The role of CYP forms in the metabolism and metabolic activation of HCFCs and other halocarbons

The use of hydrochlorofluorocarbons (HCFCs) such as HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane) and HCFC-141b (1,1-dichloro-1-fluoroethane) is becoming widespread as replacements for the ozone depleting chlorofluorocarbons. Hepatic activation of HCFC-123 or the unsaturated perchloroethylene through oxidative pathways leads to the formation of the electrophiles trifluoroacetyl chloride or trichloroacetyl chloride, respectively. These can react with epsilon-NH(2) functions of lysine in proteins and give rise to neoantigens. In the case of HCFC-123, this reaction is catalysed primarily by CYP2E1 and to a much lesser extent by the constitutive CYP2C19, CYP2B6 and CYP2C8. For perchloroethylene, the extent of activation is less and the reaction is catalysed primarily by the CYP2B family. While acute hepatotoxicity has been seen in humans exposed to HCFC-123 or halothane, little short- or long-term toxicity in rodents is observed. No immunological related toxicity of perchloroethylene has been reported in exposed humans. Long-term exposure of rats can lead to renal tubule carcinomas and in mice, hepatocellular carcinomas. These toxic reactions do not appear to be directly related to the formation of the putative trichloroacetyl chloride intermediate.

Bioactivation and toxicity in vitro of HCFC-123 and HCFC-141b: role of cytochrome P450

The bioactivation and cytotoxicity in vitro of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1,1-dichloro-1-fluoroethane (HCFC-141b), two replacements for some ozone-depleting chlorofluorocarbons (CFC), were investigated in rat liver microsomes and isolated rat hepatocytes. Both compounds were activated by cytochrome P450 to reactive metabolites, as indicated by: (i) the depletion of exogenous and cellular glutathione, (ii) the increased LDH release from hepatocytes, (iii) the loss of microsomal P450 content and activities, and (iv) the formation of free radical species observed in the presence of the two compounds. Moreover, the formation of two stable metabolites and an increased production of conjugated dienes, a marker of lipid peroxidation, were observed for both HCFC-123 and HCFC-141b. The biotransformation of both compounds by pyridine- and phenobarbital-induced rat liver microsomes and the inhibition of LDH release by 4-methylpyrazole and troleandomycin indicate that P450 2E1, 2B and, possibly, also 3A are the isoforms involved in the bioactivation and toxicity of HCFC-123 and HCFC-141b in the rat.

Neoantigen formation and clastogenic action of HCFC-123 and perchloroethylene in human MCL-5 cells

In this study, the metabolic activation of 2,2-dichloro-1,1,1-trifluoroethane (hydrochlorofluorocarbons-123, HCFC-123), halothane or 1,1-dichloro-1-fluoroethane (HCFC-141b) was compared to that of perchloroethylene, using lymphoblastoma derived cell lines expressing human CYP1A1, CYP1A2, CYP2E1, CYP2A6 and CYP3A4 (MCL-5 cells). A dose dependent increase in micronucleus formation was detected over a nominal concentration range of 0.05-2 mM for HCFC-123 and halothane, but this was not seen with HCFC-141b. No dose response for HCFC-123 was seen in a control cHo1 cell line not expressing this cytochrome P450's. Cell lines expressing individual human cytochrome P-450 (CYP) forms were also used to define the enzymes responsible for the clastogenic events and to investigate the formation of immunoreactive protein by microsomal fractions. It was shown that CYP2E1 or CYP2B6 catalysed the clastogenic response, but CYP2D6, CYP3A4, CYP1A2 or CYP1A1 all appeared to be inactive. The formation of neoantigenic trifluoroacetylated protein adducts by microsomal mixtures incubated with HCFC-123 and NADPH was catalysed primarily by CYP2E1 and to a lesser extent by CYP2C19, whereas, only trace levels of immunoreactive protein were seen with microsomes expressing CYP2B6 or CYP2C8. With perchloroethylene as a substrate, the extent of activation was low in comparison with HCFC-123, as judged by the absence of micronuclei formation in the MCL-5 cell line and the weak immunoreactivity of proteins following Western blotting. CYP1A2, CYP2B6 and CYP2C8 appeared to be responsible for perchloroethylene immunoreactivity and in contrast to the findings with the HCFC's, no activation of perchloroethylene by CYP2E1 could be detected. These results show that even though both saturated and unsaturated halocarbons can result in neoantigen formation, there is a marked difference in the specificity of the CYP enzymes involved in their metabolic activation.

Comparisons of the effects of tamoxifen, toremifene and raloxifene on enzyme induction and gene expression in the ovariectomised rat uterus

This study compares the actions of oestradiol, tamoxifen, toremifene and raloxifene on enzyme and gene expression in uterine tissues of ovariectomised rats over 72 h. The time-course for the induction of ornithine decarboxylase by the compounds showed a rapid biphasic response, while for creatine kinase brain type (BB) there was a continued increase over 72 h. The efficacy of induction showed that, with both markers, oestradiol gave the highest induction level, followed by tamoxifen or toremifene and then raloxifene. RT-PCR demonstrated that all compounds decreased oestrogen receptor (ER) alpha, ERbeta and ERbeta2 gene expression, 8-24 h after the first dose, suggesting that down-regulation of ER is not the primary cause of the difference in efficacy between these compounds. Using cDNA arrays, expression of 512 genes was examined in the uteri of oestradiol- or tamoxifen-treated rats. Both compounds resulted in the up-regulation of heat-shock protein 27, telomerase-associated protein 1 and secretin. However, most surprising was the marked down-regulation of Wilms' tumour and retinoblastoma genes. We speculate that this may result in a loss of regulation of the transition from the G1 to the S phase in the cell cycle and may make cells more vulnerable to the carcinogenic effects of tamoxifen in this tissue.

Adenomyosis--a result of disordered stromal differentiation

Adenomyosis is a fairly frequent disorder in adult women characterized by the haphazard location of endometrial glands and stroma deep within the myometrium of the uterus. This study compared the effects on uterine development of the selective estrogen receptor modulators, tamoxifen, toremifene, and raloxifene with estradiol when given orally to female mice on days 2 to 5 after birth. Uterine adenomyosis was found in all (14 of 14) mice dosed with tamoxifen and most mice (12 of 14) treated with toremifene, but in none of the vehicle-dosed controls, in only one animal treated with raloxifene at 42 and 90 days after dosing and in none of the mice treated with estradiol at 42 days. At 6 days, the uterus in the groups that developed a high incidence of adenomyosis showed histological evidence of disturbed differentiation of the myometrium. Gene-expression XY-scatterplots using Clontech mouse 1.2 Atlas mouse cDNA expression arrays analyzing total uterine RNA showed nerve growth factor-alpha, preadipocyte factor-1, and insulin-like growth factor-2 were key genes differentially modified by tamoxifen or toremifene treatment, relative to the controls. As these genes may play an important role in regulating differentiation and development of the myometrium, these data suggest that adenomyosis may be caused primarily by defects in the formation of the myometrium.

Bioactivation and cytotoxicity of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) in isolated rat hepatocytes

The bioactivation and cytotoxicity of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123), a replacement for some ozone-depleting chlorofluorocarbons, were investigated using freshly isolated hepatocytes from non-induced male rats. A time- and concentration-dependent increase in the leakage of lactate dehydrogenase and a concentration-dependent loss of total cellular glutathione were observed in cells incubated with 1, 5 and 10 mM HCFC-123 under normoxic or hypoxic (about 4% O2) conditions. Lactate dehydrogenase leakage was completely prevented by pretreating the cell suspension with the free radical trapper N-t-butyl-alpha-phenylnitrone. The aspecific cytochrome P450 (P450) inhibitor, metyrapone, totally prevented the lactate dehydrogenase leakage from hepatocytes, while two isoform-specific P450 inhibitors, 4-methylpyrazole and troleandomycin (a P450 2E1 and a P450 3A inhibitor, respectively), provided a partial protection against HCFC-123 cytotoxicity. Interestingly, pretreatment of cells with glutathione depletors, such as phorone and diethylmaleate, did not enhance the HCFC-123-dependent lactate dehydrogenase leakage. Two stable metabolites of HCFC-123, 1-chloro-2,2,2-trifluoroethane and 1-chloro-2,2-difluoroethene, were detected by gas chromatography/mass spectrometry analysis of the head space of the hepatocyte incubations carried out under hypoxic and, although at a lower level, also normoxic conditions, indicating that reductive metabolism of HCFC-123 by hepatocytes had occurred. The results overall indicate that HCFC-123 is cytotoxic to rat hepatocytes under both normoxic and hypoxic conditions, due to its bioactivation to reactive metabolites, probably free radicals, and that P450 2E1 and, to a lower extent, P450 3A, are involved in the process.

