Characterization of novel murine anti-CD20 monoclonal antibodies and their comparison to 2B8 and c2B8 (rituximab)

Rituximab is the first anti-cancer antibody approved by the FDA for the treatment of B-cell non-Hodgkin lymphoma (B-NHL), alone or in combination with chemotherapeutic drugs. Further, rituximab is now being examined in a variety of CD20+ neoplastic diseases as well as B-cell-induced autoimmune diseases. The clinical response to rituximab is significant, resulting not only in tumor regression but also prolongation of survival. However, a subset of patients does not initially respond to rituximab or develops resistance to its further treatment. Therefore, alternative therapies for these patients are strongly desired. Rituximab activity has been thought to be by antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity and apoptosis, and studies in model systems established the role of rituximab in cell signaling-induced perturbation of anti-apoptotic survival pathways, suggesting that the patients unresponsive to rituximab may be overcome with other CD20 antibodies with different activities. This study investigated eight novel murine antibodies directed against CD20 for their physical and biological properties in comparison with 2B8 and c2B8 (rituximab). These antibodies were derived by various antigenic and immunization procedures and selected for CD20 activity. Analysis of these antibodies revealed that they all bound to various B-cell lines and CD20-transfected CHO cells. Six of the eight antibodies shared similar variable-region amino acid sequences that were also shared by 2B8 while two monoclonal antibodies did not. Of them, 1K1791 has a distinct heavy chain and both 1K1791 and 1K1782 have distinct light chains. Not all of the antibodies inhibited cell growth and only two antibodies reacted with fixed GST-CD20 recombinant fusion protein. Noteworthy, 1K1791 was found to inhibit cell proliferation and also induced caspase-independent apoptosis in the absence of cross-linker. These findings identified new antibodies with properties and epitope specificities different from 2B8. The potential clinical application of such antibodies in the treatment of B-NHL and rituximab-resistant B-NHL is discussed.