e14676 Background: Ovarian cancer is the leading cause of death due to gynecologic malignancy. Biomarkers to predict chemoresponse and novel therapies to target these proteins would be practice changing. We aim to establish serum and tissue GAS6 as a predictive biomarker of chemoresponse and to determine if AXL inhibition through sequestration of its ligand, GAS6, with AVB-S6-500 (AVB) can improve chemoresponse. 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 (OVCAR8, OVCAR5, COV62, and POV71-hTERT) and stromal cells (CAF86) treated with chemotherapy +/- AVB. Mouse models (OVCAR8, PDX, OVCAR5) were used to determine if the combination of chemotherapy + AVB reduced tumor burden. Immunofluorescent assays targeting ɣH2AX were used to evaluate DNA damage. Results: Patients with high pretreatment tumor GAS6 expression ( > 85%, n = 7) or serum GAS6 concentrations ( > 25ng/mL, n = 13) were more likely to be resistant to neoadjuvant chemotherapy than those with low tumor GAS6 expression ( < 45%, n = 4) (P = 0.010) or low serum GAS6 concentrations ( < 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 < 0.05) in all tumor and stromal cell lines. Synergism was seen between carboplatin+AVB and paclitaxel+AVB with a weighted combination index < 1. In vivo tumor mouse models treated with chemotherapy+AVB had significantly smaller subcutaneous and intraperitoneal (IP) tumors than those treated with chemotherapy alone (3.1mg vs 64mg, P = 0.003 OVCAR8; 62mg vs 157mg, P = 0.0108 PDX subcutaneous model; 0.05mg vs 0.3669mg, P < 0.001 OVCAR5 IP model). Increased DNA damage was noted in tumor and stromal cells treated with carboplatin+AVB compared to carboplatin alone (OVCAR8, COV362, CAF86 P < 0.001). Conclusions: High GAS6 is associated with lack of neoadjuvant chemoresponse in HGSOC patients. The combination of chemotherapy with AVB decreases tumor cell viability, tumor growth, and an increase in DNA damage response.