Abstract P1-03-08: Simultaneous analyses of HER2 gene and protein status can more precisely predict pathological complete response (pCR) to neoadjuvant trastuzumab with chemotherapy in primary HER2-positive breast cancer

Abstract

Abstract Background: HER2-positive breast cancer is characterized by overexpression of HER2 protein and/or amplification of the HER2 gene, assessed by immunohistochemistry (IHC) and in situ hybridization (ISH), respectively. According to the 2013 ASCO/CAP guidelines on HER2 testing in breast cancer, both protein and gene status can be classified into three categories: positive, equivocal and negative. Gene-protein assay (GPA) is a newly developed technique, in which IHC and dual color ISH (DISH) are simultaneously performed on a single slide. HER2 GPA slides allow bright-field analyses of both protein and gene expression status of each cell. Breast cancer cells can be classified into nine types based on the combination of protein and gene expression with GPA. Table 1. Three by three classification of cancer cells by the HER2 GPA analysis HER2 protein 3+ (positive)2+ (equivocal)0, 1+ (negative)HER2 gene copy number≥6 (positive)ABC 4, 5 (equivocal)DEF <4 (negative)GHI Purpose: We aimed to elucidate the relationship between the results of HER2 GPA analysis and the therapeutic effects of trastuzumab-based treatment. Methods: Fifty three (53) patients with HER2-positive breast cancer, who underwent neoadjuvant trastuzumab with chemotherapy, were analyzed for HER2 status and clinical outcome. First, HER2 protein and gene status in a pre-therapeutic biopsy material from each patient were separately assessed according to the ASCO/CAP guidelines using HER2 GPA slides. Second, all cancer cells in five representative microscopic images of GPA slides were assessed to determine protein expression and to count the gene copy number at individual cell levels. Finally, we investigated the relationship between the mixture composition of the nine cell types and pathological complete response (pCR) of trastuzumab-treated breast cancer. Results: The GPA results were concordant with the results of IHC in 98% of samples, and with DISH results in 100%. Two hundred eighty nine (289) cancer cells per patient were assessed on average, ranging from 137 to 490 cells. The mean proportion of type A cancer cells was 64%, ranging from 0% to 98%. Patients who had 50% or more of type A cancer cells achieved pCR in 72% of cases (28/39), while patients with less than 50% of type A cancer cells achieved pCR in 7% of cases (1/14) (P<0.0001). The mean proportion of type C cancer cells was 7%, ranging from 0% to 60%. Patients who had 10% or more of type C cancer cells achieved pCR in 10% of cases (1/10), while patients with less than 10% of type C cancer cells achieved pCR in 65% of cases (28/43) (P=0.0016). Conclusion: HER2 GPA analyses can more precisely predict therapeutic effect of trastuzumab-based treatment in primary HER2-positive breast cancer. Our study suggests that the HER2 GPA approach would further improve precision therapy in HER2-positive breast cancer. Citation Format: Horii R, Nitta H, Ito Y, Iwase T, Ohno S, Akiyama F. Simultaneous analyses of HER2 gene and protein status can more precisely predict pathological complete response (pCR) to neoadjuvant trastuzumab with chemotherapy in primary HER2-positive breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-03-08.