PROLONGED TAMOXIFEN EXPOSURE SELECTS A BREAST-CANCER CELL CLONE THAT IS STABLE IN-VITRO AND IN-VIVO

The effects of long-term tamoxifen exposure on cell growth and cell cy cle kinetics were compared between oestrogen receptor (ER)-positive (M CF-7) and ER-negative (MDA-MB-231) cell lines. In the MCF-7 cell line, prolonged tamoxifen exposure (0.5 mumol/l for > 100 days) blocked cel ls in GO-G1 of the cell cycle, and slowed the doubling time of cells f rom 30 to 59 h. These effects corresponded to an increase in the cellu lar accumulation of tamoxifen over time [mean area under concentration curve (AUC) = 77.92 mumoles/10(6)/cells/day]. In contrast, in the MDA -MB-231 cell fine, long-term tamoxifen exposure had no obvious effect on the doubling time, and reduced cellular tamoxifen accumulation (mea n AUC = 50.50 mumoles/10(6)/cells/day) compared to the MCF-7 cells. Fl ow cytometric analysis of MDA-MB-231 cells demonstrated that a new tet raploid clone emerged following 56 days of tamoxifen exposure. Inocula tion of the MDA-MB-231 tetraploid clone and MDA-MB-231 wildtype cells into the opposite flanks of athymic nude mice resulted in the rapid gr owth of tetraploid tumours. The tetraploid tumours maintained their pl oidy following tamoxifen treatment for nine consecutive serial transpl antations. Histological examination of the fifth transplant generation xenografts revealed that the tetraploid tumour had a 25-30 times grea ter mass, area of haemorrhage and necrosis, a slightly higher mitotic index and was more anaplastic than the control neoplasm. The control w ildtype MDA-MB-231 tumours maintained a stable ploidy following tamoxi fen treatment until the eighth and ninth transplantation, when a tetra ploid population appeared, suggesting that tamoxifen treatment may sel ect for this clone in vivo. These studies suggest that prolonged tamox ifen exposure may select for new, stable, fast growing cell clones in vitro as well as in vivo.