Recruitment of histone deacetylases and DNA hypermethylation of promoter regions of specific genes are two mechanisms of transcriptional repression and gene silencing which have been linked, and are implicated in differentiation block in AML. We hypothesized that the histone deacetylase inhibitor (HDI) depsipeptide could result in transcriptional de-repression, upregulation of specific target genes and differentiation of the leukemic clone in AML. Eighteen patients (pts), median age 60 years (range 25–77) with relapsed or refractory AML were enrolled on a multicenter Phase II study of depsipeptide in AML. Patients were stratified into 2 groups on study entry: Group A (n=14) included patients without specific chromosomal abnormalities known to recruit histone deacetylases. Group B (n=4) included patients with chromosomal aberrations such as the t(8;21), inv 16 and t(15;17) known to recruit histone deacetylases. Depsipeptide was administered intravenously at a dose of 18mg/m2/d on days 1, 8 and 15 of a 28 day cycle. Peripheral blood mononuclear cells were obtained prior to (hour 0), and after 4 (hr 4) and 24 hrs (hr 24), on days 1 and 8 of the first cycle of therapy for evaluation of histone acetylation by flow cytometry, and gene re-expression by REAL-time RT-PCR. Target genes of interest include MDR1, a target of HDI mediated upregulation, and p15INK4B (p15), a target of DNA hypermethylation in AML. MDR1 and p15 copy numbers are expressed as a normalized quotient of MDR1 and p15, respectively, to the housekeeping gene ABL. The drug has been well tolerated. 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, 2 of 4 pts (50%) in Group B, have had a disappearance of bone marrow blasts (blast percentage < 5%) in the setting of a normocellular marrow, with concomitant recovery of near-normal hematopoiesis following 1 and 2 cycles of therapy respectively. This anti-leukemic effect was short-lived, with both pts developing an increase in bone marrow blasts within 30 days of the initial response. Both of these patients also had translocations involving the AML1 gene {1 had t(8;21) and the other had a novel translocation t(4;21)}. Interestingly both of these responding pts and one other pt (75%) in cohort B demonstrated an increase in H3 acetylation at 4 and/or 24 hrs, in contrast to 4 of 14 pts (28%) in cohort A. There was an overall mean increase of 41% in MDR1 expression at hr 4 on days 1 and 8 (p=0.04). p15 expression was also upregulated at hr 4 on days 1 and 8 (91% mean increase, p=0.01).

We conclude that the HDI, depsipeptide, may have anti-leukemic activity in specific cytogenetic subsets of AML known to recruit histone deacetylases, and this is associated with a concomitant increase in histone acetylation. In addition, upregulation of specific target genes occurred in patient derived mononuclear cells, following depsipeptide treatment. The study remains open to accrual for pts with specific chromosomal abnormalities known to recruit histone deacetylases.

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