P4-07-16: Development of Circulating Tumor Cell-Endocrine Therapy Index in Metastatic Breast Cancer Patients.

Introduction: Only ∼ 50% of patients (pts) with estrogen receptor (ER) positive metastatic breast cancer (MBC) benefit from endocrine therapy (ET). Currently only clinical judgment can be used to identify pts with endocrine-refractory MBC, who are better palliated with chemotherapy. Circulating Tumor Cells (CTC) are reliably enumerated using an automated immunomagnetic system (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 positive MBC who are unlikely to benefit from ET and may be better served with chemotherapy. CTC-ETI scores are assigned based on CTC levels coupled with the relative percent and degree of marker positivity on the CTC. We report preliminary results from a pilot single institutional study. Methods: CellSearch® has 4 fluorescence channels. Three distinguish CTC from WBC (DAPI, anti-cytokeratin, anti-CD45). The 4 th “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. Forty ml of blood was drawn into 4 CellSave® tubes from pts with progressive MBC. Whole blood from 4 tubes was pooled and divided into 4 different 7.5 ml aliquots of blood, which were processed and characterized for CTC counts and each of the four molecular markers using the CXC CellSearch® kit. Results: 21 pts have been accrued to the feasibility study. One patient was ineligible. Five of 20 pts had low CTC counts ( Conclusions: ER, BCL-2, HER-2, and Ki-67 can be accurately determined on CTC using the 4 th channel in the CellSearch® system to calculate CTC-ETI. 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 women starting a new endocrine therapy 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). Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P4-07-16.