Short-term dosing of alpha-hydroxytamoxifen results in DNA damage but does not lead to liver tumours in female Wistar/Han rats

It is now generally accepted that activation of tamoxifen occurs as a result of metabolism to alpha-hydroxytamoxifen. In this study, alpha-hydroxytamoxifen was given to female Wistar/Han rats (0.103 or 0.0103 mmol/kg, intraperitoneally, daily for 5 days). This resulted in liver DNA damage, determined by (32)P-post-labelling, of 3333 +/- 795 or 343 +/- 68 adducts/10(8) nucleotides, respectively (mean +/- SD, n = 4). Following HPLC separation, the retention times of the major alpha-hydroxytamoxifen DNA adducts were similar to those seen following the administration of tamoxifen. However, after rats were treated with alpha-hydroxytamoxifen (0.103 mmol/kg) for 5 days and the animals kept for up to 13 months, no liver tumours developed (0/7 rats), even with phenobarbital promotion (0/5 rats). GST-P foci were detected in the liver, but only after 13 months was their number or area significantly increased over the corresponding controls. When alpha-hydroxytamoxifen was given to female lambda/lacI transgenic rats (0.103 mmol/kg orally for 10 days) and the animals killed 46 days later, there was an approximate 1.8-fold increase in mutation frequency but no significant increase in G:C to T:A transversions as described after tamoxifen treatment. It is concluded that DNA damage alone, resulting from the short-term administration of alpha-hydroxytamoxifen, is not sufficient to initiate liver tumours even with phenobarbital promotion. As with tamoxifen, long-term exposure may be required to allow promotion and progression of transformed cells.

Anti-oestrogenic drugs and endometrial cancers

Tamoxifen is one of the most effective drugs to be used in the treatment of women with breast cancer and as a chemopreventive agent in women 'at risk' from this disease. Tamoxifen can be regarded as a paradigm for a new range of selective oestrogen receptor modulators that include toremifene, used in the treatment of metastatic breast cancer and raloxifene, presently approved for use in postmenopausal women for the treatment of osteoporosis. Tamoxifen treatment of women leads to a small increase in the incidence of endometrial cancers. It is important to understand the mechanism for this side effect in order to predict the likely human risk for other drugs of this class. Two such mechanisms have been proposed: (1) conversion of the drug to electrophilic metabolites that damage cellular DNA; and (2) an oestrogen agonist action on the uterus, promoting endogenous lesions. In rats, long-term tamoxifen treatment results in liver cancer via a genotoxic mechanism. However, it seems most likely that, in women treated with tamoxifen, endometrial cancer is related to an oestrogen agonist effect of this drug, promoting uterine cell proliferation.

Tamoxifen mutagenesis and carcinogenesis in livers of lambda/lacI transgenic rats: selective influence of phenobarbital promotion

Administration of tamoxifen (TAM) (20 mg/kg per day p.o.) for 6 weeks to female lambda/lacI transgenic rats caused a 4-fold increase in mutation frequency (MF) at the lacI gene locus in the livers of dosed animals compared with controls. After cessation of dosing, the MF showed a further increase with time at 2, 12 and 24 weeks, respectively. Phenobarbital promotion of similarly treated animals resulted in no increase in mutation frequency compared with TAM alone. Treatment with phenobarbital or TAM+phenobarbital resulted in time-dependent increases in liver weight compared with the corresponding controls. There was an increase in cell proliferation in the phenobarbital and TAM+phenobarbital groups, and at 24 weeks in the TAM dosed animals compared with controls. There was also a progressive increase in the number of GST-P expressing foci in the livers of TAM and TAM + phenobarbital rats compared with controls. The induction of cell proliferation and GSTP foci in the rat liver by phenobarbital is consistent with its ability to promote tamoxifen-initiated liver tumours in the rat. If the lacI gene is regarded as being representative of the rat genome in general (albeit that the gene is bacterial) the above observations suggest that promotion by tamoxifen confers selective advantage on mutated genes at loci that contribute to the tumour phenotype and that promotion of rat liver tumours by tamoxifen is not dependent simply upon the enhancement of cellular proliferation.

Major inter-species differences in the rates of O-sulphonation and O-glucuronylation of alpha-hydroxytamoxifen in vitro: a metabolic disparity protecting human liver from the formation of tamoxifen-DNA adducts

Tamoxifen is a hepatic genotoxin in rats and mice but a hepatocarcinogen only in rats. It is not associated with DNA adducts and liver tumours in patients. The proposed major pathway for its bioactivation in rats involves alpha-hydroxylation, O-sulphonation and generation of a carbocation that reacts with DNA. Rat liver microsomes catalyse alpha-hydroxylation at approximately 2- and 4-fold the rate achieved by human and murine liver microsomes, respectively. O-glucuronylation will deactivate alpha-hydroxytamoxifen and compete with sulphonation. Rates of O-sulphonation of alpha-hydroxytamoxifen in hepatic cytosol have been determined by a HPLC assay of substrate-dependent 3'-phosphoadenosine 5'-phosphate production. The rank order of O-glucuronylation in hepatic microsomes was estimated by HPLC-mass spectrometry. The rate of sulphonation of trans-alpha-hydroxytamoxifen (25 microM) in cytosol from adult female Sprague-Dawley rats and CD1 mice was 5.3 +/- 0.8 and 3.9 +/- 0.5 pmol/min/mg protein (mean +/- SD, n = 3), respectively. In cytosol fractions from women aged 40-65 years, the rate was 1.1 +/- 0.4 pmol/min/mg protein (mean +/- SD, n = 6). The K(m) for trans-alpha-hydroxytamoxifen in rat, mouse and human cytosol was 84. 6 +/- 3.8, 81.4 +/- 4.6 and 104.3 +/- 5.6 microM (mean +/- SD, n = 3), respectively; the corresponding V:(max) values were 22.4 +/- 3.4, 17.1 +/- 3.1 and 6.3 +/- 1.9 pmol/min/mg protein. These K:(m) were similar to a value obtained by others using purified rat liver hydroxysteroid sulphotransferase a. Turnover of the cis epimer was too slow for accurate determination of rates. Sulphonation of trans-alpha-hydroxytamoxifen in human uterine cytosol was undetectable. The rank order of O-glucuronylation of trans-alpha-hydroxy- tamoxifen in liver microsomes was human > > mouse > rat. In combination, lower rates of alpha-hydroxylation and O-sulphonation and a higher rate of O-glucuronylation in human liver would protect patients from the formation of tamoxifen-DNA adducts.

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.

Association of tamoxifen biliary excretion rate with prior tamoxifen exposure and increased mdr1b expression

ATPase transporter proteins are commonly found in the hepatocyte canalicular membrane. Some of these, in particular the multidrug resistance (mdr1b) gene, have been previously demonstrated to be inducible genes. In this study, we found that tamoxifen induced expression of the mdr1b gene in the liver up to 40-fold after 14 days' exposure to tamoxifen in the diet at a concentration of 420 ppm. As tamoxifen and its metabolites are primarily excreted into the bile, we investigated if the increased expression of mdr1b in the liver following tamoxifen exposure had any effect on its excretion in rats. We found that the excretion of tamoxifen and its metabolites into bile was increased from 8 +/- 1% to 51 +/- 18% (mean +/- SD) of an administered dose of 180 nmol/kg over a collection period of 3 hr in rats that had received tamoxifen (35 mg/kg) orally for 12 days (plus a 3-day rest) prior to the experiment. These data suggest that prolonged treatment with tamoxifen may result in lower serum and tumour concentrations, due to a self-mediated enhancement of excretion via mdr1b gene-encoded P-glycoprotein. This may have implications for other drugs sharing the same route of excretion and co-administered with tamoxifen.

