Abstract P4-01-02: Prospective characterization of HER2-positive circulating tumor cells in patients with HER2-negative metastatic breast cancer

Background: We have previously shown feasibility by CLIA validation of HER2 FISH on circulating tumor cells (CTCs). Several small retrospective studies have identified HER2 amplification in CTCs in a subset of patients (pts) with clinically HER2 negative metastatic breast cancer (MBC). While these findings potentially have profound implications for CTCs as a predictive biomarker, prospective validation and functional characterization of this subgroup is necessary. Methods: We enrolled a prospective cohort of pts with MBC that was HER2 negative by IHC and/or FISH on all available primary and metastatic biopsies. Pts had ≥1 line of prior chemotherapy for MBC. Blood samples were collected at study entry. CTCs were enumerated based on standard criteria (Cytokeratin (CK)+/CD45- staining) as well as by FISH for HER2 using the OncoCEE-BR™ CTC test (Biocept, Incorporated). This test employs a proprietary antibody cocktail for capture followed by CK/CD45 staining, and FISH analysis directly within a microfluidic device. Samples were reported as positive if the HER2/CEP17 ratio was ≥ 2.0. HER2+ CTCs were classified into two categories: CK+/CD45-/HER2+ and CK-/CD45-/HER2- Results: CTCs were observed in 208 out of 323 pts (64%). Median number of CTCs was 10 (range 1 to > 34195). 75 pts (23%) had HER2+ CTCs, with a median number of 3 HER2+ CTCs (range 1 to 21). 36% (27/75) of these pts had CK+/HER2+ CTCs and 43% (32/75) of pts had only CK-/HER2+ CTCs. The remaining pts (21%) had both CK+/HER2+ and CK-/HER2+ CTCs present. Conclusion: HER2 amplified CTCs are present in a subset (23%) of pts with clinically HER2-negative breast cancers. Interestingly, we observed a high prevalence of pts with only CK-/HER2 amplified CTCs (32 out of 323; 10%). The unique multi-antibody CTC capture method used here thus allows for detection of a prevalent population of CK-/HER2+ CTCs that may be largely undetected by other current adopted technologies. The functional significance of CK-/HER2+ and CK+/HER2+ CTCs in patients with clinically HER2 negative breast cancer is currently being evaluated in a prospective study with HER2-directed therapy. Citation Format: Ian E Krop, Erin Macrae, Sarah R Galler, Farideh Bischoff, Romeo Fauni, Edgar Sales, Lan Huynh, Christine Mitchell, Trisky Clarin-Tamayo, Mark Anderson, Leslie Abad, Eric Winer. Prospective characterization of HER2-positive circulating tumor cells in patients with HER2-negative metastatic 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 P4-01-02.