Enhanced Anti-Tumor Efficacy through a Combination of Integrin αvβ6-Targeted Photodynamic Therapy and Immune Checkpoint Inhibition

"Training" the host immune system to recognize and systemically eliminate residual tumor lesions and micrometastases is a promising strategy for cancer therapy. In this study, we investigated whether integrin αvβ6-targeted photodynamic therapy (PDT) of tumors using a phthalocyanine dye-labeled probe (termed DSAB-HK) could trigger the host immune response, and whether PDT in combination with anti-PD-1 immune checkpoint inhibition could be used for the effective therapy of primary tumors and metastases. By near-infrared fluorescence imaging, DSAB-HK was demonstrated to specifically target either subcutaneous tumors in a 4T1 mouse breast cancer model or firefly luciferase stably transfected 4T1 (4T1-fLuc) lung metastatic tumors. Upon light irradiation, PDT by DSAB-HK significantly inhibited the growth of subcutaneous 4T1 tumors, and in addition promoted the maturation of dendritic cells and their production of cytokines, which subsequently stimulated the tumor recruitment of CD8(+) cytotoxic T lymphocytes. Furthermore, DSAB-HK PDT of the first tumor followed by PD-1 blockade markedly suppressed the growth of a second subcutaneous tumor, and also slowed the growth of 4T1-fLuc lung metastasis as demonstrated by serial bioluminescence imaging. Together, our results demonstrated the synergistic effect of tumor-targeted PDT and immune checkpoint inhibition for improving anti-tumor immunity and suppressing tumor growth/metastasis.