Liposomal <scp> ¹⁸⁸ Rhenium </scp> Plus Macrophage Depletion Enhances Anti‐ <scp>PD</scp> ‐ <scp>L1</scp> Efficacy and B Cell Infiltration Against Lung Metastatic Cancer

Radionuclides such as Rhenium-188 (Re188) hold promise for treating metastatic cancers due to their cytotoxic effects and potential to stimulate systemic anti-tumor immunity. However, mononuclear phagocyte system-mediated clearance of liposome encapsulated Re188 (Lipo-Re188) limits its tumor delivery. This study aimed to enhance the therapeutic effect of Lipo-Re188 against lung metastases through macrophage depletion and immune checkpoint blockade. A lung metastatic colon cancer model was established via intravenous injection of CT26-luciferase cells and then treated with Lipo-Re188 (11.1 MBq, 30% of MTD), liposomal clodronate (Lipo-clod) for macrophage depletion, and/or anti-PD-L1 antibody. Tumor progression was monitored by bioluminescence imaging, and radionuclide biodistribution was assessed at 1, 24, and 48 h post-injection. Flow cytometry was used to assess immune cell populations in the spleen and tumor microenvironment (TME). Cytokine levels were measured using a bead-based multiplex assay and analyzed by flow cytometry. Macrophage depletion significantly enhanced tumor accumulation of Lipo-Re188 while reducing hepatic uptake and prolonging survival. The combination of Lipo-clod and Lipo-Re188 promoted B cells, restored functional T cells, and suppressed MDSC in both spleen and TME. Notably, IL-1α and GM-CSF levels were significantly elevated in the combination group. Triple therapy with Lipo-clod, Lipo-Re188, and anti-PD-L1 provided the greatest survival benefit, highest intratumoral B cell accumulation, and lowest interstitial macrophage levels, with no significant biological toxicity. Our study reveals that triple therapy overcomes immunosuppressive feedback and promotes a tumor-suppressive microenvironment. These findings support a rational combination strategy integrating radiopharmaceutical therapy with immune modulation for metastatic cancer treatment.