Pure antiestrogens and breast cancer.

Tamoxifen, which is the most commonly used drug for treatment of breast cancer, has both estrogen agonist and antagonist actions. Pure antiestrogens are devoid of any estrogen agonist effects. ICI 182,780 (fulvestrant) (Faslodex) and ICI 164,384 are competitive inhibitors of estrogen by binding to the estrogen receptor (ER). Preclinical and clinical studies show that fulvestrant and ICI 164,384 are more potent than tamoxifen in inhibiting the growth of breast cancer cells. They are devoid of any estrogen-agonist action on the uterus and vagina but lack the beneficial effects of tamoxifen on the bone and serum lipid profile. Fulvestrant is the first pure antiestrogen to complete phase III clinical trials. Such studies have shown that fulvestrant is at least as good as anastrozole in the treatment of post-menopausal women with advanced breast cancer who had relapsed or progressed on prior endocrine therapy. The drug was well tolerated and only minor side-effects were reported. Its potential role in the adjuvant setting will be determined by its adverse effects on bone mass and serum lipids. EM-800 and EM-652 are the most potent pure antiestrogens and EM-652 has the highest affinity of all antiestrogens to ER. They have no stimulatory effects on the uterus or vagina. It seems reasonable to expect that pure antiestrogens will be good alternatives to tamoxifen and aromatase inhibitors in the treatment of breast cancer.