FLT3 Inhibitors
Fms-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase (RTK) primarily expressed on hematopoietic stem and progenitor cells, playing a critical role in their proliferation, differentiation, and survival. Under normal physiological conditions, FLT3 is activated upon binding of its ligand, FLT3 ligand (FLT3L), leading to receptor dimerization, autophosphorylation of tyrosine residues within the intracellular domain, and subsequent activation of downstream signaling pathways. Key pathways include the STAT5 pathway, the RAS/MAPK pathway, and the PI3K/AKT/mTOR pathway. These signaling cascades are essential for normal hematopoiesis. However, activating mutations in FLT3, particularly internal tandem duplications (ITDs) in the juxtamembrane domain or point mutations in the tyrosine kinase domain (TKD), lead to constitutive, ligand-independent activation of the receptor. This uncontrolled signaling drives the proliferation and survival of leukemic cells, making FLT3 a significant oncogenic driver. The primary disease associated with FLT3 mutations is acute myeloid leukemia (AML). FLT3-ITD mutations are found in approximately 25-30% of AML patients, making it one of the most common genetic alterations in this aggressive blood cancer. FLT3-TKD mutations are less frequent, occurring in about 5-10% of AML cases.
Patients with FLT3-ITD positive AML typically have a poorer prognosis, characterized by higher relapse rates and shorter overall survival, highlighting the critical need for targeted therapies. The research progress and status of FLT3 inhibitors have been transformative for AML treatment. Early multi-kinase inhibitors, such as midostaurin and sorafenib, which also target other RTKs like KIT and VEGFR, demonstrated modest activity in FLT3-mutated AML. Midostaurin was the first FLT3 inhibitor approved in 2017 for newly diagnosed FLT3-mutated AML in combination with chemotherapy. A major breakthrough came with the development of more potent and selective next-generation FLT3 inhibitors, such as gilteritinib and quizartinib. Gilteritinib was approved in 2018 for relapsed/refractory FLT3-mutated AML, showing superior efficacy and a more favorable safety profile compared to chemotherapy. Quizartinib was approved in 2023 for newly diagnosed FLT3-ITD positive AML in combination with chemotherapy. Numerous other FLT3 inhibitors are in various stages of preclinical and clinical development, exploring novel combinations and strategies to overcome resistance. FLT3 inhibitors are commonly classified into first-generation (multi-kinase inhibitors like midostaurin, sorafenib) and second-generation (more selective and potent inhibitors like gilteritinib, quizartinib).
They can also be categorized by their binding mode: Type I inhibitors bind to the active conformation of the kinase, while Type II inhibitors bind to the inactive conformation. Most approved FLT3 inhibitors are small molecule tyrosine kinase inhibitors. The increasing selectivity of newer agents has been crucial in improving their therapeutic index. Other relevant general knowledge includes the significant challenge of acquired resistance, which can arise from secondary FLT3 mutations (e.g., FLT3-TKD D835Y, F691L) or activation of alternative signaling pathways (e.g., RAS/MAPK, KIT). This necessitates the development of inhibitors active against resistant mutations and combination therapy strategies. The importance of FLT3 mutation testing at diagnosis is paramount for patient selection. Common side effects can include myelosuppression, QT prolongation (with some agents), and gastrointestinal disturbances. Research tools include specific antibodies for FLT3 detection and various AML cell lines harboring FLT3 mutations. Combination with conventional chemotherapy remains a standard approach, and ongoing trials are exploring combinations with venetoclax or other novel agents to achieve deeper and more durable responses.
