Chemotherapy-Refractory Diffuse Large B-Cell Lymphomas (DLBCL) Are Effectively Killed by Ofatumumab-Induced Complement-Mediated Cytoxicity.

The combination of chimeric CD20 monoclonal antibody (mAb) rituximab (R) with chemotherapy (CHOP) has significantly improved clinical outcome in patients with diffuse large B-cell lymphoma (DLBCL). However, the efficacy of R-CHOP is variable and DLBCL remains fatal in 30–40% of the patients. We recently developed the novel fully human CD20 mAb, ofatumumab (2F2) that is currently in clinical development for CLL and follicular lymphoma (phase III) and rheumatoid arthritis (phase II). In this study, we compared ofatumumab with rituximab for their ability to induce complement dependent cytotoxicity (CDC) of isolated lymphoma cells of chemotherapy-refractory DLBCL patients. Human type II CD20 mAb 11B8 does not induce CDC and was used as a control. Ofatumumab and rituximab induced CDC of all DLBCL samples tested, including the DLBCL cell lines SUDHL4, SUDHL5 and HT and lymphoma cells derived from ten chemotherapy-refractory DLBCL patients. Ofatumumab was significantly more effective in inducing CDC in nine out of ten DLBCL tumor samples when compared to rituximab (p=0.001, Two-way-Anova). The lethal doses (LD₅₀) for ofatumumab (0.1 ± 2.8 μg/ml) was significantly lower when compared to the LD₅₀ for rituximab (6.4 ± 4.9 μg/ml, p=0.04, Mann-Whitney test). The control CD20 mAb 11B8 showed no CDC induction of DLBCL cells. Sensitivity of DLBCL patient cells to ofatumumab- and rituximab-induced CDC negatively correlated with expression of complement defense molecule CD59, but not with expression of CD46, CD55, as determined by immunohistochemistry or apoptosis inhibitors Bcl-2 and XIAP. Functional inhibition of CD55 and CD59 using blocking mAb demonstrated that ofatumumab-induced CDC of DLBCL tumor cells was less sensitive to expression of these complement defense molecules than rituximab-induced CDC. We conclude that chemotherapy-refractory DLBCL cases are sensitive to CD20 mAb induced CDC with ofatumumab being the most effective mAb, especially in patients expressing high levels of CD59. Based on our results ofatumumab should be considered as a valuable therapy option for chemotherapy-refractory DLBCL patients.