Circulating Tumor Cells Revisited

Abstract

DESPITE THE USE OF MODERN HIGH-RESOLUTION imaging technologies, it is not possible to detect tumor cell metastasis at a single cell level. To date, cancer treatment is initiated only after the clinical presentation of disease. This approach generally is unsuccessful and translates into the dogma that metastasis is a terminal process, generally viewed as fatal. Recent advances showing prolongation of survival in some tumor types and mitigation of clinical symptoms have stimulated both translational and clinical research to detect and characterize micrometastatic disease in the form of circulating tumor cells (CTCs) and disseminated tumor cells (DTCs), both of which are considered to be early accessible cellular determinants of subsequent overt metastasis. In this Commentary, we describe recent advances in CTC research, which raise the possibility that treatments with curative intent might be applied successfully to patients with micrometastatic cancer (ie, evidence of disease on a cellular level), thereby potentially preventing subsequent fatal metastatic disease. The first documented research on tumor cells in the peripheral blood circulation was reported in 1869 by Ashworth. Since then, the existence, origin, and clinical significance of CTCs have increasingly attracted investigators’ interest. However, realization of the potential application of CTC is only beginning because the main limitation for its clinical consideration has been the need for development and robust validation of CTC enrichment and detection technologies. This technique relies on detection and quantification of extremely low numbers of CTCs. Most procedures involve immunomagnetic bead separation and using tumor cell antigens (eg, epithelial cell adhesion molecule [EpCAM]) or hematopoietic cell antigens (eg, common leukocyte antigen [CD45]) for purified cell suspensions, followed by conventional immunostaining for typing epithelial origin. Currently, only 1 standardized and validated assay has been approved by the US Food and Drug Administration. With this assay, a small peripheral blood sample is exposed to ferrofluid-coated antibodies directed against the EpCAM to extract EpCAM-positive cells through a magnetic field. For subsequent computer-generated (semiautomated fluorescence–based microscopy system) classification of immunostained cells as CTCs, the enriched EpCAM-positive cell fraction is validated image by image against the presence of epithelial-specific anticytokeratin staining, proper nucleus staining, and the absence of leukocyte-specific CD45 staining. Using this technology, CTCs have been investigated in several tumor types including breast, prostate, and colorectal cancers, and trials in breast cancer have documented the prognostic significance of CTCs. In a trial of 177 patients with metastatic breast cancer who were initiating systemic treatment, a cut point of 5 CTCs per 7.5 mL differentiated patients with higher CTC counts and both shorter progression-free and overall survival of 2.7 and 10.9 months, respectively, from patients classified below this threshold with 7.0 months progression-free survival (P=.0001) and 21.9 months overall survival (P .0001). In subsequent post hoc analyses, continued CTC monitoring provided more accurate prognostic information than high-resolution anatomical imaging of metastatic tumor burden and higher accuracy than functional imaging. In future trial settings, both CTC quantity and molecular features may provide information relevant for improved patient treatment as recently suggested for patients with advanced prostate cancer and metastatic colorectal cancer. Other technologies have been introduced in attempt to limit these technological shortcomings. A microfluidic chip technology was applied to samples from patients with non– small cell lung cancer, and demonstrated the feasibility for CTC monitoring and assessment of genetic marker– guided treatment effects. The method enabled interaction between CTCs and microposts coated with an antibody against EpCAM under precisely controlled laminar-flow conditions, thus aiming to reduce the level of false negativity due to cell loss during immunomagnetic separation processes, which appears to be achievable given the high CTC recovery rate. However, the limitation of this approach and most other investigational technologies is the requirement of antibody