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

Key Inclusion Criteria:

1. Participant with a body mass index (BMI) within the 5th to 95th percentiles for the age and gender (based on World Health Organization BMI charts). Participant must have a minimum weight of 15 kg.
2. Participant must meet the International League of Associations for Rheumatology classification for 1 of the following categories and have, according to the investigator's judgment, moderately to severely active disease that is not adequately controlled with his/her current therapy.

1. Rheumatoid factor (RF)-positive polyarthritis
2. RF-negative polyarthritis
3. Oligoarthritis
4. Psoriatic arthritis
5. Enthesis-related arthritis (ERA) Note: Historical Human leukocyte antigen B-27 (HLA-B27) results are considered appropriate for ERA diagnosis during screening.
6. Systemic JIA with active arthritis without active systemic features, or with active systemic features that are stable in the prior 6 months of time of enrollment
7. Participant with intolerance or a history of inadequate response to at least one of the following medications for the treatment of JIA, administered for at least 12 weeks, based on current treatment guidelines: conventional synthetic disease-modifying antirheumatic drugs and biological disease-modifying antirheumatic drugs (including methotrexate) and non-steroidal anti-inflammatory drugs for ERA and psoriatic arthritis.
8. Female participants of childbearing potential (i.e. who have passed menarche) must have a negative highly sensitive urine pregnancy test.

Key Exclusion Criteria:

1. Participant with persistent oligoarthritis.
2. Participant with undifferentiated arthritis.
3. Participant with any other any other rheumatic, inflammatory, or immunologic disease (e.g. inflammatory bowel disease, hypogammaglobulinemia, systemic lupus erythematosus, or uncontrolled uveitis).
4. Active infection that is clinically significant, as per judgment of the investigator.
5. Participant with a history of complicated herpes zoster infection (with multi-dermatomal, disseminated, ophthalmic, or central nervous system involvement).
6. 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.