The Role of FLT3 Inhibitors in the Treatment of FLT3 Mutated Acute Myeloid Leukemia

Amir T. Fathi, MD and Yi-Bin Chen, MD

Abstract

FLT3 mutations are present in about one third of patients with AML. Several FLT3 inhibitors have been used in clinical trials and these include midostaurin, sorafenib, quizartinib, crenolanib and gilteritinib. Monotherapy with early TKIs did not have much success, however, later generation agents have shown more promising results. Combination with conventional chemotherapy may have benefit as evidenced by recently presented results, and data from ongoing trials are eagerly awaited. Several trials are also evaluating TKI given after HSCT and a large international randomized trial is planned. We may be close to an era of targeted therapy where the standard of care for this biologically defined subset will involve incorporation of a FLT3 TKI during induction chemotherapy and after HSCT. It is important that our community continue to collaborate to conduct well designed clinical trials to properly define the role of FLT3 TKIs in therapy for FLT3 mutant AML.

Keywords: FLT3, Tyrosine kinase inhibitor, AML, Maintenance therapy

Introduction:

FLT3 mutations Acute myeloid leukemia (AML) is an aggressive hematologic malignancy associated with a poor prognosis, but in recent years, the identification of an increasing array of cytogenetic and mutational abnormalities has led to improvements in prognostication and risk stratification. Among the more common genetic alterations in AML are those impacting the FMS-like tyrosine kinase 3 (FLT3) gene, detected in approximately a third of patients.1-6 The FLT3 gene is located on chromosome 13q12, and the resultant protein is a member of the class III receptor-tyrosine kinase (RTK) family, sharing structural homology with other RTKs, including KIT, VEGFR and PDGFR.7,8 The FLT3 RTK appears to play a key role in the differentiation and maturation of hematopoietic precursors, and its expression is normally limited to early myeloid progenitors.9,10 The FLT3 ligand (FL), in similar fashion to other RTKs, binds the receptor, leading to dimerization at the membrane, autophosphorylation, and activation of downstream signaling cascades, which mediate differentiation and growth. These pathways include the key intermediary proteins RAS, MEK, PI3K, AKT, and STAT-5.2,11-19
The most prevalent FLT3 mutation is the internal tandem duplication alteration (FLT3-ITD). FLT3/ITD mutations may be of variable lengths, alter the juxtamembrane portion of the RTK, and occur in approximately a quarter of those with de novo AML.1,5,6,19-22 Point mutations of the tyrosine kinase domain (TKD) involving the activation loop of the receptor (FLT3/TKD), are less common, impacting approximately 7%23,24 of AML cases. FLT3/ITD mutations are associated with a significantly higher rate of relapse, and in this fashion, adversely impact overall prognosis. The length of the ITD mutations and an increase in the ratio of mutant to wildtype (WT) FLT3 alleles are also linked to worse overall survival.25,26 In contrast to the adverse impact of FLT3/ITD alterations on prognosis, this association is less clear with FLT3/TKD AML.26,27
In recent years, there have been increasing efforts to develop targeted inhibitors of the FLT3 tyrosine kinase. Despite initial challenges with the first generations of tyrosine kinase inhibitors (TKIs), which included significant toxicities, suboptimal pharmacokinetics, off-target effects, and insufficient target inhibition, there have been a series of recently reported clinical trials that have engendered excitement. This review will summarize the use of FLT3 inhibitors for the treatment of FLT3-mutant AML. We will first discuss FLT3 inhibitors when used as monotherapy or in combination with conventional chemotherapy, and then review their role as therapy after allogeneic hematopoietic stem cell transplantation (HSCT).

