Abstract 7074: Expression and clinical impact of the prostate specific membrane antigen and programmed death-ligand 1 on disseminated tumor cells from early triple negative breast cancer patients

Abstract Background: Triple-negative (TN) breast cancer (BC) is the most aggressive BC subtype resulting in worse outcome. Currently, neoadjuvant chemotherapy (NACT) in combination with an immune checkpoint inhibitor is the standard of care in cases of high-risk early (e)TNBC. With regard to the unmet need of new biomarker research, we recently demonstrated that the presence of the prostate specific membrane antigen (PSMA) as well as the programmed death-ligand 1 (PD-L1) on circulating tumor cells in the blood of TNBC patients (pts) were poor prognostic factors for disease outcome. The present study aimed to investigate the presence and persistence of these markers on disseminated tumor cells (DTCs), already present in the bone marrow (BM) of eTNBC pts, to elucidate their value for current and future treatment strategies. Methods: 10-20 ml BM of 55 pts (n=55 before and n=48 after NACT; N=44 paired samples) diagnosed between 2014 and 2017, were aspirated and underwent Ficoll-Hypaque density gradient centrifugation, followed by Immunofluorescence triple staining experiments (CK/PSMA/CD45 and CK/PD-L1/CD45) and subsequent VyCAP analysis to identify PSMA and PD-L1 expression on epithelial, cytokeratin (CK)-positive DTCs. Statistical analysis was conducted using the IBM SPSS Software. Results: In general, PSMA and PD-L1 were upregulated in TNBC pts’ DTCs. Before NACT, the PSMA+CK+CD45- phenotype was identified as the most predominant one in 38/55 pts (69%) but rarely present after NACT [4/47 pts (9%); (69% vs 9%, p<0.001)]. In 3/4 pts, the PSMA+CK+CD45 phenotype persisted over the course of NACT, while only in one patient, PSMA-positive DTCs were present exclusively after therapy. The PD-L1+CK+CD45- phenotype was detected in 21/50 pts (42%) before and in 13/48 pts (27%) after NACT with a persistence of these cells in 6/21 pts. In 13/21 pts, PD-L1+CK+CD45- DTCs were only present before NACT and 6/13 pts expressed this phenotype after NACT but not at primary diagnosis. The presence of PSMA+CK+CD45- DTCs before NACT was associated with a shorter progression-free (log-rank p=0.002, HR=16.1) and overall survival (log-rank p=0.016, HR=2.1), respectively. The presence and/or persistence of PD-L1-positive DTCs did not result in any significant correlations with regard to outcome. Conclusions: PSMA-positive DTCs were identified as a poor prognostic factor with regard to outcome which confirms our data in circulating tumor cells of TNBC. The presence and persistence of PD-L1-positive DTCs is a promising biomarker to stratify pts for checkpoint inhibition and estimate its success. Citation Format: Anastasios Tolios, Vasileios Vardas, Ann-Kathrin Bittner, Oliver Hoffmann, Rainer Kimmig, Galatea Kallergi, Sabine Kasimir-Bauer. Expression and clinical impact of the prostate specific membrane antigen and programmed death-ligand 1 on disseminated tumor cells from early triple negative breast cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 7074.