Key Points Axl inhibition by BGB324 is active in FLT3-mutated and FLT3 wild-type AML, and presence of Axl and Gas6 are required for therapeutic efficacy. AML cells educate BMDSCs to secrete Gas6, which mediates leukemia cell proliferation and therapy resistance.

The protein kinase Bcr is a negative regulator of cell proliferation and oncogenic transformation. We identified Bcr as a ligand for the PDZ domain of the cell junction and Ras-interacting protein AF-6. The Bcr kinase phosphorylates AF-6, which subsequently allows efficient binding of Bcr to AF-6, showing that the Bcr kinase is a regulator of the PDZ domain-ligand interaction. Bcr and AF-6 colocalize in epithelial cells at the plasma membrane. In addition, Bcr, AF-6, and Ras form a trimeric complex. Bcr increases the affinity of AF-6 to Ras, and a mutant of AF-6 that lacks a specific phosphorylation site for Bcr shows a reduced binding to Ras. Wild-type Bcr, but not Bcr mutants defective in binding to AF-6, interferes with the Ras-dependent stimulation of the Raf/MEK/ERK pathway. Since AF-6 binds to Bcr via its PDZ domain and to Ras via its Ras-binding domain, we propose that AF-6 functions as a scaffold-like protein that links Bcr and Ras to cellular junctions. We suggest that this trimeric complex is involved in downregulation of Ras-mediated signaling at sites of cell-cell contact to maintain cells in a nonproliferating state.

Ben Batalla, I. et al. (2017) Axl blockade by BGB324 inhibits BCR-ABL tyrosine kinase inhibitor-sensitive and -resistant chronic myeloid leukemia. Clinical Cancer Research, 23(9), pp. 2289-2300. (doi:10.1158/1078-0432.CCR-16-1930 This is the author's final accepted version.There may be differences between this version and the published version. You are advised to consult the publisher's version if you wish to cite from it.http://eprints.gla.ac.uk/131767/ STATEMENT OF TRANSLATIONAL RELEVANCEABL tyrosine kinase inhibitors (TKI) are employed for chronic myeloid leukemia (CML) treatment and induce durable remissions in the majority of patients. However, resistant disease and persistence of BCR-ABL1-independent leukemia stem and progenitor cells remain clinical challenges.Furthermore, ABL-TKI are associated with the risk of severe side effects in long term use. Therefore, novel therapeutic strategies are warranted to target persisting stem cells and resistant clones alone or in combination with ABL-TKI.We demonstrate here the activity of an Axl inhibitor, BGB324, against TKI-sensitive and -resistant BCR-ABL1 positive cells, including T315I-mutated and ponatinib-resistant primary cells. Inhibition of Axl with BGB324 exerted therapeutic effects in complementary T315I-mutated and ponatinib-resistant preclinical mouse models. BGB324 was well-tolerated in Phase 1 clinical trials in healthy volunteers and AML patients (BGBC001, BGBC003 ongoing). Thus our findings can be rapidly translated to the clinics and BGB324 holds promise to improve treatment of CML patients.