Induction of CYP2B1 and 3A1, and associated monoxygenase activities by tamoxifen and certain analogues in the livers of female rats and mice

Xenobiotic metabolism in the fish hepatic cell lines Hepa-E1 and RTH-149, and the gill cell lines RTgill-W1 and G1B: Biomarkers of CYP450 activity and oxidative stress

The use of fish cell cultures has proven to be an effective tool in the study of environmental and aquatic toxicology. Valuable information can be obtained from comparisons between cell lines from different species and organs. In the present study, specific chemicals were used and biomarkers (e.g. 7-Ethoxyresorufin-O-deethylase (EROD) activity and reactive oxygen species (ROS)) were measured to assess the metabolic capabilities and cytotoxicity of the fish hepatic cell lines Hepa-E1 and RTH-149, and the fish gill cell lines RTgill-W1 and G1B. These cell lines were exposed to β-naphthoflavone (BNF) and benzo[a]pyrene (BaP), the pharmaceutical tamoxifen (TMX), and the organic peroxide tert-butylhydroperoxide (tBHP). Cytotoxicity in gill cell lines was significantly higher than in hepatic cells, with BNF and TMX being the most toxic compounds. CYP1-like associated activity, measured through EROD activity, was only detected in hepatic cells; Hepa-E1 cells showed the highest activity after exposure to both BNF and BaP. Significantly higher levels of CYP3A-like activity were also observed in Hepa-E1 cells exposed to TMX, while gill cell lines presented the lowest levels. Measurements of ROS and antioxidant enzymes indicated that peroxide levels were higher in gill cell lines in general. However, levels of superoxide were significantly higher in RTH-149 cells, where no distinctive increase of superoxide-related antioxidants was observed. The present study demonstrates the importance of selecting adequate cell lines in measuring specific metabolic parameters and provides strong evidence for the fish hepatocarcinoma Hepa-E1 cells to be an excellent alternative in assessing metabolism of xenobiotics, and in expanding the applicability of fish cell lines for in vitro studies.

Bioactivation of the cancer chemopreventive agent tamoxifen to quinone methides by cytochrome P4502B6 and identification of the modified residue on the apoprotein

The nonsteroidal antiestrogen tamoxifen was introduced as a treatment for breast cancer 3 decades ago. It has also been approved as a chemopreventive agent and is prescribed to women at high risk for this disease. However, several studies have shown that use of tamoxifen leads to increased risk of endometrial cancer in humans. One potential pathway of tamoxifen toxicity could involve metabolism via hydroxylation to give 4-hydroxytamoxifen (4OHtam), which may be further oxidized to form a quinone methide. CYP2B6 is a highly polymorphic drug-metabolizing enzyme, and it metabolizes a number of clinically important drugs. Earlier studies from our laboratory have shown that tamoxifen is a mechanism-based inactivator of CYP2B6. The aim of the current study was to investigate the possible formation of reactive intermediates through detection of protein covalent binding and glutathione ethyl ester adduct (GSHEE) formation. The incubation of tamoxifen with 2B6 gave rise to an adduct of 4OHtam with glutathione, which was characterized as the 4OHtam quinone methide + GSHEE with an m/z value of 719, and the structure was characterized by liquid chromatography-tandem mass spectrometry. The metabolic activation of tamoxifen in the CYP2B6 reconstituted system also resulted in the formation of an adduct to the P4502B6 apoprotein, which was identified using liquid chromatography mass spectrometry. The site responsible for the inactivation of CYP2B6 was determined by proteolytic digestion and identification of the labeled peptide. This revealed a tryptic peptide ¹⁸⁸FHYQDQE¹⁹⁴ with the site of adduct formation localized to Gln193 as the site modified by the reactive metabolite formed during tamoxifen metabolism.

Induction of hepatic cytochrome P450 enzymes: methods, mechanisms, recommendations, and in vitro-in vivo correlations

Induction of drug-clearance pathways (Phase 1 and 2 enzymes and transporters) can have important clinical consequences. Inducers can (1) increase the clearance of other drugs, resulting in a decreased therapeutic effect, (2) increase the activation of pro-drugs, causing an alteration in their efficacy and pharmacokinetics, and (3) increase the bioactivation of drugs that contribute to hepatotoxicity via reactive intermediates. Nuclear receptors are key mediators of drug-induced changes in the expression of drug-clearance pathways. However, species differences in nuclear receptor activation make the prediction of cytochrome P450 (CYP) induction in humans from data derived from animal models problematic. Thus, in vitro human-relevant model systems are increasingly used to evaluate enzyme induction. In this review, the authors' current understanding of the mechanisms of enzyme induction and the in vitro methods for assessing the induction potential of new drugs will be discussed. Relevant issues and considerations surrounding proper study design and the interpretation of in vitro results will be discussed in light of the current US Food and Drug Administration (FDA) recommendations.

