IDO (indoleamine 2,3-dioxygenase) enzyme inhibitors have entered clinical trials for cancer treatment based on preclinical studies indicating that they can defeat immune escape and broadly enhance other therapeutic modalities. However, clear genetic evidence of IDO’s impact on tumorigenesis in physiologic models of primary or metastatic disease is lacking. Investigating the impact of Ido1 gene disruption in mouse models of oncogenic KRAS-induced lung carcinoma and breast carcinoma-derived pulmonary metastasis, we have found that IDO-deficiency resulted in reduced lung tumor burden and improved survival in both models. Micro-CT imaging further revealed that the density of the underlying pulmonary blood vessels was significantly reduced in Ido1-nullizygous mice. During lung tumor and metastasis outgrowth, IL6 induction was greatly attenuated in conjunction with the loss of IDO. Biologically, this resulted in a consequential impairment of pro-tumorigenic MDSCs (myeloid-derived suppressor cells), as restoration of IL6 recovered both MDSC suppressor function and metastasis susceptibility in Ido1-nullizygous mice. Together, our findings define IDO as a prototypical integrative modifier that bridges inflammation, vascularization and immune escape to license primary and metastatic tumor outgrowth.

β-lapachone is a naturally occurring 1,2-naphthoquinone-based compound that has been advanced into clinical trials based on its tumor-selective cytotoxic properties. Previously, we focused on the related 1,4-naphthoquinone pharmacophore as a basic core structure for developing a series of potent indoleamine 2,3-dioxygenase 1 (IDO1) enzyme inhibitors. In this study, we identified IDO1 inhibitory activity as a previously unrecognized attribute of the clinical candidate β-lapachone. Enzyme kinetics-based analysis of β-lapachone indicated an uncompetitive mode of inhibition, while computational modeling predicted binding within the IDO1 active site consistent with other naphthoquinone derivatives. Inhibition of IDO1 has previously been shown to breach the pathogenic tolerization that constrains the immune system from being able to mount an effective anti-tumor response. Thus, the finding that β-lapachone has IDO1 inhibitory activity adds a new dimension to its potential utility as an anti-cancer agent distinct from its cytotoxic properties, and suggests that a synergistic benefit can be achieved from its combined cytotoxic and immunologic effects.

First identified as a mediator of acquired immune tolerance of the ‘foreign’ fetus from maternal immunity, the tryptophan-catabolizing enzyme IDO (indoleamine 2,3-dioxygenase) has since been implicated in tumor escape from the host immune system. Insight into the intricate role of IDO in the classical DMBA/TPA skin carcinogenesis model suggested that inflammatory tumor environments can induce IDO production resulting in de novo tumor development. In the genetically deficient model of IDO, mice show resistance to tumor formation. This provided a basis for our current studies exploring the importance of IDO in the microenvironment of the lung. To this end, we have investigated both primary tumor formation and metastatic disease in the lungs of IDO-deficient mice using the KRAS-induced lung adenocarcinoma and the metastatic 4T1 breast cancer models. Elevation of the inflammatory cytokine IL6 was associated with tumor outgrowth in the lungs in both models but was greatly attenuated with the loss of IDO, consistent with the in vitro demonstration that IDO activity markedly potentiates IL6 production. MDSCs (myeloid derived suppressor cells) exhibited reduced T cell suppressive activity when isolated from tumor-bearing, IDO-deficient animals that could be rescued by ectopic production of IL6 in the tumor. IL6 production could likewise reverse the pulmonary metastasis resistance exhibited by IDO-deficient mice. Interestingly, while there is a clear role of the immune system in lung tumor and metastatic outgrowth, IDO-deficient mice appear to have reduced vascularization in the lung which may partly contribute to reduced tumor formation. Together, these findings genetically validate IDO as a therapeutic target in the settings of pulmonary cancer and metastasis and establish the importance of IDO as a driver of IL6 production and MDSC function. Furthermore, the correlation of IDO to angiogenesis may be a new insight into the role of this enzyme in cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 295. doi:1538-7445.AM2012-295

The tryptophan-catabolizing enzyme IDO1 (indoleamine 2,3-dioxygenase 1) has been implicated as a mediator of immune tolerance in the reproductively essential process of shielding the ‘foreign’ fetus from maternal immunity. By extrapolation, we and others hypothesized that tumors might elevate IDO1 under selective pressure by the immune system. Our finding that loss of the Bin1 tumor suppressor gene potentiates the superinduction of IDO1 provided the first discreet example of a molecular pathway through which this immune escape process can occur. However, the involvement of IDO1 in tumorigenesis has turned out to be far more complex. Tumors that inherently lack IDO1 expression have been demonstrated to induce IDO1 expression in antigen presenting cells of the host, providing an alternative mechanism for immune escape. We have also found from studies in the classical DMBA/TPA skin carcinogenesis model that IDO1 can be induced by the inflammatory tumor-promoting process itself independent of the presence of an initiated tumor. Thus, IDO1 can be a factor in tumor promotion throughout the entire immunoediting process. With the rapid pace of development of IDO inhibitors, which are currently being evaluated in clinical trials, we are interested in determining whether mouse tumor models might provide additional insight into the optimal therapeutic application of these agents based on the underlying biology. In current studies, we have found that IDO1-nullizygous mice are resistant to both KRAS-induced lung adenocarcinomas and pulmonary breast carcinoma metastases. Micro-computed tomographic imaging confirmed that lung tumor burden was correspondingly lower in IDO1-nullizygous mice. Surprisingly, this analysis also revealed a significantly reduced pulmonary blood vessel density in IDO1-nullizygous mice. Elevation of the inflammatory cytokine IL6 (interleukin 6) was greatly attenuated in conjunction with the loss of IDO1, consistent with in vitro evidence that IDO1 potentiates IL6 production. MDSCs (myeloid derived suppressor cells) from IDO1-nullizygous animals exhibited reduced T cell suppressive activity that could be rescued by IL6. IL6 could likewise reverse the pulmonary metastasis resistance exhibited by IDO1-nullizygous mice. Together, our findings provide support for the emerging concept of IDO1 as a prototypical, integrative immune modifier that bridges inflammation, vascularization and immune escape to foster the establishment of a pathogenic, tumor-promoting environment. Citation Format: Alexander J. Muller, Courtney Smith, Mee Young Chang, James DuHadaway, Arpita Mondal, Hollie Flick, Katherine Parker, Daniel Beury, Suzanne Ostrand-Rosenberg, George C. Prendergast. IDO1 is an integrative determinant of tumor-promoting, pathogenic inflammation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3665. doi:10.1158/1538-7445.AM2014-3665