Single Cell-Level Signaling Profiling of Acute Myeloid Leukemia Following Treatment with Axl Kinase Inhibitor BGB324
Authors: Hellesøy Monica , Wnuk-Lipinska Katarzyna , Boniecka Anna , Nævdal Eline Milde , Reikvam Hakon , Loges Sonja , Cortes Jorge E. , Skavland Jørn , Bruserud Øystein , Lorens Jim , Micklem David , Gausdal Gro , Gjertsen Bjorn T. 
Published: October 2019

Abstract
Axl is a member of the Tyro3, Axl, Mer (TAM) receptor tyrosine kinase family that regulate a wide range of cellular functions, including cell survival, proliferation, migration/invasion and adhesion. Axl has been shown to play a key role in the survival and metastasis of many tumors, and has also been found to be upregulated and constitutively active in human AML. Indeed, Axl has been reported as an independent prognostic marker and a potential novel therapeutic target in AML.
BGB324 is a first-in-class highly selective small molecule inhibitor of Axl. BGB324 has been shown to be safe and well tolerated in clinical safety studies in healthy volunteers at doses up to 1500 mg/day with a predictable PK profile and long plasma half-life, and is currently in phase I b clinical trials for AML and non-small cell lung cancer.
In this study, we use phosphoflow cytometry to measure changes in signal transduction nodes in single AML cells treated with BGB324. We are applying this approach to monitor signaling profiles in primary AML cells harvested from patients undergoing BGB324 treatment.
Results:
The human AML cell line MOLM13 was treated in vitro with BGB324 (0.5 and 1µM for 1 hour) and analyzed for signal transduction changes by phosphoflow cytometry. We found a significant reduction in phosphorylation of Axl (pY779), Akt(pS473), Erk1/2(pT202/Y204) and PLCɣ1(pY783).
Next we established a systemic MOLM13 preclinical AML model in NOD/SCID mice. The mice were treated with 25 or 50 mg/kg BGB324 until moribund (up to 16 days). We found a dose-dependent and significant increase in overall survival in BGB324-treated mice. We further investigated intracellular signaling in BGB324-treated cells in vivo. Mice carrying systemic AML disease (MOLM13) were treated with BGB324 at 50mg/kg for 4 days, and we monitored CD33/45-positive MOLM13 cells harvested from spleen and bone marrow by flow cytometry. BGB324-treated mice showed a significant reduction in pErk and pPLCɣ1 relative to mice in the control group.
PBMCs from peripheral blood of AML patients treated with BGB324 400 mg x1 at day 1 and 2, and thereafter 100 mg daily were collected for single cell signal profiling of signal transduction changes by conventional flow cytometry (phospho-flow) and mass cytometry (CyTOF). Preliminary phopho-flow analyses show decrease of pAkt(T308) and pPLCgamma1(Y783) in one patient. Further analyses are ongoing and will be presented.
Figure 1. In vitro response to 1 hour BGB324 treatment in human AML cell line MOLM13 at 0.5 and 1µM doses. Response was evaluated in pAxl, pErk1/2, pAkt and pPLCγ1. n=3, *p≤0.05, **p≤0.005. Figure 1. In vitro response to 1 hour BGB324 treatment in human AML cell line MOLM13 at 0.5 and 1µM doses. Response was evaluated in pAxl, pErk1/2, pAkt and pPLCγ1. n=3, *p≤0.05, **p≤0.005. Figure 2. Dose-dependent response in overall survival in a MOLM13 systemic xenograft model (n=10). Figure 2. Dose-dependent response in overall survival in a MOLM13 systemic xenograft model (n=10). Figure 3. Response to BGB324-treatment in pErk, pPLCγ1 and pAkt in CD33/CD45-positive cells harvested from spleens (left) and bone marrows (right) of mice with systemic MOLM13 xenografts. n=5, *p≤0.05, **p≤0.005. Figure 3. Response to BGB324-treatment in pErk, pPLCγ1 and pAkt in CD33/CD45-positive cells harvested from spleens (left) and bone marrows (right) of mice with systemic MOLM13 xenografts. n=5, *p≤0.05, **p≤0.005.

Disclosures
Hellesøy: BerGenBio AS: Other: Previous employee. Stock option holder. Wnuk-Lipinska:BerGenBio AS: Employment. Boniecka:BerGenBio AS: Employment. Nævdal:BerGenBio AS: Employment. Loges:BerGenBio: Honoraria, Other: travel support, Research Funding. Cortes:Teva: Research Funding; BMS: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; BerGenBio AS: Research Funding; Ariad: Consultancy, Research Funding; Astellas: Consultancy, Research Funding; Ambit: Consultancy, Research Funding; Arog: Research Funding; Celator: Research Funding; Jenssen: Consultancy. Lorens:BerGenBio AS: Employment, Equity Ownership. Micklem:BerGenBio AS: Employment, Equity Ownership. Gausdal:BerGenBio AS: Employment. Gjertsen:Haukeland University Hospital: Research Funding.