Introduction: Glutathione S-transferase 1 (GSTP1) has been reported to function as tumor suppressor gene in various types of human cancers. GSTP1 inactivation is associated with CpG island promoter hypermethylation in the majority of prostate cancers (PCs). The initiation of minimal residual disease (MRD) and especially the detection of circulating tumor cells (CTCs) in patients’ peripheral blood represents a negative prognostic parameter for recurrence-free survival. The aim of the present study was to assess the methylation status of the GSTP1 gene in CTCs that were isolated, using the CellCollector® (GILUPI, GmbH), a novel clinical device designed for the in vivo isolation of EpCAM-positive CTCs. Patients and methods: In-vivo isolation of CTCs was performed by using CellCollector® from high-risk prostate cancer patients (n=97) and 20 healthy volunteers. For all these patients, the Ab coated region of the CellCollector® was washed in PBS , cut, and stored in Trizol reagent till analysis and DNA was further prior to the analysis isolated DNA was modified by sodium bisulfite (SB) and subjected to a real time MSP assay specific for GSTP1 methylation. In all cases, peripheral blood was also collected and used for CTC analysis by Immunostaining and the CellSearch® system. Results: All DNA samples were first checked for their quality. Based on the quality evaluation of all available DNA samples, only 63 DNAs were further qualified for analysis. GSTP1 promoter was found methylated in 12/63 (19%) the EpCAM positive fraction of in-vivo isolated CTCs. Moreover, in 5/12 (41.7%) patients for which GSTP1 promoter was found methylated, CTCs were also detected by the CellSearch® and 7 /12 (58.3%) for which GSTP1 promoter was also found positive, CTCs were also detected by the Immunostaining. Conclusion: GSTP1 promoter is methylated in in-vivo isolated CTCs from high-risk prostate cancer patients. GSTP1 promoter methylation in in-vivo isolated CTCs should be prospectively validated as a novel tumor biomarker for prostate cancer patients in a large cohort of patients. Acknowledgements: This research has been co-financed by the European Union (European Regional Development Fund - ERDF) and Greek national funds through the Operational Program ‘‘Competitiveness and Entrepreneurship’’ of the National Strategic Reference Framework (NSRF) - Research Funding Program: “ERA-NET on Translational Cancer Research (TRANSCAN) Joint Transnational Call for Proposals 2011 (JTC 2011) on: “Validation of biomarkers for personalised cancer medicine

Citation Format: Athina N. Markou, Panagiotis Paraskevopoulos, Marifili Lazaridou, Shukun Chen, Thomas Kroneis, Monika Świerczewska, Joanna Budna, Andra Kuske, Tobias M. Gorges, Maciej Zabel, Peter Sedlmayr, Catherine Alix-Panabieres, Klaus Pantel, Evi Lianidou. GSTP1 promoter methylation in in-vivo isolated CTCs from high-risk prostate cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1716. doi:10.1158/1538-7445.AM2017-1716