Pre-Clinical Development of Suberoylanilide Hydroxamic Acid (SAHA, Vorinostat ™) for the Treatment of Pediatric Acute Leukemias.

Acute leukemia is the most common malignancy of childhood and despite a good outcome for the majority of patients, a proportion of patients has high-risk features, and a very poor prognosis for ultimate cure. As such, novel approaches to therapy and new agents are clearly needed to improve the efficacy and quality of treatment available. We are investigating the utility of several histone deacetylase inhibitors (HDACIs) in the treatment of pediatric acute leukemias. Two HDACIs, suberoylanilide hydroxamic acid (SAHA, Vorinostat™) and MS-275, exhibited broad spectrum anti-tumor activity against a panel of 12 pediatric ALL and 8 AML cell lines representing different cell lineages and stages of development, with clinically achievable IC50 values of approximately 200–900nM. Both agents induced apoptosis in a dose-dependent manner. However, SAHA and MS-275 had differential effects on cell cycle progression and differentiation. Treatment with MS-275 resulted in sustained accumulation of cells in G1 phase and cell line-specific changes in the expression of multiple hematopoietic differentiation markers, including HLA-DR, CD11b, CD13, CD33, and CD61. Differentiation of both AML and ALL cell lines was induced. In contrast, treatment with SAHA either had no effect on cell cycle distribution or caused a transient arrest in G1 phase, which was reversible within 24–48 hours. In addition, only minimal changes in the expression of defined hematopoietic differentiation markers were observed following treatment with SAHA. These data suggest that the anti-leukemia effects mediated by SAHA and MS-275 occur by different biochemical and cellular mechanisms. Consistent with this hypothesis, SAHA and MS-275 exhibited synergistic anti-leukemic activity in vitro in a sequence dependent-manner, with a mean combination index of 0.666 +/− 0.048 based on median effect analysis. Sequence-dependent synergy was noted in a variety of T-cell and pre-B cell lines, including cell lines possessing MLL translocations. Both SAHA and MS-275 exhibited synergistic interactions with drugs representing the topoisomerase II, DNA methyltransferase, and proteosome inhibitor classes based on median effect analysis and/or the universal response surface approach. Detailed data demonstrating the mechanism(s) of HDACI-mediated anti-leukemic synergy and sequence-dependence with these other agents will be presented.