Fig. 2

Efficacy of CDK4/6 inhibitor combined with endocrine therapy in the ZR-75-1 cell line in vitro and in vivo. (A,B) CCK-8 and colony formation assays were performed to detect the effects of various concentrations of CDK4/6 inhibitor (palbociclib) combined with endocrine therapy (100 nM, fulvestrant) on the proliferation of HR⁺/HER2-low breast cancer cells. The effect was not as significant as in HR⁺/HER2-0 breast cancer. For 200 nM CDK4/6i + 100 nM ET: HER2-low vs. HER2-0, P = 0.003. For 400 nM CDK4/6i + 100 nM ET: HER2-low vs. HER2-0, P = 0.0001. For 800 nM CDK4/6i + 100 nM ET: HER2-low vs. HER2-0, P = 0.0001. For 100 nM CDK4/6i + 100 nM ET: HER2-low vs. HER2-0, P = 0.0001. (C–E) CDK4/6 inhibitor (palbociclib, 150 mg/kg, i.g.) combined with endocrine therapy (fulvestrant, 100 mg/kg, i.m.) inhibited tumor growth in HR⁺/HER2-low breast cancer xenografted mice, but the effect was not as significant as in the HR⁺/HER2-0 subtype. For HER2-low tumor volume on day 21: normal saline vs. ET + CDK4/6i, P = 0.001. For HER2-0 tumor volume on day 21: normal saline vs. ET + CDK4/6i, P = 0.0001. For tumor inhibition rate on day 21: HER2-low vs. HER2-0, P = 0.0001. (F) ER, PR, and HER2 expression levels in tumor tissues of HR⁺/HER2-0 and HR⁺/HER2-low breast cancer xenografted mice, measured by immunohistochemistry, the arrow points to the positive area and scale bar = 100 μm. **P < 0.01; ***P < 0.001; n = 6. ET endocrine therapy, CDK4/6i CDK4/6 inhibitor, ER: estrogen receptor, PR progesterone receptor.