Metabolism of tamoxifen by isolated rat hepatocytes: anti-estrogenic activity of tamoxifen N-oxide

Tissue distribution of 4-hydroxy-N-desmethyltamoxifen and tamoxifen-N-oxide

Tamoxifen dosage is based on the one-dose-fits-all approach. The anticancer effect of tamoxifen is believed to be due to the metabolites, 4-hydroxytamoxifen (4OHtam), and 4-hydroxy-N-desmethyltamoxifen (4OHNDtam/endoxifen). These demethylated metabolites of tamoxifen have been associated with its side effects, whereas the effect mediated by tamoxifen-N-oxide (tamNox) is still poorly understood. Our objective was to improve the therapeutic index of tamoxifen by personalizing its dosage and maintaining serum tamoxifen metabolite concentrations within a target range. We examined the levels of tamoxifen, 4OHtam, 4OHNDtam, N-desmethyltamoxifen (NDtam), N-desdimethyltamoxifen (NDDtam), and tamNox in serum and in breast tumors specimens of 115 patients treated with 1, 5 or 20 mg/day of tamoxifen for 4 weeks before surgery in a randomized trial. Furthermore, the metabolism of tamNox in MCF-7 breast cancer cells was also studied. The concentrations of tamoxifen and its metabolites in tumor tissues were significantly correlated to their serum levels. Tumor tissue levels were 5–10 times higher than those measured in serum, with the exception of tamNox. In MCF-7 cells, tamNox was converted back to tamoxifen. In contrast to the tissue distribution of tamNox, the concentrations of 4OHtam and 4OHNDtam in tumor tissues corresponded to their serum levels. The results suggest that implementation of therapeutic drug monitoring may improve the therapeutic index of tamoxifen. Furthermore, the tissue distribution of tamNox deviated from that of the other tamoxifen metabolites.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Rapid On-line Profiling of Estrogen Receptor Binding Metabolites of Tamoxifen

Here we present a high-resolution screening (HRS) methodology for postcolumn on-line profiling of metabolites with affinity for the estrogen receptor alpha (ERalpha). Tamoxifen, which is metabolized into multiple metabolites, was used as the model compound. Most of the 14 metabolites detected exhibited affinity for the ERalpha. The HRS methodology shows great potential for metabolite bio-affinity profiling and application in drug discovery and development.

OXIDATION OF TAMOXIFEN BY HUMAN FLAVIN-CONTAINING MONOOXYGENASE (FMO) 1 AND FMO3 TO TAMOXIFEN-N-OXIDE AND ITS NOVEL REDUCTION BACK TO TAMOXIFEN BY HUMAN CYTOCHROMES P450 AND HEMOGLOBIN

Tamoxifen (TAM), used as the endocrine therapy of choice for breast cancer, undergoes metabolism primarily forming N-desmethyltamoxifen, 4-hydroxytamoxifen, alpha-hydroxytamoxifen, and tamoxifen-N-oxide (TNO). Our earlier studies demonstrated that flavin-containing monooxygenases (FMOs) catalyze the formation of TNO. The current study demonstrates that human FMO1 and FMO3 catalyze TAM N-oxidation to TNO and that cytochromes P450 (P450s), but not FMOs, reduce TNO to TAM. CYP1A1, CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4 all reduced TNO, with CYP2A6, CYP1A1, and CYP3A4 producing the greatest reduction. A portion of TAM formed by CYP3A4-mediated reduction of TNO was further metabolized, but not TAM formed by the other P450s. TNO reduction by P450s is extremely rapid with considerable TAM formation detected at the earliest time point that products could be measured. TAM formation exhibited a lack of linearity with incubation time but increased linearly as a function of TNO and P450 concentration. TNO was converted into TAM by reduced hemoglobin (Hb) and NADPH-P450 oxidoreductase, suggesting involvement of the same heme-Fe(2+) complex in both Hb and P450s. The findings raise the question of whether the reductive activity may be nonenzymatic. Results of this in vitro study demonstrate the potential of TAM and TNO to be interconverted metabolically. FMO seems to be the major enzymatic oxidant, whereas several P450 enzymes and even reduced hemoglobin are capable of reducing TNO back to TAM. The possibility that these processes may comprise a metabolic cycle in vivo is discussed in this article.

