IDO1 and non-small cell lung carcinoma: More interestingly, growing preclinical evidence suggests that increase in IDO activity can be involved in resistance to checkpoint inhibition [15] In the present study, we aimed to investigate whether IDO activity, expressed as kyn/trp ratio, can inform about response to nivolumab, particularly identifying those patients resistant to PD-1 blockade in a cohort of previously treated NSCLC.