Antiestrogens: future prospects.

Tamoxifen is currently the endocrine therapy of choice for early and advanced breast cancer. Attempts to improve the therapeutic efficacy have included altering the triphenylethylene ring structure of tamoxifen, forming new nonsteroidal ring structures or creating steroidal estradiol analogs with greater antiestrogenic activity. There are now six nonsteroidal compounds either on the market or in clinical development and one steroidal "pure" antiestrogen has entered clinical trials. A number of these agents show improved estrogen-receptor binding affinity, antiestrogenic activity, and/or antitumor activity compared with tamoxifen. Preclinical and clinical data on these compounds are discussed and compared with tamoxifen when possible.