Abstract 4351: p53 is a critical determinant of the antileukemic interactions between histone deacetylase inhibitors and chemotherapy drugs in pediatric acute myeloid leukemia cells
Authors: Xie Chengzhi , Edwards Holly , Xu Xuelian , LoGrasso Salvatore B. , Stout Mark L. , Buck Steven , Matherly Larry H. , Taub Jeffrey , Ge Yubin 
Published: June 2012

Abstract
Acute myeloid leukemia (AML) accounts for one-fourth of acute leukemias in children, but it is responsible for more than half of the leukemia deaths in this patient population. Resistance to ara-C/anthracycline-based chemotherapy is a major cause of treatment failure in this disease. Among the newer agents that have been investigated in high-risk AML, clofarabine and valproic acid (VPA) are particularly notable. Clofarabine is a new FDA-approved drug for treating pediatric relapsed acute lymphoblastic leukemia and is currently being tested in pediatric AML. VPA is a FDA-licensed drug for treating both children and adults with epilepsy that has been shown to act as a histone deacetylase inhibitor (HDACI). We hypothesized that VPA and clofarabine might exhibit synergistic antileukemic activity in pediatric AML. In this study, we demonstrated highly synergistic antileukemic activities of combined clofarabine and VPA in 9 diagnostic blast samples from children with de novo AML. In contrast, both additive-to-synergistic [in Kasumi-1 which harbors t(8;21) and MV4-11 which harbors t(4;11)] and antagonistic (in THP-1 and CMS) results were obtained in 4 pediatric AML cell lines. AML cells which harbored t(8;21) were substantially more sensitive to VPA and showed greater responses to the drug combination than non-t(8;21) AML cells. Essentially the same results were obtained when clofarabine was combined with SAHA (a FDA-licensed pan-HDACI) in the pediatric AML cell lines. Co-treatment with clofarabine and VPA resulted in synergistic induction of apoptosis in Kasumi-1 cells, accompanied by synergistic activation of caspase-3 and cleavage of PARP. Our mechanistic studies revealed cooperative induction of DNA double-strand breaks (as reflected by induction of gH2AX), phosphorylation and acetylation of p53, and activation of Bax by clofarabine and VPA. Automated DNA sequencing revealed a p53 deficiency in THP-1 and CMS cell lines which may account for the antagonistic cytotoxic interactions between clofarabine and VPA, whereas the rest of the cells harbored either wild-type or mutant p53. Interestingly, treatments of Kasumi-1 cells with ara-C and VPA also resulted in cooperative activation of p53 and Bax, which may explain the differential responses of pediatric AML cells to combined ara-C and VPA observed in our previous study (Xie C et al. Clin Cancer Res 2010). Collectively, our results provide compelling evidence that p53 plays a key role in the synergistic cytotoxic effects of HDACIs combined with chemotherapy drugs in pediatric AML cells.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4351. doi:10.1158/1538-7445.AM2011-4351