Identification of small molecule inhibitors of the kynurenine pathway

Abstract

The immune system functions as an effective barrier against tumor development. However, malignant cells can avoid elimination by the immune system by acquiring certain characteristics that alter the immune system in the tumor microenvironment (TME). For instance, cancer cells can actively induce immune tolerance by expression of the immuno-suppressive enzyme indoleamine 2,3-dioxygenase 1 (IDO1). IDO1 expression results in degradation of the essential amino acid tryptophan (Trp) and production of kynurenines (Kyn) via the Kyn pathway. Targeting the Kyn pathway has emerged to be an attractive target in immuno-oncology. In this thesis, two compound classes were discovered through a cell-based screening that reduce Kyn levels in cancer cells upon stimulation with the cytokine interferon-γ (IFN-γ) by two distinct mechanisms. The indole-tetrahydropyridine pseudo-natural products (PNPs) called apoxidoles inhibit IDO1 directly by binding and stabilizing apo-IDO1 in vitro and in cells. The displacement of the IDO1 cofactor heme catalytically inactivates the enzyme and thereby decreases Kyn levels. The second small molecule class of N-substituted indoles called epoxykynins represents an alternative approach to modulate the Kyn pathway by not directly targeting IDO1. Instead, epoxykynin inhibits the catalytic activity of the C-terminal fatty acid hydrolase domain of the soluble epoxide hydrolase (sEH-H). As part of the arachidonic acid (AA) cascade, the highly active enzyme sEH-H plays an important role in the hydrolysis of CYP epoxygenase-derived fatty acid epoxides. Thereby, it contributes to the regulation of bioavailable epoxides and controls a variety of biological processes, such as inflammation, vasodilation, angiogenesis, neuropsychiatries and pain. Thereby, it contributes to the regulation of bioavailable epoxides and controls a variety of biological processes, such as inflammation, vasodilation, angiogenesis, neuropsychiatries and pain. The results obtained throughout this thesis uncover a cross-talk between sEH-H and the Kyn pathway. The discovery of new Kyn pathway inhibitors is in high demand, since the most advanced holo-IDO1 inhibitor epacadostat has recently failed in clinical trials. Additionally, the identification of epoxykynins has deepened the understanding of the functional link between the Kyn pathway and the AA metabolism. These findings might enable novel strategies to design immunotherapies and enhance the host’s immune system to overcome cancer-induced immune tolerance.