Momelotinib is a type I FLT3 inhibitor and effectively suppresses leukemic progression in mice. (A) Ribbon depiction of a structural model of FLT3 momelotinib was built using quizartinib (PDB: 4XUF) and gilteritinib (PDB: 6JQR) coordinates. A stick representation of momelotinib (cyan) and gilteritinib (brown) shows their binding to the ATP site. (B) Close-up view of the active site of the FLT3 kinase showing the interaction of momelotinib with Cys694 (C694) from the kinase hinge region and Ser618 (S618) from the P-loop through 3 hydrogen bonds (upper panel). Similar binding of gilteritinib and momelotinib (lower panel). (C) Surface depiction of FLT3 kinase with gilteritinib showing the binding of momelotinib and gilteritinib. The activation loop residues D835 (pink), D839 (brown), and Y842 (pink) are frequently mutated in patients treated with type II inhibitors. (D) Model of FLT3 inactive conformation docked with momelotinib and gilteritinib showing steric clash with Phe830 and broken hydrophobic spine. (E) Model of FLT3 active kinase with a stable hydrophobic spine showing the binding of momelotinib and gilteritinib, which provides an explanation why they favor active kinase conformation for binding. (F) Luminescence imaging of NSGS mice transplanted with MOLM13 (upper panels) and MV4-11 (lower panels) at 2 weeks posttransplantation. One million MOLM13-luciferase-Cherry cells (upper panels) and MV4-11-luciferase-Cherry cells (lower panels) were transplanted by tail vein injection into NSGS mice. Momelotinib treatment (100 mg/kg daily in phosphate-buffered saline by intraperitoneal injection) was started 2 days posttransplant. Control mice were injected with vehicle (phosphate-buffered saline).