Comparison of the biological effects of tamoxifen and a new antioestrogen (LY 117018) on the immature rat uterus.

The uterotrophic and antiuterotrophic activities of tamoxifen and 6-hydroxy-2-(p-hydroxyphenyl)-benzo(b)thien-3-yl p- less than 2-(1-pyrrolidinyl) ethoxyphenyl ketone (LY 117018) in the immature rat uterus have been evaluated. The antioestrogens were administered alone, concurrently or sequentially with or without oestradiol. LY 117018 administered alone was less uterotrophic (oestrogenic) than tamoxifen. At high doses, when administered concurrently with oestradiol, LY 117018 was more antiuterotrophic (antioestrogenic) than tamoxifen. When uterine growth was maximally stimulated by prior treatment with oestradiol, tamoxifen and LY 117018 were equally effective in reducing uterine weight. However, when uterine growth was induced with a dose of oestradiol producing an oestrogenic effect equivalent to that of tamoxifen (but less that produced by LY 117018) LY 117018 was more effective than tamoxifen in reversing the uterotrophic effect of oestradiol. In animals pretreated with LY 117018 a further increase in uterine weight occurred on treatment with tamoxifen. The increase in uterine weight after tamoxifen was progressively reversed by increasing doses of LY 117018. The hypothesis that tamoxifen and LY 117018 may act by different mechanisms, based on the apparent failure of LY 117018 to antagonize the uterotrophic action of tamoxifen, is not supported by these studies.