Moreover, the presence of KMO activity in GBM cell lines could support the formation and accumulation of tryptophan catabolites, which have been described as toxic to immune populations, such as NK cells and T and B lymphocytes, since reports suggest that KP products 3-HK, 3-HANA, and QUIN decrease proliferation and increase apoptosis in these immune populations. Here, NPPA is linked to glioblastoma.