Abstract P4-01-11: Circulating tumor cells (CTCs) detect HER2+ status and phenotypic heterogeneity in metastatic breast cancer (MBC)

Abstract

Abstract Background Circulating tumor cells (CTCs) are epithelial cells that can be found circulating in the blood of MBC patients and may represent a heterogeneous population including epithelial cells and cancer stem cells (CSC) shed from the tumor. Their detection and enumeration has prognostic significance and can be used for longitudinal monitoring of response to treatment. Recent advances have allowed for the detection of HER2 protein expressing CTCs using the CellSearch™ platform. HER2 expression has been associated with CSCs phenotype in absence of gene amplification (Korkaya et al, Oncogene 2008) and particularly in Luminal B disease (Ithimakin et al, Cancer Res, 2014). We hypothesized that HER2+/CTCs are detectable in patients with MBC irrespective of their HER2 status and this information can be potentially be used for treatment selection. Targeting HER2+ CSCs may result in clinical benefit and improved outcome. Methods This is a prospective analysis of 40 patients with locally advanced or MBC whose blood was analyzed for the baseline detection of CTCs as part of their disease initial evaluation. Blood was drawn for CTC detection on eligible patients at initiation of a new line of therapy; CTCs monitoring was repeated at progression or change in therapy. The 7.5mL of whole blood was collected in a CellSave™ Preservative Tube, and CTC isolation, enumeration and characterization were performed using the FDA-approved CellSearch™ System (Janssen Diagnostics, USA). The CellSearch™ tumor phenotyping reagent HER-2/neu (Fluorescein-conjugated) was used to determine CTC with HER-2/neu expression using the CellTrack™ Analyzer II. Results Most patients in this study had metastatic disease (90%). According to disease subtype, 43% of patients were ER+/HER2- (Luminal A), 17% ER+/HER2+ (Luminal B), 20% ER-/HER2+ (HER2) and 20% ER-/HER2- (TNBC). Moreover, 55% had a clinical diagnosis of Inflammatory Breast Cancer (IBC). Twenty-two patients had CTCs detected (55%), the average number of CTCs was 8.6 (0-135) with a median follow up of two months. Of the patients who had CTCs detected and had HER2+ disease (IHC/FISH), 83% (5/6) had concordance in HER2+ CTCs. Interestingly, in the subset of patients who had HER2 negative disease (ICH/FISH) and had detectable CTCs, there was discordance in HER2 status: 44% (7/16) had HER2+ CTCs and all but one had Luminal A disease. Two patients have been started on HER2 targeted therapy based on finding HER2+ CTCs. One of these patients had 22 CTCs, 8 of which were HER2+, was initiated on HER-2 combined regimen and at a repeat evaluation in 3 months demonstrated 0 CTCs and clinical response. Conclusions CTCs offer a new and innovative approach to detect HER2+ cells in MBC. Tissue analysis with IHC and FISH has been the gold standard but these methods are unable to account for disease phenotyopic hetereogeity and identify patients who have CSCs (HER2+) and therefore would benefit from HER2 targeted therapy. This warrants further investigation in a prospective trial in Luminal disease to formally compare these methods and correlate the results with clinical outcomes. Citation Format: Laura Austin, Zhaomei Mu, Tiffany Avery, Rebecca Jaslow, Carmela Paolillo, Angela Toss, Paolo Fortuna, Ye Zhong, Hushan Yang, Massimo Cristofanilli. Circulating tumor cells (CTCs) detect HER2+ status and phenotypic heterogeneity in metastatic breast cancer (MBC) [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 P4-01-11.