Tamoxifen induces endometrial and vaginal cancer in rats in the absence of endometrial hyperplasia

Tamoxifen was administered orally to neonatal rats on days 2-5 after birth and the subsequent effects on the uterus were characterized, morphometrically, over the following 12 months. Tamoxifen inhibited development of the uterus and glands in the endometrium, indicating a classical oestrogen antagonist action. Between 24 and 35 months after tamoxifen treatment there was a significant increase in the incidence (26%) of uterine adenocarcinomas and a 9% incidence of squamous cell carcinomas of the vagina/cervix in the absence of any oestrogen agonist effect in the uterus. This demonstrates that an oestrogen agonist effect is not an absolute requirement for the carcinogenic effect of tamoxifen in the reproductive tract of the rat. The unopposed oestrogen agonist effect of tamoxifen on the endometrium may not be the only factor involved in the development of endometrial cancers. It is possible that tamoxifen causes these tumours via a genotoxic mechanism similar to that seen in rat liver. However, using (32)P-post-labelling we failed to find evidence of tamoxifen-induced DNA adducts in the uterus. Tamoxifen may affect hormonal imprinting of oestrogen receptor responses in stem cells of the uterus, causing reproductive tract cancers to arise at a later time, in the same way as has been proposed for diethylstilbestrol. If these rodent data extrapolate to humans, then women who are taking tamoxifen as a chemopreventative may have an increased risk of vaginal/cervical cancer, as well as endometrial cancer.

Activation of transcription by estrogen receptor alpha and beta is cell type- and promoter-dependent

Tamoxifen acts as a strong estrogen antagonist in human breast but as an estrogen agonist in the uterus. The action of tamoxifen is mediated through estrogen receptors (ERalpha and ERbeta), which bind to a variety of responsive elements, to activate transcription. To examine the role of these varied elements in the response to antiestrogens, we studied the activation of a panel of differing promoters, by these compounds, in human breast, bone, and endometrial derived cell lines. No agonistic activity was observed in breast cells, whereas all antiestrogens, particularly tamoxifen, exhibited agonistic effects in uterine cell lines. All antiestrogens studied were agonistic in co-transfections of a collagenase reporter gene and ERbeta, but tamoxifen alone was agonistic with ERalpha in (uterine) HEC-1-A cells. The ERalpha mediated, agonism of tamoxifen was not observed in primary cultures of human uterine stromal cells, whereas the ERbeta-mediated agonism of all selective estrogen receptor modulators was present. This suggests that the two receptors operate by distinct pathways and that the response of cells to antiestrogens is dependent on the ER subtypes expressed.

Further characterization of the DNA adducts formed in rat liver after the administration of tamoxifen, N-desmethyltamoxifen or N, N-didesmethyltamoxifen

The present study compares the formation of DNA adducts, determined by (32)P-postlabelling, in the livers of rats given tamoxifen and the N-demethylated metabolites N-desmethyltamoxifen and N, N-didesmethyltamoxifen. Results show that after 4 days treatment (0.11 mmol/kg i.p.), similar levels of DNA damage were seen after treatment with either tamoxifen or N-desmethyltamoxifen [109 +/- 40 (n = 3) and 100 +/- 33 (n = 4) adducts/10(8) nucleotides, respectively], even though the concentration of tamoxifen in the livers of tamoxifen-treated rats was about half that of N-desmethyltamoxifen in the N-desmethyltamoxifen-treated animals (51 +/- 16 and 100 +/- 8 nmol/g, respectively). Administration of N, N-didesmethyltamoxifen to rats resulted in a 5-fold lower level of damage (19 adducts/10(8) nucleotides, n = 2). Following (32)P-postlabelling and HPLC, hepatic DNA from rats treated with tamoxifen and its metabolites showed distinctive patterns of adducts. Treatment of rats with N,N-didesmethyltamoxifen gave a major product that co-eluted with one of the minor adduct peaks seen in the livers of rats given tamoxifen. Following dosing with N-desmethyltamoxifen, the major product co-eluted with one of the main peaks seen following treatment of rats with tamoxifen. This suggests that tamoxifen can be metabolically converted to N-desmethyltamoxifen prior to activation. However, analysis of the (32)P-postlabelled products from the reaction between alpha-acetoxytamoxifen and calf thymus DNA showed two main peaks, the smaller one of which ( approximately 15% of the total) also co-eluted with that attributed to N-desmethyltamoxifen. This indicates that N-desmethyltamoxifen and N,N-didesmethyltamoxifen are activated in a similar manner to tamoxifen leading to a complex mixture of adducts. Since an HPLC system does not exist that can fully separate all these (32)P-postlabelled adducts, care has to be taken when interpreting results and determining the relative importance of individual adducts and the metabolites they are derived from in the carcinogenic process.

The tamoxifen dilemma

The anti-oestrogen tamoxifen is widely used for adjuvant therapy in the treatment of women with breast cancer and has a low incidence of serious side-effects. It could also play a role as a breast cancer chemopreventive agent. However, epidemiological studies in both tamoxifen-treated breast cancer patients and in healthy women have shown that treatment results in a small increase in the incidence of endometrial cancers. While the use of tamoxifen in breast cancer patients is clearly justified, the situation for its use as a chemopreventive agent in healthy women is not so clear cut. Reasons for caution come from studies in rats that show that tamoxifen is a genotoxic mutagenic liver carcinogen. Initiation of tumours in the rat is the result of metabolic activation of tamoxifen by CYP enzymes to an electrophile(s) that binds irreversibly to DNA. This is not related to the oestrogen receptor status of the tissue. The extent of DNA damage, detected by 32P-post-labelling or accelerator mass spectrometry, is dependent both on the dose and the length of exposure. Studies have been carried out to see if such binding occurs in the uterine endometrium from tamoxifen-treated women. Results are presently inconclusive, but if such irreversible DNA binding occurs, it is at very low levels. Based on a mechanistic understanding of tamoxifen-induced liver carcinogenesis in the rat, it seems that in humans hepatic DNA damage will be close to the limit of detection by 32P-post-labelling and liver cancer will not be a significant carcinogenic risk. We cannot be certain of the mode of action of tamoxifen that results in the increase in endometrial cancers in treated women but it seems unlikely that this will be associated with a classical genotoxic mechanism.

Alpha-hydroxytamoxifen, a genotoxic metabolite of tamoxifen in the rat: identification and quantification in vivo and in vitro

The metabolic formation of a-hydroxytamoxifen, a reactive metabolite of tamoxifen in rat liver, was characterized and quantified in vitro (hepatic microsomal incubations) and in vivo (bile-duct cannulated animals). This minor metabolite was identified by chromatographic and mass spectral comparisons with the authentic compound. The rates of formation of alpha-hydroxytamoxifen in incubations (30 min) of tamoxifen (25 microM) with liver microsomal preparations from women (pool of six), female CD1 mice or female Sprague-Dawley rats, as quantified by liquid chromatography-mass spectrometry (LC-MS), were 1.15+/-0.03, 0.30+/-0.05 and 2.70+/-0.35 pmol/min/mg protein, respectively. Selective inhibition of microsomal P450 indicated that alpha-hydroxylation was catalysed predominantly by CYP3A in humans. Bile-duct cannulated and anaesthetized female rats and mice given [14C]tamoxifen (43 micromol/kg, i.v.) excreted, respectively, 24 and 21% of the administered radioactivity in bile over 5 and 3.5 h. The major radiolabelled biliary metabolite in rats, characterized by LC-MS after enzymic hydrolysis of conjugates, was the glucuronide of 4-hydroxytamoxifen (10% of dose) and only 0.1% of the dose was recovered as alpha-hydroxytamoxifen. After administration of alpha-hydroxytamoxifen (43 micromol/kg, i.v.) to rats, only 1.19% of the administered compound was recovered from a glucuronide metabolite in bile, indicating a possible 0.84% alpha-hydroxylation of tamoxifen in vivo. There was, however, no indication of the presence in bile of either O-sulphonate or glutathione conjugates derived from alpha-hydroxytamoxifen. This study shows for the first time that alpha-hydroxytamoxifen can be glucuronylated in rat liver. Whereas sulphonation results in electrophilic genotoxic intermediates, glucuronidation may represent a means of detoxifying alpha-hydroxytamoxifen.

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.

Evaluation of tamoxifen and alpha-hydroxytamoxifen 32P-post-labelled DNA adducts by the development of a novel automated on-line solid-phase extraction HPLC method

A novel HPLC system has been developed that has allowed the separation of tamoxifen DNA adducts formed in the livers of rats and mice treated with this drug. At least 13 different peaks have been separated from 32P-post-labelled DNA, with two major peaks jointly accounting for >60% of the total adducts formed by tamoxifen in the livers of treated rats and mice. This is a great improvement on the resolution obtained by thin layer chromatography, which separates the adducts into one main product consisting of a group of major adduct spots eluting together, plus several other minor spots. Identification of the nature of some of the peaks has been investigated. Comparisons of the products formed when alpha-acetoxytamoxifen is reacted with DNA in vitro with 32P-post-labelled liver DNA adducts from rats treated with tamoxifen or alpha-hydroxytamoxifen in vivo, appear to confirm that a major route of activation of tamoxifen in vivo is via alpha-hydroxylation. The resolving power of this HPLC system has further extended this result to show that six of the peaks, including the two major peaks, are formed by the reaction of an activated alpha-hydroxytamoxifen with DNA. Activation of 4-hydroxytamoxifen by the peroxidase/H2O2 system in vitro gives a more polar DNA adduct seen only at trace levels in liver DNA from tamoxifen-treated rats and mice.

