Abstract 5665: HX301 (ON1232580) a novel kinase inhibitor with potent activity against CSF1R and FLT3, shows strong anti-AML activity in defined preclinical models
Authors: An Xiaoyu , Li Henry , Xue Linda , Liu Jinping , Xiong Lingxin , Ke Hang , Chen Cen , Gao Bing , Zheng Jia , Bao Zhengzheng , Guo Sheng , Zhang Lei , Zhang Faming 
Published: June 2023

Abstract
Acute myeloid leukemia (AML) is an aggressive leukemia of myeloid lineage with different subtypes of varying treatments/outcomes and a global annual mortality ~150,000. The first-line treatment of AML is usually induction chemotherapy, followed by further chemo-/radiation therapies or stem cell transplant. Targeted therapy tailored for specific driver mutations could be alternative treatment options, e.g., new inhibitors targeting IDH mutations and FLT3-ITR CSF1R is a receptor tyrosine kinase (RTK) responsible for the growth, survival and polarization of certain myeloid lineages of cells including macrophages. It is also frequently expressed in certain AML patient populations, thus implicated in the pathogenesis, or a new possible drug target of AML.
To test this hypothesis, a novel kinase inhibitor, HX301 with strong anti-CSF1R activity (IC50 of ~0.7nM) as well as anti-FLT3 (IC50 of ~7nM), were assessed for anti-AML activity. First, an in vitro proliferation assay was performed using primary macrophages and a panel of AML cell lines, confirming that HX-301 has high potency in primary macrophage culture (IC50 of ~70nM) and also among cultures of a subsets of AML lines (e.g. IC50 &lt; 1µM), such as MV4-11 (medium CSF1R expressing and FLT3-ITD), EOL1, MOL13, etc. Next, HX301 was pharmacologically modeled using four preclinical models to test anti-leukemia activity in vivo, including subcutaneous xenograft of MV4-11 cells (CDX) and systemically engrafted PDX models AM8096 (high CSF1R expression but wild type FLT3), AM7577 (FLT3-ITD but little CSF1R expression) and AM5512 (wild-type FLT3 and little CSF1R expression).
Our data demonstrated that HX301 partially inhibited MV4-11 tumor growth as measured by tumor volume, consistent with the in vitro observation, possibly due to the inhibition of CSF1R or FLT3-ITD, or both. On the other hand, HX301 completely suppressed leukemogenesis of AM8096, likely due to the inhibition of CSF1R. This observation suggested that CSF1R is likely the driving mechanism for the leukemogenesis of this model; or in another word, for being a PDX, CSF1R likely drives pathogenesis in the original patient. HX301 also suppressed AM7577 growth, likely due to the suppression of FLT3-ITD activity since we previously reported FLT3-ITD being the driver mutation in this model which responded to AC220, a FLT3 TKi. Lastly, HX301 has little activity against AM5512.
All the data suggests that HX301 can potentially be explored for the treatment of AML, at least a subset of the patients with CSF1R and FLT3-ITD as leukemogenic drivers. Further preclinical/translational studies are being conducted in order to reveal predictive biomarkers, in addition to FLT3 mutation and CSF1R expression/mutations. We believe that HX301 could be a potential candidate for treating subset of AML, warranting further clinical investigation.
Citation Format: Xiaoyu An, Henry Li, Linda Xue, Jinping Liu, Lingxin Xiong, Hang Ke, Cen Chen, Bing Gao, Jia Zheng, Zhengzheng Bao, Sheng Guo, Lei Zhang, Faming Zhang. HX301 (ON1232580) a novel kinase inhibitor with potent activity against CSF1R and FLT3, shows strong anti-AML activity in defined preclinical models. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5665.