Development of combination strategies for focal adhesion kinase inhibition in diffuse gastric cancer.

e16057 Background: Diffuse gastric cancer (DGC) is an aggressive and frequently lethal pathological subtype of gastric cancer (GC). Since DGC often lacks genomic aberrations that indicate clear candidate therapeutic targets, it has been challenging to develop target therapies for this GC subtype. Our previous study highlighted the contribution of focal adhesion kinase (FAK) in the tumorigenesis of DGC, and the potential for small molecule FAK inhibitors to have efficacy in DGC. Currently, FAK inhibitors are under active clinical evaluation in several tumor types. To date, FAK inhibitor monotherapy has been tolerated in the clinic, but has shown only modest clinical efficacy in cancer treatment, which inspired us to explore targets for the combination with FAK inhibitors in DGC. Methods: We constructed the engineered murine DGC model of Cdh1 -/- RHOA Y42C/+ organoids, with Cdh1 (E-Cadherin) loss and RHOA Y42C mutation, in our previous study. Using this DGC organoids model, we performed genome-scale libraries of open reading frames (ORF) screen to identify candidate mechanisms of resistance to FAK inhibitors (defactinib or PF-573228). For the top hits found in ORF screen, further validations were performed in Cdh1 -/- RHOA Y42C/+ organoids and human DGC cell lines of NUGC4 and SNU668 to check whether they could function as candidates for the combination with FAK inhibitors. Besides, we used Digiwest, a high-throughput protein analysis assay, to investigate pathway alterations of Cdh1 -/- RHOA Y42C/+ organoids treated with FAK inhibitors (defactinib or PF-573228). We then explored whether the activated pathways were the candidates for the drug combination. Zero Interaction Potency (ZIP) model was used to measure the synergistic effect between the drugs. The combination strategies were also tested in vivo. Results: We identified that cyclin-dependent kinase 6 (CDK6) promotes FAK inhibitors (defactinib and PF-573228) resistance according to ORF screen, and CDK4/6 inhibitor, Palbociclib, enhances the efficacy of defactinib for DGC models. Furthermore, we demonstrated that FAK inhibitors (defactinib and PF-573228) treatment in DGC models leads to compensatory activation of MAPK pathway in DGC, and the RAF/MEK dual inhibitor VS-6766 effectively enhances the antitumor efficacy of FAK inhibitors in vitro and in vivo. Conclusions: These data suggest that combinations of FAK inhibitors with MAPK inhibitors or CDK4/6 inhibitors are promising treatment strategies for DGC that warrant further testing in clinical trials.