Gastric and esophagogastric junction cancers are heterogeneous and aggressive tumors with an unpredictable response to cytotoxic treatment. New methods allowing for the analysis of drug resistance are needed. Here, we describe a novel technique by which human tumor specimens can be cultured ex vivo, preserving parts of the natural cancer microenvironment. Using a tissue chopper, fresh surgical tissue samples were cut in 400 μm slices and cultivated in 6‐well plates for up to 6 days. The slices were processed for routine histopathology and immunohistochemistry. Cytokeratin stains (CK8, AE1/3) were applied for determining tumor cellularity, Ki‐67 for proliferation, and cleaved caspase‐3 staining for apoptosis. The slices were analyzed under naive conditions and following 2–4 days in vitro exposure to 5‐FU and cisplatin. The slice culture technology allowed for a good preservation of tissue morphology and tumor cell integrity during the culture period. After chemotherapy exposure, a loss of tumor cellularity and an increase in apoptosis were observed. Drug sensitivity of the tumors could be assessed. Organotypic slice cultures of gastric and esophagogastric junction cancers were successfully established. Cytotoxic drug effects could be monitored. They may be used to examine mechanisms of drug resistance in human tissue and may provide a unique and powerful ex vivo platform for the prediction of treatment response.

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76 Background: GC and AEG have an unpredictable response to cytotoxic treatment and a poor prognosis. There is an urgent need for new research methods allowing for the determination of chemotherapy sensitivities, the analysis of resistance mechanisms and tumor heterogeneity. Here, we describe a novel technique extending our recent findings in other tumors (Gerlach et al. 2014; Merz et al. 2013), by which cancer specimens can be cultured in vitro and maintained in their natural micro-environment. Methods: Using a tissue chopper, fresh surgical and endoscopic tissue samples from GC and AEG were cut in 400 µm thick slices and cultivated in 6-well plates for up to 6 days. The slices were then fixed, embedded in paraffin and cut for routine histopathology and immunohistochemistry. Cytokeratin stains (CK8 and AE1/3) were used for determining tumor cellularity, ki-67 for proliferation, and cleaved caspase 3 staining for apoptosis. The slices were examined under naïve condition and following in-vitro exposure to 5-FU, cisplatin or docetaxel over a period of 2-4 days. Results: GC and AEG slice cultures from resection specimens (n=14) and endoscopic biopsies (n=17) revealed a good preservation of tissue morphology and tumor cell integrity during the culture period in most cases. The stroma and the tumor cellularity remained stable over at least 4 days, proving the viability of cancer in slice cultures. The amount of sampled tissue from endoscopic biopsies was identified as a critical determinant for the feasibility of slice cultures. During treatment of cultures with chemotherapy, a significant loss of tumor cellularity and an increase of apoptotic cells were observed, although a systematic and reproducible read-out still needs to be established. Conclusions: Slice cultures of GC and AEG were successfully established. They can be expected to provide a unique and powerful in vitro platform for the determination of sensitivities of a given tumor towards chemotherapy, to examine mechanisms of drug-resistance and to analyze tumor heterogeneity in patient samples.