Abstract P3-01-17: PD-L1 expression on circulating epithelial tumor cells (CETCs) correlates with aggressiveness of tumor in breast cancer patients

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Abstract Background: Strategies to improve the efficacy of the immune system against malignant tumors represent a major innovation focusing on the programmed death-1 receptor (PD-1), with its two ligands PD-L1 and PD-L2. The expression of PD-L1 has been evaluated in a number of different tumor types and can be used as a predictive biomarker for PD-1/PD-L1 checkpoint inhibitor treatment response. We used a non-invasive, real-time liquid biopsy to better characterize PD-L1 expression on circulating epithelial tumor cells (CETCs) in breast cancer patients. Methods: CETCs were determined from blood of 72 patients suffering from breast cancer. The number of vital CETCs and their expression of PD-L1 was investigated using the maintrac® method. Results: PD-L1 expressing CETCs were detected in 94.5 % of breast cancer patients. Breast cancer patients with metastatic disease had significantly more PD-L1 positive CETCs as compared to patients without metastasis (median 75% vs. 61.1%; p<0.05). Furthermore, patients with positive lymph node status had significantly more PD-L1 positive CETCs as compared to patients with negative lymph nodes (median 60% vs. 93%, p<0,05). Moreover, we observed a significant heterogeneity in PD-L1 immunostaining intensity across CETCs from the same patients. Conclusion: Breast cancer patients have detectable CETCs with a high frequency of PD-L1 which correlates with progression of cancer disease. PD-L1 seems to be a major factor in immune evasion and may be a promising target of anticancer therapies. Monitoring the frequency of PD-L1 positive CETCs could reflect individual patient's response to an anti-PD-1/PD-L1 therapy. Citation Format: Schott D, Pizon M, Pachmann U, Pachmann K. PD-L1 expression on circulating epithelial tumor cells (CETCs) correlates with aggressiveness of tumor in breast cancer patients [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-01-17.