The First FLT3 Inhibitors

Early FLT3 inhibitors, including sunitinib, sorafenib, midostaurin, and lestaurtinib, were not specifically designed to target FLT3, also inhibiting key enzymes such as JAK2, PDGFR, VEGFR, and 2,16,17,28-30 Sunitinib and sorafenib were both initially approved for use in solid tumor malignancies, such as metastatic renal cell carcinoma, gastrointestinal stromal tumors, and hepatocellular carcinoma.31-34 In early phase I clinical trials in AML, sunitinib demonstrated modest activity, however, responses were often transient, and patients frequently experienced toxicities.30,35,36 Sorafenib, in contrast, has been associated with greater therapeutic promise. In multiple phase I trials, sorafenib was better tolerated and consistently associated with reductions in peripheral blood and marrow leukemic blasts.37-39 Sorafenib has therefore been extensively used over the last decade as therapy for FLT3/ITD patients who have been unable to participate in trials of other FLT3 TKIs. Indeed, for many such patients, sorafenib brought about enough disease control to allow HSCT.40 Subsequent studies specifically assessed the combination of sorafenib with conventional cytotoxic therapies, and suggested both tolerability and safety.41,42 A randomized, placebo-controlled phase II study of 276 younger German patients, regardless of FLT3 mutational status, demonstrated an improvement in event free survival (EFS) for those receiving sorafenib combined with conventional induction. Intriguingly, the benefit was seen across all patients, and FLT3 mutations did not predict for improved outcomes with combination therapy.43 Other trials have assessed the role of sorafenib in combination with azacitidine, revealing significant promise in the relapsed and refractory setting. A phase II study of sorafenib and azacitidine for R/R FLT3/ITD patients reported an overall response rate of 46%, and a CR rate of 27% in this high risk population.44 A phase II study of the same combination in the upfront setting is now also ongoing (NCT02196857).
Lestaurtinib is another rather non-specific TKI with potent activity against the FLT3 tyrosine kinase. Like sorafenib, early phase trials of lestaurtinib demonstrated short-lived reductions in circulating and marrow myeloblasts45,46 particularly when the FLT3 RTK was suppressed in a potent and sustained fashion, as revealed by correlative ex vivo pharmacodynamic studies.47 A multi-center phase III clinical trial followed, evaluating lestaurtinib combined with re-induction therapy in relapsed or refractory (R/R) FLT3-mutant patients. No difference was noted in complete remission rate or overall survival between the two arms, although sustained inhibition of the FLT3 RTK was found to be sub-optimal for the majority of patients.48 More recently, results from another randomized study of lestaurtinib in combination with chemotherapy, this as upfront induction and consolidation therapy for FLT3-mutant AML among younger patients, were presented. No difference in OS was noted at 5 years, but sub-group analysis suggested that optimizing target inhibition was important. Survival was particularly improved among patients who experienced sustained FLT3 inhibition of greater than 85%.49
Similarly, midostaurin monotherapy was associated with transient decreases in marrow and blood myeloblasts in early-phase trials of FLT3-mutant patients.50,51 However, the success of midostaurin in combination with chemotherapy has been much more pronounced. In a phase I study of newly diagnosed younger patients, midostaurin was safely administered with conventional cytotoxic chemotherapy, resulting in high rates of complete remission (CR) across all patients, including those harboring FLT3 mutations.52 The long-awaited results of a randomized, phase III study of midostaurin plus induction chemotherapy was recently presented. Among FLT3-mutant patients younger than 60 years, midostaurin prolonged overall survival when compared to placebo.
The benefit of midostaurin was preserved across all FLT3-mutant subgroups, including those with FLT3/TKD mutations.53 Much has been learned from the clinical study of the earlier generation of non-specific FLT3 inhibitors. Potent and sustained inhibition of the FLT3 RTK was noted to be essential to successful suppression of circulating and marrow myeloblasts in early trials of TKI monotherapy. Toxicities, as well as suboptimal pharmacokinetics and pharmacodynamics, have also limited the clinical utility and development of certain TKIs. The combination of chemotherapy with FLT3 inhibition has been shown to be a promising approach, although concurrent chemotherapy does raise circulating FL levels, hypothetically curbing the efficacy of FLT3 inhibitors.54

The Newer Generation of FLT3 Inhibitors

