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Background: Aberrant activation of Wnt/β-catenin signaling plays an important role in both carcinogenesis and modulation of the tumor microenvironment. E7386, an investigational anti-cancer agent, inhibits the binding of β-catenin to its transcriptional co-activator, CREB binding protein (CBP), thereby modulating Wnt/β-catenin signaling. We previously reported that E7386 suppressed tumor growth in preclinical mouse models with aberrant activation of Wnt/β-catenin signaling. Moreover, the combination of E7386 plus lenvatinib, a multiple receptor tyrosine kinase inhibitor that mainly targets vascular endothelial growth factor receptors and fibroblast growth factor receptors, showed stronger antitumor and antiangiogenic activities than each treatment alone. Here, we investigated the molecular mechanism of the combination activities of E7386 plus lenvatinib. Methods: We examined antitumor activity of E7386 plus lenvatinib in multiple human tumor xenograft and mouse syngeneic models. Tumor samples underwent RNA sequencing (RNA-seq) followed by weighted gene correlation network analysis. Furthermore quantitative PCR was used to assess changes in the expression of some hypoxia-related genes. The protein interaction between β-catenin and HIF1 was analyzed by immunoprecipitation-western blotting. Results: RNA-seq analysis of the effects of E7386, lenvatinib, and E7386 plus lenvatinib on the tumor microenvironment in the Hepa1-6 hepatocellular carcinoma syngeneic tumor model in mice showed that lenvatinib suppressed the expression of angiogenesis-related genes and that E7386 plus lenvatinib enhanced this suppression. Although lenvatinib increased the expression of hypoxia-related genes, E7386 plus lenvatinib attenuated this phenomenon. According to these results, we hypothesized that E7386 inhibits the formation of the β-catenin/HIF-1 complex. To confirm the effect of E7386 on the hypoxic response, we conducted an in vitro quantitative PCR assay under hypoxic conditions in Hepa1-6 cells and the human hepatocellular carcinoma cell lines, HepG2 and Hep3B. In accordance with the RNA-seq result, E7386 significantly suppressed the expression levels of hypoxia-related genes in the HepG2 and Hep3B cells. Finally, data from immunoprecipitation-western blotting demonstrated that β-catenin formed a complex with HIF-1 and that E7386 clearly inhibited this complex formation in HepG2 cells. Conclusion: These results suggest that E7386 enhances angiogenesis inhibition when used in combination with lenvatinib and that E7386 suppresses the hypoxic response induced by angiogenesis inhibition. Further analysis is required to clarify the precise mechanisms of actions of the combination of E7386 plus lenvatinib. Citation Format: Mizuki Kimura, Yusaku Hori, Megumi Kuronishi, Takayuki Kimura, Ryoga Ishida, Kenji Ichikawa, Yoichi Ozawa, Yu Kato. E7386, a CREB binding protein (CBP)/β-catenin interaction inhibitor, suppresses the hypoxic response induced by angiogenesis inhibition in hepatocellular carcinoma models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1437.