Administration of FLT3 inhibitors effectively induces apoptosis of EZH2^Mut leukemic cells in vitro and prolongs the lifespan of a xenograft mouse model with anti-leukemia activity in vivo. (A-D) Kaplan-Meier plots of survival in NOD-SCID mice inoculated with (A-B) EZH2^Mut or (C-D) EZH2^WT T-ALL patient-derived leukemic cells via the tail vein. When the leukemic proportion extended up to 5% in peripheral blood, mice were intraperitoneally injected with vehicle (DMSO), sorafenib (60 mg/kg), or quizartinib (30 mg/kg) for 5 consecutive days. Each treatment group contained 5 animals. Day 0 was the day when cells were implanted. (E) Flow cytometric analyses of phosphorylated pSTAT5, pAKT, and pERK performed for isolated tumor cells from patient-derived xenograft mice (EZH2^Mut and EZH2^WT) after being treated with vehicle (DMSO), sorafenib, or quizartinib for 5 days. (F-G) Cell growth and viability of tumor cells (harvested from drug-free xenograft mice) assessed in vitro by MTT assays after being treated with designated concentrations of (F) sorafenib or (G) quizartinib for 24 hours in parallel studies. (H-J) Cell death analyzed in the bulk blast population using an annexin V (AV)/propidium iodide (PI) staining assay in vitro after being exposed to sorafenib or quizartinib for 24 hours. Additional analyses of the late-stage apoptotic cell percentage were carried out by concentration titrated (I) sorafenib and (J) quizartinib. All the cells harvested for in vitro studies were incubated in RPMI-1640 with 10% FBS and 5 ng/mL FLT3 ligand overnight (16 hours) for synchronization before treatment started. Data are presented as the mean ± SD. Error bars: SD of 3 independent experiments. *P < .05; ****P < .00001.