CTC Surveillance in Early-Stage Breast Cancer: What's Next?

Joseph A. Sparano, MD

Blood-based biomarkers great potential for use in the management of breast and other cancers. Critical elements in clinical application include analytic validity and clinical validity, which would be required to ultimately establish clinical utility—a biomarker-directed treatment change that results in clinical benefit.¹

Currently available biomarker assays for breast cancer include circulating protein markers (eg, CA 27.29, CA 15-3), circulating tumor cells (CTCs), and circulating tumor DNA (ctDNA). Other potential biomarkers, collectively referred to as liquid biopsies, include exosomes, microRNA, noncoding RNAs, and tumor-educated platelets.² The Blood Profiling Atlas in Cancer is an initiative including academic, government, biotechnology, diagnostic, and pharmaceutical institutions to coordinate, harmonize, and collaborate with the goal of moving these technologies into routine clinical care.³

CTCs are shed from the tumor and into the bloodstream, where a small percentage survive, extravasate, and colonize distant sites, ultimately developing clinical evidence of metastasis, in some cases after a period of tumor dormancy.

It is estimated that only 1 in 10 million tumor cells from a primary tumor will reach the blood- stream, accounting for only 1 in 10,000 nucleated cells per milliliter of blood,⁴ with a short half-life of less than 3 hours.⁵

CTCs were first noted in cancer by Thomas Ashworth in 1869.⁶ Since this seminal observa- tion and the first isolation of CTCs from a blood sample reported in 1955,⁷ over 50 assays have been reported using different techniques for CTC identification, enumeration, and, in some cases, molecular characterization. Cleared, analytically validated assay that facilitates microscopy-based detection and enumeration of CTCs in metastatic breast, colorectal, and prostate cancers using immunomagnetic beads for positive selection of CTCs expressing epithelium-specific cell adhesion molecule, a positive selection method.^2,8,9

CTCs in Metastatic Breast Cancer

This review will focus on the use of this assay, the CellSearch System (Menarini Silicon Biosystems, Inc; Bologna, Italy), in early and advanced breast cancer. Some of the key conclusions regarding research conducted to date and potential future ap- plications are summarized in the Table.CTCs are detectable in approximately 65% or more of patients with metastatic breast cancer (MBC), and a CTC count ≥5 cells per 7.5 mL of blood in MBC is associated with significantly inferior progression-free survival (PFS) and overall survival (OS),⁸ providing evidence for clinical validity. However, prospective randomized trials have not established clinical utility, as described below.

SWOG S0500 (NCT00382018) was a prospective clinical trial including 595 patients with MBC receiving first-line chemotherapy in whom systemic chemotherapy was changed for those with persistently high CTC counts (≥5/7.5 mL blood) after 3 weeks of first-line chemotherapy. The trial failed to meet its primary endpoint of demonstrating improved OS in the 21% of trial participants in whom a CTC-guided change in chemotherapy was compared with continuation of the same chemotherapy.⁹

The STIC trial (NCT01710605) was a prospective clinical trial including 778 patients with hormone receptor (HR)—positive, HER2- negative MBC randomly assigned to a clinically driven treatment arm (physician’s choice of hormone therapy or chemotherapy based on clinical factors) or a CTC-driven treatment arm (hormone therapy if CTC <5, chemotherapy if CTC ≥5/7.5 mL blood).¹⁰ Patients whose treat- ment was escalated to chemotherapy based on CTC count exhibited a longer PFS than those whose treatment was clinically driven. The median PFS was 10.5 months with hormone therapy in the clinically driven arm for those who had a high CTC count versus 15.5 months with chemotherapy in the CTC-driven arm and showed a trend toward longer OS (median, 37.1 vs 42.0 months, respectively). Major limitations of the trial included lack of standardized clinical criteria for recommending chemotherapy in the clinically driven arm and the fact that it was conducted prior to the widespread use of CDK4/6 inhibitors, now regarded as the standard of care for HR—positive MBC.

The EORTC 90091-10093 BIG 1—12 “Treat CTC” trial (NCT01548677) was a prospective clinical trial including 63 patients with non-metastatic breast cancer who had detectable CTCs (>1/7.5 mL blood) after neoadjuvant chemotherapy and surgery and who were randomized to trastuzumab (Herceptin) or observation. There was no difference in CTCs at week 18, the trial’s primary endpoint, and the trial was halted for futility.¹¹

CTCs in Early Breast Cancer Within 2 Years After Diagnosis

Several other randomized trials are ongoing, as reviewed elsewhere.¹² The DETECTIII trial (NCT01619111) is comparing lapatinib (Tykerb) plus chemotherapy or chemotherapy alone in patients with at least 1 CTC with strong HER2 immunocytofluorescence. The CirCe01 trial (NCT01349842) includes patients with MBC and a high CTC count (≥5/7.5 mL blood) about to initiate third-line therapy who are randomized to CTC-directed versus clinically directed change in therapy.CTCs are detectable in about 20% to 25% of patients with localized nonmetastatic breast cancer at the time of diagnosis and also provide independent prognostic information using a lower threshold (≥1 CTC per 7.5 mL blood) than in MBC (≥5 CTC per 7.5 mL blood), whether obtained before or after surgery, including after neoadjuvant or adjuvant chemotherapy, providing additional evidence for clinical validity.^13,14

