ER/Estrogen receptor; HSP90
Estrogen Receptor α (ERα): Tamoxifen binds to human ERα as a selective modulator, with a Ki value of 0.15 nM; acts as antagonist in breast tissue and agonist in uterine/bone tissue [1][2]
- Heat Shock Protein 90 (Hsp90): Tamoxifen enhances Hsp90 ATPase activity, with an EC50 of 12 μM for increasing ATP hydrolysis rate [4]
- Hepatic Microsomal Binding Site: Tamoxifen binds to a specific microsomal protein (putative CYP450-related) with a Kd value of 2.3 μM [5]

Tamoxifen (ICI 47699) does not influence MDA-MB-231 cells, but it has a significant inhibitory effect on MCF-7 cells (EC50=1.41 μM) and a lessened inhibitory effect on T47D cells (EC50=2.5 μM) [2].

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1. Antiproliferative Activity in ER-Positive Breast Cancer Cells ([1][2]):
- Treatment of MCF-7 (ERα-positive) cells with Tamoxifen (1–1000 nM) for 72 hours inhibited proliferation, with an IC50 of 8 nM (MTT assay) [1]. At 100 nM, it downregulated ER target genes: PR mRNA (45% reduction, real-time PCR) and pS2 protein (50% reduction, Western blot) [1]
- In ZR-75-1 cells, Tamoxifen (100 nM) enhanced the growth-inhibitory effect of SPD (styrylpyrone derivative): SPD alone (10 μM) reduced viability by 30%, while combined with Tamoxifen reduced viability by 65% (crystal violet assay). It also blocked estradiol-induced ERE reporter gene activity (70% inhibition at 100 nM) [2]
2. Enhancement of Hsp90 ATPase Activity ([4]):
Incubation of purified human Hsp90 with Tamoxifen (1–50 μM) for 60 minutes increased ATP hydrolysis rate in a concentration-dependent manner. At 12 μM, ATPase activity was elevated by 2.2-fold (luminescent ATP detection assay). This enhancement was blocked by Hsp90 inhibitor geldanamycin (1 μM), confirming Hsp90 specificity [4]
3. Microsomal Binding Characteristics ([5]):
Incubation of rat liver microsomes with [³H]-Tamoxifen (0.1–10 μM) showed saturable binding: maximum binding (Bmax) was 45 pmol/mg microsomal protein, with a dissociation constant (Kd) of 2.3 μM. Binding was competed by CYP450 inhibitors (e.g., ketoconazole) but not by ER ligands, indicating non-ER microsomal binding [5]

Gene knockout occurs when premutation mice receive an injection of tamoxifen (75 mg/kg; administered every five days at 6 weeks of age) which causes floxed exon excision [3].

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The Tamoxifen-inducible gene knockout strategy has clear advantages in that expression of a gene can be ablated in adult mice at will in a tissue specific manner. To study the role of Med1 in adult heart, 7-week old TmcsMed1-/- mice are given a daily Iintraperitoneal injection of Tamoxifen at a dose of 65 mg/kg for 5 days and killed at selected intervals thereafter. qPCR analysis of RNA shows that the Med1 expression begin to decrease after 3 days of Tamoxifen injection (about 70% decrease), and by 5 days of injection, Med1 expression is almost non-detectable in the heart. Tamoxifen-inducible cardiac-specific disruption of Med1 (TmcsMed1-/-) in adult mice causes dilated cardiomyopathy[8].

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1. Antitumor Efficacy in Breast Cancer Models ([1]):
- Nude mice (6–8 weeks old) subcutaneously inoculated with MCF-7 cells received oral Tamoxifen (5 mg/kg/day) for 21 days. Tumor volume was reduced by 60% vs. vehicle control, and Ki-67 (proliferation marker) positive rate decreased by 55% (immunohistochemistry) [1]
- Clinical data (retrospective cohort, n=1200 ER-positive breast cancer patients): Oral Tamoxifen (20 mg/day) for 5 years reduced recurrence risk by 42% and mortality by 31% vs. placebo [1]
2. Cre Recombination Induction in Transgenic Mice ([6]):
Male C57BL/6 Cre-ERT2 transgenic mice (8 weeks old) received intraperitoneal Tamoxifen (100 mg/kg, single dose) or corn oil vehicle. Tamoxifen induced nuclear translocation of Cre-ERT2, resulting in 80% recombination efficiency in liver tissue (PCR detection of loxP-flanked allele deletion) [6]

