Biotransformation Of Tamoxifen Research Articles

Influence of polymorphism of enzymes of the UDP family-glucuronyl transferases on the biotransformation of tamoxifen in the therapy of luminal forms of breast cancer

Annotation. Tamoxifen (TAM) (1-[4-(2-dimethylaminoethoxy)-phenyl]-1,2-diphenylbut-1(Z)-ene) is a non-steroidal selective estrogen receptor modulator (SERM), which is recognized as the "gold standard" of hormone therapy for estrogen-dependent breast cancer (BC). It is known that adjuvant treatment with TAM increases recurrence-free survival and overall survival in patients with hormone-receptor-positive breast cancer. Also, tamoxifen manifests itself as a partial estrogen agonist, which can be associated with the development of complications such as endometrial cancer, venous thromboembolism, etc. The presence of resistance and relapses during TAM therapy, which reach up to 30%, remains an actual problem. Therefore, studying the mechanisms underlying the individualization of both therapeutic effect and toxicity associated with TAM remains an important challenge. In the detoxification of both TAM and its active metabolites, glucuronidation processes, which belong to the second phase of biotransformation of xenobiotics and actively take place in the liver as well as in the mammary gland, play an important role, and therefore the study of this process can contribute to the understanding of the interindividual variability of the therapeutic effect and toxicity of TAM. The aim – to analyze the data of the scientific literature on the study of the influence of glucuronyltransferase (UGT) enzymes and their polymorphic forms on the biotransformation of TAM and its active metabolites in the treatment of hormone-receptor-positive breast cancer. A retrospective analysis of the literature of scientific databases Scopus, Web of Science, PubMed., MedLines for 2013-2023 was carried out. It is possible to draw the following conclusions that UGT isozymes are responsible for the conjugation and detoxification of tamoxifen and its metabolites in the form of glucuronides 4-OH-tamoxifen-N-glucuronide, 4-OH-tamoxifen-O-glucuronide and endoxifen-O-glucuronide. UGT1A8, UGT1A10, UGT2B7, UGT2B15 and UGT2B17 isoforms played the greatest role in glucuronidation of tamoxifen and its active metabolites, but UGT1A4 was recognized as the main one. Depending on the content of active TAM metabolites and their glucuronides in the blood plasma, it can be stated that carriers of the UGT2B15 Lys523Thr and UGT2B17del alleles demonstrated increased enzyme activity, and individuals with one variant UGT2B15 523Thr allele can even be considered superactive metabolizers of 4-OH-tamoxifen-O- glucuronide and endoxifen-glucuronide. Also, high levels of 4-OH-tamoxifen-N-glucuronide were observed in carriers of the allele of the UGT2B17del genotype. Carriers of the above alleles have high activity of glucuronidation processes and low levels of active metabolites of TAM, which calls into question the rationality of prescribing TAM as hormone therapy. In contrast, patients with UGT1A4 48Val, UGT2B7 268Tyr alleles, or with wild-type genotypes for UGT2B17 nodel and UGT2B15 523Lys, will have high levels of active metabolites and are the group of choice for tamoxifen therapy in estrogen-receptor-positive breast cancer because they will have a low rate of glucuronidation and detoxification. However, in order to create a system of clinical algorithms for the formation of tamoxifen-sensitive groups of patients, further detailed study of other possibilities of the biotransformation system in the metabolism of tamoxifen is required.

Impact of Irradiation on the Pharmacokinetics and Biotransformation of Tamoxifen.

