AG-221, an Oral, Selective, First-in-Class, Potent IDH2-R140Q Mutant Inhibitor, Induces Differentiation in a Xenotransplant Model

Somatic point mutations in the active site of IDH (isocitrate dehydrogenase) 1 and IDH2 genes are observed in acute myeloid leukemia (AML). These mutations lead to the production and accumulation of R-2-hydroxyglutarate (2-HG) in the tumor blast cells as well as in the plasma of patients. High levels of 2-HG have been shown to inhibit alpha-ketoglutarate-dependent dioxygenases, including epigenetic regulators (i.e. histone or DNA demethylases), resulting in altered cellular differentiation. Ex vivo experiments in primary cells from AML IDH2 mutant patients have demonstrated that IDH2 mutant inhibitors are able to revert this phenotype. In order to test the biological activity of AG-221, an oral, reversible and selective inhibitor of mutated IDH2 currently in phase I trials, we developed primary human AML xenograft models.

Primary blast cells from 3 AML patients with normal karyotype (n=3), NPM1 mutation (n=3) and FLT3ITD (n=1) were injected into immunodeficient NOD SCID IL2R gamma null (NSG) mice by intrafemoral injection after sub-lethal irradiation. Medullar and peripheral tumoral engraftment was monitored by flow cytometry (using species-specific antibodies) and serum 2-HG measurements. Engraftment of IDH2-R140Q human blast cells was not lethal, but these AML cells persisted over time. Twelve months after injection, IDH2-R140Q blast cell-engrafted mice were randomized into two groups, for treatment with AG-221 (30 mpk) or vehicle (0.5% methylcellulose / 0.2% Tween 80, in water) (n=10 mice; 5 mice per condition). Treatments were administered by oral gavage for 38 days twice per day (BID). Peripheral blood was collected at multiple time points for the determination of pharmacokinetics (PK) and pharmacodynamics (PD), and the assessment of the differentiation effects by AG-221 on human tumor cells.

PK/PD analyses showed good plasma exposure of AG-221 and reduction of 2-HG. Furthermore, AG-221 administration also induced a burst of proliferation of human blasts followed by myeloid differentiation starting at day 20 in peripheral blood as measured by the expression of CD11b, CD14, CD15, CD24 and cell morphology. No effects on proliferation or differentiation were seen in the absence of AG-221 administration. Histological analyses of hematopoietic organs in treated animals showed a decrease of infiltrating human cells, as well as obvious morphological changes in the human cell population in AG-221-treated animals compared with vehicle-treated animals. Flow cytometry confirmed the differentiation of the human cells in the spleen and bone marrow of the AG-221-treated mice. Furthermore, cells undergoing differentiation retained the R140Q mutation, demonstrating the differentiating effect of the compound.

In conclusion, AG-221 reduced serum 2-HG levels and triggered differentiation of leukemic blast cells engrafted in NSG mice. These results are consistent with what has been observed clinically in IDH2 mutant AML patients treated with AG-221 in a phase I dose escalation trial.