Tamoxifen Levels Research Articles

Pharmacokinetics of tamoxifen in premenopausal and postmenopausal women with breast cancer

We measured the serum levels of tamoxifen and 5 of its metabolites in serum from 7 premenopausal and 9 postmenopausal women during the first 56 days of treatment. The serum levels of N-desdimethyltamoxifen were higher in postmenopausal women compared to premenopausal women ( P < 0.02). A similar trend was observed for N-desmethyltamoxifen ( P = 0.06).

Determination of panomifene in human plasma by high-performance liquid chromatography

An ion-pair HPLC method was developed for the determination of the antiestrogenic drug panomifene, ( E)-1,2-diphenyl-1-{4-[2-(2-hydroxyethylamino)etboxylphenyl}-3,3,3-trifluoropropene, in human plasma. Tamoxifen, 20 ng in 1 ml of plasma, was used as an internal standard. The compounds were isolated from plasma by liquid-solid extraction. Fluorescence detection was achieved by on-line photochemical conversion of the compounds into highly fluorescent phenanthrene derivatives. The sensitivity of the method was 1 ng/ml. The within-day and between-day precision, linearity, extraction recovery and stability of panomifene in plasma and in deproteinized plasma were determined for validation of the method. The method is suitable for measuring plasma levels of panomifene and tamoxifen and for pharmacokinetic studies.

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

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

Acquired tamoxifen resistance in human breast cancer and reduced intra-tumoral drug concentration

Reduced intra-tumoral drug concentrations have been investigated as a mechanism of tamoxifen resistance in 51 patients with locally recurrent breast cancer. Serum tamoxifen was similar in patients with acquired and de-novo resistance, but intra-tumoral concentrations were significantly lower in patients with acquired resistance. Tumour oestrogen-receptor concentrations at relapse did not support the hypothesis that selective outgrowth of oestrogen-receptor-negative cells is a major mechanism for acquired tamoxifen resistance. Reduced intra-tumoral tamoxifen levels during prolonged therapy may be an important mechanism for acquired resistance in breast cancer.

Titration of the in vivo uptake of 16α-[ 18F]fluoroestradiol by target tissues in the rat: Competition by tamoxifen, and implications for quantitating estrogen receptors in vivo and the use of animal models in receptor-binding radiopharmaceutical development

We have measured in vivo the uptake of 16α-[ 18F]estradiol (FES) by target tissues in the immature rat at increasing dose levels (obtained by dilution of [ 18F]FES with unlabeled estradiol). This was done to examine the binding capacity of target tissues in vivo and to determine whether the uptake in receptor-rich tissues was flow limited, as this has implications concerning the appropriateness of using receptor-rich tissues in experimental animals as models for FES uptake by receptor-poor breast tumors in humans. We also wanted to establish the dose level of the anti-estrogen tamoxifen required to block target tissue uptake of FES. We found that in untreated rats, specific uptake in the uterus saturated at c. 180 pmol/g, in the ovary at c. 54 pmol/g and in the muscle at c. 2 pmol/g. At an intermediate dose of tamoxifen (570 μg/kg), uptake saturated at somewhat lower levels, and at a high tamoxifen dose (1710 μg/kg), yet lower specific uptake was evident. In the FES titrations at low dose levels of FES, both the uterus and the ovaries, but not the muscle, showed characteristics of flow-limited uptake, i.e. the uptake-to-dose ratio reached a maximum level. This flow limitation suggests that only when receptor levels are sufficiently low will the FES uptake be related to receptor concentration. While receptor-rich tissues such as the rat uterus will show this flow limitation, the receptor concentration in most primary and metastatic human breast tumors is sufficiently low, so that the uptake should parallel receptor content. In in vivo distribution studies, target tissues (or tumors) with low receptor content will be more fully saturated and ligand more readily displaced. Also, uptake by secondary target tissues (i.e. those with a lower content of estrogen receptor, such as muscle, thymus and kidney) may be better models for assessing the effectiveness of new breast tumor imaging agents than uptake by receptor-rich tissues.