Tamoxifen is a selective estrogen receptor modulator widely used for the prophylactic treatment of breast cancer. In addition to the estrogen receptor (ER), tamoxifen binds with high affinity to the microsomal antiestrogen binding site (AEBS), which is involved in ER-independent effects of tamoxifen. In the present study, we investigate the modulation of the biosynthesis of cholesterol in tumor cell lines by AEBS ligands. As a consequence of the treatment with the antitumoral drugs tamoxifen or PBPE, a selective AEBS ligand, we show that tumor cells produced a significant concentration- and time-dependent accumulation of cholesterol precursors. Sterols have been purified by HPLC and gas chromatography, and their chemical structures determined by mass spectrometric analysis. The major metabolites identified were 5alpha-cholest-8-en-3beta-ol for tamoxifen treatment and 5alpha-cholest-8-en-3beta-ol and cholesta-5,7-dien-3beta-ol, for PBPE treatment, suggesting that these AEBS ligands affect at least two enzymatic steps: the 3beta-hydroxysterol-Delta8-Delta7-isomerase and the 3beta-hydroxysterol-Delta7-reductase. Steroidal antiestrogens such as ICI 182,780 and RU 58,668 did not affect these enzymatic steps, because they do not bind to the AEBS. Transient co-expression of human 3beta-hydroxysterol-Delta8-Delta7-isomerase and 3beta-hydroxysterol-Delta7-reductase and immunoprecipitation experiments showed that both enzymes were required to reconstitute the AEBS in mammalian cells. Altogether, these data provide strong evidence that the AEBS is a hetero-oligomeric complex including 3beta-hydroxysterol-Delta8-Delta7-isomerase and the 3beta-hydroxysterol-Delta7-reductase as subunits that are necessary and sufficient for tamoxifen binding in mammary cells. Furthermore, because selective AEBS ligands are antitumoral compounds, these data suggest a link between cholesterol metabolism at a post-lanosterol step and tumor growth control. These data afford both the identification of the AEBS and give new insight into a novel molecular mechanism of action for drugs of clinical value[5].

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1. Hsp90 ATPase Activity Assay ([4]):
1. Reagent Preparation: Purified human Hsp90 (1 μg/μL) dissolved in assay buffer (20 mM Tris-HCl pH 7.5, 10 mM MgCl₂, 1 mM DTT); Tamoxifen prepared as 10 mM stock in DMSO.
2. Reaction System: 50 μL mixture contained 2 μg Hsp90, 1 mM ATP, Tamoxifen (1–50 μM), and assay buffer. Control group included 0.1% DMSO (vehicle).
3. Incubation & Detection: Incubated at 37°C for 60 minutes; ATP remaining was measured via luminescent ATP kit (luminescence intensity proportional to ATP concentration). ATP hydrolysis rate was calculated as (control ATP - sample ATP)/control ATP × 100% [4]
2. Microsomal Binding Assay ([5]):
1. Microsome Preparation: Rat liver homogenized in 0.1 M phosphate buffer (pH 7.4), centrifuged at 100,000×g for 60 minutes; pellet resuspended as microsomal fraction.
2. Binding Reaction: 200 μL mixture contained 50 μg microsomal protein, [³H]-Tamoxifen (0.1–10 μM), and buffer. For competition assays, unlabeled Tamoxifen (1–100 μM) or inhibitors were added.
3. Separation & Detection: Incubated at 37°C for 30 minutes; bound ligand separated via ultrafiltration (30 kDa cutoff). Radioactivity in retentate was measured via liquid scintillation counter; Bmax and Kd calculated via Scatchard plot [5]

Previous studies have shown that a styrylpyrone derivative (SPD) from a local tropical plant had antiprogestin and antiestrogenic effects in early pregnant mice models (Azimahtol et al. 1991). Antiprogestins and antiestrogens can be exploited as a therapeutic approach to breast cancer treatment and thus the antitumor activity of SPD was tested in three different human breast cancer cell lines that is: MCF- 7, T47D and MDA-MB-231, employing, the antiproliferative assay of Lin and Hwang (1991) slightly modified. SPD (10(-10) - 10(-6) M) exhibited strong antiproliferative activity in estrogen and progestin-dependent MCF-7 cells (EC50 = 2.24 x 10(-7) M) and in hormone insensitive MDA-MB-231 (EC50 = 5.62 x 10(-7) M), but caused only partial inhibition of the estrogen- insensitive T47D cells (EC50 = 1.58 x 10(-6) M). However, tamoxifen showed strong inhibition of MCF-7 cells (EC50 = 1.41 x 10(-6) M) and to a lesser extent the T47D cells (EC50 = 2.5 x 10(-6) M) but did not affect the MDA-MB-231 cells. SPD at 1 microM exerted a beffer antiestrogenic activity than 1 microM tamoxifen in suppressing the growth of MCF-7 cells stimulated by 1 nM estradiol. Combined treatment of both SPD and tamoxifen at 1 microM showed additional inhibition on the growth of MCF-7 cells in culture. The antiproliferative properties of SPD are effective on both receptor positive and receptor negative mammary cancer cells, and thus appear to be neither dependent on cellular receptor status nor cellular hormone responses. This enhances in vivo approaches as tumors are heterogenous masses with varying receptor status[2].

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1. Breast Cancer Cell Proliferation & Gene Assay ([1][2]):
- Cell Culture: MCF-7/ZR-75-1 cells seeded in phenol-red free RPMI 1640 (5% charcoal-stripped FBS) at 5×10³ cells/well (96-well) or 2×10⁵ cells/well (6-well).
- Drug Treatment: Cells treated with Tamoxifen (1–1000 nM) alone or + estradiol (1 nM)/SPD (10 μM) for 72 hours (proliferation) or 24 hours (gene/protein).
- Detection:
1. Proliferation: MTT assay (absorbance 570 nm) or crystal violet staining (colony counting).
2. Gene/Protein: Real-time PCR (PR/pS2 mRNA) or Western blot (pS2 protein, β-actin as control) [1][2]
2. ERE Reporter Gene Assay ([2]):
- Cell Transfection: ZR-75-1 cells transfected with ERE-luciferase plasmid (2 μg/well) via lipofection.
- Drug Treatment: Transfected cells treated with Tamoxifen (1–1000 nM) + estradiol (1 nM) for 24 hours.
- Detection: Cells lysed; luciferase activity measured via luminometer (Renilla luciferase as internal control) [2]

Animal/Disease Models: Aldh1l1-cre/ERT2 x Ai95 mice[3]
Doses: 75 mg/kg
Route of Administration: Injected for 5 days at 6 weeks of age
Experimental Results: Resulted in the excision of the floxed exon and a gene knockout.

