New tamoxifen analogues were tested for their antiproliferative activity both in vitro and in vivo. Binding studies showed that both 4-iodotamoxifen and pyrrolidino-4-iodotamoxifen and 2.5-fold higher affinities for the estrogen receptor compared with tamoxifen. Pyrrolidino-4-iodotamoxifen was also 1.5-fold more effective in causing inhibition of estrogen-induced growth of MCF-7 cells compared with tamoxifen at 10−6m. The 4-iodotamoxifen analogue was similar to tamoxifen in its inhibitory action at 10−6m. Antiproliferative activities of these drugs were tested using the nitrosomethylurea-induced rat mammary tumor model. Pyrrolidino-4-iodotamoxifen caused regression in 92% of rats, whereas tamoxifen caused regression in 75% of rats. The agonist activity of the analogues was determined using the immature rat and mouse uterotrophic assays. Both tamoxifen and 4-iodotamoxifen had similar partial agonist activity, and this was greater than that seen with pyrrolidino-4-iodotamoxifen. Furthermore, pyrrolidino-4-iodotamoxifen caused a dose-dependent inhibition of estrogen-induced vaginal cornification, whereas tamoxifen and 4-iodotamoxifen did not. These studies demonstrate that pyrrolidino-4-iodotamoxifen is more effective than tamoxifen in inhibiting tumor regression and that its reduced uterotrophic activity and increased estrogen receptor binding may give it significant clinical advantages over the parent compound.


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