FLT3 ligand impedes the efficacy of FLT3 inhibitors in vitro and in vivo

Abstract

We examined in vivo FLT3 inhibition in acute myeloid leukemia patients treated with chemotherapy followed by the FLT3 inhibitor lestaurtinib, comparing newly diagnosed acute myeloid leukemia patients with relapsed patients. Because we noted that in vivo FLT3 inhibition by lestaurtinib was less effective in the relapsed patients compared with the newly diagnosed patients, we investigated whether plasma FLT3 ligand (FL) levels could influence the efficacy of FLT3 inhibition in these patients. After intensive chemotherapy, FL levels rose to a mean of 488 pg/mL on day 15 of induction therapy for newly diagnosed patients, whereas they rose to a mean of 1148 pg/mL in the relapsed patients. FL levels rose even higher with successive courses of chemotherapy, to a mean of 3251 pg/mL after the fourth course. In vitro, exogenous FL at concentrations similar to those observed in patients mitigated IntroductionAcute myeloid leukemia (AML) patients who harbor the FLT3/ITD mutation have an exceptionally poor prognosis. 1,2 During the past decade, efforts have been underway to develop FLT3 inhibitors in the hopes of improving outcomes for these patients. 3 Several agents have now been studied as monotherapy for this disease, and the results have been only modestly successful. Although there have been a few remissions reported, the responses are usually limited to clearance of peripheral blasts, with persistence of disease in the marrow. In light of this, attention has turned to incorporating these agents into existing chemotherapy regimens, on the hypothesis that FLT3 inhibition will synergize with chemotherapy in inducing cytotoxicity. 4 Alternately, others have postulated that mobilizing the leukemia cells from the marrow might enhance the efficacy of FLT3 inhibition and have therefore tested FLT3 inhibitors in combination with CXCR4 inhibition. 5 Several large trials of chemotherapy administered in combination with FLT3 inhibitors are either actively accruing or have recently completed accrual. Because chemotherapy may alter the pharmacokinetics of FLT3 inhibitors and therefore affect target inhibition in vivo, we examined FLT3 inhibition in patients receiving lestaurtinib, an indolocarbazole FLT3 inhibitor, from 2 separate trials in which the agent was combined with chemotherapy. We noted a discrepancy in the degree of FLT3 inhibition, as measured by a plasma inhibitory activity assay, between the 2 groups of patients. We hypothesized that high levels of FLT3 ligand (FL), a cytokine that is known to increase after myelosuppressive therapy, could be interfering with FLT3 inhibition in these trials. We have therefore examined FL levels in response to chemotherapy and FLT3 inhibition, as well as the effect of FL levels on the efficacy of FLT3 inhibitors in vitro and in vivo. Our findings may explain why blasts in the bone marrow are more resistant to FLT3 inhibitors and furthermore have important implications both for FLT3 mutant AML as a disease as well as for efforts to incorporate FLT3 inhibitors into AML therapy. Methods ...

NADPH Oxidase-generated Hydrogen Peroxide Induces DNA Damage in Mutant FLT3-expressing Leukemia Cells

“…Here we show for the first time that the stimulation of FLT3-WT with FL results in an up-regulation of p22 phox , an increase in total and nuclear H 2 O 2 as well as DNA DSBs. It has been reported that FLT3 inhibitors were demonstrated to induce expression and release of FL (48). We suggest that the presence of FL in the bone marrow could lead to genomic instability through ROS generation.…”

High activity of sorafenib in FLT3-ITD-positive acute myeloid leukemia synergizes with allo-immune effects to induce sustained responses

“…As one mechanism it has been suggested that AML chemotherapy leads to elevated serum levels of the FLT3-ligand that presumably induces kinase inhibitor drug resistance by autocrine stimulation of FLT3-ITD. 30,31 Thus, although FLT3-ITD has been recognized as a bona fide target for therapeutic intervention with kinase inhibitors, beneficial treatment combinations, optimal treatment time points and algorithms to integrate FLT3 inhibitors are still undefined.…”

Targeting the FMS-like tyrosine kinase 3 in acute myeloid leukemia

“…60 This may have been related to elevations in plasma FLT3 ligand and a-1 acid glycoprotein levels in response to chemotherapy. 61 Samples from patients in four separate lestaurtinib trials demonstrated that FLT3 ligand levels rise substantially after induction chemotherapy. This was particularly apparent in relapsed patients, who had mean FLT3 ligand levels of 1148 pg/ml on day 15 of induction therapy vs 488 pg/ml in newly diagnosed patients.…”

“…81 Resistance to FLT3 inhibitors can occur, primarily through upregulation of the FLT3 receptor, 82 point mutations 83 or increased FLT3 ligand levels. 60,61 In vitro screens have identified four mutations in the ATP-binding pocket of FLT3 that confer varying degrees of resistance to FLT3 inhibitors. 83 FLT3 clones with mutations at alanine 627, asparagine 676 or phenylalanine remain sensitive to higher concentrations of most inhibitors, but a glycine-to-arginine mutation at position 697 conferred resistance to all nine tyrosine kinase inhibitors tested and, when it arises, may necessitate alternative therapeutic approaches.…”

“…It is clear that FLT3 ligand can interfere with tyrosine kinase inhibition of FLT3. 61 Myeloablative radiation and chemotherapy induce marked increases in FLT3 ligand. [84][85][86] This is especially problematic in heavily treated patients with relapsed disease.…”