Abstract P1-01-16: Circulating tumor cells (CTCs) biomarker evaluation from patients with metastatic breast cancer (MBC) utilizing the TargetSelectorTM platform
Authors: Hamilton EP , Yardley DA , Burris III HA , Shastry M , Huynh L , Bhikha L , Singh VM 
Published: May 2019

Background: Circulating blood biomarkers represent the promise of non-invasive, real-time surrogates for tumor tissue-based biomarkers as well as afford monitoring opportunities over the course of therapy as tumors evolve and acquire resistance to treatment. Circulating tumor cells (CTCs) are cells that disseminate from tumors and can be identified in peripheral blood. CellSearch® is an FDA approved methodology for detecting and enriching this rare CTC population. The prognostic value of CTCs has been established, however the potential to characterize biomarkers on CTCs to inform treatment decisions remains an active area of investigation. Among the emerging platforms for detection and characterization of CTCs is the TargetSelectorTM system. While CellSearch® is limited to capture and detection of epithelial derived CTCs based on EpCAM and cytokeratin (CK) respectively, Biocept's TargetSelectorTM platform utilizes a novel microfluidic system for CTC enrichment based on an antibody capture cocktail that allows for enumeration of CTCs with variable phenotypes (including CK- CTCs). The CTCs are captured in transparent microfluidic chambers and cells can be viewed in situ by fluorescence microscopy and analyzed via immunocytochemistry (ICC), fluorescence in situ hybridization (FISH) and PCR analyses.

Methods: Sixty-one patients with metastatic breast cancer consented and provided blood for utilization in the TargetSelectorTM platform. Based on the molecular analysis of tissue biopsies, 92% of these patients had ER+ breast tumors. Biomarker expression on captured CTCs was determined by ICC for ER and by FISH for HER2. Concordance between these results and biomarker expression on archival tumor tissue from these patients was calculated.

Results: CTCs were detected in 60 of 61 patient blood samples (range 2–4471); 68% had both CK+ and CK- CTCs, and 32% had only CK- CTCs. None had only CK+ CTCs. Of those with CK+ CTCs, concordance for ER expression between the tissue and blood analyses was 85% (35/41). Concordance was much lower for patients with only CK- CTCs (32%, 6/19). Concordance for HER2 amplification in CK+ patients was 93% (38/41), and 68% (13/19) in CK- patients. For this study, the liquid biopsy was obtained over an extended period of time from when solid tumor ER and HER2 assessments were obtained; this latency period may have influenced concordance levels. For HER2, there was a significantly longer time interval between biopsies for non-concordant than for concordant pairs of samples (58.1 ± 19.7 vs. 30.9 ± 4.3 months) regardless of the CK status. No such difference was seen for the ER analysis.

Conclusions: In this exploratory analysis of 61 patients with MBC, we observed a high rate of detectable CTCs as well as CTC concordance for ER (85%) and HER2 amplification (93%) for patients who had CK+ CTCs. Concordance was less if patients only had CK- CTCs. This may be attributable to heterogeneity in the breast cancer phenotypes associated with these CK- CTCs in addition to inherent issues with testing cell surface markers in this population of cells. The variable latency between the between the collection of tissue and blood samples for these analyses may account for the discrepancies observed.

Citation Format: Hamilton EP, Yardley DA, Burris III HA, Shastry M, Huynh L, Bhikha L, Singh VM. Circulating tumor cells (CTCs) biomarker evaluation from patients with metastatic breast cancer (MBC) utilizing the TargetSelectorTM platform [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 P1-01-16.