A Phase 1, Open-label, Dose-escalation Study (ELEVATE-1) to Evaluate the Safety, Tolerability, Pharmacokinetics, Pharmacodynamics, and Immunogenicity of a CD19/CD20 T-cell Engager in Participants With B-cell Driven Autoimmune Rheumatic Diseases (ARD)
This is a 2-part study of GSK5926371 in participants with autoimmune rheumatic diseases (ARD). In part 1, participants will receive different doses of GSK5926371 to find a suitable priming dose. In part 2, participants will receive GSK5926371 at doses based on data from part 1. The study is aimed at testing if GSK5926371 is safe, well-tolerated, how the body processes the study drug, how it works in the body, and whether it triggers any immune responses.
• Part 1 will enroll adult participants with systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA).
• Part 2 will enroll adult participants with SLE, RA, idiopathic inflammatory myopathies (IIM) or Sjogren's disease (SjD).
• Participants must be 18 to 70 years of age inclusive at the time of signing the informed consent form.
• Body mass index (BMI) between 18-35 kilograms per square meter (kg/m\^2) inclusive with a body weight of greater than or equal to (\>=) 45 kilograms (kg).
• A female participant is eligible to participate if she is not pregnant or breastfeeding, and one of the following conditions applies:
• Is a participant of non-childbearing potential (PONCBP), OR
• Is a participant of childbearing potential (POCBP) and using a contraceptive method that is highly effective, with a failure rate of less than (\<) 1 percent (%), 28 days prior to and during the study intervention period and for at least 28 weeks after the first dose of GSK5926371. The investigator should evaluate potential for contraceptive method failure (e.g. noncompliance, recently initiated) in relationship to the first dose of study intervention.
⁃ A POCBP must have a negative highly sensitive pregnancy test (urine or serum, as required by local regulations) within 24 hours before the first dose of study intervention.
⁃ If a urine test cannot be confirmed as negative (e.g. an ambiguous result), a serum pregnancy test is required. In such cases, the participant must be excluded from participation if the serum pregnancy result is positive.
⁃ \- Signed and dated informed consent form indicating that the participant is willing and able to comply with hospitalization, clinic visits and scheduled study assessments as detailed in the protocol.