Therapeutic Drug Monitoring of Oral Anti-Hormonal Drugs in Oncology.

Oral anti-hormonal drugs are essential in the treatment of breast and prostate cancer. It is well known that the interpatient variability in pharmacokinetic exposure is high for these agents and exposure-response relationships exist for many oral anti-hormonal drugs. Yet, they are still administered at fixed doses. This could lead to underdosing and thus suboptimal efficacy in some patients, while other patients could be overdosed resulting in unnecessary side effects. Therapeutic drug monitoring (TDM), individualized dosing based on measured blood concentrations of the drug, could therefore be a valid option to further optimize treatment. In this review, we provide an overview of relevant clinical pharmacokinetic and pharmacodynamic characteristics of oral anti-hormonal drugs in oncology and translate these into practical guidelines for TDM. For some agents, TDM targets are not well established yet and as a reference the median pharmacokinetic exposure could be targeted (exemestane: minimum plasma concentration (Cmin) 4.1 ng/mL and enzalutamide: Cmin 11.4 mg/L). However, for most drugs, exposure-efficacy analyses could be translated into specific targets (abiraterone: Cmin 8.4 ng/mL, anastrozole: Cmin 34.2 ng/mL, and letrozole: Cmin 85.6 ng/mL). Moreover, prospective clinical trials have shown TDM to be feasible for tamoxifen, for which the exposure-efficacy threshold of its active metabolite endoxifen is 5.97 ng/mL. Based on the available data, we therefore conclude that individualized dosing based on drug concentrations is feasible and promising for oral anti-hormonal drugs and should be developed further and implemented into clinical practice.

Stearoyl-CoA desaturase-1, a novel target of omega-3 fatty acids for reducing breast cancer risk in obese postmenopausal women.

BACKGROUND/OBJECTIVES: Conversion of saturated fatty acids to monounsaturated fatty acids by the enzyme stearoyl-Co-A-desaturase (SCD-1) is emerging as a major factor in promoting carcinogenesis including breast cancer. The aim of our study was to explore the regulation of SCD-1 by Raloxifene and omega-3 fatty acids in women at increased risk of breast cancer based on high breast density. SUBJECTS/METHODS: As a reflection of SCD-1 activity, we measured the ratios of palmitoleic acid (C16:1n7) to palmitic acid (C16:0) (SCD-16) and oleic acid (C18:1n9) to steric acid (C18:0) (SCD-18) in plasma samples of postmenopausal women enrolled in our clinical trial (NCT00723398) designed to test the effects of the antiestrogen, Raloxifene and/or the omega-3 preparation Lovaza, on breast density, a validated biomarker of breast cancer risk. RESULTS: We report that Lovaza but not Raloxifene-reduced SCD-16 and SCD-18 for the 2-year duration of the trial. Importantly, decreasing levels of SCD-16 and SCD-18 were associated with a progressive reduction in breast density but only in obese women (body mass index ⩾30). CONCLUSIONS: Body mass index-related factors play an important role in the reduction of breast density and hence breast cancer risk by omega-3 fatty acids. SCD-1 may be a useful biomarker in future clinical trials testing the benefit of nutritional interventions in reducing obesity-associated breast cancer risk.

Tamoxifen and estradiol improved locomotor function and increased spared tissue in rats after spinal cord injury: their antioxidant effect and role of estrogen receptor alpha.