Suppression of mammary gland carcinogenesis by post-initiation treatment of rats with tamoxifen or indole-3-carbinol or their combination

This study examined whether suppression of mammary gland carcinogenesis elicited by low doses of tamoxifen (TAM) can be enhanced by concomitant treatment of rats with indole-3-carbinol (I3C), a component of cruciferous vegetables and a dietary supplement used for its putative antiestrogenicity. Two weeks after one oral dose of 7,12-dimethylbenz[a]anthracene (DMBA) at 65 mg/kg body weight, female Sprague-Dawley rats started treatment with TAM (10 microg/rat) by subcutaneous injection, I3C (250 mg/kg body weight) by oral gavage, TAM+I3C or their respective vehicles three times per week, for up to 20 weeks. Significant increases in the median latency of malignant mammary tumors and decreases in the mean tumor mass per rat were due to TAM. Significant decreases in the mean tumor number per rat in TAM, I3C and TAM+I3C-treated rats indicated a cooperative effect of the two compounds. In both DMBA-initiated and uninitiated rats, significant increases in the ratios of liver to body weight in I3C and TAM+I3C-treated groups coincided with I3C-dependent increases of hepatic cytochrome P450 levels and activities (1A1, 1A2 and 2B1/2). The ratios of uterus to body weight decreased with the number of treatments and the decreases effected by TAM were greater than those by I3C. The levels of circulating estrone were increased in response to I3C treatment and were greater in DMBA-initiated rats than in uninitiated rats, which may contribute to the preventive effect of I3C. Chemoprevention may be accomplished through up-regulation of apoptotic enzyme (caspase) activities in the mammary gland or mammary tumors. Treatment with TAM, I3C or TAM+I3C had no effect on caspase-3&7, caspase-6, caspase-8 and caspase-9 activities in the mammary tumors or mammary gland of tumor-bearing rats or that of uninitiated rats. In the mammary gland of DMBA-initiated tumor-free rats, however, I3C treatment increased the levels of caspase-3&7 and caspase-9 activities, suggesting an I3C-mediated protective effect. Even though I3C alone is a much less effective suppressing agent of mammary carcinogenesis than TAM, I3C in combination with TAM does not weaken but may foster the benefits of chemoprevention with TAM.

Biotransformation of letrozole in rat liver microsomes: effects of gender and tamoxifen

The in vitro metabolic kinetics of letrozole were investigated by incubating letrozole (10-500 microM) in female or male rat liver microsomes to assess the effect of gender and to predict the in vivo biotransformation characteristics of letrozole in rats. The effects of tamoxifen (TAM) on the metabolic kinetics of letrozole were also examined by incubating letrozole in female rat liver microsomes in the presence or absence of TAM. The effects of chronic pretreatment of female rats with TAM (0.5, 1.0, 5.0 mg/kg/day, i.p. for 7 consecutive days) on liver microsomal protein content and metabolic activity were also examined. The formation rate of the carbinol metabolite of letrozole, CGP44 645, was significantly higher (p<0.05) in male rat liver microsomes in comparison to female. The V(max)/K(m) ratio for letrozole metabolism in female rat liver microsomes did not change significantly (p>0.05) in the presence of TAM. After chronic pretreatment of female rats with TAM (up to a dose of 1.0mg/kg/day), the hepatic microsomal protein content was significantly increased but the formation rate of CGP44 645, when normalized for protein content, did not change significantly. These results suggest that there is a marked gender difference in letrozole metabolism in rats. It also appears that acute treatment of female rat liver microsomes with TAM produces negligible inhibitory effect on the CYP mediated metabolic clearance of letrozole. However, chronic pretreatment of female rats with TAM appear to induce CYPs, but does not significantly impact the metabolic activities of the enzymes associated with the formation of the carbinol metabolite of letrozole.

CYP Induction-Mediated Drug Interactions: in Vitro Assessment and Clinical Implications

Cytochrome P450 (CYP) induction-mediated interaction is one of the major concerns in clinical practice and for the pharmaceutical industry. There are two major issues associated with CYP induction: a reduction in therapeutic efficacy of comedications and an induction in reactive metabolite-induced toxicity. Because CYP induction is a metabolic liability in drug therapy, it is highly desirable to develop new drug candidates that are not potent CYP inducer to avoid the potential of CYP induction-mediated drug interactions. For this reason, today, many drug companies routinely include the assessment of CYP induction at the stage of drug discovery as part of the selection processes of new drug candidates for further clinical development. The purpose of this article is to review the molecular mechanisms of CYP induction and the clinical implications, including pharmacokinetic and pharmacodynamic consequences. In addition, factors that affect the degree of CYP induction and extrapolation of in vitro CYP induction data to in vivo situations will also be discussed. Finally, assessment of the potential of CYP induction at the drug discovery and development stage will be discussed.

