Selective inhibition of indoleamine and tryptophan 2,3-dioxygenases: Comparative study on kynurenine pathway in cell lines via LC-MS/MS-based targeted metabolomics

In the last decade, the kynurenine pathway, which is the primary metabolic route for tryptophan (TRP) catabolism, has sparked great interest in the pharmaceutical sciences due to its role in immune regulation and cancer immunoediting. In this context, the development of cell-based assays might represent a tool to: i) characterize the cell secretome according to cell types; ii) gain more insight into the role of kynurenines in different disease scenarios; iii) screen hIDO1 (human indoleamine 2,3-dioxygenase) inhibitors and evaluate their effect on downstream TRP-catabolizing enzymes. This paper reports a validated Liquid Chromatography with tandem mass spectrometry (LC-MS/MS) method to simultaneously quantify TRP, L-kynurenine (KYN), xanthurenic acid (XA), 3-hydroxykynurenine (3OHKYN), kynurenic acid (KA), 3-hydroxyanthranilic acid (3OHAA), anthranilic acid (AA), 5-hydroxytryptamine (serotonin, 5HT) and tryptamine (TRYP) in Dulbecco's Modified Eagle and Eagle's Minimum Essential Media (DMEM and EMEM, respectively). The quantitative method was validated according to FDA, ICH and EMA guidelines, later applied: i) to assess the impact of selective inhibition of hIDO1 or hTDO (human tryptophan 2,3-dioxygenase) on the kynurenine pathway in A375 (melanoma), MDA-MB-231 (breast cancer), and U87 (glioblastoma) cell lines using multivariate analysis (MVA); ii) to determine the IC50 values of both well-known (i.e., epacadostat, linrodostat) and the novel hIDO1 inhibitor (i.e., BL5) in the aforementioned cell lines. The proposed LC-MS/MS method is reliable and robust. Furthermore, it is highly versatile and suitable for applications in the preclinical drug research and in vitro assays.