Reduced occurrence of mdr-1 amplification in stage-IV breast-cancer patients treated with tamoxifen compared with other endocrine treatments.

Amplification of mdr-1 occurs in breast cancer patients receiving endocrine treatment. We have compared a group of patients receiving tamoxifen and a group receiving second-line endocrine treatment after failure of tamoxifen treatment. Chronological tumor samples were obtained by fine-needle biopsies from the same tumor lesion of each patient and PCR was used to detect the appearance of gene amplification. Three of 20 patients receiving tamoxifen developed amplified mdr-1 whereas 7 of 17 patients receiving second-line endocrine treatment developed mdr-1 amplification.