The effect of tamoxifen on the pharmacokinetics of letrozole in female rats

The effects of single doses of tamoxifen (TAM; 0.5-5 mg/kg, i.v.) and chronic pretreatment with TAM (0.1-5.0 mg/kg/day, i.p. for 7 consecutive days) on letrozole (0.5 mg/kg, i.v.) pharmacokinetics were evaluated in female Sprague-Dawley rats. The plasma concentration-time profiles of letrozole (0.1-2.0 mg/kg) after single i.v. doses were analysed by the non-compartment model with terminal half-lives (t(1/2,lambdaz)) ranging from 34.3 to 37.5 h. The volume of distribution at the terminal phases (Vd(lambdaz)) ranged from 1.9 to 2.1 l/kg and clearance (CL) varied from 0.036 to 0.042 l/(h.kg). After co-administration of TAM and letrozole intravenously, the t1/2, Vd(lambdaz) and CL of letrozole were not significantly altered. Chronic pretreatment with TAM significantly decreased the t1/2 of letrozole by about 33%, and increased its clearance by an average of 40%. However, TAM pretreatment did not significantly affect the Vd(lambdaz) of letrozole in female rats. Co-administration of letrozole and TAM orally increased the absorption half-life of letrozole threefold although the absolute bioavailability remained unchanged. These observations suggest that single oral doses of TAM delay the absorption of letrozole while chronic pretreatment with TAM accelerates the elimination of letrozole, probably due to induction of cytochrome P450 enzymes in rats.

Persistent suppression of hepatic CYP2A1 expression and serum triiodothyronine levels by tamoxifen in intact female rats: dose-response analysis and comparison with 4-hydroxytamoxifen, fulvestrant (ICI 182,780), and 17beta-estradiol-3-benzoate

Tamoxifen, a nonsteroidal antiestrogen, is used widely in the treatment of breast cancer and is undergoing evaluation as a chemopreventive agent. In this study, we investigated several long-term effects of tamoxifen in intact adult female rats following acute treatment at various dosages. The effects of tamoxifen on somatic growth, growth hormone (GH) levels, thyroid hormone levels, and on hepatic cytochrome P450 (P450) expression were compared with those of fulvestrant (ICI 182,780), 17beta-estradiol-3-benzoate, and 4-hydroxytamoxifen under the same experimental conditions. Each compound was injected s.c. for two consecutive days, and rats were killed 37 days after treatment. Tamoxifen decreased body weight and serum triiodothyronine (T3) levels at dosages ranging from 0.5 to 200 mg/kg. Ovary weight, uterus weight, peak plasma GH concentration, and hepatic CYP2A1 content were decreased 37 days after treatment with tamoxifen at a dosage of 20 mg/kg, but expression of other P450 enzymes was not affected. However, tamoxifen and 4-hydroxytamoxifen could not be detected in plasma by high performance liquid chromatography analysis at this time, which suggests that the effects of tamoxifen were mediated indirectly. 4-Hydroxytamoxifen exhibited effects similar to those of tamoxifen, indicating that this metabolite contributes to the in vivo activity of tamoxifen. Estradiol benzoate decreased CYP2A1 and increased CYP3A hepatic levels, but had no effect on serum T3 concentration. In contrast, treatment with ICI 182,780 had little or no effect on the endpoints measured. In summary, 2-day tamoxifen treatment of intact adult female rats resulted in persistent suppression of somatic growth, serum T3 levels, and hepatic CYP2A1 expression.

Induction of cytochrome P450 3A4 in primary human hepatocytes and activation of the human pregnane X receptor by tamoxifen and 4-hydroxytamoxifen

