Background:

Estrogen receptors (ER) are members of the superfamily of ligand-modulated nuclear receptors that mediate the actions of steroid hormones, vitamin D, retinoids, and thyroid hormones. ER is activated in vivo when bound by naturally occurring estrogens such as 17α-estradiol. In addition to regulating these physiological processes, estrogen also plays a central role in stimulating breast cancer growth. (Z)-Tamoxifen is a first generation selective ER modulators that is currently approved by the FDA and is widely used to treat estrogen-dependent breast cancers. Its active metabolite, (Z)-4-Hydroxytamoxifen, is a potent estrogen receptor modulator.

In vitro: (Z)-4-hydroxytamoxifen binds to ER with 8-fold higher affinity than tamoxifen. It was found that only the Z isomer has the required antiestrogenic activity; the (E)-4-hydroxytamoxifen has only about 5% of its affinity for the ER [1].

In vivo: The antioestrogenic activities of (Z)-4-hydroxytamoxifen and tamoxifen were determined after oral administration. (Z)-4-hydroxytamoxifen was administered to groups of immature rats which also received s.c. injections of 0-2 μg oestradiol. Both compounds produced a dose-related decrease in uterine wet weight when compared with the oestradiol-treated controls. At a dose of 1 μg/day, the antiuterotrophic effects of (Z)-4-hydroxytamoxifen and tamoxifen were not significantly different but at 5μg/day, (Z)-4-hydroxytamoxifen was more active (P< 0.01). (Z)-4-hydroxytamoxifen therefore appears to retain its potent antioestrogenic activity after oral administration [2].

Clinical trial: Up to now, (Z)-4-Hydroxytamoxifen is still in the preclinical development stage.

Reference:
[1] Donna D.  Yu and Barry M. Forman. Simple and Efficient Production of (Z)-4-Hydroxytamoxifen, a Potent Estrogen Receptor Modulator. J. Org. Chem. 2003, 68, 9489-9491
[2] Jordan VC, Collins MM, Rowsby L, Prestwich G.  A monohydroxylated metabolite of tamoxifen with potent antioestrogenic activity. J Endocrinol. 1977 Nov;75(2):305-16.