Abstract LB_B05: Mapping therapy resistant phenotypes in SCLC liquid biopsies with single-cell proteomics at a personalized level
Authors: Bhattacharyya Sayantan , Stewart Catherine A , Victorian Ashley M , Ehsan Shafqat F , Lujan Ester F , Duarte Alberto , Heymach John V , Wang Jing , Byers Lauren A , Karacosta Loukia G 
Published: December 2023

Abstract
Small-cell lung cancer (SCLC) accounts for ~15% cases of all lung cancer cases and is characterized by aggressive growth and dismal prognosis. Although patients initially respond favorably to platinum and immune checkpoint blockade therapy, most develop therapy resistance and relapse within 1 year. Leveraging liquid biopsies and circulating tumor cells (CTCs) for evaluating SCLC status is regarded as a promising approach especially in SCLC where biopsies are scarce. SCLC CTC numbers are higher compared to other cancers, presenting a unique opportunity to utilize them for assessing SCLC therapy response and resistance and identifying new targetable markers. In this study we leverage mass cytometry (i.e., CyTOF), a multiplex cytometric approach that enables interrogation of 30-50 protein markers per single cell, allowing us to identify and analyze a significant number of CTCs and their heterogeneous expression profiles. Our goal was to utilize CyTOF to help us map changes in CTC expression profiles in the treatment naïve and relapsed setting and thus be able to assess therapy response and resistance at a personalized level. To achieve this, we optimized a CyTOF antibody panel by using SCLC cell lines and blood specimens from healthy donors and SCLC patients. We show that by using this approach, we are able to identify and phenotype SCLC CTCs by using the combination of CD45 (negative expression) and CD56 positive expression with further validation achieved by TTF1 and SCLC subtype transcription factor expression (e.g. NEUROD1, POU2F3). This approach avoids enriching for specific CTC epithelial phenotypes that express surface markers that fluctuate during epithelial-mesenchymal transition (EMT) (e.g. EpCAM, a common marker for isolating CTCs). We also evaluated markers that describe immune suppression activity (PD-L1), EMT status (E-Cadherin, Vimentin, MUC1, Slug), as well as markers that can be therapeutically targeted/are indicative of resistance (e.g., p-YAP). For parallel immune profiling we assessed among others CD8/CD4 (T-cytotoxic/T-helpers respectively) and CD11b and HLA-DR (Myeloid-derived suppressor cells, MDSCs) expression. We found that p-YAP and MUC1 are significantly increased in CTCs detected in blood specimens from SCLC relapsed patients compared to treatment naïve patients. Both YAP and MUC1 have been shown to be play a role in therapy resistance and YAP inhibition is considered a promising therapeutic target. Our analysis also showed a significant increase of MDSC (CD33+CD11b+HLA-DR-) percentages and a decrease in the CD4/CD8 T cell ratio in the relapsed setting. Finally, when we used PHENOSTAMP, a previously developed reference EMT map for assessing EMT phenotypes in lung cancer clinical specimens at a personalized level, we observed an increase of EMT heterogeneity in CTCs in relapsed patients. Our data highlight the translational power of our approach in tracking therapy resistant cells during therapy directly in SCLC patient liquid biopsies towards assessing therapy resistance at a personalized level.
Citation Format: Sayantan Bhattacharyya, Catherine A Stewart, Ashley M Victorian, Shafqat F Ehsan, Ester F Lujan, Alberto Duarte, John V Heymach, Jing Wang, Lauren A Byers, Loukia G Karacosta. Mapping therapy resistant phenotypes in SCLC liquid biopsies with single-cell proteomics at a personalized level [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr LB_B05.