Abstract PR-8: XIAP inhibitors prime acute leukemia cells for TRAIL‐ or chemotherapy‐induced apoptosis, bypass Bcl‐2‐imposed resistance and exert antileukemic activity in a NOD/SCID mouse model of pediatric acute leukemia

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” (IAP) proteins, 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. Therefore, we investigated the effect of small molecule XIAP inhibitors alone and in combination with the death receptor ligand TRAIL or chemotherapeutic drugs on apoptosis induction in ALL cell lines, primary leukemic blasts from children with ALL, normal peripheral blood lymphocytes and in a mouse model of pediatric ALL engrafted in NOD/SCID mice. XIAP inhibitors at subtoxic concentrations, but not a structurally related control compound, synergize with TRAIL to induce apoptosis in several ALL cell lines. 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 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, which are currently used in clinical protocols for the treatment of children with acute leukemia such as cytarabine, doxorubicin, etoposide and 6‐mercaptopurine, as well as for CD95‐triggered apoptosis. Notably, XIAP inhibitors alone and combined with TRAIL induce apoptosis in leukemic blasts from children with ALL ex vivo. 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 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 novel and effective approach to sensitize childhood acute leukemia cells for TRAIL‐ or chemotherapy‐induced apoptosis. These findings have important implications for the development of apoptosis‐based therapies for childhood leukemia. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):PR-8.