Inclusion Criteria:

• Male or female.
• Age ≥ 18 years old.
• 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:

• Individuals < 18 years old.
• 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.

Eligibility last updated 4/18/22. Questions regarding updates should be directed to the study team contact.