb \< 7g/dL. * Severe leukocytosis with WBC \> 15,000/mm\^3.