In recent years, a series of more selective and potent FLT3 inhibitors have entered clinical trials, and demonstrated significant promise. Among these is quizartinib, which was first identified through a small molecule inhibitor screen. Quizartinib also demonstrated favorable pharmacokinetic features, including an optimal half-life in vivo, as well as sustained and potent target inhibition.55 A phase I trial of 76 patients with R/R AML, regardless of FLT3 mutational status, demonstrated safety as well as a suggestion of efficacy. Clinical responses were noted in 23 patients (30%), among whom 10 cases were considered CR or CRi (complete remission with incomplete hematologic recovery). Specifically, among the 17 FLT3/ITD patients, 4 cases of CR or CRi were seen.55 Despite the tolerability of the drug, a subset of patients experienced gastrointestinal symptoms, myelosuppression, and QT prolongation. Some of the off-target effects may have been related to the inhibition of KIT by quizartinib, one of the few other targets for this very selective agent.56 Data from phase II evaluation of quizartinib have been presented for two populations of R/R patients. In 154 older (over age 60) patients, a 51% rate of composite complete remission (CRc – including CR and CRi) was seen. The CRc rate was higher among FLT3/ITD patients (57%), than that of the 44 FLT3-WT patients (36%).57,58 The second group consisted of younger patients (n=137), and once again demonstrated an impressive CRc rate of 44% among FLT3/ITD participants, with more than a third of patients successfully proceeding to HSCT after clinical response.59 Quizartinib has also been studied in combination with conventional chemotherapy for newly diagnosed AML, and data from a British study reported a CR rate of 79% among 42 evaluable patients.60
There are now multiple ongoing trials of quizartinib as monotherapy and in combination with conventional regimens. A phase II randomized study of two quizartinib doses for R/R FLT3/ITD patients (NCT01565668) has completed accrual. Additionally, a phase I trial of quizartinib combined with conventional induction chemotherapy followed by cytarabine consolidation and maintenance (NCT01390337) has completed accrual, and another is assessing the tolerability and therapeutic promise of quizartinib in combination with hypomethylating agents (HMA) (NCT01892371). Quizartinib is also being studied in the QuANTUM-R study, a randomized phase III trial comparing quizartinib monotherapy versus conventional salvage therapies among relapsed/refractory FLT3/ITD mutant patients (NCT02039726). Finally, a phase III placebo-controlled trial, the QuANTUM-First study, is planning to enroll more than 500 patients at 250 centers world-wide to fully assess the role
Another newer FLT3 TKI being studied is crenolanib, which is not only a potent inhibitor of the FLT3/ITD mutation but has also demonstrated activity against the FLT3/TKD mutation. This may be particularly important as resistance to quizartinib and other FLT3 inhibitors, among FLT3/ITD patients, are increasingly due, at least in part, to acquired TKD mutations.62,63 Crenolanib as a single agent appears to be safe and efficacious among R/R FLT3-mutant (both ITD and TKD variants) patients.64 In a recent phase II study, data on 69 patients were presented, including 36 who had progressed beyond other FLT3 inhibitors. Among those with prior exposure to TKI therapy, the rate of CRi was 39% and the rate of partial response (PR) was 11%. Among those patients with prior exposure to FLT3 TKIs, a subset (41%) also achieved response, with the CRi rate being 17%. Common toxicities included nausea, transaminitis, and fluid retention.65 Multiple clinical trials of crenolanib are now under way, some in combination with conventional therapies. A multi-center, safety and tolerability study of crenolanib combined with induction chemotherapy, is currently recruiting newly diagnosed FLT3-mutant patients (NCT02283177). Another phase I/II trial is assessing the combination of crenolanib with various re-induction regimens for R/R FLT3-mutant patients (NCT02626338). Additional combination trials are accruing patients (NCT02400281) in the US, and larger, more advanced phase studies are planned internationally.
Most recently, the novel compound gilteritinib has emerged as a potent, selective inhibitor of the FLT3- mutant receptor (both ITD and TKD mutations). A phase I/II dose-escalation trial of R/R FLT3-mutant patients revealed optimal pharmacokinetic and pharmacodynamic profiles.66 Among the 215 patients studied (73% of whom were FLT3-mutant), the CRc rate was 46%. Among those with FLT3/ITD mutations, the CRc rate was higher at 49%. Remarkably, a subset of patients with FLT3/TKD mutations also achieved CRc (29%). Diarrhea, fatigue, and transaminitis were the most common toxicities .67 Several studies of gilteritinib as monotherapy and in combination are now actively accruing. A phase III, randomized study of gilteritinib versus conventional salvage chemotherapy is accruing relapsed/refractory FLT3-mutant patients at multiple centers (NCT02421939).68 In addition, studies investigating the promise of gilteritinib in combination with HMA therapy (NCT02752035), with conventional induction chemotherapy (NCT02236013), and as maintenance therapy following achievement of first remission (NCT02927262) are ongoing. In summary, in the current era, the off-protocol options for FLT3-mutant patients are limited to sorafenib, a potent but nonspecific FLT3 inhibitor. However, other FLT3 inhibitors will likely soon become available, and others are under intensive study with very promising results (Table 1).

