A Study to Assess Adverse Events, How the Drug Moves Through the Body and Effectiveness of Intravenous Infusions and Subcutaneous Injections of ABBV-519 in Adult Participants With Systemic Lupus Erythematosus (SLE) or Rheumatoid Arthritis (RA)

Systemic lupus erythematosus (SLE) is a chronic, systemic autoimmune disease characterized by B cell hyperactivity. Rheumatoid Arthritis (RA) is a chronic inflammatory disease causing pain, stiffness, swelling and loss of joint function. The purpose of this study is to assess the pharmacokinetics, pharmacodynamics and safety of ABBV-519 in adult participants with SLE or RA. This is a single ascending dose study in an estimated 30 adult participants with moderate SLE or RA. The total duration of the study will be approximately 425 days (60-day Screening Period, 1-day Treatment Period, and a 52 week Follow-up Period) at approximately 15 to 20 sites globally. There may be higher treatment burden for participants in this trial compared to their standard of care (due to study procedures). Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.