e22030 Background: Metastatic spread of cancer cells is a key event in tumor progression and in determining the prognosis of patients with malignant disease. This study evaluated the expression of prognostic and predictive molecules in primary and metastatic gastric cancer. Our objectives were to correlate the expression pattern with the clinicopathological features and patient survival and confirm the results using a coculture spheroid model. Methods: The clinicopathological features of 94 patients (57,4% male, 42,6% female, median age of 66,2 years) with gastric cancer were characterized using multivariate and univariate analysis. Further we compared the expression profile of biomarkers, namely EPCAM, CD44s, HLA-ABC and the associated β chain, HLA-DR, ICAM-1, LFA-3 and E-Cadherin in primary gastric tumors (n=94) and their synchronous metastases (regional lymph nodes n=32, liver n=14, peritoneum n=17) using immunoperoxidase staining. A coculture in vitro model, i.e. the homo- and heterotypic spheroid model were performed as a functional test system to confirm the microenvironmental impact on the biomarker expression level. Results: Strong HLA-ABC expression was found in both primary and metastatic tumors. High expression of HLA-ABC in liver metastases significantly correlated with metastatic disease (UICC TNM stage IV, p=0,026). Contrary, the detection of HLA-DR on the cancer cells correlated with the degree of the inflammatory infiltrate (CD45, p=0,019). With respect to CD44s, the prognostic marker was upregulated in the metastatic tumors independently of their localisation (p=0.009) compared to the primary lesions. E-cadherin and LFA-3 expression were preferentially increased in distant metastases, but not in locally advanced gastric tumors (liver: E-cadherin, p=0.001; liver and peritoneum: LFA-3, p=0.0067). E-cadherin expression was upregulated in the heterotypic spheroid model coculturing primary liver cells and N87 gastric cancer cells (80% E-cadherin positive N87 cells) compared to the homotypic spheroid model consisting of N87 cells alone (40% E-cadherin positive N87 cells). Conclusions: These data suggest the consideration of the metastatic protein profile for the selection of cancer patients in new molecular therapeutic strategies. No significant financial relationships to disclose.