Investigation of the Mechanism of Tamoxifen-Stimulated Breast Tumor Growth With Nonisomerizable Analogues of Tamoxifen and Metabolites

The nonsteroidal anti-estrogen tamoxifen (TAM) is the front-line endocrine treatment for breast cancer, but disease recurrence is common. Treatment failure may occur because tumors become insensitive to TAM. Alternatively, resistance may occur because tumors become stimulated rather than inhibited by TAM. TAM-stimulated growth of MCF-7 human breast tumors has been observed in athymic mice after prolonged treatment with TAM. Our purpose was to examine the mechanism of treatment failure by determining whether TAM-stimulated tumors acquire the ability to excrete TAM and its anti-estrogenic metabolites or to convert them to estrogenic compounds with weakened antiestrogenic activity. We used high-pressure liquid chromatography to quantitate TAM and its metabolites in serum and tumors from ovariectomized athymic mice and in MCF-7 cells grown in vitro. We treated tumor-bearing mice with subcutaneous sustained-release preparations of estradiol, TAM, or a nonisomerizable (fixed-ring) analogue and then assessed the activity of these compounds on TAM-inhibited parental MCF-7 tumors and on TAM-stimulated MCF-7 TAM tumors. We found negligible differences in intratumoral TAM levels between TAM-inhibited parental MCF-7 tumors and TAM-stimulated MCF-7 TAM variants. We did not detect metabolite E (Met E), an estrogenic TAM metabolite, in serum or tumors. Using MCF-7 cells in vitro, we determined that the (Z) isomer of Met E, the form directly produced by TAM metabolism, must be present in the cell at a concentration of over 1000 ng/g to overcome growth inhibition by physiological levels of TAM and antiestrogenic metabolites, but the (E) isomer of Met E was effective at 10 ng/g. We reasoned that conversion of Met E from the (Z) (a weak estrogen) to (E) isomer (a potent estrogen) would be required if formation of Met E were responsible for TAM-stimulated growth. However, fixed-ring TAM, which can only form (Z) Met E, was shown to be as capable as TAM of initiating and maintaining anti-estrogen-stimulated growth of MCF-7 tumors in athymic mice. Metabolism and isomerization of TAM to estrogenic compounds is not the mechanism of TAM-stimulated growth in our model. Other potential mechanisms for TAM-stimulated growth, such as estrogen receptor mutation, must be investigated so that effective strategies can be devised to control breast cancer once therapy fails.

Tamoxifen as an antifertility agent in the long-tailed macaque ( Macaca fascicularis)

The potential use of tamoxifen, a nonsteroidal antiestrogen, as an antifertility agent was studied in the long-tailed macaque ( Macaca fascicularis). Twenty-six cycling females were bred, then treated with a single oral dose of tamoxifen (5 mg/kg) ( N = 13) or vehicle ( N = 13) on day 4 post-ovulation. Serum progesterone (P 4) and tamoxifen concentrations were evaluated on post-ovulation days 4, 8, 12, 16, and 18. No effects of treatment were observed on P 4 concentrations or on the fertility rate — pregnancy was achieved in 4/13 controls (31%) and 6/13 treated females (46%). Analysis for serum tamoxifen concentrations in samples collected during the fertility and a supplemental pharmacokinetic study ( N = 3; single oral dose of 10 mg/kg; urine and serum evaluated) failed to reveal any detectable tamoxifen levels. It was concluded that (1) absorption of tamoxifen may be negligible under the described treatment regimens or (2) tamoxifen metabolism/clearance occurs at a rapid rate.

Tamoxifen induces hepatocellular carcinoma in rat liver: A 1-year study with two antiestrogens

The effects of equimolar doses of the triphenylethylene antiestrogens tamoxifen and toremifene on female Sprague-Dawley rat liver were studied in a 52-week toxicity study which included a 13-week recovery period. Liver tumors were found in four out of five rats at the highest dose level of tamoxifen (45 mg/kg per day) after 52 weeks of dosing, and these appeared to be hepatocellular carcinomas in three rats. After the 13-week recovery period all surviving rats in the highest tamoxifen dose group had large liver tumors (diameter up to 2 cm) which appeared to be hepatocellular carcinomas in five out of six rats. No tumor was observed in the toremifene-treated rats (48 mg/kg per day) either after 52 weeks of dosing or after the recovery period. Electron microscopic morphometric analysis after 52 weeks of dosing revealed that at the tamoxifen high dose level, the volume densities of the peroxisomes, mitochondria, and residual bodies were elevated in the nonneoplastic hepatocytes of the rats. In the neoplastic hepatocytes of the tamoxifen-treated rats the volume density of nuclei was slightly elevated. The slight proliferation of peroxisomes and mitochondria might be related to tumor development in the tamoxifen treated rats.

