The main objective is to assess the safety and tolerability of budoprutug in adults with SLE. Pharmacokinetics, pharmacodynamics, and preliminary clinical efficacy will also be assessed.

Budoprutug is a humanized, immunoglobulin (Ig) G1 monoclonal antibody that selectively binds to CD19 and is projected to deplete targeted cells through antibody-dependent cellular cytotoxicity. This Phase 1b, open-label study will evaluate budoprutug administered as a single intravenous infusion in ascending dose cohorts of patients aged 18 years and above with active, seropositive SLE and inadequate response to standard therapy. The study will also assess the pharmacokinetics, pharmacodynamics and early indications of efficacy of budoprutug in SLE, where pharmacodynamics will be evaluated as the change in the number of B cells and immunoglobulins (antibodies) in the blood over time following a single infusion.

Inclusion Criteria:

1. Aged 18 to 65 years at the time of consent.
2. Diagnosis of SLE according to the 2019 European League Against. Rheumatism and the American College of Rheumatology (ACR) classification criteria.
3. Active, seropositive disease, with SLEDAI 2K >=8.
4. Inadequate response to at least 2 therapeutic interventions, including at least one oral immunosuppressive or biologic standard-of care therapy.

Exclusion Criteria:

1. Active neuropsychiatric SLE.
2. History of inflammatory or autoimmune diseases including, but not limited to, rheumatoid arthritis, scleroderma, myositis, vasculitis, inflammatory bowel disease, or other conditions that require immune suppressive therapy. Subjects with stable concurrent Sjogren's, asthma, or autoimmune thyroid disease may be considered for participation.
3. Active systemic infection or history of chronic, recurrent, latent, or recent serious infections.