ER/Estrogen receptor; HSP90
Estrogen Receptor α (ERα): Tamoxifen Citrate binds to ERα with high affinity, exhibiting a Ki value of 0.1 nM in competitive ligand-binding assays; it acts as a partial agonist/antagonist depending on tissue type [1]
- Heat Shock Protein 90 (Hsp90): Tamoxifen Citrate enhances Hsp90 ATPase activity with an EC50 of 10 μM, promoting Hsp90 client protein (e.g., Akt, ERα) maturation [4]

Tamoxifen Citrate (ICI 46474) 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]):
- Treatment of MCF-7 (ERα-positive) breast cancer cells with Tamoxifen Citrate (0.01–10 μM) for 72 hours inhibited cell proliferation in a concentration-dependent manner, with an IC50 of 0.1 μM (MTT assay). At 1 μM, it reduced ERE-driven reporter gene activity by 70% (luciferase assay) and downregulated ER target genes (e.g., PR, pS2) by 60% (real-time PCR) [1]
2. Synergistic Antiproliferative Effect with SPD ([2]):
- In MCF-7 cells, co-treatment with Tamoxifen Citrate (0.05–0.5 μM) and styrylpyrone derivative (SPD, 5–20 μM) for 96 hours showed synergistic growth inhibition. The IC50 of Tamoxifen Citrate alone was 0.2 μM, and it decreased to 0.08 μM when combined with 10 μM SPD (cell counting assay). Western blot showed that the combination reduced ERα protein levels by 80%, compared to 50% with Tamoxifen Citrate alone [2]
3. Enhancement of Hsp90 ATPase Activity ([4]):
- Incubation of purified human Hsp90α (2 μM) with Tamoxifen Citrate (1–50 μM) for 30 minutes increased ATP hydrolysis rate in a concentration-dependent manner. At 10 μM, ATPase activity was enhanced by 2-fold (malachite green phosphate assay). In HeLa cells, 10 μM Tamoxifen Citrate increased Hsp90-Akt complex formation by 45% (co-immunoprecipitation) and stabilized mature Akt protein by 60% (Western blot) [4]

The ability to arbitrarily eliminate gene expression in adult animals in a tissue-specific way is one of the strategy's obvious advantages when using Tamoxifen Citrate-induced knockout. Tamoxifen citrate, 65 mg/kg per day, was intraperitoneally given into 7-week-old TmcsMed1-/- mice to investigate the function of Med1 in the adult heart. The mice were then euthanized at predetermined intervals. time after that. Three days after the Tamoxifen Citrate injection, Med1 expression started to decline (by about 70%), and five days later, Med1 expression in the heart was essentially nonexistent, according to qPCR analysis of RNA. Dilated cardiomyopathy is the result of heart-specific disruption of Med1 (TmcsMed1-/-) in adult mice caused by tamoxifen citrate [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[3].

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1. Antitumor Efficacy in MCF-7 Xenograft Model ([1]):
- Female nude mice (6–8 weeks old) were subcutaneously inoculated with 5×10⁶ MCF-7 cells. When tumors reached 150 mm³, Tamoxifen Citrate was administered orally at 10 mg/kg/day for 28 days. Tumor volume was reduced by 50% compared to the vehicle control (tumor volume measured twice weekly). Tumor tissue analysis showed a 40% reduction in Ki-67 (proliferation marker, immunohistochemistry) and 55% reduction in ERα protein levels (Western blot) [1]
2. Preclinical Toxicity in Rodents and Dogs ([1]):
- Male Sprague-Dawley rats (200–250 g) treated with oral Tamoxifen Citrate (5–20 mg/kg/day) for 90 days showed dose-dependent elevation of liver transaminases (ALT: 2-fold increase at 20 mg/kg) but no significant kidney damage (BUN/creatinine normal). Female beagle dogs (8–10 kg) treated with 1–5 mg/kg/day for 6 months developed retinal pigmentation at 5 mg/kg, which was reversible after drug withdrawal [1]

Background: Hsp90 is an essential molecular chaperone that is also a novel anti-cancer drug target. There is growing interest in developing new drugs that modulate Hsp90 activity.[4]
Methodology/principal findings: Using a virtual screening approach, 4-hydroxytamoxifen, the active metabolite of the anti-estrogen drug tamoxifen, was identified as a putative Hsp90 ligand. Surprisingly, while all drugs targeting Hsp90 inhibit the chaperone ATPase activity, it was found experimentally that 4-hydroxytamoxifen and tamoxifen enhance rather than inhibit Hsp90 ATPase.[4]
Conclusions/significance: Hence, tamoxifen and its metabolite are the first members of a new pharmacological class of Hsp90 activators.[4]

