AG-221, a Small Molecule Mutant IDH2 Inhibitor, Remodels the Epigenetic State of IDH2-Mutant Cells and Induces Alterations in Self-Renewal/Differentiation in IDH2-Mutant AML Model in Vivo

Somatic mutations in isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) are observed in patients with acute myeloid leukemia (AML). Leukemia-associated IDH1/2 mutations result in aberrant accumulation of the oncometabolite 2-hydroxyglutarate (2-HG). The observation that IDH1/2 mutations are mutually exclusive with TET2 mutations led to the finding that IDH1/2-mutant production of 2-HG inhibits TET2 function and induces changes in DNA methylation. These data suggested that small molecule inhibition of mutant IDH enzymes might reverse the aberrant epigenetic remodeling of IDH-mutant leukemia cells and restore normal hematopoietic differentiation.

We therefore investigated the in vivo efficacy of AG-221, a potent and selective mutant IDH2 inhibitor in early-phase clinical trials, in murine models of IDH2-mutant leukemia. We first assessed the impact of AG-221 on 2-HG production in hematopoietic cells expressing mutant IDH2-R140Q. AG-221 treatment (10mg/kg or 100mg/kg bid) led to a reduction in 2-HG in vivo (96.7% below pre-treatment levels). Moreover, AG-221 treatment restored megakaryocyte-erythroid progenitor (MEP) differentiation that is suppressed by mutant IDH2 expression in vivo (mean MEP% mean, 39% Veh vs 50% AG-221).

We next investigated the impact of mutant IDH2 inhibition with AG-221 on DNA methylation in vivo. We used eRRBS, a bisulfite-based next-generation sequencing platform, to assess the effect of AG-221 therapy on DNA methylation. AG-221 or vehicle therapy treated LSK stem cells (lin^- Sca⁺ c-Kit⁺) were sorted from mice expressing IDH2-R140Q and evaluated by eRRBS. AG-221 therapy reversed the effects of mutant IDH2; we observed a significant reduction in DNA methylation, including 180 genes that had 20 or more hypomethylated differentially methylated cytosines (DMCs) following treatment. 84 of these genes had reduced methylation at 10 or more DMCs in the gene promoter with AG-221 therapy compared to vehicle. Mutant IDH2 inhibition with AG-221 reversed aberrant methylation at many genes with a known role in hematopoietic proliferation and differentiation, including the master transcriptional factor RUNX1.

We next assessed in vivo effects of the small-molecule IDH2-R140Q inhibitor in a mouse model of IDH2-mutant leukemia. We generated mice that simultaneously expressed a constitutive Flt3^ITD knock-in allele and a conditional mutant IDH2^R140Q knock-in allele. As reported recently using retroviral/transgenic models, Mx1-Cre IDH2^R140QFlt3^ITD developed fully penetrant, transplantable AML with expansion of c-Kit⁺ positive blasts in the peripheral blood, and widespread leukemic infiltration. AG-221 inhibited the serial replating capacity of IDH2^R140QFlt3^ITD expressing cells in vitro. We competitively transplanted IDH2^R140QFlt3^ITD AML cells and normal bone marrow cells into secondary recipients, and then assessed the effect of AG-221 therapy on leukemia in vivo and on disease burden. AG-221 (100mg/kg bid) treatment of mice engrafted with Mx1-Cre IDH2^R140QFlt3^ITD AML cells markedly reduced 2HG levels consistent with on target inhibition in vivo. AG-221 therapy induced differentiation of leukemic cells, with an increase in the CD11b⁺ population and a decrease in the c-Kit⁺ population in the peripheral blood at 2wks. We next assessed the impact of treatment with both AG-221 therapy with AC220, a potent, specific Flt3 inhibitor in late phase clinical trials. Combined IDH2^R140Q and Flt3^ITD inhibition resulted in a marked decrease in leukemic burden to vehicle-treated mice, with a significant reduction in leukemic cell chimerism in vivo in the setting of combined inhibition at 2 wks, (mean 45.2 fraction 88% veh, 73% AG-221, p<.01).

These data demonstrate that AG-221 inhibits mutant IDH2-mediated 2-HG production in vivo and reverses the effects of mutant IDH2 on DNA methylation in mutant stem/progenitor cells. AG-221 induces differentiation and impairs self-renewal of IDH2-mutant leukemia cells, effects that are further enhanced by simultaneous inhibition of Flt3^ITD. Clinical trials combining IDH2 inhibitors with other targeted AML therapies are warranted in order to increase therapeutic efficacy.