The goal of this clinical trial is to investigate oral TTI-0102 to evaluate its pharmacokinetics, safety, and preliminary efficacy in patients with Leigh syndrome spectrum, in three sequences. First sequence will be placebo-controlled and include 12 adolescents and adults weighing more than 40 kg. Second sequence will be open-label and include 3 adults and 3 children weighing 25-40 kg. Third sequence will also be open-label and will include 3 children weighing 20-25 kg. The main questions clinical trial aims to answer are:
* Characterize the relationship between TTI-0102 drug levels by measuring
1. cysteamine pharmacokinetic (PK) parameters 2. pharmacodynamic (PD) efficacy measures * Generate a population PK model and determine dose level for later dosing groups * collect data to inform the design of a subsequent phase 2b/3 study of TTI-0102 in this patient population
Participants will be asked to:
* Take the study drug twice daily for 12 weeks * Visit the clinic for checkups and testing 7 times in total: once prior to study drug dosing, then 5 times over the 12-week treatment period and one more time a month after the last study drug dose.
Individuals aged 5 years to 55 years (inclusive) and weight-eligible, at the time of informed consent / assent and enrollment
Confirmed diagnosis of Leigh syndrome or Leigh-like disorder (Leigh syndrome spectrum [LSS]) with neurodevelopmental manifestations, which include documented developmental delay, developmental regression, or abnormal neurologic exam findings
Willing and able to comply with required study schedule and procedures, including daily TTI-0102 study drug dosing requirements, i.e., able to ingest solution orally
Adequate liver function as evidenced by total bilirubin < 1.5x upper limit of normal (ULN) and liver function tests < 3x ULN
Documented diagnosis of concurrent inborn errors of metabolism
Non-elective hospitalization related to their Leigh syndrome or mitochondrial disease within 60 days prior to screening
Treatment with taurine during the previous 30 days of screening, and not willing to discontinue for the duration of the study
Severe gastrointestinal disease including gastroparesis