Targeting CDK4/6 in Combination with Phage-Based Anti-HER2 Vaccination Enhances the Anticancer Response by Overcoming Immune Evasion in Breast Cancer

Background/Objectives: Cancer vaccines represent the next frontier in immunotherapy, aiming to elicit long-lasting protective anti-tumor protective immune responses. Human epidermal growth factor receptor 2 (HER2) is a well-established therapeutic target in breast cancer. Active immunization with HER2-displaying M13 bacteriophages can induce a therapeutic immune response against HER2-positive breast cancer, offering a promising alternative to trastuzumab. However, the duration of anticancer immune protection triggered by anti-HER2 phage-based vaccines is limited by tumor-immune suppressive mechanisms. Methods: In this study, two vaccination cycles with ECTM phages displaying the extracellular (EC) and transmembrane (TM) domains of human HER2 were combined with palbociclib, a CDK4/6 inhibitor, to enhance antitumor im-munity in the clinically relevant Δ16HER2 transgenic preclinical model of breast cancer. Results: The proposed combination treatment resulted in a better and long-lasting control of tumor growth rate and multiplicity than either palbociclib or phage vaccination alone, correlating with a significantly stronger anti-HER2 humoral response (IgG2a isotype). Analysis of the tumor immune infiltrate revealed an increased presence of CD8⁺ T cells concomitant with a reduction in FoxP3⁺ regulatory T cells (Tregs) in tumors explanted from mice receiving the combination therapy. Conclusions: These preclinical results provide a rationale for the clinical translation of CDK4/6 inhibitors combined with anti-HER2 active immunotherapies in breast cancer, as they may yield sustained antitumor responses by reverting the immunosuppressive tumor environment.