Targeting Myeloid Metabolism to Reprogram the Tumor Microenvironment of Glioblastoma

IDH1R132H mutation is recognized as the most common initiating event for low-grade glioma and IDH1- mutant grade IV astrocytoma. Our recent study (Stegh and colleagues, Cell Report, 2017) indicated that wild-type IDH1 (wt-IDH1) is overexpressed in 2/3 of glioblastoma (GBM) that lack IDH1R132H mutation. Both RNAi-mediated knockdown and pharmacological inhibition of wt-IDH1, alone and in combination with radiation therapy, slowed the growth of patient-derived GBM xenografts, while overexpression of wt-IDH1 promoted intracranial HGG growth. Diminished wt-IDH1 activity in GBM tumor cells correlated with reduced α-KG and NADPH levels and was paralleled by enhanced histone methylation, the expression of transcripts associated with cellular differentiation, and weakened defense against oxidative stress. Our analyses of GBM single-cell RNA sequencing (scRNA-Seq) datasets now demonstrate that in addition to GBM tumor cells, wt-IDH1 is also highly expressed in tumor-associated myeloid cells (TAMCs). To determine the role of wt-IDH1 expression in TAMCs, we generated immunocompetent mice with conditional wt-IDH1 gain- (lox-stop-lox wt-IDH1; wt-IHD1LSL) and loss-of-function (floxed wt-IDH1; wt-IDH1L/L). These genetically engineered mice express or delete wt-IDH1 after tamoxifen-induced LysM-creERT2-mediated recombination selectively in the myeloid compartment. The genetic ablation of wt-IDH1 in murine myeloid cells increased their proinflammatory polarization and tumoricidal activity, similar to the pharmacological inhibition of wt-IDH1 using the wt-IDH1-specific and brain-penetrant small molecule inhibitor 13i, developed by AbbVie4. To assess the effects of myeloid wt-IDH1 on glioma growth, we implanted murine tumor cells into tamoxifen-treated LysM-creERT2; IDH1L/L recipient mice or into wild-type recipients that, upon tumor establishment, were treated systemically with 13i. Genetic ablation or pharmacological inhibition of wt-IDH1 promoted inflammatory myeloid phenotypes, increased CD8+ T cells in the TME, as assessed by scRNA- Seq, and resulted in long-term animal survival and anti-tumor memory. Our central hypothesis is that wt-IDH1, through metabolic and epigenetic reprogramming of TAMCs, maintains an immunosuppressive TME and that the pharmacological inhibition of wt-IDH1, using 13i, represents a novel strategy to increase the efficacy of existing immunotherapies for GBM treatment. We will test these hypotheses in two Specific Aims by determining the cellular, epigenomic, and metabolic mechanisms by which wt-IDH1 controls TAMC phenotypes (Aim 1) and by developing treatment regimens combining 13i with prioritized immunotherapies, including inhibitors of PD1 immunosuppressive signaling (Aim 2). Impact: We will define wt-IDH1 as a metabolic checkpoint of myeloid cell activation and credential the pharmacologic inhibition of wt-IDH1 as a novel and safe immunotherapeutic strategy for patients with GBM. Project Number: 1R01CA294649-01A1 | Fiscal Year: 2025 | NIH Institute/Center: National Cancer Institute (NCI) | Principal Investigator: Alexander Stegh | Institution: WASHINGTON UNIVERSITY, SAINT LOUIS, MO | Award Amount: $2,467,799 | Activity Code: R01 | Study Section: Therapeutic Immune Regulation Study Section[TIR] View on NIH RePORTER: https://reporter.nih.gov/project-details/11226360