B-Cell Targeted Therapies in Systemic Lupus Erythematosus
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Systemic lupus erythematosus is a multisystem autoimmune disease characterized by the formation of autoantibodies that target a variety of self antigens. B cells are fundamental to the development of these antibodies and are a target for intervention in the disease. This review discusses four therapies that target B cells by inducing B-cell depletion, reduction in B-cell proliferation and differentiation, or modulation of B-cell function. Rituximab is an anti-CD20 chimeric monoclonal antibody that depletes B cells but not plasma cells. Systematic reviews of open label studies, particularly in lupus patients refractory to conventional therapy, have suggested that rituximab can be an effective treatment for non-renal lupus and lupus nephritis. However, randomized, double-blind, controlled trials comparing rituximab with placebo in addition to standard of care therapy for non-renal lupus and lupus nephritis over 12 months failed to demonstrate efficacy using the planned primary endpoints, although there were some post-hoc analyses suggesting that rituximab may have beneficial effects that would be worthy of further study as no significant toxicity has been demonstrated. Treatment with belimumab, a humanized monoclonal antibody targeted against B lymphocyte stimulator (BLys), was more efficacious than placebo and had no significant increase in adverse events in two non-renal, phase III lupus trials when given in addition to standard of care therapy for 52 weeks. Belimumab is licensed for the management of lupus in the US and in Europe. Atacicept is a humanized fusion protein that binds BLys and APRIL (a proliferation-inducing ligand) that might be more effective than belimumab in the management of lupus. Unfortunately a phase II/III trial of atacicept in lupus nephritis had to be stopped due to the development of low immunoglobulin levels and pneumonias in some patients. However, in retrospect these complications may have been due to concomitant treatment with mycophenolate mofetil and results of a 52-week, non-renal, phase III trial with atacicept are awaited. Epratuzumab is a humanized monoclonal antibody that targets CD22 on B cells and results in modulation of B-cell function and migration, as CD22 regulates adhesion and inhibits B-cell receptor (BCR) signalling. Epratuzumab at a cumulative dose of 2,400 mg over 4 weeks has been shown to improve lupus disease activity compared with placebo 12 weeks after initiation of therapy in a phase II study, and a 12-month phase III study is on-going. B-cell targeted therapies are an attractive prospect for treating lupus disease and the results of current phase III trials are eagerly awaited. Finding the most appropriate trial design to demonstrate efficacy in lupus trials has been a challenge. The SRI (SLE response index) used in the belimumab studies and the BICLA (British Isles Lupus Assessment Group-based Composite Lupus Assessment) used in the epratuzumab studies are currently the promising trial designs for non-renal studies. For lupus nephritis it is important that trials are of adequate duration to be able to demonstrate benefit of new therapies over conventional therapy.