(E/Z)-Endoxifen (Synonyms: (E/Z)-N-desmethyl-4-hydroxy Tamoxifen)

Endoxifen is a key active metabolite of tamoxifen (TAM) with higher affinity and specificity to estrogen receptor that also inhibits aromatase activity.

Endoxifen, a hydroxylated tamoxifen metabolite, is approximately 100-fold more potent as an antagonist of the ER than tamoxifen. It also suggests that endoxifen but not 4-hydroxytamoxifen results in ER-alpha degradation in addition to its effects on the ER at the level of transcription[1]. Endoxifen, is a potent antiestrogen that targets estrogen receptor α for degradation in breast cancer cells. Additionally, it is showed that Endoxifen blocks ERA transcriptional activity and inhibits estrogen-induced breast cancer cell proliferation even in the presence of tamoxifen, N-desmethyl-tamoxifen, and 4-hydroxytamoxifen[2]. Endoxifen is strongly growth inhibitory at 10 μM for all the breast cancer cell lines except for moderate inhibition for MDAMB-468.Cytotoxic effects are quite significant at 10 μM concentration for MCF7, HS 578T, and BT-549 cells. At lower Endoxifen concentrations (0.01-1 μM), the inhibitory effects are not as significant as 10 μM, whereas 100 μM Endoxifen concentration found to be lethal for all tested cells[3].

Orally administered Endoxifen is rapidly absorbed and systemically available when tested in female rats. The Endoxifen-treated rats show 787% higher exposure (AUC0–∞) and 1,500% higher concentration (Cmax) levels of Endoxifen when compared with Tamoxifen. Oral Endoxifen administration once a day for 28 consecutive days at dosages 2, 4, and 8 mg/kg proves safe and results in progressive inhibition of the growth of the human mammary tumor xenografts in female mice[3].

References:
[1]. Goetz MP, et al. Tamoxifen, endoxifen, and CYP2D6: the rules for evaluating a predictive factor. Oncology (Williston Park). 2009 Dec;23(14):1233-4, 1236.
[2]. Wu X, et al. The tamoxifen metabolite, endoxifen, is a potent antiestrogen that targets estrogen receptor alpha for degradation in breast cancer cells. Cancer Res. 2009 Mar 1;69(5):1722-7.
[3]. Wu X, et al. The tamoxifen metabolite, endoxifen, is a potent antiestrogen that targets estrogen receptor alpha for degradation in breast cancer cells. Cancer Res. 2009 Mar 1;69(5):1722-7.