Background: Acute myeloid leukemia (AML) is a heterogeneous disorder with aberrant regulation of a variety of signal pathways. Therefore, simultaneous targeting of two or even more deregulated signal transduction pathways is needed to overcome drug resistance. Previously, it was reported that SNS-032, a selective cyclin-dependent kinase inhibitor, is an effective agent for treatment of AML; however, the molecular mechanisms of SNS-032-induced cell death of AML cells are not yet fully understood. The aim of the study was to characterize the effects in vitro of SNS-032, used alone and in combination with an Akt inhibitor perifosine, against AML cells and to identify the mechanism involved.
Results: SNS-032 significantly induced cytotoxicity in human AML cell lines and blasts from patients with newly diagnosed or relapsed AML. However, Kasumi-1 cells and some of leukemic samples (14.9%) from AML patients were resistant to SNS-032-mediated cell death. Western blot analysis showed that SNS-032 strongly inhibited the phosphorylation of mammalian target of rapamycin (mTOR) on Ser 2448 and Ser2481, and that removal of SNS-032 resulted in partial recovery of cell death and reactivation of phosphorylation of mTOR. Moreover, exogenous insulin-like growth factor-1 (IGF-1) did not reverse SNS-032-induced cell growth inhibition and downregualtion of phosphor-mTOR at Ser2448 and Ser2481 although slight suppression of IGF-1R expression was triggered by the agent. Furthermore, SNS-032 at a lower concentration (60-80 nM) enhanced AML cell cytotoxicity induced by perifosine, an Akt inhibitor. Importantly, SNS-032 treatment reduced colony formation ability of AML cells, which was significantly increased when two agents were combined. This combination therapy led to almost complete inhibition of Akt activity.
Conclusion: We conclude that SNS-032 might directly target mammalian target of rapamycin complex 1 (mTORC1)/mTORC2. Our results further provide a rationale for combining SNS-032 with perifosine for the treatment of AML.