The effects of intermittent progesterone upon tamoxifen inhibition of tumor growth in the 7,12-dimethylbenzanthracene rat mammary tumor model.

The development of endometrial cancer is a potential risk during long-term tamoxifen therapy for breast cancer. In order to protect the uterus, progestin treatment has been proposed for these patients. However, within the 7,12-dimethylbenzanthracene-induced rat mammary model, progesterone is known to reverse the antitumor effects of tamoxifen. This study shows that progesterone administered intermittently still reverses the antitumor effects of tamoxifen in this model. This effect of progesterone is not due to a decrease in the tissue levels of tamoxifen, and may be direct, via the progesterone receptor.

Recombinant human interferon-alpha 2a increases hormone receptor level of a human breast carcinoma xenograft in nude mice and enhances the anti-proliferative activity of tamoxifen.

The effect of recombinant human interferon‐α2a (rhIFN‐α2a) on the hormone receptor level and antitumor activity of tamoxifen (TAM) was investigated in nude mice using ZR‐75‐1, an estrogen receptor (ER)‐positive, and progesterone receptor (PgR)‐negative human breast carcinoma xenograft. ER levels (maximum binding sites) of tumors treated with rhIFN‐α2a at a dose of 6 × 104 U/mouse/ day for 1 or 3 wk were not significantly different from the control, whereas those with rhIFN‐α2a at a dose of 6 × 104 U/mouse/day for 1 or 3 wk were higher than the control (3.9‐ to 4.4‐fold) with a significant difference at P < 0.01. The increase of ER by rhIFN‐α2a was investigated using a sucrose density gradient method. The peak was only seen at 8S in both rhIFN‐α2a‐treated tumor and control ER, and the sedimentation patterns were almost the same, suggesting that both ERs were essentially equivalent. On the other hand, PgR of all the treated groups could be detected, while that of the control group was undetectable. The antitumor effect of the combination treatment of rhIFN‐α2a and TAM was compared with those of single treatments. While rhIFN‐α2a at a dose of 6 × 105 U/mouse/day and TAM did not show a combination effect, rhIFN‐α2a at a dose of 6 × 104 U/mouse/day and TAM showed a synergistic combination effect, and ER was decreased to the threshold of detection by the combination treatment. These findings indicated that a low dose of rhIFN‐α2a increased the ER levels of ER‐positive human breast cancer in vivo as well as in vitro and enhanced the anti‐proliferative effect of TAM, and the newly synthesized ER was essentially the same as the original ER.

Intermittent high-dose tamoxifen as a potential modifier of multidrug resistance

In vitro tamoxifen reverses multidrug resistance (MDR). To evaluate the clinical potential of using tamoxifen in this way, intermittent high-dose tamoxifen was combined with oral etoposide in 86 patients. At 320 mg/day tamoxifen for 6 days, mean plasma levels of tamoxifen in 11 patients increased from 453 ng/ml (range 269–664) on day 2 to 984 ng/ml (578–1336) on day 6. Of 31 patients who had plasma tamoxifen measured during the time of etoposide administration (days 4–6), 13 (43%) were over 1111 ng/ml (3 μmol/l), an active in vitro level. Potentially active levels of the principal metabolite, N-desmethyl tamoxifen, were also obtained but accumulation was slower. Emesis and thromboembolism were toxicities. Tamoxifen is a modifier of MDR, a role that warrants further clinical studies.

Tamoxifen and the isomers of 4-hydroxytamoxifen in tamoxifen-resistant tumors from breast cancer patients.

