Association of ERα-36 expression with the de novo resistance of tamoxifen in ER-positive breast cancer.

Background Approximately 70-80% of breast cancer express ER-alpha and hormonal therapies, given significant improvements in patient survival. About 50% of ER-positive breast cancer patients with advanced disease insensitive to tamoxifen treatment when diagnosed. Recent studies have shown that ERα-36 is a crucial factor in the resistance of tamoxifen. Objective This study aims to determine the association between ERα-36 expression and de novo resistance of tamoxifen in patients with ER-positive breast cancer. Methods This study was an observational study using a cross-sectional method and was conducted at Wahidin Sudirohusodo Hospital and Unhas Hospital. ERα-36 protein expression was assessed using an immunohistochemistry assay. The association of ERα-36 expression and resistance of tamoxifen was tested with the Chi-square test. Results A total of 50 locally advanced breast cancer cases were included in this study, 22 cases (44%) had overexpression of ERα-36, and 28 cases (56%) had not, 24 cases (48%) had experience resistance to tamoxifen and 26 cases (52%) had not. There was a significant association between ERα-36 expressions and resistance of tamoxifen (p = 0.000). Conclusions There was an association between the expression of ER-α36 with de novo resistance of tamoxifen in ER-positive breast cancer. ER-α36 could act as a worth considering biomarker for de novo resistance of tamoxifen in therapeutic strategies.