17ß-Estradiol is a multi-active steroid that imparts neuroprotection via diverse mechanisms of action. However, its role as a neuroprotective agent after spinal cord injury (SCI), or the involvement of the estrogen receptor-alpha (ER-α) in locomotor recovery, is still a subject of much debate. In this study, we evaluated the effects of estradiol and of Tamoxifen (an estrogen receptor mixed agonist/antagonist) on locomotor recovery following SCI. To control estradiol cyclical variability, ovariectomized female rats received empty or estradiol filled implants, prior to a moderate contusion to the spinal cord. Estradiol improved locomotor function at 7, 14, 21, and 28 days post injury (DPI), when compared to control groups (measured with the BBB open field test). This effect was ER-α mediated, because functional recovery was blocked with an ER-α antagonist. We also observed that ER-α was up-regulated after SCI. Long-term treatment (28 DPI) with estradiol and Tamoxifen reduced the extent of the lesion cavity, an effect also mediated by ER-α. The antioxidant effects of estradiol were seen acutely at 2 DPI but not at 28 DPI, and this acute effect was not receptor mediated. Rats treated with Tamoxifen recovered some locomotor activity at 21 and 28 DPI, which could be related to the antioxidant protection seen at these time points. These results show that estradiol improves functional outcome, and these protective effects are mediated by the ER-α dependent and independent-mechanisms. Tamoxifen׳s effects during late stages of SCI support the use of this drug as a long-term alternative treatment for this condition.

Effects of cytochrome P450 inhibitors and inducers on the metabolism and pharmacokinetics of ospemifene.

PURPOSE: The objectives were to determine the cytochrome P450 (CYP) enzymes involved in the metabolism of ospemifene and its main hydroxylated metabolites and to examine the effects of CYP inhibitors and inducers on ospemifene pharmacokinetics. METHODS: In vitro metabolism studies were conducted using human liver microsomes; CYP-selective inhibitors and CYP-specific substrates were used to determine the roles of nine CYP isoforms in ospemifene metabolism. Two Phase 1 clinical trials were conducted in healthy postmenopausal women; crossover designs examined the effects of pretreatment with the CYP modulators rifampicin, ketoconazole, fluconazole and omeprazole on ospemifene pharmacokinetics. RESULTS: Although several CYP inhibitors decreased the in vitro formation of ospemifene metabolites, none of them completely blocked metabolism. Roles for CYP3A4, CYP2C9, CYP2C19 and CYP2B6 in the metabolism of ospemifene and its two main metabolites, 4--hydroxyospemifene and 4'-hydroxyospemifene, were confirmed. The in vivo experiments demonstrated that ospemifene serum concentrations were decreased by rifampicin pretreatment, increased by ketoconazole or fluconazole pretreatment, and minimally affected by omeprazole pretreatment. CONCLUSIONS: The clinical pharmacokinetic findings and in vitro data suggest that CYP3A4 is important for ospemifene metabolism, but other CYP isoforms and metabolic pathways also contribute. Strong CYP3A or CYP2C9 inducers (e.g. rifampicin) would be expected to decrease the exposure to ospemifene. Ospemifene should be used with caution when coadministered with the modest CYP3A inhibitor ketoconazole and should not be coadministered with the potent CYP3A/CYP2C9/CYP2C19 inhibitor fluconazole. The potent CYP2C19 inhibitor omeprazole is unlikely to cause clinically significant changes in ospemifene pharmacokinetics.

Endoxifen, the active metabolite of tamoxifen, is a substrate of the efflux transporter P-glycoprotein (multidrug resistance 1).

