Everolimus for Treating Hormone Receptor-positive Metastatic Breast Cancer Previously Treated With Cyclin-dependent Kinase 4/6 Inhibitors

Background/Aim: Cyclin-dependent kinase (CDK) 4/6 inhibitors have become the standard of care as the first -and second-line treatments for hormone receptor-positive (HR+) or human epidermal growth factor receptor 2 (HER2)-negative metastatic and recurrent breast cancers. Although CDK 4/6 inhibitors can markedly prolong progression-free survival, there is no clear evidence of their post-treatment effects. The option of developing mammalian target of rapamycin (mTOR) inhibitors, such as everolimus, has been discussed as a post-treatment option for such patients. This study aimed at determining everolimus' efficacy as a post-treatment option following CDK4/6 inhibitor administration in clinical settings. Patients and Methods: Thirteen patients who received everolimus as a post-treatment after CDK4/6 inhibitor therapy from December 2017 to July 2021 were retrospectively reviewed. The primary endpoint was progression-free survival; the secondary endpoints were overall response rate, clinical benefit rate, and overall survival. Results: The median patient age, progression-free survival, and overall survival were 59 years (range=44-76 years), 9.1 months (95% confidence interval=2.3 to not reached), and 37.4 months (95% confidence interval=12.3-37.4), respectively. The overall response rate and clinical benefit rate were 15.3% (2/13) and 46.2% (6/13),