Tamoxifen is a widely utilized antiestrogen in the treatment and chemoprevention of breast cancer. Clinical studies document that tamoxifen administration markedly enhances the systemic elimination of other drugs. Additionally, tamoxifen enhances its own clearance following repeated dosing. The mechanisms that underlie these clinically important events remain unresolved. Here, we report that tamoxifen and its metabolite 4-hydroxytamoxifen markedly induce cytochrome P450 3A4, a drug-metabolizing enzyme of central importance, in primary cultures of human hepatocytes. Tamoxifen and 4-hydroxytamoxifen (1-10 microM) significantly increased the CYP3A4 expression and activity (measured as the rate of testosterone 6beta-hydroxylation). Maximal induction was achieved at the 5 microM level. At this level, tamoxifen and 4-hydroxytamoxifen caused a 1.5- to 3.3-fold (mean, 2.1-fold) and 3.4- to 17-fold (mean, 7.5-fold) increase in the CYP3A4 activity, respectively. In comparison, rifampicin treatment resulted in a 6- to 16-fold (mean, 10.5-fold) increase. We also observed corresponding increase in the CYP3A4 immunoreactive protein and mRNA levels. Furthermore, tamoxifen and 4-hydroxytamoxifen efficaciously activated the human pregnane X receptor (hPXR; also known as the steroid xenobiotic receptor), a key regulator of CYP3A4 expression. The efficacy of tamoxifen and 4-hydroxytamoxifen relative to rifampicin for hPXR activation was approximately 30 and 60%, respectively. Our results indicate that the mechanism of tamoxifen-mediated alteration in drug clearance pathways in humans may involve CYP3A4 induction by the parent drug and/or its metabolite. Furthermore, the CYP3A4 induction may be a result of hPXR activation. These findings have important implications for optimizing the use of tamoxifen and in the development of newer antiestrogens.

Bilirubin and uroporphyrinogen oxidation by induced cytochrome P4501A and cytochrome P4502B. Role of polyhalogenated biphenyls of different configuration

In previous work it was shown that hepatic microsomes from rats treated with 3-methylcholanthrene and similar inducers had increased bilirubin-degrading activity. The activity was further stimulated by addition of 3,4-tetrachlorobiphenyl (TCB), a response specifically dependent on CYP1A1. Here, we compared the effect of adding PCBs of either planar or non-planar configuration on rate of bilirubin degradation, monooxygenase activity and NADPH/O(2) consumption by liver microsomes from animals treated with either phenobarbital or 3-methylcholanthrene/beta-naphthoflavone. We also examined the oxidation of uroporphyrinogen (hexahydro-uroporphyrin) (URO'gen) under these conditions. Polychlorinated biphenyl (PCBs) stimulated the rate of bilirubin and URO'gen oxidation with microsomes expressing high levels of either CYP2B or CYP1A, inhibiting at the same time their monooxygenase activities (PROD and EROD, respectively); however, non-planar di-ortho-substituted PCBs were preferentially active with phenobarbitone-induced microsomes, in contrast to those active with 3-methylcholanthrene/beta-naphthoflavone microsomes, where a planar configuration was required for activity. An antibody raised against CYP2B1 markedly inhibited the PCB-dependent bilirubin degradation and PROD activities of phenobarbital-induced microsomes with similar dose-response curves for the two effects. Increased microsomal utilizations of NADPH and O(2) were also caused by PCBs with both types of induced microsomes and here again PCBs of different configuration were preferentially active. It is concluded that PCBs of the appropriate configuration may interact with either CYP1A1 or CYP2B1, increase production of oxidative species by an uncoupling mechanism, and lead to oxidation of target molecules in the cell, among these uroporphyrinogen and bilirubin.

Molecular and pharmacokinetic evaluation of rat hepatic and gastrointestinal cytochrome p450 induction by tamoxifen

Tamoxifen (TAM) is a first-line endocrine treatment for all stages of postmenopausal breast cancer. The cytochrome P450 (CYP) enzymes catalyze the majority of TAM's primary metabolism, producing N-desmethyltamoxifen (DMT) and 4-hydroxytamoxifen (4-OH-TAM) in both humans and rats. CYP 3A isoforms are the predominant subfamily involved in the formation of DMT and recent studies have shown that TAM induces hepatic forms of these enzymes. TAM's inductive effect on gastrointestinal CYP 3A has not been previously reported. The current studies investigated TAM's induction of CYP isoforms (3A and 2B) in female rat gastrointestinal and hepatic tissue at the mRNA, protein, and catalytic level. Since previous studies have not addressed whether TAM induction causes changes to the overall pharmacokinetics (PKs), a rat PK model was used to determine if TAM induced its own metabolism, and/or the metabolism of a CYP 3A substrate, midazolam (MDZ). Phenobarbital (PB) and/or dexamethasone (DEX) were used as positive controls for all studies. TAM significantly induced, or caused a trend towards induction of all studied parameters for hepatic CYP 3A and 2B, whereas intestinal CYP 3A and 2B analysis did not show significant induction by TAM at any level. A study evaluating time-dependent alterations in the PK profile of TAM showed no change in apparent oral clearance (Cl(app)) during two weeks of chronic dosing with TAM. However, the Cl(app) for MDZ was shown to trend towards an increase after two weeks of dosing with TAM, in a second PK study. These combined investigations suggest that TAM is an inducer of rat hepatic CYP 3A and 2B isoforms, and this agent has the potential of influencing the PK of coadministered 3A substrates.

