Abstract 2911: Small molecule XIAP inhibitors sensitize acute leukemia cells for TRAIL- or chemotherapy-induced apoptosis, overcome Bcl-2-mediated resistance and reduce leukemic burden in vivo in NOD/SCID mice
Authors: Fakler Melanie , Löder Sandra , Vogler Meike , Schneider Katja , Jeremias Irmela , Debatin Klaus-Michael , Fulda Simone 
Published: February 2011

Abstract
Defects in apoptosis contribute to poor outcome in high risk pediatric acute lymphoblastic leukemia (ALL), calling for novel strategies that counter apoptosis resistance. XIAP, a member of “Inhibitor of Apoptosis” (IAP) proteins, presents a promising target, since it is expressed at high levels in acute leukemia. Therefore, we investigated the effect of small molecule XIAP inhibitors alone and in combination with TRAIL or chemotherapeutics in ALL cell lines, primary leukemic blasts, normal peripheral blood lymphocytes and in a mouse model of pediatric ALL.
Here, we report that XIAP inhibitors at subtoxic concentrations, but not a structurally related control compound, cooperate with TRAIL to induce apoptosis in ALL cells in a highly synergistic manner as calculated by combination index. Also, XIAP inhibitors act in concert with TRAIL to reduce clonogenic growth of ALL cells demonstrating that they suppress longterm survival. Analysis of signaling pathways reveals that XIAP inhibitors enhance TRAIL-induced activation of caspases, loss of mitochondrial membrane potential and cytochrome c release in a caspase-dependent manner, indicating that they promote a caspase-dependent mitochondrial amplification loop. Intriguingly, XIAP inhibitors overcome Bcl-2-mediated resistance to TRAIL by promoting Bcl-2 cleavage, Bak conformational change and caspase-3-driven mitochondrial perturbations. Thus, the combination of XIAP inhibitors and TRAIL can even break Bcl-2-imposed resistance, a defect in the apoptotic pathway that frequently occurs in acute leukemia and predicts poor prognosis. Moreover, XIAP inhibitors sensitize ALL cells for chemotherapy, e.g. cytarabine, doxorubicin, etoposide and 6-mercaptopurine. Intriguingly, this chemosensitization is inhibited by a TNFalpha-neutralizing antibody, demonstrating that it depends on XIAP inhibitor-triggered production of TNFalpha. Notably, XIAP inhibitors potently trigger cell death in leukemic blasts from children with ALL ex vivo and also sensitize primary leukemia cells for TRAIL- or chemotherapy-induced cell death. In contrast to malignant cells, XIAP inhibitors and TRAIL at equimolar concentrations are non-toxic to normal peripheral blood lymphocytes despite expression of the apoptosis-inducing TRAIL receptors on the cell surface, pointing to a therapeutic window. Most importantly, XIAP inhibitors exert potent anti-leukemic activity in vivo in a mouse model of pediatric ALL engrafted in NOD/SCID mice.
These findings provide first evidence that XIAP inhibitors present a novel strategy to trigger apoptosis in childhood leukemia and to prime acute leukemia cells for TRAIL- or chemotherapy-induced cell death. These data have important clinical implications for the development of apoptosis targeted therapies for childhood leukemia.
Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2911.