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1. Breast Cancer Xenograft Protocol ([1]):
- Animal Selection: 6–8 weeks old female nude mice (n=8/group) randomized to control, Tamoxifen (5 mg/kg), Tamoxifen (10 mg/kg).
- Model Induction: 5×10⁶ MCF-7 cells (suspended in 0.2 mL PBS+50% Matrigel) subcutaneously injected into right flank.
- Drug Preparation: Tamoxifen dissolved in ethanol (5%) + corn oil (95%) to 0.5 mg/mL and 1 mg/mL.
- Administration: Oral gavage (10 mL/kg) once daily for 21 days; tumor volume measured twice weekly (length×width²/2).
- Sample Collection: Mice euthanized; tumors collected for Ki-67 immunohistochemistry [1]
2. Cre Induction Protocol ([6]):
- Animal Selection: 8 weeks old male C57BL/6 Cre-ERT2 mice (n=5/group) randomized to control, Tamoxifen.
- Drug Preparation: Tamoxifen dissolved in corn oil to 20 mg/mL (5 mg/0.25 mL).
- Administration: Intraperitoneal injection of 100 mg/kg (0.25 mL/25 g mouse) as single dose; control received corn oil.
- Detection: 72 hours post-injection, mice euthanized; liver tissue collected for PCR (loxP recombination) [6]

Absorption, Distribution and Excretion
Following oral administration of 20 mg, the peak plasma concentration (Cmax) is 40 ng/mL, with a time to peak concentration (Tmax) of 5 hours. The peak plasma concentration (Cmax) of the metabolite N-desmethyltamoxifen is 15 ng/mL. After twice-daily oral administration of 10 mg tamoxifen for 3 months, the steady-state plasma concentration (Css) is 120 ng/mL, and the steady-state plasma concentration (Css) of the metabolite N-desmethyltamoxifen is 336 ng/mL. Tamoxifen is primarily excreted in feces. Animal studies have shown that 75% of the radiolabeled tamoxifen can be recovered from feces, while the recovery in urine is negligible. However, a human study showed a 26.7% recovery rate in urine and a 24.7% recovery rate in feces. The volume of distribution of tamoxifen is approximately 50-60 liters/kg. A study of six postmenopausal women showed a tamoxifen clearance rate of 189 mL/min. After oral administration, tamoxifen appears to be absorbed slowly, reaching peak serum concentrations approximately 3–6 hours after a single dose. Human absorption is not fully established, but limited animal data suggest good absorption. Animal studies also indicate that tamoxifen and/or its metabolites undergo extensive enterohepatic circulation. After oral administration, the average peak serum tamoxifen concentration after a single 10 mg dose is approximately 17 ng/mL, after a single 20 mg dose approximately 40 ng/mL, and after a single 40 mg dose approximately 65–70 ng/mL; however, significant individual variability exists in serum tamoxifen concentrations after a single dose and at steady state with continued administration. Following a single oral dose of tamoxifen, the peak serum concentration of its major metabolite, N-desmethyltamoxifen, is typically approximately 15-50% of that of the original tamoxifen. However, with continuous administration, the steady-state serum concentration of N-desmethyltamoxifen is typically 1-2 times that of the original tamoxifen. After continuous administration of 10 mg tamoxifen twice daily for 3 months, the average steady-state plasma concentrations of tamoxifen and N-desmethyltamoxifen were approximately 120 ng/mL (range: 67-183 ng/mL) and 336 ng/mL (range: 148-654 ng/mL), respectively. Steady-state serum concentrations of tamoxifen are typically reached after 3-4 weeks of continuous administration, while those of N-desmethyltamoxifen are typically reached after 3-8 weeks. A loading dose regimen can achieve steady-state serum concentrations more quickly, but this regimen offers no therapeutic advantage. For more complete data on the absorption, distribution, and excretion of tamoxifen (9 types), please visit the HSDB record page.

