Abstract P1-01-01: Circulating tumor cell number and CTC-endocrine therapy index predict clinical outcomes in ER positive metastatic breast cancer patients: Results of the COMETI Phase 2 trial

Introduction: Only half of hormone receptor positive (HR+) metastatic breast cancer (MBC) patients (pts) benefit from endocrine therapy (ET). Circulating tumor cells (CTC) are prognostic in pts with MBC using CellSearch® technology. The CTC-endocrine therapy index (CTC-ETI) provides semi-quantitative analyses of CTC-ER (estrogen receptor), BCL2, HER2, and Ki67 expression. We hypothesized that CTC-ETI high (elevated CTC number and/or low expression of ER and BCL2, and high expression of HER2 and Ki-67) might predict resistance to ET in a prospective, multi-institutional clinical trial: COMETI-P2-2012.0 (NCT01701050). Methods: 121 pts with ER+, HER2 negative (-), and progressive MBC after one or more lines of ET or within 12 months (mos) of completing adjuvant ET, who were initiating a new ET, were enrolled after informed consent. CTC and CTC-ETI were determined as previously reported (Paoletti C et al, CCR 2015) at baseline (BL), 1, 2, 3, and 12 mos, and/or at the time of progression. Imaging was performed every 3 mos. Association of CTC levels and CTC-ETI with patient outcomes (progression free survival (PFS); rapid progression (RP) defined as progression within 3 mos) was assessed using logrank and Fisher9s exact tests. Trial design estimated 85 PFS and 51 RP events, providing >90% power (2-sided a=0.05); pts with unsuccessful BL CTC-ETI or ineligible were unevaluable. Only baseline (BL) data are reported in this abstract. Results: 32% of enrolled pts had progression within 12 mos of completing adjuvant ET, whereas 40%, 20%, and 8% had 1, 2, ≥3 lines of ET for MBC. CTC-ETI was successfully determined in 93% of pts (90% CI, 88% to 97%). CTC were ≥5 CTC/7.5 ml whole blood in 37/108 (34%) pts evaluable for clinical validity. Elevated CTC was associated with worse PFS (median (m) PFS: 3.3 vs. 5.9 mos; P Conclusions: In this multi-institutional, prospective study, CTC-ETI was accurately determined, confirming the previously established analytical validity of the assay, meeting the primary objective of the trial. Elevated CTC and CTC-ETI high compared to low were associated with poor outcomes to ET. CTC-ETI distribution resulted in a small number of patients assigned to the intermediate group, restricting our ability to associate this group with outcomes. These results suggest that CTC-biomarker phenotype and enumeration have clinical validity. CTC-ETI may identify ER+ HER2– MBC pts who are unlikely to benefit from ET and might be better treated with ET in combination with other therapies or proceed to chemotherapy. Further analyses including CTC-ETI at serial time points during ET are planned. Citation Format: Paoletti C, Regan MM, Liu MC, Marcom PK, Hart LL, Smith II JW, Tedesco KL, Amir E, Krop IE, DeMichele AM, Goodwin PJ, Block M, Aung K, Cannell EM, Darga EP, Baratta PJ, Brown ME, McCormack RT, Hayes DF. Circulating tumor cell number and CTC-endocrine therapy index predict clinical outcomes in ER positive metastatic breast cancer patients: Results of the COMETI Phase 2 trial [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 P1-01-01.