Tamoxifen is widely prescribed to patients with estrogen receptor-positive breast cancer, and it is a prodrug that requires bioactivation by cytochrome P450 enzymes CYP2D6 and 3A4 to generate the active metabolite, endoxifen. Large interpatient variability in endoxifen plasma levels has been reported, and polymorphisms in CYP2D6 have been implicated as a major determinant of such variability. However, little is known regarding the role of drug transporters such as P-glycoprotein [multidrug resistance 1 (MDR1), ATP-binding cassette B1 (ABCB1)] to endoxifen disposition and response. Therefore, we determined the ability of P-glycoprotein to transport endoxifen in vitro, using a polarized human P-glycoprotein-overexpressing cell line. Markedly higher transport of endoxifen was observed in the basal-to-apical direction, which was abrogated in the presence of the potent and specific P-glycoprotein inhibitor (2R)-anti-5-{3-[4-(10,11-difluoromethanodibenzo-suber-5-yl)piperazin-1-yl]-2-hydroxypropoxy}quinoline trihydrochloride (LY335979). To validate the in vivo relevance of P-glycoprotein to endoxifen disposition, plasma and tissue concentrations were also determined in Mdr1a-deficient mice after oral administration of endoxifen. Plasma endoxifen levels did not significantly differ between wild-type and Mdr1a-deficient mice. However, brain concentrations of endoxifen were nearly 20-fold higher in Mdr1a-deficient mice compared to wild-type mice. Because P-glycoprotein is highly expressed at the blood-brain barrier and in some breast cancer tumors, variation in expression and function of this transporter may alter central nervous system entry and the attained intracellular concentration in such breast cancer cells and therefore may prove to be of relevance to therapeutic outcome.

Synthesis, radiolabeling and In Vivo tissue distribution of an anti-oestrogen glucuronide compound, (99m)Tc-TOR-G.

Toremifene (TOR) has been used as an anti-oestrogen drug for the treatment and prevention of human breast cancer. The aim of this study was the addition of the hydrophilic groups diethylenetriamine pentaacetic acid (DTPA) and glucuronic acid to the starting substance TOR and to label it with technetium-99m ((99m)Tc) radionuclide and to investigate radiopharmaceutical potential of the new compound. The synthesis reactions are completed in four steps, including enzymatic reaction, with the following substeps; preparation of microsomal fraction from Hutu 80 cell line and subsequent purification of UDP-glucuronyl transferase (UDPGT), estimation of protein quantity in microsomal samples and glucuronidation reaction. The results indicate that (99m)Tc-TOR-G may be proposed as a new anti-oestrogen glucuronide imaging agent for ovarian tumours.

Identifying subsets of complex mixtures most associated with complex diseases: polychlorinated biphenyls and endometriosis as a case study.

BACKGROUND: Exploratory statistical analyses have been conducted on an epidemiologic data set in which the relationship was examined between exposure to polychlorinated biphenyl (PCB) mixtures and risk of endometriosis in women. In that study, the association between endometriosis and the sum of 4 antiestrogenic PCBs (PCBs 105, 114, 126, and 169) was borderline significant (P = 0.079), whereas an association was not found (P = 0.681) with the sum of 12 estrogenic PCBs. This finding was inconsistent with the widely held notion that endometriosis is an estrogen-dependent disease, prompting further statistical analyses to explore these associations in more detail. METHODS: As an alternative method of data reduction, an optimization algorithm was developed to determine weights in a linear combination of scaled PCB levels that has the strongest possible association with the risk of endometriosis. RESULTS: Application of this method to the antiestrogenic PCB subgroup revealed that PCB 114 was responsible for nearly 100% of the association. The fact that PCB 114 is neither the most potent nor abundant antiestrogen in the mixture suggests that PCB 114 might be estrogenic or that the association may be driven by a different mechanism. Use of this statistical weighting method for further analyses of 12 estrogenic PCBs showed that any association with endometriosis was driven mainly by PCBs 99 and 188 and possibly a few others. CONCLUSION: Although the role of PCB mixtures in endometriosis remains unclear, these results demonstrate how the integration of refined statistical methods coupled with toxicologic and biologic interpretation can generate testable hypotheses that might not otherwise have been generated.

Evaluation of the safety and tolerability of oral TAS-108 in postmenopausal patients with metastatic breast cancer.

