Everolimus-based combination therapies for HR+, HER2- metastatic breast cancer

Metastatic breast cancer (MBC) is the leading cause of cancer-related morbidity and mortality among women worldwide. Endocrine therapy is the standard of care for the most common subtype of MBC, hormone-receptor positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) disease. Advances in treating this type of MBC have focused on improving the efficacy of endocrine therapy by adding agents that target specific molecular pathways of breast cancer cell growth and survival. The combination of the aromatase inhibitor exemestane and the mammalian target of rapamycin inhibitor, everolimus, more than doubled median progression-free survival compared with exemestane alone (7.8 vs 3.2 months, respectively; hazard ratio 0.45 [95% confidence interval 0.38-0.54]; log rank P < 0.0001) in the BOLERO-2 study in postmenopausal women with HR+, HER2- locally advanced or metastatic breast cancer that had recurred or progressed on prior non-steroidal aromatase inhibitor therapy. In addition, everolimus plus exemestane was associated with a manageable safety profile. The results of BOLERO-2 led to regulatory approval of everolimus plus exemestane. Additional everolimus-based combinations have been or are under investigation in the HR+, HER2- MBC setting, including combinations with letrozole, fulvestrant, ribociclib, tamoxifen, and chemotherapy. This review summarizes key data on everolimus-based combinations focusing on efficacy, safety, biomarkers, quality of life, and health economic outcomes. These data are discussed in the context of the changing MBC treatment algorithm to provide insights into the clinical relevance of everolimus-based combinations.