The Histone Deacetylase Inhibitor (HDI) Depsipeptide Has Differential Activity in Core Binding Factor AML.

Recruitment of histone deacetylases and CpG island methylation at promoter regions of specific genes are two mechanisms of epigenetic silencing which have been linked and are implicated in differentiation block in AML. We hypothesized that the HDI depsipeptide could cause transcriptional de-repression and upregulation of specific target genes in AML, with subsequent differentiation of the leukemic clone. Twenty-one patients (pts), median age 60 years (range 25–77) were enrolled on a multicenter Phase II study of depsipeptide in AML. Pts were stratified into 2 groups on study entry: Group A (n= 14) included pts without specific chromosomal abnormalities known to recruit histone deacetylases. Group B (n=7) included pts with chromosomal aberrations such as the t(8;21) and inv 16 known to recruit histone deacetylases. All 7 pts in cohort B had translocations involving CBFα or AML1 (n=6) or CBFβ (n=1). Depsipeptide was administered intravenously at a dose of 13mg/m2/d on days 1, 8 and 15 of a 28 day cycle. Peripheral blood mononuclear cells were obtained prior to, and after 4 and 24 hrs, on days 1 and 8 of the first cycle of therapy for evaluation of histone (H3) acetylation by flow cytometry, and gene re-expression by Quantitatative real-time RT-PCR (RQ-PCR). Target genes of interest include MDR1, a target of HDI mediated upregulation, p15^INK4B(p15) a target of DNA hypermethylation in AML, and p14ARF (p14), a target of AML1-ETO mediated transcriptional repression. H3 acetylation at the p15 and MDR1 promoters was analyzed by chromatin immunoprecipitation, followed by Q- PCR. The most common adverse effects noted included grade 1/2 nausea, vomiting and fatigue. No objective evidence of response (CR or PR) or other evidence of antileukemic activity has been seen in group A. In contrast, in group B, antileukemic activity has been observed in 4 of 7 (57%) of pts. These include 2 pts with clearance of bone marrow (BM) blasts in the setting of a normocellular marrow, and 2 other pts with a significant decrease (>50% decrease) BM blasts. This effect was short-lived, with all 4 pts developing evidence of disease progression within 30 days of the initial response. Interestingly 5 of 7 pts (including all 4 pts with evidence of an antileukemic response) in cohort B demonstrated an increase in global H3 acetylation at 4 and/or 24 hrs, in contrast to 4 of 14 pts (28%) in cohort A. Furthermore, in cohort B, at 24hrs, there was a 75% mean increase in MDR1 expression (p=0.005), a 162% mean increase in p15 (p=0.01) and a 106% mean increase in p14 (p<0.0001). Although there was a trend towards upregulation of MDR1, p15 and p14 expression by hr 24 in cohort A (41%, 29% and 34% mean increase in expression respectively), these changes were not statistically significant. In 4 pts analyzed, there was evidence of an increase in H3 acetylation by hr 4 at the MDR1 promoter (p=0.03) and at the p15 promoter (p=0.02).

We conclude that the HDI, depsipeptide, may have anti-leukemic activity in Core Binding Factor AML, and this is associated with a concomitant increase in histone acetylation and upregulation of specific target genes. Further development of this agent in this subset of AML should focus on combination strategies particularly those that involve other agents that target epigenetic changes such as DNA hypomethylating agents.