The purpose of this study is to assess plasma bimekizumab concentrations following subcutaneous (sc) bimekizumab administration.

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.

Data from this trial may be requested by qualified researchers six months after product approval in the US and/or Europe, or global development is discontinued, and 18 months after trial completion. Investigators may request access to anonymized individual patient-level data and redacted trial documents which may include: analysis-ready datasets, study protocol, annotated case report form, statistical analysis plan, dataset specifications, and clinical study report. Prior to use of the data, proposals need to be approved by an independent review panel at www.Vivli.org and a signed data sharing agreement will need to be executed. All documents are available in English only, for a prespecified time, typically 12 months, on a password protected portal. This plan may change if the risk of re-identifying trial participants is determined to be too high after the trial is completed; in this case and to protect participants, individual patient-level data would not be made available.

Data from this trial may be requested by qualified researchers six months after product approval in the US and/or Europe or global development is discontinued, and 18 months after trial completion.

Qualified researchers may request access to anonymized IPD and redacted study documents which may include: raw datasets, analysis-ready datasets, study protocol, blank case report form, annotated case report form, statistical analysis plan, dataset specifications, and clinical study report. Prior to use of the data, proposals need to be approved by an independent review panel at www.Vivli.org and a signed data sharing agreement will need to be executed. All documents are available in English only, for a pre-specified time, typically 12 months, on a password protected portal.