IFNG and head and neck squamous cell carcinoma: HNSCC cell lines were cultured and treated with IFNγ for 24 h and 72 h, then treated with cytostatic drugs including 5‐fluorouracil (5‐FU), Cisplatin, Gemcitabine and Cetuximab. IDO1 immunofluorescence was performed on the treated cells and kynurenine pathway (KP) metabolites in the cell culture supernatant was quantified by liquid chromatography tandem mass spectrometry