Determination of DNA damage in F344 rats induced by geometric isomers of tamoxifen and analogues

To investigate the activation mechanisms involved in tamoxifen carcinogenicity, analogues of tamoxifen isomers modified at the ethyl group were synthesized and assessed for their ability to induce hepatic DNA damage following their administration to female F344 rats. The cis isomer was prepared by acid-catalyzed isomerization of tamoxifen and isolated by preparative HPLC. The active metabolite alpha-hydroxytamoxifen and geometric isomers of bromotamoxifen and C-desmethylenetamoxifen, analogues in which the ethyl group has been replaced by a bromine atom and methyl group, respectively, were synthesized according to published procedures. The levels of hepatic DNA adducts induced were determined by 32P-postlabeling. Bromotamoxifen and tamoxifen 1,2-epoxide caused no detectable DNA damage relative to controls. Trans isomers of tamoxifen, C-desmethylenetamoxifen, and alpha-hydroxytamoxifen all produced DNA adducts at a 5-90-fold higher level than the corresponding cis isomers. In contrast, both the cis and trans isomers of alpha-hydroxytamoxifen showed similar reactivity toward calf thymus DNA in vitro. Molecular models of alpha-hydroxytamoxifen isomers suggest this difference in DNA adduct-forming ability is due to steric hindrance of the enzymes involved in the activation of this metabolite. There were high adduct levels in the liver, but no uterine DNA adducts were detected in rats treated with alpha-hydroxytamoxifen. This suggests that in contrast to the liver, alpha-hydroxytamoxifen is not further activated in rat uterus. This may help to explain the absence of uterine tumors in rats following long-term tamoxifen treatment.

Antiestrogen therapy: uncertainties and risk assessment

Tamoxifen is by far the most clinically tested antiestrogenic drug currently used as adjuvant therapy for breast cancer and it continues to provide considerable benefit in this setting. The balance from clinical trials indicates a strong association between the use of tamoxifen and an increase in uterine tumors (three to sixfold). In rats, tamoxifen is a mutagenic, genotoxic hepatocarcinogen. These actions are not related to its estrogen antagonist activity but have been shown to be as a result of metabolic activation of this drug by cytochrome P450 enzymes, resulting in irreversible binding to cellular DNA. The mechanism of endometrial cancer associated with tamoxifen treatment is unclear, although there are two plausible hypotheses: (1), tamoxifen causes damage and mutation to DNA in uterine cells or (2), it promotes the development of endometrial tumors through its estrogen agonist activity. The evidence for a genotoxic effect of tamoxifen in the uterus is highly contentious and, on balance, we have concluded that it is more likely that the estrogenic effects of tamoxifen promote tumor development.

Investigation of the formation and accumulation of liver DNA adducts in mice chronically exposed to tamoxifen

Tamoxifen was administered to three strains of female mice (B6C3F1, C57BL/6 and DBA/2) in short- and long-term studies to determine their ability to activate tamoxifen and cause hepatic DNA damage. 32P-Postlabelling of liver DNA from mice treated for 4 days showed a group of major adducts that increased in a dose-dependent manner and co-chromatographed with the major adducts detected in rat liver. On cessation of dosing, the majority of adducts were cleared within 3 days. Binding of [14C]tamoxifen to DNA nucleotides was demonstrated by the use of accelerator mass spectrometry. In long-term studies of 12 months to 2 years duration, dependent on strain, tamoxifen was administered continuously in the diet to give a daily dose of approximately 40 mg/kg. DNA adducts were detected after 3 months, although the number of adducts decreased with time and by 2 years were not detectable in the tamoxifen treated mice. None of the treated groups showed a significantly increased incidence of liver tumours, with or without phenobarbital promotion and there was no sustained liver cell proliferation. Tamoxifen was detected in the mouse livers, but at levels 50 times lower than those reported in a comparable rat study. These results suggest that, in contrast to the rat, tamoxifen is non-carcinogenic in mice because it does not cause sufficient cumulative DNA damage, or act as a promoter by causing cell proliferation.

Comparisons of the binding of [14C]radiolabelled tamoxifen or toremifene to rat DNA using accelerator mass spectrometry

Tamoxifen, widely used as adjuvant therapy in the treatment of breast cancer, is now undergoing trials as a cancer chemopreventative agent. Previous work has shown an association between 32P-postlabelled adducts in rat liver DNA and the development of liver tumours. With the use of accelerator mass spectrometry, [14C]tamoxifen was shown to bind to liver DNA of female rats in a dose-dependent manner and was linear over 0.1-1 mg/kg, compatible with the therapeutic dose used in women (20 mg/person per day). Radiolabel could also be detected in extrahepatic organs, including reproductive and GI-tract, where levels were about 18 and 46%, respectively those seen in liver. Following enzymatic hydrolysis of liver DNA, normal nucleotides by HPLC showed < 2% incorporation of the [14C]radioactivity while > 80% appeared as non-polar products. In contrast, when animals were given an equivalent dose of [14C]toremifene, binding to DNA was an order of magnitude lower than that seen with tamoxifen and no evidence of non-polar adducted nucleotides following HPLC. However, in vitro, using human, rat or mouse liver microsomal preparations, NADPH-dependent binding of both toremifene and tamoxifen to calf thymus DNA could be demonstrated, suggesting that under favourable circumstances toremifene is capable of undergoing conversion to reactive intermediates.

Identification and mechanism of formation of potentially genotoxic metabolites of tamoxifen: study by LC-MS/MS

On-line high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI MS) and tandem mass spectrometry (MS/MS) have been applied to the study of tamoxifen metabolism in liver microsomes and to the identification of potentially genotoxic metabolites. The results showed that the hydroxylated derivatives, including 4-hydroxytamoxifen and alpha-hydroxytamoxifen are detoxication metabolites, while arene oxides, their free radical precursors or metabolic intermediates, are the most probable species involved in DNA-adduct formation.

Tamoxifen causes gene mutations in the livers of lambda/lacI transgenic rats

Tamoxifen, a rat liver carcinogen, was administered to female lambda/lacI transgenic rats at a dose of 20 mg/kg body weight by gavage for 6 weeks, and the animals were sacrificed 2 weeks later. Tamoxifen induced liver DNA adducts and caused a significant increase in mutation frequency (MF) of approximately 3-fold at the lacI gene in liver DNA. Liver DNA from animals dosed with tamoxifen at 10 mg/kg also showed a similar increase in MF. The mutations were characterized by a raised proportion of: (a) G:C to T:A transversions; (b) insertions of base pairs; and (c) deletions of pairs of G:C base pairs. These observations indicate that tamoxifen induces a distinct spectrum of mutations compared with that found in controls. Toremifene, a noncarcinogenic analogue of tamoxifen with similar estrogenic/antiestrogenic properties examined at 20 mg/kg body weight using the same dosing regime as tamoxifen was not mutagenic. A single oral dose of the rat liver carcinogen aflatoxin B1 (0.5 mg/kg) also significantly raised the MF. In conclusion, although tamoxifen is not mutagenic in regulatory short-term tests, it is a gene mutagen in the rat liver.

