The caspase-8 inhibitor emricasan combines with the SMAC mimetic birinapant to induce necroptosis and treat acute myeloid leukemia

Abstract

Resistance to chemotherapy is a major problem in cancer treatment, and it is frequently associated with failure of tumor cells to undergo apoptosis. Birinapant, a clinical SMAC mimetic, had been designed to mimic the interaction between inhibitor of apoptosis proteins (IAPs) and SMAC/Diablo, thereby relieving IAP-mediated caspase inhibition and promoting apoptosis of cancer cells. We show that acute myeloid leukemia (AML) cells are sensitive to birinapant-induced death and that the clinical caspase inhibitor emricasan/IDN-6556 augments, rather than prevents, killing by birinapant. Deletion of caspase-8 sensitized AML to birinapant, whereas combined loss of caspase-8 and the necroptosis effector MLKL (mixed lineage kinase domain-like) prevented birinapant/IDN-6556-induced death, showing that inhibition of caspase-8 sensitizes AML cells to birinapant-induced necroptosis. However, loss of MLKL alone did not prevent a caspase-dependent birinapant/IDN-6556-induced death, implying that AML will be less likely to acquire resistance to this drug combination. A therapeutic breakthrough in AML has eluded researchers for decades. Demonstrated antileukemic efficacy and safety of the birinapant/emricasan combination in vivo suggest that induction of necroptosis warrants clinical investigation as a therapeutic opportunity in AML.

Combination of IAP antagonist and IFNγ activates novel caspase-10- and RIPK1-dependent cell death pathways

“…HT29 cells treated with IFNγ/SM also upregulated caspase-10, and various cleaved forms were detected (Figure 6a). Addition of the pancaspase inhibitor IDN-6556, a more potent caspase inhibitor than QVD, 49 prevented the formation of the smallest processed product of caspase-10 (p25), which served as the clearest signature of caspase-10 activation (Figure 6a). As expected, IFNγ/SM/IDN-6556 treatment also induced MLKL phosphorylation (Figure 6a).…”

RIPK1 and RIPK3 - emerging targets in cancer?

“…On the one hand, there is little to suggest that oncogenic transformation per se renders cells significantly more sensitive to RIPK1/RIPK3 responses in a cell autonomous manner. On the other hand, data using combination of SMAC mimetics or TLR3 agonist poly(I:C) in combination with pan-caspase inhibitors suggest that such necroptosis-inducing stimuli may be well tolerated and efficacious (66,80). One possibility is that only a very limited degree of RIPK1/RIPK3 cell death may be needed to get the immune system fired up to target the cancer, as some of the positive responses were not observed in immune-deficient animals (35, 80).…”

“…Finding conditions to drive necroptosis activation by these molecules emerged as one productive approach to achieve excellent anti-tumor activity. As mentioned above, Brumatti et al (66) showed that combining birinapant with pan-caspase inhibitor IDN6556 resulted in greatly increased efficiency of cell death.This combination induced necroptosis, compared to apoptosis induced by birinapant alone, and was effective even in the cells specifically selected in vitro to be resistant to birinapant. This also translated into birinapant+IDN combination causing improved tumor regression and survival of animals transplanted with MLL-ENL murine leukemia cells.…”

“…Activation of RIPK1/RIPK3-dependent apoptosis or necroptosis typically requires additional modifiers, such as inhibition of cIAPs for activation of RIPK1 kinase-dependent apoptosis (73,74) and/or inhibition of caspase-8 for RIPK1/RIPK3 kinasedependent necroptosis (64,66,75,76). Consequently, Moriwaki et al examined cell death in response to a panel of commonly used chemotherapeutic agents and did not observe any changes in cell death upon knockdown of either RIPK1 or RIPK3 in RIPK3-expressing cells or upon overexpression of RIPK3 in cells that lost RIPK3 expression(37), suggesting that RIPK1/RIPK3 may not be generally involved in chemotherapy-induced cell death.…”

Killing AML: RIPK3 leads the way

“…This finding is consistent with independent studies showing that deletion of Ripk3 did not alter disease progression of MLL-ENL-, MLL-AF9-, NUP98-HoxA9-, and HoxA9/Meis1-induced leukemia. 6,7 Furthermore, the authors showed that these leukemia subsets were sensitive to killing by the SMAC mimetic birinapant in a RIPK1/TNFR1-dependent manner. Therefore, our work suggests that alternative therapeutic approaches will be required for AML subsets with reduced RIPK3 expression.…”