In vitro activity: In vitro, among the 78 tyrosine kinases tested, Gilteritinib at 1 nM concentration inhibits FLT3, leukocyte tyrosine kinase (LTK), anaplastic lymphoma kinase (ALK), and AXL kinases by over 50% with an IC50 value of 0.29 nM for FLT3, and is about 800-fold more potent than for c-KIT. Gilteritinib inhibits eight of the 78 tested kinases by over 50% at concentrations of either 1 nM (FLT3, LTK, ALK, and AXL) or 5 nM (TRKA, ROS, RET, and MER). The IC50 values are 0.29 nM for FLT3 and 0.73 nM for AXL. Gilteritinib inhibits FLT3 at an IC50 that is approximately 800-fold more potent than the concentration required to inhibit c-KIT (230 nM). The antiproliferative activity of Gilteritinib is evaluated against MV4-11 and MOLM-13 cells, which endogenously express FLT3-ITD. After 5 days of treatment, Gilteritinib inhibits the growth of MV4-11 and MOLM-13 cells with mean IC50s of 0.92 nM (95% CI: 0.23-3.6 nM) and 2.9 nM (95% CI: 1.4-5.8 nM), respectively. Growth suppression of MV4-11 cells is accompanied by inhibition of FLT3 phosphorylation. Relative to vehicle control cells, phosphorylated FLT3 levels are 57%, 8%, and 1% after 2 h of treatment with 0.1 nM, 1 nM, and 10 nM Gilteritinib, respectively. In addition, doses as low as 0.1 nM or 1 nM result in the suppression of phosphorylated ERK, STAT5, and AKT, all of which are downstream targets of FLT3 activation. To investigate the effects of Gilteritinib on AXL inhibition, MV4-11 cells that expressed exogenous AXL are treated with Gilteritinib. At concentrations of 1 nM, 10 nM, and 100 nM for 4 h, Gilteritinib treatment decreases phosphorylated AXL levels by 38%, 29%, and 22%, respectively.

Kinase Assay: Gilteritinib was tested in a panel of 78 tested kinases using ATP concentrations that are approximately equal to the Km value for each kinase in a TK-ELISA or off-chip mobility shift assay. Initially, two concentrations of Gilteritinib (1 nM and 5 nM) are tested to assess each compound’s inhibitory effect on TK activity. Further studies are then conducted using a dose range of Gilteritinib to determine IC50values for kinases in which activity is inhibited by>50% with 1 nM Gilteritinib as well as for c-KIT. TK-ELISA and MSA assays are used to conduct IC50 studies for FLT3, LTK, AXL, and c-KIT; the HTRF KinEASE-TK assay is performed to assess the IC50 value of echinoderm microtubule-associated protein-like 4-ALK (EML4-ALK).

Cell Assay: MV4-11 cells treated with DMSO or increasing concentrations of gilteritinib are incubated for 2 hours. Immunoprecipitation and immunoblot for phosphorylated FLT3 and total FLT3 are performed.