The antiestrogen tamoxifen is effective in therapy for breast cancer. However, its use is limited by the eventual development of acquired tamoxifen resistance in many patients. The mechanisms responsible for tamoxifen resistance remain unknown; loss of estrogen receptor (ER), selection of hormone-independent breast cancer clones, or alterations in serum tamoxifen levels after long-term use do not explain acquired resistance in most patients. Using an experimental model in which human breast cancer cells develop resistance in athymic mice treated with tamoxifen, we have recently shown that acquired resistance is associated with markedly reduced cellular concentrations of tamoxifen and by isomerization of the trans-4-hydroxy metabolite to the less potent cis isomer. Using a sensitive high-performance liquid chromatography (HPLC) assay, we have now measured levels of tamoxifen and its major metabolites in a series of 14 tumors from patients treated with tamoxifen. The duration of therapy ranged from 1 month to 6 years. Tumor tamoxifen levels varied over a wide range. Low concentrations were observed in tumors from eight patients, all demonstrating progressive disease at the time of biopsy after a minimum duration of treatment of 6 months. Six tumors had moderate to high tamoxifen levels, two from patients responding to tamoxifen, one from a patient with stable disease, and three from patients with disease progression. Both the cis and trans isomers of the potent antiestrogenic metabolite 4-hydroxy-tamoxifen were detected in 11 tumors. Six tumors had high ratios of the cis to trans isomer (1.10:2.06), all from patients not responding to tamoxifen. The five tumors with low cis:trans ratios included the two tumors from responding patients and three from patients with progression. All but one of the 11 nonresponding patients had either a low tumor tamoxifen level, a high cis:trans ratio, or both. This study clearly demonstrates a wide range of tumor tamoxifen levels and accumulation of the less antiestrogenic cis isomer of 4-hydroxytamoxifen in some patients on tamoxifen therapy. Additional study is necessary to determine if these metabolic profiles are related to the development of tamoxifen resistance.

Reversible control of oestradiol-stimulated growth of MCF-7 tumours by tamoxifen in the athymic mouse

We investigated the ability of high concentrations of oestradiol to reverse the growth inhibitory action of tamoxifen on MCF-7 breast cancer cells in vivo. Tamoxifen inhibits the oestradiol stimulated growth of MCF-7 cells in athymic mice. Using a sustained release preparation of tamoxifen we consistently achieved serum concentrations of the drug in the 40 to 50 ng ml-1 range and much higher levels in tissues. These serum levels are sufficient to inhibit the oestrogen stimulated growth of MCF-7 tumours exposed to physiologic (i.e. 300-600 pg ml-1 serum oestradiol concentrations). However, by administering dosages that increase serum oestradiol concentrations to 900-2000 pg ml-1, mimicking the increase often observed clinically in premenopausal women taking tamoxifen, we show that the growth inhibitory action of tamoxifen can be partially reversed. Serum tamoxifen levels were elevated to nearly 400 ng ml-1 by injecting 1 mg day-1 tamoxifen (IP 3 x weekly); this dosage was more effective at inhibiting oestradiol stimulated tumour growth than subcutaneous tamoxifen capsules alone. Our data suggest that at low serum levels tamoxifen may not act optimally. There may be a need to monitor tamoxifen levels in premenopausal patients to ensure that they are high enough not to be overcome by a tamoxifen induced increase in ovarian steroidogenesis.

Acquired Tamoxifen Resistance: Correlation With Reduced Breast Tumor Levels of Tamoxifen and Isomerization of Trans-4-Hydroxytamoxifen

Acquired tamoxifen resistance represents a major cause of treatment failure in breast cancer. We implanted estrogen receptor-positive MCF-7 human breast cancer cells in athymic nude BALB/c mice as a model to study in vivo acquired tamoxifen resistance. After 4-6 months of tumor growth suppression by trans-tamoxifen, tumor progression was observed despite continued tamoxifen administration. Acquired resistance was not due to loss of estrogen receptors, to alterations in serum or tumor estrogen levels, or to changes in tamoxifen or its major metabolites in serum. Tamoxifen-resistant tumors remained estrogen dependent in vivo. However, resistance was also associated with the ability of tamoxifen to stimulate tumor growth. Resistant tumors were characterized by markedly lower intracellular tamoxifen levels and by isomerization of the potent antiestrogenic metabolite trans-4-hydroxy-tamoxifen to the less potent cis isomer. Metabolic tolerance, as manifested by alterations in cellular concentrations of tamoxifen and its metabolites, may thus be one mechanism for acquired tamoxifen resistance in breast cancer.

