Prognostic impact of indoleamine 2,3-dioxygenase 1(IDO1)mRNA expression on circulating tumour cells of patients with head and neck squamous cell carcinoma

Background We sought to determine the prognostic role of indoleamine 2,3-dioxygenase 1 (IDO1) by evaluatingIDO1expression in circulating tumour cells (CTCs) at baseline and after completion of chemoradiotherapy in patients with locally advanced (LA) head and neck squamous cell carcinoma (HNSCC) treated with curative intent. Methods In a prospective cohort of 113 patients with LA HNSCC, we evaluated expression ofIDO1in the EpCAM+ CTC fraction at baseline and after cisplatin chemoradiation. The prognostic value of combinedprogrammed cell death ligand-1(PDL-1) andIDO1expression was assessed. Results IDO1was significantly overexpressed at baseline compared with the post-treatment counterparts (p=0.007).IDO1messenger RNA (mRNA) expression at baseline was associated with better survival in terms of progression-free survival (PFS) (HR=0.19, p=0.017). Post-treatmentIDO1mRNA levels were correlated with unfavourable prognosis in terms of overall survival (OS) (HR=3.27, p=0.008). Patients with combined decreased expression levels ofPDL-1andIDO1after treatment exhibited superior PFS (p=0.043) and OS (p=0.021). Conclusions Our results strongly suggest thatIDO1mRNA expression is an independent prognostic factor for clinical outcome. Our study provides useful information for future trials combining chemoradiation with immune checkpoint inhibitors and IDO1 inhibitors.