Murine monoclonal antibody 2B8 specifically recognizes the CD20 phosphoprotein expressed on the surface of normal B lymphocytes and B- cell lymphomas. The light- and heavy-chain variable regions of 2B8 were cloned, after amplification by the polymerase chain reaction, into a cDNA expression vector that contained human IgG1 heavy chain and human kappa-light chain constant regions. High-level expression of chimeric- 2B8 antibody (C2B8) was obtained in Chinese hamster ovary cells. Purified C2B8 exhibited antigen binding affinity and human-tissue reactivity similar to the native murine antibody. In vitro studies showed the ability of C2B8 to bind human C1q, mediate complement- dependent cell lysis of human B-lymphoid cell lines, and lyse human target cells through antibody-dependent cellular cytotoxicity. Infusion of macaque cynomolgus monkeys with doses ranging from 1.6 mg/kg to 6.4 mg/kg resulted in greater than 98% depletion of peripheral blood (PB) B cells and 40% to 70% depletion of lymph node B cells. Recovery of PB B cells usually started at 2 weeks after treatment and required 60 to greater than 90 days to reach normal levels. As much as 95% depletion of B cells in peripheral lymph nodes and bone marrow was observed following weekly injections of 16.8 mg/kg antibody. No toxicity was observed in any of the animals. These results offer the possibility of using an “immunologically active” chimeric anti-CD20 antibody as an alternative approach in the treatment of B-cell lymphoma.

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Purpose: Rituximab [chimeric anti-CD20 monoclonal antibody], alone or combined with chemotherapy, is used in the treatment of non-Hodgkin's lymphoma (NHL). Rituximab binds to CD20 and inhibits intracellular survival/growth pathways leading to chemo/ immunosensitization of tumor cells in vitro. The contribution of rituximab Fc-FcR interaction in signaling is not known. This study examined the role of Fc-FcR interactions in rituximab-induced signaling using rituximab (Fab') 2 fragments as well as rituximab devoid of the CH2 Fc-binding domain (CH2 -). Experimental Design: Rituximab (CH2 -) and rituximab (Fab') 2 were tested for their activity on B-NHL cell lines. Cell signaling and sensitization to chemotherapy and immunotherapy were examined. The in vitro studies were validated in mice bearing tumor xenografts. Results: Although the modified antibodies were defective in antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity functions, they retained all other biological activities such as inhibition of cell proliferation, induction of cell aggregation, and apoptosis induction. In addition, similar to rituximab, the modified antibodies inhibited the activity of cell survival/growth pathways and their associated transcription factors (e.g., NF-κB, YY1, SP-1), and signal transducers and activators of transcription 3 (STAT-3), and downregulated the expression of antiapoptotic gene products, such as Bcl-2/Bcl XL , which regulate drug resistance. The modified antibodies, similar to rituximab, sensitized resistant B-NHL cells to both CDDP and Fas ligandinduced apoptosis. Furthermore, treatment of nude mice bearing Raji tumor cell xenografts with the combination of rituximab (Fab') 2 or rituximab and CDDP resulted in similar and significant inhibition of tumor growth. Conclusion: These findings reveal that rituximab-mediated inhibition of intracellular signaling pathways and leading to chemo/immuno-sensitization of resistant B-NHL is Fc independent. (Clin Cancer Res 2009;15(21):6582-94) Rituximab (chimeric mouse anti-human CD20 monoclonal antibody) is a genetically engineered monoclonal antibody used for the treatment of patients with relapsed or refractory low-grade or follicular B-cell non-Hodgkin's lymphoma (NHL) and in patients with relapsed stage III/IV follicular lymphoma (1, 2). Rituximab is composed of murine Variable (V) and human IgG1 Constant (C) regions. Rituximab functions by binding to the CD20 antigen expressed on the surface of normal and malignant B cells. The overall response in patients is 50% when it is used as a single agent (3), and the response rate

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