Low-Dose Metronomic Gemcitabine Pretreatments Overcome the Resistance of Breast Cancer to Immune Checkpoint Therapy

Aims: Immunotherapy has revolutionized cancer management. However, response to immunotherapy is heterogeneous. Thus, strategies to improve antitumor immune responses in resistant tumors, such as breast cancer, are urgently needed. Methods: Established murine tumors were treated with anti-CTLA4 or anti-PD-1 alone or combined with metronomic gemcitabine (met-GEM). Tumor vascular function, immune cell tumor infiltration and gene transcription were determined. Results: Low-dose met-GEM (2 mg/kg) treatments improved tumor vessel perfusion and increased tumor-infiltrating T cells. Notably, low-dose met-GEM pretreatments converted resistant tumors to respond to immunotherapy. Moreover, combined therapy reduced tumor vessel density, improved tumor vessel perfusion, increased T-cell tumor infiltration and upregulated the expression of some anticancer genes. Conclusion: Low-dose met-GEM pretreatment reconditioned the tumor immune microenvironment and improved immunotherapy efficacy in murine breast cancer.