e20086 Background: 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. Methods: 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). CTCs were detected after immunofluorescence staining using antibodies against the DLL3, cytokeratins (CK), CD45 and vimentin (Vim). Results: Before treatment, 74.1% of patients had detectable CTCs (DLL3⁺/CD45^-). One-treatment cycle significantly decreased both the detection rate of DLL3⁺/CD45^- CTCs ( p< 0.001) and their absolute number ( p< 0.001). 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 of DLL3⁺/CD45^- CTCs as well as their number were significantly increased compared to post-1^st 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). Conclusions: 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 treatment efficacy and patients’ clinical outcome.