Molecular mechanisms of resistance to tamoxifen therapy in breast cancer.

Clinical data suggest that the use of adjuvant tamoxifen citrate (Nolvadex) for a minimum of 5 years, and possibly indefinitely, will result in maximal antitumor benefit. There is concern that long-term tamoxifen maintenance therapy may result in the induction of drug resistance. This article reviews the potential molecular mechanisms of resistance to tamoxifen and explores the possibility of tamoxifen-stimulated tumor growth.