Prospective Validation of a Pharmacological Biomarker for Low-Dose Rituximab in Rheumatoid Arthritis

Rituximab is an anti-CD20 monoclonal antibody which is effective in RA. The dose indicated in the marketing authorisation is 1000 mg twice per cycle. However, rituximab is associated with adverse events, and more specifically the risk of infection, which seems to be dose-dependent. The dose of 1000 mg once a cycle has been evaluated and is non-inferior for maintenance treatment. Lower doses than 1000 mg once could also be sufficient, but there is a lack of predictive criteria to guide clinicians in their search for the minimal effective dose for a given patient. In our center, a PK-PD model including several parameters (body surface area, serum IgG concentration, residual rituximab concentrations and CD4+ T-cell count) has enabled to quantify part of the concentration-effect relationship of rituximab in this indication and at standard dose (1000 mg twice at 15-day intervals). On the basis of this model, a model for predicting clinical response has been developed.with reliable prediction of clinical response. The aim of this study is to apply this model to patients receiving low-dose rituximab (1000 mg once per cycle) and to evaluate the prediction of clinical response to low-dose rituximab using this model. Correlation and coefficient of determination between DAS28-CRP observed 6 months after a second low-dose cycle and DAS-28 predicted by a PK-PD model taking into account gender, body surface area, IgG concentration, rituximab concentration and CD4 T-cell count.