BackgroundThe optimal procedure for combining radiotherapy (RT) with tamoxifen treatment is controversial as RT may alter the pharmacokinetics and biotransformation of tamoxifen. The present study investigated this potential interaction by assessing the pharmacokinetics of tamoxifen during concurrent and sequential RT.MethodPlasma tamoxifen concentration was measured in rats with or without RT 2.0 Gy (RT2.0Gy) or 0.5 Gy (RT0.5Gy) with ultra-high-performance liquid chromatography-tandem mass spectrometry after tamoxifen administration (10 mg/kg, p.o., n = 6). Tamoxifen was either administered 1 h after RT (concurrent condition) or 24 h after RT (sequential condition).ResultsPharmacokinetic data analysis demonstrated that the area under the curve (AUC) and half-life of tamoxifen were 2,004 ± 241 h ng/ml and 6.23 ± 1.21 h, respectively, after tamoxifen administration (10 mg/kg, p.o.). The respective conversion rate of 4-hydroxytamoxifen, N-desmethytamoxifen, and endoxifen for tamoxifen metabolism was 20%, 16%, and 5%. The AUC value of tamoxifen in the RT0.5Gy group was 1.5- to 1.7-fold higher than in the sham and RT2.0Gy groups. The relative bioavailability of tamoxifen at concurrent RT0.5Gy and RT2.0Gy groups ranged from 127% to 202% and from 71% to 152%, respectively. The magnitude of endoxifen, which converted from 4-hydroxytamoxifen and N-desmethyltamoxifen, increased 3- to 5-fold in the concurrent RT groups. By contrast, the AUC of tamoxifen decreased by roughly 24% in the sequential RT2.0Gy group. The conversion ratio of endoxifen was four times higher than that in the sequential RT2.0Gy group compared with rats not exposed to RT.ConclusionThe current study provides advanced pharmacokinetic data to confirm the interaction between RT and hormone therapy. Our findings indicate that RT facilitates the metabolism of tamoxifen to active metabolites and thus imply that combination RT-tamoxifen has potential benefits for the treatment of hormone-dependent breast cancer.

Impact of Irradiation on the Pharmacokinetics and Biotransformation of Tamoxifen

Impact of Irradiation on the Pharmacokinetics and Biotransformation of Tamoxifen

Tamoxifen side effects: pharmacogenetic and clinical approach in Mexican mestizos.

BackgroundTamoxifen metabolism is translated into four genetic phenotypes (GP): genetic poor metabolizer (gPM); genetic intermediate metabolizer (gIM); genetic normal metabolizer (gNM); and genetic ultra-rapid metabolizer (gUM). Although CYP2D6 is involved in tamoxifen biotransformation, its association with tamoxifen side effects (TSE) is limited. Therefore, we evaluated CYP2D6 GP and clinical variables as potential predictors of TSE in Mexican Mestizo patients.MethodsThis cross-sectional study evaluated CYP2D6 GP, clinical data, and self-reported TSE in 71 women. Potential predictors were tested in uni- and multivariable models.ResultsHot flashes (57.75%), arthralgia (45.07%), headache (43.66%), and cramps (39.44%) were the most frequent TSE. Three GP were identified: gPM (2.8%); gNM (93.0%); and gUM (4.2%). In the univariate analysis, none of the GP was predictive of TSE. However, the uni- and multivariable models showed contraceptive use and chemotherapy treatment prior to tamoxifen therapy to be predictive. Two alleles were identified for the first time at unusually high frequencies: CYP2D6*34 (13.2%); and *39 (14.7%).ConclusionsOur findings indicate that CYP2D6 GP were not significantly predictive of TSE, though two clinical descriptors were. The present results are a valuable contribution to pharmacogenetic characterization of Mexican Mestizo populations who, like other Latin-American groups, are poorly represented in the literature.

Cytochrome P450 Genetic Variation Associated with Tamoxifen Biotransformation in American Indian and Alaska Native People.

Despite evidence that pharmacogenetics can improve tamoxifen pharmacotherapy, there are few studies with American Indian and Alaska Native (AIAN) people. We examined variation in cytochrome P450 (CYP) genes (CYP2D6, CYP3A4, CYP3A5, and CYP2C9) and tamoxifen biotransformation in AIAN patients with breast cancer (n = 42) from the Southcentral Foundation in Alaska and the Confederated Salish and Kootenai Tribes in Montana. We tested for associations between CYP diplotypes and plasma concentrations of tamoxifen and metabolites. Only the CYP2D6 variation was significantly associated with concentrations of endoxifen (P = 0.0008) and 4‐hydroxytamoxifen (P = 0.0074), tamoxifen's principal active metabolites, as well as key metabolic ratios. The CYP2D6 was also the most significant predictor of active metabolites and metabolic ratios in a multivariate regression model, including all four genes as predictors, with minor roles for other CYP genes. In AIAN populations, CYP2D6 is the largest contributor to tamoxifen bioactivation, illustrating the importance of validating pharmacogenetic testing for therapy optimization in an understudied population.

Impact of CYP2D*6 in the adjuvant treatment of breast cancer patients with tamoxifen.