Quantification of tamoxifen and three metabolites in plasma by high-performance liquid chromatography with fluorescence detection: application to a clinical trial

A sensitive and reproducible assay employing liquid-liquid extraction and high-performance liquid chromatography with fluorescence detection for the quantification of tamoxifen, N-desmethyltamoxifen, 4-hydroxytamoxifen, and Z-4-hydroxy-N-desmethyltamoxifen in human plasma is described. The compounds and internal standard, propranolol, were separated with a cyano column and a mobile phase of acetonitrile-20 mM potassium phosphate buffer (pH 3; 35:65, v/v) then detected with fluorescence using a modified version of a method originally described by Fried and Wainer [J. Chromatogr. B 655 (1994) 261]. The coefficients of variation for the midpoint of the standard curve for each compound were less than 10%. This method was applied to a pharmacokinetic study of tamoxifen disposition in breast cancer patients.

Effect of tamoxifen on the enzymatic activity of human cytochrome CYP2B6

Tamoxifen is primarily used in the treatment of breast cancer. It has been approved as a chemopreventive agent for individuals at high risk for this disease. Tamoxifen is metabolized to a number of different products by cytochrome P450 enzymes. The effect of tamoxifen on the enzymatic activity of bacterially expressed human cytochrome CYP2B6 in a reconstituted system has been investigated. The 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of purified CYP2B6 was inactivated by tamoxifen in a time- and concentration-dependent manner. Enzymatic activity was lost only in samples that were incubated with both tamoxifen and NADPH. The inactivation was characterized by a K(I) of 0.9 microM, a k(inact) of 0.02 min(-1), and a t(1/2) of 34 min. The loss in the 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity did not result in a similar percentage loss in the reduced carbon monoxide spectrum, suggesting that the heme moiety was not the major site of modification. The activity of CYP2B6 was not recovered after removal of free tamoxifen using spin column gel filtration. The loss in activity seemed to be due to a modification of the CYP2B6 and not reductase because adding fresh reductase back to the inactivated samples did not restore enzymatic activity. A reconstituted system containing purified CYP2B6, NADPH-reductase, and NADPH-generating system was found to catalyze tamoxifen metabolism to 4-OH-tamoxifen, 4'-OH-tamoxifen, and N-desmethyl-tamoxifen as analyzed by high-performance liquid chromatography analysis. Preliminary studies showed that tamoxifen had no effect on the activities of CYP1B1 and CYP3A4, whereas CYP2D6 and CYP2C9 exhibited a 25% loss in enzymatic activity.

Identification of the cytochrome P450 IIIA family as the enzymes involved in the N-demethylation of tamoxifen in human liver microsomes

The antiestrogen tamoxifen (Tam or Nolvadex, ICI)-Z-1-[4-[2-(dimethylamino) ethoxy]phenyl]-1,2-diphenyl-1-butene is widely used in treatment of hormone-dependent breast cancer. The drug is extensively metabolized by cytochrome P450 dependent hepatic mixed function oxidase in man, yielding mainly the N-desmethyl metabolite (DMT). This study has been carried out to determine the P450 enzyme involved in the N-oxidative demethylation of Tam in microsomal samples from 25 human livers (23 adults, two children). This metabolic step was inhibited by carbon monoxide up to 75%. Tam was demethylated into DMT with an apparent Km of 98 +/- 10 microM; rates varied between 37 and 446 pmol/min/mg microsomal protein. These metabolic rates were strongly correlated with 6 beta-hydroxylation of testosterone (r = 0.83) and erythromycin N-demethylase (r = 0.75), both activities known to be associated with P450 IIIA enzyme. To further assess whether or not the Tam demethylation pathway is catalysed by the same P450, the inhibitory effect of TST on this reaction was determined. The competitive inhibition had an apparent Ki of 100 +/- 10 microM. Drugs such as erythromycin, cyclosporin, nifedipine and diltiazem were shown to inhibit in vitro the metabolism of tamoxifen. Furthermore the P450 IIIA content of liver microsomal samples, measured by Western blot technique using a monoclonal P450NF (nifedipine) antibody, was strongly correlated with DMT formation (r = 0.87). Tam N-demethylase activity was inhibited by more than 65% with polyclonal anti-human anti-P450NF. All these in vitro observations establish that a P450 enzyme of the IIIA sub-family is involved in the oxidative demethylation of tamoxifen in human liver.

