40P Development of Circulating Tumor Cell-Endocrine Therapy Index (CTC-ETI) in Metastatic Breast Cancer Patients

ABSTRACT Introduction Only ∼50% of patients (pts) with estrogen receptor (ER) positive (+) metastatic breast cancer (MBC) benefit from endocrine therapy (ET). Circulating Tumor Cells (CTC) are reliably enumerated using CellSearch (Veridex, LLC). High CTC levels predict rapid progression in pts with MBC. We have developed a multi-parameter assay, the CTC-Endocrine Therapy Index (CTC-ETI) using CellSearch that may identify pts with ER+ MBC who are unlikely to benefit from ET and may be better served with chemotherapy. CTC-ETI scoring is based on CTC levels coupled with the relative percent and degree of marker positivity on CTC. We report preliminary results from a feasibility study. Methods CellSearch has 4 fluorescence channels: 3 distinguish CTC from WBC and the 4th “empty” channel was used to measure ER, BCL-2, HER-2, and Ki-67 expression with fluorescent-labeled antibodies. These 4 markers reflect sensitivity (ER, BCL-2) or resistance (HER-2, Ki-67) to ET. Blood from pts with progressive MBC was processed and characterized for CTC counts and each of the 4 markers using the CXC CellSearch kit. Results 34 pts have been accrued. One was ineligible. Ten of 33 pts had low CTC counts ( Conclusions CTC-ETI can be determined in MBC pts using the 4th channel in the CellSearch system. We predict that lower CTC-ETI scores (low or no CTC, or CTC with high CTC ER and BCL-2 and low CTC HER-2 and Ki-67) could be associated with favorable response to ET. Successful completion of the feasibility study will lead to a prospective trial to determine if high CTC-ETI at baseline predicts resistance and rapid progression on ET in pts starting new ET for MBC. Supported by Veridex, LLC, Fashion Footwear Charitable Foundation of New York/QVC Presents Shoes on Sale™ (DFH), Associazione Sandro Pitigliani and by a studentship from FIRC (CP). Disclosure M.C. Connelly: I am an employee of Veridex, LLC. D.A. Chianese: I am employed by Johnson and Johnson who in part funded this research. I am in addition, a shareholder of Johnson and Johnson. D.F. Hayes: I receive research support for laboratory and clinical research from Veridex LLC. All other authors have declared no conflicts of interest.