Leukemia (2012) 26, 2176–2185 & 2012 Macmillan Publishers Limited All rights reserved 0887-6924/12 www.nature.com/leu REVIEW Targeting the FMS-like tyrosine kinase 3 in acute myeloid leukemia R Swords1, C Freeman2 and F Giles3 Acute myeloid leukemia (AML) is a highly heterogenous disease with multiple signaling pathways contributing to its pathogenesis. A key driver of AML is the FMS-like tyrosine kinase receptor-3 (FLT3). Activating mutations in FLT3, primarily the FLT3-internal tandem duplication (FLT3-ITD), are associated with decreased progression-free and overall survival. Identification of the importance of FLT3-ITD and the FLT3 pathway in the prognosis of patients with AML has stimulated efforts to develop therapeutic inhibitors of FLT3. Although these inhibitors have shown promising antileukemic activity, they have had limited efficacy to date as single agents and may require use in combination with cytotoxic chemotherapies. Here, we review clinical and preclinical results for the clinically mature FLT3 inhibitors currently in development. We conclude that multitargeted FLT3 inhibitors may have more utility earlier in the course of disease, when in vitro evidence suggests that AML cells are less dependent on FLT3 signaling, perhaps because of upregulation of multiple other signaling pathways. More potent agents may have greater utility in relapsed and heavily pretreated patients, in whom high levels of circulating FLT3 ligand may necessitate use of an agent with a very favorable pharmacokinetic/ pharmacodynamic profile. Novel combination regimens are also discussed. Leukemia (2012) 26, 2176–2185; doi:10.1038/leu.2012.114 Keywords: FLT3; tyrosine kinase inhibitors; acute myeloid leukemia; internal tandem duplication INTRODUCTION internal tandem duplication (ITD) in the FMS-like tyrosine kinase 10 Adult acute myeloid leukemia (AML) accounts for B30% of receptor-3 gene (FLT3), partial tandem duplication in the mixed 11 12 leukemias and 440% of leukemia-related deaths.1 AML is a highly lineage leukemia gene, Wilms tumor gene mutations and 13 heterogenous disease characterized by several karyotypic abnor- overexpression of the BAALC gene. Other mutations that may be malities (Table 1).2,3 Karyotypic abnormalities associated with a associated with poor prognosis in patients with AML include relatively good prognosis are present in B15% of patients 10–11-translocation oncogene family member 2, isocitrate and include translocation of chromosomes 15 and 17, t(15;17) dehydrogenase-1, isocitrate dehydrogenase-2 and additional 14 (q22;q21), translocation of chromosomes 8 and 21, t(8;21) sex comb-like 1. Alternatively, CCAAT/enhancer-binding (q22;q22) and inversion of chromosome 16, inv(16)(p13q22)/ protein a (CEBPA) and nucleophosmin 1 (NPM1) mutations (in t(16;16)(p13;q22). Prognostically bad karyotypic abnormalities, FLT3-ITD-negative patients) have been associated with improved 14,15 also present in B15% of patients, include deletion of chromo- prognosis. some 7, deletion of chromosome 5q and more than three In this review, we evaluate the prognostic implications of gene chromosomal abnormalities.4,5 Other karyotypic abnormalities mutations and cytogenetic risk factors in patients with AML, that have an unclear effect on prognosis have been observed, focusing on the FLT3-ITD mutation. We discuss established and including trisomy 8 and trisomy 21d. These abnormalities are emerging approaches using FLT3 inhibitors for the treatment generally considered to be in the intermediate-risk prognosis of AML. category along with normal karyotype. The identification of these markers led to the development of cytogenetic risk classification systems by several cooperative groups internationally, including the Cancer and Leukemia Group B,5 Eastern Cooperative Oncology STRUCTURE/FUNCTION OF THE FLT3 RECEPTOR and Southwest Oncology Groups,6 the UK Medical Research FLT3 receptor Council2 and Fro¨hling et al.4 in Germany. Mutations in FLT3 are among the most commonly occurring Most patients with AML have poor rates of survival, particularly and clinically relevant mutations in patients with AML. FLT3 is a those with a poor-risk karyotype. Five-year survival rates are receptor tyrosine kinase expressed on the leukemic blasts of B55%, 24% and 5% in patients with favorable-, intermediate- and 70–100% of patients with AML16,17 that is increasingly recognized poor-risk cytogenetics, respectively.5 Even among patients with as having a role in the pathogenesis of this disease. Upon binding normal karyotype AML, however, clinical outcomes are of the FLT3 ligand, FLT3 dimerizes and undergoes a conformational heterogenous. Among other patient-related factors, mutations in change whereby its activation loop opens, revealing the ATP- key genes involved in AML pathogenesis lead to large interpatient binding pocket. The receptor becomes autophosphorylated, variability in prognostic risk (Table 2).7–9 There is a plethora of inducing cell growth and inhibiting apoptosis through the mutations associated