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/2a, open-label study will evaluate budoprutug 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. Budoprutug will be administered as two (2) IV infusions 14 days apart in ascending dose cohorts.

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 >=6 at screening
4. Inadequate response to at least one therapeutic intervention

Exclusion Criteria:

1. Active neuropsychiatric SLE.
2. Active lupus nephritis type III or IV that is expected to require induction therapy during the study or recently treated with induction therapy within 12 weeks.
3. Prior diagnosis of, or fulfills diagnostic criteria for, other autoimmune or inflammatory disease that may confound clinical assessments or increase subject risk in the study
4. Active systemic infection or history of chronic, recurrent, latent, or recent serious infections.