FLT3 Inhibitors after HSCT

Although the recent results of clinical trials combining various TKIs with conventional chemotherapy are compelling, many centers continue to routinely recommend HSCT as consolidation in first remission (CR1) for patients with FLT3-mutant AML, especially those with the FLT3/ITD mutation. This is based on the historically dismal outcomes with chemotherapy-based approaches in FLT3/ITD AML69 and the results of several retrospective analyses suggesting a benefit to HSCT in CR1.70-72 Nevertheless, even with HSCT in CR1, the risk of disease relapse for patients with FLT3/ITD AML remains unacceptably high and is the leading cause of long-term failure. This has led to the rationale to use FLT3 TKIs as potential maintenance therapy after HSCT in hopes of decreasing the risk of disease relapse and improving long-term survival.
We initially conducted a phase I trial using sorafenib as maintenance therapy after HSCT for any patient with FLT3/ITD mutant AML in CR after HSCT. Any conditioning, donor source and GVHD prophylaxis was allowed and patients could be in first or subsequent remission. Sorafenib was started between days 45 and 120 after HSCT and dose escalation proceeded in separate cohorts from 200 mg twice daily to 400 mg twice daily. Therapy was continued for 12 28-day cycles and the primary endpoint was to define the maximum tolerated dose (MTD) of sorafenib in this setting. The MTD was established at 400 mg twice daily without accruing any drug-related DLTs. With a median follow-up of 16.7 months, 1-year PFS was 85% (90% CI, 66%-94%) and OS was 95% (90% CI, 79%99%). For patients in CR prior to HSCT, 1-year PFS was 95% and 1-year OS was 100% with one case of disease relapse observed.73
Other single-arm studies presented in abstract form have showed similarly encouraging results. Pratz et al. reported on the use of peri-transplant and maintenance sorafenib (given both before and after HSCT) in 28 patients with FLT3-ITD AML undergoing HSCT in CR1. With a median follow-up of 450 days, there were only 5 relapses and 6 deaths.74 A number of other retrospective studies have also reported encouraging results using sorafenib as maintenance, pre-emptive or therapy for relapsed disease after HSCT.75-77 In addition, Sandmaier et al. reported preliminary results on 13 patients treated with maintenance quizartinib who had AML in CR showing encouraging safety.78
We recently conducted a retrospective analysis comparing consecutive patients with FLT3ITD AML in CR1 who underwent HSCT who received sorafenib maintenance versus those who did not. Given that the median time to initiation of sorafenib was day 68 after HSCT, a landmark analysis was performed comparing sorafenib patients (n=26) and control patients (n=43) who were alive and without relapse at this time. The use of maintenance sorafenib was associated with improved 2-year PFS (82% vs. 53%, p=0.0081) and 2-year OS (81% vs. 62%, p=0.029) with the benefit driven primarily by a significant decrease in rate of disease relapse (8.2% vs. 37.7%, p=0.0077).
There were no significant differences in rates of chronic GVHD or non-relapse mortality.79 Two phase II randomized trials, one using sorafenib (EudraCT 2010-018539-16) and the other using midostaurin as maintenance after HSCT (NCT01883362), are ongoing, and the results are eagerly awaited. Based on such compelling results, the upcoming BMT Clinical Trials Network (CTN) 1506 trial is a large phase III prospective randomized placebo-controlled trial studying the potential benefit of the FLT3 inhibitor gilteritinib as maintenance therapy after HSCT for patients with FLT3/ITD AML in CR1. Any conditioning, donor, graft source and GVHD prophylaxis is allowed with stratification based on: 1) age, 2) minimal residual disease (MRD) status and 3) time from HSCT to randomization. Patients will be randomized to gilteritinib vs. placebo for a period of 24 months. The primary endpoint will be a comparison between leukemia-free survival between the two arms with secondary endpoints including overall survival, safety and toxicity, cumulative incidence of acute and chronic GVHD, respectively, and event-free survival. This study will potentially be the first trial to prove a benefit of a maintenance therapy following HCT and, importantly, also validate the use of a next-generation sequencing based assay to detect minimal residual disease (MRD) in this population. The validation of an MRD assay and correlation of MRD with outcomes is a tremendously important aspect of this trial as it can lead to eventually changing the endpoint of all future trials in FLT3/ITD mutant AML. More importantly, we can then potentially transition TKI therapy from a maintenance approach to a pre-emptive approach after HSCT to minimize the number of patients who are overtreated.
