Key Points• Enasidenib, a selective inhibitor of mutant-IDH2 enzymes, was safe and welltolerated in patients with IDH2-mutated myeloid malignancies.• Enasidenib induced hematologic responses in patients with relapsed/refractory AML in this dose-escalation and expansion study. ABSTRACTRecurrent mutations in isocitrate dehydrogenase 2 (IDH2) occur in ~12% of patients with acute myeloid leukemia (AML). Mutated IDH2 proteins neomorphically synthesize 2-hydroxyglutarate resulting in DNA and histone hypermethylation, leading to blocked cellular differentiation. Enasidenib (AG-221/CC-90007) is a first-in-class, oral, selective inhibitor of mutant-IDH2 enzymes. This first-in-human, phase 1/2 study assessed the maximum tolerated dose (MTD), pharmacokinetic and pharmacodynamic profiles, safety, and clinical activity of enasidenib in patients with mutant-IDH2 advanced myeloid malignancies. We assessed safety outcomes for all patients (N=239) and clinical efficacy in the largest patient subgroup, those with relapsed or refractory AML (n=176), from the phase 1 dose-escalation and expansion phases of the study. In the doseescalation phase, an MTD was not reached at doses ranging from 50-650 mg daily.Enasidenib 100 mg daily was selected for the expansion phase based on pharmacokinetic and pharmacodynamic profiles and demonstrated efficacy. Grade 3-4 enasidenib-related adverse events included indirect hyperbilirubinemia (12%) and IDHinhibitor-associated differentiation syndrome (IDH-DS; 7%). Among patients with relapsed or refractory AML, overall response rate was 40.3%, with median response duration of 5.8 months. Responses were associated with cellular differentiation and maturation, typically without evidence of aplasia. Median overall survival among relapsed/refractory patients was 9.3 months, and for the 34 patients (19.3%) who attained complete remission was 19.7 months. Continuous daily enasidenib treatment was generally well-tolerated and induced hematologic responses in patients who had failed prior AML therapy. Inducing differentiation of myeloblasts, not cytotoxicity, appears to drive the clinical efficacy of enasidenib.

A number of human cancers harbor somatic point mutations in the genes encoding isocitrate dehydrogenases 1 and 2 (IDH1 and IDH2). These mutations alter residues in the enzyme active sites and confer a gain-of-function in cancer cells, resulting in the accumulation and secretion of the oncometabolite (R)-2-hydroxyglutarate (2HG). We developed a small molecule, AGI-6780, that potently and selectively inhibits the tumor-associated mutant IDH2/R140Q. A crystal structure of AGI-6780 complexed with IDH2/R140Q revealed that the inhibitor binds in an allosteric manner at the dimer interface. The results of steady-state enzymology analysis were consistent with allostery and slow-tight binding by AGI-6780. Treatment with AGI-6780 induced differentiation of TF-1 erythroleukemia and primary human acute myelogenous leukemia cells in vitro. These data provide proof-of-concept that inhibitors targeting mutant IDH2/R140Q could have potential applications as a differentiation therapy for cancer.

The pathophysiology of IDH mutations in tumorigenesis is increasingly described, yet the prognostic significance of IDH1 and IDH2 mutations in AML remains controversial. The primary objective of this study was to define the natural history and prognosis of patients with AML and IDH1 or IDH2 mutations and provide historical survival expectations. A total of 826 patients treated from 2010 to 2014 at a single institution were evaluated, including 167 patients (20%) with AML and IDH1 or IDH2 mutations. Median age was 62 years (range 18–92). There were 59 IDH1-R132, 83 IDH2-R140, and 23 IDH2-R172 mutations. Clinicopathologic characteristics associated with IDH-mutations included older age, less frequent therapy-related status, and increased incidence of intermediate-risk cytogenetics, FLT3-ITD mutations, and NPM1 mutations. Remission rates (CR/CRi) by AML treatment status were: induction, 68%; Salvage-1 (S1), 42%; and Salvage-2 and beyond (S2+), 27%. No difference in response was identified by IDH mutation status. Similarly, overall survival (OS) was not dependent on IDH status within any cohort. The median OS was 15.4 months in induction, 8.7 months in S1, and 4.8 months in S2+. This analysis defines the clinical outcome associated with IDH-mutations in both the front-line and salvage AML treatment settings, and confirms that response rate and OS for both IDH-mutated and IDH wild-type AML patients is comparable. This provides contemporary data to be used for comparison with results of novel investigational (e.g., selective IDH inhibitor) strategies.