Evidence for superoxide formation during hepatic metabolism of tamoxifen

Spin trapping of free radicals during the hepatic metabolism of tamoxifen was investigated; the spin trap employed in this study was 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). The spin adduct 2-hydroxy-5,5-dimethyl-1-pyrrolidinyloxyl (DMPO-OH) was detected in an in vitro incubation mixture of phenobarbital-treated rat hepatocytes containing tamoxifen, dimethyl sulfoxide, and DMPO. However, since the spin adduct 2,5,5-trimethyl-1-pyrrolidinyloxyl (DMPO-CH3) was not observed, the DMPO-OH resulted from the cellular bioreduction of 2-hydroperoxy-5,5-dimethyl-1-pyrrolidinyloxyl (DMPO-OOH) by glutathione peroxidase. Addition of superoxide dismutase (SOD) to the in vitro system indicated that superoxide production was intracellular. When noninduced hepatocytes were utilized, free radical production was not evident. Thus, the cytochrome P450 monooxygenase system was responsible, in part, for the intermediacy of superoxide anion during hepatic metabolism.

Metabolism of the 4-iodo derivative of tamoxifen by isolated rat hepatocytes. Demonstration that the iodine atom reduces metabolic conversion and identification of four metabolites

The 4-iodo derivative of tamoxifen, which has been reported to possess improved oestrogen receptor affinity and effectiveness as an inhibitor of breast tumour cell growth in vitro, was metabolized by hepatocytes isolated from rats pretreated with phenobarbital four times more slowly than tamoxifen and there was very little formation of glucuronide conjugates. Four principal metabolites were isolated. Examination of mass spectra revealed desmethyl-4-iodotamoxifen, 4-iodotamoxifen N-oxide, and alpha-hydroxydesmethyl-4-iodotamoxifen (4-[4-[2-(methylamino)ethoxy]phenyl]-4-(4-iodophenyl)-3-phenyl-but-3- (Z)-en-2-ol). Their identification was confirmed by comparison with synthesized samples. The structure of the fourth metabolite, 4'-hydroxy-4-iodotamoxifen was revealed by 1H NMR spectroscopy. The iodophenyl moiety is thus retained in all the metabolites. The iodine atom not only blocks metabolism in its vicinity but also reduced the rate of side-chain demethylation and N-oxidation by three-fold. It can be predicted from this study that the presence of the iodine atom should give the compound a greater duration of action in vivo.

Determination of 2-methyl derivatives of tamoxifen in cell culture medium using high-performance liquid chromatography and electrochemical detection

The 2-methyl derivatives of tamoxifen (2-methyltamoxifen and 2-methyl-4-hydroxytamoxifen) were extracted from a cell culture medium at pH 5.4 (Earle's Minimum Essential Medium) with an internal standard (tamoxifen) on a phenyl sorbent cartridge. The compounds were then separated by high-performance liquid chromatography on a nitrile column eluted with acetonitrile-methanol-0.05 M sodium dihydrogenphosphate (19:4:11.6:69,v/v) containing 0.11 mmol/l disodium EDTA and determined by electrochemical detection at +1.1 V vs. Ag/AgCl/3 M NaCl. The absolute detection limits were 50 pg for 2-methyl-4-hydroxytamoxifen and 100 pg for tamoxifen and 2-methyltamoxifen at a sensitivity of 1 nA/V.

The metabolism of propranolol (ICI 45,520, Inderal) and xamoterol (ICI 118,587, Corwin) by isolated rat hepatocytes: in vivo-in vitro correlations

1. The metabolism of two compounds which undergo predominantly Phase I (propranolol) and Phase II (xamoterol) metabolism in vivo has been studied in isolated rat hepatocytes. 2. Propranolol was rapidly metabolized by rat hepatocytes to a number of metabolites which correlated well with those observed in vivo. The effect of saturable metabolism on the in vitro clearance of propranolol at high substrate concentrations was very similar to the changes observed in vivo. 3. Xamoterol was metabolized by rat hepatocytes to produce mainly xamoterol glucuronide, with the sulphate conjugate of xamoterol representing a minor component. The low rate of formation of xamoterol sulphate is probably due to the low affinity of xamoterol for the sulphotransferase enzyme, since supplementation with inorganic sulphate did not significantly alter the rate of sulphation; the sulphotransferase system of these hepatocytes was however shown to be active in the metabolism of phenol. 4. The correlations observed between the known routes of metabolism of propranolol and xamoterol in vivo and those observed in isolated hepatocytes support the utility of isolated hepatocytes as a predictive model of metabolic events in vivo.

