Endoxifen

Endoxifen, also known as 4-hydroxy-"N"-desmethyltamoxifen, is a chemical that is under development for estrogen receptor-positive breast cancer. It is also being evaluated as an antipsychotic for treatment of mania and other psychotic disorders. 
Endoxifen is a nonsteroidal selective estrogen receptor modulator (SERM) of the triphenylethylene group. It is an active metabolite of tamoxifen and has been found to be effective in patients that have failed previous hormonal therapies (tamoxifen, aromatase inhibitors, and fulvestrant). The prodrug tamoxifen is metabolized by the CYP2D6 enzyme to produce endoxifen and afimoxifene (4-hydroxytamoxifen).
In the first study to evaluate the pharmacology of endoxifen, it showed 25% of the affinity of estradiol for the estrogen receptor (ER) while afimoxifene had 35% of the affinity of estradiol for the ER. The antiestrogenic actions of endoxifen and afimoxifene in this study were very similar. In another study, the affinity of endoxifen for the ERα was 12.1% and its affinity for the ERβ was 4.75% relative to estradiol. For comparison, afimoxifene had relative binding affinities for the ERα and ERβ of 19.0% and 21.5% compared to estradiol, respectively. In yet another investigation, both endoxifen and afimoxifene had 181% of the affinity of estradiol for the ER whereas tamoxifen had 2.8% and "N"-desmethyltamoxifen had 2.4%.