Abstract P3-06-26: Measurement of domain-specific HER2 (ERBB2) expression classifies benefit from Trastuzumab in breast cancer
Authors: Carvajal-Hausdorf Daniel E , Schalper Kurt A , Pusztai Lajos , Psyrri Amanda , Kalogeras Konstantine T , Kotoula Vassiliki , Fountzilas George , Rimm David L 
Published: May 2015

Abstract
Background: The ASCO/CAP guidelines consider chromogen-based immunohistochemistry (IHC) as the primary assay to determine HER2 status in breast cancer. Studies have shown that antibodies targeting different protein domains (intracellular [ICD] or extracellular domain [ECD]) are equivalent using traditional methods. Here we define a new method for standardization of domain specific measurements, then determined their effect on trastuzumab benefit in the adjuvant setting.
Methods: We measured HER2 protein using quantitative immunofluorescence (QIF) in a standardization tissue microarray (TMA) with CLIA-lab defined HER2 status with 2 antibodies targeting the ICD (CB11 and A0485) and 2 against the ECD (SP3 and D8F12). Cut-points were generated using Joinpoint software. HER2 IHC and FISH results were used as reference to determine sensitivity and specificity. Finally, domain-specific HER2 levels were measured in 180 samples from a clinical trial of adjuvant chemotherapy followed by trastuzumab (HeCOG 10/05).
Results: HER2 ICD showed slightly higher sensitivity to predict HER2 gene amplification than the ECD, while the ECD was more specific and had higher positive predictive value. Fifteen percent of trastuzumab-treated patients from HeCOG 10/05 showed discordant results using ICD and ECD antibodies. ECD-high status was significantly associated with longer disease-free survival (DFS) (log-rank P=0.049, HR=0.31, 95% CI: 0.144-0.997), while ICD status was not. In patients with low ECD, there was no difference in DFS between ICD-low and ICD-high. However, when ICD was positive, high ECD was significantly associated with longer DFS (log-rank P=0.027, HR=0.23, 95% CI: 0.037-0.82) compared to low ECD. Since this trial was not randomized for trastuzumab, interaction could not be tested, but neither ICD nor ECD showed prognostic value in the 462 patients that were traditionally classified as HER2-negative and did not receive trastuzumab.
Conclusion: Determination of HER2 status in breast cancer tumor tissue samples using a standardized system and antibodies against the ECD or ICD suggests a biological difference in the tumors. High ECD was associated with benefit from trastuzumab, while elevated ICD alone was not. This observation could be useful in developing a new HER2 assay that can subclassify traditionally HER2-positive patients into groups for subsequent antibody (ECD) vs tyrosine kinase inhibitor (ICD) therapies.
Citation Format: Daniel E Carvajal-Hausdorf, Kurt A Schalper, Lajos Pusztai, Amanda Psyrri, Konstantine T Kalogeras, Vassiliki Kotoula, George Fountzilas, David L Rimm. Measurement of domain-specific HER2 (ERBB2) expression classifies benefit from Trastuzumab in breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-06-26.