The Kynurenine Pathway: A Primary Resistance Mechanism in Patients with Glioblastoma

The failure of chemotherapy and radiation therapy to achieve long-term remission or cure in patients with glioblastoma (GBM) is, in a large part, due to the suppression of the immune system induced by the tumors themselves. These tumors adapt to treatment with chemotherapy or radiation therapy by stimulating secretion of molecules that cause tryptophan metabolism to be disrupted. Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are produced, accelerating metabolism along the kynurenine pathway and resulting in excess levels of quinolinic acid, 3-hydroxyanthranilic acid and other neurotoxic molecules. IDO and TDO also act as checkpoint molecules that suppress T-cell function. GBM is particularly associated with severe immunosuppression, and this tumor type might be thought to be the ideal candidate for checkpoint inhibitor therapy. However, treatment with checkpoint inhibitors now in clinical use for peripheral solid tumors, such as those inhibiting cytotoxic T-lymphocyte-associated protein-4 (CTLA4) or programmed cell death-1 (PD1) receptors, results in further abnormalities of tryptophan metabolism. This implies that to obtain optimal results in the treatment of GBM, one may need to add an inhibitor of the kynurenine pathway to therapy with a CTLA4 or PD1 inhibitor, or use agents which can suppress multiple checkpoint molecules.