The study has been designed with three components. Part A is an open label PK study followed by a randomized trial component (Part B) followed by open label Long Term Extension (LTE).

The initial PK analysis is first done in adolescent subjects (12 to <18 years) before initiating the PK study in younger cohort (6 to <12 years)

Inclusion Criteria:

• Subject must be 6 to < 18 years of age, of either sex, of any race/ ethnicity, must weight greater than or equal to 15Kg.
• Diagnosis of predominantly plaque psoriasis for ≥6 months (as determined by subject interview and confirmation of diagnosis through physical examination by investigator).
• Moderate to severe psoriasis at baseline defined as: at least 10% Body Surface Area (BSA) involvement, PGA score ≥ 3, and PASI score ≥ 12
• Subject must be considered a candidate for systemic therapy and/or phototherapy.
• Subject is considered to be eligible according to tuberculosis (TB) screening criteria
• A maximum of 2 QuantiFERON tests will be allowed. A re-test is only permitted if the first is indeterminate; the result of the second test will then be used.

Exclusion Criteria:

• Subject has predominantly non-plaque forms of psoriasis specifically erythrodermic psoriasis, predominantly pustular psoriasis, medication-induced or medication-exacerbated psoriasis, or new-onset guttate psoriasis
• Subject has laboratory abnormalities at screening including any of the following: Alanine transaminase (ALT) or aspartate transaminase, (AST) ≥2X the upper limit of normal, Creatinine ≥1.5X the upper limit of normal serum direct bilirubin ≥ 1.5 mg/dL, white blood cell count < 3.0 x 103/μL, and any other laboratory abnormality, which, in the opinion of the Investigator, will prevent the subject from completing the study or will interfere with the interpretation of the study results
• Subject who is expected to require topical therapy, phototherapy, or additional systemic therapy for psoriasis during the trial
• Female subjects of childbearing potential who are pregnant, intend to become pregnant (within 6 months of completing the trial), or are lactating. (Sexually active adolescent girls will be required to use contraception)
• Subject with presence of any infection or history of recurrent infection requiring treatment with systemic antibiotics within 2 weeks prior to Screening, or severe infection (e.g. pneumonia, cellulitis, bone or joint infections) requiring hospitalization or treatment with IV antibiotics within 8 weeks prior to Screening
• Positive human immunodeficiency virus (HIV) test result, hepatitis B Virus (HBV) test results, or hepatitis C virus (HCV) test result
• Subjects who have a high risk of suicidality at the Screening assessment as indicated by the C-SSRS (Columbia Suicide Severity Rating Scale), or based on the Investigator's judgment.
• Subject who has received any of the prohibited medications, supplements or substances during the study.

The study will follow three distinct components: Open-label PK, blinded Randomized Trial Component (RCT) with open-label comparator followed by the open-label extension.