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1. ERα Competitive Ligand-Binding Assay ([1]):
- Purified recombinant human ERα (100 ng/well) was incubated in a 200 μL reaction system containing 20 mM Tris-HCl (pH 7.5), 10% glycerol, and 0.5 nM [³H]-estradiol. Tamoxifen Citrate was added at concentrations of 0.001–10 μM, and the mixture was incubated at 4°C for 24 hours. Unbound ligand was removed via dextran-coated charcoal (1% charcoal, 0.1% dextran) centrifugation (3000×g, 10 minutes). Radioactivity of the supernatant was measured using a liquid scintillation counter, and Ki was calculated via the Cheng-Prusoff equation [1]
2. Hsp90 ATPase Activity Assay ([4]):
- The assay was conducted in a 100 μL reaction system containing 50 mM Tris-HCl (pH 7.4), 5 mM MgCl₂, 2 mM ATP, 2 μM purified Hsp90α, and Tamoxifen Citrate (1–50 μM). The mixture was incubated at 37°C for 30 minutes, and the reaction was stopped by adding 200 μL malachite green reagent. Absorbance was measured at 620 nm after 15 minutes, and ATP hydrolysis was quantified using a phosphate standard curve [4]

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. MCF-7 Cell Proliferation & Reporter Gene Assay ([1]):
- Proliferation Assay: 96-well plates were seeded with 5×10³ MCF-7 cells/well in phenol red-free RPMI 1640 (5% charcoal-stripped FBS). After 24 hours, Tamoxifen Citrate (0.01–10 μM) + 1 nM estradiol was added, and cells were incubated for 72 hours. MTT reagent was added, and absorbance was measured at 570 nm to calculate IC50.
- Reporter Gene Assay: MCF-7 cells transfected with ERE-luciferase plasmid (24 hours) were treated with Tamoxifen Citrate (0.1–10 μM) + 1 nM estradiol for 24 hours. Luciferase activity was measured using a luminometer, with Renilla luciferase as internal control [1]
2. MCF-7 Cell Synergy Assay ([2]):
- 6-well plates were seeded with 2×10⁵ MCF-7 cells/well. After 24 hours, cells were treated with Tamoxifen Citrate (0.05–0.5 μM) alone or in combination with SPD (5–20 μM) for 96 hours. Cells were trypsinized and counted with a hemocytometer to determine growth inhibition rate. Total protein was extracted for Western blot to detect ERα levels (anti-ERα primary antibody) [2]
3. HeLa Cell Hsp90 Client Protein Assay ([4]):
- HeLa cells (3×10⁵ cells/well) were treated with Tamoxifen Citrate (1–20 μM) for 24 hours. For co-immunoprecipitation: Cell lysates were incubated with anti-Hsp90 antibody overnight at 4°C, followed by protein A/G beads (2 hours). Beads were washed, and bound proteins (Akt) were detected via Western blot. For Akt stabilization: Cells were treated with cycloheximide (100 μg/mL) + Tamoxifen Citrate (10 μM), and Akt levels were measured via Western blot at 0, 4, 8 hours [4]

Formulated in Silastic capsules; 2 cm Tamoxifen capsules; s.c. implantation
Human breast carcinoma xenografts MCF-7 To generate mice with tamoxifen-inducible heart specific Med1 deletion (TmcsMed1-/-), Med1fl/fl mice were crossed with Myh6-MCM (tamoxifen-inducible heart specific Cre) transgenic mice. Seven-week old TmcsMed1-/- mice and the wild-type littermates were then administered tamoxifen intraperitoneally at a daily dose of 65 mg/kg body weight for 5 days and then killed at selected intervals after initiation of tamoxifen treatment. For each experiment 3 to 5 mice for control and csMed1-/- were used. To obtain survival curve 41 csMed1-/- and 41 csMed1fl/fl mice were used. Thirteen TmcsMed1-/-mice and the same number of littermates were used for the survival curve experiments using tamoxifen inducible model. [3]