Dose-Dependent Effects of Tamoxifen on Long Bones in Growing Rats: Influence of Ovarian Status*

Tamoxifen is a nonsteroidal antiestrogen which has been reported by various investigators to have estrogen agonist and antagonist effects on rat bone. These different interpretations may be due to differences in the ovarian status, estrogen levels, and/or tamoxifen levels of the rats. To address this issue, a dose response was determined for the effects of tamoxifen on bone histomorphometry in intact female and ovariectomized (OVX) rats. The results were compared with those obtained after treatment of OVX rats with estrogen alone or a combination of estrogen and tamoxifen. OVX resulted in increases in growth rate (weight gain) and periosteal bone formation rate and decreases in uterine weight and cancellous bone fractional volume (BV/TV). Treatment of OVX rats with estrogen resulted in dose-dependent decreases in growth rate and periosteal bone formation rate as well as dose-dependent increases in uterine weight and BV/TV. Similarly, tamoxifen treatment resulted in dose-dependent decreases in overall growth rate and periosteal bone formation rate in both OVX and intact rats. Tamoxifen treatment prevented the decrease in BV/TV after OVX, although the highest dose of tamoxifen resulted in a small decrease in BV/TV in intact female rats. In contrast to estrogen, tamoxifen treatment prevented the increase in uterine weight in intact female rats as well as the decrease in uterine weight in OVX rats. Tamoxifen treatment did not alter the effects of 17 beta-estradiol on the periosteal bone formation rate in OVX rats, but reduced the increase in BV/TV to values similar to those in intact rats. These results are consistent with tamoxifen behaving as a partial estrogen agonist on rat bone.

Regulation of proliferation of rat cartilage and bone by sex steroid hormones

We have demonstrated previously that 17β-estradiol (E 2) stimulates proliferation of skeletal tissues, both in vivo and in vitro, as measured by increased DNA synthesis and creatine kinase (CK) specific activity. The effect of E 2 on bone is sex specific. E 2 is active only in females and androgens only in males. By contrast, in cartilage of both sexes, dihydrotestosterone (DHT) as well as E 2 stimulates CK specific activity and DNA synthesis. In bone, we find that sex steroids stimulate skeletal cell proliferation in gonadectomized as well as in immature rats. Ovariectomized (OVX) rats, between 1 and 4 weeks after surgery, show stimulation of CK by E 2. The basal activity and response of CK changes with the varying endogenous levels of E 2 in cycling rats, in which the highest basal activity is at proestrus and estrus and the highest response is in diestrus. In rats of all ages tested, both the basal and stimulated specific activity of CK is higher in diaphysis and epiphysis than in the uterus, or in the adipose tissue adjacent to the uterus, which has a response similar to that of the uterus itself. The effect of E 2 in vivo, and in chrondroblasts and osteoblasts in vitro, is inhibited by high levels of the antiestrogen tamoxifen which, by itself, in similar high concentrations, shows stimulatory effects. In addition to the sex steroids, skeletal cells are also stimulated by secosteroid and peptide calciotrophic hormones. The interactions of the sex steroids and the other calciotrophic hormones. These results provide the first steps towards understanding the regulation of bone cell proliferation and growth by the concerted action of a variety of hormones and growth factors.

Tamoxifen metabolites in patients on long-term adjuvant therapy for breast cancer

Serum concentrations of tamoxifen, 4-OH-tamoxifen, N-desmethyltamoxifen, and metabolites E and Y were assayed to assess the variation of tamoxifen-metabolism during short-term and long-term endocrine treatment for breast cancer. Once steady-state was achieved, serum levels of tamoxifen and its metabolites in individual patients were stable in the short (10 weeks) and long term (over 7 years) (coefficient of variation [CV], 10–15%), but the variation between individuals (CV 50–70%) was high. Serum tamoxifen and N-desmethyltamoxifen levels were not correlated with indices of obesity. Thus this does not explain the large variation between individuals. In addition to the metabolites that were measured, 4-hydroxy-N-desmethyltamoxifen was tentatively identified in patients' serum. Overall, this study demonstrated that the metabolites of tamoxifen are stable (i.e. no metabolic tolerance) for up to 10 years of drug administration.

