Background and Objectives: Breast cancer is the most common malignant neoplasm worldwide and the most prevalent one among women. It represents the leading cause of cancer-related death among females. Cyclin-dependent kinase 4 and 6 inhibitors disrupt the cell cycle, inducing cellular senescence and, ultimately, apoptosis. Consequently, they have become a novel type of adjuvant therapy for the treatment of advanced or metastatic breast cancer characterised by positive hormone receptors and human epidermal growth factor receptor 2 (HER-2) negative. Methods: A systematic review was conducted, analysing the available literature on cyclin-dependent kinase 4 and 6 inhibitors published over the last five years. The aim was to evaluate the efficacy and safety of adding these drugs to the standard endocrine therapy for this pathology. Results: The combination of cyclin-dependent kinase 4 and 6 inhibitors with endocrine therapy was shown to improve progression-free survival, overall survival, and chemotherapy-free intervals in patients who received this combination therapy. Conclusions: The addition of CDK4/6 inhibitors to endocrine therapy in the treatment of advanced or metastatic breast cancer with positive hormone receptors and HER-2 negative significantly improved PFS, median survival, and chemotherapy-free intervals compared with the use of hormonal treatments alone or in combination with a placebo. Currently, CDK4/6 inhibitors are becoming established as a new standard treatment for this pathology, offering lower toxicity than chemotherapy. However, it is necessary to deeply investigate the mechanisms of treatment resistance and develop effective therapies to overcome them.

Background/Objectives: Breast cancer is the most common malignant neoplasm worldwide and the most prevalent among women. It represents the leading cause of cancer-related death among females. Cyclin-dependent kinase 4 and 6 inhibitors disrupt the cell cycle, inducing cellular senescence and, ultimately, apoptosis. Consequently, they have become a novel adjuvant therapy for the treatment of advanced or metastatic breast cancer characterised by positive hormone receptors and human epidermal growth factor receptor 2 (HER-2) negative. Methods: A systematic review was conducted, analysing the available literature on cyclin-dependent kinase 4 and 6 inhibitors published over the last five years. The aim was to evaluate the efficacy and safety of adding these drugs to the standard endocrine therapy for this pathology. Results: The combination of cyclin-dependent kinase 4 and 6 inhibitors with endocrine therapy has been shown to improve progression-free survival, overall survival, and chemotherapy-free intervals in patients who received this combination therapy. Conclusions: The addition of CDK4/6 inhibitors to endocrine therapy in the treatment of advanced or metastatic breast cancer with positive hormone receptors and HER-2 negative has significantly improved PFS, median survival, and chemotherapy-free intervals compared to the use of hormonal treatments alone or in combination with placebo. Currently, CDK4/6 inhibitors are becoming established as a new standard treatment for this pathology, offering lower toxicity than chemotherapy. However, it is necessary to deeply investigate the mechanisms of treatment resistance and develop effective therapies to overcome them.