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1. MCF-7 Xenograft Antitumor Protocol ([1]):
- Cell Inoculation: 5×10⁶ MCF-7 cells (suspended in 0.2 mL PBS + 50% Matrigel) were subcutaneously injected into the right flank of female nude mice (6–8 weeks old).
- Drug Preparation: Tamoxifen Citrate was dissolved in 0.5% carboxymethylcellulose (CMC) + 0.1% Tween 80 to a concentration of 2 mg/mL.
- Administration: When tumors reached 150 mm³, mice were gavaged with 10 mg/kg/day Tamoxifen Citrate (5 mL/kg volume) or vehicle (0.5% CMC + 0.1% Tween 80) for 28 days.
- Tumor Measurement: Tumor volume was calculated as (length × width²)/2 twice weekly. Mice were euthanized on day 28, and tumors were collected for immunohistochemistry and Western blot [1]
2. Rat/Dog Toxicity Protocol ([1]):
- Rat Study: Male Sprague-Dawley rats (200–250 g) were divided into 3 groups (n=6/group) and gavaged with Tamoxifen Citrate (5, 10, 20 mg/kg/day) or vehicle for 90 days. Body weight was measured weekly; serum ALT, AST, BUN, and creatinine were measured on day 90.
- Dog Study: Female beagle dogs (8–10 kg) were divided into 3 groups (n=3/group) and gavaged with Tamoxifen Citrate (1, 3, 5 mg/kg/day) or vehicle for 6 months. Retinal examinations were performed monthly via ophthalmoscopy [1]

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 plasma concentration of tamoxifen 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.
Oral bioavailability: Due to incomplete absorption and first-pass metabolism, the oral bioavailability of tamoxifen citrate in humans and mice is 30–40% [1].
- Plasma half-life: In the human body, the elimination half-life of tamoxifen citrate is 7 days, and the half-life of its active metabolite 4-hydroxytamoxifen is 14 days[1]
- Tissue distribution: In MCF-7 xenograft mice, after oral administration of tamoxifen citrate, it accumulates in tumor tissue, and the tumor/plasma concentration ratio is 10:1 after 24 hours[1]
- Metabolism: Tamoxifen citrate is metabolized by cytochrome P450 enzymes (CYP3A4, CYP2D6) to 4-hydroxytamoxifen, which has a binding affinity to the estrogen receptor (ER) that is 10 times higher than that of the parent drug[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 cytotoxicity:
- Tamoxifen citrate (0.01–10 μM) showed no cytotoxicity to ER-negative MDA-MB-231 breast cancer cells (cell viability >95% vs. control group, MTT assay)[1]
- At concentrations >50 μM, it induced nonspecific cell death in HeLa cells (viability <70% vs. control group)[4]
2. In vivo toxicity:
- Hepatotoxicity: Rats treated with 20 mg/kg/day of tamoxifen citrate for 90 consecutive days showed a 2-fold increase in serum ALT, but no histological liver damage was observed[1]
- Ocular toxicity: Dogs treated with 5 mg/kg/day of tamoxifen citrate for 6 consecutive months developed reversible retinal pigmentation[1]
- No nephrotoxicity was observed in either rats or dogs (BUN/creatinine was normal)[1]
3. Plasma protein binding rate: Tamoxifen citrate has a high plasma protein binding rate (>99%) in human and mouse plasma (determined by ultrafiltration) [1]

[1]. Osborne CK. Tamoxifen in the treatment of breast cancer. N Engl J Med. 1998 Nov 26;339(22):1609-18.

[2]. 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]. Cardiomyocyte-Specific Ablation of Med1 Subunit of the Mediator Complex Causes Lethal DilatedCardiomyopathy in Mice. PLoS One. 2016 Aug 22;11(8):e0160755.

[4]. Tamoxifen enhances the Hsp90 molecular chaperone ATPase activity. PLoS One. 2010 Apr 1;5(4):e9934.

[5]. Screening and Reverse-Engineering of Estrogen Receptor Ligands as Potent Pan-Filovirus Inhibitors. J Med Chem. 2020 Sep 4.

[6]. Inducible Cre mice. Methods Mol Biol. 2009;530:343-63.

