Zasocitinib matching placebo.

Zasocitinib.

AUC0-Last of zasocitinib in plasma will be assessed.

The sPGA is an assessment by the investigator of the overall disease severity at the time of evaluation. Erythema (E), induration (I), and desquamation (D) are scored on a 5-point scale ranging from 0 (none) to 4 (severe). The sPGA composite score ranges from 0 to 4 and is calculated as Clear (0) = 0 for all three; Almost clear (1) = mean greater than (>) 0, less than (<) 1.5; Mild (2) = mean >= 1.5, <2.5; Moderate (3) = mean >=2.5, <3.5; and Severe (4) = mean >=3.5. The percentage of participants achieving an enhanced sPGA response of Clear (0) at Week 16 will be assessed.

The PASI is a measure of the average redness, thickness and scaliness of psoriatic skin lesions (each graded on a 0 to 4 scale; 0 = none to 4 = very severe), weighted by the area of involvement (head, upper extremities, trunk, and lower extremities). The PASI score ranges from 0 to 72, with higher PASI scores denoting more severe disease activity (less than or equal to [<=] 3 representing mild disease, >= 3 to 15 representing moderate disease, and >= 15 indicating severe disease). The PASI-100 is defined as 100% improvement from baseline in PASI score. The percentage of participants achieving 100% improvement or complete clearance of psoriasis lesions in the PASI score from baseline at Week 16 will be assessed.

ssPGA assesses the overall severity of active psoriasis on the participant's scalp. Scalp lesions will be evaluated in terms of clinical signs of erythema, induration, and scaling and scored on 5-point ssPGA scale where 0=absence of disease and 4=severe disease. Higher scores indicate more severe scalp psoriasis. The percentage of participants achieving an ssPGA score of at least 3 at baseline who show an ssPGA score of 0 or 1 with a >= 2-point decrease at Week 16 will be reported.

Psoriasis BSA will be assessed by means of the handprint method, where the surface of the palm and 5 digits of the participant's hand represents 1% BSA. The sum of handprints equates to the total surface area of involvement. The change from baseline in BSA affected by psoriasis at Week 16 will be assessed.

Psoriasis BSA will be assessed by means of the handprint method, where the surface of the palm and 5 digits of the participant's hand represents 1% BSA. The sum of handprints equates to the total surface area of involvement. The percent change from baseline in BSA affected by psoriasis at Week 16 will be assessed.

The DLQI is a 10-item validated questionnaire completed by participants aged 16 years or older at screening, which is used to assess the impact of skin disease on the participant's quality of life (QoL) during the previous week. The questionnaire covers 6 domains, including symptoms and feelings, daily activities, leisure, work and school, personal relationships and treatment. Each question is scored from 0=not at all, 1=a little, 2=a lot, and 3=very much, giving a total score ranging from 0 to 30. A high score is indicative of a poor QoL. DLQI scores indicate: 0-1 (no effect on participant's life), 2-5 (small effect on participant's life), 6-10 (moderate effect on participant's life), 11-20 (very large effect on participant's life), 21-30 (extremely large effect on participant's life). The percentage of participants with a baseline DLQI Score >= 2 who achieve DLQI Score of Zero (0) or One (1) at Week 16 will be assessed.

The CDLQI is a 10-item validated questionnaire completed by participants aged 4 to <16 years of age at screening, which is used to assess the impact of skin disease on the participant's quality of life (QoL) during the previous week. The questionnaire covers 6 domains designed to measure the impact of skin disease on the child's quality of life, including symptoms and feelings, daily activities, leisure, school, personal relationships and treatment. The CDLQI score ranges from 0 to 30 points, higher score indicates greater severity. The percentage of participants with a baseline CDLQI Score >=2 who achieve CDLQI Score of 0 or 1 at Week 16 will be assessed.

The DLQI is a 10-item validated questionnaire completed by participants aged 16 years or older at screening, which is used to assess the impact of skin disease on the participant's quality of life (QoL) during the previous week. The questionnaire covers 6 domains, including symptoms and feelings, daily activities, leisure, work and school, personal relationships and treatment. Each question is scored from 0=not at all, 1=a little, 2=a lot, and 3=very much, giving a total score ranging from 0 to 30. A high score is indicative of a poor QoL. DLQI scores indicate: 0-1 (no effect on participant's life), 2-5 (small effect on participant's life), 6-10 (moderate effect on participant's life), 11-20 (very large effect on participant's life), 21-30 (extremely large effect on participant's life). The change from baseline in DLQI at Week 16 will be assessed.

