Abstract 597: The combination of valproic acid and fludarabine treatment induces a synergistic apoptotic response in chronic lymphocytic leukemia (CLL) patients involving the lysosomal protease cathepsin B.

Abstract

Abstract Chronic Lymphocytic Leukaemia (CLL) is the most common hematological malignancy in the western world. Fludarabine, a nucleoside analogue, is commonly used in combinational treatments for CLL. However, patients commonly develop resistance to fludarabine based therapies. Valproic Acid (VPA), an inhibitor of histone deacetylases (HDACs), showed synergistic apoptotic responses when combined with fludarabine in human leukemic cells and primary CLL cells. The mechanism for this synergistic apoptotic response is unknown. We found that fludarabine and VPA treatment increased activates caspases-8, -9, -3, and caspase-2 and a decreased in expression of anti-apoptotic proteins Mcl-1 and XIAP. Treatment with fludarabine alone or in combination with VPA led to the loss of lysosome integrity suggesting a leakage of the lysosomal content into the cytosol in response to these drugs. Chemical inhibition of a specific lysosomal protease, cathepsin B, using CA074-Me was sufficient to stabilize Mcl-1 and XIAP while reducing caspase activation and apoptosis. Addition of purified cathepsin B to leukemic cell lysates led to the reduction in protein levels of Mcl-1, XIAP and pro-caspase-2, thus suggesting that the re-localization of cathepsin B into the cytosol is sufficient to degrade these proteins. VPA treatment increased cathepsin B levels in both leukemic cell lines and primary CLL cells. VPA also increased cathepsin B activity. In addition, six relapsed CLL patients who had received at least one prior therapy with fludarabine were treated with VPA alone or in combination with fludarabine. No responses were seen after 28 days using VPA alone but in five patients who continued on VPA with fludarabine, three patients showed reduced lymphocyte count after receiving at most five cycles of the combination therapy. When the CLL cells from VPA-treated CLL patients were examined, VPA administration induced increased levels of histone-3 acetylation and cathepsin B expression in vivo. Thus, VPA and fludarabine synergistic apoptotic response is mediated by cathepsin B activation leading to a decrease in anti-apoptotic proteins in CLL cells and likely contributes to the clinical response observed in relapsed, fludarabine-refractory CLL patients. Citation Format: Ju Yoon Yoon, David Szwajcer, Ganchimeg Ishdorj, Pat Benjaminson, Rajat Kumar, James B. Johnston, Spencer B. Gibson. The combination of valproic acid and fludarabine treatment induces a synergistic apoptotic response in chronic lymphocytic leukemia (CLL) patients involving the lysosomal protease cathepsin B. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 597. doi:10.1158/1538-7445.AM2013-597