Abstract C168: Use of cellular antibody binding data to guide assessment of monoclonal antibodies as potential antibody-drug conjugates.

Although current therapies that target driver mutations often show dramatic short-term benefit, resistance to therapy is common as cancer cells respond by shifting to a reliance on other driver mutations not affected by treatment. Antibody-drug conjugates (ADCs) represent an alternative therapeutic modality that delivers a general toxin selectively to tumors by virtue of linkage to a monoclonal antibody that binds specifically to antigens expressed on the surface of cancer cells. ADCs are now being widely developed, with greater than 20 currently under evaluation in clinical trials. The mechanism of action of ADCs involves a specific sequence of events, all of which are requisite for efficient payload delivery: (1) the ADC must bind native antigen expressed on the cancer cell surface; (2) the resulting antigen-ADC complex must be internalized upon binding; (3) the internalized ADC must be trafficked to the lysosome; and (4) the payload must be successfully released to engage its biological target. We demonstrate here a method to evaluate naked antibodies as potential ADC candidates by determining the relative, cell-surface expression of target antigens across cancer types and subtypes, and to relate this to the growth inhibition achieved by targeting and internalization of a known toxin. We profiled trastuzumab (anti-HER2 mAb) in >200 cancer cell lines and collected quantitative antibody binding measurements by high-content imaging to establish comparative, cell-surface HER2 expression across the panel. We then selected a subset of cell lines displaying a range of HER2 expression levels, and tested the cytotoxic activity of trastuzumab when linked to an anti-human secondary ADC comprised of a single Fab fragment coupled to Saporin (FabZAP). The dose response results were then used to compute excess over bliss independence to identify synergistic activity between trastuzumab and FabZAP, which indicated that the trastuzumab-ADC complex was internalized and caused cell death. In general, a positive relationship between cell surface HER2 expression and synergism was observed within HER2-positive cell lines, although not all high HER2 expressing lines displayed strong synergy suggesting some tumors may be refractory to payload delivery or to the payload itself. No synergistic activity was observed in cell lines lacking HER2 expression. These data demonstrate that this approach is a useful method to profile naked antibodies when considering them for further development as ADCs. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C168. Citation Format: Katie Snead, Jonathan M. Crane, Karen Bernards, Brian Nelson, Alison R. Angione, Keith McKinley, Kate Waikins, Usha Warrior, O. Jameel Shah. Use of cellular antibody binding data to guide assessment of monoclonal antibodies as potential antibody-drug conjugates. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C168.