Inclusion Criteria:
- Study participant must be 2 to <18 years of age inclusive, at the Baseline Visit.
- Study participants who have confirmed diagnosis of enthesitis-related arthritis (ERA; including juvenile-onset ankylosing spondylitis (JAS)) and/or juvenile psoriatic arthritis (JPsA) according to the juvenile-International League of Associations for Rheumatology (JIA-ILAR) classification criteria of at least 3 months duration prior to the Screening Visit.
- Study participants who have active disease (ERA \[including JAS\] and/or JPsA) defined as having at least 3 active joints, each of which needs to be included in the joints assessed in the JADAS27, and for ERA at least 1 site of enthesitis at Baseline or documented by history.
- Study participants with inadequate response (at least 1 month) or intolerance to at least 1 nonsteroidal anti-inflammatory drug (NSAID).
- Study participants taking concomitant methotrexate or sulfasalazine are allowed to continue the medication if it has been used for the past 12 weeks with a stable dose for the 4 weeks prior to Baseline, with no change in dose for the first 16 weeks of treatment foreseen. (Note: prior or concomitant use of methotrexate or sulfasalazine is NOT required for study participation.)
- Study participants with no concomitant use of second line agents such as disease-modifying and/or immunosuppressive drugs with the exception of methotrexate or sulfasalazine.
- Body weight of ≥10kg.
- Male and female.
- A female study participant will be eligible to participate if she is not pregnant, not breastfeeding, and at least 1 of the following conditions applies:
1. Not a woman of childbearing potential (WOCBP) OR
2. A WOCBP who agrees to follow the contraceptive guidance during the Initial Treatment Period, the Open-label Extension (OLE) Period, and for at least 20 weeks after the final dose of investigational medicinal product (IMP; ie, the Safety Follow-up (SFU) Period)
- Capable of giving/having parent(s) or legal representative provide signed informed consent/assent (where appropriate), which includes compliance with the requirements and restrictions listed in the Informed Consent Form (ICF) and assent and in this protocol.
Exclusion Criteria:
- Study participants fulfilling any International League of Associations for Rheumatology (ILAR) diagnostic juvenile idiopathic arthritis (JIA) category other than enthesitis-related arthritis (ERA; including juvenile-onset ankylosing spondylitis (JAS)) and/or juvenile psoriatic arthritis (JPsA).
- Study participant has history of inflammatory bowel disease (IBD) or signs/symptoms suggestive of IBD.
- Study participant has active uncontrolled uveitis.
- Study participant has history of active tuberculosis (TB) unless successfully treated, latent TB unless prophylactically treated.
- Study participant has had major surgery (including joint surgery) within the 3 months prior to the Baseline Visit or has planned major surgery within 6 months after entering the study.
- Study participant has laboratory abnormalities at Screening defined in the Protocol.
- Study participant has an active infection or history of infections (such as serious infection, chronic infections, opportunistic infections, unusually severe infections).
- Study participant has received drugs listed in the protocol outside the specified timeframes relative to the Baseline Visit or receives prohibited concomitant treatments.
- Study participant had previous therapy with bimekizumab or prior treatment with other IL-17 biologic response modifier.
- Study participant had prior treatment with more than one biologic response modifier (other than an IL-17).
- Presence of active suicidal ideation, or positive suicide behavior.
- Study participant has been diagnosed with severe depression in the past 6 months.