Microfluidics and Circulating Tumor Cells (2012)

Circulating tumor cells (CTCs) are shed from cancerous tumors, enter the circulatory system, and migrate to distant organs to form metastases that ultimately lead to the death of most patients with cancer. Identification and characterization of CTCs provides a means to study, monitor, and potentially interfere with the metastatic process. Isolation of CTCs from blood is challenging because CTCs are rare and possess characteristics that reflect the heterogeneity of cancers. Various methods have been developed to enrich CTCs from many millions of normal blood cells. Microfluidics offers an opportunity to create a next generation of superior CTC enrichment devices. This review focuses on various microfluidic approaches that have been applied to date to capture CTCs from the blood of patients with cancer. Circulating tumor cells (CTCs) are shed from cancerous tumors, enter the circulatory system, and migrate to distant organs to form metastases that ultimately lead to the death of most patients with cancer. Identification and characterization of CTCs provides a means to study, monitor, and potentially interfere with the metastatic process. Isolation of CTCs from blood is challenging because CTCs are rare and possess characteristics that reflect the heterogeneity of cancers. Various methods have been developed to enrich CTCs from many millions of normal blood cells. Microfluidics offers an opportunity to create a next generation of superior CTC enrichment devices. This review focuses on various microfluidic approaches that have been applied to date to capture CTCs from the blood of patients with cancer. Circulating tumor cells (CTCs) are defined as tumor cells in the circulation. The dissemination of cancer cells from the primary tumor is a first step of the metastatic process in distant organs and is the leading cause of death in patients with cancer. Approximately 90% of patients with epithelial cancers, accounting for approximately 80% of all cancers throughout the world, die of metastatic disease rather than of the direct effects of the primary tumor. An average tumor may release an estimated million cells per day into the bloodstream1Butler J.E. Ni L. Nessler R. Joshi K.S. Suter M. Rosenberg B. Chang J. Brown W.R. Cantarero L.A. The physical and functional behavior of capture antibodies adsorbed on polystyrene.J Immunol Methods. 1992; 150: 77-90Crossref PubMed Scopus (272) Google Scholar; however, most dispersed cancer cells do not survive. Nevertheless, the cells that do survive clearly pose an existential threat to the host organism. From the moment that CTCs were first identified in 1869, an underlying hypothesis has emerged that these cells originate from primary tumors and perhaps from metastatic sites. For solid tumors, the presence of these cells is evident late in disease and is most apparent when metastatic disease sites are already established. Identification and characterization of CTCs offers an opportunity to study, monitor, and, ultimately, alter the metastatic process. CTCs are exceedingly rare cells. Ordinary blood cell types are present in tremendous numbers by comparison, and patients with cancer may have altered levels of other blood cell types to consider. These other cell types may comprise leukocytes (approximately 7 million/mL of blood) and red blood cells (approximately 5 billion/mL of blood). Assuming that all CTCs shed from the primary tumor remain in the circulatory system, an estimated yield of <200 CTCs/mL of blood will be derived from an average male patient (5 L of total blood volume). Therefore, CTCs should, theoretically, be present at <0.004% of all mononucleated cells in the blood. However, evidence thus far indicates that not all shed CTCs remain in the circulation on comparison of CTC numbers in blood near the site of primary tumor and peripheral blood.2Okumura Y. Tanaka F. Yoneda K. Hashimoto M. Takuwa T. Kondo N. Hasegawa S. Circulating tumor cells in pulmonary venous blood of primary lung cancer patients.Ann Thoracic Surg. 2009; 87: 1669-1675Abstract Full Text Full Text PDF PubMed Scopus (75) Google Scholar A working hypothesis is that these cells are in the bloodstream for ≤24 hours.3Stott S.L. Lee R.J. Nagrath S. Yu M. Miyamoto D.T. Ulkus L. Inserra E.J. Ulman M. Springer S. Nakamura Z. Moore A.L. Tsukrov D.I. Kempner M.E. Dahl D.M. Wu C.