On the promoting action of tamoxifen in a model of hepatocarcinogenesis induced by p-dimethylaminoazobenzene in CF1 mice

BACKGROUND AND AIMS

Tamoxifen (TMX) has proven to be an effective palliative treatment for advanced breast cancer with low reported incidence of side effects. TMX has been demonstrated to be an initiator and/or a promoter in the rat model of hepatocarcinogenesis. To document the long-term effect of TMX in mice treated with p-dimethylaminoazobenzene (DAB), we have investigated the time response action of these drugs on different biochemical parameters.

METHODS

A group of animals was placed on dietary DAB (0.5%, w/w) during a period of 28 weeks. Control animals received a standard laboratory diet. Two other groups of non-treated and DAB-treated animals received TMX citrate (0.025%, w/w) in the diet since day 20.

RESULTS

The activities of the enzymes involved in heme synthesis and degradation as evaluated in the DAB group was not further affected by TMX. DAB and/or TMX treatment significantly increased the content of total cytochrome P450 and also the activity of glutathione S-transferase indicating liver damage. In all treated groups oxidative stress and an adaptive response of the natural defense system (catalase and superoxide dismutase) were demonstrated. Histological and morphological studies revealed liver cell hyperplasia in DAB treated group; however, only in the DAB+TMX group solid, trabecular and acinar hepatocellular carcinoma was confirmed at the end of the experimental trial.

CONCLUSION

We have demonstrated that TMX produced changes in hepatic enzyme activities which may be relevant for the metabolism and disposition of this and/or other drugs. Because liver tumors could be initiated and promoted by several agents which need to be activated, the possible hazard of TMX should be considered. This study reports that long-term treatment with TMX enhances hepatocarcinogenesis induced by DAB. The widespread use of TMX as an anticancer agent adds to the significance of this study.

Chemoprevention of breast cancer by tamoxifen: risks and opportunities

The antiestrogen tamoxifen is widely used in the adjuvant therapy of breast cancers in women and helps to prevent the occurrence of breast tumors in healthy women. However, epidemiological studies have shown tamoxifen treatment to be associated with a 2- to 5-fold increased risk of endometrial cancer. In rats but not in mice, long-term administration of tamoxifen results in an increase in hepatocellular carcinomas. Mechanistically, this occurs through metabolic activation of the drug, mainly by the CYP3A family, to an electrophilic species, that causes DNA damage in target tissues, and subsequently leads to gene mutations. It is controversial whether low levels of DNA damage occur in human uterine tissues, and there is no evidence that this can be causally related to the mechanisms of carcinogenesis. In healthy women, the risk:benefits for the use of tamoxifen is in part related to the risk of developing breast cancer. The results from the carcinogenicity studies in rats do not predict the likelihood that women will develop liver cancer or indeed cancers in other organs. The mechanism of endometrial cancer in women remains unresolved, but the experience with tamoxifen has highlighted the potential problems that need to be addressed in the assessment of future generations of selective estrogen receptor modulators.

Comparison of the DNA adducts formed by tamoxifen and 4-hydroxytamoxifen in vivo

Tamoxifen is a liver carcinogen in rats and has been associated with an increased risk of endometrial cancer in women. Recent reports of DNA adducts in leukocyte and endometrial samples from women treated with tamoxifen suggest that it may be genotoxic to humans. One of the proposed pathways for the metabolic activation of tamoxifen involves oxidation to 4-hydroxytamoxifen, which may be further oxidized to an electrophilic quinone methide. In the present study, we compared the extent of DNA adduct formation in female Sprague-Dawley rats treated by gavage with seven daily doses of 54 micromol/kg tamoxifen or 4-hydroxytamoxifen and killed 24 h after the last dose. Liver weights and microsomal rates of ethoxyresorufin O-deethylation, 4-dimethylaminopyrine N-demethylation and p-nitrophenol oxidation were not altered by tamoxifen or 4-hydroxytamoxifen treatment. Uterine weights were decreased significantly and uterine peroxidase activity was decreased marginally in treated as compared with control rats. DNA adducts were assayed by 32P-post-labeling in combination with HPLC. Two major DNA adducts were detected in liver DNA from rats administered tamoxifen. These adducts had retention times comparable with those obtained from in vitro reactions of alpha-acetoxytamoxifen and 4-hydroxytamoxifen quinone methide with DNA. Hepatic DNA adduct levels in rats administered 4-hydroxytamoxifen did not differ from those observed in control rats. Likewise, adduct levels in uterus DNA from rats treated with tamoxifen or 4-hydroxytamoxifen were not different from those detected in control rats. These data suggest that a metabolic pathway involving 4-hydroxytamoxifen is not a major pathway in the activation of tamoxifen to a DNA-binding derivative in Sprague-Dawley rats.

