Abstract A13: Therapeutic AXL inhibition with AVB-S6-500 improves response to chemotherapy and induces a homologous recombination deficiency in ovarian cancer
Authors: Mullen Mary Margaret , Lomonosova Elena , Noia Hollie , Wilke Daniel , Oplt Alyssa , Guo Lei , Kuroki Lindsay , Hagemann Andrea , McCourt Carolyn , Thaker Premal , Mutch David , Powell Matthew , Fuh Katherine 
Published: September 2020

Abstract
Background: Over 80% of patients with advanced ovarian cancer will develop chemoresistance and die within 5 years. Biomarkers to predict chemoresponse and novel therapies to target chemoresistance would be practice changing. We aim to establish serum and tissue GAS6 as predictive biomarkers of chemoresponse and to determine if AXL inhibition through sequestration of its ligand, GAS6, with AVB-S6-500 (AVB) can improve chemoresponse. Further, we desired to understand the affect AVB has on DNA damage response (DDR).
Methods: AVB was supplied by Aravive Biologics. High-grade serous ovarian cancer (HGSOC) tumor samples were obtained pre- and post-neoadjuvant chemotherapy. AXL and GAS6 expression were evaluated by immunohistochemistry and serum concentration. In vitro viability and clonogenic assays were performed on chemoresistant tumor cells (OVCAR8, OVCAR5, COV62, and POV71-hTERT) treated with chemotherapy +/- AVB. Mouse models (OVCAR8, PDX, OVCAR5) were used to determine if the combination of chemotherapy + AVB reduced tumor burden. Immunofluorescent (IF) assays targeting γH2AX were used to evaluate DNA damage and additional assays targeting 53BP1, RAD51, BRCA1, and BRCA2 were used to evaluate DNA damage response in cells treated with AVB, carboplatin, and/or AVB+carboplatin. Flow cytometry was used to evaluate RPA binding and cell cycle.
Results: Patients with high pretreatment tumor GAS6 expression (&gt;85%, n=7) or serum GAS6 concentrations (&gt;25ng/mL, n=13) were more likely to be resistant to neoadjuvant chemotherapy than those with low tumor GAS6 expression (&lt;45%, n=4) (P=0.010) or low serum GAS6 concentrations (&lt;15ng/mL, n=5) (P=0.002). Carboplatin plus AVB (2μM, 5μM) and paclitaxel plus AVB (1μM) resulted in decreased cell viability and clonogenic growth compared to chemotherapy alone (p&lt;0.05) in all tumor cell lines. In vivo tumor mouse models treated with chemotherapy+AVB had significantly smaller subcutaneous and intraperitoneal (IP) tumors than those treated with chemotherapy alone (P&lt;0.001). Increased DNA damage occurred in tumor cells treated with AVB than controls and in tumor cells treated with carboplatin+AVB than carboplatin alone (OVCAR8, COV362, CAOV3, OVCAR3-TPMES P&lt;0.001). Cells treated with AVB either alone or in combination with carboplatin demonstrated decreased binding of protein involved in homologous recombination including RPA, RAD51, BRCA1, and BRCA2 (P&lt;0.05). Conversely, these same cells demonstrated increased binding of the nonhomologous end joining protein 53BP1 (P&lt;0.05). There were no differences in cell cycle among the treatment groups.
Conclusions: High GAS6 is associated with poor neoadjuvant chemoresponse in HGSOC patients. The combination of chemotherapy with AVB decreases tumor cell viability and tumor growth. AVB imparts a homologous recombination deficiency through downregulation of homology-directed DNA repair with an associated upregulation of nonhomologous end joining.
Citation Format: Mary Margaret Mullen, Elena Lomonosova, Hollie Noia, Daniel Wilke, Alyssa Oplt, Lei Guo, Lindsay Kuroki, Andrea Hagemann, Carolyn McCourt, Premal Thaker, David Mutch, Matthew Powell, Katherine Fuh. Therapeutic AXL inhibition with AVB-S6-500 improves response to chemotherapy and induces a homologous recombination deficiency in ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr A13.