FLT3 inhibition as therapy in acute myeloid leukemia: a record of trials and tribulations

Abstract

Acute myeloid leukemia (AML) is a hematologic malignancy with a poor prognosis. Approximately one quarter of the patients with AML also carry an internal tandem duplication (ITD) mutation in the gene encoding FMS-like tyrosine kinase 3 (FLT3), which has a significantly deleterious impact on prognosis. The ITD mutation renders FLT3 constitutively active and leads to uncontrolled proliferation of the leukemic blast. Over the course of the last decade, a variety of compounds have been developed in preclinical and clinical studies as potent inhibitors of FLT3. Many of the earlier agents under investigation, such as lestaurtinib, midostaurin, and sunitinib, were initially developed as inhibitors of other tyrosine kinases and as targeted therapies in a variety of malignancies. These compounds have been demonstrated to have some efficacy in clinical trials of AML, mainly manifesting as transient decreases in circulating blasts correlating with effective in vivo suppression of the FLT3 target. Nevertheless, the cumbersome pharmacokinetics of some compounds and the suboptimal specificity and potency of others have limited their therapeutic efficacy. In the last few years, newer, more potent and specific agents have been under investigation, with the leading example being AC220. This agent has shown significant promise in early phases of clinical investigation, and is currently in more advanced clinical trials. Hope remains that FLT3 inhibition will be become an effective therapeutic adjunct to our current treatment approach to AML.

Figure 1.

Simplified diagram of signaling cascades downstream of FLT3 that are thought to promote leukemogenesis. Abbreviations: BAD, Bcl-2-associated death promoter; ERK, extracellular signal–related kinase; FL, FLT3 ligand; FLT3, FMS-like tyrosine kinase 3; Grb2, growth factor receptor-bound protein 2; MEK, mitogen-activated protein kinase/ERK kinase; mTOR, mammalian target of rapamycin; PI3K, phosphoinositide 3-kinase; PIM1, proto-oncogene serine/threonine-protein kinase 1; PIP₂, phosphatidylinositol-bisphosphate; PIP₃, phosphatidylinositol-trisphosphate; Rheb, Ras homolog enriched in brain; SOS, son of sevenless; STAT-5, signal transducer and activator of transcription 5; TSC, tuberous sclerosis protein. Figure derived from one obtained courtesy of Dr. Mark Levis, Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins, Baltimore, MD.