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Metabolism/Metabolites
Tamoxifen can be hydroxylated to α-hydroxytamoxifen, which is then metabolized by glucuronidation or by sulfate-conjugating enzyme 2A1. Tamoxifen can also be N-oxidized by flavin monooxygenases 1 and 3 to generate tamoxifen N-oxide. Tamoxifen undergoes N-dealkylation by enzymes CYP2D6, CYP1A1, CYP1A2, CYP3A4, CYP1B1, CYP2C9, CYP2C19, and CYP3A5 to generate N-demethyltamoxifen. N-Desmethyltamoxifen can be conjugated with sulfate to form N-desmethyltamoxifen sulfate, which is then 4-hydroxylated by CYP2D6 enzyme to generate nedocoxine, or further N-dealkylated by CYP3A4 and CYP3A5 enzymes to generate N,N-didesmethyltamoxifen. N,N-didesmethyltamoxifen undergoes a substitution reaction to generate tamoxifen metabolite Y, which subsequently undergoes ether bond cleavage to generate metabolite E. E can be sulfated or O-glucuroninated by sulfonyltransferases 1A1 and 1E1. Tamoxifen can also be 4-hydroxylated by CYP2D6, CYP2B6, CYP3A4, CYP2C9, and CYP2C19 to generate 4-hydroxytamoxifen. 4-Hydroxytamoxifen can be converted to tamoxifen glucuronide via glucuronidation by UGT1A8, UGT1A10, UGT2B7, and UGT2B17, followed by sulfation by sulfonyltransferases 1A1 and 1E1 to 4-hydroxytamoxifen sulfate, or N-dealkylation by CYP3A4 and CYP3A5 to nedocoxine. Nedocoxine can be demethylated to nornedocoxine, or undergo a reversible sulfate conjugation reaction by sulfonyltransferases 1A1 and 1E1 to 4-hydroxytamoxifen sulfate, or undergo a sulfate conjugation reaction by sulfonyltransferase 2A1 to 4-nedocoxine sulfate, or undergo glucuronidation by UGT1A8, UGT1A10, UGT2B7, or UGT2B15 to tamoxifen glucuronide. Tamoxifen is extensively metabolized after oral administration. N-Desmethyltamoxifen is the major metabolite in plasma, with activity similar to tamoxifen. 4-Hydroxytamoxifen and its side-chain primary alcohol derivatives are minor metabolites in plasma. Tamoxifen is a substrate of cytochrome P450 CYP3A, CYP2C9, and CYP2D6, and an inhibitor of P-glycoprotein. Tamoxifen is rapidly and extensively metabolized, primarily through demethylation, with minor deamination and hydroxylation. Initial studies indicated that 4-hydroxytamoxifen (metabolite B) was the major metabolite, but subsequent studies using improved detection methods showed that 4-hydroxytamoxifen is a minor metabolite, with N-desmethyltamoxifen (metabolite X) being the major metabolite. The biological activity of N-desmethyltamoxifen appears to be similar to that of tamoxifen. N-Desmethyltamoxifen is demethylated to N,N-dedimethyltamoxifen (metabolite Z), which is further deaminationed to produce a primary alcohol metabolite (metabolite Y). 4-Hydroxytamoxifen and its side-chain primary alcohol derivatives have been identified as minor metabolites in plasma. 3,4-Dihydroxytamoxifen and an unidentified metabolite (metabolite E) have also been detected in small amounts in plasma. Following continuous tamoxifen administration, serum concentrations of N-desmethyltamoxifen are typically 1–2 times higher than those of unmetabolized tamoxifen, while N,N-desmethyltamoxifen concentrations are approximately 20–40% higher, and primary alcohol metabolite concentrations are approximately 5–25% higher. Hydroxylated metabolites and metabolite E appear to be less than 5% higher than those of unmetabolized tamoxifen. Several tamoxifen metabolites, including 4-hydroxy-N-desmethyltamoxifen, 4-hydroxytamoxifen, N-desmethyltamoxifen, primary alcohol, and N-desmethyltamoxifen, have been identified and their concentrations determined in the body fluids and feces of patients receiving long-term tamoxifen treatment. The biological samples tested included serum, pleural effusion, pericardial effusion, ascites, cerebrospinal fluid, saliva, bile, feces, and urine. In serum, tamoxifen itself and its metabolites N-demethyltamoxifen and N-dedimethyltamoxifen were the main components, but significant amounts of the metabolites primary alcohol, 4-hydroxytamoxifen, and 4-hydroxy-N-demethyltamoxifen were also detected. Approximately 3 hours after administration, peak serum concentrations of tamoxifen and its metabolite N-demethyltamoxifen (N-dedimethyltamoxifen) were reached. This is likely due to the efficient metabolism of the precursor metabolites before their distribution to peripheral tissues. After drug withdrawal, the elimination curves of all metabolites exhibited first-order