ICI-164384, also known as N-n-butyl-N-methyl-11-(3,17β-dihydroxyestra-1,3,5(10)-trien-7α-yl)undecanamide, is a steroidal antiestrogen and a synthetic derivative of estradiol which is closely related to fulvestrant and was never marketed.[1] [2] It is a silent antagonist of the estrogen receptor (ER) with no intrinsic estrogenic activity and hence is a pure antiestrogen, unlike selective estrogen receptor modulators (SERMs) like tamoxifen. The drug was under development by AstraZeneca for the treatment of breast cancer but was discontinued in favor of fulvestrant,[3] which is very similar to ICI-164384 but is more potent in comparison.[4]