Characterization of DLL3-positive circulating tumor cells (CTCs) in patients with small cell lung cancer (SCLC) and evaluation of their clinical relevance during front-line treatment

The aim of the study was to characterize and evaluate the presence of DLL3-positive Circulating Tumor Cells (CTCs) in SCLC patients receiving front-line chemotherapy and assess their clinical relevance.Peripheral blood was obtained from treatment-naïve patients with SCLC (n = 108 patients), after one etoposide/platinum cycle (n = 68 patients) and on disease progression (n = 48 patients). Immunofluorescence staining using antibodies against the DLL3, cytokeratins (CK), CD45 and vimentin (Vim) was used for the detection and characterization of CTCs.Before treatment, 74.1% of patients had detectable DLL3+/CD45- CTCs. One-treatment cycle significantly decreased both the detection rate (p < 0.001) and the absolute number (p < 0.001) of DLL3+/CD45- CTCs. Triple immunofluorescence staining using anti-CK, anti-Vim and anti-DLL3 antibodies revealed an important CTC heterogeneity since DLL3 could be detected in Vim+, Vim-, CK+ and CK- CTCs. On disease progression, both the detection rate and the absolute number of DLL3+/CD45- CTCs were significantly increased compared to post-1st cycle values (p < 0.001 and p = 0.002, respectively). In addition, 22.7% of patients had detectable DLL3+/CD45- cells which could not be captured by the CellSearch assay. In multivariate analysis, the detection of DLL3+/CD45- CTCs at baseline was significantly associated with decreased progression-free survival (HR = 10.8; p = 0.005) whereas their detection on disease progression was associated with decreased overall survival (HR: 28.2; p = 0.016).These findings demonstrate an important heterogeneity of CTCs, based on the expression of CK, Vim and DLL3, in patients with SCLC and the changes of DLL3+/CD45- CTCs during treatment seem to be a dynamic biomarker associated with patients' clinical outcome.