The CDLQI is a 10-item validated questionnaire completed by participants aged 4 to <16 years of age at screening, which is used to assess the impact of skin disease on the participant's quality of life (QoL) during the previous week. The questionnaire covers 6 domains designed to measure the impact of skin disease on the child's quality of life, including symptoms and feelings, daily activities, leisure, school, personal relationships and treatment. The CDLQI score ranges from 0 to 30 points, higher score indicates greater severity. The change from baseline in CDLQI at Week 16 will be assessed.

The Itch NRS is a single-item participant-reported measure. Participants indicate itch severity at its worst over a 24-hour recall period on an 11-point scale anchored at 0, representing 'no itching' and 10, representing 'worst itch imaginable'. The percentage of participants in Cohort 1 with a baseline Itch NRS Score >=4 who achieve a >=4-Point improvement in Itch NRS Score at Week 16 will be assessed.

The Itch NRS is a single-item participant-reported measure. Participants indicate itch severity at its worst over a 24-hour recall period on an 11-point scale anchored at 0, representing 'no itching' and 10, representing 'worst itch imaginable'. The change from baseline in Itch NRS at Week 16 will be assessed.

The Itch NRS is a single-item participant-reported measure. Participants indicate itch severity at its worst over a 24-hour recall period on an 11-point scale anchored at 0, representing 'no itching' and 10, representing 'worst itch imaginable'. The percent change from baseline in Itch NRS at Week 16 will be assessed.

The PASI is a measure of the average redness, thickness and scaliness of psoriatic skin lesions (each graded on a 0 to 4 scale; 0 = none to 4 = very severe), weighted by the area of involvement (head, upper extremities, trunk, and lower extremities). The PASI score ranges from 0 to 72, with higher PASI scores denoting more severe disease activity (less than or equal to [<=] 3 representing mild disease, >= 3 to 15 representing moderate disease, and >= 15 indicating severe disease). The PASI-75 is defined as 75% improvement from baseline in PASI score. The percentage of participants achieving at least 75% or greater reduction in the PASI score from baseline during open-label period will be reported.

The PASI is a measure of the average redness, thickness and scaliness of psoriatic skin lesions (each graded on a 0 to 4 scale; 0 = none to 4 = very severe), weighted by the area of involvement (head, upper extremities, trunk, and lower extremities). The PASI score ranges from 0 to 72, with higher PASI scores denoting more severe disease activity (less than or equal to [<=] 3 representing mild disease, >= 3 to 15 representing moderate disease, and >= 15 indicating severe disease). The PASI-90 is defined as 90% improvement from baseline in PASI score. The percentage of participants achieving at least 90% or greater reduction in the PASI score from baseline during open-label period will be reported.

The PASI is a measure of the average redness, thickness and scaliness of psoriatic skin lesions (each graded on a 0 to 4 scale; 0 = none to 4 = very severe), weighted by the area of involvement (head, upper extremities, trunk, and lower extremities). The PASI score ranges from 0 to 72, with higher PASI scores denoting more severe disease activity (less than or equal to [<=] 3 representing mild disease, >= 3 to 15 representing moderate disease, and >= 15 indicating severe disease). The PASI-100 is defined as 100% improvement from baseline in PASI score. The percentage of participants achieving 100% improvement or complete clearance of psoriasis lesions in the PASI score from baseline during open-label period will be reported.

The sPGA is an assessment by the investigator of the overall disease severity at the time of evaluation. Erythema (E), induration (I), and desquamation (D) are scored on a 5-point scale ranging from 0 (none) to 4 (severe). The sPGA composite score ranges from 0 to 4 and is calculated as Clear (0) = 0 for all three; Almost clear (1) = mean greater than (>) 0, less than (<) 1.5; Mild (2) = mean >= 1.5, <2.5; Moderate (3) = mean >=2.5, <3.5; and Severe (4) = mean >=3.5. The percentage of participants achieving an sPGA of Clear (0) or Almost Clear (1) with a >=2-point decrease from baseline will be reported.

The sPGA is an assessment by the investigator of the overall disease severity at the time of evaluation. Erythema (E), induration (I), and desquamation (D) are scored on a 5-point scale ranging from 0 (none) to 4 (severe). The sPGA composite score ranges from 0 to 4 and is calculated as Clear (0) = 0 for all three; Almost clear (1) = mean greater than (>) 0, less than (<) 1.5; Mild (2) = mean >= 1.5, <2.5; Moderate (3) = mean >=2.5, <3.5; and Severe (4) = mean >=3.5. The percentage of participants achieving an enhanced sPGA of Clear (0) during open-label period will be reported.

ssPGA assesses the overall severity of active psoriasis on the participant's scalp. Scalp lesions will be evaluated in terms of clinical signs of erythema, induration, and scaling and scored on 5-point ssPGA scale where 0=absence of disease and 4=severe disease. Higher scores indicate more severe scalp psoriasis. The percentage of participants achieving an ssPGA response of clear (0) or almost clear (1) with a >=2-point decrease from baseline for participants with a baseline ssPGA >=3 during open-label period will be reported.

