we observed that in certain cell contexts the expression of Mcl 1 may be decreased by pharmacologic inhibition of the mitogen-activated protein kinase pathway, resulting in sensitization to apoptosis induction by ABT 737, we are anxious that in certain cancer cells Mcl 1 expression may be independent of MAPK and therefore cannot be downregulated by MAPK inhibitors. Moreover, from the mechanistic point of view, Foretinib VEGFR inhibitor analyzing how direct Mcl 1 antagonism modulates apoptosis in leukemia cells, alone or in combination with other therapeutic approaches, is of utmost importance for the development of novel therapeutic approaches. Here, we report that obatoclax, a BH3 mimetic currently in clinical trials that displays an alternative binding affinity than that of ABT 737 to antiapoptotic Bcl 2 members of the family, is beneficial in inducing apoptosis in AML cell lines and primary samples. Like ABT 737, obatoclax induced apoptosis in a timedependent and dose dependent fashion and apoptosis induction occurred at doses that reflected the appreciation of the agent for Bcl 2 family proteins. Mechanistically, apoptosis induction by obatoclax was preceded by liberation of Bak from Mcl 1, dissociation of Bim from Bcl 2 and Mcl 1, and the synthesis of a complex of conformationally altered Bak, previously claimed to promote apoptosis, with Bax. Interestingly, the Bak and Bax advanced formation caused by obatoclax has also been observed in peripheral blood mononuclear cells of patients with refractory Figure 5. Obatoclax induces apoptosis in primary AML samples. A, main AML samples were treated with increasing levels of obatoclax, and Annexin V was measured in the CD34 positive area by flow cytometry after 24 h. As described in Materials and Techniques specific apoptosis was established natural product library. B, key AML samples were cultured with obatoclax as suggested, and clonogenicity was determined as described in Materials and Practices. D, the potential of normal bone marrow samples treated with obatoclax was determined as above. N, Bcl 2 was immunoprecipitated from a main AML sample treated with obatoclax, and the presence of Bim was considered by Western blot. Benefits are representative of three independent samples examined. Cancer Research Cancer Res 2008, 68:. Might 1, 2008 3418. aacrjournals. org chronic lymphocytic leukemia within a single representative phase I trial. Unlike observed for ABT 737, apoptosis induced by obatoclax was decreased, however not abolished, in the lack of Bak/Bax, suggesting that additional target apart from Bcl 2 subscribe to the activation of the intrinsic pathway by this cycloprodigiosin derivative. This effect could be temporally separated from the effects of obatoclax in washout experiments and occurred in the absence of Bax/Bak/Bim proteins, suggesting that this agent has multiple targets.