Discovery and characterization of potent and selective AXL receptor tyrosine kinase inhibitor AB801

Background: AXL receptor tyrosine kinase (AXL) is a transmembrane protein that is overexpressed in a variety of tumors and correlates with poor prognosis in cancer patients. AXL is expressed in cancer and stromal cells and has been implicated in the development of resistance to chemotherapy, targeted therapies & immunotherapies. Activation of AXL by its ligand, growth arrest specific protein 6 (GAS6), or ligand-independent dimerization facilitates AXL phosphorylation, initiates signaling cascades that promote cancer cell proliferation, survival, and an immunosuppressive microenvironment. Here we present the discovery and characterization of a novel, highly potent and selective AXL inhibitor, AB801. Materials and methods: The potency and specificity of AB801 against AXL and other kinases were determined using a panel of HTRF® KinEASE-TK assays. The effects of AB801 on AXL were further assessed by p-AXL ELISA utilizing HEK293T cells transiently transfected with NanoLuc®-tagged AXL. Kinome selectivity was determined through a competition binding assay utilizing DNA-tagged kinases. Pharmacokinetics were evaluated in preclinical species. AB801 was also characterized in routine in vitro safety assays, including hERG inhibition. Results: The novel AXL inhibitor AB801 is potent, reversible, and selective. AB801 exhibits a double-digit picomolar inhibitory constant and retains significant activity in 100% human serum cell-based assays. Additionally, good selectivity was observed against MERTK, TYRO3, and the overall kinome. Importantly, the molecule does not show significant inhibition of the major CYP450 isoforms or the hERG potassium channel. Studies in preclinical species demonstrate a favorable pharmacokinetic profile, consistent with a projected once-a-day oral administration in humans. Conclusions: AXL inhibition is a promising therapeutic mechanism for impairing the growth and metastasis of chemotherapy- and immunotherapy-resistant tumors. AB801 exhibits improved potency, selectivity, and safety profiles compared to other AXL inhibitors currently advancing through clinical development. Conflict of interest: Ownership: Shareholder(s) in Arcus Biosciences Other Substantive Relationships: Current employees of Arcus Biosciences.