Steroidal pure antioestrogens.

The effects of some novel 7 alpha-alkyl amide analogues of oestradiol on the rat and mouse uterus have been measured. The compound ICI 164,384 [N-n-butyl-N-methyl-11-(3,17 beta-dihydroxyoestra-1,3,5(10)-trien-7 alpha-yl) undecamide] was entirely devoid of oestrogenic activity in the rat and mouse uterus but completely blocked the uterine stimulatory effects of oestradiol and of tamoxifen. Biological activity was confined to 7 alpha-isomers. The affinity of ICI 164,384 for the rat uterus oestrogen receptor (0.19) was substantially greater than that of tamoxifen (0.025 c.f. oestradiol = 1). The compound inhibited oestradiol-induced growth of ZR-71-1 cells in a dose-dependent manner in vitro. ICI 164,384 thus has the characteristics of a pure antagonist of oestrogen action.