Study to Characterise Pharmacokinetics (PK), Pharmacodynamics (PD), and Safety of Anifrolumab Administered Subcutaneously (SC) or Intravenously (IV)

The purpose of this study is to assess the PK, PD, and safety of anifrolumab as SC administration or IV infusion in adult participants with systemic lupus erythematosus (SLE).

Inclusion Criteria: * Participants who have a diagnosis of adult SLE according to the European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) 2019 criteria for ≥ 24 weeks prior to signing the informed consent form (ICF). * "Clinical" Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K) ≥ 4 points at Screening. * Antinuclear antibody (ANA)-positive at Screening. * Must be on stable background standard therapy with oral corticosteroids (OCS), antimalarials, and/or immunosuppressants alone or in combination. Exclusion Criteria: * Active severe or unstable neuropsychiatric SLE. * Active severe SLE-driven renal disease. * Known history of primary immunodeficiency, splenectomy, or any underlying condition predisposing to infection, or a positive result for Human Immunodeficiency Virus (HIV) infection confirmed by central laboratory at Screening. * At Screening, confirmed positive test for hepatitis B serology or positive test for hepatitis C antibody. * Any severe case of herpes zoster infection at any time prior to Week 0 (Day 1). * Opportunistic infection requiring hospitalisation or IV antimicrobial treatment within 3 years of randomisation * History of cancer, apart from: 1. Squamous or basal cell carcinoma of the skin treated with documented curative success ≥ 3 months prior to Day 1. 2. Cervical cancer in situ treated with apparent curative success ≥ 1 year prior to Day 1.