As we await results of ongoing trials giving TKIs after HSCT, many questions remain unanswered, including the phase of therapy during which TKIs should be used, choice of specific agent and duration of therapy. Moreover, even though we intend to specifically inhibit the mutant FLT3 tyrosine kinase with these agents, the mechanism of action behind the putative therapeutic benefit of FLT3 TKIs remains in question. The majority of agents used to date are not specific FLT3 TKIs, and thus, off-target effects may be playing a therapeutic role. Interestingly, in our maintenance sorafenib trial, it was observed that many patients developed a rash which resembled cutaneous graft-versus-host disease (GVHD), yet significantly improved when stopping sorafenib.73 In addition, some preclinical evidence does suggest that FLT3 is present on dendritic cells and stimulation of such leads to expansion of regulatory T-cells.80 Thus, inhibition via FLT3 inhibitors may exert an immunomodulatory effect which may contribute in part to its observed efficacy when given after HSCT.
In conclusion, the use of FLT3 TKIs for FLT3 mutant AML remains an extremely active area of investigation. Single-agent early generation TKIs in the relapsed / refractory setting did not appear to bring about durable remissions, however, later generation agents are having more success in ongoing trials. Combination with conventional chemotherapy may have benefit as evidenced by recently presented results, and data from ongoing trials are eagerly awaited. Perhaps, the setting with the most potential benefit for FLT3 TKIs is use as maintenance therapy after HSCT and a large international prospective randomized trial (BMT CTN 1506) is planned. Remarkably, in a relatively short period of time since the discovery of the prognostic implications of FLT3 mutations in AML, we may be close to an era of targeted therapy where the standard of care for this biologically defined subset will involve incorporation of a FLT3 TKI during induction chemotherapy and after HSCT. We hope that the community at large is able to continue to collaborate to conduct these important trials in order to bring about such improvements in care, and ultimately, outcomes for our patients.

References:

1. Gilliland DG, Griffin JD: The roles of FLT3 in hematopoiesis and leukemia. Blood 100:1532-42, 2002
2. Levis M, Small D: FLT3: ITDoes matter in leukemia. Leukemia 17:1738-52, 2003
3. Schnittger S, Schoch C, Dugas M, et al: Analysis of FLT3 length mutations in 1003 patients with acute myeloid leukemia: correlation to cytogenetics, FAB subtype, and prognosis in the AMLCG study and usefulness as a marker for the detection of minimal residual disease. Blood 100:59-66, 2002
4. Frohling S, Schlenk RF, Breitruck J, et al: Prognostic significance of activating FLT3 mutations in younger adults (16 to 60 years) with acute myeloid leukemia and normal cytogenetics: a study of the AML Study Group Ulm. Blood 100:4372-80, 2002
5. Kottaridis PD, Gale RE, Frew ME, et al: The presence of a FLT3 internal tandem duplication in patients with acute myeloid leukemia (AML) adds important prognostic information to cytogenetic risk group and response to the first cycle of chemotherapy: analysis of 854 patients from the United Kingdom Medical Research Council AML 10 and 12 trials. Blood 98:1752-9, 2001
6. Kiyoi H, Towatari M, Yokota S, et al: Internal tandem duplication of the FLT3 gene is a novel modality of elongation mutation which causes constitutive activation of the product. Leukemia 12:1333-7, 1998
7. Carow CE, Kim E, Hawkins AL, et al: Localization of the human stem cell tyrosine kinase-1 gene (FLT3) to 13q12–>q13. Cytogenetics & Cell Genetics 70:255-7, 1995
