Study to Measure Filgotinib in the Blood of Children and Teenagers With Arthritis Taking Filgotinib (SCALESIA)

Key Inclusion Criteria:

• Participant with a body mass index (BMI) within the 5th to 95th percentiles for theage and gender (based on World Health Organization BMI charts). Participant musthave a minimum weight of 15 kg.

• Participant must meet the International League of Associations for Rheumatologyclassification for 1 of the following categories and have, according to theinvestigator's judgment, moderately to severely active disease that is notadequately controlled with his/her current therapy.

• Rheumatoid factor (RF)-positive polyarthritis

• RF-negative polyarthritis

• Oligoarthritis

• Psoriatic arthritis

• Enthesis-related arthritis (ERA) Note: Historical Human leukocyte antigen B-27 (HLA-B27) results are considered appropriate for ERA diagnosis duringscreening.

• Systemic JIA with active arthritis without active systemic features, or withactive systemic features that are stable in the prior 6 months of time ofenrollment

• Participant with intolerance or a history of inadequate response to at least one ofthe following medications for the treatment of JIA, administered for at least 12weeks, based on current treatment guidelines: conventional syntheticdisease-modifying antirheumatic drugs and biological disease-modifying antirheumaticdrugs (including methotrexate) and non-steroidal anti-inflammatory drugs for ERA andpsoriatic arthritis.

• Female participants of childbearing potential (i.e. who have passed menarche) musthave a negative highly sensitive urine pregnancy test.

Key Exclusion Criteria:

• Participant with persistent oligoarthritis.

• Participant with undifferentiated arthritis.

• Participant with any other any other rheumatic, inflammatory, or immunologic disease (e.g. inflammatory bowel disease, hypogammaglobulinemia, systemic lupuserythematosus, or uncontrolled uveitis).

• Active infection that is clinically significant, as per judgment of theinvestigator.

• Participant with a history of complicated herpes zoster infection (withmulti-dermatomal, disseminated, ophthalmic, or central nervous system involvement).

• Currently on any therapy for chronic infection (such as pneumocystis,cytomegalovirus, herpes simplex, or atypical mycobacteria).

Note: Other protocol defined Inclusion/ Exclusion criteria may apply.