Abstract PS2-11: Ctc-her2+ a novel subset in stage IVaggressive: Molecular correlations, outcome and clinical characteristics in metastatic breast cancer
Authors: D'Amico Paolo , Zhang Qiang , Gerratana Lorenzo , Zhang Youbin , Shah Ami , Reduzzi Carolina , Davis Andrew , Jacob Saya , Wehbe Firas , Behdad Amir , Curigliano Giuseppe , Cristofanilli Massimo 
Published: February 2021

Abstract
Background The enumeration of Circulating Tumor Cells (CTCs) in the peripheral blood of patients (pts) with metastatic breast cancer (MBC) showed prognostic value independent of clinic-pathological characteristics and proposed as a new tool for Stage IV stratification. Molecular characterization of CTCs with analysis of standard HER2 and estrogen-receptor (ER) status provides additional predictive value. The presence of HER2 positive CTCs (CTCs-HER2+) has been documented in pts with HER2 negative MBC and suggestive of tumor heterogeneity. The molecular features associated with the detection of these cells and the clinical significance of CTCs-HER2+ remains of unclear clinical significance. The lack of validated prospective trials limits the use of this immunophenotypic characterization as a prognostic and predictive factor. We conducted an analysis of a large cohort of patients with MBC evaluated for CTCs enumeration (CTCs+), molecular analysis of CTCs (CTCs-HER2+) and ctDNA. Methods The IRB-approved study prospectively analyzed blood samples of pts with MBC enrolled before starting a new line of therapy. Samples were collected from pts treated at Northwestern University (Chicago, IL) between 2016 and 2020. ctDNA was analyzed using the Guardant360 NGS assay (Guardant Health) with a specific focus on the detection of ERBB2 mutations. CTCs were enumerated through CellSearch™ (Menarini Silicon Biosystems, Bologna, Italy), and characterized for HER2 expression using the CellSearch CXC Kit. HER2+ CTCs were divided in 4 different categories (0,1+,2+,3+), leaning on fluorescence intensity, as previously reported by Riethdorf et al. Pts with &lt;5 CTCs (stage IV indolent ) were excluded from the analysis. The cHER2 ratio, defined as the sum of 2+ CTCs and 3+ CTCs divided by the total number of HER2+ CTCs was used to split the remaining pts in 2 different subgroups: cHER2 ratio&gt; 0.75 (cHER2high), cHER2 ratio&lt;0.75 (cHER2low). Pts' characteristics and genomic alterations were compared between these two subgroups and a long-rank test was performed to analyze the outcome (progression-free survival, PFS). Results A total of 247 pts with MBC about to start new therapy were enrolled. 136 patients (55%) demonstrated CTCs count ≥ 5 (stage IV aggressive). Of these, 111 pts had a blood sample bearing HER2+ CTCs. 88 pts were classified as cHER2low, while 32 were allocated in the second subgroup (cHER2high). The clinic subtype distribution of the two groups was as follows: 65% ER+, 19% HER2+ and 16% TNBC among cHER2low, compared to 55% ER+, 38% HER2+ and 7% TNBC among cHER2high pts. The HER2+ subtype was significantly more represented in the cHER2high group (p&lt;0.05), along with ERBB2 mutations (p&lt;0.05). We observed a total of 89 events (progression to treatment), 67 and 22 respectively. The median PFS was 87 days in the cHER2low and 209 days among cHER2high pts (log Rank test, p&lt;0.001). Conclusion This study shows a significant association between the cHER2 ratio and outcome in pts with Stage IV aggressive disease, suggesting a potential predictive/prognostic role of this biomarker among MBC pts bearing CTC-HER2+. cHER2high pts, in which blood was collected a prevalent number of CTC-HER2+ 2+ and 3+, were more likely to have a HER2 FISH amplification by tissue biopsy and ERBB2 gene alteration by ctDNA. These findings, taken together, suggest a potential role of these subsets of CTC-HER2+ in the characterization of the HER2+ metastatic disease. The cHER2low is intrinsically characterized by a higher percentage of CTC-HER2+ 1+. This CTC-HER2+ subset of cells is commonly detected among all subtypes but, more frequently in ER+ disease and their significance remains unclear. Exploring the underlying biological nature of the CTC-HER2+ 1+ could allow to better understand the role these cells in the natural evolution of the MBC.
Citation Format: Paolo D'Amico, Qiang Zhang, Lorenzo Gerratana, Youbin Zhang, Ami Shah, Carolina Reduzzi, Andrew Davis, Saya Jacob, Firas Wehbe, Amir Behdad, Giuseppe Curigliano, Massimo Cristofanilli. Ctc-her2+ a novel subset in stage IVaggressive: Molecular correlations, outcome and clinical characteristics in metastatic breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS2-11.