BACKGROUND: The potential of TAS-108 for the treatment of breast cancer has been shown by preclinical studies. We therefore investigated the safe dosage, tolerability, and effectiveness on hormone levels and bone metabolism markers and the pharmacokinetics of TAS-108 administered in postmenopausal Japanese women with metastatic breast cancer. PATIENTS AND METHODS: The subjects had previously undergone standard endocrine therapeutic modalities. TAS-108 was given repeatedly to five patients each, at three dose levels (40, 80, and 120 mg p.o.) once a day after the first daily meal for a scheduled 8 weeks. Plasma concentrations of TAS-108 and its metabolites were measured at the scheduled time points. RESULTS: Fifteen patients received TAS-108 treatment. Orally administered TAS-108 was well tolerated at doses up to 120 mg and did not cause notable changes either in hormone levels or bone metabolism markers. Pharmacokinetic results indicated dose-dependent increases in plasma levels of TAS-108 and its metabolites. A steady state was achieved by 2 weeks at all dose levels, suggesting no marked accumulation. Clinical benefits were confirmed in 5 of 15 patients. CONCLUSIONS: Repeated oral administration of TAS-108 at doses up to 120 mg was well tolerated, and the plasma level of this compound increased dose-dependently.

The growth-promoting action of individual women's sera on mammary carcinoma cells.

In vitro studies concerning the growth-stimulating effect of hormones, especially of estradiol and its metabolites, have mainly been performed using pure substances and breast cancer cell lines. In order to take into account the metabolism of inactive into active hormones or drugs and vice versa which occurs in several tissues, the influence of individual patients' sera on the growth of breast cancer cells in vitro was tested. Besides measuring the growth promoting action of several hormone replacement therapies, the antiestrogenic effect was determined by measuring the effect of 10(-10) M estradiol added to the culture medium (E2-sensitivity). Influence on proliferation and stimulatability was similar in MCF-7 and T47-D cells. Growth-promoting potential correlated significantly with patient age, being higher in young ladies than in older ones. The converse was true for E2 sensitivity. From the different steroid hormones tested, only higher estradiol levels were associated with increased growth stimulation and diminished E2 sensitivity. Hormone replacement therapy (HRT) of different types did not significantly increase growth potential of serum, however these results are preliminary. Treatment with tamoxifen of breast cancer patients led to a decrease of E2 sensitivity, whereas growth potential was not affected significantly. For the aromatase inhibitor Arimidex, a tendency towards growth inhibition and increased E2 sensitivity was observed. Our in vitro system allows identifying differences between individual persons and groups of women of different age or treatment with respect to stimulation of growth or influence on estrogen sensitivity of breast cancer cells by serum. It is speculated that results might reflect the personal risk or the risk under treatment to develop breast cancer.

Metabolism, distribution, and excretion of a next generation selective estrogen receptor modulator, lasofoxifene, in rats and monkeys.

Disposition of lasofoxifene (LAS; 6-phenyl-5-[4-(2-pyrrolidin-1-yl-ethoxy)-phenyl]-5,6,7,8-tetrahydro-naphthalen-2-ol. tartrate) was investigated in rats and monkeys after oral administration of a single oral dose of [(14)C]LAS. Total mean recoveries of the radiocarbon were 96.7 and 94.3% from rats and monkeys, respectively. The major route of excretion in both species was the feces, and based on a separate study in the bile duct-cannulated rat, this likely reflects excretion in bile rather than incomplete absorption. Whole-body autoradioluminography suggested that [(14)C]LAS radioequivalents distributed rapidly in the rat with most tissues achieving maximal concentrations at 1 h. Half-life of radioactivity was longest in the uvea (124 h) and shortest in the spleen ( approximately 3 h). LAS was extensively metabolized in both rats and monkeys because no unchanged drug was detected in urine and/or bile. Based on area under the curve((0-24)) values, >78% of the circulating radioactivity was due to the metabolites. A total of 22 metabolites were tentatively identified by liquid chromatography-tandem mass spectrometry. Based on the structures of the metabolites, six metabolic pathways of LAS were identified: hydroxylation at the tetraline ring, hydroxylation at the aromatic ring attached to tetraline, methylation of the catechol intermediates by catechol-O-methyl transferase, oxidation at the pyrrolidine ring, and direct conjugation with glucuronic acid and sulfuric acid. LAS and its glucuronide conjugate (M7) were the major circulating drug-related moieties in both rats and monkeys. However, there were notable species-related qualitative and quantitative differences in the metabolic profiles. The catechol (M21) and its sulfate conjugate (M10) were observed only in monkeys, whereas the glucuronide conjugate of the methylated catechol (M8) and hydroxy-LAS (M9) were detected only in rats.