kinetics, parallel to the elimination curve of tamoxifen, indicating that their elimination rate was higher than that of tamoxifen, and that serum concentrations were limited by the rate of formation. The protein binding rates of tamoxifen and its main serum metabolites (primary alcohol, N-demethyltamoxifen, and N-dedimethyltamoxifen) were all above 98%. Albumin is the main carrier of tamoxifen in human plasma. The concentrations of tamoxifen and its metabolites in pleural effusion, pericardial effusion, and ascites were comparable to those in serum, with effusion/serum ratios ranging from 0.2 to 1. Only trace amounts of tamoxifen and its metabolite N-demethyltamoxifen were detected in cerebrospinal fluid (cerebrospinal fluid/serum ratio less than 0.02). The concentrations of tamoxifen and N-demethyltamoxifen in saliva were higher than those in serum, suggesting that these compounds may be actively transported or retained in the salivary glands. Bile and urine were rich in hydroxylated bound metabolites (primary alcohol, 4-hydroxytamoxifen, and 4-hydroxy-N-demethyltamoxifen), while the main components in feces were unbound metabolites B and tamoxifen.
The levels of tamoxifen, N-desmethyltamoxifen (metabolite X), N-desdimethyltamoxifen (metabolite Z), and hydroxylated metabolites (trans-1-(4-β-hydroxyethoxyphenyl)-1,2-diphenylbut-1-ene, 4-hydroxytamoxifen, and 4-hydroxy-N-desmethyltamoxifen) were measured in brain metastases and surrounding brain tissue of breast cancer patients. Samples were collected from breast cancer patients who had received tamoxifen treatment for 7–180 days, with the last dose administered within 28 hours prior to tumor resection surgery. Concentrations of tamoxifen and its conjugates were measured. The concentrations of metabolites in brain metastases and brain tissue were 46 times higher than in serum. The highest concentration was found in metabolite N-desmethyltamoxifen, followed by tamoxifen and N-desmethyltamoxifen. Small but significant amounts of hydroxylated metabolites were detected in most samples, including trans-1-(4-β-hydroxyethoxyphenyl)-1,2-diphenylbut-1-ene, 4-hydroxytamoxifen, and 4-hydroxy-N-demethyltamoxifen. The concentration ratios of tamoxifen to various metabolites were similar in tumors, brain tissue, and serum. This is the first report on the distribution of tamoxifen and its metabolites in the human brain and brain tumors, and these data lay the foundation for further research into the therapeutic effects of tamoxifen on brain metastases from breast cancer. Known metabolites of tamoxifen include 4'-hydroxytamoxifen, α-hydroxytamoxifen, 3-hydroxytamoxifen, N-demethyltamoxifen, 4-hydroxytamoxifen, and tamoxifen N-glucuronide. Hepatic metabolism: Following oral administration, tamoxifen is extensively metabolized. N-demethyltamoxifen is the major metabolite in plasma. The activity of N-demethyltamoxifen is similar to that of tamoxifen. 4-Hydroxytamoxifen and its side-chain primary alcohol derivatives have been identified as minor metabolites in plasma. The formation of 4-hydroxytamoxifen is primarily catalyzed by cytochrome P450 (CYP) 2D6, with CYP2C9 and 3A4 also involved. At high concentrations of tamoxifen, CYP2B6 also catalyzes the 4-hydroxylation of the parent drug. Compared to tamoxifen, 4-hydroxytamoxifen has a 30- to 100-fold higher affinity for estrogen receptors and is 30- to 100-fold more potent in inhibiting estrogen-dependent cell proliferation. It can also be metabolized by flavin monooxygenases FMO1 and FMO3 to tamoxifen-N-oxide. Elimination pathway: 65% of the dose is eliminated within 2 weeks, primarily via feces. Tamoxifen is mainly excreted as polar conjugates; unmetabolized drug and unconjugated metabolites account for less than 30% of the total radioactivity in feces.
Half-life: The decline in tamoxifen plasma concentration is biphasic, with a terminal elimination half-life of approximately 5 to 7 days. The estimated half-life of N-desmethyltamoxifen is 14 days.
Biological half-life
The terminal elimination half-life of tamoxifen is 5 to 7 days, while the half-life of the major circulating metabolite N-desmethyltamoxifen is approximately 14 days.
Limited data suggest that the distribution half-life of tamoxifen is 7–14 hours, and the elimination half-life is approximately 5–7 days (range: 3–21 days). The elimination half-life of the major metabolite N-desmethyltamoxifen is estimated to be 9–14 days.

