The history and mechanism of action of fulvestrant.

Fulvestrant is a pure antiestrogen that emerged from a systematic medicinal chemistry strategy of modification of long-chain alkyl substitutes in the 7a-position of estradiol. Fulvestrant has no uterotrophic effects on the immature or ovariectomized rat and blocks the agonistic effects of estradiol and tamoxifen in a dose-dependent manner. In in vivo and in vitro breast cancer models, fulvestrant has anticancer activity at least as good as tamoxifen and is superior to tamoxifen in some models. Fulvestrant requires intramuscular administration in a proprietary formulation of castor oil and alcohols. When fulvestrant binds to estrogen receptor monomers it inhibits receptor dimerization, activating function 1 (AF1) and AF2 are rendered inactive, translocation of receptor to the nucleus is reduced, and degradation of the estrogen receptor is accelerated. This results in pure antiestrogenic effects. There is substantial preclinical evidence that the nonsteroidal hormone-dependent mechanisms of estrogen receptor activation and regulation via growth factor receptors and their signal transduction pathways are important in the development of breast cancer hormonal resistance. Methods of exploiting the interactions between these nonsteroidal hormone-dependent mechanisms of resistance and hormonal agents such as fulvestrant are an active area for drug development and clinical investigation.