MEK1/2 inhibitors sensitize BCR/ABL+ human leukemia cells to the dual ABL/SRC inhibitor BMS354825 Free

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Interactions between dual Bcr/Abl and Src inhibitor dasatinib and MEK1/2 inhibitors have been examined in CML cell lines and primary specimens, including those resistant to imatinib mesylate. Co-treatment of K562 or LAMA cells with marginally toxic concentrations of the MEK1/2 inhibitors PD184352 or U0126 and extremely low concentrations of dasatinib (e.g., sub-nanomolar) synergistically potentiated mitochondrial damage, caspase activation, and apoptosis. Similar interactions were observed in CD34+ cells from a CML patient-derived but not in normal human CD34+ bone marrow cells. These interactions were associated with multiple perturbations in survival signaling pathways, including inactivation of Bcr/Abl, STAT5, ERK1/2, down-regulation of Bcl-xL, Mcl-1, and dephosphorylation/activation of BIM. These perturbations were also associated with BAX and BAK translocation, resulting in activation of caspases as well as mitochondrial dysfunction. BIM knockdown by shRNA suppressed both BAX and BAK conformational change and also protected the cells from dasatinib/PD184352/toxicity, suggesting that BIM plays a functional role in the lethality of the dasatinb/PD184352 regimen. Furthermore, K562 ectopically expressing Mcl-1 or Bcl-xL was significantly less susceptible to dasatinib/PD184352, suggesting that Mcl-1 and Bcl-xL contribute functionally to the lethality of the dasatinb/PD184352 regimen. Notably, the dasatinib/PD184352 regimen was active against leukemic cells exhibiting various forms of imatinib mesylate resistance, including Bcr/Abl overexpression, Lyn activation, and several Bcr/Abl kinase domain mutations (e.g., E255K, M351T), but not against cells expressing the T315I mutation. Evidence from early clinical trials involving MEK1/2 inhibitors indicate that it is feasible to achieve the desired pharmacodynamic effect (e.g., inactivation ERK1/2) at well tolerated MEK inhibitor doses. These considerations, along with evidence that interrupting the MEK1/2/ERK1/2 pathway dramatically sensitizes Bcr/Abl+ leukemia cells, including those resistant to imatinib mesylate, to extremely low concentrations of dasatinib through multiple mechanisms make this strategy an attractive one to pursue in imatinib mesylte-refractory CML.