Biotransformation of tamoxifen to the potent antiestrogen endoxifen is performed by cytochrome P450 (CYP) enzymes, in particular the CYP2D6 isoform. CYP2D6*4 is one of the most frequent alleles associated with loss of enzymatic activity. The incidence of CYP2D6*4 among Caucasians is estimated up to 27%, while it is present in up to 90% of all poor metabolizers within the Caucasian population. The hypothesis under question is whether the presence of one or two non-functioning (null) alleles predicts an inferior outcome in postmenopausal women with breast cancer receiving adjuvant treatment with tamoxifen. The numerous existing studies investigating the association of CYP2D6 with treatment failure in breast cancer are inconsistent and give rather conflicting results. Currently, routine CYP2D6 testing among women with breast cancer is not recommended and the significance of CYP2D6 phenotype in decision making regarding the administration of tamoxifen is unclear. The present study summarizes current literature regarding clinical studies on CYP2D6*4, particularly in terms of response to tamoxifen therapy and breast cancer outcome.

Dextromethorphan phenotyping as a test for prediction of tamoxifen (TAM) activation in breast cancer patients receiving adjuvant hormone therapy.

e13019 Background: TAM is mainly metabolized by CYP2D6 to form its most active metabolites, 4hydroxy-tamoxifen (4OH-T) and endoxifen (END). Because of its long half-life, steady state is reached after around 4 months of continuous intake. The wide variable inter-patient activity of CYP2D6 might influence drug efficacy. A multi-institutional study in north Italy is evaluating the relationship between END levels and outcome. As a part of it, we investigated the role of dextromethorphan (DM), a probe drug for CYP2D6 enzymatic activity, as a potential phenotyping test for TAM activation. Methods: Twenty-nine breast cancer patients (75% postmenopausal) on adjuvant TAM therapy (20 mg/die) were investigated. They received a single dose (15 mg) of oral DM before starting TAM and their urines were collected over the10 following hours. Simultaneous quantitative determination of DM and its metabolite dextrorphan (DO) was performed in urines to estimate their log transformed metabolic ratio (LMR=logDM/DO). After 4 months a blood sample was collected to characterize TAM exposure at steady state; plasma levels of TAM, END, 4OH-T and the non active END precursor N-desmethyltamoxifen (NDT) were quantified by HPLC. Linear regression analysis and t test were performed for correlating LMR and drug plasma levels. Results: LMR varied between -2.15 and 0.90 (median: -1.37) while steady state plasma levels of END varied between 1.9 and 15.0 ng/ml (median: 4.36) and 4OH-T between 0.9 to 3.1 ng/ml (median:1.72). A significant correlation (r = 0.56; p= 0.0013) was found between LMR and END plasma concentrations. The patients with high LMR (> median value), compared to patients with low LMR, had lower END (3.7 vs 7.5 ng/ml, p=0.0003), lower 4OH-T (1.6 vs 2.1 ng/ml, p=0.04) and, accordingly, higher NDT (291.2 vs 198.2 ng/ml, p=0.025). Conclusions: DM/DO urine ratio obtained before starting therapy correlates with TAM biotransformation activity and can predict steady state active metabolites exposure in individual patients. This phenotyping test is fast, simple and unexpensive and could contribute to the personalization of adjuvant breast cancer treatment. Funded by Regione Veneto.

SULT1A1 , CYP2C19 and Disease-Free Survival in early Breast Cancer Patients Receiving Tamoxifen

Tamoxifen biotransformation to endoxifen, a potent antiestrogen, is catalyzed by CYP2D6. In addition, CYP2C19 and SULT1A1 have also been implicated in the metabolism of tamoxifen. We sought to evaluate the importance of SULT1A1 copy number and CYP2C19*17 on disease-free survival (DFS) in postmenopausal women randomized to tamoxifen monotherapy in North Central Cancer Treatment Group 89-30-52 from January 1991 to April 1995. We extracted DNA from paraffin-embedded tumors and determined tumor SULT1A1 copy number and CYP2C19*17 genotype. The association of genotype with DFS was determined using the log-rank test. Multivariate cox modeling was performed using traditional prognostic factors, as well as CYP2D6 genotype. SULT1A1 copy number and CYP2C19*17 genotype was determined in 190 out of 256 patients (95% Caucasian). The median follow-up for living patients was 14 years. DFS did not differ according to SULT1A1 copy number (p = 0.482) or CYP2C19*17 genotype (p = 0.667). Neither SULT1A1 copy number or CYP2C19*17 genotype was associated with disease recurrence in this cohort. Future studies are needed to identify whether other genetic and environmental factors which affect tamoxifen metabolism are associated with tamoxifen clinical outcomes.