with poor prognosis in AML. These include activation of a signaling cascade involving proteins such as the 1Sylvester Comprehensive Cancer Center, University of Miami Leonard M. Miller School of Medicine, Miami, FL, USA; 2Barts and The London NHS Trust, St Bartholomew’s Hospital, London, UK and 3HRB Clinical Research Facilities, National University of Ireland Galway and Trinity College Dublin, Dublin, Ireland. Correspondence: Dr FJ Giles, HRB Clinical Research Facility, National University of Ireland, University Road, Galway, Ireland. E-mail: [email protected] Received 28 December 2011; revised 22 March 2012; accepted 10 April 2012; accepted article preview online 27 April 2012; advance online publication, 22 May 2012 Role of FLT3 in AML R Swords et al 2177 Ras-GTPase-activating protein, phospholipase C, signal transducer Table 1. Karyotypic abnormalities in patients with AML, classified 18 5 and activator of transcription 5 and ERK1/2 (Figure 1). according to cytogenetic risk by the Cancer and Leukemia Group B, 19,20 Eastern Cooperative Oncology Group/Southwest Oncology Group6 Whereas, FLT3 ligand is ubiquitously expressed, the FLT3 and Medical Research Council3 receptor is localized primarily in hematopoietic and neural tissues.21 FLT3 is integral to hematopoiesis through its Cytogenetic risk group Abnormality Incidence, % regulation of proliferation, differentiation and apoptosis of hematopoietic progenitor cells. Knockout mice models have Favorable Inv(16) or t(16;16) 3.5–8.7 shown that lack of FLT3 ligand and FLT3 leads to reduced Del(9q) 1.6–2.8 hematopoietic precursors,22,23 demonstrating the importance of Del(16q) NR these proteins in hematopoietic expansion. t(8;21) 6.7–8.2 t(15;17) 4.4–12 FLT3 mutations Intermediate Normal karyotype 40–48 Several different mutations can occur in the FLT3 receptor in À 7 3.8–8.5 À Y 3.3–4.8 patients with AML. The most common of these is an ITD on exon Del(5q) 1.7–5.9 14 of the FLT3 gene, which varies from 3 to 4400 base pairs but is Del(7q) 1.6–8.5 typically in frame and occurs in the juxtamembrane region of the Del(9q) 1.6–2.8 receptor.24 The ITD disrupts the autoinhibitory function of the Del(12p) NR juxtamembrane domain, resulting in constitutive autophosphory- Del(20q) 1.3 lation of FLT3 and activation of its downstream effectors.25,26 þ 6NRThe FLT3-ITD mutation produces growth factor-independent þ 8 8.7–10.1 proliferation in leukemia cell lines and a fatal myeloproliferative 11 1.6 þ syndrome in murine models.27 Interestingly, however, when FLT3- þ 13 2.4 ITD was expressed in cells from knockout mice lacking FLT3 þ 21 2.3–2.8 28 t(6;9) 0.7–1.8 ligand, the receptor was only weakly activated. This suggests t(9;11) 2.2 that the ITD leads to gain of function mainly by inducing 11q23 3.7 hyperresponsivity of the FLT3 receptor to FLT3 ligand rather than through autoactivation of the receptor. a Unfavorable/adverse Complex 5.9–11.7 Activating mutations within the kinase domain of FLT3 are also Abnormal 3q 2.5 observed in patients with AML. The most common is mutation of Inv(3) or t(3;3) 1.0–2.0 the aspartate 835 (D835) mutation in the activation loop of the 5 1.6–5.9 À tyrosine kinase domain, which occurs in B7% of patients with À 7 3.8–8.5 29,30 Del(5q) 1.7–5.9 AML. Del(7q) 1.6–8.5 t(6;9) 0.7–1.8 t(9;22) 1.3 FLT3 IN THE CLINIC a It has become clear that the presence of FLT3-ITD, which is Abbreviations: AML, acute myeloid leukemia; NR, not reported. Complex 31,32 karyotype defined as X3 cytogenetic abnormalities. observed in B25% of patients with AML, is a key indicator of poor long-term prognosis. This has led to a concerted effort to Table 2. Prognostically relevant mutations in patients with AML Abnormality Prognostic outcome Study results P-value Negative patient outcome FLT3-ITD (compared with non-ITD FLT3) Shorter median OS15 10 vs 15.5 months 0.015 Shorter median DFS15 8 vs 12.5 months 0.033 MLL partial tandem duplication Shorter duration of CR117 7.1 vs 23.2 months 0.01 (compared with MLL WT) Shorter median OS11 5 vs 12 months 0.006 Shorter median RFS11 4 months vs NR o0.001 Shorter 2-year EFS118 20% vs 66% 0.03 Overexpression of BAALC (compared Shorter median OS13 1.7 vs 5.8 years 0.02 with low BAALC expression) Shorter median EFS13 0.8 vs 4.9 years 0.03 Shorter median DFS13 1.4 vs 7.3 years 0.03 Positive patient outcome Mutated CEBPA (compared with Longer median duration of CR46 NR vs 26 months 0.01 nonmutated CEBPA) Longer median OS46 NR vs 19 months 0.05 NPM1 mutations (FLT3-ITD À pts Higher rates of CR44 86% vs 63% o0.001a compared with FLT3-ITD þ pts) Longer median OS43 1183 vs 321 days 0.014 Longer median EFS43 773 vs 234 days 0.001 Abbreviations: AML, acute myeloid leukemia; CEBPA, CCAAT/enhancer-binding protein a; CR, complete response; DFS, disease-free survival; EFS, event-free survival; FLT3, FMS-like tyrosine kinase receptor-3 gene; ITD, internal tandem duplication; MLL, mixed lineage leukemia; NPM1, nucleophosmin 1; NR, not reached; OS, overall survival; Pts, patients; RFS, relapse-free survival; WT, wild-type.