The HDACi SAHA also induced down-regulation of Mcl-1. Furthermore, Mcl-1 protein is a direct cleavage substrate of activated caspase-3. We noted that the amount of Mcl-1 correlated with iuduction of activated caspase-3. Our results demonstrate that SAHA enhances BPR1J-340 inhibition activity in FLT3-ITD + cells and suggests that the enhancement is due to HDACi-induced reduction of FLT3-ITD, STAT5, and Mcl-1. However, the underlying mechanism of enhanced action by combination treatment remains to be further elucidated. The maximum achievable plasma concentration of BPR1J-340 after a single 1.5 mg/kg in rat is more than 272-fold above the IC50 for FLT3-ITD inhibition in biochemical and cellular assays. Even at 24 hour after the single dosing, the plasma levels of BPR1J-340 were close to the IC50 value for inhibition of FLT3ITD. In addition, the high Vss indicated that the distribution of BPR1J-340 into deep tissue compartments, including tumor tissue, is expected. These LY2835219 citations pharmacokinetic properties suggest that BPR1J-340 dosing once a day is sufficient for continuous inhibition of FLT3 activity in rats or mice. To examine whether BPR1J-340 exhibits antitumor activity in vivo, MOLM-13 cells were subcutaneously implanted into nude mice. Our results demonstrated that BPR1J-340 administration resulted in significant tumor regression and tumor shrinkage in this MOLM-13 tumor model. In comparison with sulfonamide BPR1J-97in the same model, BPR1J340 results in a higher CR ratioat a lower dose. These data demonstrated that BPR1J-340 is superior to the sulfonamide compound BPR1J-097 in an in vivo efficacy study. In conclusion, results from this study demonstrate that BPR1J340 exhibits high potency and excellent selectivity against FLT3 kinase, strong suppression of 
the FLT3-ITD survival signaling pathway, favorable pharmacokinetic properties, and complete tumor regression in a FLT3-ITD xenograft model. These data together support further clinical investigation of PR1J-340 in patients with AML. In addition, the BPR1J-340 potentiated the anti-proliferative activity of the HDAC inhibitor SAHA against human leukemia cells. The combination of SAHA and BPR1J-340 should be a good candidate therapy to develop as a treatment in AML and further investigation in clinical study is warranted. Chronic myeloid leukemiais a myeloproliferative disorder of hematopoietic stem cells GDC-0879 caused by the presence of the BCR-ABL oncogene in the so-called Philadelphia chromosome. The BCR-ABL fusion protein is a constitutively active tyrosine kinase and activator of downstream molecules such as Myc and signal transducer and activator of transcription. BCR-ABL activity promotes the growth of leukemic cells and enhances malignant expansion of hematopoietic stem cells. The clinical outcome for patients with CML is improved by imatinib mesylate. Imatinib was the first ABL tyrosine kinase inhibitorthat was identified to reduce BCRABL kinase activity. In the International Randomized Study of Interferon and STI571trial, imatinib treatment resulted in a high level of cytogenetic response. However, some patients developed resistance to imatinib, which could be attributed to point mutations in the kinase domain of BCR-ABL. These BCR-ABL mutations directly impede contact between the BCRABL protein and imatinib. Recently, second-generation ABL TKIs dasatiniband nilotinibhave been increasingly used for patients resistant to or intolerant of imatinib therapy, and have been approved for front line use in patients with chronic phase CML. However, one point mutation, T315I, located in the gatekeeper region of the ATP-binding site, confers resistance to imatinib, dasatinib, and nilotinib.