Abstract 276: Targeting AXL sensitizes non-small cell lung cancer to ATR inhibitors by enhancing replication stress
Authors: Ramkumar Kavya , Tong Pan , Fan You-Hong , Peng David , Heymach John V. , Gibbons Don L. , Wang Jing , Byers Lauren A. 
Published: August 2019

Abstract
Therapeutic resistance limits effective treatment of non-small cell lung cancer (NSCLC) and a better understanding of mechanisms contributing to resistance and strategies to overcome these are urgently needed. AXL, a TAM family receptor tyrosine kinase, has emerged as a key determinant of intrinsic and acquired resistance to chemotherapy, radiation and targeted therapies in NSCLC and other cancers, through its roles in mediating epithelial-mesenchymal transition (EMT) and immune escape. We previously showed that AXL may also play a role in DNA damage repair and that AXL overexpression mediated primary as well as acquired resistance to inhibitors of WEE1, a replication stress response kinase, in small cell lung cancer. In the present study, we further investigated the role of AXL in replication stress response.
We found that, in NSCLC cell lines, AXL inhibition with the selective small-molecule AXL inhibitor BGB324 caused replication stress checkpoint activation, as indicated by increased RPA32 hyper-phosphorylation and ATR-mediated CHK1 phosphorylation. We further screened ATR inhibitors, VX-970 and AZD6738, in a panel of 20 NSCLC cell lines and correlated drug sensitivity with baseline expression of over 200 phosphorylated and total proteins, measured by reverse phase protein array. Notably, AXL was one of the biomarkers of resistance to the ATR inhibitor VX-970 (rho=0.52, p&lt;0.05). Therefore, we hypothesized that AXL plays a unique role in regulating replication stress and targeting AXL will sensitize NSCLC cells to ATR inhibitors. Combination of BGB324 and ATR inhibitors (VX-970 and AZD6738) significantly decreased cell proliferation in a panel of human and GEMM-derived NSCLC cell lines as compared to single agents alone. In NSCLC cells with primary resistance to ATR inhibition, co-targeting AXL and ATR significantly increased RPA32 hyper-phosphorylation, concomitantly with increased DNA double strand breaks and inactivated G2/M checkpoint, resulting in mitotic catastrophe. AXL knockdown in a GEMM-derived Kras/Trp53 mutant NSCLC model also showed similar results. Notably, NSCLC cell lines with low levels of SLFN11 (a DNA/RNA helicase that induces replication arrest following DNA damage independently of ATR) were more sensitive to AXL/ATR co-targeting.
In conclusion, these findings suggest that AXL may play a novel and unexpected role in regulating replication stress. Furthermore, our results show that targeting AXL sensitizes NSCLC cell lines with primary resistance to ATR inhibitors and that AXL/ATR inhibitor combinations could be useful in treating platinum- and PARP inhibitor-resistant SLFN11-low tumors.
Citation Format: Kavya Ramkumar, Pan Tong, You-Hong Fan, David Peng, John V. Heymach, Don L. Gibbons, Jing Wang, Lauren A. Byers. Targeting AXL sensitizes non-small cell lung cancer to ATR inhibitors by enhancing replication stress [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 276.