Reimagining IDO Pathway Inhibition in Cancer Immunotherapy via Downstream Focus on the Tryptophan-Kynurenine-Aryl Hydrocarbon Axis

Abstract

Significant progress has been made in cancer immunotherapy with checkpoint inhibitors targeting programmed cell death protein 1 (PD-1)-programmed death-ligand 1 signaling pathways. Tumors from patients showing sustained treatment response predominately demonstrate a T cell-inflamed tumor microenvironment prior to, or early on, treatment. Not all tumors with this phenotype respond, however, and one mediator of immunosuppression in T cell-inflamed tumors is the tryptophan-kynurenine-aryl hydrocarbon receptor (Trp-Kyn-AhR) pathway. Multiple mechanisms of immunosuppression may be mediated by this pathway including depletion of tryptophan, direct immunosuppression of Kyn, and activity of Kyn-bound AhR. Indoleamine 2,3-dioxygenase 1 (IDO1), a principle enzyme in Trp catabolism, is the target of small-molecule inhibitors in clinical development in combination with PD-1 checkpoint inhibitors. Despite promising results in early-phase clinical trials in a range of tumor types, a phase III study of the IDO1-selective inhibitor epacadostat in combination with pembrolizumab showed no difference between the epacadostat-treated group versus placebo in patients with metastatic melanoma. This has led to a diminution of interest in IDO1 inhibitors; however, other approaches to inhibit this pathway continue to be considered. Novel Trp-Kyn-AhR pathway inhibitors, such as Kyn-degrading enzymes, direct AhR antagonists, and tryptophan mimetics are advancing in early-stage or preclinical development. Despite uncertainty surrounding IDO1 inhibition, ample preclinical evidence supports continued development of Trp-Kyn-AhR pathway inhibitors to augment immune-checkpoint and other cancer therapies.

Figure 1.

Tryptophan depletion-dependent signaling. Depletion of tryptophan suppresses activity in the mTORC1 signaling pathway, leading to autophagy in T cells, and releases GCN2-mediated phosphorylation of eIF-2, inducing cell cycle arrest and death in T cells.

Figure 2.

IDO1-Kynurenine-AhR signaling in TME immunosuppression. A, IDO1 in tumor cells, dendritic cells, and fibroblasts. TDO in hepatocytes are the rate-limiting enzymes in the conversion of tryptophan to Kynurenine and Kynurenine derivatives. Kynurenine binds to and activates the AhR, a ligand-activated transcription factor, in regulatory T cells, NK cells and dendritic cells. B, Activation and nuclear translocation of the AhR (1) in dendritic cells induces synthesis and release of IL-10 and inhibits IFNβ signaling, (2) in NK cells induces synthesis and release of IL-10 and IFNγ, and (3) in Tregs promotes Treg development. C, Tregs and IL-10 promote immunosuppression within the TME, whereas inhibition of IFNβ by AhR releases regulation of immunosuppression from inhibitory IFNβ signaling. In addition, both IL-10 and IFNγ promote IDO1 activity, establishing a positive feedback loop for IDO1-Kynurenine-AhR signaling.

Figure 3.

Trp-Kyn pathway inhibitors in current or prior clinical development

Figure 4.. Expression of PD1 is positively correlated with immunotherapy relevant target genes across solid tumors from TCGA

. (A) Heatmap of Pearson’s product-moment correlation coefficient r between PD1 and immune target genes by tumor type. Immune target genes were separated into those strongly or less correlated with PD1 expression. Methods: level 3 RNA-Seq data (release date February 4, 2015) were downloaded for 30 solid tumor types from TCGA and processed as previously described (87). Acute myeloid leukemia, diffuse large B-cell lymphoma, and thymoma were excluded because of high tumor intrinsic immune cell transcripts. Skin cutaneous melanoma had both primary and metastatic samples available, whereas the other 29 cancers had only primary tumors available. A non-comprehensive list of 171 immune molecules representative of the interactions between tumor cells and immune cells in the tumor microenvironment were selected and correlated with PD1 (alias PDCD1) gene expression. For each tumor type, Pearson’s r was computed between each immune molecule and PD1 and used for clustering the genes by hierarchical unsupervised clustering with Euclidean distance. Two distinct groups are shown, consisting of (1) strongly correlated genes and (2) less correlated genes. (B and C) Correlation plots of PD1 vs IDO1, IDO2, KYNU, TDO2, AHR and GCN2 (alias EIF2AK4) (highlighted in red in A) in (B) metastatic melanoma and (C) NSCLC. Patients were categorized into T cell-inflamed (red), non-T cell-inflamed (blue) and intermediate groups using a defined T cell-inflamed gene signature (87). Each data point represents one patient. NSCLC = non-small cell lung carcinoma.