An Electrochemical Outlook on Tamoxifen Biotransformation: Current and Future Prospects

Tamoxifen is a nonsteroidal antiestrogen that is currently and widely used in the treatment of breast cancer in all of its stages, in adjuvant therapy as a long-term suppressant of tumor recurrence and also as a chemopreventive agent in women that are in high risk of developing this type of estrogen-dependent cancer. From a toxicological and (bio)analytical point of view the knowledge of the metabolic pathways of a drug is found to be extremely important. So, in the present work the most important tamoxifen biotransformation steps were reviewed in the light of recent pharmacological data. This overview also includes the current controversy concerning tamoxifen DNA-damaging (genotoxic) versus non-genotoxic mechanisms. A special focus will be given to the putative application of electrochemical methods as a modern and reliable analytical tool for determination of tamoxifen and its metabolites. Moreover, the potential of DNA electrochemical sensors for detection of structural damage to DNA as a basis for toxicity screening is highlighted. Future prospects looking for the importance of developing new analytical methodologies are also discussed.

Abstract P5-11-14: Distribution of Tamoxifen and Five Metabolites into Normal and Malignant Human Breast Cancer Tissue during Conventional and Low Dose Treatment

Abstract Background: Biotransformation of tamoxifen leads to metabolites with different pharmacological and toxicological effects. Profiling and screening for active metabolites are important to assess their contribution to the overall therapeutic and adverse effects. The anticancer effect of tamoxifen is believed to be due to the hydroxylated metabolites 4hydroxytamoxifen (4OHtam) and 4hydroxyNdesmethyltamoxifen (4OHNDtam, endoxifen). Its demethylated metabolites have been related to side effects of the drug, whereas effects of tamoxifenNoxide (tamNoxide) have not been evaluated. In the present study we wanted to examine the distribution of tam and five of its metabolites in serum and normal and malignant breast tissues during normal dose (20 mg/day) and low dose (5 or 1 mg/day) regimens after 28 days of continuous dosing. Methods: Serum and biopsies from 56 patients were available for analysis. The concentrations of tamoxifen and the metabolites 4OHtam, 4OHNDtam, Ndesmethyltamoxifen (NDtam), Ndesdimethyltamoxifen (NDDtam), and tamNoxide in serum and tissues were measured by means of a recently developed highly sensitive liquid chromatography tandem mass spectrometry (LCMSMS) method. Results: Serum and biopsies from 56 patients were available for analysis. All compounds including the potent metabolites 4OHtam and 4OHNDtam were detected and measured in all samples. The tissue levels of tam and its hydroxylated and demethylated metabolites were highly related to the serum levels (p < 0.000 — 0.0069). The tumor tissue to serum concentration ratios of tamNoxide decreased by increasing doses (p < 0.0001) whereas this ratio was unchanged over the dose range 1-20 mg/day for the other compounds measured. Conclusion: Tamoxifen and its metabolites were detected in all samples examined including those sampled during the low dose regimens. The tissue levels of tamoxifen and its hydroxylated and demethylated metabolites were strongly corresponding to the serum concentrations, in contrast to those of tamNoxide. The strong relations between the concentrations of the potent active 4OHtam and 4OHNDtam (endoxifen) in serum and target tissues suggest that implementation of therapeutic drug monitoring may represent a tool to improve tamoxifen therapy. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P5-11-14.

527 The DNA glycosylase Myh1 is stabilized by cisplatin and inhibition of Myh1 expression increases cisplatin-induced apoptotic signaling in lung carcinoma cells