Mutations in the gene encoding isocitrate dehydrogenase 2 (IDH2) occur in several types of cancer, including acute myeloid leukemia (AML). In model systems, mutant IDH2 causes hematopoietic differentiation arrest. Enasidenib, a selective small-molecule inhibitor of mutant IDH2, produces a clinical response in 40% of treated patients with relapsed/refractory AML by promoting leukemic cell differentiation. Here, we studied the clonal basis of response and acquired resistance to enasidenib treatment. Using sequential patient samples, we determined the clonal structure of hematopoietic cell populations at different stages of differentiation. Before therapy, IDH2-mutant clones showed variable differentiation arrest. Enasidenib treatment promoted hematopoietic differentiation from either terminal or ancestral mutant clones; less frequently, treatment promoted differentiation of nonmutant cells. Analysis of paired diagnosis/relapse samples did not identify second-site mutations in IDH2 at relapse. Instead, relapse arose by clonal evolution or selection of terminal or ancestral clones, thus highlighting multiple bypass pathways that could potentially be targeted to restore differentiation arrest. These results show how mapping of clonal structure in cell populations at different stages of differentiation can reveal the response and evolution of clones during treatment response and relapse.

Key Points• IDH2 R140Q expression in TF-1 cells can induce DNA and histone hypermethylation that mirrors human IDH2 mutant acute myeloid leukemia.• The hypermethylation can be reversed on treatment with AGI-6780, an IDH2 mutantspecific small-molecule inhibitor.Mutations of IDH1 and IDH2, which produce the oncometabolite 2-hydroxyglutarate (2HG), have been identified in several tumors, including acute myeloid leukemia. Recent studies have shown that expression of the IDH mutant enzymes results in high levels of 2HG and a block in cellular differentiation that can be reversed with IDH mutant-specific smallmolecule inhibitors. To further understand the role of IDH mutations in cancer, we conducted mechanistic studies in the TF-1 IDH2 R140Q erythroleukemia model system and found that IDH2 mutant expression caused both histone and genomic DNA methylation changes that can be reversed when IDH2 mutant activity is inhibited. Specifically, histone hypermethylation is rapidly reversed within days, whereas reversal of DNA hypermethylation proceeds in a progressive manner over the course of weeks. We identified several gene signatures implicated in tumorigenesis of leukemia and lymphoma, indicating a selective modulation of relevant cancer genes by IDH mutations. As methylation of DNA and histones is closely linked to mRNA expression and differentiation, these results indicate that IDH2 mutant inhibition may function as a cancer therapy via histone and DNA demethylation at genes involved in differentiation and tumorigenesis. (Blood. 2015;125(2):296-303) IntroductionActive site mutations in IDH1 (R132) and IDH2 (R172 and R140) that produce high levels of 2-hydroxyglutarate (2HG) have been identified in several human cancers.1-3 IDH mutations have been shown to cause DNA hypermethylation in both gliomas and leukemias via inhibition of methylcytosine dioxygenase TET2.4,5 Mutant IDH can also promote histone hypermethylation through competitive inhibition of a-ketoglutarate (aKG)-dependent Jumonji-C histone demethylases, thereby activating or deactivating expression of associated genes. 4,6,7 We have shown that mutant IDH1 and IDH2 can affect cell differentiation in solid and liquid tumors. [8][9][10] An IDH1 R132H inhibitor, AGI-5198, delayed growth and promoted differentiation of glioma cells while reducing histone H3K9 trimethylation. 8 Leukemic cell differentiation was also induced in primary human patient samples harboring an IDH2 R140Q mutation when they were treated ex vivo with AGI-6780, an IDH2 R140Q allosteric inhibitor.9 However, the mechanism by which IDH2 mutant activity and 2HG levels contribute to cellular differentiation and tumorigenesis is not fully understood. High levels of 2HG have been shown to competitively inhibit aKGdependent dioxygenases, leading to broad epigenetic changes. Therefore, we sought to investigate the global and gene-specific effects of mutant IDH2 inhibition in TF-1 cells expressing IDH2 R140Q. Probing the effects of IDH2 R140Q expression on histone and DNA methylation and gene expres...