Increased antitumor potential of the raloxifene prodrug, raloxifene diphosphate.

Raloxifene (RAL) significantly reduced the incidence of breast cancer in women at high risk of developing the disease. Unlike tamoxifen (TAM), an increased incidence of endometrial cancer was not observed in women treated with RAL. However, RAL, having two hydroxyl moieties, can be conjugated rapidly through phase II metabolism and excreted, making it difficult to achieve adequate bioavailability by oral administration in humans. As a result, higher doses must be administered to obtain an efficacy equivalent to that achieved with TAM. To improve oral bioavailability and antitumor potential, RAL diphosphate was prepared as a prodrug. RAL diphosphate showed several orders of magnitude lower binding potential to both ER alpha and ER beta and weak antiproliferative potency on cultured human MCF-7 and ZR-75-1 breast cancer cells, as compared to RAL. However, RAL diphosphate has a much higher bioavailability than RAL, endowing it with higher antitumor potential than RAL against both 7,12-dimethylbenz(a)anthracene-induced mammary carcinoma in rats and human MCF-7 breast cancer implanted in athymic nude mice. The RAL prodrug may provide greater clinical benefit for breast cancer therapy and prevention.

CYP2D6 genotype, antidepressant use, and tamoxifen metabolism during adjuvant breast cancer treatment.

BACKGROUND: The efficacy of tamoxifen therapy for the treatment of breast cancer varies widely among individuals. Plasma concentrations of the active tamoxifen metabolite endoxifen are associated with the cytochrome P450 (CYP) 2D6 genotype. We examined the effects of concomitant use of selective serotonin reuptake inhibitor antidepressants, which are CYP2D6 enzyme inhibitors commonly prescribed to treat hot flashes in women who take tamoxifen, and genotypes for genes that encode tamoxifen-metabolizing enzymes on plasma concentrations of tamoxifen and its metabolites. METHODS: Eighty patients with newly diagnosed with breast cancer who were beginning tamoxifen therapy (20 mg/day orally), 24 of whom were taking CYP2D6 inhibitors, were genotyped for common alleles of the CYP2D6, CYP2C9, CYP3A5, and sulfotransferase (SULT) 1A1 genes. Plasma concentrations of tamoxifen and its metabolites were measured after 1 and 4 months of tamoxifen therapy. Differences in plasma concentrations of tamoxifen and its metabolites between genotype groups were analyzed by the Wilcoxon rank sum test. All statistical tests were two-sided. RESULTS: Among all women, plasma endoxifen concentrations after 4 months of tamoxifen therapy were statistically significantly lower in subjects with a CYP2D6 homozygous variant genotype (20.0 nM, 95% confidence interval [CI] = 11.1 to 28.9 nM) or a heterozygous genotype (43.1 nM, 95% CI = 33.3 to 52.9 nM) than in those with a homozygous wild-type genotype (78.0 nM, 95%CI = 65.9 to 90.1 nM) (both P = .003). Among subjects who carried a homozygous wild-type genotype, the mean plasma endoxifen concentration for those who were using CYP2D6 inhibitors was 58% lower than that for those who were not (38.6 nM versus 91.4 nM, difference = -52.8 nM, 95% CI = -86.1 to -19.5 nM, P = .0025). The plasma endoxifen concentration was slightly reduced in women taking venlafaxine, a weak inhibitor of CYP2D6, whereas the plasma endoxifen concentration was reduced substantially in subjects who took paroxetine (a potent inhibitor of CYP2D6). Genetic variations of CYP2C9, CYP3A5, or SULT1A1 had no statistically significant associations with plasma concentrations of tamoxifen or its metabolites. CONCLUSION: Interactions between CYP2D6 polymorphisms and coadministered antidepressants and other drugs that are CYP2D6 inhibitors may be associated with altered tamoxifen activity.