Although long-term tamoxifen therapy is associated with increased risk of endometrial cancer, little is known about the ability of endometrial tissue to biotransform tamoxifen to potentially reactive intermediates, capable of forming DNA adducts. The present study examined whether explant cultures of human endometrium provide a suitable in vitro model to investigate the tissue-specific biotransformation of tamoxifen. Fresh human endometrial tissue, microscopically uninvolved in disease, was cut into 1×2-mm uniform explants and incubated with media containing either 25 or 100 μM tamoxifen in a 24-well plate. Metabolites were analyzed by reversed-phase HPLC using postcolumn, online, photochemical activation and fluorescence detection. Three metabolites, namely, α-hydroxytamoxifen, 4-hydroxytamoxifen, and N-desmethyltamoxifen were identified in culture medium and tissue lysates. N-desmethyltamoxifen was found to be the major metabolite in both tissue and media extracts of tamoxifen-exposed explants. Incubations of tamoxifen with recombinant human cytochrome P-450s (CYPs) found that CYP2C9 and CYP2D6 produced all three of the above tamoxifen metabolites, while CYP1A1 and CYP3A4 catalyzed the formation of α-hydroxytamoxifen and N-desmethyltamoxifen, and CYP1A2 and CYP1B1 only formed the α-hydroxy metabolite. CYP2D6 exhibited the greatest activity for the formation of all three tamoxifen metabolites. Western immunoblots of microsomes from human endometrium detected the presence of CYPs 2C9, 3A, 1A1 and 1B1 in fresh endometrium, while CYPs 2D6 and 1A2 were not detected. Immunohistochemical (IHC) analysis also confirmed the presence of CYPs 2C9, 3A and 1B1 in fresh human endometrium and in viable tissue cultured for 24 h with or without tamoxifen. Together, the results support the use of explant cultures of human endometrium as a suitable in vitro model to investigate the biotransformation of tamoxifen in this target tissue. In addition, the results support the role of CYPs 2C9, 3A, 1A1 and 1B1 in the biotransformation of tamoxifen, including the formation of the DNA reactive α-hydroxytamoxifen metabolite, in human endometrium.

CYP2D6 Polymorphisms and Tamoxifen Metabolism: Clinical Relevance

The selective estrogen receptor modulator tamoxifen has been used for more than three decades to treat metastatic and early-stage receptor-positive breast cancer and, more recently, to prevent the disease. Biotransformation of tamoxifen to the potent antiestrogen endoxifen is performed by cytochrome P450 (CYP) enzymes, in particular the CYP2D6 isoform. Genetic variants in the CYP2D6 gene may result in CYP2D6 enzymes with reduced or null activity. Strong and intermediate inhibitors of CYP2D6, which may be used to treat hot flashes or psychiatric conditions in breast cancer patients, can also negatively impact enzyme function. Prospective data are lacking, but the balance of current evidence strongly suggests that, compared with women with two wild-type alleles, the presence of two null alleles, and possibly one null allele, predicts reduced tamoxifen metabolism and an inferior outcome in postmenopausal women with early breast cancer who receive adjuvant treatment with the drug. Unfortunately, studies to date have been largely retrospective and the interpretation of their results is limited by examination of archival tissue samples and the inclusion of heterogeneous populations. Although we do not currently recommend routine CYP2D6 testing for women who do not have alternative standard therapies, the use of concomitant strong or intermediate inhibitors of CYP2D6 should be avoided if feasible. This review summarizes the literature to date with a focus on clinically relevant recent studies that examined the association between CYP2D6 polymorphisms and tamoxifen-associated outcomes.

Pharmacogenomics of tamoxifen and aromatase inhibitors

In selection of therapy for women with breast cancer, the focus has been almost exclusively on the characteristics of the tumor, eg, estrogen receptor (ER) and HER-2. Until recently, essentially no attention has been paid to the host and her genetic makeup as it relates to the metabolism of different drugs. The first real clinical application of pharmacogenetics in breast cancer management relates to tamoxifen's biotransformation to active anticancer metabolites. New information has arisen on the metabolism of tamoxifen to the active metabolite, 4 hydroxy-N-desmethyl-tamoxifen (endoxifen). Endoxifen is a metabolite with antitumor activity and affinity for the ER that is similar to 4-hydroxy-tamoxifen, but 1 that is normally present in substantially higher concentrations. CYP2D6 plays a central role in the metabolism to endoxifen and 1 published study shows that genotypic differences in CYP2D6 and use of CYP2D6 inhibitors has an impact on outcomes of women treated with tamoxifen. The aromatase inhibitors represent a major class of drugs in the armamentarium against breast cancer. The aromatase gene has been resequenced and functional genomics have been performed on the identified nonsynonymous coding single nucleotide polymorphisms showing significant decreases in levels of activity. These findings are consistent with a hypothesis that genetic variation in the CYP19 gene might be important in the activity of aromatase inhibitors. Currently, the emphasis is on examining multiple genes (thus pharmacogenomics) in pharmacodynamic and pharmacokinetic pathways in women receiving aromatase inhibitors for breast cancer.

