The Novel Inhibitor PLX3397 Effectively Inhibits FLT3-Mutant AML,
Authors: Burton Elizabeth , Wong Bernice , Zhang Jiazhong , West Brian , Bollag Gideon , Habets Gaston , Galanis Allison , Nguyen Hoa , Arowojolu Omotayo , Rajhowa Trivikram , Levis Mark J. 
Published: October 2019

Abstract
Abstract 3632
Mutations of FLT3 (FMS-like tyrosine kinase 3) are frequently present in acute myeloid leukemias (AML) and associated with poor prognosis. Such mutations alter the conformation and constitutively activate the FLT3 tyrosine kinase. PLX3397 is a novel, orally active, selective small molecule inhibitor of the FLT3-mutant as well as FMS and KIT kinases.
The high selectivity of PLX3397 has been demonstrated in biochemical assays against a panel of over 200 recombinant kinases. FLT3, FMS and KIT are significantly inhibited at low nanomolar concentrations. The majority of kinases screened were not inhibited by PLX3397 (IC50=>10 μM) and the only other kinase with significant sub-micromolar inhibition was KDR. PLX3397 exhibits favorable pharmaceutical properties and demonstrated minimal off-target activity when tested in a broad array of 71 targets in 8 families (Neurotransmitter-related, Steroids, Ion Channels, Nitric Oxide, Prostaglandins, Growth Factors, Brain/Gut Peptides, and Enzymes).
In cellular assays PLX3397 effectively inhibited ligand-stimulated autophosphorylation of the endogenous receptor tyrosine kinases FMS (IC50=20 nM) and KIT (IC50=120 nM), but not ligand-stimulated wild-type FLT3 (IC50=1.7 μM). In SEMK2 cells, which over-express wild type FLT3, PLX3397 inhibited FLT3 autophosphorylation with an IC50 of 240 nM. The phosphorylation of the activated FLT3-ITD protein in MV-4-11 and MOLM-14 cells harboring mutations (FLT3-ITD) of the juxtamembrane region of FLT3 was efficiently inhibited (IC50 26 nM and 30 nM respectively), indicating a possible application in this major subset of FLT3 mutated AML. In parallel with inhibition of FLT3-mutant autophosphorylation, the phosphorylation of known downstream effectors (STAT5, AKT, MEK, and ERK) is also inhibited. However, no inhibition was observed for KDR phosphorylation.
PLX3397 potently inhibited of mutant-FLT3 driven proliferation in both MV-4-11 and MOLM-14 AML cells in culture (IC50=100–200 nM), but was less effective against HL60 cells expressing wild-type FLT3. MV-4-11 cells grown as subcutaneous xenograft tumors in mice were also highly sensitive to PLX3397 dosed orally at 10 or 30 mg/kg qd, with tumor regression observed at the higher dose.
Primary samples collected from AML patients at relapse were tested in culture. While no significant effects were seen in FLT3 wild type samples below 1 μM, a clear dose response to PLX3397 was observed in samples with FLT3-ITD mutations. Because the estimated protein binding of PLX3397 in plasma is >99%, we also tested the drug against the MV-4-11 cell line in plasma and found the IC50 for inhibition of autophosphorylation to be 2.8 μM. In humans trough levels of PLX3397 (10 μM) capable of inhibiting FLT3-mutants can be safely achieved. A Phase I/II study is planned to evaluate the safety and explore the efficacy of PLX3397 in patients with FLT3-mutant- AML.

Disclosures:
Burton: Plexxikon Inc.: Employment. Wong:Plexxikon Inc.: Employment. Zhang:Plexxikon Inc.: Employment. West:Plexxikon Inc.: Employment. Bollag:Plexxikon Inc.: Employment. Habets:Plexxikon Inc.: Employment. Nguyen:Plexxikon Inc.: Employment. Levis:Plexxikon Inc.: Honoraria.