Abstract: Background and purpose: Radiotherapy and endocrine therapy are both important parts of adjuvant therapy for breast cancer, yet few studies have been conducted focusing on the interaction between radiation and endocrine therapy. Up to now, no conclusion has been drawn on the timing sequence of adjuvant radiation and endocrine therapy, which is indeed crucial in clinical practice. This study intended primarily to investigate the effect of concurrent or sequential exemestane combined with radiation on radiosensitivity of MCF-7 cells and its possible mechanism, and further to provide rationale for optimal clinical treatment modality. Methods: MCF-7 cells were arranged into three trial groups: the radiation group, exemestane sequenced with radiation group and exemestane followed radiation group. Radiosensitivity was evaluated by clonogenic assay, cell proliferation was measured by MTT assay, the ability to induce cell apoptosis was evaluated by DAPI staining assay, the changes of Bcl-2 and Bax were detected by Western blot. Results: Sensitive enhancement ratios (SER) were 1.51 and 1.37 in the exemestane sequenced with radiation group and exemestane followed radiation group, respectively. Compared with the radiation group, the percentage of cellular proliferation inhibition and apoptosis increased obviously in the exemestane sequenced with radiation group and exemestane followed radiation group. Exemestane combined with radiation made the Bax protein increase obviously and the Bcl-2 protein lowered significantly. Conclusion: Exemestane can enhance the radiosensitivity of MCF-7 cells, whose mechanism might be relevant to the promotion of cellular apoptosis. However, the treatment sequence does not affect the outcome.

Key words: Exemestane, MCF-7 cell, Radiosensitivity