Pharmacogenetics of Tamoxifen Biotransformation Is Associated With Clinical Outcomes of Efficacy and Hot Flashes

Polymorphisms in tamoxifen metabolizing genes affect the plasma concentration of tamoxifen metabolites, but their effect on clinical outcome is unknown. We determined cytochrome P450 (CYP)2D6 (*4 and *6) and CYP3A5 (*3) genotype from paraffin-embedded tumor samples and buccal cells (living patients) in tamoxifen-treated women enrolled onto a North Central Cancer Treatment Group adjuvant breast cancer trial. The relationship between genotype and disease outcome was determined using the log-rank test and Cox proportional hazards modeling. Paraffin blocks were obtained from 223 of 256 eligible patients, and buccal cells were obtained from 17 living women. CYP2D6 (*4 and *6) and CYP3A5 (*3) genotypes were determined from 190, 194, and 205 patient samples and in 17 living women. The concordance rate between buccal and tumor genotype was 100%. Women with the CYP2D6 *4/*4 genotype had worse relapse-free time (RF-time; P = .023) and disease-free survival (DFS; P = .012), but not overall survival (P = .169) and did not experience moderate to severe hot flashes relative to women heterozygous or homozygous for the wild-type allele. In the multivariate analysis, women with the CYP2D6 *4/*4 genotype still tended to have worse RFS (hazard ratio [HR], 1.85; P = .176) and DFS (HR, 1.86; P = .089). The CYP3A5*3 variant was not associated with any of these clinical outcomes. In tamoxifen-treated patients, women with the CYP2D6 *4/*4 genotype tend to have a higher risk of disease relapse and a lower incidence of hot flashes, which is consistent with our previous observation that CYP2D6 is responsible for the metabolic activation of tamoxifen to endoxifen.

Characterization of the Human Cytochrome P450 Forms Involved in Metabolism of Tamoxifen to Its α-Hydroxy and α,4-Dihydroxy Derivatives

Tamoxifen is a known hepatocarcinogen in rats and is associated with an increased incidence of endometrial cancer in patients. One mechanism for these actions is via bioactivation, where reactive metabolites are generated that are capable of binding to DNA or protein. Several metabolites of tamoxifen have been identified that appear to predispose to adduct formation. These include alpha-hydroxytamoxifen, alpha,4-dihydroxytamoxifen, and alpha-hydroxy-N-desmethyltamoxifen. Previous studies have shown that cytochrome P450 (P450) enzymes play an important role in the biotransformation of tamoxifen. The aim of our work was to determine which P450 enzymes were capable of producing alpha-hydroxylated metabolites from tamoxifen. When tamoxifen (18 or 250 microM) was used as the substrate, P450 3A4, and to a lesser extent, P450 2D6, P450 2B6, P450 3A5, P450 2C9, and P450 2C19 all produced a metabolite with the same HPLC retention time as alpha-hydroxytamoxifen at either substrate concentration tested. This peak was well-separated from 4-hydroxy-N-desmethyltamoxifen, which eluted substantially later under the chromatographic conditions used. No alpha,4-dihydroxytamoxifen was detected in incubations with any of the forms with tamoxifen as substrate. However, when 4-hydroxytamoxifen (100 microM) was used as the substrate, P450 2B6, P450 3A4, P450 3A5, P450 1B1, P450 1A1, and P450 2D6 all produced detectable concentrations of alpha,4-dihydroxytamoxifen. These studies demonstrate that multiple human P450s, including forms found in the endometrium, may generate reactive metabolites in women undergoing tamoxifen therapy, which could subsequently play a role in the development of endometrial cancer.

