Cell experiment:

MCF7 and Ishikawa cells are grown in 10% triple charcoal-stripped serum-containing medium for 3 d. The cells are then plated at a density of 2,000 cells per well in 96-well tissue culture plates and treated as indicated every 48 h. Cell proliferation assays are conducted 8 days after the first treatment using a CellTiter-Glo Luminescent Cell Viability kit[1].

Animal experiment:

Mice[2] GLAST::CreERT2 mice are used. Tamoxifen is injected as active metabolite 4-hydroxytamoxifen (4-OH-TAM), or 4-hydroxy-N-desmethyltamoxifen (Endoxifen). 4-OH-TAM is dissolved at 1 mM concentration in 2% ethanol/PBS and 5 μL injected into the ventricles. Endoxifen has not been used before for the induction of the ERT2 system. Endoxifen hydrochloride hydrate has a recommended solubility of over 10 mg/mL (25 mM) in 100% DMSO, corresponding to a maximum concentration of 25 mM or a dose of 125 nmol in a 5 μL volume. At this concentration, a proportion of mice developed neurological signs suggestive of neurotoxicity. Subsequently, concentrations of 2.5, 5, 10, 12.5 and 25 mM, dissolved in 10, 20, 40, 50 and 100% DMSO, respectively, corresponding to a drug dose of 12.5, 25, 50, 62.5 and 125 nmol are used. For testing DMSO toxicity, groups of three mice are injected with 5 uL PBS, 50% DMSO in PBS or 100% DMSO in PBS. Mice are culled 9 days after injection and coronally sectioned brains are stained for cleaved caspase 3, GFAP and Iba1.

Endoxifen Z-isomer is the most important Tamoxifen metabolite responsible for eliciting the anti-estrogenic effects of this drug in breast cancer cells expressing estrogen receptor-alpha (ERα). Endoxifen inhibits hERG tail currents at 50 mV in a concentration-dependent manner with IC50 values of 1.6 μM.IC50 value: 1.6 μM [1]Target: hERG Potassium Channel, Estrogen Receptor/ERREndoxifen Z-isomer is considered a prodrug, since it has a much higher potency for the estrogen receptor than its parent drug. Endoxifen inhibits the hERG channel protein trafficking to the plasma membrane in a concentration-dependent manner with Endoxifen being more potent than Tamoxifen. [1] Endoxifen is also shown to be a more potent inhibitor of estrogen target genes when ERβ is expressed. Additionally, low concentrations of Endoxifen Z-isomer observed in Tamoxifen treated patients with deficient CYP2D6 activity (20 to 40 nM) markedly inhibit estrogen-induced cell proliferation rates in the presence of ERβ, whereas much higher Endoxifen Z-isomer concentrations are needed when ERβ is absent.[2]

References:
[1]. Chae YJ, et al. Endoxifen, the active metabolite of tamoxifen, inhibits cloned hERG potassium channels. Eur J Pharmacol. 2015 Apr 5;752:1-7.
[2]. Wu X, et al. Estrogen receptor-beta sensitizes breast cancer cells to the anti-estrogenic actions of endoxifen. Breast Cancer Res. 2011 Mar 10;13(2):R27.