Ofatumumab Is More Efficient than Rituximab in Lysing B Chronic Lymphocytic Leukemia Cells in Whole Blood and in Combination with Chemotherapy

Abstract

Ofatumumab (OFA) is a human anti-CD20 Ab approved for treatment of fludarabine-refractory B chronic lymphocytic leukemia (B-CLL). The efficacy of different immunotherapeutic strategies is best investigated in conditions that are as physiologic as possible. We have therefore compared the activity OFA and rituximab (RTX), alone or in combination with chemotherapeutic agents in unmanipulated whole blood assays, using flow cytometry. OFA (10–100 μg/ml) lysed B-CLL targets in whole blood more efficiently and with faster kinetics than RTX, with a mean 56% lysis at 24 h compared with 16%. This activity of OFA was fully complement dependent, as shown by >99% inhibition by anti-C5 Ab eculizumab and a lack of NK cell activation in whole blood. OFA-mediated NK cell activation was blocked by complement. OFA-mediated lysis could be increased an additional 15% by blocking CD55 and CD59 complement inhibitors. Interestingly, OFA-mediated lysis correlated significantly with CD20 expression levels (r2 = 0.79). OFA showed overlapping dose response curves similar to those for RTX in phagocytosis assays using either human macrophages or neutrophils. However, phagocytosis was inhibited in the presence of serum or whole blood. Finally, combined treatment with mafosfamide and fludarabine showed that these therapeutic drugs are synergistic in B-CLL whole blood assays and show superior activity when combined with OFA compared with RTX. These results confirm in B-CLL samples and in physiologic conditions the superior complement mediated cytotoxicity induced by OFA alone compared with RTX, the lack of NK cell activation, and phagocytosis in these conditions and suggest effective chemoimmunotherapy strategies using this new generation anti-CD20 Ab.

Regulation of complement and modulation of its activity in monoclonal antibody therapy of cancer

“…The mAb ofatumumab is capable of lysing a range of rituximab-resistant targets, in particular CD20 low expressing cells. 49,50 In B-CLL cells, expressing both CD52 and low levels of CD20, alemtuzumab-mediated CDC can further be enhanced by the addition of ofatumumab. 51 The clinical response rate to rituximab has been shown to rely on the CD20 expression level, which is consistent with data on in vitro CDC induction.…”

Lessons for the clinic from rituximab pharmacokinetics and pharmacodynamics

“…1A). [6][7][8][9][10][11][12][13] These mechanisms depend on the Fc portion of the antibody binding to FcγRs on immune cells. In addition, RTX and other anti-CD20 antibodies can activate signaling pathways after binding of the Fab portion to CD20 on B cells and induce homotypic adhesion (aggregation of target cells) and/or cell death to a variable extent (Fig.…”

Safety and activity of BTK inhibitor ibrutinib combined with ofatumumab in chronic lymphocytic leukemia: a phase 1b/2 study

“…23 It exhibits more potent complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) compared with rituximab in B-cell lines including CLL cells. [24][25][26] Ofatumumab is approved in the United States for treatment of CLL refractory to fludarabine and alemtuzumab 27 and in combination with chlorambucil for previously untreated CLL where fludarabine-based treatment is inappropriate. 28,29 Studies with single-agent ibrutinib showed early lymphocytosis in patients with CLL, [30][31][32] which is considered a pharmacodynamic effect of ibrutinib resulting in mobilization of lymphocytes into the peripheral blood from tissue compartments.…”