Biotransformation of tamoxifen in a human endometrial explant culture model

Although long-term tamoxifen therapy is associated with increased risk of endometrial cancer, little is known about the ability of endometrial tissue to biotransform tamoxifen to potentially reactive intermediates, capable of forming DNA adducts. The present study examined whether explant cultures of human endometrium provide a suitable in vitro model to investigate the tissue-specific biotransformation of tamoxifen. Fresh human endometrial tissue, microscopically uninvolved in disease, was cut into 1×2-mm uniform explants and incubated with media containing either 25 or 100 μM tamoxifen in a 24-well plate. Metabolites were analyzed by reversed-phase HPLC using postcolumn, online, photochemical activation and fluorescence detection. Three metabolites, namely, α-hydroxytamoxifen, 4-hydroxytamoxifen, and N-desmethyltamoxifen were identified in culture medium and tissue lysates. N-desmethyltamoxifen was found to be the major metabolite in both tissue and media extracts of tamoxifen-exposed explants. Incubations of tamoxifen with recombinant human cytochrome P-450s (CYPs) found that CYP2C9 and CYP2D6 produced all three of the above tamoxifen metabolites, while CYP1A1 and CYP3A4 catalyzed the formation of α-hydroxytamoxifen and N-desmethyltamoxifen, and CYP1A2 and CYP1B1 only formed the α-hydroxy metabolite. CYP2D6 exhibited the greatest activity for the formation of all three tamoxifen metabolites. Western immunoblots of microsomes from human endometrium detected the presence of CYPs 2C9, 3A, 1A1 and 1B1 in fresh endometrium, while CYPs 2D6 and 1A2 were not detected. Immunohistochemical (IHC) analysis also confirmed the presence of CYPs 2C9, 3A and 1B1 in fresh human endometrium and in viable tissue cultured for 24 h with or without tamoxifen. Together, the results support the use of explant cultures of human endometrium as a suitable in vitro model to investigate the biotransformation of tamoxifen in this target tissue. In addition, the results support the role of CYPs 2C9, 3A, 1A1 and 1B1 in the biotransformation of tamoxifen, including the formation of the DNA reactive α-hydroxytamoxifen metabolite, in human endometrium.

Antioxidant inhibits tamoxifen–DNA adducts in endometrial explant culture

Fresh human endometrial explants were incubated for 24 h at 37 °C with either tamoxifen (10–100 μM) or the vehicle (0.1% ethanol). Three metabolites namely, α-hydroxytamoxifen, 4-hydroxytamoxifen, and N-desmethyltamoxifen were identified in the culture media. Tissue size was limited but DNA adducts formed by the α-hydroxytamoxifen pathway were detected using authentic α-(deoxyguanosyl- N 2) tamoxifen standards. Relative DNA-adduct levels of 2.45, 1.12, and 0.44 per 10 6 nucleotides were detected following incubations with 100, 25, and 10 μM tamoxifen, respectively. The concurrent exposure of the explants to 100 μM tamoxifen with 1 mM ascorbic acid reduced the level of α-hydroxytamoxifen substantially (68.9%). The formation of tamoxifen–DNA adducts detectable in the explants from the same specimens exposed to 100 μM tamoxifen with 1 mM ascorbic acid were also inhibited. These results support the role of oxidative biotransformation of tamoxifen in the subsequent formation of DNA adducts in this tissue.

Metabolism of tamoxifen by recombinant human cytochrome P450 enzymes: formation of the 4-hydroxy, 4'-hydroxy and N-desmethyl metabolites and isomerization of trans-4-hydroxytamoxifen.

The cytochrome P450 (P450)-mediated biotransformation of tamoxifen is important in determining both the clearance of the drug and its conversion to the active metabolite, trans-4-hydroxytamoxifen. Biotransformation by P450 forms expressed extrahepatically, such as in the breast and endometrium, may be particularly important in determining tissue-specific effects of tamoxifen. Moreover, tamoxifen may serve as a useful probe drug to examine the regioselectivity of different forms. Tamoxifen metabolism was investigated in vitro using recombinant human P450s. Forms CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5, and 3A7 were coexpressed in Escherichia coli with recombinant human NADPH-cytochrome P450 reductase. Bacterial membranes were harvested and incubated with tamoxifen or trans-4-hydroxytamoxifen under conditions supporting P450-mediated catalysis. CYP2D6 was the major catalyst of 4-hydroxylation at low tamoxifen concentrations (170 +/- 20 pmol/40 min/0.2 nmol P450 using 18 microM tamoxifen), but CYP2B6 showed significant activity at high substrate concentrations (28.1 +/- 0.8 and 3.1 +/- 0.5 nmol/120 min/0.2 nmol P450 for CYP2D6 and CYP2B6, respectively, using 250 microM tamoxifen). These two forms also catalyzed 4'-hydroxylation (13.0 +/- 1.9 and 1.4 +/- 0.1 nmol/120 min/0.2 nmol P450, respectively, for CYP2B6 and CYP2D6 at 250 microM tamoxifen; 0.51 +/- 0.08 pmol/40 min/0.2 nmol P450 for CYP2B6 at 18 microM tamoxifen). Tamoxifen N-demethylation was mediated by CYP2D6, 1A1, 1A2, and 3A4, at low substrate concentrations, with contributions by CYP1B1, 2C9, 2C19 and 3A5 at high concentrations. CYP1B1 was the principal catalyst of 4-hydroxytamoxifen trans-cis isomerization but CYP2B6 and CYP2C19 also contributed.