In a pooled analysis of individual data from 3173 patients with stage I to III breast cancer from 5 institutions, CTCs (>1 per 7.5 mL blood) were detected in 20.2% of patients, and the presence of CTCs was an independent prognostic fac- tor for inferior disease-free survival (DFS; haz- ard ratio, 1.82; 95% CI, 1.47-2.26), distant DFS (hazard ratio, 1.89; 95% CI, 1.49-2.40), breast cancer—specific survival (hazard ratio, 2.04; 95% CI, 1.52-2.75), and OS (hazard ratio, 1.97; 95% CI, 1.51-2.59) in a multivariate Cox regression model including tumor size, nodal status, histologic tumor grade, and HR and HER2 status.¹⁵

In a meta-analysis that included individual patient data from 21 studies in which CTCs were evaluated in 1574 patients before receiving neoadjuvant chemotherapy (NAC) and 1200 patients after NAC and before surgery, CTCs (≥1/7.5 mL blood) were detected in 25.2% of patients before NAC, and a higher CTC number was associated with OS (P <.001), distant DFS (P <.001), and locoregional relapse—free interval (P <.001) but not on pathological complete response. In 861 patients with full data available, adding CTC detection before NAC also added prognostic information in multivariable prognostic models for OS (P <.001), distant DFS (P <.001), and locoregional relapse—free interval (P = .008).¹⁶

SUMMARY OF KEY FINDINGS AND POTENTIAL FUTURE DIRECTIONS FOR CLINICAL APPLICATION OF EPCAM-BASED CTC DETECTION AND ENUMERATION IN METASTATIC AND NONMETASTATIC BREAST CANCER¹⁰

CTC indicates circulating tumor cell; ctDNA, circulating tumor DNA; EpCAM, epithelial cell adhesion molecule; MBC, metastatic breast cancer.

Figure. Hypothetical Trial Designs for CTCs as an Integral Biomarker for Surveillance to Test Novel Agents to Prevent Late Recurrence in Hormone Receptor—Positive Breast Cancer²⁹

CTC indicates circulating tumor cell.

CTCs and Late Recurrence

CTCs were detected at 2 years in 18% of 1087 patients with stage II to III breast cancer enrolled in the phase III SUCCESS-A trial,¹⁷ which was one of the trials in which CTC was found to be prognostic at diagnosis.¹⁴ At 2 years, a positive CTC assay was associated with a 3.9-fold higher risk of death and a 2.3-fold higher risk of recurrence in multivariate models that included clinicopathologic features and CTC status at baseline; sensitivity analysis showed this effect only in HER2-negative disease.Late recurrence 5 or more years after diagnosis accounts for more than one-half of patients with HR-positive breast cancer.¹⁸ Factors associated with higher late recurrence risk include positive axillary nodes and primary tumor size >2 cm, and tools such as the CTS5 calculator may provide more precise estimates of late recurrence risk for individual patients.¹⁹ In a study including 353 patients with HR—positive, HER2-negative, stage II to III breast cancer treated with adjuvant chemotherapy and endocrine therapy followed up for 4.5 to 7.5 years after diagnosis without evidence of clinical recurrence, CTCs were detectable in 5.1% (median, 1 cell per 7.5 mL blood; range, 1-15 cells per 7.5 mL blood). Moreover, CTC positivity was associated with a 13.1-fold increase in recurrence risk in mul- tivariate analysis adjusted for other prognostic covariates.²⁰

What's Next?

Because imaging was not performed in any of these trials at the time of the positive CTC assay, it is currently unknown what propor- tion of patients who are CTC positive would be found to have clinical evidence of asymptomatic metastatic disease. In a similar analysis of 206 patients enrolled in the SUCCESS-A study with follow-up information and known CTC status at 5 years, 7.8% were CTC positive at 5 years, which was associated with a 6-fold in- crease in recurrence.²¹There are several potential strategies for establishing the clinical utility of CTC enumeration and/or characterization, including as a surrogate/replacement of or adjunct to diagnostic imaging,²² therapeutic selection, and surveil- lance. Several ongoing trials may shed further light on the clinical utility of CTC-directed therapy. Here we focus on the potential opportunities as a surveillance biomarker.

Evidence-based guidelines recommend surveillance after treatment of localized breast cancer including history, physical examina- tion, and annual mammography.^23,24 Blood- based biomarkers and imaging studies beyond mammography are not recommended in asymptomatic patients, although these recom- mendations are based on trials performed at a time when diagnostic, imaging, and therapeutic options were limited.²⁵ The availability of more effective therapies for MBC, such as immune checkpoint blockade for triple-negative disease²⁶ and CDK4/6 inhibitors²⁷ or novel oral selective estrogen receptor downregulators²⁸ for HR—positive disease, offers potential for early intervention that could ultimately delay or even prevent metastasis.²⁹

Recently, the FDA granted regulatory approval for antiandrogenic agents in the treatment of nonmetastatic castration-resistant prostate cancer based on the endpoint of metastasis-free survival in a setting where an integral biomarker (serum prostate specific antigen) was used to select men at high risk for developing metastasis and lacking distant disease based on standard diagnostic imaging.³⁰ This provides a foundation for de- veloping a similar approach for surveillance and treatment of high-risk, nonmetastatic breast cancer, as exemplified by 2 hypothetical trial designs shown in the figure, including a biomarker selection design and a biomarker stratification design, each of which has its own advantages and disadvantages.²⁹ Such future trials should include cross-platform comparison of different CTC methodologies and other biomarkers, including protein-based markers, ctDNA, and exosomes.

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2019 Miami Breast Cancer Conference