CDK4-6 Inhibitors: Evaluation of Efficacy in Cases of Hormone Receptor-Positive HER2-Negative Breast Cancer with Only Bone Metastasis

Objective: Bone is the most common site of metastasis in patients with hormone receptor (HR)-positive breast cancer.However, 17-37% of these patients with metastatic disease develop metastasis only in the bone.In this context, the present study aimed to compare the CDK4-6 inhibitors palbociclib and ribociclib in terms of their efficacy in treating HR-positive human epidermal growth factor receptor 2 (HER-2)-negative breast cancer patients with only bone metastases detected at diagnosis. Material and Methods:The study was conducted as a retrospective observational study of 31 patients with HR-positive and HER2-negative breast cancer and only bone metastases who were treated with CDK4-6 inhibitors.The patients were divided into two groups based on the CDK4-6 inhibitor used and subjected to overall survival (OS) analysis. Results:The median age of the patients included in the present study was 57 years (36-76).The median OS in the ribociclib group was 25.46 months [confidence interval (CI) was not reached in the Kaplan-Meier analysis].The median OS in the palbociclib group was 16.07 months (95% CI: 7.88-24.25).The difference in OS between the two groups was statistically significant (p=0.043).Among the other variables with the potential of affecting the OS of these patients, the N stage and survival values were observed to be significantly different (p=0.033) between the two groups.The multivariate analysis revealed the N stage (p=0.011)and the type of CDK4-6 inhibitor used (p=0.023)as the independent risk factors that affected the OS of these patients. Conclusion:In patients with hormone-positive HER2-negative breast cancer with only bone metastasis, ribociclib administration achieved increased OS compared to the use of palbociclib.