Species differences in the metabolic activation of tamoxifen into genotoxic derivatives: risk assessment in women

Rats and Rhesus monkeys are compared in their response to tamoxifen treatment, with particular reference to tamoxifen-related liver DNA damage and bioactivation of tamoxifen by isolated microsomes in vitro. Monkeys, treated with tamoxifen, accumulate in their livers a metabolite of tamoxifen, N,N-didesmethyl tamoxifen, with powerful inhibitory activity on cytochrome P450-dependent drug metabolism. The accumulation of this metabolite in the monkeys may limit the cytochrome P450-dependent conversion of tamoxifen into reactive derivatives and, in this way, protect against the formation of DNA adducts. This metabolite is also found in the liver and serum of patients taking tamoxifen, but more work is needed to determine whether inhibition of tamoxifen bioactivation also exists in the human patient in vivo and, if so, to what extent and in which organ.

Expression of cytochrome P450 genes encoding enzymes active in the metabolism of tamoxifen in human uterine endometrium

Long-term tamoxifen therapy is associated with increased risk of uterine endometrial cancer and benign alterations. Tamoxifen is metabolized to reactive intermediates by endometrial tissue, and tamoxifen therapy-induced DNA adducts have been found in human endometrium. Since metabolic activation is often catalyzed by cytochrome P450 (CYP) enzymes, the expression profile of individual xenobiotic-metabolizing CYP genes was studied in human uterine endometrium by reverse transcriptase-polymerase chain reaction. The following CYP mRNAs were detected: CYP2B6, CYP2C, CYP2E1, CYP3A4, CYP3A5, CYP4B1, and CYP11A. Amplification of CYP1A1, CYP1A2, CYP2A6, CYP2D6, CYP2F1, CYP3A7, and CYP19 was not found. CYP3A5 and CYP4B1 transcripts were found only in samples from premenopausal women. These data suggest that the human endometrial epithelium has the potential of producing CYP enzymes known to generate genotoxic intermediates from tamoxifen and metabolites that affect oestrogen receptors.

Extrahepatic cytochrome P450: role in in situ toxicity and cell-specific hormone sensitivity

It is clear that members of the Cytochrome P450 supergene family are responsible for the majority of activations of procarcinogens to ultimate carcinogens in the body. These procarcinogens include the food mutagens (heterocyclic amines), pesticides, polycyclic aromatic hydrocarbons and nitrosamines. The Cyp P450 profile in a cell can indicate the capacity of that cell to form reactive metabolites. Furthermore, environmental factors, through their action on P450s, influence the fate of procarcinogens in a cell. This is because different isoforms of P450 are regulated differently by ethanol, diet and environmental inducers, have different substrate specificities and different propensity to be inhibited or activated by dietary components. Cyp P450 (through steroid inactivation), can also influence sensitivity of cells to hormones. Age and hormone related regulation of P450 isoforms such as 1A1, 2B1 and 2A3 in the breast suggest that in situ activation of carcinogens and hormone inactivation can occur in the breast. In the brain and endometrium most of the #P450 isoforms remain to be identified.

Effects of cytochrome P450 inducers on tamoxifen genotoxicity in female mice in vivo

We recently reported that administration of the antiestrogen tamoxifen (TAM) gives rise to two groups of DNA adducts in female mouse liver in vivo, as measured by 32P-postlabeling, and provided evidence that 4-hydroxytamoxifen and alpha-hydroxytamoxifen are proximate carcinogenic metabolites leading to group I and group II adducts, respectively (Randerath et al., Carcinogenesis 15: 2087-2094, 1994). Because cytochrome P450 (CYP) enzymes play an important role in TAM metabolism, in this investigation we tested the hypothesis that induction of liver CYP enzymes may affect TAM metabolism profoundly, resulting in increased or decreased TAM-DNA adduct formation in vivo. To this end, we treated female ICR mice with TAM either alone or in combination with one of several classic CYP inducers, i.e. phenobarbital (PB), beta-naphthoflavone (BNF), and pregnenolone-16 alpha-carbonitrile (PCN), and determined the levels of 32P-postlabeled TAM-DNA adducts and the activities of several CYP-dependent enzymes. Each of the inducers greatly diminished levels of group II, but did not affect group I adducts. TAM elicited induction of benzphetamine N-demethylase activity in liver, while activities of other enzymes were not affected. TAM, when given in combination with BNF, elicited a synergistic induction of ethoxyresorufin O-deethylase (EROD) (CYP1A1) and methoxyresorufin O-demethylase (MROD) (CYP1A2) activities. Likewise, PCN given along with TAM caused synergistic induction of EROD and ethylmorphine N-demethylase activities. There was no synergism between PB and TAM, however. Overall, the results further support the existence of two pathways of TAM metabolism to DNA-reactive electrophiles and strongly suggest that the classic CYP inducers tested enhance detoxication of TAM to non-genotoxic metabolites.

