Small Molecule XIAP Inhibitors Cooperate with TRAIL to Trigger Apoptosis in Childhood Acute Leukemia Cells and Overcome Bcl-2-Mediated Resistance

Children with high risk acute lymphoblastic leukemia (ALL) do not respond well to current treatments. This failure is, at least in part, due to defects in apoptosis programs. Therefore, new strategies are required that counter apoptosis resistance in order to improve the poor prognosis of high risk pediatric acute leukemia. Since XIAP, a member of “Inhibitor of Apoptosis Proteins” (IAPs), is expressed at high levels in acute leukemia and blocks apoptosis at a central point of the apoptotic machinery, XIAP may present a suitable molecular target for therapeutic intervention. Here, we report that neutralizing XIAP by small molecule inhibitors is a novel and effective approach to sensitize childhood acute leukemia cells for TRAIL- or chemotherapy-induced apoptosis. XIAP inhibitors at subtoxic concentrations, but not a structurally related control compound, synergize with TRAIL to induce apoptosis in acute lymphoblastic leukemia cells. Also, XIAP inhibitors act in concert with TRAIL to reduce clonogenic growth of ALL cells demonstrating that they suppress long-term 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 feedback mitochondrial amplification loop. Intriguingly, XIAP inhibitors overcome Bcl-2-mediated resistance to TRAIL by enhancing Bcl-2 cleavage and Bak conformational change. Thus, XIAP inhibitors combined with TRAIL even break Bcl-2-imposed resistance, a defect in the apoptotic pathway that is common in acute leukemia and associated with poor prognosis. Further, XIAP inhibitors prime ALL cells for apoptosis induced by various anti-leukemic drugs, e.g. cytarabine, doxorubicin, etoposide and 6-mercaptopurine, or by agonistic anti-CD95 antibody. Notably, XIAP inhibitors kill leukemic blasts from children with ALL ex vivo and cooperate with TRAIL to induce apoptosis. In contrast to malignant cells, XIAP inhibitors at equimolar concentrations alone or in combination with TRAIL are non-toxic to normal peripheral blood mononuclear cells despite expression of the apoptosis-inducing TRAIL receptors on the cell surface, pointing to a therapeutic window. Most importantly, XIAP inhibitors significantly reduce leukemic burden in vivo in a mouse model of pediatric ALL engrafted in NOD/SCID mice. Thus, small molecule XIAP inhibitors present a promising novel approach for apoptosis-based therapy of childhood acute leukemia.