Tamoxifen metabolism in rat liver microsomes: identification of a dimeric metabolite derived from free radical intermediates by liquid chromatography/mass spectrometry

Formation of raloxifene homo-dimer in CYP3A4, evidence for multi-substrate binding in a single catalytically competent P450 active site

Raloxifene is a polyaromatic compound which has been reported to form radicals when incubated with horseradish peroxidase resulting in formation of a homo-dimer product. Polyaromatic phenols have also been reported to undergo oxidation by P450 enzymes to form reactive intermediates, presumably through the formation of phenoxy radical species. Recently, we observed that a raloxifene homo-dimer was formed in vitro when incubated with CYP3A4. In response to this finding, a series of experiments were designed to determine whether the observed raloxifene homo-dimer was formed via solution phase chemistry similar to that previously documented with horseradish peroxidase or if generation of the homo-dimer occurred within the P450 active site. To this end, a series of experiments were carried out to determine the structure of the CYP3A4 generated raloxifene homo-dimer using analytical techniques including: high resolution MS, NMR and H/D exchange. In addition, a variety of in vitro techniques were applied to characterize the mechanism responsible for formation of the raloxifene homo-dimer. Collectively, the results of these experiments suggest that unlike the homo-dimer formed by peroxidase enzymes, raloxifene homo-dimer formation mediated by CYP3A4 is a consequence of two raloxifene molecules binding simultaneously within the active site of a catalytically competent P450 enzyme.

Uterine peroxidase-catalyzed formation of diquinone methides from the selective estrogen receptor modulators raloxifene and desmethylated arzoxifene

Long-term usage of the selective estrogen receptor modulator (SERM) tamoxifen has been associated with an increased risk of endometrial cancer. One potential mechanism of tamoxifen-induced carcinogenesis involves metabolism to reactive intermediates, such as an o-quinone, quinone methide, and carbocations. We have previously shown that the benzothiophene SERMs, raloxifene and desmethylated arzoxifene (DMA), can also be bioactivated to electrophilic quinoids by rat/human liver microsomes and rat hepatocytes [(2006) Chem. Res. Toxicol. 19, 1125-1137]. Because the uterus is a major target tissue of estrogens and antiestrogens, it was of interest to determine if quinoids could be formed from SERMs in uterine tissue potentially producing cytotoxic effects. Incubations with rat uterine microsomes showed that both raloxifene and DMA could be oxidized to electrophilic diquinone methides that were trapped as the corresponding GSH conjugates. A new raloxifene GSH-dependent conjugate was identified as raloxifene Cys-Gly that was formed from the hydrolysis of 7-glutathinyl raloxifene by gamma-glutamyl transpeptidase. Interestingly, the metabolism of raloxifene and DMA in rat uterine microsomes was not NADPH-dependent and could be inhibited by cyanide and NADPH or enhanced by H2O2. In addition, coincubations with the peroxidase substrates guaiacol or o-phenlyenediamine inhibited diquinone methide GSH conjugate formation from both SERMs. Incubations of raloxifene and DMA with horseradish peroxidase (HRP) were studied as models of the interaction between benzothiophene SERMs and peroxidase. The results showed that HRP could directly oxidize raloxifene and DMA to the corresponding dimers via the formation of phenoxyl radicals in the absence of exogenous hydrogen peroxide. In addition, GSH appears to be involved in multiple peroxidase-catalyzed oxidative metabolic pathways of benzothiophene SERMs. Finally, COATag (covert oxidatively activated tag) methodology, which involves the utilization of biotin-conjugated raloxifene and DMA, was used to identify target proteins by affinity chromatography. Incubations of raloxifene and DMA COATags with rat uterine microsomes showed several modified proteins by Western blot analysis. The protein modification could be enhanced by the addition of H2O2 and decreased by the addition of NADPH, suggesting that unlike liver metabolism the formation of quinoids in the uterus could be mediated by uterine peroxidases.

