Abstract Background: Delta-like Ligand 3 (DLL3), a Notch receptor ligand, is highly expressed in small cell lung cancer (SCLC) and has emerged as a promising therapeutic target. Circulating tumor cells (CTCs), abundant in SCLC, interact with diverse immune cell populations in peripheral blood. This study investigates the potential of DLL3 as a biomarker to enhance CTC detection and monitor immunotherapy response in SCLC. Methods: We enrolled 17 patients with extensive-stage (ES) SCLC and 5 with stage IV non-small cell lung cancer (NSCLC; as controls), all scheduled for an immune checkpoint inhibitor or a bispecific T-cell engager. Whole blood (6 mL) was collected before and during treatment. CTCs were enriched using CTCeptorTM, a fully automated, negative depletion-based continuous centrifugal microfluidic system, and classified into three phenotypes: pan-cytokeratin (CK)+/DLL3-/CD45-, CK-/DLL3+/CD45-, or CK+/DLL3+/CD45-. Baseline tumor samples were analyzed by immunohistochemistry for SCLC molecular subtypes and DLL3. Results: DLL3+/CD45- CTCs were detected in all ES-SCLC patients (100%) prior to treatment with atezolizumab plus chemotherapy or tarlatamab, with a median count of 23 (range 7-121) and a median size of 9.71 μm (7.33-12.18). In contrast, no DLL3+/CD45- CTCs were detected in NSCLC patients, where only CK+/DLL3-/CD45- CTCs were observed (1-4 cells). Among ES-SCLC patients, CK-/DLL3+/CD45- cells were the predominant subtype (63.3%, 522/825), while CK+/DLL3+/CD45- CTCs accounted for 27.7% (228/825) of all detected CTCs, representing additional CTCs not captured by CK-based detection alone. ASCL1 was the most prevalent molecular subtype (12/17), with DLL3 exhibiting the highest H-score (median, 185; range, 20-300). Notably, no correlation was observed between the number or proportion of DLL3+CTC and the DLL3 H-score (r = -0.163, p = 0.578), while DLL3+CTCs remained prevalent in NERUDO1 and POU2F3 subtypes. Furthermore, both total and DLL3+CTC counts markedly decreased within 2 cycles of atezolizumab chemotherapy or 1 week after tarlatamab treatment. Conclusion: DLL3+CTCs can be efficiently and specifically detected in ES-SCLC patients using CTCeptorTM, serving as a complementary marker to CK for CTC identification. DLL3 appears more frequently in CTCs than in tumor tissue. These findings support DLL3 as a potential biomarker for CTC-based monitoring in ES-SCLC and warrant further validation in larger cohorts, including analyses of immunologic features. Citation Format: Hyung-Joo Oh, Hyun-Ju Cho, Seung-Hee Song, Yoo-Duk Choi, Woo-Jae Son, Jin-Han Bae, In-Jae Oh, Young-Chul Kim, Hae Ung Lee, Cheol-Kyu Park. Delta-like ligand 3 (DLL3)-based enrichment enables sensitive and specific detection of circulating tumor cells during immunotherapy in small-cell lung cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1071.
Oh et al. (Fri,) studied this question.