Effect on response to neoadjuvant chemotherapy in high-grade serous ovarian cancer by inhibiting the GAS6/AXL pathway and inducing homologous recombination deficiency.

6080 Background: Less than 10% of patients with high grade serous ovarian cancer (HGSC) have a complete pathologic response to neoadjuvant chemotherapy. We aimed to identify a biomarker predictive of response to neoadjuvant chemotherapy and to determine if GAS6/AXL inhibition with AVB500 (AVB) could increase platinum response. Methods: AVB was supplied by Aravive Biologics. HGSC tumor samples were obtained pre- and post-neoadjuvant chemotherapy. GAS6 expression was measured by tissue immunohistochemistry (IHC) and serum ELISA. Four HGSC cell lines were used for all experiments. Immunofluorescent (IF) assays targeting ɣH2AX for DNA damage, RAD51, BRCA1, and BRCA2 for homologous recombination (HR) and 53BP1 for non-homologous end joining (NHEJ) were performed. Flow cytometry was used to evaluate RPA binding. DNA fiber assays were performed. In vitro clonogenic assays were done on chemoresistant ovarian tumor cells treated with carboplatin (carbo) +/- AVB and olaparib +/- AVB. Synergy assays were analyzed using Combenefit software. Mouse models were used to evaluate the combination of carboplatin + AVB and olaparib + AVB on tumor burden. Results: Patients with high pretreatment tumor GAS6 IHC expression ( > 85%) or serum GAS6 concentrations ( > 25ng/mL) were more likely to have a poor response to neoadjuvant chemotherapy than those with low GAS6 (P = 0.002). Additionally, high GAS6 concentration was associated with decreased overall survival (24.4 months versus undefined, P = 0.009). Carbo + AVB resulted in decreased clonogenic colonies compared to carbo alone (p < 0.05). In vivo tumor mouse models treated with chemotherapy + AVB had significantly less tumor burden than those treated with chemotherapy alone (50mg vs 357mg, P = 0.003). We identified an induction in HR deficiency by a decrease in RAD51, BRCA1, and BRCA2 foci and RPA binding in cells treated with carbo + AVB compared to carbo (P < 0.05). There was increase in ɣH2AX and 53BP1 foci as well as replication fork slowing in tumor cells treated with carboplatin + AVB (P < 0.01). We also AVB and carboplatin were synergistic. Olaparib + AVB resulted in decreased clonogenic colonies (P < 0.05) and decreased tumor burden in mouse models (76mg vs 171mg, P = 0.03) compared to olaparib alone. Conclusions: GAS6 is a potential biomarker predictive of poor response to neoadjuvant chemotherapy in HGSC. Inhibition of this GAS6/AXL pathway with AVB improves sensitivity to traditional neoadjuvant chemotherapy by inducing a homologous recombination deficiency.