Multiple signaling pathways induced by hexavalent, monospecific, anti-CD20 and hexavalent, bispecific, anti-CD20/CD22 humanized antibodies correlate with enhanced toxicity to B-cell lymphomas and leukemias

We have generated hexavalent antibodies (HexAbs) comprising 6 Fabs tethered to one Fc of human IgG1. Three such constructs, 20-20, a monospecific HexAb comprising 6 Fabs of veltuzumab (humanized anti-CD20 immunoglobulin G1 kappa [IgG1 kappa]), 20-22, a bispecific HexAb comprising veltuzumab and 4 Fabs of epratuzumab (humanized anti-CD22 IgG1 kappa), and 22-20, a bispecific HexAb comprising epratuzumab and 4 Fabs of veltuzumab, were previously shown to inhibit proliferation of several lymphoma cell lines at nanomolar concentrations in the absence of a crosslinking antibody. We now report an in-depth analysis of the apoptotic and survival signals induced by the 3 HexAbs in Burkitt lymphomas and provide in vitro cytotoxicity data for additional lymphoma cell lines and also chronic lymphocytic leukemia patient specimens. Among the key findings are the significant increase in the levels of phosphorylated p38 and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) by all 3 HexAbs and the notable differences in the signaling events triggered by the HexAbs from those incurred by crosslinking veltuzumab or rituximab with a secondary antibody. Thus, the greatly enhanced direct toxicity of these HexAbs correlates with their ability to alter the basal expression of various intracellular proteins involved in regulating cell growth, survival, and apoptosis, with the net outcome leading to cell death. (Blood. 2010;116(17):3258-3267)