Anti-oestrogen resistant human breast cancer cell lines are more sensitive towards treatment with the vitamin D analogue EB1089 than parent MCF-7 cells.

Most breast cancer patients treated with anti-oestrogens will eventually develop resistance towards treatment. Therefore it is important to find new therapeutic agents effective for treatment of patients relapsing on anti-oestrogen. The vitamin D analogue EB1089 (Seocalcitol(TM)) is a promising new agent for treatment of breast cancer patients with advanced disease, and in this study we show that two different anti-oestrogen-resistant human breast cancer cell lines are more sensitive towards treatment with EB1089, than the parent MCF-7 cell line. The two resistant cell lines both express a lower content of the anti-apoptotic protein Bcl-2, and we suggest that this may explain the higher sensitivity towards EB1089. The importance of Bcl-2 for response to EB1089 is supported by our observation that oestradiol abrogates the effect of EB1089 in cell lines which increase Bcl-2 in response to oestradiol treatment. Overall these results indicate that treatment with Seocalcitol(TM)may prove effective when patients become refractory to anti-oestrogen therapy, and that Bcl-2 may be used as a predictive marker.