Abstract IA15: Biology and clinical implications of circulating tumor cells and bone marrow micrometastasis
Authors: Pantel Klaus 
Published: May 2015

Abstract
Sensitive methods have been developed to detect circulating tumor cells (CTC) in the peripheral blood and disseminated tumor cells (DTC) in the bone marrow at the single cell level. CTC and DTC can be distinguished and enriched from the surrounding leukocytes by either physical properties (e.g., density and size) or biological properties (e.g., expression of epithelial proteins such as EpCAM or cytokeratins) (Pantel et al., Nat Rev Cancer 2008; Alix-Panabieres & Pantel, Ann Rev Med 2012; Parkinson et al., J Transl Med 2012). CTC/DTC are usually detected by immunostaining or RT-PCR assays, and more recently by the EPISPOT assay which measures the number of cells releasing/secreting tumor-associated marker proteins. Interestingly, detection of cell-free nucleic acids released by tumor cells into the blood might become an indirect way to detect micrometastatic disease in the future (Schwarzenbach et al, Nat Rev Cancer 2011). At present, most assays rely on epithelial markers and miss CTCs undergoing an epithelial-mesenchymal transition (EMT). New markers such as the actin bundling protein plastin-3 (Yokobori et al., Cancer Res. 2013) are not downregulated during EMT and not expressed in normal blood cells might overcome this important limitation and, therefore, increase the sensitivity of CTC assays. Recently, in vivo capture of CTCs with an antibody-coated wire placed into the peripheral arm vein has become feasible and allows now the “fishing” for CTCs from approx. 1.5 liters of blood within 30 minutes (Saucedo-Zeni, Int. J. Oncol. 2012).
Interestingly, the bone marrow seems to be a common homing organ for cells derived from various epithelial tumours including breast and prostate cancer (Pantel et al., Nat Rev Cancer 2004; Braun et al., NEJM 2005; Köllermann et al., JCO 2008; Janni et al., CCR 2011). However, a significant fraction of DTC remain over years in a “dormant” stage, and little is known about the conditions required for the persistence of dormancy or the escape from the dormant phase into the active phase of metastasis formation (Uhr & Pantel, PNAS 2011; Kang & Pantel, Cancer Cell, 2013). Our recent findings indicated that the subset of EpCAMlow, CD44high, CD47+, c-Met+ CTCs obtained from the peripheral blood of breast cancer patients might have an increased ability to colonize bone marrow in immunodeficient mice (Baccelli et al, Nature Biotech. 2013). However, it is unclear whether these CTCs are metastasis-inducer cells because they were obtained from patients with advanced metastatic disease and extraordinarily high CTC counts.
Sequential peripheral blood analyses, however, are more convenient for patients than bone marrow analyses and many research groups are currently assessing the clinical utility of CTC for assessment of prognosis and monitoring of systemic therapy (Cristofanilli et al., NEJM 2004; DeBono et al., CCR 2008; Cohen et al, JCO 2008; Riethdorf et al., CCR 2010; Zhang et al., CCR 2012). In particular, monitoring of CTC during and after systemic adjuvant therapy (e.g., chemotherapy, hormonal therapy, antibody therapy) might provide unique information for the clinical management of the individual cancer patient and allow an early change in therapy years before the appearance of overt metastases signals incurability. There is an unmet need for biomarkers for real-time monitoring of the efficacy of systemic adjuvant therapy in individual patients. In particular, early changes in CTC counts might indicate success or failure of a particular therapy given to an individual patient.
Although systemic therapies are aimed to eliminate metastatic cells, the current stratification is usually performed on the primary tumors for practical reasons. However, there is increasing evidence that the phenotype and genotype of primary and metastatic cancer cells are discordant (Heitzer et al., Cancer Res., 2013). Thus, the molecular analysis of CTC isolated from peripheral blood samples as “liquid biopsy” will reveal characteristics of metastatic cancer cells (Gasch et al., Clin. Chem. 2012; Alix-Panabieres & Pantel, Clin. Chem. 2013). This information can be used as companion diagnostics to improve the stratification of therapies and to obtain insights into therapy-induced selection of cancer cells. At present, CTC are included as biomarker in more than 400 clinical trials.
In conclusion, research on DTC and CTC opens a new avenue for detecting, understanding and fighting early metastatic spread of tumor cells with important implications for future therapies.
Citation Format: Klaus Pantel. Biology and clinical implications of circulating tumor cells and bone marrow micrometastasis. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr IA15.