Circulating Tumor Cells in Patients with Neuroendocrine Neoplasms

Dear Editor,

One characteristic feature that makes a malignant tumor insidious and dangerous is its property to metastasize. Patients do not exclusively die due to the primary cancer that is normally completely removed surgically in case of a solid tumor; usually they die from metastatic spread, which frequently manifests after successful initial therapy. This phenomenon can be seen for many cancer types, including neuroendocrine neoplasms (NENs). NENs describe a group of het­erogenous tumors arising from the cells of the neuroendocrine system, affecting most commonly the gastrointestinal tract, pancreas, and the lung. A basis for metastasis of tumors is the activation of angiogenesis for oxygen supply, entering blood vessels, and the ability to extravagate to distant organs [1]. Once, the tumor cells circulate in the peripheral blood, they are called “circulating tumor cells” (CTCs) and are supposed to be a useful marker for clinical application. Studies are ongoing to investigate the value of CTC enumeration and characterization for general clinical prognosis, as a guide for treatment selection, and the treatment course (reviewed in [2]). Potential characteristics of CTCs include their size and their expression of EpCAM (epithelial cell adhesion molecule), which makes them distinguishable from the peripheral blood cells. EpCAM is a 39 to 42kDa transmembrane molecule known to be overexpressed in, for example, human insulinomas and adenocarcinomas [3] [4]. Up to now, there is only rare data available regarding the characteristics (e.g., EpCAM expression) and the number of CTCs in NEN patients. Recently, Khan and colleagues studied the potential of CTCs as prognostic markers in neuroendocrine tumors by using the CellSearch (Veridex LLC) platform to enumerate CTCs of 175 patients with metastatic NENs [5]. Regardless of the diverse patients’ clinical characteristics, Khan et al. could demonstrate CTCs of NEN patients expressing EpCAM [6] and being potentially associated with progressive disease. Presence of CTCs was associated with increased tumor burden, increased tumor grade, and elevated serum chromogranin A concentrations [5]. In parallel, we had started to investigate the existence of CTCs in NEN patients. For these experiments, n=18 NEN patients were included into the study [9 females, mean age 48 years (range, 25–80 years); 9 males, mean age 63 years (range, 48–77 years)] suffering from pancreatic and midgut NEN, pheochromocytoma, bronchopulmonary NEN, NEN of the uterus, and one NEN with unknown origin. Twelve patients showed morphological detectable metastatic spread (67%). NEN patients’ peripheral blood monocuclear cells (PBMCs) were investigated by immunohistochemical staining, defining CTCs to be CD45 negative and EpCAM positive. CTC analyses of NEN patients revealed CTC existence within the peripheral blood in 8 (44%) NEN patients (mean number of CTCs: one per 100 000 PBMC’s, [Fig. 1]). This was the case in NEN patients with pancreatic tumor, midgut tumor, pheochromocytoma, and in one NEN patient with unknown origin. An association of CTC number and metastatic spread was seen by trend as 6 out of 8 patients (75%) showed metastasis whereas 2 patients were proved to be in a nonmetastatic stage. Irrespectively of the small number of patients, our results confirm the work of Khan et al. [5] demonstrating CTC of NEN expressing EpCAM. Furthermore, the studies by Khan et al. and us represent an important research advance in the field of NENs and give a hint of the value of CTC enumeration for clinical outcome prediction. CTC measurement might be used for monitoring anticancer therapies also in NEN patients [7] [8]. In addition, further research is needed to characterize CTCs in NEN patients in detail, as was the case for lung, breast, and colon cancer [9] [10].

Fig. 1 CTCs in NEN patients. a Shows the frequency of CTCs in the peripheral blood of all investigated NEN patients (n=18) and in subgroups (grouped by the location of the primarius). b Shows a representative immunofluorescent staining for DAPI, EpCAM, and CD45 of a CTC in a NEN patient.

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