Clastogenic and aneugenic effects of tamoxifen and some of its analogues in hepatocytes from dosed rats and in human lymphoblastoid cells transfected with human P450 cDNAs (MCL-5 cells)

Tamoxifen and its analogues 4-hydroxytamoxifen, toremifene, 4-hydroxytoremifene, clomifene and droloxifene were tested for clastogenic effects in a human lymphoblastoid cell line (MCL-5) expressing elevated native CYP1A1 and containing transfected CYP1A2, CYP2A6, CYP2E1 and CYP3A4 and epoxide hydrolase and in a cell line containing only the viral vector (Ho1). MCL-5 or Ho1 cells were incubated with 4-hydroxytamoxifen, 4-hydroxytoremifene, clomifene or droloxifene and the incidence of micronuclei estimated. With MCL-5 cells there was an increase in micronuclei with 4-hydroxytamoxifen, 4-hydroxytoremifene and clomifene but not with droloxifene. With Ho1 cells only 4-hydroxytamoxifen and 4-hydroxytoremifene caused an increase in micronuclei. MCL-5 cells were incubated with tamoxifen, 4-hydroxytamoxifen, toremifene, droloxifene, clomifene or diethylstilbestrol (0.25-10 microg/ml) for 48 h and subjected to 3 h treatment with vinblastine (0.25 microg/ml) to arrest cells in metaphase. The incidence of cells with chromosomal numerical aberrations (aneuploidy) was increased in cells treated with tamoxifen, 4-hydroxytamoxifen, toremifene, clomifene and diethylstilbestrol but not droloxifene. The frequency of cells with structural abnormalities (excluding gaps) was increased in cells treated with tamoxifen and toremifene but not 4-hydroxytamoxifen, clomifene, droloxifene or diethylstilbestrol. The clastogenic activities of tamoxifen (35 mg/kg), toremifene (36.3 mg/kg), droloxifene (35.2 mg/kg) and diethylstilbestrol (25 mg/kg) were compared in groups of four female Wistar rats. Each chemical was dissolved in glycerol formal, administered as a single dose by gavage and hepatocytes isolated by collagenase perfusion 24 h later. The cells were cultured in the presence of epidermal growth factor (40 ng/ml) for 48 h, colchicine (10 microg/ml) being added for the final 3 h of incubation. At least 100 chromosomal spreads were examined from each animal for the presence of numerical and structural abnormalities. The incidences of aneuploidy following treatment were: tamoxifen 81%, toremifene 46%, droloxifene 9.6%, diethylstilbestrol 45.7%, vehicle control 5.3%. The incidences of chromosomal structural abnormalities excluding gaps were: tamoxifen 4.3%, toremifene 0.8%, droloxifene 0.5%, diethylstilbestrol 0.8%, control 0.5%. The incidence of chromosomal structural aberrations excluding gaps in the treated animals was not statistically significantly different from controls except in the tamoxifen-treated group. Tamoxifen (35 mg/kg per os) and toremifene (36.3 mg/kg per os) were dosed to rats for 4 weeks and chromosomal spreads made from hepatocytes. The incidences of aneuploidy were: tamoxifen 94%, toremifene 57%, control 6.5%. The incidences of chromosomal aberrations excluding gaps were: tamoxifen 12%, toremifene 1%, control 0.5%. The incidence of tamoxifen-induced chromosomal structural abnormalities was significantly elevated compared with control levels. The results demonstrate that tamoxifen and toremifene are the only two drugs tested in the study that cause chromosomal structural and numerical aberrations in vitro and tamoxifen is the only drug that induces both these effects in rat liver cells stimulated to divide in culture following oral dosing. Since chromosomal mutations require cell division for their manifestation and tamoxifen is the only compound of those tested that causes hyperplasia in the rat liver, chromosomal aberrations and aneuploidy in the rat liver would only be expected to occur following treatment with tamoxifen alone, although aneuploidy could be induced by toremifene in conjunction with a promoter such as phenobarbitone.

Peroxidase activation of 4-hydroxytamoxifen to free radicals detected by EPR spectroscopy

4-Hydroxytamoxifen is a major metabolite of the antiestrogenic drug tamoxifen used in the treatment of women with breast cancer. 4-Hydroxytamoxifen is broken down by a horseradish peroxidase/H2O2 system very much more rapidly than tamoxifen and causes much greater DNA damage determined by 32P-postlabelling. EPR spin trapping of 4-hydroxytamoxifen reaction products in the presence of the free radical trap 5,5-dimethyl-1-pyrroline N-oxide, together with glutathione as a hydrogen donor, resulted in the generation of a species with the characteristics of the glutathione thiyl radical (aN approximately 15.3 G, aH approximately 16.2 G). Support for the creation of thiyl radicals comes from the close to stoichiometric time dependent formation of glutathione disulfide concomitant with the loss of glutathione. Similar results were obtained using 4-hydroxytoremifene but no radical formation or glutathione loss could be detected using 3-hydroxytamoxifen (droloxifene). On-line LC-ESI MS analysis of the incubation products from 4-hydroxytamoxifen has identified three products with a protonated molecular mass of 773, consistent with the formation of dimers of 4-hydroxytamoxifen. The role that radical mechanisms have in the carcinogenic effects of tamoxifen in the endometrium or other target organs of women taking this drug remains to be established.

Effect of tamoxifen feeding on metabolic activation of tamoxifen by the liver of the rhesus monkey: does liver accumulation of inhibitory metabolites protect from tamoxifen-dependent genotoxicity and cancer?

Tamoxifen induces hepatocellular carcinomas in rats and is converted by rat hepatic cytochrome P450 enzymes into reactive metabolites capable of forming adducts with nucleic acids, proteins and chromosomal aberrations. In rats tamoxifen has also been shown to induce liver cytochrome P450 enzymes, to stimulate its own metabolism leading to greater covalent binding and to induce a higher degree of unscheduled DNA synthesis. This suggests that, at least in the rat, a sensitive species, tamoxifen may contribute significantly to its genotoxic and carcinogenic potential, by assisting its own metabolic activation. We have now investigated the effect of feeding tamoxifen to male and female Rhesus monkeys. A marked induction of the hepatic cytochrome(s) P450 is found in the monkey but, in spite of this, the in vitro metabolism of 7-ethoxyresorufin by microsomes from treated animals is markedly inhibited and so is the dealkylation of two other 7-alkoxyresorufin substrates. Evidence is presented for the accumulation in the liver of monkeys treated with tamoxifen of a powerful inhibitor of drug metabolism, and the inhibitor is identified as a metabolite of tamoxifen, its N,N-didesmethyl derivative. The level of 32P-postlabelled DNA adducts was considerably higher in rats given tamoxifen than in similarly treated monkeys. Also, whereas rats responded to tamoxifen treatment with a marked increase in covalent binding to microsomal protein, in the monkeys, where accumulation of the inhibitory metabolite in the microsomal fraction was also seen, covalent binding was not greater with microsomes from treated animals than in the corresponding controls. N,N-Didesmethyl-tamoxifen, added in vitro to human and rat microsomes, reduced significantly the extent of covalent binding, suggesting that the accumulation of the metabolite observed in the liver of primates may discourage the cytochrome P450-dependent conversion of tamoxifen into reactive derivatives and in this way protect against the formation of adducts. This mechanism may also contribute to protecting the primate against tamoxifen- induced liver cancer.

Mutational spectra of tamoxifen-induced mutations in the livers of lacI transgenic rats

Tamoxifen, an important drug in breast cancer treatment, causes liver cancer in rats. The standard range of in vitro tests have failed to show that it causes DNA damage, but 32P-postlabelling and DNA-binding studies have shown that tamoxifen forms DNA adducts in rat liver. In 1995 a transgenic rat (Big Blue; Stratagene, La Jolla, CA) became available which harbours the bacterial lacI gene, thereby allowing the in vivo study of tamoxifen mutagenesis. Recently, we [Styles JA et al. (1996): Toxicologist 30; 161] showed that tamoxifen caused on increase in the mutation frequency at the lacI gene in these transgenic rats. In this study, we report on our preliminary analysis of the mutational spectra of 33 control and 38 tamoxifen-induced mutant lacI genes. Plasmid DNA containing the lacI gene was isolated from the mutant phages and its DNA sequence determined. In the control animal group, 81% of the mutant lacI genes were point mutations, whilst in the tamoxifen-treated group, 62% of the mutant lacI genes were point mutations. Of the tamoxifen-induced mutants, 43% were GC-->TA transversions and 70% of point mutations. In the control group, GC-->TA transversions were 19% of all mutations and 24% of point mutations. Thus, compared with control animals, tamoxifen treatment had significantly increased the proportion of GC-->TA transversions.

