Abstract 4420: Identification of a novel marker gene predicting recurrence after chemotherapy with DNA-damaging agents by using drug-resistant gastric cancer organoids

Abstract Despite recent advances in gastric cancer therapy, chemotherapy resistance and lack of methods for selecting combination regimens remain major problems. Organoids, which recapitulate the in vivo micro-environment, can be established from surgical specimens with a high success rate, and are widely used for drug sensitivity assays. Cancer organoids are also known to contain abundant amounts of cancer stem cells (CSCs), a subpopulation of tumor cells with self-renewal and differentiation capabilities. CSCs are thought to contribute to tumor initiation, metastasis, and acquiring resistance to chemotherapy by altering their phenotype during the treatment.In this study, we aimed to discover a novel biomarker for predicting multidrug resistance using gastric cancer organoids (GCOs), which provide in vivo-like culture system that more closely resembles tumor cell organization than traditional cell lines, to understand the mechanism of multidrug resistance in cells, including CSCs, under in vivo conditions and to identify novel biomarkers to predict chemoresistance.We evaluated 5-fluorouracil or oxaliplatin-resistant GCOs to find novel biomarkers that reflect multidrug resistance in gastric cancer. To examine the resistance mechanisms, RNA-seq analysis and ex vivo drug sensitivity testing were performed. The association of biomarkers with patient prognosis and chemotherapy efficacy was evaluated using three original cohorts with a total of 230 cases. The results were also validated with two independent public cohorts and a single-cell RNA sequence data. Increased expression of PI3/Elafin was detected in all 5-fluorouracil or oxaliplatin-resistant GCOs. Our findings suggest a potential association of PI3/Elafin expression with ribosome biosynthesis and RNA metabolism under organoid conditions. We also found that PI3/Elafin overexpression promotes 5-fluorouracil/oxaliplatin/cisplatin resistance but not paclitaxel resistance. Immunohistochemical evaluation of Elafin expression revealed that the Elafin-positive gastric cancer group had a poorer outcome, especially in terms of time-to-recurrence. Elafin positivity was also an independent predictor of relapse after chemotherapy with DNA-damaging agents. PI3/Elafin promotes DNA-damaging drug resistance through multiple downstream regulations related to RNA and ribosomal metabolism. We demonstrated that PI3/Elafin contributes to the acquisition of resistance to 5-fluorouracil and platinum drugs in in vivo-like environments and identified GOs related to ribosome and RNA metabolism as novel PI3/Elafin downstream targets. Our results provide new insights into multidrug resistance mechanisms and highlight PI3/Elafin as a novel tool for precise stratification of patients with gastric cancer. Citation Format: Kenji Harada, Naoya Sakamoto, Shingo Sakashita, Motohiro Kojima, Kazuaki Tanabe, Hideki Ohdan, Kohei Shitara, Takahiro Kinoshita, Genichiro Ishii, Wataru Yasui, Atsushi Ochiai, Shumpei Ishikawa. Identification of a novel marker gene predicting recurrence after chemotherapy with DNA-damaging agents by using drug-resistant gastric cancer organoids [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4420.