Linear-log trapezoidal method was used for evaluation. For the calculation of AUCtau, pre-dose concentration of Day 7 was used as an estimate for the concentration of 24 hours post-dose on Day 7.

Pharmacokinetic (PK) parameter analysis population: treated participants who had at least 1 of the PK parameters of primary interest. All participants with evaluable PK were included in the analysis. Two participants weren't included in analysis as they didn't have evaluable PK data (1 discontinued study intervention on Day 3 and the other had the important protocol deviation, PK samples not collected on Day 7). "Number of Participants Analyzed": participants evaluable for the outcome measure.

PK parameter analysis population: treated participants who had at least 1 of the PK parameters of primary interest. All participants with evaluable PK were included in the analysis. Two participants weren't included in analysis as they didn't have evaluable PK data (1 discontinued study intervention on Day 3 and the other had the important protocol deviation, PK samples not collected on Day 7). "Number of Participants Analyzed": participants evaluable for the outcome measure.

PK parameter analysis population: treated participants who had at least 1 of the PK parameters of primary interest. All participants with evaluable PK were included in the analysis. Two participants weren't included in analysis as they didn't have evaluable PK data (1 discontinued study intervention on Day 3 and the other had the important protocol deviation, PK samples not collected on Day 7). "Number of Participants Analyzed": participants evaluable for the outcome measure.

Clearance of a drug is a measure of the rate at which a drug is metabolized or eliminated by normal biological process. Clearance obtained after oral dose (apparent oral clearance) is influenced by the fraction of the dose absorbed.

PK parameter analysis population: treated participants who had at least 1 of the PK parameters of primary interest. All participants with evaluable PK were included in the analysis. Two participants weren't included in analysis as they didn't have evaluable PK data (1 discontinued study intervention on Day 3 and the other had the important protocol deviation, PK samples not collected on Day 7). "Number of Participants Analyzed": participants evaluable for the outcome measure.

Volume of distribution is defined as the theoretical volume in which the total amount of drug would need to be uniformly distributed to produce the desired plasma concentration of a drug. Vz/F is influenced by the fraction absorbed.

PK parameter analysis population: treated participants who had at least 1 of the PK parameters of primary interest. All participants with evaluable PK were included in the analysis. Two participants weren't included in analysis as they didn't have evaluable PK data (1 discontinued study intervention on Day 3 and the other had the important protocol deviation, PK samples not collected on Day 7). "Number of Participants Analyzed": participants evaluable for the outcome measure.

Elimination half-life (t1/2) is the time measured for the plasma concentration to decrease by one half at the elimination phase.

PK parameter analysis population: treated participants who had at least 1 of the PK parameters of primary interest. All participants with evaluable PK were included in the analysis. Two participants weren't included in analysis as they didn't have evaluable PK data (1 discontinued study intervention on Day 3 and the other had the important protocol deviation, PK samples not collected on Day 7). "Number of Participants Analyzed": participants evaluable for the outcome measure.

The pharmacodynamic (PD) parameter analysis population included all participants treated who had at least 1 of the PD parameters of primary interest. Here, "Number Analyzed" signifies number of participants evaluable for specified rows.

T lymphocytes included CD3 cells, CD4 T helper lymphocytes and CD8 T cytotoxic lymphocytes.

The PD parameter analysis population included all participants treated who had at least 1 of the PD parameters of primary interest. Here, "Number Analyzed" signifies number of participants evaluable for specified rows.

B lymphocytes included CD19 cells.

The PD parameter analysis population included all participants treated who had at least 1 of the PD parameters of primary interest. Here, "Number Analyzed" signifies number of participants evaluable for specified rows.

The PD parameter analysis population included all participants treated who had at least 1 of the PD parameters of primary interest. Here, "Number Analyzed" signifies number of participants evaluable for specified rows.

An AE is any untoward medical occurrence in a participant or clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Treatment-emergent were events between first dose to 35 days after last dose, that were absent before treatment or that worsened relative to pretreatment state.

Safety analysis set included all participants assigned to study intervention and who received at least 1 dose of study intervention.

An AE is any untoward medical occurrence in a participant or clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. Treatment-related AE was any untoward medical occurrence attributed to study drug in a participant who received study drug. Relatedness was judged by investigator.

Safety analysis set included all participants assigned to study intervention and who received at least 1 dose of study intervention.

An AE is any untoward medical occurrence in a participant or clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention. SAE was an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly.

Safety analysis set included all participants assigned to study intervention and who received at least 1 dose of study intervention.

An AE is any untoward medical occurrence in a participant or clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.

Safety analysis set included all participants assigned to study intervention and who received at least 1 dose of study intervention.

Vital signs evaluation included blood pressure and heart rate measurements. Clinical significance of any vital sign abnormality was judged by investigator.

Safety analysis set included all participants assigned to study intervention and who received at least 1 dose of study intervention.

Clinical laboratory parameters included haematology: haemoglobin, haematocrit, red blood cells count, platelet count, white blood cells count, total absolute: neutrophils, eosinophils, monocytes, basophils, lymphocytes; chemistry: urea and creatinine estimated creatinine clearance, glucose (fasting), sodium, potassium, chloride, aspartate aminotransferase (AT), alanine AT, total bilirubin, alkaline phosphatase, albumin, total protein; urinalysis: local dipstick: pH, qualitative: glucose, protein, albuminuria, blood, ketones, nitrites, leukocyte esterase and others. Clinical significance of any laboratory abnormality was judged by investigator.

Safety analysis set included all participants assigned to study intervention and who received at least 1 dose of study intervention.

The pediatric taste questionnaire included 3 questions regarding: 1) Overall Taste (likeability in tase), 2) Overall Mouthfeel (how the medicine felt) and 3) Overall Volume of Medicine (likeability of the amount of medicine). Each question ranged from 1 (most favorable) to 5 (least favorable), higher scores indicates less liking to medicine. Ritlecitinib was provided as capsules; for administration, the capsules were dissolved in water and the contents of the capsule in water were taken according to the dosing administration instructions.

Safety analysis set included all participants assigned to study intervention and who received at least 1 dose of study intervention. Here, "Number of Participants Analyzed" signifies number of participants evaluable for this outcome measure.