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-naïve 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 CellSearch; CK(+)/Ki67(+) and CK(+)/Vim(+) CTCs are associated with unfavorable clinical outcome.