8. van der Geer P, Hunter T, Lindberg RA: Receptor protein-tyrosine kinases and their signal transduction pathways. Annual Review of Cell Biology 10:251-337, 1994
9. Small D, Levenstein M, Kim E, et al: STK-1, the human homolog of Flk-2/Flt-3, is selectively expressed in CD34+ human bone marrow cells and is involved in the proliferation of early progenitor/stem cells. Proceedings of the National Academy of Sciences of the United States of America 91:459-63, 1994
10. Lyman SD, James L, Johnson L, et al: Cloning of the human homologue of the murine flt3 ligand: a growth factor for early hematopoietic progenitor cells. Blood 83:2795-801, 1994
11. Roux PP, Blenis J: ERK and p38 MAPK-activated protein kinases: a family of protein kinases with diverse biological functions. Microbiol Mol Biol Rev 68:320-44, 2004
12. Manning BD, Cantley LC: AKT/PKB signaling: navigating downstream. Cell 129:1261-74, 2007
13. Hay N, Sonenberg N: Upstream and downstream of mTOR. Genes Dev 18:1926-45,
14. Bar-Natan M, Nelson EA, Xiang M, et al: STAT signaling in the pathogenesis and treatment of myeloid malignancies. JAKSTAT 1:55-64, 2012
15. Fathi AT, Chen YB: Treatment of FLT3-ITD acute myeloid leukemia. Am J Blood Res 1:175-89, 2011
16. Fathi AT, Chabner BA: FLT3 inhibition as therapy in acute myeloid leukemia: a record of trials and tribulations. Oncologist 16:1162-74, 2011
17. Fathi A, Levis M: FLT3 inhibitors: a story of the old and the new. Curr Opin Hematol 18:71-6, 2011
18. Rombouts WJ, Blokland I, Lowenberg B, et al: Biological characteristics and prognosis of adult acute myeloid leukemia with internal tandem duplications in the Flt3 gene. Leukemia 14:675-83, 2000
19. Mizuki M, Fenski R, Halfter H, et al: Flt3 mutations from patients with acute myeloid leukemia induce transformation of 32D cells mediated by the Ras and STAT5 pathways. Blood 96:3907-14, 2000
20. Marcucci G, Haferlach T, Dohner H: Molecular genetics of adult acute myeloid leukemia: prognostic and therapeutic implications. J Clin Oncol 29:475-86, 2011
21. Schnittger S, Bacher U, Haferlach C, et al: Diversity of the juxtamembrane and TKD1 mutations (exons 13-15) in the FLT3 gene with regards to mutant load, sequence, length, localization, and correlation with biological data. Genes Chromosomes Cancer 51:910-24, 2012
22. Kayser S, Schlenk RF, Londono MC, et al: Insertion of FLT3 internal tandem duplication in the tyrosine kinase domain-1 is associated with resistance to chemotherapy and inferior outcome. Blood 114:2386-92, 2009
23. Yamamoto Y, Kiyoi H, Nakano Y, et al: Activating mutation of D835 within the activation loop of FLT3 in human hematologic malignancies. Blood 97:2434-9, 2001
24. Abu-Duhier FM, Goodeve AC, Wilson GA, et al: Identification of novel FLT-3 Asp835 mutations in adult acute myeloid leukaemia. British Journal of Haematology 113:983-8, 2001
25. Stirewalt DL, Kopecky KJ, Meshinchi S, et al: Size of FLT3 internal tandem duplication has prognostic significance in patients with acute myeloid leukemia. Blood 107:3724-6, 2006
26. Thiede C, Steudel C, Mohr B, et al: Analysis of FLT3-activating mutations in 979 patients with acute myelogenous leukemia: association with FAB subtypes and identification of subgroups with poor prognosis. Blood 99:4326-35, 2002
27. Moreno I, Martin G, Bolufer P, et al: Incidence and prognostic value of FLT3 internal tandem duplication and D835 mutations in acute myeloid leukemia. Haematologica 88:19-24, 2003
28. Levis M, Tse KF, Smith BD, et al: A FLT3 tyrosine kinase inhibitor is selectively cytotoxic to acute myeloid leukemia blasts harboring FLT3 internal tandem duplication mutations. Blood 98:885-7, 2001
