The role of hormonal therapy in the management of hormonal-receptor-positive breast cancer with co-expression of HER2

Approximately half of breast cancers that overexpress human epidermal growth factor receptor 2 (HER2) also express hormone receptors (HRs). Although HR positivity predicts efficacy of endocrine agents, preclinical and clinical data suggest that HER2 overexpression confers intrinsic resistance to hormonal treatment. In addition, HER2 overexpression is an independent adverse prognostic factor regardless of the hormonal status of the tumor, indicating that patients with HR+/HER2+ breast tumors might not derive a benefit from single-agent hormonal therapy. These data provided a strong rationale for exploring the targeting of both HR and HER2 signaling pathways in HR+/HER2+ breast tumors to optimize hormonal therapy and overcome resistance to anti-estrogen therapy. Results from a randomized clinical trial that combined hormonal treatment with targeted anti-HER2 therapy in postmenopausal women with HR+/HER2+ advanced breast cancer indicate that this novel dual-targeting strategy significantly improves outcomes compared with endocrine treatment alone. Nonetheless, other data suggest that it might achieve an inferior outcome compared with anti-HER2 therapy plus chemotherapy. Therefore, targeting both the HR and HER2 signaling pathways upfront might not be the most-effective therapeutic strategy in the management of HR+/HER2+ breast cancer. We discuss the clinical implication of resistance to endocrine therapy, and describe new insights into the management of HR+/HER2+ advanced breast cancer.