-L. Iafrate A.J. Smith M.R. Tompkins R.G. Sequist L.V. Toner M. Haber D.A. Maheswaran S. Isolation and characterization of circulating tumor cells from patients with localized and metastatic prostate cancer.Sci Transl Med. 2010; 2: 25ra23Crossref PubMed Scopus (347) Google Scholar, 4Meng S. Tripathy D. Frenkel E.P. Shete S. Naftalis E.Z. Huth J.F. Beitsch P.D. Leitch M. Hoover S. Euhus D. Haley B. Morrison L. Fleming T.P. Herlyn D. Terstappen L.W.M.M. Fehm T. Tucker T.F. Lane N. Wang J. Uhr J.W. Circulating tumor cells in patients with breast cancer dormancy.Clin Cancer Res. 2004; 10: 8152-8162Crossref PubMed Scopus (727) Google Scholar Further reports on the characterization of blood constituents indicate that larger numbers of cellular fragments are present in the circulatory system of patients with cancer along with undamaged CTCs. Enumeration of tumor cell fragments and intact CTCs can be used to accurately predict the survival of patients with cancer.5Coumans F.A. Doggen C.J. Attard G. de Bono J.S. Terstappen L.W. All circulating EpCAM+CK+CD45- objects predict overall survival in castration-resistant prostate cancer.Ann Oncol. 2010; 21: 1851-1857Crossref PubMed Scopus (170) Google Scholar The life span and load of CTCs may differ depending on the type of their originating tumor and the current treatment regimen. Ironically, surgical removal of the tumor may aid the release of tumor cells into the circulation, thus facilitating the process of metastasis. In fact, awareness of CTCs is causing leading surgeons to modify their treatment routines to minimize that possibility. Also, the ability of CTCs to interact with other cell types, such as endothelial cells or platelets, may affect the number of CTCs in the circulation.4Meng S. Tripathy D. Frenkel E.P. Shete S. Naftalis E.Z. Huth J.F. Beitsch P.D. Leitch M. Hoover S. Euhus D. Haley B. Morrison L. Fleming T.P. Herlyn D. Terstappen L.W.M.M. Fehm T. Tucker T.F. Lane N. Wang J. Uhr J.W. Circulating tumor cells in patients with breast cancer dormancy.Clin Cancer Res. 2004; 10: 8152-8162Crossref PubMed Scopus (727) Google Scholar Although the study of CTCs is extremely challenging, their detection in the circulation offers valuable information correlative with patient prognosis.6Van De Stolpe A. Pantel K. Sleijfer S. Terstappen L.W. Den Toonder J.M.J. Circulating tumor cell isolation and diagnostics: toward routine clinical use.Cancer Res. 2011; 71: 5955-5960Crossref PubMed Scopus (159) Google Scholar It is widely accepted that cells leave the evolving primary tumor to perpetuate the metastatic process. However, recent findings help to refine this working hypothesis by demonstrating that tumor cells from metastatic sites and the blood can repopulate the primary site and modify characteristics of even primary tumors.7Kim M.-Y. Oskarsson T. Acharyya S. Nguyen D.X. Zhang X.H.-F. Norton L. Massagué J. Tumor self-seeding by circulating cancer cells.Cell. 2009; 139: 1315-1326Abstract Full Text Full Text PDF PubMed Scopus (1002) Google Scholar Another reason for CTC monitoring is underscored by cases where the primary tumor (prostate or breast cancer, for example) is often completely excised or does not persist throughout the disease. Nevertheless, such patients may acquire metastatic events from dormant reservoirs of tumor cells elsewhere, such as in the bone marrow (Figure 1). Therefore, routine and sensitive monitoring of CTCs may be of tremendous value for early detection of recurrent disease and is possibly the only means to gain dynamic molecular characterization of tumor cells in the midst of advancing disease status. A primary tumor contains a heterogeneous cell population composed of tumor cells, normal tissue supporting stroma and endothelium, and inflammatory cells. All contribute to the rapid in and and with these may CTCs to be heterogeneous as evidence that of these cells may be for sites in tumor and L. J. cancer a disease of Med. PubMed Scopus Google Scholar, G. J.F. D. R. R. F. J. R. G. R. D. A. M.E. Terstappen L.W. de Bono J.S. of and in circulating tumor cells from patients with castration-resistant prostate Res. 2009; PubMed Scopus Google Scholar A of CTCs the metastatic of cells is to depending on a disease and treatment The ability to various CTCs may a of isolation approaches and characterization The of CTC detection is to cells that are of distant However, the characteristics of such CTCs are not In is that will be to cancers because is that cancers to sites. Therefore, is that a has to be for tumor Nevertheless, CTC detection has to be of numbers of tumor cells in a of millions of blood cells. For of patients with metastatic prostate cancer or CTCs per of however, only of these patients CTCs per J. M. Uhr J.W. Terstappen L.W.M.M. 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