Do we really know how to overcome trastuzumab resistance in hormone sensitive metastatic breast cancer?

Purpose: This article focuses on how the status of hormone receptors (HR) influences the efficacy of trastuzumab in patients with metastatic HER2-positive breast cancer treated with first-line trastuzumab in combination with taxane-based chemotherapy. Methods: A prospective study was carried out at the Clinic for Oncology, Clinical Centre in Nis, from January 2015 to until June 2018. A total of 121 patients were treated with first-line trastuzumab in combination with taxane-based chemotherapy. None of the patients from the HR-positive group received hormonotherapy after completion of chemotherapy with trastuzumab. Results: Clinical benefit rate was present in 76% of the patients, including partial response (PR) in 37%, stable disease (SD) in 38%, and complete response (CR) in almost 8% of the patients. Progressive disease (PD) occurred in almost a quarter of the patients, i.e. 24%. Progression-free survival (PFS) in the entire group of patients amounted to 9 months, whereas overall survival (OS) was 30 months. PFS in the HR-negative tumor group was significantly longer (13 months) compared to 8 months in the HR-positive tumor group (p<0.0001; HR 0.49;95% CI 0.31-0.69). Furthermore, OS was significantly longer in the HR-negative tumor group (34 months), compared to 26 months in the HR-positive tumor group (p=0.0073, HR 0.57; 95% CI 0.36-0.90). Conclusions: These data indicate a different response to anti-HER2 therapy in patients with HER2+ metastatic breast cancer (MBC) according to HR status, thus emphasizing that ER most likely represents an escape pathway for the response to anti-HER2 target therapy and vice versa. Combining hormonotherapy with anti-HER2 therapy surely represents a promising strategy which could help overcome resistance to trastuzumab and other anti-HER2 agents.