Hydroxylated metabolites of tamoxifen are formed in vivo and bound to estrogen receptor in target tissues.

After in vivo administration of [3H]tamoxifen to immature female rats and chickens, polar metabolites of tamoxifen were found in plasma, liver, uterus, and oviduct. 4-Hydroxytamoxifen and M2, another hydroxylated metabolite of tamoxifen, were the major tritiated compounds of the cytosol and the KCl-nuclear extract of target tissues and appeared to occupy the estrogen receptor sites since their accumulation in these fractions was saturable, resistant to charcoal, and prevented by estradiol. In the rat uterus, 4-hydroxytamoxifen was predominant during the 24 h following the [3H]tamoxifen injection, then its concentration declined while M2 became predominant. 4-Hydroxytamoxifen was also found in chicken oviduct where tamoxifen is acting as a full estrogen antagonist. Moreover, liver, chicken oviduct, and lamb uterus were able to convert tamoxifen into 4-hydroxytamoxifen in vitro. Other estrogen target tissues, such as the rat uterus, dimethylbenz (a) anthracene-induced rat mammary tumors, and MCF7 cells, did not transform tamoxifen significantly. 4-Hydroxytamoxifen formed in vitro was able to bind selectively to estrogen receptor with a high affinity and with a low dissociation rate similar to estradiol. These results demonstrate that 4-hydroxytamoxifen is formed in vivo and retained on estrogen receptor in target tissues, due to its high affinity. Since we have separately shown that 4-hydroxytamoxifen is a full anti-estrogen, more potent than tamoxifen itself in MCF7 cells, we conclude that tamoxifen is mostly acting in vivo indirectly via hydroxylated metabolite(s).