Study of Subcutaneous (SC) Belimumab in Pediatric Participants With Systemic Lupus Erythematosus (SLE)

The purpose of this study is to evaluate the pharmacokinetics (PK), safety, and pharmacodynamics (PD) of repeat doses of 200 milligrams per milliliter (mg/mL) belimumab administered via SC injection in pediatric participants 5 to 17 years of age with SLE on a background of standard of care therapy. This bridging PK study is part of an extrapolation strategy to support the use of SC belimumab in pediatric SLE participants, based on the completed adult SLE study with SC belimumab and the pediatric SLE study with intravenous (IV) belimumab. Part A is an open label 12-week treatment phase where participants will be enrolled and allocated to treatment cohorts based on their body weight at baseline. The dose and dosing regimens selected for SC administration in this pediatric population are intended to achieve a similar average exposure as observed with the weekly 200 mg SC dosing regimen in adult SLE patients. Part B is an optional 40-week open-label continuation phase, open to all participants who have completed Part A. Dosing of SC belimumab may continue at the same frequency in Part B or may require a change in frequency according to changes in participant body weight. The total duration of the study will be 68 weeks including a 12-Week open label treatment phase (Part A), an optional 40-week open-label continuation phase (Part B) and 16-week follow-up.

Inclusion Criteria: * Participant must be between 5 and 17 years of age inclusive, at the time of Day 1. * Participants who meet the 1997 American College of Rheumatology (ACR) criteria for the classification of SLE; * Have or have had in series 4 or more of the 11 ACR criteria for the classification of SLE. * Have active SLE disease defined as a safety of estrogen in lupus erythematosus national assessment (SELENA) systemic lupus erythematosus disease activity index (SLEDAI) score \>=6 at screening. * Have documented positive autoantibody test results within the study screening period, defined as an anti-nuclear antibody (ANA) titre \>= 1:80 and/or a positive anti-dsDNA (\>=30 international units per milliliter \[IU/mL\]) serum antibody test based on either the study's central laboratory results or the local laboratory results. Only unequivocally positive values as defined in the laboratory's reference range are acceptable; borderline values will not be accepted * Are on a stable SLE treatment regimen, "Stable treatment at Baseline" consists of any of the following medications (alone or in combination) administered for a period of at least 30 days prior to Day 1; * Corticosteroids \[prednisone or prednisone equivalent up to 0.5 milligram per kilogram per day (mg/kg/day)\], for those participants on alternating day doses of steroids, use the average of 2 daily doses to calculate the average daily steroid dose. * Other immunosuppressive or immunomodulatory agents including methotrexate, azathioprine, leflunomide, mycophenolate (including mycophenolate mofetil, mycophenolate mofetil hydrochloride, and mycophenolate sodium), calcineurin inhibitors (e.g. tacrolimus, cyclosporine), sirolimus, oral cyclophosphamide, 6-mercaptopurine or thalidomide. * Anti-malarials (e.g. hydroxychloroquine, chloroquine, quinacrine). * Non-steroidal anti-inflammatory drugs (NSAIDs) * New SLE therapy must not be added within 30 days of Day 1. * Body weight \>=15 kg. * Male and/or female; * Contraceptive use by men or women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. * No contraceptive measures are required for male participants. * A female participant is eligible to participate if she is not pregnant or breastfeeding, and at least one of the following conditions applies, Is not a woman of childbearing potential (WOCBP) or Is a WOCBP and is using a contraceptive method that is highly effective, with a failure rate of \