FLT3 inhibition and mechanisms of drug resistance in mutant FLT3-positive AML

Abstract

An appealing therapeutic target in AML is constitutively activated, mutant FLT3, which is expressed in a subpopulation of AML patients and is generally a poor prognostic indicator in patients under the age of 65. There are currently several FLT3 inhibitors that are undergoing clinical investigation. However, the discovery of drug-resistant leukemic blast cells in FLT3 inhibitor-treated AML patients has prompted the search for novel, structurally diverse FLT3 inhibitors that could be alternatively used to circumvent drug resistance. Here, we provide an overview of FLT3 inhibitors under preclinical and clinical investigation, and we discuss mechanisms whereby AML cells develop resistance to FLT3 inhibitors, and the ways in which combination therapy could potentially be utilized to override drug resistance. We discuss how the cross-talk between major downstream signaling pathways, such as PI3K/PTEN/Akt/mTOR, RAS/Raf/MEK/ERK, and Jak/STAT, can be exploited for therapeutic purposes by targeting key signaling molecules with selective inhibitors, such as mTOR inhibitors, HSP90 inhibitors, or farnesyltransferase inhibitors, and identifying those agents with the ability to positively combine with inhibitors of FLT3, such as PKC412 and sunitinib. With the widespread onset of drug resistance associated with tyrosine kinase inhibitors, due to mechanisms involving development of point mutations or gene amplification of target proteins, the use of a multi-targeted therapeutic approach is of potential clinical benefit.

Introduction

There are around 15,000 newly diagnosed acute myelocytic leukemia (AML) patients in the US each year. This hematopoietic malignancy is characterized by aberrant proliferation of myeloid progenitor cells, coupled by a partial block in cellular differentiation (McKenzie, 2005). Permeation of bone marrow and peripheral blood with immature leukemic myeloblasts is the outcome of the abnormal survival advantage of leukemic cells, and causes such symptoms as bleeding, anemia, and infection.

Current therapies for AML often do not succeed because of therapy-induced mortality or drug resistance (Estey, 2001). The use of conventional chemotherapeutic agents as a single treatment approach is coupled to a low therapy-induced mortality, however a high risk of relapse due to drug resistance (Mathews and DiPersio, 2004). In contrast, allogeneic transplantation (alloBMT) has a high therapy-induced mortality, and yet a lower risk of relapse (Mathews and DiPersio, 2004). Due to the fact that alloBMT shows more promise in younger patients, it has an overall small impact on the majority of AML patients, who tend to be aged 65 and older (Witherspoon and Deeg, 1999).

In AML, the activation of signaling pathways results from a range of genetic modifications leading to mutation of signaling molecules, such as receptor tyrosine kinases. Approximately 30% of AML patients, as well as a portion of ALL patients, harbor a mutant form of the class III receptor tyrosine kinase, FLT3 (Fms-Like Tyrosine kinase-3; STK-1, human Stem Cell Tyrosine Kinase-1; or FLK-2, Fetal Liver Kinase-2) (Stirewalt and Radich, 2003). Internal tandem duplications (ITD) within the juxtamembrane domain represent the most common form of constitutively activated FLT3, occurring in approximately 20–25% of AML patients and in less than 5% of myelodysplastic syndrome (MDS) patients (Nakao et al., 1996, Horiike et al., 1997, Kiyoi et al., 1998, Kondo et al., 1999, Rombouts et al., 2000, Kelly et al., 2002a, Kelly et al., 2002b). Indeed, a rapidly lethal myeloproliferative disorder in mice results from the in vivo transplantation of murine bone marrow cells infected with a FLT3-ITD-expressing retrovirus (Kelly et al., 2002a, Kelly et al., 2002b).

Also identified in AML patients are point mutations within the “activation loop” of FLT3 (Yamamoto et al., 2001). For example, a missense mutation in the aspartic acid residue at position 835 is believed to induce the activation loop into an “activated” configuration. Additional, albeit less prevalent, mutations in the kinase domain includes N841I (Jiang et al., 2004) and Y842C (Kindler et al., 2005).

Generally, the existence of a FLT3-ITD mutation translates into a poorer prognosis in both disease-free survival and overall survival (Mattison et al., 2007). In fact, patients harboring both a nucleophosmin 1 (NMP1) mutation, which is typically a positive prognostic indicator, and mutant FLT3 tend to have poorer outcomes (Mattison et al., 2007).

There are several inhibitors of FLT3 currently in clinical trials, and a number of novel inhibitors under preclinical investigation. However, the FLT3 inhibitors tested thus far clinically generally induce only partial and transient responses in patients when used as single agents. This suggests a need for the development of novel agents conferring higher potency and/or less toxicity that can either be used effectively as single agents or that can be effectively combined with other agents to suppress disease progression and prolong the lifespan of patients.

In addition to identifying and developing potent FLT3 inhibitors representative of novel and unique structural classes, there is a push toward gaining a better understanding of the mechanisms underlying drug resistance in AML. Clinical trial data with tyrosine kinase inhibitors show that while the peripheral blood blasts decline well, bone marrow responses are less common. Stromal cells have been implicated in this mode of resistance, as they provide viability signals to leukemic cells that protect them from the effects of the inhibitor. Other mechanisms of drug resistance include the emergence of point mutations in the target protein, and deregulation of signaling molecules associated with apoptotic signaling leading to a survival advantage in leukemic cells.

There are several strategies that may be effective in preventing relapse due to the emergence of point mutations in target proteins, as well as in overcoming drug resistance believed to be caused by stromal-mediated viability signals or deregulation of apoptotic signaling. These include the combined use of more than one FLT3 inhibitor, providing their interaction with FLT3 signaling or the FLT3 protein target is distinct enough for the two inhibitors to synergize. Alternatively, FLT3 inhibitors can be combined with small molecule inhibitors that interact with key components of major signaling pathways that play a significant role in AML. Finally, FLT3 inhibitors can be combined with standard chemotherapy as an approach to achieve maximum efficacy in patients.