Combined CDK4/6 Inhibition and Radiation: Effects on Cellular Senescence, Cell Cycle Regulation, and Cell Death in Mammary Carcinoma Cells

CDK4/6 inhibitors such as palbociclib, ribociclib and abemaciclib are commonly used in the clinical treatment of HR-positive, HER2-negative metastatic or locally advanced breast cancer. Patients with metastatic disease often receive palliative radiotherapy for symptom control of bone metastases and/or local lesions, typically administered in close temporal proximity to CDK4/6 inhibitor therapy, although treatment with the inhibitors may be temporarily paused during the radiotherapy period in some cases. In this study, we investigated the extent to which senescence is induced by CDK4/6 inhibitors, ionizing radiation, and the combination of the two, compared to other types of cell fate. Eight breast cancer cell lines with different molecular subtypes and two healthy cell lines (fibroblasts and keratinocytes) were treated with CDK inhibition using palbociclib, ribociclib or abemaciclib and with or without a single dose of 2 Gy ionizing radiation. Cellular senescence, cell death in form of apoptosis and necrosis, and the cell cycle were analyzed using flow cytometry. We focused mainly on understanding how CDK inhibition can trigger cellular senescence. Our data showed that in many cell lines —but not all—the use of CDK inhibitors induced senescence much more strongly than cell death. Except for one cell line, significantly more cell lines died necrotically than apoptotically. Neither apoptosis nor necrosis was responsible for a major cell fate after CDK inhibition. Combination therapy with irradiation did not show a clear additive effect. In cell lines, senescence is clearly triggered by CDK4/6 inhibitors and even more so when in combination with ionizing radiation, which, when transferred to patients, could lead to less damage caused by cell loss, such as necrotic areas. However, it could also lead to more senescence-specific side effects, such as inflammation-induced tumors and fibrosis.