Abstract 5426: Assessing factors predictive of response to ADCs for companion diagnostic strategies

One premise of antibody-drug conjugates (ADC) is that the bound mAb-antigen complex on the cell surface will internalize and be metabolized by lysosomal proteases to release the free drug. Thus, the efficacy of an ADC is dependent not only on the presence of cell surface antigens, but also in-tact delivery of the conjugated drug, and an active pathway of receptor-mediated endocytosis. On a cellular level, the biological mechanisms behind receptor internalization and turnover have not been elucidated for novel therapeutic targets. On a tissue level, vascularization and hypoxic profile will affect delivery of the antibody into the tissue. Varying expression of the antibody target within the tissue (antigen density) will also affect the uptake of the ADC. Furthermore, the extracellular stability of the ADC may be affected by the activity of the various proteases in the tumor microenvironment (TME), outside of the lysozome. These concepts are critical for efficacy, and thus are especially important for companion diagnostic approaches (CDx) meant to predict response to ADCs. Thus, a predictive assay which would account not only for the degree of cell surface expression of the target, but also receptor internalization and potential effects of tumor microenvironment is required. To answer this need, Flagship Biosciences has invented several proprietary approaches for measuring critical properties of the therapeutic target on the cell surface or inside the cell, as well as properties of the TME which could be used to understand and predict efficacy to an ADC using FFPE biopsies. These quantitative pathology approaches are based on image analysis approaches which been designed specifically for ADC CDx programs to answer these critical questions: 1) Defining cell surface target expression independent of cytoplasmic expression; 2) Estimating receptor flux (turnover, internalization) based on staining profile; 3) Assessing critical factors in the TME which may affect delivery of the in-tact drug to the intracellular target; 4) Assaying vascular properties which may affect delivery of the drug; and 5) Heterogeneity of the target within a tumor. We are able to provide these as discrete evaluations or multiplex these evaluations for an integrative answer which can be derived from a typical clinical biopsy. Incorporation of these novel tools will enable ADC developers to create efficacy and patient stratification paradigms which incorporate the critical biological endpoin