Abstract B042: Molecular assessment of cell surface targets using integrated circulating tumor cell (CTC) RNAseq and single CTC phenotyping

Abstract Background: Androgen-targeted therapies are a mainstay of treatment for metastatic prostate cancer and have significantly improved patient outcomes. However, development of treatment resistance remains universal, occurring through androgen receptor (AR) alterations driving constitutive AR signaling, or lineage state transitions that bypass AR entirely and culminate in a small cell/neuroendocrine phenotype (NEPC). Cell surface targeted therapies, including antibody drug conjugates and targeted radioligand therapies, represent a new therapeutic class that has shown promise for treatment of androgen-resistant metastatic prostate cancer (mCRPC). Circulating tumor cells (CTCs) present an accessible source of tumor material to identify expression of known and novel targets for therapeutic development, as well as the unique potential for longitudinal profiling to understand the evolution of target expression and association with clinical resistance to cell surface targeted therapies. We have developed a CTC platform allowing for gene expression and protein profiling of cell surface targets on CTCs. Methods: Live CTCs were isolated with our automated microfluidic technology integrating negative and positive selection for CTC enrichment with a cohort of 273 samples. CTCs were captured immunomagnetically followed by RNA isolation on chip and RNA-seq, or protein staining on chip for single cell fluorescent quantification of prostate adenocarcinoma and NEPC cell surface targets including PSMA, TROP2, B7H3 and DLL3. Results: CTC RNA sequencing can be used to understand the expression of cell surface targets across different CTC phenotypes. In CTCs from patient with mCRPC, canonical prostate adenocarcinoma cell surface targets STEAP1, KLK2 and FOLH1 and epithelial cell surface target TACSTD2 had the highest expression, while prostate adenocarcinoma target STEAP2, pan-tumor target ERBB2, and tumor immune checkpoint cell surface protein CD276 (B7H3) had intermediate expression. Expression of DLL3 and SSTR2, targets associated with neuroendocrine differentiation, was lowest, though still detected in a subset of CRPC CTCs. Comparison of expression levels between CRPC and NEPC CTCs demonstrated lower expression of most adenocarcinoma cell surface targets in NEPC CTCs as expected, and a trend towards higher expression of DLL3 and SSTR2. Cell surface expression is being assessed for proteins including Trop-2, PSMA and DLL3. Evaluation of DLL3 at the protein level in a subset of patients including both CRPC (n=13) and NEPC (n=12) confirmed that DLL3 was detected in both adenocarcinoma and neuroendocrine CTCs, with a median of 45.8% DLL3-positive CTCs per sample. Conclusions: Biomarkers are needed to better predict response and resistance to targeted therapies. CTC RNAseq and single CTC phenotyping hold potential for predictive and on-treatment biomarkers of response to cell surface targeted therapies. Further study of mechanisms driving response and resistance can be evaluated with these biomarkers. Citation Format: Jamie M. Sperger, Marina N. Sharifi, Katherine R. Kaufmann, Matthew L. Bootsma, Shannon R. Reese, Viridiana Carreno, Alex H. Chang, Luke A. Nunamaker A. Nunamaker, Charlotte Linebarger, Amy K. Taylor, Katherine E Tippins, Kyle T. Helzer, Shuang G. Zhao, Joshua M Lang. Molecular assessment of cell surface targets using integrated circulating tumor cell (CTC) RNAseq and single CTC phenotyping [abstract]. In: Proceedings of the AACR Special Conference: Liquid Biopsy: From Discovery to Clinical Implementation; 2024 Nov 13-16; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2024;30(21_Suppl):Abstract nr B042.