Internal tandem duplication (ITD) in Fms-like tyrosine kinase 3 (FLT3) is frequently observed in acute myeloid leukemia (AML). Quizartinib, gilteritinib, and midostaurin are inhibitors against FLT3-ITD that have good efficacy for FLT3-ITD-positive AML patients. Long-term administration leads to drug resistance through acquired tyrosine kinase domain (TKD) mutations in FLT3-ITD, such as N676K, F691L, D835V, and Y842C. Here, our screen to detect inhibitors capable of overcoming resistance to FLT3 inhibitors identified heat shock protein (HSP) 90 inhibitors as potential candidates. Although Ba/F3 cells expressing FLT3-ITD with TKD mutations (Ba/F3-ITD+N676K, Ba/F3-ITD+F691L, Ba/F3-ITD+D835V, and Ba/F3-ITD+Y842C) showed various resistance patterns to FLT3 inhibitors compared with Ba/F3-ITD cells that express FLT3-ITD lacking TKD mutations, they were more sensitive to HSP90 inhibitors than Ba/F3 cells. Notably, the Ba/F3-ITD+D835V cells were the most sensitive to HSP90 inhibitors. Treatment with HSP90 inhibitors downregulated FLT3 and its downstream signaling and induced G1 arrest followed by apoptosis in Ba/F3-ITD+N676K, Ba/F3-ITD+F691L, Ba/F3-ITD+Y842C, and especially Ba/F3-ITD+D835V cells at lower concentrations compared with Ba/F3-ITD cells. The downregulation of FLT3-ITD+D835V was caused by rapid proteolysis in autophagy. Similar results were also observed in the quizartinib-resistant MV4-11 cells, QR1 and QR2, which were established by culturing cells in the presence of quizartinib and harbored FLT3-ITD+D835H and FLT3-ITD+D835V, respectively, in a single allele. Interestingly, the efficacies of HSP90 inhibitors in QR cells are reversely correlated with that of quizartib, but not to gilteritinib and midostaurin. Collectively, HSP90 inhibitors are good candidates to overcome drug resistance in AML with various FLT3-ITD TKD mutations.
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