ETH-155008, a Novel Selective Dual Inhibitor of FLT3 and CDK4/6 in Preclinical Treatment of Acute Myeloid Leukemia

Fms-like tyrosine kinase 3 (FLT3) is part of a family of receptor tyrosine kinases (RTKs), and a molecular therapeutic target of acute myeloid leukemia (AML). There are three FTL3 inhibitors, midostaurin, gilteritinib, and quizartinib have been approved for clinical treatment of AML. We have evaluated the inhibition of cell proliferation in vitro and antitumor activity in vivo of ETH-155008, a novel triple inhibitor of FLT3, Pim-3 and CDK4/6. ETH-155008 was found to have a strong inhibitory activity against key target enzymes (FLT3 IC₅₀ at 0.5nM; Flt3 -Itd IC₅₀ at 0.6 nM; CDK4/D1 IC₅₀ at 0.8nM, CDK6/D1 IC₅₀ at 1.4nM), which were more potent than the tested positive controls gilteritinib, quizartinib, and AMG-925. ETH-155008 showed to have highly potent antiproliferative activities against cultured human acute myeloid leukemia cells (MV4-11 and Molm-13) with IC50 at 4.1nM and 12 nM, respectively, and showed remarkable antitumor efficacy in vivo.

The in vivo antitumor activity of ETH-155008 was evaluated and compared with midostaurin and gilteritinib in the treatment of the two models with mutant forms in TKD and gatekeeper region, namely BaF3 FLT3-ITD-F691L and BaF3 FLT3-ITD-D835Y, respectively. Treatment with ETH-155008 (40 mg/kg/daily x 18, oral) significantly delayed tumor growth and resulted in a smaller mean tumor size (tumor growth inhibition (TGI) of 86% (p<0.001 compared with the control group) in the BaF3 FLT3-ITD-F691L model, and cured all of the tumor-bearing mice in the BaF3 FLT3-ITD-D835Y model. The antitumor activity of ETH-155008 was comparable to gilteritinib, but much better than midostaurin, the latter showed only mild antitumor activity with ~25% of TGI in the two FLT3 mutant models. We also tested the therapeutic efficacy of ETH-155008 in the human AML xenograft model MV4-11, ETH-155008 cured all of the tumor-bearing mice (mean tumor size at time of treatment 126 mm³), at a dose of 40mg/kg/daily for 14 days, that showed very little toxicity (only 2% body weight loss) which were well below the maximum tolerated dose.

ETH-155008 represents a novel dual inhibitor that may yet fulfill the promise of effective AML therapy through dual targeting of FLT3 and CDK4/6. These preclinical results supported the safety and efficacy observed in the preclinical studies and worthy of further clinical evaluation.