Environmental PCB exposure and risk of endometriosis.

BACKGROUND: Hormonally active environmental agents have recently been associated with the development of endometriosis. METHODS: We undertook a study to assess the relationship between endometriosis, an estrogen-dependent gynaecological disease, and 62 individual polychlorinated biphenyl (PCBs) congeners. We enrolled 84 eligible women aged 18-40 years undergoing laparoscopy for study, which included an interview and blood specimen (n=79; 94%). Thirty-two women had visually confirmed endometriosis at laparoscopy while 52 did not. Blood specimens were run in batches of 14 including four quality control samples for toxicological analysis. Each PCB congener was adjusted for recovery; batch-specific reagent blanks were subtracted. All PCB concentrations were log transformed and expressed in ng/g serum first as a sum and then as tertiles by purported estrogenic or anti-estrogenic activity of PCB congeners. RESULTS: Using unconditional logistic regression analysis, a significantly elevated odds ratio (OR) was observed for women in the third tertile of anti-estrogenic PCBs [OR 3.77; 95% confidence interval (CI) 1.12-12.68]. Risk remained elevated after controlling for gravidity, current cigarette smoking and serum lipids (OR 3.30; 95% CI 0.87-12.46). CONCLUSIONS: These data suggest that anti-estrogenic PCBs may be associated with the development of endometriosis.

Therapeutic strategies using the aromatase inhibitor letrozole and tamoxifen in a breast cancer model.

BACKGROUND: The antiestrogen tamoxifen has potent activity against estrogen receptor-positive breast cancer, but two nonsteroidal aromatase inhibitors, letrozole and anastrozole, show considerable advantages over tamoxifen with respect to patient survival and tolerability. To determine the optimal way to use letrozole and tamoxifen, we studied their effects on a breast tumor xenograft model, MCF-7Ca, that is responsive to both antiestrogens and aromatase inhibitors. METHODS: Female ovariectomized BALB/c athymic nude mice carrying xenograft tumors were treated daily subcutaneously with one of the following first-line therapies for varying durations: no drug (control), tamoxifen (100 microg/day) alone, letrozole (10 microg/day) alone, both drugs at the same time, or alternating 4-week courses of each drug (beginning with a course of tamoxifen or beginning with a course of letrozole). Tumor volumes and weights were estimated using linear mixed-effects models. The time to tumor doubling was calculated, and tumor weights in the treatment groups were compared, with adjustments for multiple comparisons being made with either Tukey's or Dunnett's procedure. Second-line therapies (with tamoxifen, letrozole, or fulvestrant) were initiated when tumors doubled in size under first-line therapies. All statistical tests were two-sided. RESULTS: The times for doubling of tumor volume were as follows: control, 3-4 weeks; tamoxifen alone, 16 weeks; tamoxifen alternating with letrozole, 17-18 weeks; tamoxifen plus letrozole, 18 weeks; letrozole alternating with tamoxifen, 22 weeks; letrozole alone, 34 weeks. First-line treatment with letrozole was superior to treatment with tamoxifen alone or with the two drugs combined (at week 16, both P<.001). Alternating tamoxifen and letrozole and alternating letrozole and tamoxifen were also not as effective as letrozole alone (at week 16, P =.002 and P<.001, respectively). Tumors progressing on tamoxifen remained sensitive to second-line therapy with letrozole compared with those remaining on tamoxifen at the end of treatment (week 28, P<.001), whereas tumors progressing on letrozole were unaffected by second-line treatment with the antiestrogens tamoxifen or fulvestrant. CONCLUSIONS: First-line letrozole therapy extends time for tumor progression in this model relative to the other treatment regimens tested. However, further studies are needed to determine the most effective second-line therapy for tumors that progress on letrozole.