Polymorphisms in cytochrome P4503A5 (CYP3A5) may be associated with race and tumor characteristics, but not metabolism and side effects of tamoxifen in breast cancer patients

Tamoxifen (TAM) is widely used for treatment and prevention of breast cancer. TAM is metabolized by cytochrome P450 (CYP450) enzymes, including CYP3A5. Although two genetic polymorphisms in CYP3A5 are known (CYP3A5*3 and CYP3A5*6), the effects of these polymorphisms on TAM metabolism, TAM side effects, and tumor characteristics are unknown. Thus, this work tested the hypothesis that CYP3A5 polymorphisms are associated with differential TAM levels, TAM side effects, and tumor characteristics in breast cancer patients. Postmenopausal women with breast cancer (n=98) were recruited from a single cancer center. Polymorphic status was established using polymerase chain reactions (PCR). The associations between polymorphic status, race, TAM levels, side effects, and tumor characteristics were assessed using t-tests and logistic regression models. The data indicate that 40.7% of the breast cancer patients had the CYP3A5*3 polymorphism, and 9.1% had the CYP3A5*6 polymorphism. In addition, Caucasian women were 26 times more likely to carry the CYP3A5*3 polymorphism than African American (AA) women, whereas AA women were nine times more likely to carry the CYP3A5*6 polymorphism than Caucasian women. No significant differences were seen in TAM or TAM metabolite levels or TAM side effects by polymorphic status. There was a significant difference, however, in mean tumor size in women with the CYP3A5*6 polymorphism (3.6+/-0.98 cm) compared to those without the polymorphism (2.0+/-0.18 cm) (P<0.02). Taken together, these data suggest that racial differences in CYP3A5 polymorphisms exist although the polymorphisms do not appear to be associated with levels of TAM metabolites and side effects.

Medication use, tamoxifen (TAM), and TAM metabolite concentrations in women with breast cancer

Tamoxifen (TAM) is commonly used as an adjuvant treatment for breast cancer. Although patients taking TAM are often taking medications for comorbidities, data regarding the interaction of TAM with other medications are limited. Thus, this study was carried out to determine whether medications co-prescribed with TAM significantly influence the plasma concentrations of TAM and its metabolites (N-desmethyltamoxifen; N-DMT and 4-hydroxytamoxifen; 4-OHT) in 98 women diagnosed with breast cancer. Participants taking diuretics had significantly higher plasma concentrations of TAM and N-DMT than participants not taking a diuretic. Arthritis/pain medication intake was negatively associated with plasma TAM concentrations. Chemotherapeutic agents, allergy drugs, anti-depressants, and diabetes medications did not significantly alter plasma TAM or metabolite concentrations. This suggests that diuretic or an arthritis/pain medication may affect TAM metabolism.

Association of tamoxifen (TAM) and TAM metabolite concentrations with self-reported side effects of TAM in women with breast cancer

The positive effects of tamoxifen (TAM) on breast cancer recurrence and survival as well as on overall mortality have led to its use as the predominant adjuvant therapy among women with breast cancer. However, the association of TAM intake with undesirable side effects has been reported in numerous studies. This analysis was carried out to assess whether the concentrations of TAM or TAM metabolites, N -desmethyltamoxifen ( N -DMT) and 4-hydroxytamoxifen (4-OHT), were associated with self-reported side effects of TAM. Participants were 99 breast cancer patients who had been taking TAM for at least 30 days. Each participant completed a questionnaire that was used to ascertain whether she experienced certain specific symptoms while taking TAM. In addition, each woman provided a blood sample that was used to measure plasma concentrations of TAM, N -DMT, and 4-OHT by high performance liquid chromatography. Results of the analysis showed that women who experienced at least one TAM-related side effect had significantly higher levels of TAM than women not experiencing any TAM-related side effects. Furthermore, women who reported experiencing visual problems had significantly higher levels of both TAM and N -DMT compared to those women who reported experiencing no visual problems. The levels of 4-OHT were negatively associated with the occurrence of vaginal discharge. The results of this study suggest that the self-reported occurrence of certain symptoms during TAM treatment is related to TAM metabolism. Future studies should assess subgroups of women with specific TAM and TAM metabolite profiles to determine whether alternate, equally effective therapies would decrease their risk of experiencing certain undesirable side effects.

Aging May Be Associated with Concentrations of Tamoxifen and Its Metabolites in Breast Cancer Patients

BACKGROUND

Although tamoxifen (TAM) is the predominant adjuvant therapy for estrogen receptor positive (ER(+)) breast tumors, 50% of breast cancer patients do not respond positively to this therapy, or they experience adverse side effects. This variability in TAM responsiveness may be due to differences in TAM metabolism that stem from differences in race, age, and body mass index (BMI). Thus, the purpose of this study was to test the hypothesis that race, age, and BMI are associated with the metabolism of TAM to two primary metabolites, N-desmethyltamoxifen (N-DMT) and 4-hydroxytamoxifen (4-OHT).

METHODS

The study design was cross-sectional, and data were analyzed using independent sample t tests and multiple linear regression models. Breast cancer patients (n = 99) taking TAM for at least 30 days were recruited from a local hospital clinic. Each participant provided informed consent, completed a questionnaire, and donated a blood sample. The questionnaire was used to ascertain race, age, and BMI. The blood samples were used to measure plasma concentrations of TAM, N-DMT, and 4-OHT.

