Expression of the Prostate Specific Membrane Antigen (PSMA) and Programmed Death-Ligand 1 (PD-L1) on Disseminated Tumor Cells from Early Triple Negative Breast Cancer Patients and their Clinical Impact

Background Triple-negative breast cancer (TNBC) is the most aggressive breast cancer (BC) subtype, with a worse outcome, highlighting the need for novel biomarkers to treat patients accordingly. This study aimed to evaluate the presence of prostate-specific membrane antigen (PSMA) and programmed death-ligand 1 (PD-L1) on DTCs in the bone marrow of TNBC patients before and after neo-adjuvant chemotherapy, to provide critical insights into their prognostic and therapeutic potential. Methods We investigated the expression of PSMA and PD-L1 in disseminated, cytokeratin (CK)-positive tumor cells (DTCs) from the bone marrow of TNBC patients before (55 patients) and after (47 patients) neo-adjuvant treatment using triple immunofluorescence stainings and VyCAP platform analysis. A weighted scoring system was developed to quantify PSMA and PD-L1 expression on DTCs, categorizing patients into negative, low/intermediate, and high expression groups. Statistical analyses included Spearman’s correlation, Wilcoxon signed-rank, chi-square, and McNemar’s tests, with significance set at p  < 0.05. Results At baseline, 69% of patients exhibited CK + PSMAhighCD45- DTCs, which significantly decreased to 9% post-therapy ( p  < 0.001). Similarly, CK + PD-L1highCD45- DTCs declined from 43–27% after therapy ( p  = 0.045). Based on our scoring system, most patients initially classified as PSMA-high transitioned to PSMA-negative (n = 20, p  < 0.001), while only one patient remained PSMA-high post-therapy. Post-NACT, a strong inverse correlation emerged between CK + PD-L1highCD45- and CK + PSMA-CD45- cells ( p  = 0.010), and dual-positive phenotypes decreased to 4% ( p  < 0.001). Presence of PSMA + DTCs at diagnosis was associated with shorter progression-free ( p  = 0.002, HR = 16.1) and overall survival ( p  = 0.016, HR = 2.1). Patients transitioning from PSMA-high to PSMA-negative post-therapy had improved overall survival compared to those remaining PSMA-high ( p  < 0.001, HR = 1). Conclusions PSMA and PD-L1 are frequently overexpressed on DTCs of TNBC patients, predominantly before the administration of NACT. PSMA-positive DTCs emerged as a significantly poor prognostic factor, holding promise as a biomarker for identifying individuals at higher risk of relapse. It also provides a potential therapeutic target. Similarly, the presence and persistence of PD-L1-positive DTCs suggests their utility as biomarkers to stratify patients for ICI therapies.