Characterization of new estrogen receptor destabilizing compounds: effects on estrogen-sensitive and tamoxifen-resistant breast cancer.

BACKGROUND: Antiestrogens of the selective estrogen receptor modulator (SERM) type, such as tamoxifen, have two major limitations: their mixed agonist and antagonist profile and the development of tumor resistance. We characterized two new pure antiestrogens-ZK-703 and ZK-253-that belong to the class of specific estrogen receptor destabilizers (SERDs), which includes fulvestrant, and compared their activity with that of fulvestrant and tamoxifen. METHODS: Effects of antiestrogens on the growth of estrogen-dependent breast tumors in vivo were determined using several mouse xenograft models (including the tamoxifen-sensitive tumors MCF7, T47D, and MV3366 and the tamoxifen-resistant tumors ZR75-1 and MCF7/TAM) and chemically induced (nitrosomethyl urea [NMU] and dimethylbenzanthracene [DMBA]) rat breast cancer models (groups of 10 animals). We determined the initial response and effects on hormone receptor levels and the time to relapse after treatment (i.e., time to reach a predetermined tumor size threshold). Estrogen receptor (ER) levels were determined by immunoassay. RESULTS: ZK-703 (administered subcutaneously) and ZK-253 (administered orally) were more effective than tamoxifen or fulvestrant at inhibiting the growth of ER-positive breast cancer in all xenograft models. For example, MCF7 tumors relapsed (i.e., reached the size threshold) in 10 weeks in mice treated with tamoxifen but in 30 weeks in mice treated with ZK-703. ZK-703 and ZK-253 also prevented further tumor progression in tamoxifen-resistant breast cancer models to a similar extent (more than 30 weeks in mice with ZR75-1 and MCF7/TAM tumors). In the chemically induced rat breast cancer models, orally administered ZK-703 and ZK-253 caused a nearly complete (>80%) inhibition of tumor growth. ER levels were dramatically reduced in MCF7 tumors after 5 weeks of ZK-703 treatment compared with ER levels in vehicle-treated tumors; by contrast, ER levels in tamoxifen-treated tumors were higher than those in control tumors. CONCLUSION: ZK-703 and ZK-253 are potent, long-term inhibitors of growth in both tamoxifen-sensitive and tamoxifen-resistant breast cancer models.

The in vivo metabolism of tibolone in animal species.

The in vivo tissue distribution and metabolism of tibolone was studied in different animals to further investigate the compound's tissue-specificity. Tibolone's metabolism was studied in vivo in rats and rabbits by administration of [16-3H]-tibolone and the metabolic pattern was determined in urine and faeces after oral administration to female rats and dogs. The main excretory pathway was found to be excretion in the faeces. Important phase-I metabolic routes were the reduction of the 3-keto to the 3a- or 3beta-hydroxy functions with a preference for 3alpha-OH in rats and for 3beta-OH in dogs. To a lesser extent, hydroxylation reactions at C2 and C7, and a shift of the delta5(10)-double bond to a delta4(5)-position also occurred. The main phase-II metabolic route was sulphate conjugation of the hydroxyl groups at C3 and C17. Since the oxidation reactions form only a minor part of the metabolism of tibolone, it is concluded that the cytochrome P450 enzymes do not play an important role in tibolone's metabolism. For both phases, quantitative differences were found between the species. In human similar metabolites are found. Profiling of the target organs in female rats and rabbits showed a tissue-specific distribution of metabolites. The majority of the metabolites existed as sulphate conjugates and no glucuronidated conjugates were observed. The same metabolites were found in both the circulation and the tissues. However, different tissues had quantitatively different metabolic profiles.