Metabolism of steroid-modifying anticancer agents

The application of steroid-modifying drugs as a strategy for the treatment of hormone-dependent cancers has gained increasing popularity during the past decade. However, it is important to point out and emphasize that very few of the agents were originally designed for their current application. Most were designed for other purposes, predominantly fertility control (e.g. LHRH agonists and the antiestrogens). Nevertheless, now it is possible to integrate their actions to design rational therapies. There are many reasons for the current interest in antisteroidal drugs. The initial euphoria over the potential ability of combination chemotherapy to cure breast and prostatic carcinoma has proved to be premature. Combination chemotherapy has many severe side-effects which limits patient acceptability, especially if the patient realizes that the likelihood of a cure is remote. In the main, antisteroidal therapies do not have many side-effects and those that do, e.g. aminoglutethimide, are the focus of increased efforts in drug design to produce increased drug specificity. Finally, there is a growing realization that hormone-dependent cancer control with a nontoxic, antisteroidal therapy may be the most acceptable approach currently available for early disease management. Chemotherapy would then be reserved as the final option for treatment. The description of drug metabolism has been central to the development of synthetic LHRH analogs and an understanding of the mode of action of nonsteroidal antiestrogens and antiandrogens. The discovery of steroid synthetic pathways has been essential for the development of the aromatase inhibitors. This whole area of endeavor has now become a major focus of attention for the medicinal chemist. A new generation of agents is entering clinical evaluation which will provide a wealth of valuable information about the successful (or unsuccessful?) methods to control hormone-dependent disease. Since the success or failure of a drug can often depend upon formulation, pharmacokinetics, bioavailability or metabolism, it is our hope that this overview might help solve some of the future problems.

Metabolism of tamoxifen by isolated rat hepatocytes. Identification of 1-[4-(2-hydroxyethoxy)phenyl]-1-(4-hydroxyphenyl)-2-phenyl-1-butene and the dependence of N-oxidation on oxygen availability

Metabolism of tamoxifen by rat hepatocytes and hydrolysis of the resulting polar metabolites corresponding to conjugates with beta-glucuronidase gave a major component which was identified as 1-[4-(2-hydroxyethoxy)phenyl]-1-(4-hydroxyphenyl)-2-phenyl-1-butene by comparison of mass spectral properties with those of synthetic material. This compound, which was not observed as a phase I metabolite, is believed to have been found previously in rat bile and in human faeces (metabolite F) but its structure had been incorrectly assigned. Its binding affinity for the estrogen receptor was greater than that of tamoxifen but less than that of 4-hydroxytamoxifen, and it possessed a corresponding degree of antitumour activity against the MCF-7 breast cancer cell line. By carrying out the hepatocyte incubation separately under oxygen and air, it has been shown that the N-oxidation of tamoxifen is favoured by a high concentration of oxygen during in vitro metabolism but that the rate of 4-hydroxylation is not dependent on oxygen availability.

Aspects of metabolism of tamoxifen by rat liver microsomes. Identification of a new metabolite: E-1-[4-(2-dimethylaminoethoxy)-phenyl]-1, 2-diphenyl-1-buten-3-ol N-oxide

Metabolism of tamoxifen N-oxide by phenobarbitone-induced rat liver microsomes gave a major metabolite which was identified as E-1-[4-(2-dimethylaminoethoxy)phenyl]-1, 2-diphenyl-1-buten-3-ol N-oxide (alpha-hydroxytamoxifen N-oxide) by comparison of mass spectral properties with synthetic material. This new metabolite was also formed from tamoxifen. Tamoxifen epoxide was synthesised; its microsomal metabolism gave the corresponding N-oxide. Neither tamoxifen epoxide nor its N-oxide was detected as a product of tamoxifen metabolism.

