GPER-1 expression is associated with a decreased response rate to primary tamoxifen therapy of breast cancer patients.

Abstract

Endocrine therapies using tamoxifen and/or aromatase inhibitors are important therapeutic options for the targeted treatment of hormone-responsive breast cancer. In addition to nuclear estrogen receptors ERα and β, G-protein-coupled estrogen receptor GPER-1 is a third receptor-mediating estrogen effects in breast cancer cells. The aim of this study was to examine to what extent GPER-1 expression might affect the efficacy of primary endocrine treatment of breast cancer. GPER-1 expression was determined in tissue samples from patients with early breast cancer by means of immunohistochemistry and a GPER-1 score of ≥ 3 was considered to be positive. In a total of 165 patients, the response to a primary therapy with tamoxifen (TAM) or aromatase inhibitors (AI) was assessed by ultrasound imaging for up to 6months. The primary endpoint of this study was the response to treatment evaluated by RECIST 1.1 criteria. GPER-1 expression was observed in 127 (77.0%) out of 165 cases. Based on GPER-1 expression and the type of endocrine treatment, the patients were divided into 4 groups: GPER-1 negative/TAM (12.1%), GPER-1 negative/AI (10.9%), GPER-1 positive/TAM (44.8%), and GPER-1 positive/AI (32.1%). The groups were well balanced regarding different clinical and pathological factors. After 4 and 6months of treatment, a high level of stable disease or progressive disease was observed in the GPER-1 positive/TAM group only (p < 0.0001), whereas in the other three groups of patients, the most common objective response was classified as partial response. We observed a continuous reduction of mean tumor size in patients treated with aromatase inhibitors irrespective of the GPER-1 status and in GPER-1 negative patients treated with TAM. In contrast, in GPER-1 positive patients treated with TAM, a reduction of mean tumor size was observed only in the first 2months after beginning of treatment. Four and six months after start of treatment, no reduction, but even a slight increase of tumor size was observed in this patients group. GPER-1 expression is significantly associated with a reduced effect of primary treatment with tamoxifen in breast cancer patients.