The NPMDS evaluates the progression of mitochondrial disease in pediatric patients in 4 domains:

I - Current Function (vision, hearing, communication, feeding, and mobility) with scores ranging from 0 to 21; II -System Specific Involvement (seizures, encephalopathy, bleeding diathesis or coagulation defects, gastrointestinal, endocrine, respiratory, cardiovascular, renal, liver, and blood) with scores ranging from 0 to 30.

III - Current Clinical Assessment (growth and development over past 6 months, vision, strabismus and eye movement, myopathy, ataxia, pyramidal, extrapyramidal, and neuropathy) with scores ranging from 0 to 28; and IV - Quality of Life with scores ranging from 0 to 25. For sections I-III, higher scores reflect more severe disease. For Section IV, a higher score reflects a lower quality of life.

Completers Analysis Set included all participants who received at least one dose of study drug (RP103) with at least one post-baseline NPMDS assessment and an evaluable Week 24 NPMDS assessment within the protocol specified window.

The pharmacodynamic (PD) analysis set included all participants who received at least one dose of study drug and had at least one post-baseline PD assessment. Participants with available data at each time point are included in the analysis.

PD analysis set participants with available data at each time point.

PD analysis set participants with available data at each time point.

The investigator selected the two most preeminent symptoms for each participant during the screening visit from the following: Myopathy, dystonia, ataxia, retarded motor development, reduced activities of daily living, and vision. The 2 symptoms selected for each participant were then assessed at each subsequent study visit.

Myopathy was assessed using the 6 minute walk test, which measures the distance walked in a 6 minute walk test.

Participants who received at least one dose of study drug (RP103) and had at least one post-baseline 6 minute walk test assessment and for whom myopathy was prespecified as a preeminent symptom and with available data at each time point.

The investigator selected the two most preeminent symptoms for each participant during the screening visit from the following: myopathy, dystonia, ataxia, retarded motor development, reduced activities of daily living, and vision. The 2 symptoms selected for each participant were then assessed at each subsequent study visit.

Myopathy was assessed using standard grip strength evaluation, which measures hand strength in both hands using a Jamar dynamometer.

Participants who received at least one dose of study drug (RP103) and had at least one post-baseline Jamar hand strength assessment and for whom myopathy was prespecified as a preeminent symptom and with available data at each time point.

The investigator selected the two most preeminent symptoms for each participant during the screening visit from the following: myopathy, dystonia, ataxia, retarded motor development, reduced activities of daily living, and vision. The 2 symptoms selected for each participant were then assessed at each subsequent study visit.

Dystonia symptoms were assessed using the Barry-Albright Dystonia Scale for Dystonia. Participants were assessed for dystonia in each of the following regions: eyes, mouth, neck, trunk, and each upper and lower extremity (8 body regions) on a scale from 0 (absent) to 4 (severe symptoms). The individual scores were summed to calculate the total score which ranges from 0 (dystonia absent) to 32 (severe dystonia).

Participants who received at least one dose of study drug (RP103) and had at least one post-baseline dystonia assessment and for whom dystonia was prespecified as a preeminent symptom and with available data at each time point.

The investigator selected the two most preeminent symptoms for each participant during the screening visit from the following: myopathy, dystonia, ataxia, retarded motor development, reduced activities of daily living, and vision. The 2 symptoms selected for each participant were then assessed at each subsequent study visit.

Ataxia was assessed using the Friedreich Ataxia Rating Scale (FARS). FARS comprises a functional ataxia staging score of overall mobility (score 0 to 6), an assessment of the activities of daily living (ADL) (score 0 to 36) and a neurological assessment (score from 0 to 117) which is composed of bulbar (score 0-11), upper limb (score 0- 36) and lower limb (score 0-16), peripheral nerve (score 0-26) and upright stability/gait (score 0-28). The scores were summed to calculate the total score which ranges from 0 to 159. A higher score indicates a greater level of disability.

Participants who received at least one dose of study drug (RP103) and had at least one post-baseline ataxia assessment and for whom ataxia was prespecified as a preeminent symptom and with available data at each time point.

The investigator selected the two most preeminent symptoms for each participant during the screening visit from the following: myopathy, dystonia, ataxia, retarded motor development, reduced activities of daily living, and vision. The 2 symptoms selected for each participant were then assessed at each subsequent study visit.

Retarded motor development was assessed using the Gross Motor Function Measure (GMFM)-88 which consists of 88 items scored on a scale of 0 to 3:

0: Does not initiate the task;

1. Initiates the task (completes < 10%);
2. Partially completes the task (10 to 99%);
3. Completes the task (100%).

The 88 items are grouped into five dimensions: 1) lying and rolling, 2) sitting, 3) crawling and kneeling, 4) standing, and 5) walking, running and jumping. Scores are expressed as a percentage of the maximum score for that dimension. The total score is the average of the 5 the percentage scores where higher scores indicate better performance.

Participants who received at least one dose of study drug (RP103) and had at least one post-baseline gross motor function assessment and for whom retarded motor development was prespecified as a preeminent symptom and with available data at each time point.

The investigator selected the 2 most preeminent symptoms for each participant during the screening visit from the following: myopathy, dystonia, ataxia, retarded motor development, reduced activities of daily living, and vision. The 2 symptoms were assessed at each subsequent study visit.

Reduced activities of daily living was assessed using the modified Lansky Play Performance Scale, completed by parents based on their child's activity in the past week, where 100=fully active; 90=minor restrictions in strenuous physical activity; 80=active, gets tired more quickly; 70=greater restriction of play, less time spent in play activity; 60=up and around, active play minimal; quieter activities; 50=lying around much of the day; no active playing, all quiet play and activities; 40=mainly in bed; quiet activities; 30=bedbound; needs assistance even for quiet play; 20=sleeps often; play limited to very passive activities; 10=doesn't play or get out of bed; 5=unresponsive 0=dead

Participants who received at least one dose of study drug (RP103) and had at least one post-baseline Lansky play performance scale assessment and for whom reduced activities of daily living was prespecified as a preeminent symptom and with available data at each time point.