XIAP Inhibitors Present a Promising New Strategy to Sensitize Childhood Acute Leukemia Cells for Chemotherapy-Induced Apoptosis.

Abstract 3791

Poster Board III-727

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. Increasing evidence suggests that high levels of “Inhibitor of Apoptosis” (IAP) proteins may represent a key antiapoptotic mechanism in cancer cells including acute leukemia. Among the IAP family members, it is especially X-linked inhibitor of apoptosis (XIAP) that is known for its antiapoptotic function. Since XIAP blocks apoptosis at a central point of the apoptotic machinery by inhibiting activation of effector caspases, we explored whether XIAP presents a suitable molecular target for therapeutic intervention in childhood leukemia.

Here, we report that neutralizing XIAP by small molecule inhibitors is a novel and effective approach to sensitize childhood acute leukemia cells for chemotherapeutic drugs, which are currently used in clinical protocols for the treatment of children with acute leukemia. Subtoxic concentrations of XIAP inhibitors synergize with various anticancer drugs, for example Cytarabine, Vincristine, Doxorubicin, Etoposide and cyclophosphamide, to induce apoptosis in ALL and also in AML cells. By comparison, no sensitization for chemotherapy-induced apoptosis is observed in the presence of a structurally related control compound that only weakly binds to XIAP, demonstrating the specificity of the sensitization effect of XIAP inhibitors. In addition, XIAP inhibitors act in concert with anticancer drugs to reduce clonogenic growth of ALL cells demonstrating that they also suppress long-term survival. Analysis of signaling pathways reveals that XIAP inhibitors enhance chemotherapy-induced activation of caspases, loss of mitochondrial membrane potential and cytochrome c release in a caspase-dependent manner. Further, XIAP inhibitors cause rapid and profound downregulation of cIAP1 accompanied by activation of NF-κB. Of note, inhibition of RIP1 kinase by necrostatin or caspases by the broad range caspase inhibitor zVAD.fmk also significantly reduces the XIAP inhibitor-mediated sensitization to cytotoxic drugs. Intriguingly, the addition of TNFα blocking antibodies also significantly decreases apoptosis upon combined treatment with XIAP inhibitors and chemotherapeutic drugs, indicating that paracrine/autocrine production of TNFα is involved in this synergistic interaction. In support of this notion, addition of soluble recombinant TNFα further increases apoptosis that is induced by XIAP inhibitors and anticancer drugs. Importantly, XIAP inhibitors kill leukemic blasts from children with ALL ex vivo and act in concert with chemotherapeutic drugs to trigger apoptosis. In contrast to malignant cells, XIAP inhibitors at equimolar concentrations alone or in combination with chemotherapeutics are non-toxic to normal peripheral blood lymphocytes, pointing to a therapeutic window. By demonstrating that XIAP inhibitors present a promising novel approach to enhance the efficacy of chemotherapy in childhood acute leukemia, our findings have important implications for the development of innovative treatment strategies to overcome apoptosis resistance in children with high risk leukemia. This approach could be translated into clinical application in childhood ALL, since IAP inhibitors have recently entered evaluation in early clinical trials, thus underscoring the feasibility to incorporate XIAP inhibitors into chemotherapy protocols.