Abstract 5456: Discovery of OTS447, a highly potent and selective inhibitor of FLT3 for the treatment of AML patients with FLT3-ITD/TKD mutations
Authors: Okamoto Yasuhide , Takamatsu Naofumi , Miyamoto Takashi , Taira Akiko , Toyota Kozue , Imai Yukiho , Fujii Shinobu , Ono Rumiko , Adachi Kyoko , Matsuo Yo , Chung Suyoun , Park Jae-Hyun 
Published: June 2022

Abstract
FMS-like tyrosine kinase 3 (FLT3) is one of the most frequently mutated genes in acute myeloid leukemia (AML), and its activating mutations including internal tandem duplications (ITDs) and missense point mutations in the tyrosine kinase domain (TKD) are found in approximately 30% of AML patients. Although the patients initially well responded to FLT3 inhibitors such as gilteritinib, the most cases relapsed within a few months after the initiation of treatment. For these patients who relapsed after FLT3 inhibitor-based therapy, no effective treatment is available. Mechanisms of resistance are not fully elucidated, but FLT3-ITD-TKD double mutant is considered as one of possible mechanisms. We have discovered a novel FLT3 inhibitor OTS447 that shows the potent and selective inhibition in both FLT3-ITD and FLT3-ITD-TKD double mutants.
OTS447 was identified through the screening of our in-house proprietary compound library. It showed potent inhibitory activity against FLT3 with an IC50 value of 0.19 nM. The selectivity of OTS447 was investigated by human kinase profile assay. Among 371 human kinases tested, there were only seven (including FLT3) whose activity were inhibited by 80% or more at 5 nM of OTS447. We then examined the anti-proliferative effects in FLT3-ITD and -wild type (WT) AML cell lines. The proliferation of FLT3-ITD cell lines, MV4-11 and MOLM13, was more strongly suppressed than that of FLT3-WT cell lines. We also investigated the inhibitory effects to FLT3-ITD-TKD double mutants using Ba/F3 cells. FLT3-ITD-D835Y mutant was inhibited as strongly as FLT3-ITD mutant, and FLT3-ITD-F691I mutant was more strongly inhibited than Ba/F3 parental cells. These data suggest that OTS447 has selective inhibitory activity against FLT3 mutants. To confirm that FLT3 inhibition by OTS447 leads to the anti-proliferative effect, FLT3 autophosphorylation and phosphorylation of downstream molecules such as STAT5, ERK and AKT were examined. Both autophosphorylation of FLT3 and phosphorylation of downstream molecules were decreased by OTS447 in dose-dependent manner. Moreover, treatment of OTS447 induced the apoptosis and increased sub-G1 population in MV4-11 cell. Finally, we tested OTS447 anti-tumor effect using MV4-11 mouse xenograft model. OTS447 showed potent growth inhibition against MV4-11 tumor in dose-dependent manner.
In summary, we discovered OTS447, a potent and selective FLT3 inhibitor, that can inhibit not only ITD mutation but also ITD-TKD mutations of the FLT3 gene. OTS447 possesses cytotoxic activity induced by inhibition of FLT3 signaling pathway and has anti-tumor activity in mouse xenograft model. We are pursuing further optimization of the inhibitor aiming for the effective treatment of AML patients with several types of FLT3-activating mutations.
Citation Format: Yasuhide Okamoto, Naofumi Takamatsu, Takashi Miyamoto, Akiko Taira, Kozue Toyota, Yukiho Imai, Shinobu Fujii, Rumiko Ono, Kyoko Adachi, Yo Matsuo, Suyoun Chung, Jae-Hyun Park. Discovery of OTS447, a highly potent and selective inhibitor of FLT3 for the treatment of AML patients with FLT3-ITD/TKD mutations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5456.