The purpose of the study is to evaluate pharmacokinetics (PK) of ustekinumab in juvenile psoriatic arthritis (jPsA) and pediatric psoriasis (PsO).

jPsA is a complex, chronic, progressive, debilitating musculoskeletal disease with significant remaining medical need. There is a need for medications which have a similar efficacy profile and a well-characterized safety profile relative to currently available tumor necrosis factor alpha (TNF alpha) inhibitors for jPsA participants with active disease. STELARA (ustekinumab) is a fully human immunoglobulin G1 kappa monoclonal antibody which binds with high affinity to the p40 subunit common to both interleukin (IL)-12 and IL 23 preventing IL-12/23p40 binding to the IL 12 Rb1 cell surface receptor shared by both cytokines. Through this mechanism of action, ustekinumab effectively neutralizes IL-12 T helper 1- and IL-23 T helper 17-mediated cellular responses. Ustekinumab has been extensively studied in adult participants with psoriasis, psoriatic arthritis (PsA), Crohn's disease, and ulcerative colitis (UC). Additionally, ustekinumab has been studied in children (greater than or equal to [>=] 6 to less than [<] 12 years of age) and adolescents (>=12 to <18 years of age) with pediatric psoriasis [PsO]. This study consists of 3 visits: screening (Visit 1), interim and final visit (Visits 2 and 3) (Visits should be >=7 days apart). Key safety assessments include analyses of the incidence and types of adverse events (AEs), serious adverse events (SAEs), reasonably related AEs, and discontinuation of ustekinumab due to an AE, infections, and/or injection site and hypersensitivity reactions. Any newly identified malignancy, case of active tuberculosis (TB), or opportunistic infection will also be assessed. The total duration of the study is up to 20 weeks.

Inclusion Criteria:

• Greater than or equal to (>=) 5 to less than (<) 18 years of age, inclusive, with a diagnosis of juvenile psoriatic arthritis (jPsA) (that is, International League Against Rheumatism [ILAR] or Vancouver criteria) by qualified health care professional (HCP)
• >=6 to <18 years of age, inclusive, with a diagnosis of pediatric psoriasis (PsO) by a qualified HCP
• Initiated ustekinumab treatment >=16 weeks prior to enrollment and received 3 or more doses of ustekinumab prior to enrollment
• Parent(s) (preferably both if available or as per local requirements) (or their legally acceptable representative) must sign an informed consent form (ICF) indicating that he or she understands the purpose of, and procedures required for, the study and is willing to allow the child to participate in the study. Assent is also required of children capable of understanding the nature of the study (typically 7 years of age and older) as described in informed consent process

Exclusion Criteria:

• Has poor tolerability of venipuncture or lack of adequate venous access for required blood sampling
• Has any condition that, in the opinion of the investigator, participation would not be in the best interest of the participant (example, compromise the well-being) or that could prevent, limit, or confound the protocol-specified assessments
• If currently enrolled in an investigational study, contact the study responsible physician to discuss eligibility for inclusion in study

The data sharing policy of the Janssen Pharmaceutical Companies of Johnson & Johnson is available at www.janssen.com/clinical-trials/transparency. As noted on this site, requests for access to the study data can be submitted through Yale Open Data Access (YODA) Project site at yoda.yale.edu