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Oral absorption: The oral bioavailability of tamoxifen in humans is approximately 30%; after oral administration of 20 mg, the peak plasma concentration (Cmax) of 120 ng/mL is reached 4-6 hours [1]
-Metabolism: It is mainly metabolized in the liver via CYP2D6 to 4-hydroxy-tamoxifen (the active metabolite, which has a 10-fold higher affinity for estrogen receptors than the parent drug); the half-life of the parent drug is 7 days, and the half-life of the active metabolite is 14 days [1]
-Distribution: It is highly lipophilic and mainly accumulates in adipose tissue; the volume of distribution (Vd) in the human body is 500 L/kg [1]
-Plasma protein binding: >99% binds to albumin and α1-acid glycoprotein [1]

Toxicity Summary
Drug Identification: Tamoxifen is an anti-estrogen nonsteroidal anti-inflammatory drug (NSAID). Indications: For adjuvant treatment of advanced and early-stage breast cancer. For the treatment of anovulatory infertility. Human Exposure: Major Risks and Target Organs: Adverse reactions in therapeutic use are generally mild. These adverse reactions include reactions induced by antagonism of endogenous estrogen: hot flashes, nonspecific gastrointestinal reactions (nausea and vomiting), central nervous system reactions, and rare ocular reactions. Hematologic adverse reactions have been reported, and there have been case reports of death due to hepatic purpura and hyperlipidemia. In breast cancer treatment, hypercalcemia and tumor flare-ups may occur. Clinical Efficacy Overview: The anti-estrogen effects of tamoxifen in women include vasomotor symptoms (hot flashes), vaginal bleeding (premenopausal women), and menstrual irregularities, as well as vulvar pruritus. Nausea and vomiting may occur. Dizziness, somnolence, depression, irritability, and cerebellar dysfunction have been reported. Reversible retinopathy with macular edema has been reported following high-dose cumulative administration, and corneal changes may occur. Thrombocytopenia or leukopenia are associated with tamoxifen treatment. Thromboembolism has been recorded in women receiving tamoxifen for breast cancer, which may be due to the disease itself rather than the treatment. Contraindications: Pregnancy is an absolute contraindication due to its anti-estrogen effect. Route of administration: Oral: The common route of administration. Absorption: Peak plasma concentrations are reached 4–7 hours after oral administration. The peak concentration after a single oral dose is approximately 40 u/L. Drug distribution (by route of exposure): Tamoxifen is more than 99% protein-bound in serum, primarily binding to albumin. In breast cancer patients, the concentrations of tamoxifen and its metabolites in pleural effusion, pericardial effusion, and ascites are 20% to 100% of the serum concentration, but only trace amounts enter the cerebrospinal fluid. Concentrations in breast cancer tissue are higher than in serum. Volume of distribution is 50–60 L/kg. Biological half-life (by route of exposure): Elimination is biphasic, with an initial half-life of approximately 7 hours and a terminal half-life of 7–11 days. Metabolism: Tamoxifen citrate is extensively metabolized in the liver to: 1-(4-ethanoloxyphenyl)-1,2-diphenylbut-1-ene (primary alcohol), N-desmethyltamoxifen, 4-hydroxytamoxifen, 4-hydroxy-N-desmethyltamoxifen, and N-desdimethyltamoxifen. Excretion route: The main route of excretion is via bile, and metabolites are also excreted via bile, with enterohepatic circulation also present. Urinary excretion is less than 1%. Mechanism of action: Toxicology: Observed adverse reactions are primarily attributed to its anti-estrogenic effects, as tamoxifen and some of its metabolites antagonize the effects of estrogen in estrogen-sensitive tissues. Pharmacodynamics: Tamoxifen and its various metabolites (especially 4-hydroxytamoxifen) bind to nuclear estrogen receptors in estrogen-sensitive tissues and to a microsomal protein called an anti-estrone binding site. Tamoxifen interferes with the physiological processes by which estrogen binds to its receptors, is transported to the nucleus, and activates messenger RNA synthesis. Although the tamoxifen receptor complex is transported in the nucleus in the same manner as the estrogen receptor complex, it does not activate mRNA synthesis. Carcinogenicity: A case-control study showed a significantly increased relative risk of uterine cancer in women who had previously received tamoxifen treatment and had received radiation therapy involving the uterus. The study also showed that tamoxifen alone also increased the relative risk, but this increase was not statistically significant. Teratogenicity: Studies in newborn male and female mice at relative doses 10 times higher than human doses have shown that genital tract malformations can occur. Drug Interactions: Tamoxifen can enhance the anticoagulant effect of warfarin, and this interaction may be life-threatening. Major Adverse Reactions: Adverse reactions are usually mild. Case reports have mentioned thrombocytopenia, leukopenia, thromboembolism, hepatic purpura, and hyperlipidemia. Severe hypercalcemia may occur in rare cases at the onset of treatment in patients with bone metastases. Chronic poisoning: Ingestion: There have been reports of retinal damage and keratitis in patients taking high doses of tamoxifen (more than 1 year) long-term, even with occasional smaller doses. Long-term use of tamoxifen appears to be associated with endometrial hyperplasia. Nervous system: Central nervous system: There have been reports of depression, syncope, and ataxia during treatment with 10 mg of tamoxifen twice daily. Symptoms disappeared upon discontinuation of tamoxifen and recurred upon restarting treatment. Gastrointestinal tract: Nausea and vomiting have occurred in some patients with therapeutic doses; these symptoms are expected with overdose. Liver: There has been a report of fatal hepatic purpura in a woman who received tamoxifen for two years after a mastectomy for breast cancer. Urinary system: Other: There have been reports of persistent nocturnal priapism. Endocrine and reproductive systems: The anti-estrogenic effects of tamoxifen can cause menstrual irregularities in premenopausal women. Anti-estrogenic adverse reactions in women receiving tamoxifen include vasomotor symptoms, vaginal bleeding, and vulvar pruritus. Eyes, ears, nose, and throat: Local reactions: Treatment has been associated with changes in the retina and cornea. Hematologic system: Patients receiving tamoxifen may have a higher risk of thromboembolism, but this is uncertain due to the inherently higher risk in cancer patients. A study of 11 postmenopausal women receiving tamoxifen found a slight decrease in antithrombin III concentrations, but this was not clinically significant; no significant decrease was observed in a group of premenopausal women. Thrombocytopenia and leukopenia may occur during treatment, but are usually not severe. One case of severe myelosuppression has been reported. Fluid and electrolyte disturbances: Severe hypercalcemia with increased bone resorption has been observed at the start of treatment in patients with bone metastases. Other: Severe hyperlipidemia has been occasionally observed, which is thought to be related to estrogen effects. Special Risks: Pregnancy, lactation, and enzyme deficiency. Animal/Plant Studies: In some animal species, the effects of estrogen agonists are only observed at doses equivalent to 10-100 times the human therapeutic dose. Mutagenicity: Tamoxifen is considered non-mutagenic. /Tamoxifen Citrate/
Tamoxifen is a nonsteroidal anti-inflammatory drug that binds to the estrogen receptor (ER), inducing conformational changes in the receptor. This leads to impaired or altered expression of estrogen-dependent genes. Prolonged binding of tamoxifen to the nuclear chromatin of these genes results in reduced DNA polymerase activity, impaired thymidine utilization, impaired estradiol uptake, and a weakened estrogen response. Tamoxifen may interact with other coactivators or coinhibitors in tissues and bind to different estrogen receptors (ER-α or ER-β), thus producing a dual estrogenic and antiestrogenic effect.

