Introduction
Acute myeloid leukemia (AML) represents a group of hematopoietic stem cell malignancies that affect approximately two to three adults per 100,000 each year in Western countries. Outcomes for the majority of patients remain poor. Cytogenetic aberrations represent one of the most important independent prognostic factors in AML. Over the past decade, significant progress has been made in the understanding of the cytogenetic and molecular determinants of AML pathogenesis. This has allowed development of risk-stratified treatment options in order to offer more adapted treatment options to patients at high risk.
FMS-like tyrosine kinase 3
FMS-like tyrosine kinase 3 (FLT3) is one of the most commonly mutated genes in AML. FLT3 is a transmembrane tyrosine kinase that belongs to the class 3 split-kinase domain family of receptor tyrosine kinase [1]. FLT3 is mutated in about 30% of AML cases [2, 3]. Two major classes of activating mutations have been identified: internal tandem duplications (ITDs) (24%) which affect the juxtamembrane domain of the receptor [4] and tyrosine kinase domain (TKD) point mutations in the activation loop, with a majority at the D835 residue (7%) [2, 5]. Both types of mutations lead to constitutive activation of FLT3, and the FLT3-ITD mutations in particular were clearly identified as being associated with a worse prognosis [6, 7]. Next-generation sequencing studies have established that these mutations occur relatively late in leukemogenesis [8]. However, FLT3 is of importance in maintaining the transformed state [9]. Inhibition of the receptor tyrosine kinases using small molecules represents an attractive therapeutic target.
FLT3 inhibitors
Non-selective inhibitors
In the last decade, several molecules with activity against FLT3 have been tested. Six of them have been or are currently tested in randomized trials. The first-generation FLT3 inhibitors (lestaurtinib, midostaurin, sorafenib) are multikinase inhibitors. The relative non-selectivity of some of these agents, and suboptimal pharmacokinetics associated with others has led to unimpressive results as single agents. Most are not inhibitors of FLT3 specifically, but often have multiple kinase targets. As single agents, they also demonstrated only limited activity with best responses on peripheral blast clearances. However, they showed interesting results in combination with intensive chemotherapy. Sorafenib has demonstrated activity against AML with FLT3-ITD mutation in combination with chemotherapy for relapsed but also newly diagnosed patients with AML [10–13]. Sorafenib has also shown promising efficacy in combination with hypomethylating agents (azacitidine) [14], low-dose homoharringtonine [15], or low-dose cytarabine [16] in primary refractory FLT3-ITD+ AML. The first results with midostaurin suggested further investigation of midostaurin in combination with other agents [17]. In the recent CALGB 10603 (RATIFY) study, the addition of midostaurin to standard chemotherapy followed by one year of midostaurin maintenance therapy conferred an equal benefit across all FLT3 subgroups (even those with FLT3-TKD mutations) for both overall survival (OS)and event-free survival in both uncensored and censored for transplant analyses [18]. However, combinations with chemotherapy were not all successful. Trials conducted by the Medical Research Council did not suggest an overall survival or event-free survival benefit for the combination of lestaurtinib with chemotherapy [19].
Selective inhibitors
More recently, more selective and more potent FLT3 inhibitors have been developed (quizartinib, crenolanib, gilteritinib). The second-generation FLT3 inhibitor, quizartinib (which also inhibits c-KIT stem cell factor CD117 expressed in more than 70% of AML) has demonstrated encouraging results based on a high response rate when administered as monotherapy [20–22]. However, patients may experience greater clinical benefit when quizartinib is administered in combination with intensive chemotherapy [23, 24]. Among current clinical trials, a phase I/II trial with quizartinib in combination with 5-azacitidine or low-dose cytarabine is ongoing in patients >60 years of age with previously untreated AML (NCT01892371). An ongoing phase III (QuANTUM-R), open-label study of quizartinib monotherapy versus salvage chemotherapy in patients with FLT3-ITD mutations who are refractory or relapsed in six months with or without hematopoietic stem cell transplantation (HSCT) is also still enrolling (NCT02039726). A study of quizartinib as maintenance therapy is also ongoing but showed promising early efficacy data in patients with AML who have been allografted in first or second remission [25]. The QuANTUM-FIRST protocol, a phase III, double-blind, placebo-controlled study of quizartinib administered with induction and consolidation, and administered as maintenance therapy in subjects 18 to 75 years old with newly diagnosed FLT3-ITD+ AML has just opened (NCT02668653). Other second- or third-generation FLT3 tyrosine kinase inhibitors are under investigation. They could even be more promising in the way they could show higher cytotoxicity and overcome the emergence of therapeutic resistance that could be observed with quizartinib and first-generation FLT3 inhibitors. Crenolanib suppresses growth of leukemic cells harboring both FLT3-ITD and FLT3-TKD mutations, the latter of which are increasingly seen to emerge as resistant mutations after FLT3 inhibitor therapy [26, 27]. Crenolanib can bind the active or inactive conformations of receptor tyrosine kinases, unlike type II inhibitors which only bind the inactive form. Crenolanib is currently being investigated in combination with intensive chemotherapy in relapsed/refractory patients with FLT3 activation mutation (NCT02298166) and in patients with newly diagnosed AML with a FLT3-ITD or TKD mutation (NCT02283177). Gilteritinib is a potent inhibitor of both FLT3-ITD and FLT3-TKD mutations. In the FLT3 mutant adult patient population, the overall response rate was 57% [28]. A differentiation response to gilteritinib has recently been demonstrated among relapsed or refractory FLT3-mutated patients with NPM1 and DNMT3A mutations [29]. Four important studies are currently ongoing: one phase II/III trial in newly diagnosed AML patients comparing gilteritinib versus gilteritinib plus azacitidine versus azacitidine alone (NCT02752035), one phase III study in relapsed/refractory AML with FLT3 activation mutation comparing gilteritinib versus preselected salvage chemotherapy (NCT02421939), and two phase III studies following first CR achievement in patients with FLT3-ITD mutation comparing gilteritinib versus placebo either as maintenance therapy (NCT02927262) or in case of HSCT after time of engraftment for two years (NCT02997202).
