THERAPEUTIC RESPONSE AND TOXICITY OF CYCLIN- DEPENDENT KINASE 4/6 INHIBITORS IN THE TREATMENT OF HORMONE RECEPTOR-POSITIVE, HUMAN EPIDERMAL GROWTH FACTOR RECEPTOR-2 NEGATIVE METASTATIC BREAST CANCER – A SINGLE-CENTER EXPERIENCE

Background: The aim of the study was to determine the therapeutic response and toxicity of CDK 4/6 inhibitor therapy in patients with hormone-positive HER2-negative metastatic breast cancer. Additional aims were to determine whether there is a difference in survival and response to therapy in relation to age, ECOG status, other visceral diseases, the length of time waiting for drug approval, side effects, and consequently, treatment interruption and dose reduction. Methods: The study analyzed all available medical documentation for 65 patients who were treated with CDK 4/6 inhibitors for hormone receptor-positive HER2-negative metastatic breast cancer at the Oncology Clinic of the University Clinical Hospital Mostar during the period from January 1, 2020, to December 31, 2023. Data for analysis were obtained by processing the medical documentation. Additionally, data were obtained from the Federal Fund of the Health Insurance and Reinsurance Institute of the Federation of Bosnia and Herzegovina. A retrospective cohort study was conducted based on the obtained data. Principal conclusion: There was no statistically significant difference in response to therapy regarding PFS between patients who received CDK 4/6 inhibitors in four different therapeutic lines. However, a statistically significant difference in OS was found, i.e., patients who received this therapy in the first and second lines of treatment had longer survival compared to those who received therapy in later lines, partially confirming the hypothesized hypothesis. A statistically significant majority of patients on CDK 4/6 inhibitor therapy did not experience side effects, thus confirming the hypothesis. Keywords: Breast Neoplasms, Neoplasm Metastasis, Cyclin-Dependent Kinase Inhibitor