Selection of B-cell chronic lymphocytic leukemia cell variants by therapy with anti-CD20 monoclonal antibody rituximab

Objective Anti-CD20 chimeric monoclonal antibody rituximab (Mabthera; IDEC-C2B8) is currently tested in several clinical trials for the treatment of B-cell chronic lymphocytic leukemia (B-CLL). In the present study, we investigated whether rituximab therapy may select for CD20− subclones. Materials and Methods Leukemic B-CLL cells were isolated from patients with B-CLL and sensitivity to rituximab-induced cell death was examined. Levels of CD20 protein and mRNA were determined using flow cytometry and real-time PCR, respectively. Clonality analyses of leukemic cells throughout rituximab therapy were performed by GeneScan analysis of patient clone specific rearrangements of the complementarity determining region III of the heavy chain immunoglobulin. Results Cytotoxicity of rituximab in vitro did not depend on the protein levels of CD20. During therapy with rituximab CD20+ B-CLL cells were depleted and CD20− leukemic cells emerged. After treatment, the initial CD20+ B-CLL cell clone reexpanded. CD20− B-CLL cells retained their capacity to synthesize the CD20 molecule. Conclusions These data support the concept that in B-CLL rituximab treatment may not lead to the emergence of CD20− leukemic variants. Our findings support clinical studies investigating the benefit of prolonged period of rituximab therapy in B-CLL disease.