Acute myeloid leukemia (AML) is the second most-common form of leukemia in children. Although, the 5-year survival rate for children with AML is estimated at 64%, nearly half of the patients have refractory disease. Novel targeted therapeutics with minimal side effects are required to improve the outcome in kids with refractory disease. One class of molecular targeted therapeutics that shows specific anti-leukemic effects while sparing normal hematopoeitic progenitor cells includes inhibitors of epigenetic modifications such as DNA methylation and histone deacetylation. Although DNA methyltransferase (DNMT) inhibitors are currently on clinical trials for AML, their efficacy as single agents is limited and the mechanism of action is unknown. This limited efficacy could be due to co-regulatory effects of DNA and histone modifications on gene expression. Therefore, DNA hypomethylating agents when used in combination with other epigenetically active agents such as histone deacetylase (HDAC) inhibitors should achieve greater efficacy by optimal re-expression of tumor suppressor genes silenced in AML. Our data shows a synergistic induction of cytotoxicity upon treatment with azacytidine (DNMT inhibitor) and panobinostat (HDAC inhibitor) with combination indices ranging from 0.6 to 0.8 in a variety of pediatric AML cell lines bearing distinct chromosomal abnormalities. Furthermore, immune-deficient (NSG-B2m) mice transplanted with MV4;11 cells via the tail vein were treated i.p. with 20 doses of maximally tolerated dose of 2.5 mg/Kg azacytidine and/or 2.5 mg/Kg panobinostat over a period of 33 days. The mice treated with azacytidine or panobinostat increased median survival by 26 and 6 days respectively. However, mice treated with both drugs showed a drastic reduction in leukemic burden leading to complete remission till the end of their life (around 2 years). Reduced leukemic burden and prolonged survival was also observed in AML-193 xenografted mice treated with the drug combination. In an effort to understand the mechanism of synergism between azacytidine and panobinostat, an Infinium Human Methylation 450K Bead Chip array was performed to identify methylation status of 400,000 CpG islands spanning promoter regions and island shores. Azacytidine treatment reduced the number of hypermethylated CpG islands by 30%, while panobinostat had no effect. However, treatment with drug combination did not change the number of hypermethylated CpG islands indicating that the observed synergy both in vitro and in vivo might result from specific activation of a subset of gene targets. Identification and validation of targets that mediate these synergistic effects is in progress to improve the selection of patients most likely to benefit from combination therapy.

Citation Format: Anilkumar Gopalakrishnapillai, E Anders Kolb, Sonali P. Barwe. Combination of epigenetic modifiers achieves complete remission in xenograft models of pediatric acute myeloid leukemia. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-213. doi:10.1158/1538-7445.AM2015-LB-213