FLT3: A 35-Year Voyage from Discovery to the Next Generation of Targeted Therapy in AML

FMS-like tyrosine kinase 3 (FLT3) is a crucial regulator of normal hematopoiesis, with high expression in hematopoietic stem and progenitor cells. Beyond its role in stem cell survival and proliferation, FLT3 signaling is essential for immune regulation, particularly dendritic cell differentiation and NK cell expansion. In acute myeloid leukemia (AML), FLT3 mutations-most commonly internal tandem duplications (FLT3-ITD) and tyrosine kinase domain (FLT3-TKD) substitutions-are among the most frequent genetic alterations, driving constitutive activation of proliferative and antiapoptotic pathways and conferring adverse prognosis. The clinical development of FLT3 inhibitors has been a decades-long endeavor. Early multikinase agents established proof-of-concept but were hampered by off-target effects and incomplete efficacy. The subsequent generation of potent and selective inhibitors has transformed outcomes, culminating in FDA approvals of midostaurin, quizartinib, and gilteritinib. Together with allogeneic transplantation, these agents have reshaped the treatment paradigm for FLT3-mutant AML, converting a historically high-risk subset into one with realistic prospects for long-term survival. Despite these advances, challenges remain. Resistance emerges through cell-intrinsic mechanisms such as acquisition of secondary TKD or RAS pathway mutations, metabolic reprogramming, and antiapoptotic shifts, as well as cell-extrinsic mechanisms mediated by the bone marrow microenvironment, including cytokine support, stromal CYP3A4 metabolism, and retinoid inactivation. These pathways sustain measurable residual disease (MRD), the key predictor of relapse. Rational combination strategies and MRD-directed approaches are therefore essential to fully realize the curative potential of FLT3 inhibition.