Phenotypic characterization of circulating tumor cells in the peripheral blood of patients with small cell lung cancer

Background To evaluate the phenotypic heterogeneity of circulating tumor cells (CTCs) based on the expression of proliferative, apoptotic and Epithelial-to-Mesenchymal Transmission (EMT) markers during front-line treatment in patients with small cell lung cancer (SCLC) and to evaluate their clinical relevance. Methods CTCs from 108 chemotherapy-naive patients with SCLC were analyzed by double immunofluorescence staining using anti-Ki67, anti-M30, anti-Vimentin along with anti-CKs antibodies. In 83 patients CTCs were also enumerated using the CellSearch. Results Sequential samples were available from 76 and 48 patients after one-treatment cycle and on disease progression (PD), respectively, for immunofluorescence and from 50 and 36 patients after one-cycle and on PD, respectively, for CellSearch. At baseline, 60.2% of the patients had detectable CTCs by either method. Both proliferative (CK67(+)) and non-proliferative (Ki67(-)), apoptotic (M30(+)) and non-apoptotic (M30(-)) as well as EMT (Vim(+)) CTCs were present in the same patient. Among 22 patients without detectable CTCs by CellSearch, CK+/Ki67(+) and CK+/Vim(+) CTCs could be detected in 6 (27.3%) and 6 (27.3%) patients, respectively. One-chemotherapy cycle reduced both the incidence of detection (p<0.001) and the absolute number (p<0.001) of CTCs; conversely, on PD both the incidence of detection and the number of CTCs were significantly increased (p = 0.002 and p = 0.04, respectively). Multivariate analysis revealed that the increased number of Vim(+) CTCs at baseline and of non-apoptotic CTCs on PD could be emerged as independent prognostic factors associated with decreased OS(p = 0.009 and p = 0.023, respectively). Conclusions CK+/Ki67(+), CK+/M30(+) and CK+/Vim(+) CTCs represent distinct subpopulations of CTCs in patients with SCLC, can be detected even in the absence of detectable CTCs by Cell-Search; CK+/Ki67(+) and CK+/Vim(+) CTCs are associated with unfavorable clinical outcome.