Abstract 487: A high percentage of LCNEC shows DLL3 expression in association with molecular subtypes and neuroendocrine markers

Background Pulmonary large cell neuroendocrine carcinoma (LCNEC) can be subdivided in two types: the co-mutated TP53 and RB1 subtype, and the TP53 and STK11/KEAP1 mutated subtype (Derks et al. Clin Cancer Res 2018; J Thorac Oncol 2018). DLL3 is a member of the Notch ligand family and a possible treatment target for neuroendocrine carcinoma. We investigated DLL3 and NE marker expression in mutational subtypes of metastatic LCNEC. Methods Immunohistochemical (IHC) analysis for DLL3 (clone SC16.65) was performed on 94 pathological reviewed pretreatment stage IV LCNEC. Samples were scored positive if ≥1% tumor cells showed cytoplasmic or dotlike DLL3 immunostaining. Also an H-score was calculated by multiplying intensity by percentage of positive cells. Targeted next generation sequencing (TP53, RB1, STK11, KEAP1) could be performed in 66 patients. Results DLL3 was expressed in 70/94 (75%) LCNEC, 56 of which showed cytoplasmic immunostaining. 37/70 (53%) samples had an H-score>100. DLL3 staining was more often seen in STK11 and/or KEAP1 mutated LCNEC (13/14;93%) than in tumors with wildtype STK11/KEAP1 (36/52;69%) (trend;p=0.093). In addition, DLL3 positivity was associated with expression of ≥2 NE-markers (64/79 LCNEC (81%) vs 3/8 (37%) tumors with Conclusions A high percentage (75%) of stage IV LCNEC shows cytoplasmatic DLL3 epression in association with NE markers, half of which with H-scores>100. Interestingly, DLL3 positivity was observed in almost all STK11/KEAP1 mutated LCNEC, which is in agreement with recent data (George et al. Nat Commun 2018). It thus might be worthwhile to investigate the efficacy of DLL3 targeted therapy in LCNEC. Citation Format: Ernst-Jan M. Speel, Bregtje C. Hermans, Jules L. Derks, Erik Thunnissen, Robert Jan van Suylen, Michael A. den Bakker, Harry J. Groen, Egbert F. Smit, Ronald A. Damhuis, Esther C. van den Broek, Cecile M. Stallinga, Guido M. Roemen, Anne-Marie C. Dingemans. A high percentage of LCNEC shows DLL3 expression in association with molecular subtypes and neuroendocrine markers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 487.