Targeting AXL Kinase Sensitizes Acute Myeloid Leukemia Stem and Progenitor Cells to Venetoclax Treatment

Acute myeloid leukemia (AML) is a heterogeneous haematological cancer characterized phenotypically by the rapid clonal growth of myeloid cells and an accumulation of blasts in the peripheral blood and bone marrow of patients. Despite the major progress that has been made in categorizing different genetic and molecular AML subgroups, therapies and long-term patient outcomes have not changed significantly over the past four decades. Recently, venetoclax (ABT-199), a BH3-mimetic and selective BCL-2 inhibitor, was approved for the treatment of older patients with AML. However, the limited efficacy, drug resistance in complex karyotype AML and disease progression on venetoclax as well as the inherent resistance of leukemic stem cells (LSCs) to therapy pose significant clinical challenges, warranting identification of novel targets and improved treatment strategies.

One candidate target is AXL, a member of the TYRO3/AXL/MER (TAM) family of receptor tyrosine kinases. AXL and its ligand growth arrest-specific gene 6 (GAS6) are elevated in AML patients and LSCs, and associated with poor prognosis. To test whether targeting of the AXL/GAS6 pathway is a feasible treatment strategy for AML, in particular to eradicate LSCs, we developed SLC-391, a novel, potent and selective AXL inhibitor. In vitro and in vivo evaluations of the pharmaceutical properties of SLC-391 indicated reasonable solubility, excellent metabolic stability as well as desirable bioavailability in mice and rats. In silico molecular docking analysis showed that SLC391 can adopt a conformation with surface and charge complementary to the active site of the AXL kinase, potentially engaging in hydrophobic ring-mediated interactions. Further, cell-based studies discovered that SLC-391 targets AML cells with high AXL/GAS6 expression, particularly MLL+ AML cells, and synergizes with venetoclax in cell viability and apoptosis assays (CI<0.6). In addition, simultaneous AXL and BCL-2 inhibition reduced the clonal short- and long-term growth of primitive AML patient cells in CFC re-plating and LTC-IC assays compared to single or control treatments (20-95% inhibition). Moreover, a combination of AXL inhibition and venetoclax treatment was able to target LSCs and AML blasts in two different preclinical patient-derived xenotransplantation (PDX) models, extending the mean survival of these mice by 14-30 days compared to single agents (P<0.025). Mechanistically, single-cell RNA-sequencing and functional validation studies revealed that AXL inhibition perturbs oxidative metabolism, and differentially targets signaling pathways to synergize with venetoclax in leukemic cell killing. Importantly, the combination of AXL inhibition plus venetoclax treatment was not toxic to normal BM cells from healthy donors. Hence, our findings identify a promising, improved and specific treatment strategy for AML, particularly patients with high AXL/GAS6 expression.