Trifunctional antibody ertumaxomab: Non-immunological effects on Her2 receptor activity and downstream signaling

The trifunctional antibody ertumaxomab bivalently targets the human epidermal growth factor receptor 2 (Her2) on epithelial (tumor) cells and the T cell specific CD3 antigen, and its Fc region is selectively recognized by Fcγ type I/III receptor-positive immune cells. As a trifunctional immunoglobulin, ertumaxomab therefore not only targets Her2 on cancer cells, but also triggers immunological effector mechanisms mediated by T and accessory cells (e.g., macrophages, dendritic cells, natural killer cells). Whether molecular effects, however, might contribute to the cellular antitumor efficiency of ertumaxomab are largely unknown.

Potential molecular effects of ertumaxomab on Her2-overexpressing BT474 and SK-BR-3 breast cancer cells were evaluated. The dissociation constant Kd of ertumaxomab was calculated from titration curves that were recorded by flow cytometry. Treatment-induced changes in Her2 homodimerization were determined by flow cytometric fluorescence resonance energy transfer measurements on a cell-by-cell basis. Potential activation / deactivation of Her2, ERK1/2, AKT and STAT3 were analyzed by western blotting, Immunochemistry and immunofluorescent cell staining.

The Kd of ertumaxomab for Her2-binding was determined at 265 nM and the ertumaxomab binding epitope was found to not overlap with that of the therapeutic anti-Her2 monoclonal antibodies trastuzumab and pertuzumab. Ertumaxomab caused an increase in Her2 phosphorylation at higher antibody concentrations, but changed neither the rate of Her2-homodimerization /-phosphorylation nor the activation state of key downstream signaling proteins analyzed.

The unique mode of action of ertumaxomab, which relies more on activation of immune-mediated mechanisms against tumor cells compared with currently available therapeutic antibodies for breast cancer treatment, suggests that modular or sequential treatment with the trifunctional bivalent antibody might complement the therapeutic activity of other anti-Her2/anti-ErbB receptor reagents.