Tamoxifen is metabolized in the liver by the cytochrome P450 isoform CYP2D6 and CYP3A4 into active metabolites such as afimoxifene (4-hydroxytamoxifen; 4-OHT) and endoxifen (N-desmethyl-4-hydroxytamoxifen) which have 30 to 100 times greater affinity for the ER than tamoxifen itself.
Endoxifen (EDX) is a key active metabolite of tamoxifen (TAM) with higher affinity and specificity to estrogen receptors that also inhibits aromatase activity. Tamoxifen is a largely inactive pro-drug, requiring metabolism into its most important metabolite endoxifen. Since the cytochrome P450 (CYP) 2D6 enzyme is primarily involved in this metabolism, genetic polymorphisms of this enzyme, but also drug-induced CYP2D6 inhibition can result in considerably reduced endoxifen formation and as a consequence may affect the efficacy oftamoxifen treatment.
As noted, tamoxifen is extensively metabolized predominantly by the cytochrome P450 (CYP) system to several primary and secondary metabolites, some of which exhibit more antiestrogenic effects in breast cancer cells than tamoxifen itself. Tamoxifen metabolism mostly occurs via two pathways, 4-hydroxylation and N-demethylation, both of which result in the very potent secondary metabolite, endoxifen. Originally, the 4-hydroxylation path, which is catalyzed by multiple CYPs including CYP2D6, was given much attention because the immediately resulting metabolite, 4-hydroxy-tamoxifen, had been shown to be approximately 30- to 100-fold more potent as an antiestrogen than tamoxifen However, this pathway only contributes approximately 7% of tamoxifen metabolism. N-demethylation to N-desmethyltamoxifen, catalyzed primarily by CYP3A4 and CYP3A5, contributes approximately 92% of tamoxifen metabolism. N-desmethyltamoxifen is further oxidized to a number of metabolites that appear important to tamoxifen activity, the most important being endoxifen. Endoxifen, first identified in human bile is formed from N-desmethyltamoxifen through hydroxylation by CYP2D6, and from 4-hydroxy-tamoxifen through demethylation by CYP3A4. While 4-hydroxytamoxifen and endoxifen have similar potencies in terms of antiestrogenic activity, endoxifen plasma concentrations in those receiving tamoxifen therapy is, on average, over ten-fold higher than that of 4-hydroxytamoxifen with large interpatient variability. In addition to the estrogen receptor inhibition exhibited by endoxifen, it also uniquely targets ERα (coded for by ESR1 gene) for proteasomal degradation. Endoxifen alone causes a decrease in ERα protein levels while the other metabolites of tamoxifen merely stabilize them. Because of this added effect, endoxifen is likely to be the primary metabolite responsible for the success seen in tamoxifen treatment.
Tamoxifen/Endoxifen kinetics:Following a single oral dose of 20 mg tamoxifen, an average peak plasma concentration of 40 ng/mL (range 35 to 45 ng/mL) occurred approximately 5 hours after dosing. The decline in plasma concentrations of tamoxifen is biphasic with a terminal elimination half-life of about 5 to 7 days. The average peak plasma concentration of N-desmethyl tamoxifen is 15 ng/mL (range 10 to 20 ng/mL). Chronic administration of 10 mg tamoxifen given twice daily for 3 months to patients results in average steady-state plasma concentrations of 120 ng/mL (range 67-183 ng/mL) for tamoxifen and 336 ng/mL (range 148-654 ng/mL) for N-desmethyl tamoxifen. The average steady-state plasma concentrations of tamoxifen and N-desmethyl tamoxifen after administration of 20 mg tamoxifen once daily for 3 months are 122 ng/mL (range 71-183 ng/mL) and 353 ng/mL (range 152-706 ng/mL), respectively. After initiation of therapy, steady state concentrations for tamoxifen are achieved in about 4 weeks and steady-state concentrations for N-desmethyl tamoxifen are achieved in about 8 weeks, suggesting a half-life of approximately 14 days for this metabolite.
Drug distribution in tissue is extensive both for the parent compound (tamoxifen) and the metabolites (specifically, endoxifen). Reports of tissue concentrations 10 to 60 fold higher than plasma concentrations in the liver, brain, pancreas, skin and bones have been reported in animal studies.
Endoxifen Z (cis)-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  Target: hERG Potassium Channel, Estrogen Receptor/ERR Endoxifen Z-isomer is considered a prodrug, since it has a much higher potency for the estrogen receptor than its parent drug, tamoxifen. Endoxifen inhibits the hERG channel protein trafficking to the plasma membrane in a concentration-dependent manner with Endoxifen being more potent than Tamoxifen. 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.
However, direct oral administration of endoxifen may present the problem of low bioavailability due to the rapid first-pass metabolism via O-glucuronidation.
Z-endoxifen has been administer directly to the breast, 10mg daily (5 mg per breast) and 20mg daily (10 mg per breast). This is essentially transdermal administration and based on the partition coefficient of endoxifen breast concentration should be10 to 60 times the plasma concentrations –the drawback it will be absorbed transdermally and will first pass into the systemic circulation before it partitions to the breast tissue –thus it is likely to have side effects that are identical to tamoxifen.