Most pts with AML are not cured with conventional chemotherapy regimens. Gain of function mutations & overexpression of tyrosine kinase (TK) receptors (ie KIT & FLT3) are associated with leukemogenesis and poor outcome & represent therapeutic targets. The use of TK inhibitors in AML has been disappointing. Recently we showed that BOR downregulates KIT in AML cells by upregulating miR-29b that disrupts a KIT transactivating complex comprising SP1 and NFκB (p65). In silico we found SP1 & p65 binding sites also in the FLT3 promoter region & speculated that FLT3 may be regulated similarly. Utilizing chromatin immunoprecipitation (CHIP) in MV4-11 cells expressing high FLT3 levels we showed enrichment of SP1 (1.6 fold) & p65 (3.1 fold) vs IgG control on the FLT3 promoter. Cotransfection of a luciferase reporter harboring the FLT3 promoter with SP1 or p65 vectors in 293T cells increased luciferase activity (3.1 & 3.0. fold); siRNA mediated knock-down of SP1 or p65 decreased activity (0.5 & 0.7 fold). In KG1 cells with low FLT3 levels, overexpression of SP1 or p65 increased FLT3 (1.7 & 1.6 fold); in MV4-11 cells siRNA-mediated knockdown of SP1 or p65 decreased FLT3 (0.05 & 0.3 fold) mRNA & protein. Since miR-29b can disrupt the SP1/p65 complex we overexpressed miR-29b in MV4-11 cells, which consequently downregulated FLT3 mRNA & protein level in MV4-11 cells to barely detectable levels. Both BOR & DEC can upregulate miR-29b; thus these drugs potentially decrease FLT3. Indeed BOR (60nM at 24h) or DEC (2.5µM at 48h) led to decreased FLT3 in MV4-11 cells (<.001 & 0.5 fold) & in primary AML blasts (0.28 & 0.42 fold). We concluded that BOR & DEC target the expression of FLT3 via upregulation of miR-29b & disruption of the SP1/p65 complex. To validate these results, we assessed the expression of FLT3 in AML pts enrolled on a Phase I clinical trial (NCT00703300) that tested the BOR & DEC combination. Pts with relapsed/refractory AML or age >60 years with previously untreated disease received DEC at 20mg/m2 IV daily on days 1-10. BOR was gradually dose escalated to 1.3 mg/m2 given on days 5, 8, 12 & 15. Cycles were repeated every 28 days. Pretreatment and day 26 expression levels of FLT3 and miR-29b were assessed. 19 patients were enrolled with a median age of 69 years (range: 32-83). Pts received a median of 2 cycles of therapy (range, 1-14 cycles). Febrile neutropenia & infectious complications were frequent. BOR & DEC at target doses were tolerable; neuropathy following repetitive cycles of BOR limited its administration. 7 pts achieved disease remission. Consistent with our preclinical results, miR-29b expression increased (median fold change: 3.9) and FLT3 expression levels decreased (median fold change: 0.4). As the FLT3 targeting results are encouraging a phase II study of BOR & DEC combination is being planned with modifications of BOR dosing to avoid neuropathy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr CT-06. doi:1538-7445.AM2012-CT-06