FLT3/BTK inhibitor combinations show additive effects. (A-B) Proliferation analysis of Molm13 and MV4-11 cells in the presence of quizartinib, ibrutinib, or DMSO. Results (from 4 independent experiments; mean ± SD) are shown for cells that had been treated for 3 days. *P < .05 between the different treatment groups compared with DMSO using Student t test. (C) Colony formation assays of MV4-11 and Molm13 cells in the presence of DMSO, ibrutinib, quizartinib, and ibrutinib/quizartinib. Results (mean ± SD) are from 4 independent experiments. *P < .05 between the different treatment groups compared with DMSO using Student t test. (D) FFM40 and FFM41 cells were exposed to quizartinib continuously (20 nM) or ibrutinib (1 µM) for 1 hour (followed by wash-out) in suspension culture or coculture with bone marrow stroma and analyzed for annexin V binding by flow cytometry. Results from 4 independent experiments. *P < .05 between the different treatment groups compared with the respective DMSO controls using Student t test. (E) Cleared cellular lysates of FFM40 cells derived from bone marrow stroma cocultures and treated as indicated were subjected to immunoblotting with antibodies against pERK (upper panel) and ERK (lower panel).