Development of Circulating Tumor Cell-Endocrine Therapy Index in Patients with Hormone Receptor–Positive Breast Cancer

Abstract

Background Endocrine therapy (ET) fails to induce a response in one-half of patients with hormone receptor (HR) positive metastatic breast cancer (MBC) and almost all will eventually become refractory to ET. Circulating Tumor Cells (CTC) are associated with worse prognosis in MBC patients, but enumeration alone is insufficient to predict the absolute odds of benefit from any therapy, including ET. We developed a multi-parameter CTC-Endocrine Therapy Index (CTC-ETI), which we hypothesize may predict resistance to ET in patients with HR positive MBC. Methods The CTC-ETI combines enumeration and CTC expression of four markers: estrogen receptor (ER), B-cell lymphoma 2 (BCL-2), Human Epidermal Growth Factor Receptor 2 (HER2), and Ki67. The CellSearch® System and reagents were used to capture CTC and measure protein expression by immunofluorescent staining on CTC. Results The feasibility of determining CTC-ETI was initially established in vitro and then in a prospective single-institution pilot study in MBC patients. CTC-ETI was successfully determined in 44/50 (88%) patients. Eighteen (41%), 9 (20%), and 17 (39%) patients had low, intermediate, and high CTC-ETI scores, respectively. Inter-observer concordance of CTC-ETI determination was 94–95% (Kappa statistic 0.90–0.91). Inter- and cell-to-cell intra-patient heterogeneity of expression of each of the CTC-markers was observed. CTC biomarker expression was discordant from both primary and metastatic tissue. Conclusions CTC expression of ER, BCL-2, HER2, and Ki67 can be reproducibly measured with high analytical validity using the CellSearch® System. The clinical implications of CTC-ETI, and of the heterogeneity of CTC-biomarker expression, are being evaluated in an ongoing prospective trial.

Circulating Tumor Cells and Circulating Tumor DNA: Challenges and Opportunities on the Path to Clinical Utility

“…To that end, a score based on CTC enumeration and characterization for estrogen receptor (ER), Bcl-2, HER2, and Ki67, the CTC-Endocrine Therapy Index (CTC-ETI; ref. 40), is currently being tested in the COMETI Phase II study (NCT01701050 ; Table 3). …”

Intratumoral Heterogeneity: From Diversity Comes Resistance

“…Single-cell analysis of CTCs in ER þ metastatic breast cancer patients showed that in nearly half of evaluable patients, all CTCs were ER À , and the remaining patients harbored CTCs with mixed ER expression. Trials are currently under way to assess whether the endocrine therapy index of CTCs is correlated with patient outcome (28). Heterogeneous expression of the ERBB3 (HER3) and its ligand NRG1 have been reported in a variety of human cancers, including breast cancer, lung cancer, and glioblastoma (23,29,30).…”

Improving the CellSearch® system

“…Most of these studies show that a number of patients develop Her-2-positive CTCs, while the primary tumor was negative indicating a new possible treatment target for this group of patients. A recent article from Paoletti et al shows the use of the CXC kit for studying the development of resistance of breast cancer patients to the estrogen receptor downregulating drug fulvestrant [90,91]. This small study suggests a heterogeneous cause of resistance based on the expression of estrogen receptor (ER) and B-cell lymphoma 2 (BCL-2) on CTCs.…”