Circulating tumor cells in small cell lung cancer: ex vivo expansion.

Abstract

Lung cancer is the leading cause of cancer-related deaths worldwide, and the small cell lung cancer (SCLC) histotype is its most aggressive variant. Yu et al. recently reviewed the detection methods and clinical applications of circulating tumor cells (CTCs) in lung cancer in detail [1]. CTCs are shed from primary tumors or metastatic sites into the peripheral blood and may serve as liquid tumor biopsies for early diagnosis, risk evaluation, and monitoring of therapy [1, 2]. As CTCs are normally rare, detection methods comprise enrichment steps followed by identification with selective markers. The Cell-Search system is the only diagnostic assay approved by the US Food and Drug Administration and was employed to detect SCLC CTCs. In particular, CTCs were defined as nucleated cells coexpressing EpCAM and cytokeratins and negative for the white blood cell surface marker CD45 [2]. In SCLC patients, the detection rates of the CellSearch system ranged from 14.3 to 68.6 % [1]. However, during cancer invasion and metastasis, some cells undergo epithelial– mesenchymal transition (EMT), thus increasing the probability of false-negative CTC results. Among primary lung cancer patients, SCLC patients showed a higher CTC count than non-small cell lung cancer (NSCLC) cases [1]. A higher stage of lung cancer usually correlates with a higher number of CTCs. Yet, only a minority of these disseminated cells can develop into metastases [2]. Lung cancer cells shed into the circulation during surgery only persist for a short time [1]. Numerous investigations have evaluated the prognostic value of CTCs in lung cancer; however, the conclusions of these studies are controversial. For example, Naito et al. indicated that SCLC patients with[8 CTCs/7.5 ml blood had a significantly shorter survival time than the remaining patients with\8 CTCs after therapy [3]. Hou et al. drew the same conclusion using 50 CTCs/7.5 ml blood as the cut-off value, citing an overall survival of 5.4 versus 11.5 months for the high CTC cohort [4]. More studies using standardized methods are needed to validitate a correlation between CTC count and prognosis/disease response. In the latter study, CTCs were present in 85 % of patients and were abundant (range 0–44,896, mean ± SD: 1589 ± 5565), although a significant subpopulation of the CTCs was apoptotic [4]. Molecular characterization of CTCs may provide novel insights into progression of SCLC but investigations are hampered by insufficient tissue for research, because surgical resection and/or serial biopsies are rarely performed. In order to identify the functional properties of different CTC subsets, expansion of CTCs in cell culture systems is required. First ex vivo culture of CTCs has been described in breast cancer by Zhang et al. and Yu et al. [5]. However, these cultures required very high CTC concentrations in suitable blood samples ([1000 cells/7.5 ml in breast cancer), which were so far only found in a few patients. In colon cancer, CTCs isolated from the blood of one patient with a rapidly progressing tumor (out of 71 patients) displaying a CTC count[2300 cells/7.5 ml blood gave rise to a stable colon CTC line [6]. Another approach to expand the number of CTCs is xenotransplantation of patientderived CTCs into immunodeficient mice. Only blood & Gerhard Hamilton gerhard.hamilton@toc.lbg.ac.at