Predictive role of HER2-status on the effectiveness of endocrine adjuvant treatment in postmenopausal breast cancer patients: a population-based cohort study

PURPOSE: There are conflicting results on the potential role of HER2-status on the efficacy of aromatase inhibitors (AIs) and tamoxifen (TAM) in patients with hormone receptor (HR)-positive breast cancer (BC). The purpose of this population-based cohort study was to investigate the potential benefit of AIs compared to TAM as adjuvant therapy in postmenopausal BC patients by HER2-status in the era of modern therapy with HER2-blockade. METHODS: A population-based cohort study was performed including all postmenopausal women diagnosed with HR-positive BC without distant metastasis between 2007 and 2012 in three healthcare regions in Sweden. We analyzed the breast cancer-specific survival (BCSS) and overall survival (OS) in two distinct cohorts (HER2-negative, HER2-positive) based on the type of endocrine therapy (ET) used. A propensity score matching was performed separately in the HER2-negative and HER2-positive cohorts, respectively. RESULTS: After propensity score matching, 4368 patients with HER2-negative and 214 patients with HER2-positive BC were available for analysis. In the HER2-negative cohort, an improved BCSS [Hazard Ratio (HR): 0.51; 95% confidence interval (CI): 0.34–0.77, p value < 0.001] and a trend toward improved OS (HR: 0.66; 95% CI: 0.41–1.08, p value = 0.093) in favor of AI-based therapy was observed. In the HER2-positive cohort, no statistically significant difference between AI-based ET and TAM was found in terms of either BCSS or OS, although the direction of HR was similar as in the HER2-negative cohort (HR for BCSS: 0.84; 95% CI: 0.14–5.04, p = 0.849; HR for OS: 0.62; 95% CI: 0.10–3.38, p = 0.345). CONCLUSION: Our study results, based on propensity-matched cohorts, did not support any predictive value of HER2-status on endocrine therapy in postmenopausal BC patients. AI-based ET remains the treatment of choice for postmenopausal BC patients with HR-positive disease in the modern era of HER2-directed therapy irrespective of HER2-status.