29. Levis M, Allebach J, Tse KF, et al: A FLT3-targeted ASP2215 tyrosine kinase inhibitor is cytotoxic to leukemia cells in vitro and in vivo. Blood 99:3885-91, 2002
30. O’Farrell AM, Foran JM, Fiedler W, et al: An innovative phase I clinical study demonstrates inhibition of FLT3 phosphorylation by SU11248 in acute myeloid leukemia patients. Clin Cancer Res 9:5465-76, 2003
31. Demetri GD, van Oosterom AT, Garrett CR, et al: Efficacy and safety of sunitinib in patients with advanced gastrointestinal stromal tumour after failure of imatinib: a randomised controlled trial. Lancet 368:1329-38, 2006
32. Motzer RJ, Hutson TE, Tomczak P, et al: Sunitinib versus interferon alfa in metastatic renal-cell carcinoma. N Engl J Med 356:115-24, 2007
33. Escudier B, Eisen T, Stadler WM, et al: Sorafenib in advanced clear-cell renal-cell carcinoma. N Engl J Med 356:125-34, 2007
34. Llovet JM, Ricci S, Mazzaferro V, et al: Sorafenib in advanced hepatocellular carcinoma. N Engl J Med 359:378-90, 2008
35. Fiedler W, Serve H, Dohner H, et al: A phase 1 study of SU11248 in the treatment of patients with refractory or resistant acute myeloid leukemia (AML) or not amenable to conventional therapy for the disease. Blood 105:986-93, 2005
36. Fiedler W, Kayser S, Kebenko M, et al: A phase I/II study of sunitinib and intensive chemotherapy in patients over 60 years of age with acute myeloid leukaemia and activating FLT3 mutations. Br J Haematol 169:694-700, 2015
37. Zhang W, Konopleva M, Shi YX, et al: Mutant FLT3: a direct target of sorafenib in acute myelogenous leukemia. Journal of the National Cancer Institute 100:184-98, 2008
38. Pratz KW, Cho E, Levis MJ, et al: A pharmacodynamic study of sorafenib in patients with relapsed and refractory acute leukemias. Leukemia 24:1437-44, 2010
39. Metzelder SK, Schroeder T, Finck A, et al: High activity of sorafenib in FLT3-ITDpositive acute myeloid leukemia synergizes with allo-immune effects to induce sustained responses. Leukemia 26:2353-9, 2012
40. Sharma M, Ravandi F, Bayraktar UD, et al: Treatment of FLT3-ITD-positive acute myeloid leukemia relapsing after allogeneic stem cell transplantation with sorafenib. Biol Blood Marrow Transplant 17:1874-7, 2011
41. Ravandi F, Cortes JE, Jones D, et al: Phase I/II study of combination therapy with sorafenib, idarubicin, and cytarabine in younger patients with acute myeloid leukemia. Journal of Clinical Oncology 28:1856-62, 2010
42. Serve H, Wagner R, Sauerland C, et al: Sorafenib in combination with standard induction and consolidation therapy In elderly AML patients: Results from a randomized, placebocontrolled phase II trial. Blood (ASH Annual Meeting Abstracts) 116:Abstract 333, 2010
43. Rollig C, Serve H, Huttmann A, et al: Addition of sorafenib versus placebo to standard therapy in patients aged 60 years or younger with newly diagnosed acute myeloid leukaemia (SORAML): a multicentre, phase 2, randomised controlled trial. Lancet Oncol 16:1691-9, 2015
44. Ravandi F, Alattar ML, Grunwald MR, et al: Phase 2 study of azacytidine plus sorafenib in patients with acute myeloid leukemia and FLT-3 internal tandem duplication mutation. Blood 121:4655-62, 2013
45. Knapper S, Burnett AK, Littlewood T, et al: A phase 2 trial of the FLT3 inhibitor lestaurtinib (CEP701) as first-line treatment for older patients with acute myeloid leukemia not considered fit for intensive chemotherapy. Blood 108:3262-70, 2006
46. Smith BD, Levis M, Beran M, et al: Single-agent CEP-701, a novel FLT3 inhibitor, shows biologic and clinical activity in patients with relapsed or refractory acute myeloid leukemia. Blood 103:3669-76, 2004
