Role of Tumor Markers and Circulating Tumors Cells in the Management of Breast Cancer

Breast cancer is the most common malignancy (excluding skin cancer) affecting women in the United States and is second to lung cancer as a cause of cancer-related mortality.[1] In addition to screening, the emergence of new therapeutic agents has contributed to the significant reduction of breast cancer mortality since 1990.[1] Treatment of metastatic breast cancer (MBC) imposes a challenge to medical oncologists, as it is an incurable disease. Patients are usually treated until disease progression or unacceptable toxicity ensues. Assessment of disease progression is critical, not only to eliminate ineffective therapy, but also to avoid unnecessary toxicity. 

Therefore, the implementation of sensitive tests to monitor disease progression is crucial. Imaging studies are routinely used to evaluate treatment response of metastatic disease after initial confirmation of metastasis by pathologic assessment. However, imaging studies fall short of detecting minimal or evolving disease. Even the functional imaging modalities such as positron-emission tomography (PET) can detect metastatic disease only when it is larger than several millimeters in diameter. Although PET scan is shown to be more sensitive than conventional imaging (ie, computed tomography [CT] and magnetic resonance imaging [MRI]) in the detection of metastatic and recurrent disease, there is no compelling evidence to suggest a benefit in predicting treatment response in these patients.[2] Thus, the development of sensitive biologic marker tests has long been desirable, and remains a constant dilemma in clinical practice.

Serologic Markers Used in Breast Cancer

Serial measurement of tumor markers after primary treatment for breast cancer can detect preclinical recurrent disease with lead times of about 2 to 9 months. But the clinical significance of this finding is unknown.[3] In 1996, an American Society of Clinical Oncology (ASCO) expert panel recommended that a five- to tenfold increase of CA 15-3 above the normal limit be considered an alert for the presence of metastatic disease.[4] However, the assay lacks sensitivity and specificity, and increased levels of this antigen can be seen in individuals with no breast cancer.[4]

The following serologic markers are frequently used in current practice to monitor disease status of patients with breast cancer.

• CA 15-3
• CA 27.29
• Carcinoembryonic antigen (CEA)
• HER2 extracellular domain (ECD)

ASCO recently updated its guidelines for the use of tumor markers in breast cancer.[5] The ASCO panel recognized CA 15-3, CA 27.29, and CEA to be of clinical utility in breast cancer management. A literature review of the utility of serum tumor markers in breast cancer has been published elsewhere.[3]

Both CA 15-3 and CA 27.29 received US Food and Drug Administration (FDA) approval in 1996 to be used in surveillance for disease recurrence or metastasis in patients with stage II/III breast cancer.[6,7]

MUC1 Markers

Both commonly used tumor markers of breast cancer—CA 15.3 and CA 27.29—measure the product of the MUC1 gene. This gene codes for a very heterogeneous large (300–400 kD) mucin glycoprotein, also called polymorphic epithelial mucin (PEM), which is expressed in most glandular epithelial cells.[8] Tumors involving glandular organs (eg, breast cancer) sometimes overexpress this mucin glycoprotein. Excess mucin is subsequently shed into the circulation, making serum assays useful as tumor markers.[9] In spite of the presence of multiple assays that can measure the MUC1 glycoprotein in serum, only two assays are used in clinical practice—CA 15-3 and CA 27.29. Data comparing 10 commercial assay methods for these two markers have shown a strong correlation among these tests.[10]

One study compared CA 27.29 levels with those of urinary deoxypyridinoline (DPD [a bone resorption marker]), serum calcium, and alkaline phosphatase (ALP) in breast cancer patients with and without bone metastases. The best correlation with metastatic disease was achieved with CA 27.29.[11] Another study compared CA 27.29 and CA 15-3 in 603 patients with breast cancer and 194 healthy controls. It showed excellent correlation between high marker levels and breast cancer, with CA 27.29 achieving the highest significance level.[12] High preoperative levels of CA 15-3 were shown to be associated with an adverse patient outcome.[13,14]