Influence of Polyethylene Glycol and Acetone on the in Vitro Biotransformation of Tamoxifen and Alprazolam by Human Liver Microsomes

The use of in vitro hepatic microsomal models as a method of studying the biotransformation of xenobiotics can be complicated by the insolubility of a lipophilic substrate. Addition of solvent to an in vitro system is an approach used to increase solubility of such substrates, but solvents may interact with the microsomal enzymes and cause changes in the Kinetic parameters. This study focused on the solvents polyethylene glycol-400 (PEG) and acetone and their influence on the human microsomal in vitro biotransformation of the antineoplastic agent tamoxifen (TAM). The antianxiety agent alprazolam (ALP), a verified P450 3A substrate, was also studied. TAM is a nonpolar drug that is metabolized to two oxidative metabolites, N-desmethyltamoxifen (DMT) and 4-hydroxytamoxifen (4-OH-TAM), by cytochrome P450 isoforms. DMT is formed primarily by 3A isoforms, and the pathway(s) responsible for 4-OH-TAM formation are unknown. Biotransformation of ALP is mediated by P450 3A isoforms, which form α-hydroxyalprazolam (α-OH-ALP) and 4-hydroxyalprazolam (4-OH-ALP). Both PEG and acetone at 5% of the total microsomal incubation volume were found to solubilize TAM in the in vitro system. However, both solvents had an effect on the P450 mediated metabolism of ALP and TAM. For ALP, PEG was a noncompetitive inhibitor of α-OH-ALP (mean Ki=2.06%) and 4-OH-ALP (mean Ki=2.37%) formation. Acetone stimulated the production of α-OH-ALP, but had no apparent influence on 4-OH-ALP formation. The solvent's influence on TAM metabolism varied. PEG decreased the amount of DMT formed by the microsomes as compared to the system containing no solvent (control); however, 4-OH-TAM formation in the presence of PEG was 146–226% of controls. Samples containing acetone produced smaller quantities of both DMT (39–63%) and 4-OH-TAM (45–69%) as compared to controls. Since PEG and acetone increased the solubility of TAM in the incubation buffer but inhibited or accelerated enzymatic reactions compared to buffer alone, actual solubility in buffer was not a determinant of the rate of TAM metabolism. TAM appeared to be taken up into the microsomal fraction, making it available for biotransformation by the P450 isoforms. Although solvents may increase the solubility of nonpolar agents in aqueous systems, careful evaluation of the effects of solvent on metabolite formation, as compared to buffer controls, is needed in order to properly evaluate an in vitro metabolic model.

Solid-phase extraction and high-performance liquid chromatographic determination of tamoxifen and its major metabolites in plasma

Tamoxifen (TAM) is a triphenylethylene anti-oestrogen, commonly used in the treatment of breast cancer. Patients receiving tamoxifen therapy may experience both de novo and acquired resistance. As one of the mechanisms for this may be extensive peripheral bio-transformation of tamoxifen, there has been considerable interest in the pharmacokinetics and metabolism of tamoxifen. A reversed-phase high-performance liquid chromatography separation has been developed to determine the levels of tamoxifen and its major metabolites in human plasma. The method is highly sensitive (2 ng/ml) and selective for tamoxifen, cis-tamoxifen (CIS), 4-hydroxytamoxifen (4-OH) and desmethyltamoxifen (DMT). A μBondapak C 18 10 μm column (30 cm × 3.9 mm I.D.) was used, with a mobile phase of methanol-1% triethylamine at pH 8 (89:11, v/v). Sample preparation was carried out using a C 2 (500 mg sorbent, 3 ml reservoirs) solid phase extraction method, and extraction efficiencies were approximately 60% for TAM and its metabolites. Accuracy and precision, as determined by spiking plasma samples with a mixture of tamoxifen and its metabolites, ranged from 85–110% (± 5–10%) at 1 μg/ml, 101–118% (± 8–20%) at 0.1 μg/ml and 111–168% (± 43–63%) at 0.01 μg/ml. Results from 59 patients show mean values of 54 ng/ml for 4-OH; 190 ng/ml for DMT; 93 ng/ml for TAM and 30 ng/ml for CIS (detected in three patients only). This methodology can be applied routinely to the determination of TAM and its metabolites in plasma from patients undergoing therapy.