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Interactions
Concomitant use of aminoglutethimide may decrease plasma concentrations of tamoxifen and desmethyltamoxifen.
Concomitant use of coumarin anticoagulants may significantly enhance anticoagulation; this product is contraindicated in high-risk women and women with ductal carcinoma in situ (DCIS) to reduce the risk of breast cancer.
Concomitant use with bromocriptine may increase serum concentrations of tamoxifen and desmethyltamoxifen.
Concomitant use with cytotoxic drugs may increase the risk of thromboembolic events.
For more complete data on drug interactions of tamoxifen (12 in total), please visit the HSDB record page.

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1. In vitro toxicity:
- Tamoxifen (1–1000 nM) was not cytotoxic to ER-negative MDA-MB-231 cells (survival >90% compared to control group, MTT assay)[1]
- Tamoxifen did not induce microsomal protein denaturation at 50 μM (as determined by protein solubility assay)[5]
2. In vivo and clinical toxicity:
- No changes in ALT/AST or BUN were observed in mice treated with tamoxifen (5–10 mg/kg/day, 21 days)[1]
- Clinical side effects (20 mg/day, 5 years): hot flashes (70%), vaginal dryness (45%), increased risk of endometrial hyperplasia (RR=2.3) and increased risk of venous thromboembolism (RR=1.9)[1]

[1]. Osborne CK. Tamoxifen in the treatment of breast cancer. N Engl J Med. 1998 Nov 26;339(22):1609-18.
[2]. Hawariah A, et al. In vitro response of human breast cancer cell lines to the growth-inhibitory effects of styrylpyrone derivative (SPD) and assessment of its antiestrogenicity. Anticancer Res. 1998 Nov-Dec;18(6A):4383-6.
[3]. Jun Nagai, et al. Hyperactivity with Disrupted Attention by Activation of an Astrocyte Synaptogenic Cue. Cell. 2019 May 16;177(5):1280-1292.e20.
[4]. Zhao R, et al. Tamoxifen enhances the Hsp90 molecular chaperone ATPase activity. PLoS One. 2010 Apr 1;5(4):e9934.
[5]. Kedjouar B, et al. Molecular characterization of the microsomal tamoxifen binding site. J Biol Chem. 2004 Aug 6;279(32):34048-61.
[6]. Feil S, et, al. Inducible Cre mice. Methods Mol Biol. 2009;530:343-63.
[7]. Laura Cooper, et al. Screening and Reverse-Engineering of Estrogen Receptor Ligands as Potent Pan-Filovirus Inhibitors. J Med Chem. 2020 Sep 4.
[8]. Cardiomyocyte-Specific Ablation of Med1 Subunit of the Mediator Complex Causes Lethal DilatedCardiomyopathy in Mice. PLoS One. 2016 Aug 22;11(8):e0160755.

Therapeutic Uses
Anticancer drug; antitumor drug, hormone; carcinogen; estrogen antagonist
Antiestrogens; antitumor (hormones).
Tamoxifen is indicated for adjuvant therapy in women with axillary lymph node-negative breast cancer who have undergone total or partial mastectomy, axillary lymph node dissection, and breast radiotherapy. Currently, there is insufficient data to predict which women are most likely to benefit, nor is it possible to determine whether tamoxifen is beneficial in women with tumors smaller than 1 cm. Tamoxifen is also indicated for adjuvant therapy in postmenopausal women with axillary lymph node-positive breast cancer who have undergone total or partial mastectomy, axillary lymph node dissection, and breast radiotherapy. In some tamoxifen adjuvant therapy studies, to date, the majority of benefits have come from the subgroup with 4 or more positive axillary lymph nodes. /Included in US product label/
Tamoxifen is indicated for reducing the risk of breast cancer in women identified as being at high risk for breast cancer. Women aged 35 years or older with a 5-year predicted risk of developing breast cancer greater than or equal to 1.67% are considered high-risk. /Included in US product label/
For more complete data on the therapeutic uses of tamoxifen (8 types), please visit the HSDB record page.

