Development of betabodies: The next generation of phosphatidylserine targeting agents.

Externalized phosphatidylserine (PS) is a phospholipid and a selective marker of the tumor microenvironment (TME). It is 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 in part to suppress immune responses by binding to PS receptors expressed on tumor-infiltrating myeloid cells. PS has been targeted with antibodies, such as bavituximab, that bind the phospholipid via a cofactor, ß2-glycoprotein 1 (ß2GP1); these antibodies showed excellent specificity for tumor vasculature and induce an immune stimulatory environment. We have advanced this concept by developing the next generation of PS targeting agent, a fusion protein (betabody) constructed by linking PS-binding domain V of ß2GP1 to the Fc of an IgG2a. Betabodies bind to externalized PS with high affinity (∼1 nM), without the requirement of a co-factor and localize robustly to the TME. We demonstrate that betabodies are a direct PS-targeting agent that has the potential to be used as anti-tumor therapy, drug delivery vehicles, and tools for imaging the TME.

AXL Is a Key Factor for Cell Plasticity and Promotes Metastasis in Pancreatic Cancer.

Pancreatic ductal adenocarcinoma (PDA), a leading cause of cancer-related death in the United States, has a high metastatic rate, and is associated with persistent immune suppression. AXL, a member of the TAM (TYRO3, AXL, MERTK) receptor tyrosine kinase family, is a driver of metastasis and immune suppression in multiple cancer types. Here we use single-cell RNA-sequencing to reveal that AXL is expressed highly in tumor cells that have a mesenchymal-like phenotype and that AXL expression correlates with classic markers of epithelial-to-mesenchymal transition. We demonstrate that AXL deficiency extends survival, reduces primary and metastatic burden, and enhances sensitivity to gemcitabine in an autochthonous model of PDA. PDA in AXL-deficient mice displayed a more differentiated histology, higher nucleoside transporter expression, and a more active immune microenvironment compared with PDA in wild-type mice. Finally, we demonstrate that AXL-positive poorly differentiated tumor cells are critical for PDA progression and metastasis, emphasizing the potential of AXL as a therapeutic target in PDA. IMPLICATIONS: These studies implicate AXL as a marker of undifferentiated PDA cells and a target for therapy.