Psoriasis BSA will be assessed by means of the handprint method, where the surface of the palm and 5 digits of the participant's hand represents 1% BSA. The sum of handprints equates to the total surface area of involvement. The change from baseline in BSA affected by psoriasis during open-label period will be assessed.

Psoriasis BSA will be assessed by means of the handprint method, where the surface of the palm and 5 digits of the participant's hand represents 1% BSA. The sum of handprints equates to the total surface area of involvement. The percent change from baseline in BSA affected by psoriasis during open-label period will be assessed.

The DLQI is a 10-item validated questionnaire completed by participants aged 16 years or older at screening, which is used to assess the impact of skin disease on the participant's quality of life (QoL) during the previous week. The questionnaire covers 6 domains, including symptoms and feelings, daily activities, leisure, work and school, personal relationships and treatment. Each question is scored from 0=not at all, 1=a little, 2=a lot, and 3=very much, giving a total score ranging from 0 to 30. A high score is indicative of a poor QoL. DLQI scores indicate: 0-1 (no effect on participant's life), 2-5 (small effect on participant's life), 6-10 (moderate effect on participant's life), 11-20 (very large effect on participant's life), 21-30 (extremely large effect on participant's life). The percentage of participants with a baseline DLQI Score >= 2 who achieve DLQI Score of 0 or 1 during open-label period will be assessed.

The CDLQI is a 10-item validated questionnaire completed by participants aged 4 to <16 years of age at screening, which is used to assess the impact of skin disease on the participant's quality of life (QoL) during the previous week. The questionnaire covers 6 domains designed to measure the impact of skin disease on the child's quality of life, including symptoms and feelings, daily activities, leisure, school, personal relationships and treatment. The CDLQI score ranges from 0 to 30 points, higher score indicates greater severity. The percentage of participants with a baseline CDLQI Score >= 2 who achieve CDLQI Score of 0 or 1 during open-label period will be assessed.

The DLQI is a 10-item validated questionnaire completed by participants aged 16 years or older at screening, which is used to assess the impact of skin disease on the participant's quality of life (QoL) during the previous week. The questionnaire covers 6 domains, including symptoms and feelings, daily activities, leisure, work and school, personal relationships and treatment. Each question is scored from 0=not at all, 1=a little, 2=a lot, and 3=very much, giving a total score ranging from 0 to 30. A high score is indicative of a poor QoL. DLQI scores indicate: 0-1 (no effect on participant's life), 2-5 (small effect on participant's life), 6-10 (moderate effect on participant's life), 11-20 (very large effect on participant's life), 21-30 (extremely large effect on participant's life). The change from baseline in DLQI during open-label period will be assessed.

The CDLQI is a 10-item validated questionnaire completed by participants aged 4 to <16 years of age at screening, which is used to assess the impact of skin disease on the participant's quality of life (QoL) during the previous week. The questionnaire covers 6 domains designed to measure the impact of skin disease on the child's quality of life, including symptoms and feelings, daily activities, leisure, school, personal relationships and treatment. The CDLQI score ranges from 0 to 30 points, higher score indicates greater severity. The change from baseline in CDLQI during open-label period will be assessed.

The Itch NRS is a single-item participant-reported measure. Participants indicate itch severity at its worst over a 24-hour recall period on an 11-point scale anchored at 0, representing 'no itching' and 10, representing 'worst itch imaginable'. The percentage of participants with a baseline Itch NRS score >=4 who achieve a >=4-Point improvement in Itch NRS score during open-label period will be assessed.

The Itch NRS is a single-item participant-reported measure. Participants indicate itch severity at its worst over a 24-hour recall period on an 11-point scale anchored at 0, representing 'no itching' and 10, representing 'worst itch imaginable'. The change from baseline in Itch NRS for participants in Cohort 1 during open-label period will be assessed.

The Itch NRS is a single-item participant-reported measure. Participants indicate itch severity at its worst over a 24-hour recall period on an 11-point scale anchored at 0, representing 'no itching' and 10, representing 'worst itch imaginable'. The percent change from baseline in Itch NRS for participants in Cohort 1 during open-label period will be assessed.

Plasma concentrations of zasocitinib will be assessed using a validated liquid chromatography tandem mass spectrometry bioanalytical method.

The acceptability and palatability of zasocitinib will be assessed using the Acceptability Palatability Questionnaire. In this 5-item questionnaire, participants will be asked to evaluate the palatability and acceptability of zasocitinib considering the following elements: smell, taste, ease of swallowing, and aftertaste. This questionnaire uses an 11-point scale anchored at 0; a higher score means better acceptability and palatability.