IDO1 and glioblastoma: To improve antitumor immune responses to anti-PD-1 in GBM, many clinical studies have explored combining anti-PD-1 with other immune checkpoint blockade agents, such as IDO1 inhibitors (ClinicalTrials.gov identifier: NCT03707457) and anti-lymphocyte-activation gene 3 (LAG-3) monoclonal antibodies (NCT02658981).