Interestingly, in a phase 1 trial, the IDO-1 inhibitor PF-06840003 demonstrated disease control in 47% of patients (n = 8).40 These promising results highlight the importance of further investigating KP dysregulation in GBM patients and suggest that evaluating TRP metabolites could aid in the identification of patient cohorts most likely to benefit from targeted interventions aimed at these pathways. Here, IDO1 is linked to glioblastoma.