Sialic acid conjugate-modified liposomal platform modulates immunosuppressive tumor microenvironment in multiple ways for improved immune checkpoint blockade therapy

Immune checkpoint blockade (ICB) treatment is promising for the clinical therapy of numerous malignancies. However, most cancer patients rarely benefit from such single-agent immunotherapies because of the complexity of both the tumor and tumor microenvironment. A tumor-specific liposomal vehicle (DOX-SAL) modified with a sialic acid-cholesterol conjugate (SA-CH) and remotely loaded with doxorubicin (DOX) is herein reported for improving chemoimmunotherapy. The intravenous administration of DOX-SAL dramatically downregulates tumor-associated macrophage (TAM)-mediated immunosuppression, inhibits immunoregulatory functions, and promotes intratumoral infiltration of CD8⁺ T cells. Compared to conventional liposomes, DOX-SAL-mediated combination therapy with a PD-1-blocking monoclonal antibody (aPD-1 mAb) almost completely eliminates B16F10 tumors and efficiently inhibits 4T1 tumors. Moreover, cancer stem cells exhibit efficient tumor-initiating, tumor-propagating, and immunosuppressive tumor microenvironment-shaping capabilities. To further improve the treatment efficacy of an immunologically "cold" tumor, metformin (MET), which selectively eradicates breast cancer tumor stem cells, is co-encapsulated with DOX into liposomes to develop DOX/MET-SAL. The combination therapy with DOX/MET-SAL and aPD-1 mAb in a 4T1 orthotopic mouse model indicates their synergetic benefit on primary tumor inhibition, metastasis suppression, and survival rate improvement. Thus, the multifunctional liposomal platform has potential value for ICB combination immunotherapy.