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

Who May Qualify: 1. Be 18+ years old and legally able to consent 2. Have active psoriatic arthritis (PsA) or axial spondyloarthritis (axSpA), suitable for 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 stopped bDMARDs ≥3 months or 5 half-lives ago Who Should NOT Join This Trial: 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 nontuberculous mycobacterium (NTMB) 4. Other inflammatory diseases (e.g., rheumatoid arthritis (RA), lupus, inflammatory bowel disease (IBD)) 5. Recent live vaccines (within 8 weeks) or Bacillus Calmette-Guerin (BCG) (within 1 year) 6. Recent use of glucagon-like peptide-1 (GLP-1) agonists (within 28 weeks) Always talk to your doctor about whether this trial is right for you.