A continuous fluorometric assay for gamma-glutamyltranspeptidase

The synthesis of gamma-glutamyl-7-amino-4-(trifluoromethyl)coumarin and its use as a substrate for gamma-glutamyltranspeptidase is described. The reaction product 7-amino-4-(trifluoromethyl)coumarin was fluorescent at neutral pH values and with excitation and emission wavelengths of 400 and 490 nm, respectively, concentration was linearly related to fluorescence over the range of 10 to 300 pmol/3 ml reaction mixture. At pH 8.4, the optimum for purified gamma-glutamyltranspeptidase, continuous fluorometric determination of enzyme activity with time could be carried out. This permitted accurate estimations to be made of initial reaction rates. Low levels of gamma-glutamyltranspeptidase activity could be shown in isolated rat hepatocytes (1 x 10(5) cells). Such activity could be inhibited by the addition acivicine (alpha-amino-3-chloro-5-isoxazoleacetic acid), suggesting the absence of peptidase-like activity. Following 2-week pretreatment of rats with the antiestrogen, tamoxifen, a fourfold increase in gamma-glutamyltranspeptidase activity was observed. The present method offers a convenient, sensitive method for the determination of gamma-glutamyltranspeptidase activity without the need for elaborate workup procedures.

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.

32P-postlabelled DNA adducts in liver obtained from women treated with tamoxifen

32P-Postlabelling of DNA extracted from the livers obtained from seven women receiving tamoxifen (20 mg either once or twice daily) was compared with liver DNA from seven individuals not receiving this drug. In all but one of the treated women, tamoxifen and its N-desmethyltamoxifen metabolite could be detected in liver extracts by high performance liquid chromatography; none was detected in control samples. The total level of 32P-postlabelled DNA adducts extracted from the tamoxifen treated women ranged between 18-80 adducts/10(8) nucleotides. The pattern of 32P-postlabelled adducts was not the same as those seen in rats dosed with this drug. There was no significant difference in the level of DNA damage between the tamoxifen treated and control groups. Although only a small number of subjects has so far been examined it appears that women are less susceptible to liver DNA damage caused by tamoxifen than rats.

DNA damage as assessed by 32P-postlabelling in three rat strains exposed to dietary tamoxifen: the relationship between cell proliferation and liver tumour formation

Tamoxifen was administered in the diet (420 p.p.m.) to female F344 (Fischer), Wistar (LAC-P) and LEW (Lewis) rats to determine for each strain the early morphological and biochemical changes associated with the subsequent development of liver cancer. Hepatic DNA damage, as determined by 32P-postlabelling, showed a cumulative increase with time from 500 adducts/10(8) nucleotides at 30 days to almost 3000 adducts/10(8) nucleotides after 180 days, with little difference between strains at this time point. A significant strain difference was found in the number of adducts present in the Fischer rats at 90 days, compared to the Wistar and Lewis strains. There was a marked strain differences in the time to development of liver tumours. After 6 months treatment, both Wistar and Lewis rats had tumours while none were seen in the Fischer animals. After 11 months, all of the Wistar and Lewis rats had developed liver carcinoma, while the Fischer rats developed liver carcinoma by 20 months. Depression in cell proliferation, relative to age-matched controls, was seen in the livers of Fischer rats after six months of exposure to tamoxifen, in contrast to an increase in the Wistar and Lewis rats. This observation is consistent with the promotion of foci to tumours and the subsequent progression of tumours to carcinomas in the latter two strains. These data may assist in establishing the possible risk factors, such as extent of DNA damage and increased liver cell proliferation, to women with long-term prophylactic exposure to tamoxifen.

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.

Genotoxic effects of 3-amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233) and nitrogen mustard-N-oxide (nitromin) in Walker carcinoma cells under aerobic and hypoxic conditions

As judged by alkaline elution, exposure of Walker cells to either 3-amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233) or nitromin results in a dose-dependent increase in DNA damage due to single-strand breaks. With nitromin or SR 4233 there was little difference in the extent of DNA single-strand breaks between Walker cells incubated either hypoxically or aerobically. In contrast, there was a 24-fold enhancement in the differential hypoxic/aerobic response to SR 4233 in clonogenic studies. Following incubation of cells with nitrogen mustard, DNA cross-linking is observed. Bioreduction of nitromin would be expected to yield nitrogen mustard as the putative reactive metabolite. However, only DNA strand-breaks could be detected in Walker cells incubated with nitromin, suggesting that reduction of this pro-drug to nitrogen mustard was not a major activation pathway. In cells incubated under aerobic conditions, SR 4233 induces oxidative DNA damage, as indicated by the formation of 8-hydroxydeoxyguanosine, suggesting the involvement of futile redox cycling. In rats dosed with SR 4233 in vivo, significantly higher levels of 8-hydroxydeoxyguanosine could be detected in liver, compared to vehicle-dosed controls.

Peroxidase activation of tamoxifen and toremifene resulting in DNA damage and covalently bound protein adducts

When [14C]tamoxifen was incubated with horseradish peroxidase and H2O2, two major metabolites, separated and identified by HPLC, were N-desmethyltamoxifen and tamoxifen N-oxide. Toremifene incubated in a similar system yielded N-desmethyltoremifene and toremifene N-oxide. No 4-hydroxylated metabolites were detected with either drug. When calf thymus DNA was included in peroxidase incubation mixtures, DNA damage, as assessed by 32P-postlabelling, could also be detected. The extent of damage caused by tamoxifen and toremifene was similar. The major adducts formed following incubation of DNA with tamoxifen had similar Rf values to two of the 32P-postlabelled adducts seen following dosing of rats with tamoxifen. Peroxidase was able to activate both drugs to derivatives which covalently bound to bovine serum albumin. The pH optimum for covalent binding and N-demethylation was near to pH 6.0. Results from liquid chromatography-electrospray secondary ion mass spectrometry suggest that tamoxifen and toremifene are metabolized by peroxidase to putative reactive epoxide intermediates responsible for the genotoxic effects. It is proposed that peroxidase oxidizes tamoxifen to a carbon-centred free radical which reacts with oxygen to form peroxy radicals capable of inserting an oxygen atom into tamoxifen. Lactoperoxidase and prostaglandin synthase are also able to catalyse tamoxifen N-demethylation and binding to protein. These data show that peroxidase can activate both tamoxifen and toremifene to an intermediate(s) that can damage DNA and covalently react with protein. Since it is known that women treated with tamoxifen can develop endometrial tumours, it may be relevant to determine whether activation of tamoxifen by peroxidases may contribute to its carcinogenic action at extrahepatic sites.

Tamoxifen induces short-term cumulative DNA damage and liver tumors in rats: promotion by phenobarbital

Tamoxifen administered in the diet (420 ppm) to Wistar rats (TOX:P) for only 3 months caused cumulative hepatic DNA damage as assessed by 32P-postlabeling, consistent with the proposal that tamoxifen is a genotoxic carcinogen in this species. Promotion of tumor development with phenobarbital after discontinuation of dietary tamoxifen resulted in the formation of liver carcinomas after 9 months. At 12 and 20 months in this study, the majority of these rats had liver carcinomas. Rats treated with tamoxifen for 3 months but not promoted with phenobarbital also developed liver tumors over a longer period of time. These tumors were predominantly adenomas, with one carcinoma, and occurred at a lower incidence than the tumors produced by promotion with phenobarbital. Rats treated with phenobarbital alone did not develop tumors after 20 months. Tamoxifen-induced DNA adducts were relatively persistent, with only a 38% decrease 3 months after tamoxifen treatment had been discontinued. This demonstrates that, in a susceptible species (the rat), tamoxifen can cause initiation of liver cancer after only 3 months exposure. It is proposed that the persistence of such DNA adducts may account for the ability of phenobarbital to promote a high incidence of liver carcinoma, even after discontinuation of tamoxifen treatment. These data are relevant to the concern for women given prophylactic tamoxifen for long periods in that even if there is a relatively small amount of cumulative tamoxifen-induced liver DNA damage, liver tumors could be promoted by other agents, even after the cessation of tamoxifen treatment.

Co-chromatography of a tamoxifen epoxide-deoxyguanylic acid adduct with a major DNA adduct formed in the livers of tamoxifen-treated rats

Tamoxifen is a potent liver carcinogen in rats and has been shown to form covalent DNA adducts in the livers of several species of rodent. We have shown previously by 32P-postlabeling (Carcinogenesis, 13, 2197-2203) that > 85% of the total adducts detected and resolved by multi-directional TLC migrate as a single spot. In the present study, this material was further analysed by reverse-phase HPLC and resolved into two approximately equal components. Tamoxifen 1,2-epoxide, a postulated metabolite of tamoxifen, was reacted with DNA and polydeoxyribonucleotides and the products analysed. 32P-Postlabeling revealed three major adduct spots on TLC which comigrated with the three major adduct spots seen with DNA from livers of tamoxifen-treated rats. Moreover, the major epoxide adduct, which contained guanine as the modified base, eluted on HPLC as a single major peak coincident with one of the major peaks derived from the liver DNA of tamoxifen-treated rats. These results demonstrate that approximately 40% of the tamoxifen-DNA adducts formed in vivo are chromatographically indistinguishable with the major product of the reaction of tamoxifen epoxide with guanine residues in DNA and provide important clues to the mechanism of activation of tamoxifen to a genotoxic carcinogen.

