Treatment Algorithms for Hormone Receptor-Positive Advanced Breast Cancer: Applying the Results from Recent Clinical Trials into Daily Practice—Insights, Limitations, and Moving Forward

Hormone receptor–positive (HR+) breast cancer is the most prevalent subtype of breast cancer in both early- and advanced-stage disease. Thus, the treatment of HR+ breast cancer has had the greatest global influence in improving clinical outcomes overall. Although the first-line metastatic breast cancer (MBC) trials comparing a third-generation aromatase inhibitor (AI) to tamoxifen have favored the AI, one of the challenges in translating these findings into clinical practice stems from the influence of prior adjuvant endocrine therapy, particularly the increasing use of adjuvant AIs today, on the choice of endocrine agent in the advanced setting because of the development of acquired resistance. Because the majority of patients enrolled into these studies were either endocrine-treatment naïve or exposed to tamoxifen only, the “real-life” applicability of the evidence is unclear. Because a superior dose of the selective estrogen receptor (ER) downregulator fulvestrant has now been established, its role as first-line therapy is being re-established. We are now starting to see the promise realized with blocking cross-talking growth factor pathways in addition to the ER pathway. The greatest efficacy is seen with the mammalian target of rapamycin (mTOR) inhibitor everolimus in combination with exemestane and, perhaps to a lesser extent, anti-HER2–directed therapy in combination with an AI. Future gains will likely involve a greater understanding of the redundancy and compensation induced by blocking these pathways, trials involving blocking multiple pathways in addition to hormonal agents, and the molecular interrogation of the individual's tumor in search of predictive biomarkers and “actionable” genomic aberrations.