Abstract 1272: Aberrantly exposed phosphatidylserine as a drug delivery target in pancreatic cancer
Authors: Phinney Natalie Zara , Huang Xianming , Trawick Mary Lynn , Pinney Kevin , Brekken Rolf 
Published: April 2023

Abstract
Pancreatic ductal adenocarcinoma (PDA) has risen to become the 3rd leading cause of cancer related deaths in the United States with a 5-year survival rate of 11% and a median survival of just 4-6 months. PDA is notoriously chemo-resistant and has shown little to no sensitivity to the current immune checkpoint blockade therapy. A novel marker for the tumor microenvironment is phosphatidylserine, first characterized as a selective marker of tumor vasculature. Phosphatidylserine (PS) is a phospholipid exposed on the outer leaflet of the plasma membrane of tumor associated endothelial cells, apoptotic tumor cells, and some viable tumor cells, where it functions to suppress the immune response. Exposed PS drives immunosuppression by binding to PS receptors expressed on tumor-infiltrating macrophages and dendritic cells. To this point, PS has been targeted with antibodies, such as Bavituximab, that have shown excellent specificity for tumor vasculature and an immune stimulatory environment evidence by polarization of macrophages to a pro-inflammatory M1-like phenotype, a reduction in myeloid-derived suppressor cells, enhanced maturation of dendritic cells and increased primed T cell activity. We have advanced this concept by developing the next generation of PS targeting agent, a fusion protein (betabody) that exploits the PS-binding domain of β2-glycoprotein 1 (β2GP1), an abundant serum protein that binds PS. Betabodies were constructed by fusing domain V of β2GP1 to the Fc of an IgG2a. Betabodies are smaller than full length antibodies (~85 kDa vs 150 kDa), have good affinity for PS (~1 nM) and bind directly to PS. We have demonstrated that betabodies bind specifically to externalized PS through ELISA, ICC and flow cytometry. We established that betabodies localize robustly and specifically to the tumor microenvironment through in vivo localization experiments in tumor-bearing mice. Further studies will exploit the specificity of betabodies to deliver novel microtubule-disrupting agents to exposed PS in the microenvironment of PDA. Because betabodies are internalized inefficiently, therapeutic payloads will be linked to betabodies via enzyme cleavable linkers. This first-in-class tumor targeting therapeutic has potential to provide efficacy in therapy-resistant pancreatic tumors. In addition, similarly to PS targeting antibodies, preliminary data in vitro has shown that treatment of both bone marrow derived macrophages and Raw 264.7 with the unconjugated betabodies repolarize M2 macrophages to a M1 phenotype. The mechanism of action has not yet been fully elucidated; however, it may involve the interaction between the β2GP1 domains and endogenous LPS. This first-in-class tumor targeting therapeutic has potential to provide efficacy in therapy-resistant pancreatic tumors via targeted drug delivery as well as modulation of the immuno-microenvironment like its PS targeting predecessors.
Citation Format: Natalie Zara Phinney, Xianming Huang, Mary Lynn Trawick, Kevin Pinney, Rolf Brekken. Aberrantly exposed phosphatidylserine as a drug delivery target in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1272.