Inhibition of Flt3‐activating mutations does not prevent constitutive activation of ERK/Akt/STAT pathways in some AML cells: a possible cause for the limited effectiveness of monotherapy with small‐molecule inhibitors

Abstract

The Flt3 receptor tyrosine kinase is a critical mediator in the pathogenesis of acute myeloid leukaemia (AML). Flt3-activating mutations have been associated with poor prognosis and decreased overall survival of AML patients, thus Flt3 constitutes an ideal target for drug treatment of such disease. Unfortunately, the monotherapy with small-molecule tyrosine kinase inhibitors in clinical trials shows that remission is not permanent, presumably by resistance of Flt3 mutants to inhibitors. An alternative approach for treatment is based on the cooperation between Flt3 and additional intracellular pathways for AML transformation in some patients. Thus, the inhibition of both Flt3 and such pathways may be exploited for successful treatment of the disease. We investigated the importance of Flt3-activating mutations for the constitutive activation of intracellular pathways in primary AML cells, and their effect on cell survival. We found that the main compounds involved in the differentiation, proliferation and survival of AML (MAPK/AKT/STAT) were constitutively activated. However, only four samples showed internal tandem duplications (ITDs) for Flt3. Surprisingly, contrary to previous reports, we found that inhibition of ITD/Flt3 activity did not prevent the phosphorylation of ERK, STAT5 or Akt in some primary AML cells. In parallel, we found that in these cells, Flt3 and ERK or Akt cooperate to regulate cell survival. Our results support the hypothesis that the optimal therapeutic treatment of AML may require not only the oncogenic tyrosine kinase, but also the appropriate combination of different specific inhibitors, thus providing a more effective approach to reverse leukaemogenesis. Thus, we propose that each AML patient should have an individually tailored combination treatment.

MEK inhibition induces caspases activation, differentiation blockade and PML/RARα degradation in acute promyelocytic leukaemia

“…Moreover, it is well established that downmodulation of MEK/ERK activity (Lunghi et al, 2003;Ló pez-Pedrera et al, 2004;Siendones et al, 2007) through the activation of caspases (Allan et al, 2003). Accordingly, we found that the combined treatment with PD98059 and the caspase inhibitor (z-VAD-fmk) restored PML/RARa expression in APL cells, suggesting that caspase activation might be responsible for the degradation of PML/ RARa related to the MEK/ERK blockade.…”

“…Recent evidence indicated that blast cells from most acute myeloid leukaemias (AML), including APL, showed constitutive activation of extracellular signal-regulated kinases 1/2 (ERK 1/2) (Siendones et al, 2007), as well as of the immediately upstream kinases ERK (Platanias, 2003). The relevant role of this pathway in cell survival and suppression of apoptosis in leukaemic cells has been recently demonstrated by Milella et al (2001), who showed that the downmodulation of MEK1 phosphorylation induced apoptosis and inhibited the proliferation of primary blasts, but not healthy cells.…”

FLT3 inhibitors for acute myeloid leukemia: a review of their efficacy and mechanisms of resistance

“…In addition, mutations in Cbl can interfere with degradation of the FLT3 receptor, thereby leading to increased FLT3 activity [94]. Mutations or expression modification of genes regulating apoptosis, such as BCL2, MCL-1, and NF-jB, can render leukemic cells independent of FLT3 signaling [86,[95][96][97]. In an in vitro model, midostaurin resistance appeared to be in part governed by up-regulation of anti-apoptotic signals and down-regulation of pro-apoptotic pathways [98].…”

Drug resistance in mutant FLT3-positive AML

“…Combined treatment using FLT3 inhibitors and chemotherapeutic agents It has been shown that, in at least some primary AML cells, there is continued phosphorylation of ERK, STAT5 or AKT following inhibition of FLT3-ITD, which may contribute to the limited efficacy of FLT3 inhibitors used as single agents for the treatment of mutant FLT3-positive AML (Siendones et al, 2007). In addition, it has recently been shown that FLT3-ITD inhibition of autophosphorylation does not always result in cell death, which raises the question of the importance of FLT3 signaling for some mutant FLT3-positive AML (Pratz et al, 2010).…”