Targeting AML stem/progenitor cells by combinational therapy with SMAC mimetics and demethylating agents

Chemotherapy for acute myeloid leukemia (AML) principally induces intrinsic apoptosis in circulating leukemic blasts. Unfortunately, standard therapy is relatively ineffective in eliminating AML stem/progenitor cells that drive leukemogenesis. The inhibitors of apoptosis (IAP) protein family are critical regulators of cell survival. IAP proteins, in particular cIAP1 and cIAP2, have drawn great attention in recent years as targets for cancer therapy due to the development of SMAC mimetics. We discovered that the expressions of cIAP1, which primarily inhibits the extrinsic apoptosis pathway and a main target of SMAC mimetics, and caspase-8, the key caspase of the extrinsic apoptosis were significantly higher in AML stem/progenitor cells than in bulk AML cells. Conversely, the expression of SMAC, an endogenous antagonist of IAP proteins, was significantly lower by comparison. We investigated the therapeutic potential of targeting IAPs by SMAC mimetics in AML and reported that the novel SMAC mimetic birinapant effectively induced apoptosis in AML cells, including AML stem/progenitor cells. This effect was present even in leukemic cells co-cultured with bone marrow derived-mesenchymal stromal cells under hypoxic conditions representative of the bone marrow microenvironment. Furthermore, this anti-leukemia activity was enhanced in vitro and in vivo by combination with demethylating agents, which apparently modulated NFκB signaling and many key members of the extrinsic apoptosis pathway. The apoptosis repressor with caspase recruitment domain (ARC) protein is known to suppress the extrinsic apoptosis. We have demonstrated that ARC is an effective negative prognostic factor for AML, is regulated by cIAP1/NIK signaling in AML cells, and can be inhibited by demethylating agents. Our findings suggest that the activation of the extrinsic pathway has the potential to eradicate AML stem/progenitor cells and that mechanistic combinations of SMAC mimetics with agents that modulate the extrinsic apoptosis pathway may benefit patients with AML. In fact, our preliminary results served as the catalyst for recently-initiated phase I/II trials of birinapant in combination with 5-azacytidine in pre-leukemia myelodysplastic syndrome and AML patients.