Determination of the optimal time for tamoxifen treatment in combination with radiotherapy.

Although radiotherapy and tamoxifen have been extensively used to treat estrogen receptor α (ERα)-positive breast cancers, it is still questionable when tamoxifen should be started to maximize clinical benefits in combination with radiotherapy. Generally, clinician's opinion and experience are major determinants in scheduling concurrent or sequential tamoxifen and radiotherapy. Thus, we attempted to determine an optimal time to start tamoxifen treatment by analyzing tamoxifen responses at different times after irradiating MCF-7 cells to cumulative doses of 10 or 20-30 Gy. MCF-7 cells were irradiated with 5 Gy a week, twice (a cumulative dose of 10 Gy) followed by a period of recovery. MTT viability assay for tamoxifen was done with MCF-7 cells harvested immediately after each 5 Gy (MCF-7-5 Gy) or 10 Gy (MCF-7-10 Gy) irradiation or after subsequent culture of surviving MCF-7-10 Gy cells for 40 days (MCF-7-R1). To establish the radioresistant cells, the above cycles of irradiation were repeated for a cumulative dose of 20 Gy (MCF-7-R2) or 30 Gy (MCF-7-R3). In addition, cytotoxic effects of tamoxifen were also measured. Attenuated tamoxifen response was observed in MCF-7-5 Gy and 10 Gy cells, whereas the efficacy of tamoxifen was restored in MCF-7-R1 cells. Furthermore, these responses to tamoxifen correlated with ERα expression. However, the radioresistant MCF-7 cells (MCF-7-R2/R3) exhibited resistance to tamoxifen without change in ER expression, but the phosphorylation of AKT was increased. Taken together, our data suggest that sequential tamoxifen treatment following radiotherapy is more effective than concurrent treatment. Furthermore, the reduced efficacy of tamoxifen on radioresistant cells indicates that an additional targeted therapy, such as AKT inhibitor treatment, is required to improve tamoxifen response in radioresistant breast cancer.