Carcinoembryonic Antigen

The CEA test for MBC is less sensitive compared to the MUC1 assays. CEA can be elevated in up to 60% of patients with metastatic disease, compared with about 80% who have elevated levels of the MUC1 antigen.[15] CEA testing does not have an additive impact when used with MUC1 levels. A study of 53 patients with MBC showed that CA 15-3 and CEA levels were elevated in 94% and 69%, respectively, and CEA was elevated in only one patient with a normal CA 15-3 level.[16] A study suggested that CA 27.29 is more sensitive and specific than CEA.[17]

The ASCO management guidelines for breast cancer recommend against obtaining serum tumor markers (CA 15-3, CA 27.29, or CEA) during routine surveillance after adjuvant setting.[18] However, when treating patients with MBC, changes in these marker levels are assumed to reflect disease progression or response to therapy, albeit with low sensitivity and specificity. The ASCO guidelines for using tumor markers in breast cancer, published in 2007, suggest the use of MUC1 and CEA assays initially in patients with metastatic disease, but discourage continued measurement if the MUC1 assay is elevated. When the CEA level is used for follow-up of metastatic disease, it should be noted that this measurement may spuriously rise during the first 4 to 6 weeks of a new therapy.[5]

HER2 Extracellular Domain

Serum concentrations of the shed form of HER2 have also been widely investigated for potential prognostic value in breast cancer. A published systematic review of the literature evaluated the correlation between serum level of HER2 ECD and clinical outcome in studies that involved > 6,500 patients with breast cancer. Increased ECD predicted a poor response to hormonal therapy but predicted a good response to trastuzumab (Herceptin).[19] The analysis did not explicitly identify the ECD concentration as an independent prognostic factor. The lack of either high-quality studies or consistent results prompted the ASCO guidelines panel to exclude this test in any clinical setting of breast cancer management.[5]

Circulating Tumor Cell Test

Epithelial tumor cells have been shown to be shed from breast cancer tumors into circulation.[20] Indeed, the prognostic significance of the circulating tumor cell (CTC) test in patients with breast cancer was first questioned more than 40 years ago.[21,22] A validated methodology has recently been developed to measure the level of CTCs in the peripheral blood of patients with MBC.[23] Many clinical studies have been performed to evaluate the utility of the CTC test in breast cancer. Table 1 lists studies that will be discussed in this article.

• Method of Isolation of CTC—The US Food and Drug Administration (FDA) cleared the CellSearch assay (Veridex, Warren, NJ) for detection of CTCs. The system uses the CellSearch Epithelial Cell Kit. The procedure entails adding antibody-coated magnetic beads to the blood sample to distinguish cells using fluorescently labeled monoclonal antibodies against the epithelial-cell adhesion molecule.[24]

• Clinical Trials of CTC Testing in MBC—A prospective multicenter study tested 177 patients with MBC for the number of CTCs before and after a new line of treatment. Imaging studies were evaluated blindly to define disease status. Disease evaluation continued thereafter every 9 to 12 weeks. The control group comprised healthy women and women with benign breast diseases. The study showed that MBC patients with a baseline (before starting treatment) CTC level ≥ 5 (per 7.5 mL of whole blood) had a shorter median progression-free survival (2.7 vs 7.0 months, P < .001) and shorter overall survival (10.1 vs > 18 months, P < .001). This difference in survival remained significant when posttreatment CTC levels were evaluated as well. Multivariate analysis showed that, of all the variables, the levels of CTC both at baseline and after treatment were the most significant predictors of progression-free and overall survival.[25]

A follow-up analysis of the same study showed that serial estimation of CTC level up to 20 weeks correlates with progression-free and overall survival.[26] This suggests that elevated CTC level at any time during therapy can reflect disease progression and mortality risk for MBC patients. Another subset analysis was done on 83 (of the 177) patients who were receiving first-line treatment for metastatic disease. CTC level was assessed in these patients at baseline and monthly thereafter for up to 6 months, for a median follow-up of 12.2 months. CTC levels before and after starting therapy were strong, independent prognostic factors for both progression-free and overall survival.[27]