Quantitative measurement of HER3 total protein (H3T) and association with clinical outcome in HER2-positive metastatic breast cancer patients treated with trastuzumab

1021 Background: The heterodimerization of HER3, with HER2 and the subsequent activation of the AKT pathway, has been implicated in both regulation of tumor cell growth and in resistance to HER2 targeted therapies such as trastuzumab. Currently available methods of measuring H3T in formalin-fixed, paraffin-embedded (FFPE) samples are insufficiently sensitive or specific; thus the impact of quantitative measurement of H3T expression on clinical response to trastuzumab has not been evaluated. Methods: We have developed a highly quantitative, accurate, precise, sensitive, and reproducible H3T assay in FFPE samples based on the VeraTag technology platform. Consequently, the H3T assay displays ∼5-fold greater sensitivity than conventional IHC measurements. Additionally, the rank order and accuracy of the H3T measurements were preserved over a wide dynamic range in well-characterized cell line panels. Intra- and inter-assay reproducibility measurements demonstrated 7–11 %CV and 8–16 % CV, respectively. Results:...