Abstract
In cancer therapy, chemotherapy and immunomodulatory agents are often combined to leverage their complementary mechanisms. Chemotherapeutic drugs promote the release of tumor antigens in situ, enhancing immune recognition, whereas immunostimulants recruit and activate immune cells. However, chemotherapy also induces externalization of phosphatidylserine (PS) on tumor cells, which interacts with antigen presenting cells (APCs) in the tumor microenvironment (TME) and suppresses their responses to immunostimulants. We hypothesize that chemotherapy-induced PS exposure is a key driver of immunosuppressive TME, and that blocking this effect is essential to enable effective immunostimulation. In this study, we validated that sublethal doses of doxorubicin, a representative chemotherapeutic agent, induce PS externalization on tumor cells, which in turn impairs the responsiveness of APCs to immunostimulants. To counteract chemotherapy-induced PS exposure, we employed zinc-dipicolylamine (DPA-Zn), a cost-effective and commercially available small molecule, as a PS blocking agent. DPA-Zn bound to PS and restored the responsiveness of PS-suppressed APCs to immunostimulants, such as lipopolysaccharide and cyclic dinucleotide (CDN). DPA-Zn enhanced the antitumor activity of doxorubicin and its combination with granulocyte-macrophage colony-stimulating factor (GM-CSF) or CDN in the B16F10 melanoma model. The safety and antitumor activity of this combination were further improved with liposomal formulations of doxorubicin and CDN. These findings identify PS externalization as a mechanism of chemotherapy-induced immunosuppressive TME and demonstrate that targeting PS with DPA-Zn can potentiate chemoimmunotherapy.