Metabolism of tamoxifen and its uterotrophic activity

Tamoxifen is an estrogen agonist in mouse uterus, a partial estrogen agonist/antagonist in rat uterus, and a pure estrogen antagonist in chicken oviduct. Tamoxifen metabolism was examined both in vitro and in vivo to determine if differences in the species response to this drug resulted from the differential formation of metabolites with estrogenic or antiestrogenic activity. Animals were given a subcutaneous injection of [3H]tamoxifen, and 4 or 24 hr later tamoxifen and its metabolites were extracted from tissues and separated by TLC. The profiles of metabolites extracted from the livers of these species were qualitatively similar; the principle metabolite was 4-hydroxytamoxifen, which comprised 27, 14, and 16% of the radioactivity from mouse, rat, and chicken livers, respectively, at 24 hr. Tamoxifen, however, was the principal compound extracted from all three livers. Metabolites extracted from mouse and rat uteri were the same ones obtained from liver, although their abundance (relative to tamoxifen) was much lower in uteri than in liver. Metabolite E and bisphenol, two tamoxifen derivatives that we believed might account for the uterotrophic effect of tamoxifen in the mouse, were found not to be formed in either liver or uterus. Tamoxifen metabolism was also studied in vitro using liver microsomes from these same species; The same metabolites were formed in vitro as in vivo, although their relative abundances were lower in vitro. No clear-cut differences in metabolism were seen that would account for the disparate pharmacological effects of tamoxifen in these species.

Metabolism of clomiphene in the rat. Estrogen receptor affinity and antiestrogenic activity of clomiphene metabolites

Incubation of the nonsteroidal antiestrogen clomiphene with rat liver microsomes resulted in the formation of the 4-hydroxy-, N-desethyl-, and N-oxide metabolites, in qualitative contrast to results previously obtained analogously with rabbit microsomes, with which only the first two metabolites were detected. Metabolites were characterized by thin-layer chromatography in comparison with synthetic standards. They were similarly compared using low resolution electron ionization mass spectrometry, except for the N-oxide which was best characterized by fast atom bombardment mass spectrometry. Oral administration of clomiphene resulted in no detectable urinary elimination of the drug or its metabolites; 4-hydroxyclomiphene was the sole detectable elimination product in fecal extracts. The relative uterine cytosol estrogen receptor binding affinities, at 4 degrees, of 4-hydroxyclomiphene and the E-isomers of clomiphene, desethylclomiphene, and clomiphene N-oxide were, in turn, 331, 0.71, 0.62, and 0.88 (estradiol = 100). In the 3-day immature rat uterotropic assay, 4-hydroxyclomiphene had no significant uterotropic effect at doses up to 50 micrograms/day, but substantially inhibited that of estradiol (0.5 micrograms/day) at doses of 2 micrograms/day.

Laboratory studies to develop general principles for the adjuvant treatment of breast cancer with antiestrogens: problems and potential for future clinical applications

The general pharmacology of tamoxifen in animals and man is reviewed with particular reference to the long-term adjuvant therapy of node-positive breast cancer. Rats with dimethylbenzanthracene (DMBA)-induced mammary carcinomata have been used extensively as a laboratory model to study hormone-dependent cancer. The administration of a 30-day course of tamoxifen (50 micrograms daily) starting 5, 15, 30, or 50 days after DMBA caused a delay in tumor appearance and decrease in the cumulative number of tumors that were induced by 200 days. Similarly, the administration of increasing doses of tamoxifen (0.2, 3, 50, and 800 micrograms daily) between 30 and 60 days after DMBA produced a dose-related delay in tumor appearance and a decrease in the cumulative number of tumors at 200 days. Since the tumors that were induced after tamoxifen still responded to ovariectomy, tamoxifen appears to act as an inhibitor of the tumor cell cycle rather than as a tumoricidal agent in this model. This principle was exemplified by comparing a short course (30 day) with a continuous course (170 day) of tamoxifen initiated 30 days after DMBA. The short course of therapy only delayed tumor appearance whereas continuous therapy maintained 90% of the animals in a tumor-free state. These data strongly support the use of long-term (up to five-year) adjuvant therapy with tamoxifen in patients as a suppressive therapy for hormone-sensitive metastases.