We found dramatic synergistic growth inhibition and substantially enhanced apoptosis when combining MI503 with the specific small molecule FLT3 inhibitor AC220 (Quizartinib) compared to monotreatment or vehicle control in the FLT3ITD positive MLL-r MV4-11 and MOLM-13 cells. Similar results were obtained when MI503 was combined with other specific FLT3 inhibitors - Crenolanib and Gilteritinib.