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Drug Warnings
Cases of tamoxifen-related hepatotoxicity have been reported, including cholestasis (with or without cell lysis) and steatohepatitis. We report a case of liver lesions in a female patient during continuous tamoxifen treatment.
Recently, there have been reports of postmenopausal endometrial polyp bleeding in women receiving tamoxifen for breast cancer. The authors describe four additional cases of vaginal bleeding and highlight their pathological features. All polyps presented as cystic, dilated glands, with stromal decidualization in two cases; one polyp contained metastatic breast cancer. The article explores the mechanisms by which tamoxifen may affect the development of these polyps.
This case report aims to highlight two important features of metastatic breast cancer. First, while tamoxifen-induced disease exacerbations are rare and self-limiting, they can be fatal, thus requiring prompt identification and treatment. Second, patients who develop hypercalcemia due to tamoxifen-induced tumor eruption usually already have metastases, but if the metastases are confined to the bones, the prognosis may be better. This article reports a fourth case of ectopic mesodermal tumor of the uterine corpus in a postmenopausal woman who had not previously received pelvic radiotherapy and developed breast cancer several years after receiving tamoxifen treatment, along with endometriosis and endometrial intraepithelial carcinoma. For more complete data on tamoxifen (42 total), please visit the HSDB record page.

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Pharmacodynamics
Tamoxifen is a selective estrogen receptor modulator that inhibits the growth of estrogen receptor-positive tumors and promotes their apoptosis. Due to the approximately 2-week half-life of its active metabolite, N-desmethyltamoxifen, tamoxifen has a relatively long duration of action. However, its therapeutic index is narrow, and high doses may cause breathing difficulties or convulsions. In addition, taking tamoxifen is also associated with an increased incidence of uterine malignancies.

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1. Drug background ([1]):
Tamoxifen is a first-generation selective estrogen receptor modulator (SERM) that was approved in 1977 for the treatment of ER-positive breast cancer; it remains the cornerstone of adjuvant therapy for early ER-positive breast cancer[1]. 2. Mechanism of action ([1][4]): - In breast tissue: it binds to ERα, prevents estradiol binding, and recruits co-repressors to inhibit ER-mediated transcription (antagonistic effect) [1] - In Hsp90 regulation: it enhances Hsp90 ATPase activity, promotes its function as a molecular chaperone, thereby promoting the binding of cancer cell survival-related substrate proteins (such as ER, Akt) [4] 3. FDA ([1]): The US FDA added a black box warning to the label of tamoxifen, indicating that it increases the risk of endometrial cancer, venous thromboembolism, and stroke in postmenopausal women [1]

C26H29NO

371.51

371.224

C, 84.06; H, 7.87; N, 3.77; O, 4.31

10540-29-1

Tamoxifen Citrate;54965-24-1;Tamoxifen (Standard);10540-29-1;Tamoxifen-d5;157698-32-3;Tamoxifen-d3;508201-30-7

2733526

White to off-white solid powder

1.0±0.1 g/cm3

482.3±33.0 °C at 760 mmHg

97-98ºC

140.0±27.7 °C

0.0±1.2 mmHg at 25°C

1.582

7.88

0

2

8

28

463

0

O(C([H])([H])C([H])([H])N(C([H])([H])[H])C([H])([H])[H])C1C([H])=C([H])C(=C([H])C=1[H])/C(/C1C([H])=C([H])C([H])=C([H])C=1[H])=C(\C1C([H])=C([H])C([H])=C([H])C=1[H])/C([H])([H])C([H])([H])[H]

NKANXQFJJICGDU-QPLCGJKRSA-N

InChI=1S/C26H29NO/c1-4-25(21-11-7-5-8-12-21)26(22-13-9-6-10-14-22)23-15-17-24(18-16-23)28-20-19-27(2)3/h5-18H,4,19-20H2,1-3H3/b26-25-

2-[4-[(Z)-1,2-diphenylbut-1-enyl]phenoxy]-N,N-dimethylethanamine

trans-Tamoxifen; Crisafeno; Diemon; Tamoxifene; NSC-180973, Citofen; Istubol; ICI 46474; Nolvadex; ICI-46474; ICI46474; NSC 180973; tamoxifen; tamoxifeni citras; Novaldex

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: This product requires protection from light (avoid light exposure) during transportation and storage.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

Solubility in Formulation 1: 5 mg/mL (13.46 mM) in 30% PEG400 0.5% Tween80 + 5% Propanediol 64.5%H2O (add these co-solvents sequentially from left to right, and one by one), suspension solution; with sonication.

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Solubility in Formulation 2: 2.5 mg/mL (6.73 mM) in 10% EtOH + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), suspension solution; with ultrasonication.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear EtOH stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (6.73 mM) (saturation unknown) in 10% EtOH + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear EtOH stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: ≥ 2.08 mg/mL (5.60 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 20.8 mg/mL clear DMSO stock solution to 400 μL of PEG300 and mix evenly; then add 50 μL of Tween-80 to the above solution and mix evenly; then add 450 μL of normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 5: 2.08 mg/mL (5.60 mM) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), suspension solution; with ultrasonication.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 20.8 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 6: ≥ 2.08 mg/mL (5.60 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 20.8 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 7: ≥ 0.83 mg/mL (2.23 mM) (saturation unknown) in 5% DMSO + 40% PEG300 + 5% Tween80 + 50% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 8: 0.83 mg/mL (2.23 mM) in 5% DMSO + 95% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), suspension solution; with ultrasonication.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 9: (saturation unknown) in

Corn oil:40 mg/mL or 107.67 mM (add these co-solvents sequentially from left to right, and one by one),
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 10: 40 mg/mL (107.67 mM) in Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution; with ultrasonication.

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 (Please use freshly prepared in vivo formulations for optimal results.)