Regulation of gene expression of various phase I and phase II drug-metabolizing enzymes by tamoxifen in rat liver

The objective of the present investigation was to evaluate the effect of tamoxifen (TAM) on the gene expression of different phase I and phase II drug-metabolizing enzymes. Groups of male and female F344/NCr rats were administered either corn oil or TAM (2.8 to 45 mg/kg body wt x 14 days) dissolved in corn oil by gavage. An additional group of rats received a diet supplemented with phenobarbital (PB, 500 ppm). Northern blot analyses of total liver RNA were conducted using [32P]-labeled cDNA or oligonucleotide probes coding for different sulfotransferase (ST); UDP-glucuronosyltransferase (UGT), glutathione S-transferase (GST), epoxide hydrolase (EPH) or cytochrome P450 (CYP) mRNA transcripts. In male rats, TAM increased the levels of STel, STa and STpl mRNAs, whereas PB increased only the STel mRNA. In female rats, there was no expression of STel and STHA mRNA in either control or TAM-treated animals. TAM and PB increased UGTBe/p mRNAs in all rats, whereas UGTml mRNA was elevated only in PB-treated animals. EPH mRNA was elevated markedly in all rats treated with TAM and PB, whereas GSTya/ye mRNA was highly increased by PB, but only marginally increased by TAM. Finally, TAM increased CYP3A1 mRNA, and slightly increased CYP2B1 mRNA, whereas PB highly elevated mRNAs for both of these CYP genes. In conclusion, treatments of rats with TAM increased the mRNA levels of many phase I and phase II drug-metabolizing enzymes, and this pleiotypic response to TAM seems to be different from other prototype inducers such as PB or dioxin (TCDD).

A two-year dietary carcinogenicity study of the antiestrogen toremifene in Sprague-Dawley rats

The carcinogenic potential of the nonsteroidal triphenylethylene antiestrogen toremifene (Fareston) was evaluated in a standard 104-week rat dietary carcinogenicity study. The doses were 0, 0.12, 1.2, 5.0 and 12 mg/kg/day and the number of animals 50/sex/dose group. The body weight gain and food consumption were monitored once weekly (study weeks 1-16) or once every four weeks thereafter (study weeks 17-104). Blood samples were taken at weeks 34, 52 and 104 and the plasma concentrations of toremifene, as well as the two main metabolites (deaminohydroxy)toremifene and N-demethyltoremifene, were measured. All doses of toremifene reduced food intake and body weight gain. Toremifene caused a significant reduction in mortality, which was mainly due to reduced incidences of pituitary tumors. This was evident in all dose groups. Drug-related decrease of mammary tumors in females (at all doses) and testicular tumors in male rats (doses > or = 1.2 mg/kg/day) were also evident. The incidence of the preneoplastic foci of basophilic hepatocytes were significantly decreased in treated female groups. Toremifene induced no preneoplastic or neoplastic lesions. Based on histopathology, no obvious toxicity could be observed. Drug-related changes were observed in the genital organs, thyroid, spleen, mammary gland, adrenal, kidney, stomach and lung. These changes were due to hormonal disturbances or as a result of reduced food consumption or reduced incidences of pituitary, mammary or testicular tumors. This study indicates that toremifene is an efficient antiestrogen in long-term treatment, is well tolerated and has no tumorigenic potential in rats.

Chemoprevention of breast cancer by tamoxifen: risks and opportunities

The antioestrogen tamoxifen is of proven efficacy in inhibiting the growth of oestrogen receptor positive breast cancers in women. In rats, long-term dosing leads to the development of hepatocellular tumours. Tamoxifen in this species is a genotoxic carcinogen. Metabolic activation by cytochrome P450-dependent enzymes leads to DNA damage detectable by 32P-postlabelling. Factors important in the development of hepatocellular lesions were the nature and quantity of metabolism and promotion/progression of the DNA lesion by agents such as phenobarbital and cell proliferation. No evidence was found for tamoxifen-induced DNA damage in the livers of 7 women taking this drug therapeutically.

