To analyze the effect of IDO inhibitors on the expression of AHR and CYP1A1, RT4 and T24 cells were incubated with 1 μM INCB024360 (INCB, Tocris Bioscience, Bristol, UK) or 1 mM 1-methyl-D-tryptophan (MT, Sigma-Aldrich, St. Louis, MO) for 48 h. INCB was dissolved in DMSO (Sigma-Aldrich, St. Louis, MO) for a 1 mM stock solution. MT was dissolved in 0.1 N NaOH for a 20 mM stock solution and then neutralized to pH 7.4 with 0.1 N HCl.
Cells were incubated into 6-well plates until 80% confluence. At this time, the medium was removed and medium containing IDO inhibitors was added. After 48 h, the cells were harvested and pelleted, being frozen at nitrogen and kept at − 80 °C for storage until RNA extraction. Supernatant was collected and maintained at − 80 °C for kynurenine measurement. The experiments were carried out in triplicates and repeated three times at different times.