Abstract 4508: Targeting XIAP as a novel paradigm to induce apoptosis in chronic lymphocytic leukemia

Abstract

Abstract Evasion of apoptosis is a hallmark of chronic lymphocytic leukemia (CLL), calling for new concepts to overcome apoptosis resistance. We therefore investigated whether targeting the antiapoptotic protein XIAP by small molecule inhibitors would sensitize for apoptosis. Here, we provide first evidence that XIAP inhibitors in combination with the death receptor ligand TRAIL present a new strategy to synergistically trigger apoptosis in CLL even in subgroups with resistant disease. Analysis of apoptosis regulatory proteins reveals that XIAP, cIAP1 and cIAP2 are expressed at high levels in primary CLL samples. Proofs of concept studies in CLL cell lines demonstrate that subtoxic concentrations of several distinct XIAP inhibitors significantly enhance TRAIL-induced apoptosis. By comparison, no sensitization for death receptor-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. Importantly also in primary CLL samples, subtoxic concentrations of XIAP inhibitors act in concert with TRAIL to trigger apoptosis in 18 of 27 cases (67%), whereas primary CLL cells are resistant to treatment with TRAIL alone. Analysis of combination index reveals that this interaction of XIAP inhibitor and TRAIL is highly synergistic. Mechanistic studies in primary CLL cells show that the addition of XIAP inhibitor profoundly enhances TRAIL-induced cleavage of caspase-3 into active fragments and significantly increases caspase-3 enzymatic activity. The broad range caspase inhibitor zVAD. fmk completely blocks apoptosis in response to combination treatment with XIAP inhibitor and TRAIL, pointing to caspase-dependent apoptosis. Intriguingly, the cooperative interaction of XIAP inhibitor and TRAIL is even evident in several distinct subgroups of CLL patients with poor prognostic features, including patients with 17p deletion, TP53 mutation, chemotherapy-refractory disease or unmutated VH genes. Interestingly, we found that cases with unmutated VH genes are significantly more sensitive to XIAP inhibitor- and TRAIL-induced apoptosis compared to VH gene mutated samples, pointing to a role of B-cell receptor signaling in the regulation of apoptosis in CLL cells. In conclusion, we provide first evidence that XIAP inhibitors in combination with TRAIL present a novel strategy to trigger apoptosis even in resistant forms and poor prognostic subgroups of CLL. These findings have important implications for the development of innovative approaches to overcome the intrinsic resistance to apoptosis in CLL. Since IAP inhibitors as well as TRAIL receptor agonists as single agents are currently already under evaluation in early clinical trials, it is feasible that such combination protocols of XIAP inhibitors and TRAIL could be translated into clinical application in CLL. 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 4508.