CDK4/6 inhibitors synergize with radiotherapy to prime the tumor microenvironment and enhance the antitumor effect of anti-PD-L1 immunotherapy in triple-negative breast cancer

  • J Biomed Sci. 2025 Aug 20;32(1):79. doi: 10.1186/s12929-025-01173-3.
Wen-Chi Yang  1  2  3 ,  Ming-Feng Wei  1 ,  Ying-Chun Shen  2  4 ,  Chiun-Sheng Huang  5 ,  Sung-Hsin Kuo  6  7  8  9
Affiliations
  • 1. Division of Radiation Oncology, Department of Oncology, National Taiwan University Hospital, No. 7, Chung-Shan South Road, Taipei, 100, Taiwan.
  • 2. Graduate Institute of Oncology, National Taiwan University College of Medicine, Taipei, Taiwan.
  • 3. Department of Radiation Oncology, National Taiwan University Cancer Center, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, Taiwan.
  • 4. Department of Medical Oncology, National Taiwan University Cancer Center, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, Taiwan.
  • 5. Department of Surgery, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, Taiwan.
  • 6. Division of Radiation Oncology, Department of Oncology, National Taiwan University Hospital, No. 7, Chung-Shan South Road, Taipei, 100, Taiwan. [email protected].
  • 7. Graduate Institute of Oncology, National Taiwan University College of Medicine, Taipei, Taiwan. [email protected].
  • 8. Cancer Research Center, National Taiwan University College of Medicine, Taipei, Taiwan. [email protected].
  • 9. NTU Radiation Application and Hardness Technology Research Center, National Taiwan University, Taipei, Taiwan. [email protected].
PMID: 40836337 DOI: 10.1186/s12929-025-01173-3
Abstract

Background: Triple-negative breast Cancer (TNBC) has the highest mortality rate among all breast Cancer subtypes. Although immunotherapy shows promise, its efficacy varies. CDK4/6 inhibitors can radiosensitize and modulate the immune system, and high-dose radiotherapy (RT) can enhance the effects of immunotherapy. This study explored the combination of RT with CDK4/6 inhibitors to improve TNBC immunotherapy by modulating the tumor microenvironment.

Methods: We assessed the radiosensitizing effects of abemaciclib (a CDK4/6 inhibitor) using clonogenic assays in three human TNBC cell lines (MDA-MB-231, MDA-MB-453, and MDA-MB-468) and two murine TNBC cell lines (4T1 and EMT6). The antitumor efficacy of the treatments (control, RT, abemaciclib, anti-PD-L1 antibody [aPD-L1], abemaciclib combined with aPD-L1, abemaciclib combined with RT, aPD-L1 combined with RT, and the triple combination of abemaciclib with aPD-L1 and RT) was evaluated in both 4T1 and EMT6 cell line-derived immunocompetent mouse models. Interferon-γ (IFN-γ) levels in mouse blood were monitored to gauge the immune response. Tumor-infiltrating lymphocytes (TILs) were analyzed using flow cytometry and immunohistochemical staining.

Results: Clonogenic assays showed synergistic effects of RT and abemaciclib in all TNBC cell lines. RT increased PD-L1 expression, whereas abemaciclib did not alter PD-L1 expression. In the 4T1 and EMT6 mouse models, the triple combination treatment markedly inhibited tumor growth (P < 0.01). In the 4T1 mouse model, the triple combination group exhibited significantly greater circulating IFN-γ levels (P < 0.001) than the Other groups. TIL analysis revealed a significant increase in CD4 + and CD8 + T cells and tumor-associated macrophages (P < 0.01) in the triple combination therapy group. Immunohistochemical staining confirmed increased infiltration of CD4 + T cells, CD8 + T cells, monocyte chemoattractant protein-1, CD80-, and iNOS- positive macrophages into the tumor microenvironment of this group, with a marked reduction in CD206-positive macrophages.

Conclusion: Combining CDK4/6 inhibitors with RT enhanced the antitumor effects of aPD-L1 immunotherapy against TNBC. This effect was correlated with increased IFN-γ secretion and recruitment of CD4 + and CD8 + T cells and M1 tumor-associated macrophages, leading to modulation of the tumor microenvironment.

Keywords
CDK4/6 inhibitor; Immunotherapy; Radiotherapy.
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