A comparative study of tamoxifen metabolism in female rat, mouse and human liver microsomes

The metabolisms of tamoxifen in female rat, mouse and human liver microsomal preparations were compared. Rat, mouse and human liver microsomes were incubated with tamoxifen in the presence of NADPH and MgCl2 and the metabolites formed were analysed by on-line HPLC-electrospray ionization MS. The major metabolites formed by rat liver microsomes were 4-hydroxytamoxifen, 4'-hydroxytamoxifen, N-desmethyltamoxifen and tamoxifen N-oxide. In addition, two epoxide metabolites, 3,4-epoxytamoxifen and 3',4'-epoxytamoxifen, and their hydrolysed derivatives, 3,4-dihydrodihydroxytamoxifen and 3',4'-dihydrodihydroxytamoxifen, have been identified. The pattern of the main metabolites obtained with human liver microsomes resembles qualitatively that of rat liver microsomes. The major differences between rat and human liver microsomes were that the amount of hydroxylated metabolites were much lower in human and only traces of 3,4-epoxytamoxifen and the corresponding dihydrodihydroxy derivative were detected. No 3',4'-epoxytamoxifen was detected in human liver microsomes. The four major metabolites were also formed in much larger amounts and with faster rates of formation by mouse liver microsomes, though tamoxifen N-oxide clearly predominated in this species. Polar metabolites, 3,4-dihydroxytamoxifen and 4-hydroxytamoxifen N-oxide, which were undetectable in rat and human, were formed in significant amounts in mouse microsomes. As in human microsomes, there was only one epoxide metabolite, 3,4-epoxytamoxifen, produced by mouse liver microsomes at levels lower than that found in rat. The faster rate of metabolism and the production of polar metabolites may indicate the ability of mouse to detoxify tamoxifen by rapid elimination compared with rat and human. The production of a larger amount of potentially reactive epoxide metabolites in rat may be responsible for the liver carcinogenesis in this species.

Genotoxicity of tamoxifen, tamoxifen epoxide and toremifene in human lymphoblastoid cells containing human cytochrome P450s

The clastogenicity of tamoxifen and toremifene was tested in six human lymphoblastoid cell lines each expressing increased monooxygenase activity associated with a specific transfected human cytochrome P450 cDNA (CYP1A1, CYP1A2, CYP2D6, CYP2E1 or CYP3A4). The chemicals were also tested in a cell line (MCL-5) expressing elevated native CYP1A1 and containing transfected CYP1A2, CYP2A6, CYP2E1 and CYP3A4 and epoxide hydrolase, and in a cell line containing only the viral vector (Ho1). Dose-related increases in micronuclei were observed when cells expressing 2E1, 3A4, 2D6 or MCL-5 cells were exposed to tamoxifen. The positive responses in the cell lines were in the order MCL-5 > 2E1 > 3A4 > 2D6. Toremifene also gave positive results with 2E1, 3A4 and MCL-5 cells, although the responses were less marked and the positive effects required higher doses than with tamoxifen. A synthesized epoxide of tamoxifen was also tested in these cell lines and produced similar increases in the incidences of micronucleated cells. The increases in the responses observed with the epoxide were greater than with tamoxifen or toremifene. The P450 isoenzyme activities in these cells were in a range similar to those of human tumour-derived cell lines. Microsomes (1A1, 2A2, 2A6, 2B6, 2E1, 3A4 and 2D6) from these cells all metabolized tamoxifen. The major metabolite detected by HPLC was N-desmethyltamoxifen, and 4-hydroxytamoxifen was also detected in cells with cytochrome P450 2E1 and 2D6. These results are consistent with the following conclusions. (1) Tamoxifen requires metabolic activation to DNA-reactive species by specific CYP monooxygenases in order to exert its genotoxic effects. (2) The positive clastogenic effects elicited in lymphoblastoid cells by tamoxifen epoxide suggest that the genotoxic (and possibly the carcinogenic) effects of tamoxifen may be due to one or more epoxide metabolites that are generated intracellularly, probably in close proximity to the nucleus. (3) Tamoxifen is more genotoxic than toremifene.

Characterization of Clara and type II cells isolated from rat lung by fluorescence-activated flow cytometry

A novel procedure for the isolation of Clara cells from rat lung is described. Single-cell suspensions from male F344/TOX rat lungs, prepared by subtilisin digestion, were treated with monochlorobimane (10 microM) and anlaysed by fluorescence-activated flow cytometry. A sub-population of about 2.8% of the total, showing the highest blue fluorescence and by inference the highest GSH concentration, was isolated as Clara cells of > 95% purity. Type II cells of similar purity were sorted after treatment with Phosphine 3R. Both sub-populations were > 90% viable, as judged by Trypan Blue exclusion. Comparison of CYP (cytochrome P-450) isoenzymes between these subpopulations, using Western blotting, showed CYP1A1 to be barely detectable. In Clara cells, CYP2B1 was 10-fold higher than in Type II cells. Mono-oxygenase activity towards the O-deethylation of 3-cyano-7-ethoxycoumarin was 2-fold higher in Clara cells. No activity was detected in macrophages. Pretreating rats with the mono-oxygenase inducers phenobarbitone, 3-methylcholanthrene or Aroclor 1254 showed the last-named to be the most potent inducer of CYP1A1. In Clara cells, CYP1A1 concentration and mono-oxygenase activities were induced > 2000- and 50-fold respectively, whereas in Type II cells increases of 300- and 3.6-fold were seen. Clara cells isolated from the lungs of control rats had a concentration of GSH (2.7 nmol/10(6) cells) that was 9- and 2-fold higher than that in Type II cells or macrophages respectively. GSH depleted by monochlorobimane treatment was restored after 2-3 h incubation with 0.2 M N-acetylcysteine. gamma-Glutamyltranspeptidase activity in Clara cells was 6- and 50-fold higher than in Type II cells or macrophages respectively.

Metabolic activation of halothane to neoantigens in C57Bl/10 mice: immunochemical studies

C57Bl/10 mice were given halothane (10 mmol/kg, intraperitoneally) and microsomal proteins were analysed for the presence of trifluoroacetylated (TFA) neoantigens by SDS-gel electrophoresis followed by immunoblotting using a polyclonal anti-TFA antibody. In microsomal preparations from liver, lung and olfactory tissues, a 54 kDa TFA adduct was detectable 1 h after dosing. After 3-48 h, multiple bands were detected in liver (45-100 kDa) and in the lung (26-57 kDa) and in one experiment in which [14C]halothane was given, several immunoreactive bands from liver microsomes were shown to contain a covalently bound metabolite of the drug. In olfactory tissue, initially (1 h), a major band of 54 kDa and a less prominent component of about 50 kDa were seen. The number of bands increased at later times but the additional bands were far fewer than in liver. The rate of decay of the 54 kDa adduct was also measured in both liver and olfactory microsomes and found to be compatible with the reported turnover of total liver cytochrome P-450. 24 h after treating mice with halothane (10 mmol/kg), no TFA neoantigens could be detected on the outer cell surface of isolated viable hepatocytes when analysed by fluorescence activated flow cytometry. In contrast, non-viable cells, or those fixed in acetone were all positive. Using immunohistochemistry, TFA neoantigens were demonstrated in the centrilobular area of the liver, the non-ciliated bronchiolar epithelial (Clara) cells of the lung, proximal tubular cells of the kidney and the respiratory and olfactory epithelium of nasal tissues.

