A Study to Learn if Bimekizumab Given in Different Ways is Safe and Moves Similarly Throughout the Body Over Time in Adults

Inclusion Criteria:

1. Be 18+ years old and legally able to consent

2. Have active psoriatic arthritis (PsA) or axial spondyloarthritis (axSpA), suitablefor bimekizumab treatment

3. Weigh between 45-100 kg (females) or 50-100 kg (males).

4. Be biologic disease-modifying anti-rheumatic drug (bDMARD)-naïve or have stoppedbDMARDs ≥3 months or 5 half-lives ago

Exclusion Criteria:

1. Serious organ system disorders (e.g., heart, liver, kidney, gastrointestinal, neuro)

2. Severe psychiatric issues or substance abuse in the past year

3. Recent or chronic infections, including tuberculosis (TB) or nontuberculousmycobacterium (NTMB)

4. Other inflammatory diseases (e.g., rheumatoid arthritis (RA), lupus, inflammatorybowel disease (IBD))

5. Recent live vaccines (within 8 weeks) or Bacillus Calmette-Guerin (BCG) (within 1year)

6. Recent use of glucagon-like peptide-1 (GLP-1) agonists (within 28 weeks)