Interactions of Midostaurin (PKC412), Rapamycin and Ara-C in AML Cell Lines: Role of FLT3 Signalling.

Treatment options for patients with AML who are unfit for intensive chemotherapy are currently very limited. Commonly used treatment regimens such as low dose Ara-C result in complete remission rates of 17% and these remissions are not sustained (Burnett et al 2007). There is a rationale to examine the effect of combining small molecule inhibitors of signalling pathways with low dose Ara-C with the aim to improve its antileukaemic activity. To this end we have studied the cytotoxic interactions of the FLT3 inhibitor PKC412 and the mTOR inhibitor rapamycin with Ara-C in isobologram analysis (CalcuSyn). 4 cell lines were studied: 2 with FLT3 ITDs (MV4-11 and MOLM13) and 2 wild type for FLT3 (HL-60 and BV173). As expected synergy was seen between Ara-C and PKC412 in the FLT3 ITD positive cell lines (CI50: 0.47 MOLM13 and 0.68 MV4-11), but not in the wild type cell lines where antagonism was noted (CI50: 1.21 BV173 and 1.7 HL-60). In contrast, the Ara-C and rapamycin combination was synergistic in all cell lines, irrespective of FLT3 status. When the combination of PKC412 and rapamycin was studied, high levels of synergy were observed in the FLT3 ITD cell lines (CI50: 0.16 MOLM13 and 0.26 MV4-11) and in the BV173 cell line (CI50: 0.37) which has been shown to have autocrine activation of wild type FLT3 (Zheng et al, 2004) but not in HL-60 cells, where the combination was antagonistic (CI50: 3.4). FLT3 signalling pathways converge with the mTOR pathway via AKT and therefore our results suggest that a double hit on FLT3 signalling is likely to be beneficial in the treatment of FLT3 positive leukemias. In contrast rapamycin augments the cytotoxicity of AraC independently of Flt3 status. The combination of all 3 drugs produced no further benefit compared to the use of Ara-C and rapamycin alone. These promising drug combinations warrant further investigation in clinical studies.