An Open-label Study to Evaluate the Pharmacokinetics and Safety of Bimekizumab in Pediatric Study Participants With Active Juvenile Idiopathic Arthritis Subtypes Enthesitis-related Arthritis (Including Juvenile-onset Ankylosing Spondylitis) and Juvenile Psoriatic Arthritis

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 juvenilepsoriatic arthritis (JPsA) according to the juvenile-International League ofAssociations for Rheumatology (JIA-ILAR) classification criteria of at least 3months duration prior to the Screening Visit.

• Study participants who have active disease (ERA [including JAS] and/or JPsA) definedas having at least 3 active joints, each of which needs to be included in the jointsassessed in the JADAS27, and for ERA at least 1 site of enthesitis at Baseline ordocumented by history.

• Study participants with inadequate response (at least 1 month) or intolerance to atleast 1 nonsteroidal anti-inflammatory drug (NSAID).

• Study participants taking concomitant methotrexate or sulfasalazine are allowed tocontinue the medication if it has been used for the past 12 weeks with a stable dosefor the 4 weeks prior to Baseline, with no change in dose for the first 16 weeks oftreatment foreseen. (Note: prior or concomitant use of methotrexate or sulfasalazineis NOT required for study participation.)

• Study participants with no concomitant use of second line agents such asdisease-modifying and/or immunosuppressive drugs with the exception of methotrexateor 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 InitialTreatment Period, the Open-label Extension (OLE) Period, and for at least 20weeks after the final dose of investigational medicinal product (IMP; ie, theSafety Follow-up (SFU) Period)

• Capable of giving/having parent(s) or legal representative provide signed informedconsent/assent (where appropriate), which includes compliance with the requirementsand restrictions listed in the Informed Consent Form (ICF) and assent and in thisprotocol.

Exclusion Criteria:

• Study participants fulfilling any International League of Associations forRheumatology (ILAR) diagnostic juvenile idiopathic arthritis (JIA) category otherthan enthesitis-related arthritis (ERA; including juvenile-onset ankylosingspondylitis (JAS)) and/or juvenile psoriatic arthritis (JPsA).

• Study participant has history of inflammatory bowel disease (IBD) or signs/symptomssuggestive of IBD.

• Study participant has active uncontrolled uveitis.

• Study participant has history of active tuberculosis (TB) unless successfullytreated, latent TB unless prophylactically treated.

• Study participant has had major surgery (including joint surgery) within the 3months prior to the Baseline Visit or has planned major surgery within 6 monthsafter 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 seriousinfection, chronic infections, opportunistic infections, unusually severeinfections).

• Study participant has received drugs listed in the protocol outside the specifiedtimeframes relative to the Baseline Visit or receives prohibited concomitanttreatments.

• Study participant had previous therapy with bimekizumab or prior treatment withother 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.