OSU-03012 sensitizes Ba/F3p210^E255K and Ba/F3p210^T315I cells to imatinib-induced apoptosis. (A, left) Dose-response curves obtained by MTS assays after 48-hour exposure of Ba/F3p210^E255K (E225K) and Ba/F3p210^T315I (T315I) cells to a combination of 5 μM OSU-03012 and varying concentration of imatinib mesylate (solid curves) or varying concentrations of imatinib mesylate alone (dotted curves). (Right) Dose-response curves obtained by MTS assays after 48-hour exposure to a combination of 5 μM imatinib mesylate and varying concentration of OSU-03012 (solid curves) or varying concentrations of OSU-03012 alone (dotted curves). Each data point represents means ± SD (n = 6). (B) Flow cytometric analysis of apoptotic death in Ba/F3p210^E255K (top row) and Ba/F3p210^T315I (bottom row) cells treated with 1, 2.5, and 5 μM imatinib mesylate in combination with 5 μM OSU-03012 for 48 hours. Results are representative of at least 3 independent experiments. (C) Ba/F3p210^E255K and Ba/F3p210^T315I cells were exposed to varying concentrations of OSU-03012 and imatinib mesylate at a fixed ratio (1:1) for 48 hours, after which CI values for apoptosis were determined in relation to the fraction affected using the medium dose effect analysis. CI values less than 1 are considered as a synergistic interaction. Mutually nonexclusive CI for combination at the IC₅₀ was 0.602 for Ba/F3p210^E255K and 0.649 for Ba/F3p210^T315I. The P values for sign test comparing CI equal to 1 versus CI less than 1 are .016 for Ba/F3p210^E255K and .008 for Ba/F3p210^T315I. The curves depict the simulations of mutually nonexclusive CI values by Calcusyn.