Metabolism of 3-amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233) by purified DT-diaphorase under aerobic and anaerobic conditions

Purified DT-diaphorase [NAD(P)H (quinone acceptor) oxidoreductase (EC.1.6.99.2)] from Walker cells was used to investigate the reductive metabolism of 3-amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233) under aerobic and anaerobic conditions. In the presence of NADPH, under aerobic conditions, HPLC analysis showed the four-electron reduction product 3-amino-1,2,4-benzotriazine (SR 4330) was the major reaction product. In contrast, anaerobically, the 2-electron reduction product 3-amino-1,2,4-benzotriazine-1-oxide (SR 4317) was the predominant metabolite. Anaerobic reduction of SR 4233 to the known metabolites SR 4317 and SR 4330, catalyzed by DT-diaphorase, was 3-fold higher than reduction under aerobic conditions. Anaerobically, approximately half of the substrate utilized could not be accounted for by the formation of known products. Aerobically, the majority of the SR 4233 lost could be accounted for by its conversion to SR 4317 and SR 4330. In Walker cells incubated with SR 4233 anaerobically, SR 4317 was the major metabolite formed. Dicoumarol (100 microM) had little effect on the rate of formation of this metabolite in this cell line or in a rat liver epithelial derived (JBJ) cell line. Dicoumarol did however partially reduce the induction of unscheduled DNA synthesis caused by SR 4233 in Walker cells but not in JB1 cells, suggesting the action of dicoumarol may be specific to Walker cells. It is concluded that DT-diaphorase plays only a minor role in the overall reduction of SR 4233 in the two cell lines studied.

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

Previous studies suggest long-term feeding of tamoxifen (Z-1-[4-(2-dimethylamino-ethoxy)phenyl]1,2-diphenyl-1-butane) to rats gives rise to liver tumours, while mice are resistant. The effects of tamoxifen on cytochrome P450 isoenzymes and associated monoxygenase activities in the livers of female Fischer rats and C57Bl/6 and DBA/2 mice have been compared. Total microsomal cytochrome P450 was not induced in the livers of rats given tamoxifen (45 mg/kg daily for 4 days) and was in fact significantly reduced after 3 days treatment. In contrast, there was a 30-60-fold increase in the metabolism of benzyloxy- and pentoxyresorufins to resorufin. Ethoxyresorufin O-deethylase was induced only 2.5-fold. The regio- and stereo-specific hydroxylation of testosterone following tamoxifen pretreatment of rats showed a general time- and dose-dependent induction. 6 beta- and 16 alpha-hydroxylation of testosterone together with oxidation to androstenedione were increased 2-3-fold while 2 beta-hydroxylation was induced only marginally, suggesting that tamoxifen produces a mixed pattern of induction with a significant phenobarbitone-like component. No induction of the 2 beta- or 6 beta-hydroxylation pathway occurred in either mouse strain. In rats, immunoblotting experiments with polyclonal antibodies raised against CYP2B1 or 3A1 showed that tamoxifen pretreatment resulted in 2-3-fold increases in both CYP2B1, 2B2 and 3A1 proteins, relative to controls. Immunohistochemistry of rat liver sections showed a centrilobular localization of these induced proteins. Similar patterns of induction as measured by immunoblotting experiments and testosterone hydroxylation were seen following the administration of structurally related analogues, toremifene and droloxifene (3-hydroxytamoxifen), thought to be non-carcinogenic in the rat. No induction of these monooxygenase activities was seen in C57Bl/6 mice and only small increases in benzyloxy and pentoxyresorufin metabolism were in DBA/2 mice. It is suggested that the induction of cytochrome P450-dependent activities by tamoxifen may result in accelerated liver metabolism of this drug with important implications for the disposition of tamoxifen in vivo and also for its metabolic conversion to genotoxic metabolite(s). The difference in inducibility of cytochrome P450-dependent monooxygenase activities between rats and mice offers a plausible and testable hypothesis that the difference in tamoxifen metabolism between the two species may contribute to their carcinogenic response to tamoxifen.

Genotoxic potential of tamoxifen and analogues in female Fischer F344/n rats, DBA/2 and C57BL/6 mice and in human MCL-5 cells

Chronic administration of tamoxifen to female rats causes hepatocellular carcinomas. We have investigated damage to liver DNA caused by the administration of tamoxifen to female Fischer F344/N rats or C57B1/6 or DBA/2 mice using 32P-postlabelling. Following the administration of tamoxifen for 7 days (45 mg/kg/day) and extraction of hepatic DNA, up to 7 radiolabelled adduct spots could be detected after PEI-cellulose chromatography of the 32P-labelled DNA digests. Tamoxifen caused a time-dependent increase in the level of adduct detected up to a value of at least 1 adduct/10(6) nucleotides after 7 days dosing. A dose response relationship was demonstrated over the range of 5-45 mg/kg/day (0.013-0.12 mmol/kg/day). On cessation of dosing there was a loss of adducts from the liver DNA. These adducts were not detected in DNA from vehicle-dosed controls or in DNA from kidney, lung, spleen, uterus or peripheral lymphocytes. Pyrrolidinotamoxifen caused a similar level of adduct formation as tamoxifen. In contrast, no significant adduct formation could be detected in liver DNA from rats given droloxifene or toremifene. Mice given tamoxifen (45 mg/kg/day for 4 days) showed levels of adducts in the liver which were 30-40% of those present in rats. Exposure of rat hepatocytes to tamoxifen in vitro, resulted in induction of unscheduled DNA synthesis, when preparations from rats which had been pretreated with tamoxifen in vivo were used. No such increase could be detected in hepatocytes from control rats, suggesting tamoxifen may induce enzymes responsible for its own activation. Tamoxifen induced a significant increase in micronucleus formation in a dose dependent manner in cultures of MCL-5 cells, a human cell line that expresses 5 different human cytochrome P450 isoenzymes, as well as epoxide hydrolase.

Acute lesions in rats caused by 3-amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233) or nitromin: a comparison with rates of reduction in microsomal systems from target organs

Pathological lesions to male Fischer rats were investigated 24 h after the administration of 3-amino-1,2,4-benzotriazine-1,4- dioxide (SR 4233) or nitromin, two compounds which need to undergo bioreductive activation in order to exert their toxic effects. Although SR 4233 reduction leads to a putative free radical species while with nitromin a bifunctional alkylating agent is formed, in both instances, the bone marrow was a major target organ. However, the response of other organs to these compounds differed. SR 4233 caused lesions to the olfactory epithelium, liver, kidney and thymus. Nitromin caused focal haemorrhages on the intestine, which were reduced in germ-free rats. Rates of reduction of SR 4233 or nitromin were determined under anaerobic conditions using microsomal preparations from target tissues. With SR 4233 as a substrate, reductase activities were highest in the olfactory epithelium, 6 fold higher than in the liver. SR 4233 reductase activities generally correlated with those of NADPH:cytochrome c reductase or the concentration of cytochrome P-450 reductase protein in the affected organs while with nitromin, there appeared to be no such relationship. The present results support the concept that the expression of pathological damage in vivo is a multifactorial process and does not directly correlate with initial rates of reduction of either drug determined in vitro.

Preparation of rat lung cells for flow cytometry

Morphologically, the lung is a complex organ containing over 40 different cell types (1). Although species and strain dependent, in the Fischer rat, the most common cell types include: endothelial, 33%; Type I, 6.5%; Type II, 12%; macrophages, 8%; ciliated and nonciliated bronchiolar epithelial (Clara) cells, 1.3 and 0.7%, respectively (2,3). The lungs from an "adult" rat comprise about 4 × 10(8) cells (3). Unlike the liver, there is no one enzyme, such as collagenase, universally used for cell dispersion. Instead, a wide variety of proteases, such as Protease I or XIV (4,5), have been recommended. Enzyme cocktails have often been adjusted to suit the cell type being isolated: e.g., collagenase/trypsin/elastase for Type I cells (6), elastase/trypsin for Type II cells (7), and hyaluronidase/cytochalasin for tracheal epithelial cells (8). However, such proteolytic enzymes may lead to the loss of specific cell surface markers. There is general agreement that DNAse is necessary to minimize cellular reaggregation once the lung cells have been isolated. This procedure describes the use of the proteolytic enzyme subtilisin, recently introduced for the isolation of lung cells (9), which has been particularly effective in the preparation of functional Clara cells following their separation by flow cytometry (10). Type II cells have also been isolated using similar procedures (unpublished), and macrophages are lavaged from the lung during the perfusion process and can be recovered if required.

Fluorimetric determination of oxidised and reduced glutathione in cells and tissues by high-performance liquid chromatography following derivatization with dansyl chloride

A high-performance liquid chromatographic method utilising fluorimetric detection of oxidised and reduced glutathione, following derivatization with dansyl chloride is described. Dansyl derivatives are separated on an aminopropyl silica column with a methanol-sodium acetate gradient system giving detection limits (signal-to-noise ratio = 2) of 1 pmol. This is in the order of 100-fold more sensitive than established methods based on the ultraviolet detection of dinitrophenylglutathione derivatives. The present procedures have been used to determine oxidised and reduced glutathione in rat lung tissues and in alveolar macrophages.