A Research Study to Evaluate the Effects of a New Oral Medicine Called Cenerimod in Adults With Systemic Lupus Erythematosus

Inclusion Criteria: Inclusion criteria at screening: * Signed Informed Consent Form (ICF) prior to any study-mandated procedure. * Diagnosis of Systemic Lupus Erythematosus (SLE) made at least 6 months prior to Screening, according to 2019 European League Against Rheumatism / American College of Rheumatology Criteria. * A modified Systemic Lupus Erythematosus Disease Activity Index-2000 (mSLEDAI-2K) score ≥ 6 and clinical mSLEDAI-2K score ≥ 4 with at least 2 points for musculoskeletal or mucocutaneous manifestations (i.e., myositis, arthritis, rash, alopecia, mucosal ulcers). The mSLEDAI-2K score does not include "leukopenia". * British Isles Lupus Assessment Group-2004 (BILAG) Grade B in ≥ 2 organ systems or a BILAG Grade A in ≥ 1 organ system. * Physician's Global Assessment (PGA) score ≥ 1.0 on a 0 to 3 visual analog scale. * Currently treated with one or more of the following SLE background medications: * Anti-malarials (≤ 400 mg/day hydroxychloroquine, ≤ 500 mg/day chloroquine, ≤ 100 mg/day quinacrine). * Mycophenolate mofetil (≤ 2 g/day) / mycophenolic acid (≤1.44 g/day). * Azathioprine (≤ 2 mg/kg/day). * Methotrexate (≤ 25 mg/week). * Oral Corticosteroids (OCS): * if OCS is the only SLE background medication: ≥ 7.5 mg/day and ≤ 30 mg/day prednisone or equivalent. * if OCS is not the only SLE background medication: ≤ 30 mg/day prednisone or equivalent. * Belimumab (≤10 mg/kg every 4 weeks intravenously \[i.v.\], or 200 mg/week subcutaneously \[s.…