Species differences in the covalent binding of [14C]tamoxifen to liver microsomes and the forms of cytochrome P450 involved

Species differences in the NADPH-dependent covalent binding of [14C]tamoxifen to liver microsomes have been studied using preparations from humans, female F344 rats and DBA/2 mice. Protein binding has been used as an index of metabolic activation and as a surrogate for DNA binding in order to establish which forms of cytochrome P450 are responsible for genotoxicity. A panel of 12 human liver microsomes has been characterized and immunoquantified for nine cytochrome P450 isoenzymes. Binding of tamoxifen (45 microM) (25 +/- 2.5 pmol/15 min/mg protein, mean +/- SE) correlated (P < 0.05) with CYP3A4 and CYP2B6 content. Covalent binding of [14C]tamoxifen to microsomal preparations from human breast tumour tissue could also be detected but at levels 7-fold lower than in liver. The covalent binding of tamoxifen to mice, rat or human liver microsomal preparations increased with increasing substrate concentration. Covalent binding of [14C]tamoxifen (45 microM) in rats was 3.8-fold and mice 17-fold higher than in human liver microsomal preparations. In mice, the apparent Km (9.6 +/- 1.9 microM) was very much lower than for rats (119 +/- 41 microM). Pretreatment of female rats with phenobarbitone or dexamethasone resulted in a 4- to 5-fold increase in [14C]tamoxifen binding, relative to controls, consistent with the involvement of CYP2B1 and CYP3A1 in the metabolic activation. It cannot be distinguished at present if the same reactive metabolites are involved in protein and DNA binding. The greater potential of mouse liver microsomes to activate tamoxifen, relative to rats, does not reflect DNA damage or hepatocarcinogenicity seen following dosing with tamoxifen in vivo. It is concluded that covalent binding of tamoxifen to protein in vitro cannot be directly related to the carcinogenic potential of this compound. However, in the three species investigated, results suggest that the rat is a better model than the mouse for human liver microsomal activation of tamoxifen both with respect to kinetic parameters and the pattern of metabolic products.

Studies of tamoxifen as a promoter of hepatocarcinogenesis in female Fischer F344 rats

Tamoxifen, an antiestrogen used in the treatment of breast cancer, was assessed for carcinogenic potential in the two-stage model of experimental hepatocarcinogenesis. Groups of female Fisher F344 rats were initiated with a non-necrogenic, subcarcinogenic dose of diethylnitrosamine (DEN; 10 mg/kg, po) and fed tamoxifen at a concentration of 250 mg per kg of AIN-76A diet for 6 or 15 months. The livers of these animals exhibited an increase in size and number of altered hepatic foci compared with those animals which were initiated with DEN but not exposed to tamoxifen. This finding indicates that tamoxifen may have a carcinogenic potential in the rat liver. After 6 months of treatment, neoplastic nodules were observed in 3/8 rats in the DEN-initiated, tamoxifen-treated group. In the initiated group provided with tamoxifen for 15 months, neoplastic nodules were observed in 7/8 rats and hepatocellular carcinomas in 3/8 rats. The serum level of tamoxifen in these rats was 200-300 ng/ml. The ratio of tamoxifen, 4-hydroxy tamoxifen, and N-desmethyl tamoxifen was 1:0.1:0.5-1 in the serum. When adjusted for age-related weight increases, the serum and liver levels of tamoxifen and its N-desmethyl metabolite did not change over the 15 months. In the rat liver, the level of tamoxifen and its N-desmethyl metabolite was 10-29 micrograms/g liver after 6 or 15 months of chronic dietary administration. The ratio of tamoxifen:4-hydroxy tamoxifen:N-desmethyl tamoxifen was 1:0.1.3-3.3 in the liver. Therefore, the liver had 20- to 30-fold more tamoxifen and 4-hydroxy tamoxifen and at least 100-fold more N-desmethyl tamoxifen than the serum (assuming 1 gram of tissue is equivalent to 1 ml of serum). These results indicate that tamoxifen is a promoting agent for the rat liver at serum levels found in patients given the usual therapeutic course of tamoxifen. The high concentrations of tamoxifen attained in the rat liver indicate that actions other than its known estrogenicity for liver could contribute to its promoting action. In addition, these results indicate that the pharmacodynamic differences in tamoxifen metabolism in rats and humans and at low versus high doses should be determined. Thus, the therapeutic indications for tamoxifen should be balanced by the potential risk it may present as a promoting agent in mammalian liver.