The resulting clones were grown in the absence of interleukin-3 and analyzed by treatment in increasing concentrations of FLT3 TKI for 2 days, after which growth inhibition was assessed using the MTT assay. a growing number of FLT3 mutations that are likely to be encountered in patients. Such knowledge, combined with known remaining sensitivity to other FLT3 TKI, will be important to establish as secondary drug treatments that can be substituted when these mutants are encountered. cases.10C12 FLT3 mutations generally occur in the juxtamembrane (JM) domain or in the kinase Sh3pxd2a domain (KD). The JM mutations factor in approximately 23% of newly diagnosed cases of AML and occur as in-frame internal tandem duplications Febuxostat (TEI-6720) (ITDs) of varying length, resulting in duplication of a sequence of typically 4C50 amino acids, often accompanied by a one or two amino-acid insert.10 The crystal structure of FLT3 shows that the JM domain functions as an autoinhibitory mechanism to regulate FLT3 kinase activity, and disruption by mutations destabilize its conformation.13 KD mutations constitute about 7C10% of AML cases and usually present as missense mutations of the activation loop, most commonly Febuxostat (TEI-6720) at D835.11, 12 Because of its proliferative stimulus and frequent mutation rate in AML, FLT3 has been deemed as a highly desirable target for modulation. The impressive response of chronic myelogenous leukemia patients to BCR-ABL TKI generated enthusiasm for molecularly targeted therapies in other malignancies dependent on constitutively activated kinase signaling. However, the development of resistance to imatinib due to the acquisition of point mutations in BCR-ABL also foreshadows a similar outcome now being reported in AML patients expressing a FLT3/ITD mutation being treated with FLT3 TKI.14C16 Resistance mutations often decrease the affinity of a TKI for its target and necessitate the use of a structurally unrelated inhibitor if one is available. This expectation Febuxostat (TEI-6720) has led to investigations attempting to identify a spectrum of secondary mutations of FLT3/ITD in the laboratory, which confer resistance to FLT3 TKI prior to their emergence in the clinic. Several groups have employed various techniques to identify FLT3 resistance mutations.17C21 In contrast to the wide array of BCR-ABL resistance mutations, relatively few FLT3 resistance mutations have been identified, which may partially reflect the failure to achieve sufficient levels of inhibition of FLT3 signaling in many trials,22 In this study, we identified the F691L and Y842C mutations previously identified as well as two novel mutations, F621L and A627P, that cause resistance to select TKI. These results suggest that novel mutations arising in FLT3/ITD, perhaps by selection during the course of treatment with a TKI, may prove to be refractory to FLT3 mutant AML management using most TKIs and emphasize the need for development of FLT3 inhibitors that can overcome resistance due to mutations. MATERIALS AND METHODS Reagents and Febuxostat (TEI-6720) antibodies Lestaurtinib, midostaurin, Febuxostat (TEI-6720) sunitinib, sorafenib and AC220 were purchased from LC Labs (Westchester, PA, USA). KW2449 was from Kyowa Hakko Kirin Co., Ltd. (Tokyo, Japan). AGS324 was provided by Aviv Gazit. Recombinant human interleukin-3 was purchased from Pepro Tech, Inc. (Rocky Hill, NJ, USA). FLT3 S-18 and STAT5 antibodies were from Santa Cruz Biotechnology (Santa Cruz, CA, USA), 4G10 anti-phosphotyrosine antibody and recombinant protein A-agarose were from Upstate Biotechnology (Lake Placid, NY, USA) and CD135-phycqerythrin (PE)-conjugated and annexin V-PE antibodies were from BD Pharmingen (San Jose, CA, USA). PhosphoMAP kinase, phospho STAT5, phosphoAKT, MAP kinase and AKT antibodies were from Cell Signaling Technologies, Inc. (Beverly, MA, USA). Goat anti-mouse and goat anti-rabbit horseradish peroxidase antibodies and the enhanced chemiluminescence kit were from Amersham Biosciences (Arlington Heights, IL, USA). DNA constructs and cells BaF3 or TF-1 cells were cultured in RPMI medium supplemented with 1 ng/ml recombinant human interleukin-3 or 1 ng/ml of granulocyte-macrophage colony-stimulating factor, respectively. FLT3/ITD cells were established from a patient sample as previously described.23 FLT3 point mutations were generated by site-directed mutagenesis in the pBabe Neo vector containing FLT3/ITD complementary DNA using the QuickChange Site-Directed Mutagenesis (Stratagene, La Jolla, CA, USA) and used to transfect BaF3 cells using the Nucleofector II from Amaxa Biosystems (Walkersville, MD, USA). BaF3 cells were chosen for confirmation of resistance.