Abstract 2046: Inhibiting IDH mutations in low-grade glioma alters cellular function and the immune environment

Abstract Somatic mutations in the genes encoding the metabolic enzymes isocitrate dehydrogenase (IDH) 1 and 2 occur in many cancers, including glioma, cholangiocarcinoma, and acute myeloid leukemia. Mutant IDH (mIDH) proteins have a gain-of-function enzyme activity, catalyzing the reduction of alpha-ketoglutarate (α-KG) to the oncometabolite D-2-hydroxyglutarate (2-HG). 2-HG competitively inhibits α-KG-dependent enzymes, leading to epigenetic dysregulation and tumorigenesis. TET2 is an α-KG-dependent enzyme mediating the conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), which leads to DNA demethylation. In mIDH tumors, TET2 activity is inhibited by 2-HG, leading to DNA hypermethylation. In addition, IDH mutations are associated with immune evasion in gliomas. IDH mutations are common in low-grade gliomas (LGG; WHO grade 2/3) and include mutations in IDH1 (~80%) and IDH2 (~4%). Ivosidenib (AG-120, IVO) is a first-in-class, oral inhibitor of mIDH1, and vorasidenib (AG-881, VOR) is an oral, potent, brain-penetrant inhibitor of both mIDH1 and mIDH2. In an ongoing perioperative study (NCT03343197), IVO and VOR demonstrated brain penetrance and >90% suppression of 2-HG in resected mIDH1 gliomas after pre-operative treatment for approximately 4 weeks. In this analysis, we examined the cellular mechanisms underlying mIDH inhibition in LGG using resected tumor tissues collected following treatment with IVO and VOR. Optimal 2-HG suppression (defined by post-treatment 2-HG levels equal to or lower than the upper range of IDH wild-type tumor 2-HG levels) in IVO- or VOR-treated samples (n=17) led to a ~2-fold increase in 5hmC compared with untreated controls and banked reference samples (n=35; P=0.04). Furthermore, immunohistochemistry (IHC) analysis of the proliferation marker Ki-67 showed a ~3-fold decrease in the percentage of Ki-67-positive cells in IVO- or VOR-treated samples with optimal 2-HG suppression (n=21) compared with untreated controls (n=5; P=0.02). Tumor immune microenvironment analyses were also conducted. Comparing IVO- or VOR-treated samples (n=21) with untreated controls and banked reference samples (n=33), RNAseq analysis showed a ~2-fold decrease in the expression of tumor-associated macrophage markers CD68 (P=0.01) and CD163 (P=0.03) in mIDH oligodendrogliomas. In addition, IHC analysis from paired pre-treatment (archival) and post-treatment samples showed an increase of CD3+ T-cell infiltration (P=0.03) and CD8+ T-cell infiltration (P=0.01) in IVO- or VOR-treated samples with optimal 2-HG suppression (n=21). Single-cell RNAseq was conducted for a subset of samples, and the results will be presented. Overall, these data suggest that mIDH inhibition decreases DNA hypermethylation and tumor cell proliferation, and activates the immune microenvironment in mIDH LGG. In conclusion, these data elucidate the mechanisms underlying mIDH inhibition and support further development of mIDH inhibitors in mIDH LGG. Page 1 of 1 Citation Format: Min Lu, Ingo K. Mellinghoff, Aaron Diaz, Jennie W. Taylor, Sung Choe, Ania Tassinari, Dongwei Zhu, Katie Sellers, Kha Le, Feng Tai, Islam Hassan, Shuchi S. Pandya, Lori Steelman, Bin Wu. Inhibiting IDH mutations in low-grade glioma alters cellular function and the immune environment [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2046.