[Mechanism of acquired antiestrogen resistance and its management in breast cancer].

Although non-steroidal antiestrogen, tamoxifen(TAM), has been known to be useful for estrogen receptor(ER)-positive breast cancer, acquired tamoxifen resistance frequently occurs during the treatment. Several factors seem to be responsible for the resistance; decreased level of ER, mutation of ER, agonistic effect of TAM, ovarian steroidogenesis, altered metabolism of TAM, and increased expression level of antiestrogen binding sites. Withdrawal of TAM and second-line endocrine therapy might be effective on the tamoxifen resistant breast cancer which retains estrogen dependency. Otherwise, chemotherapy should be chosen for estrogen-independent breast cancer which has lost ER and fails to respond to other endocrine therapy. In this context, the menstrual status, serum level of estradiol, and steroid receptor status should be considered for the management of acquired antiestrogen resistance in breast cancer.