A Study Evaluating the Effects of GLPG3970 Given as Oral Treatment for 12 Weeks in Adults With Systemic Lupus Erythematosus

Inclusion Criteria: * SLE diagnosis defined by American College of Rheumatology (ACR) 1997 criteria ≥4 * Active arthritis in \>=4 active joints (according to 28 joint count) and/or cutaneous lupus erythematosus disease area and severity index (CLASI) score \>=6. * Anti-dsDNA antibodies \>15 IU/mL. * Stable standard-of-care (SoC) therapy (defined as no change in prescription for at least 2 weeks prior to first IP dosing) consisting of the following permitted SoC medications: * Corticosteroids \<=20 mg/day (prednisone or equivalent) for at least 2 weeks prior to first IP dosing; AND/OR * Non-steroidal anti-inflammatory drug (NSAIDs); AND/OR * One single antimalarial at a stable dose (hydroxychloroquine \<=5 mg/ kg/day, quinacrine 100 mg/day, or chloroquine 2.3 mg/kg/day) for at least 8 weeks prior to first IP dosing; AND/OR * One single immunosuppressant at a stable dose (azathioprine (AZA) \<=2 mg/kg/day, methotrexate (MTX) \<=20 mg/week, or mycophenolate mofetil (MMF) \<=2 g/day) for at least 8 weeks prior to first IP dosing. * estimated glomerular filtration rate (eGFR) \>=60 mL/min (according to Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI)) This list only contains the key inclusion criteria. Exclusion Criteria: * Lupus nephritis \>= Class III * Severe organ manifestation or life-threatening lupus disease (active severe central nervous system lupus, severe pleuro-pericarditis, severe vasculitis). * Severe acute respiratory syndrome coronavirus disease 2 (SARS-CoV-2) infection \>0 * Unstable condition not related to SLE * Systemic inflammatory condition other than SLE such as, but not limited to rheumatoid arthritis (RA), spondyloarthropathy, Crohn's disease, ulcerative colitis, or psoriatic arthritis * Sjögren's syndrome and/or antiphospholipid antibody syndrome who do not require treatment with any prohibited medication are NOT excluded. * Active systemic infection * Poorly controlled chronic cardiac, pulmonary, or renal disease. * Known or suspected history of or a current immunosuppressive condition, or a history of invasive opportunistic infections * Treatment with disallowed therapies This list only contains the key exclusion criteria.