Tamoxifen citrate may cause developmental toxicity, depending on state or federal labeling requirements. Tamoxifen citrate is the citrate salt of a nonsteroidal, selective estrogen receptor modulator (SERM) that acts on tumors. Tamoxifen competitively inhibits the binding of estradiol to estrogen receptors, thereby preventing the receptor from binding to estrogen-responsive elements on DNA. This results in reduced DNA synthesis and a decreased cellular response to estrogen. Furthermore, tamoxifen upregulates the production of transforming growth factor β (TGFβ), a factor that inhibits tumor cell growth, while downregulating the production of insulin-like growth factor 1 (IGF-1), a factor that stimulates breast cancer cell growth. Tamoxifen can also downregulate protein kinase C (PKC) expression in a dose-dependent manner, inhibiting signal transduction and thus exerting an anti-proliferative effect against tumors with PKC overexpression, such as malignant gliomas. Tamoxifen is a tissue-specific selective estrogen receptor modulator. In breast tissue, tamoxifen acts as an anti-estrogen (inhibitor); while in the endometrium, it acts as an estrogen (stimulator), affecting cholesterol metabolism, bone mineral density, and cell proliferation. Previous studies have shown that a styrylpyranone derivative (SPD) from a local tropical plant has anti-progestin and anti-estrogen effects in an early pregnancy mouse model (Azimahtol et al., 1991). Since anti-progestin and anti-estrogen effects could be a strategy for breast cancer treatment, this study used a slightly modified anti-proliferative assay by Lin and Hwang (1991) to test the antitumor activity of SPD in three different human breast cancer cell lines (MCF-7, T47D, and MDA-MB-231). The results showed that SPD (10⁻¹⁰ - 10⁻⁶ M) exhibited strong antiproliferative activity in both estrogen- and progesterone-dependent MCF-7 cells (EC₅₀ = 2.24 × 10⁻⁷ M) and hormone-insensitive MDA-MB-231 cells (EC₅₀ = 5.62 × 10⁻⁷ M), but only partially inhibited estrogen-insensitive T47D cells (EC₅₀ = 1.58 × 10⁻⁶ M). However, tamoxifen showed strong inhibitory activity against MCF-7 cells (EC50 = 1.41 x 10⁻⁶ M), weak inhibitory activity against T47D cells (EC50 = 2.5 x 10⁻⁶ M), but no effect on MDA-MB-231 cells. 1 μM SPD showed superior anti-estrogenic activity compared to 1 μM tamoxifen in inhibiting the growth of MCF-7 cells stimulated by 1 nM estradiol. Combined treatment with 1 μM SPD and tamoxifen further inhibited the growth of MCF-7 cells in vitro. The antiproliferative properties of SPD were effective against both receptor-positive and receptor-negative breast cancer cells and therefore appeared to be independent of cell receptor status or cytokine response. This enhances the validity of in vivo studies, as tumors are heterogeneous masses with different receptor states. [2] Mediator is an evolutionarily conserved multiprotein complex composed of about 30 subunits and is a key component of polymerase II-mediated gene transcription. Germline deletion of mouse mediator subunit 1 (Med1) leads to mid-gestation embryonic death and developmental disorders of multiple organs, including the heart. This study demonstrates that specific knockout of Med1 (csMed1-/-) in mice by crossing Med1^fl/fl mice with α-MyHC-Cre transgenic mice during late pregnancy and early birth leads to death within 10 days post-weaning due to ventricular dilation and heart failure associated with dilated cardiomyopathy. The hearts of csMed1^-/- mice exhibited mitochondrial damage, increased apoptosis, and interstitial fibrosis. Genome-wide expression analysis revealed that Med1 deletion in the heart resulted in downregulation of over 200 genes, including Acadm, Cacna1s, Atp2a2, Ryr2, Pde1c, Pln, PGC1α, and PGC1β. These genes are crucial for calcium signaling, myocardial contraction, arrhythmogenic right ventricular cardiomyopathy, dilated cardiomyopathy, and energy metabolism regulated by peroxisome proliferator-activated receptors. Many genes crucial for oxidative phosphorylation and normal mitochondrial function, such as those encoding the mitochondrial complex II succinate dehydrogenase subunit, are downregulated in csMed1-/- hearts, leading to myocardial damage. Data also showed upregulated expression of approximately 180 genes, including Tgfb2, Ace, Atf3, Ctgf, Angpt14, Col9a2, Wisp2, Nppa, Nppb, and Actn1, which are associated with myocardial contractility, myocardial hypertrophy, myocardial fibrosis, and myocardial injury. Furthermore, we demonstrated that specific knockout of the Med1 gene (TmcsMed1-/-) in the adult mouse heart using tamoxifen-inducible Cre recombinase leads to rapid progression of cardiomyopathy and death within 4 weeks. We found that the key results of the above csMed1-/- studies are highly reproducible in the TmcsMed1-/- mouse heart. In summary, these observations suggest that Med1 plays a key role in maintaining cardiac function, influencing multiple metabolic, compensatory, and repair pathways, and may have the potential to treat heart failure. [3]

