Fc receptors (FcR) on the surface of human breast cancer cells may facilitate the cytotoxic effects of hTRA-8, a new humanized apoptosis-inducing antibody.

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Introduction: A death receptor 5 (DR5) specific, mouse monoclonal antibody known as mTRA-8 was a promising therapeutic agent in mouse models of human breast cancer (Buchsbaum, et. al, Clin Can Res 9: 3731-41, 2003), and the humanized version (hTRA-8) is under study. Death receptor antibodies selectively induce apoptosis in tumor cells, a process that appears to require death receptor aggregation. Immune cells and endothelial cells within tumors express FcR and facilitate aggregation in vivo. Hypothesis: Breast cancer cells endogenously express FcR, which enhances the cytotoxicity of hTRA-8. Methods: The metastatic breast cancer cell line MDA-MB-231 (2LMP variant) was treated for 24 hrs with hTRA-8 in vitro with or without cross-linking antibody, to induce aggregation and subsequent apoptosis. ATPLite assays (n=3) measured cell viability after exposure. FcRs were measured by flow cytometry and Scatchard analyses using fluorescently labeled and Tc-99m-labeled hTRA-8, respectively, with appropriate controls. Results: Apoptosis induced by hTRA-8 was dose-dependent (ATP levels were diminished between 40-80% of controls) and an exogenous cross-linking reagent was not required at any concentration. The MDA-MB-231 cells expressed surface FcR2; control fluorescence averaged 253+55 while hTRA-8 averaged 527+17. Conclusion: Breast cancer cells may act in a paracrine or autocrine manner, facilitating the aggregation of DR5 receptors through the interaction between the DR5-hTRA-8 (receptor-ligand) complex and tumor-expressed FcR2.