Endoxifen E-isomer is the E-isomer of (Z)-Endoxifen. (Z)-Endoxifen, an active metabolite generated via actions of CYP3A4/5 and CYP2D6, is a more potent selective estrogen receptor modulator (SERM) than Tamoxifen.
Endoxifen exists as the potently anti-estrogenic (Z)-isomer and the lesser known (E)-isomer. It is assumed that (E)-Endoxifen, structurally related to (E)-4-OH-tamoxifen, have similar pharmacological properties. The (E)-isomer is an impurity in (Z)-Endoxifen drug substance and increases under certain storage conditions. (E)-Endoxifen is identified as the primary degradant[1].
[1]. Elkins P, et al. Characterization of the isomeric configuration and impurities of (Z)-Endoxifen by 2D NMR, high resolution LC-MS, and quantitative HPLC analysis. J Pharm Biomed Anal. 2014 Jan;88:174-9.