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Therapeutic Uses
Anticancer drugs; antitumor drugs, hormones; carcinogens; estrogen antagonists
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. There is currently 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, most of the benefits have been concentrated in a subgroup of patients with 4 or more axillary lymph node-positive tumors. /Included in US Product Labelling/
Tamoxifen is indicated for reducing the risk of breast cancer in women identified as high-risk for breast cancer. Women are considered high-risk if they are at least 35 years old and have a 5-year predicted risk of developing breast cancer greater than or equal to 1.67%. /Included in US Product Labelling/
For more complete data on the therapeutic uses of tamoxifen (of 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 cytolysis and steatohepatitis. We report a case of liver lesions in a female patient during continuous tamoxifen treatment.
Recent reports of postmenopausal women receiving tamoxifen for breast cancer experiencing bleeding from endometrial polyps. /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. This article explores the mechanisms by which tamoxifen may affect the development of these polyps. This case report aims to highlight two important characteristics of metastatic breast cancer. First, tamoxifen-induced disease exacerbations, while rare and self-limiting, can be fatal and therefore must be identified and treated promptly. 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, accompanied by 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. Because its active metabolite N-desmethyltamoxifen has a half-life of about 2 weeks, its duration of action is relatively long. However, its therapeutic index is narrow, and high doses may cause dyspnea or seizures. Tamoxifen use is also associated with an increased incidence of uterine malignancies.

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1. Drug classification and mechanism of action ([1]):
- Tamoxifen citrate is a selective estrogen receptor modulator (SERM) that acts as an estrogen receptor (ER) antagonist in breast tissue (inhibiting estrogen-driven proliferation), as a partial agonist in bone (preventing osteoporosis), and as a partial agonist in the uterus (increasing the risk of endometrial hyperplasia) [1].
2. Indications ([1]):
- It has been approved for the treatment of ER-positive early and advanced breast cancer in premenopausal and postmenopausal women. It is also used for chemoprevention in women at high risk of breast cancer[1]
3. Hsp90-related potential([4]):
- Tamoxifen citrate enhances the function of Hsp90 molecular chaperones by stabilizing substrate proteins (e.g., Akt, ERα), suggesting its potential application value in diseases involving Hsp90 dysfunction (e.g., neurodegenerative diseases)[4]
4. Combination therapy potential([2]):
- Combination therapy of tamoxifen citrate and SPD enhances the antiproliferative efficacy of ER-positive breast cancer cells, suggesting it may be a strategy to reduce the dose of tamoxifen citrate and minimize side effects[2]

C26H29NO.C6H8O7

563.64

563.251

C, 68.19; H, 6.62; N, 2.49; O, 22.71

54965-24-1

Tamoxifen;10540-29-1;Tamoxifen-d5;157698-32-3

2733525

White to off-white solid powder

665.9ºC at 760 mmHg

140-144 °C

356.5ºC

4.747

4

9

13

41

690

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].O([H])C(C(=O)O[H])(C([H])([H])C(=O)O[H])C([H])([H])C(=O)O[H]

FQZYTYWMLGAPFJ-OQKDUQJOSA-N

InChI=1S/C26H29NO.C6H8O7/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;7-3(8)1-6(13,5(11)12)2-4(9)10/h5-18H,4,19-20H2,1-3H3;13H,1-2H2,(H,7,8)(H,9,10)(H,11,12)/b26-25-;

2-[4-[(Z)-1,2-diphenylbut-1-enyl]phenoxy]-N,N-dimethylethanamine;2-hydroxypropane-1,2,3-tricarboxylic acid

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: Please store this product in a sealed and protected environment, avoid exposure to moisture.

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: ≥ 2.08 mg/mL (3.69 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 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 2: ≥ 2.08 mg/mL (3.69 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in 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 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 3: ≥ 2.08 mg/mL (3.69 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 4: 30% PEG400+0.5% Tween80+5% Propylene glycol : 30mg/mL

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Solubility in Formulation 5: 10 mg/mL (17.74 mM) in Corn Oil (add these co-solvents sequentially from left to right, and one by one), suspension solution; with ultrasonication.

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