Belimumab, sold under the brand name Benlysta, is a human monoclonal antibody that inhibits B-cell activating factor (BAFF),[6] also known as B-lymphocyte stimulator (BLyS).

[4][5] Common adverse effects reported with belimumab include nausea, diarrhea, and fever, as well as hypersensitivity and infusion-site reactions, which were severe in 0.9% of patients.

[medical citation needed] B-cell activating factor (BAFF), also called B-lymphocyte stimulator (BLyS), is required for the development and survival of B cells.

It interacts with three membrane receptors on B lymphocytes:[medical citation needed] When BAFF binds to BAFF-R and BCMA on B cells, levels of Bcl-2, a survival factor, are increased.

When all three BAFF receptors are stimulated, levels of NF kappa B, which contributes to cell proliferation and differentiation, are increased in the nucleus.

[18] In 2003, CAT researchers reported that, by using phage display technology, they had elicited an array of more than 1,000 distinct antibodies, half of which inhibited binding of BLyS to its receptor.

The companies would share equally in Phase III/IV development costs, sales and marketing expenses, and profits of any product commercialized under the agreement.

[18] On 13 February 2007, HGS and GSK announced the initiation of the first of two Phase III clinical trials of belimumab in patients with active lupus erythematosus.

[21] Two Phase III clinical studies were conducted, involving a total of 1,684 patients with scores of ≥6 on the SELENA-SLEDAI assessment of lupus activity.

[38] Belimumab is much more expensive than other drugs used to treat lupus, including prednisone ($140 per year), hydroxychloroquine ($132), oral methotrexate ($432), azathioprine ($468), and mycophenolate mofetil ($1,224).

[38] In the United Kingdom, the National Institute for Health and Care Excellence calculated the cost of belimumab at £61,200 per quality-adjusted life year (QALY).

BR3-Fc, a recombinant fusion protein built with the extracellular ligand-binding portion of BAFF-R, blocks activation of this receptor by BLyS and is in early-stage pharmaceutical development.