47. Levis M, Brown P, Smith BD, et al: Plasma inhibitory activity (PIA): a pharmacodynamic assay reveals insights into the basis for cytotoxic response to FLT3 inhibitors. Blood 108:3477-83, 2006
48. Levis M, Ravandi F, Wang ES, et al: Results from a randomized trial of salvage chemotherapy followed by lestaurtinib for patients with FLT3 mutant AML in first relapse. Blood 117:3294-301, 2011
49. Knapper S, Russell N, Gilkes A, et al: A randomised assessment of adding the kinase inhibitor lestaurtinib to 1st-line chemotherapy for FLT3-mutated AML. Blood, 2016
50. Fischer T, Stone RM, Deangelo DJ, et al: Phase IIB trial of oral Midostaurin (PKC412), the FMS-like tyrosine kinase 3 receptor (FLT3) and multi-targeted kinase inhibitor, in patients with acute myeloid leukemia and high-risk myelodysplastic syndrome with either wild-type or mutated FLT3. J Clin Oncol 28:4339-45, 2010
51. Stone RM, DeAngelo DJ, Klimek V, et al: Patients with acute myeloid leukemia and an activating mutation in FLT3 respond to a small-molecule FLT3 tyrosine kinase inhibitor, PKC412. Blood 105:54-60, 2005
52. Stone RM, Fischer T, Paquette R, et al: Phase IB study of the FLT3 kinase inhibitor midostaurin with chemotherapy in younger newly diagnosed adult patients with acute myeloid leukemia. Leukemia 26:2061-8, 2012
53. Stone RM, Mandrekar S, Sanford BL, et al: The multi-kinase inhibitor midostaurin (M) prolongs survival compared with placebo (P) in combination with daunorubicin (D)/cytarabine (C) induction (ind), high-dose C consolidation (consol), and as maintenance (maint) therapy in newly diagnosed acute myeloid leukemia (AML) patients (pts) age 18-60 with FLT3 mutations (muts): An international prospective randomized (rand) P-controlled double-blind trial (CALGB 10603/RATIFY [Alliance]). Blood (ASH Annual Meeting Abstracts) 126, 2015
54. Sato T, Yang X, Knapper S, et al: FLT3 ligand impedes the efficacy of FLT3 inhibitors in vitro and in vivo. Blood 117:3286-93, 2011
55. Cortes JE, Kantarjian H, Foran JM, et al: Phase I study of quizartinib administered daily to patients with relapsed or refractory acute myeloid leukemia irrespective of FMS-like tyrosine kinase 3-internal tandem duplication status. J Clin Oncol 31:3681-7, 2013
56. Kampa-Schittenhelm KM, Heinrich MC, Akmut F, et al: Quizartinib (AC220) is a potent second generation class III tyrosine kinase inhibitor that displays a distinct inhibition profile against mutant-FLT3, -PDGFRA and -KIT isoforms. Mol Cancer 12:19, 2013
57. Cortes JE, Perl AE, Dombret H, et al: Final Results of a Phase 2 Open-Label, Monotherapy Efficacy and Safety Study of Quizartinib (AC220) in Patients >= 60 Years of Age with FLT3 ITD Positive or Negative Relapsed/Refractory Acute Myeloid Leukemia Blood 120:Abstract 48,
58. Martinelli G, Perl AE, Dombret H, et al: Effect of quizartinib (AC220) on response rates and long-term survival in elderly patients with FLT3-ITD positive or negative relapsed/refractory acute myeloid leukemia. J Clin Oncol (Annual Meeting Abstracts) 31:Abstract 7021, 2013
59. Levis MJ, Perl AE, Dombret H, et al: Final Results of a Phase 2 Open-Label, Monotherapy Efficacy and Safety Study of Quizartinib (AC220) in Patients with FLT3-ITD Positive or Negative Relapsed/Refractory Acute Myeloid Leukemia After Second-Line Chemotherapy or Hematopoietic Stem Cell Transplantation Blood 120:Abstract 673, 2012
60. Burnett AK, Bowen D, Russell N, et al: AC220 (Quizartinib) can be safely combined with conventional chemotherapy in older patients with newly diagnosed acute myeloid leukaemia: Experience from the AML18 pilot trial. Blood (ASH Annual Meeting Abstracts) 122:622, 2013