Beside tyrosine kinase inhibitors currently undergoing randomized trials, novel inhibitors are in early phase of development. Pacritinib is a third-generation tyrosine kinase inhibitor with activity against a number of targets of relevance to AML. Beside a potent inhibitory activity against FLT3, pacritinib displays also activity against JAK2 [30]. The JAK-STAT pathway is frequently activated in AML and may represent a mechanism of resistance to FLT3 inhibitors. Blockade of FLT3 in conjunction with JAK2 signaling could enhance clinical benefit for AML patients harboring a FLT3-ITD mutation [31]. Ponatinib, approved as a BCR-ABL inhibitor, is a multikinase inhibitor. It is a potent type I FLT3 inhibitor with activity against FLT3 with induced point mutations [32, 33]. However, it is not currently that much developed as a FLT3 inhibitor due to a reported increased risk of vascular adverse events.
Future considerations
Overall, several FLT3 inhibitors have been therefore evaluated in clinical trials, either as single agents or in combination with chemotherapy giving in this setting promising results. Most first-generation FLT3 inhibitors either did not generate sufficient initial response or failed to sustain therapeutic benefit. Only combined therapy with chemotherapy provided some benefits. However, interpretation of clinical trials involving a FLT3 inhibitor should take into account the disease context and the agents that are coadministered. In RATIFY study, given the slightly higher complete response rate and markedly improved overall survival in patients transplanted in first remission, midostaurin may have led to a lower disease burden early on in the course of the disease. Less specific FLT3 inhibitors might be useful in the initial diagnosis context due to the polyclonal nature of the disease. The disappointing results observed with first-generation FLT3 inhibitors also question about the elucidation of the mechanisms of resistance to FLT3 inhibitors. New agents can overcome FLT3 inhibitor resistance. At present, quizartinib appears as the most potent and specific FLT3 inhibitor and is able to completely suppress FLT3-ITD autophosphorylation. The combination of its excellent potency, selectivity, and pharmacokinetic properties made it the first drug candidate with a profile that matches the characteristics desirable for a clinical FLT3 inhibitor [34]. However, like with first-generation inhibitors, resistance can occur after short periods of treatment with acquired mutations in FLT3-ITD that disrupt binding of the drug to the target [9, 35]. In contrast, type I inhibitors target the active conformation of the kinase and may be effective against FLT3-ITD with point mutations conferring resistance to type II FLT3 inhibitors. This is the case with crenolanib, which could then represent an even more promising FLT3 inhibitor and could overcome quizartinib resistance. Combining type I and type II tyrosine kinase inhibitors could also be of benefit to enhance efficacy and perhaps suppress the emergence of therapeutic resistance. Because patients with FLT3-ITD AML often relapse during consolidation therapy, it has recently been suggested that these patients did not benefit from intensifying therapy, in contrast to those with FLT3 wild type [36]. Leukemia stem cells harboring the FLT3-ITD mutation could have a survival advantage over their wild-type counterparts and emerge at relapse as a dominant clone. It has therefore been suggested that the best therapeutic approach would be to proceed as rapidly as possible to allogeneic HSCT once remission is achieved.
Conclusion
FLT3 inhibitors should represent one of many novel therapeutic approaches as a step forward for patients with FLT3-ITD mutations. However, important questions remain such as the use of specific agents or non-specific agents in FLT3-mutated AML, the use of nonspecific agents in wild-type FLT3 disease, or the role of FLT3 inhibitors as maintenance therapy and in the post-transplant setting.