RESULTS

Plasma concentrations of TAM, N-DMT, and 4-OHT differed among individual patients. Age, but not race and BMI, was positively associated with plasma concentrations of TAM and N-DMT, even after adjustment for potential confounders (p = 0.02 for TAM and p = 0.03 for N-DMT).

CONCLUSIONS

This study suggests that aging may alter the metabolism of TAM. As increased levels of TAM and TAM metabolites may provide a possible explanation for why older women taking TAM are at increased risk for adverse side effects, future studies should determine whether age-related differences in the concentrations of TAM and TAM metabolites are associated with differences in TAM toxicity or responsiveness.

Hyphenated techniques in anticancer drug monitoring. II. Liquid chromatography-mass spectrometry and capillary electrophoresis-mass spectrometry

High-performance liquid chromatography has become the separation technique of choice for the monitoring of generally thermolabile anticancer agents. With the introduction of electrospray mass spectrometry, the coupling of liquid chromatogaphy and mass spectrometry has opened the way to widely and routinely applied anticancer drug monitoring. Real-time metabolism versus degradation can now be distinguished, since derivatization is no longer obligatory. This is important for the monitoring of the anabolic and catabolic pathways of the same agent, such as 5-fluorouracil. Detection limits almost equal to those obtained with capillary gas chromatography-mass spectrometry are realistic with the latest generation of mass spectrometers, enabling quantitative analysis of various anticancer agents and their metabolites down to the low ng/ml level. Furthermore, sample clean-up and chromatography can be downscaled markedly using the latest column technologies, such as the generally applied 10 cm x 2.8 mm I.D. RP 18 columns. The coupling of capillary electrophoresis to mass spectrometry is today far from a routine application in anticancer drug monitoring. Nevertheless, interesting applications have been reported and are selected for the present review.

DNA adducts formed from 4-hydroxytamoxifen are more mutagenic than those formed by alpha-acetoxytamoxifen in a shuttle vector target gene replicated in human Ad293 cells

The drug tamoxifen, used to treat breast cancer, causes liver cancer in rats and endometrial cancer in women. Tamoxifen forms liver DNA adducts in both short- and long-term dosing of rodents, and DNA adducts have also been reported in tissues of women undergoing tamoxifen therapy. It is not known if the induction of endometrial cancer in women is through these DNA adducts or through the estrogenic nature of the drug. In this study, we have investigated the mutagenicity of two model reactive intermediates of tamoxifen, alpha-acetoxytamoxifen and 4-hydroxytamoxifen quinone methide (4-OHtamQM). These form the same DNA adducts as those found in tamoxifen-treated rats. The two compounds were used to treat the pSP189 plasmid containing the supF gene, which was replicated in Ad293 cells before being screened in indicator bacteria. Plasmid reacted with 4-OHtamQM was more likely to be mutated (2-7-fold increase) than that reacted with alpha-acetoxytamoxifen, despite having a lower level of DNA damage (12-20-fold less), as assayed by (32)P-postlabeling. The two compounds induced statistically different mutation spectra in the supF gene. The majority of mutations in alpha-acetoxytamoxifen-treated plasmid were GC -->TA transversions while GC-->AT transitions were formed in 4-OHtamQM-treated plasmid. 4-OHTamQM-treated DNA induced a larger proportion of multiple mutations and large deletions compared to alpha-acetoxytamoxifen. Sites of mutational hotspots were observed for both compounds. In conclusion, the quantitatively minor DNA adduct of tamoxifen (dG-N(2)-4-hydroxytamoxifen) is more mutagenic than the major tamoxifen DNA adduct (dG-N(2)-tamoxifen).

Liquid chromatography-mass spectrometry in the study of the metabolism of drugs and other xenobiotics

The application of liquid chromatography-mass spectrometry (LC/MS) to the study of metabolism of drugs and other xenobiotics is reviewed. Original research papers covering the period from 1998 to early 2000 and concerning the use of LC/MS in the study of xenobiotic metabolism in humans and other mammalian species are reviewed. LC/MS interfaces, sample preparation steps, column types, mobile phases and additives, and the type of metabolites detected are summarized and discussed in an attempt to identify the current and future trends in the use of LC/MS for metabolism studies. Applications are listed according to the parent xenobiotic type and include substances used in therapeutics, drug candidates, compounds being evaluated in clinical trials, environmental pollutants, adulterants and naturally occurring substances.