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Abstract In mouse models of lung carcinoma and pulmonary breast cancer metastasis, genetic ablation of the tryptophan catabolizing enzyme IDO1 (indoleamine 2, 3-dioxygenase 1) resulted in restricted tumor outgrowth associated with attenuated induction of the inflammatory cytokine IL6 (interleukin 6). Utilizing the 4T1 breast cancer metastasis model to further elucidate the mechanistic basis underlying IDO1’s pro-tumor activity in the lungs established that IDO1 acts as an integral determinant of a tumor-promoting inflammatory state that supports neovascular retention. Ido1−/− (nullizygous) mice exhibited impaired pulmonary metastases neovascularization and outgrowth that was driven by IFNγ (interferon gamma) in the absence of IDO1-dependent IL6 production. Hypoxia was likewise elevated, and these stress conditions increased the susceptibility of the metastatic tumor cells to cytotoxicity associated with administration of GSK2656157, an inhibitor of PERK (protein kinase RNA-like ER kinase), which acts as a primary sensor for activating the UPR (unfolded protein response) survival pathway. In WT (wild type) mice with established 4T1 lung metastases, administration of the IDO1 inhibitors epacadostat, navoximod or indoximod each similarly resulted in rapid collapse of the metastatic tumor neovasculature and concomitantly increased intratumoral hypoxia and sensitivity to PERK inhibition. Therefore, while IDO1 inhibition does not directly promote effective tumor cell killing in this context, the disruption of blood supply due to IFNγ-mediated neovascular regression subjects the tumor cells to hypoxic environmental stress, raising the possibility for therapeutic intervention with selectively targeted agents. Combining chemotherapy with immune checkpoint blockade (ICB) is recognized as an effective strategy for treating lung cancer, with first-line therapy for some patients including the alkylating agent carboplatin in conjunction with pembrolizumab. The challenge remains, however, that chemotherapy causes systemic side effects that can negatively impact quality of life and limit efficacy. Evofosfamide is an alkylating prodrug that was designed to be selectively activated in regions of hypoxia, thereby limiting systemic exposure. We report that enhanced metastatic tumor cell killing with evofosfamide can be achieved by co-administration of an IDO1 inhibitor. Elevated intratumoral hypoxia has also previously been linked to increased expression levels of PDL1, a favorable indicator of ICB responsiveness, and we have confirmed the upregulation of PDL1 in 4T1 lung metastases following IDO1 inhibitor administration. Our data in the lung metastasis model support future studies to evaluate whether IDO1 inhibition can be effectively combined with evofosfamide and anti-PD1 treatment to improve therapeutic outcomes. Citation Format: Shih-Chun Shen, Souvik Dey, James B. DuHadaway, Erika Sutanto-Ward, George C. Prendergast, Alexander J. Muller. IDO1 inhibition enables IFNγ-elicited responsiveness of pulmonary breast cancer metastases to hypoxia-directed tumoricidal agents abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 729.
Shen et al. (Fri,) studied this question.