Implications of tamoxifen metabolism in the athymic mouse for the study of antitumor effects upon human breast cancer xenografts

The metabolism of tamoxifen in the human has been well established and may be important in the antiestrogenic activity of this agent. This study examines whether tamoxifen metabolism in the athymic mouse xenograft model is similar to tamoxifen metabolism in the breast cancer patient.Serum taken from athymic mice 24 h after a single large oral dose (200 mg/kg) of tamoxifen contained compounds corresponding to standards of tamoxifen, 4-hydroxytamoxifen, N-desmethyltamoxifen, 4-hydroxy-N-desmethyltamoxifen and tamoxifen-N-oxide when analyzed by high pressure liquid chromatography. The administration of large single doses (200 mg/kg) of 4-hydroxytamoxifen and N-desmethyltamoxifen either alone or in combination produced the expected peaks for the administered agents and a peak confirming the identity of 4-hydroxy-N-desmethyltamoxifen. 4-Hydroxy-N-desmethyltamoxifen was detected in serum from six out of 10 breast cancer patients receiving 10 mg bid of tamoxifen. These patients had tamoxifen, 4-hydroxytamoxifen and N-desmethyltamoxifen levels of 108 ± 23, 2.6 ± 0.5, and 238 ± 58 ng/ml, respectively.Repeated large oral doses (200 mg/kg/day for 6 days) of tamoxifen to athymic mice produced a similar array of serum metabolites as seen after the single dose and in the breast cancer patient. However, levels of 4-hydroxytamoxifen (628 ± 192 ng/ml) were similar to those of tamoxifen (441 ± 208 ng/ml) whereas N-desmethyltamoxifen (1343 ± 388 ng/ml) levels were 2–3 times greater. A similar pattern of metabolites was produced with a 50 mg/kg dose of tamoxifen although levels were considerably reduced.Subcutaneous administration of tamoxifen (200 mg/kg/day for 6 days) produced serum levels of the parent compound (120 ± 19 ng/ml) in the same range as tamoxifen levels in the breast cancer patient. However, although N-desmethyltamoxifen was the major metabolite, levels (115 ± 18 ng/ml) were only equivalent to those of tamoxifen itself and 4-hydroxytamoxifen levels (26 ± 5 ng/ml) were appreciably higher than the breast cancer patient. Lowering the dose of tamoxifen (50 mg/kg) administered s.c. produced not only lower circulating tamoxifen levels (41 ± 3 ng/ml) but also changed the metabolite profile. Rather than N-desmethyltamoxifen levels equivalent to those of tamoxifen, as seen with the higher dose, they were reduced to the level of 4-hydroxytamoxifen (7 ± ng/ml). Low levels of both N-desmethyltamoxifen and 4-hydroxytamoxifen compared to the parent compound was characteristic of metabolite profiles produced by 0.25, 1.25 or 2.5 cm silastic capsules releasing an average (± S.E.) 53 ± 9, 84 ± 8 and 192 ± 12 μg of tamoxifen per day, respectively. These capsules monstrated their ability to inhibit 17 β-estradiol stimulated MCF-7 tumor growth in a dose related manner. The largest capsule (2.5 cm) produced serum levels of tamoxifen (51.8 ± 4.9 ng/ml), 4-hydroxytamoxifen (3.5 ± 0.4 ng/ml) and N-desmethyltamoxifen (3.8 ± 0.6 ng/ml) which were sufficient to produce complete inhibition of tumor growth.The athymic mouse hydroxylates oral tamoxifen more readily than the human and therefore produces a different metabolite profile.

The effect of tamoxifen on the function and lifespan of the corpus luteum and on subsequent ovarian function

The aim of the present study was to investigate the effects of tamoxifen on pituitary and luteal function and on the bleeding pattern when administered continuously in the secretory phase. The study included 16 women with regular menstrual cycles followed during one control, one treatment and one follow-up cycle. Each volunteer received 20 mg tamoxifen twice daily from cycle day 18 to menstruation in the treatment cycle. The luteal phase was slightly, but significantly prolonged during treatment, and FSH, progesterone, 17-hydroxyprogesterone, 20 alpha- dihydro progesterone, estrone, estrone sulphate and estradiol significantly elevated in comparison with corresponding data during the control cycle. The results indicate that estrogen may be of some importance for the regulation of the life span of the corpus luteum in the human. The significantly elevated levels of pregnanediol glucuronide and estrone glucuronide during the follow-up cycle are most likely a result of either a direct effect of remaining circulating tamoxifen levels on the ovary, or mediated through the increased release of FSH. If estrogens are of importance for the process of implantation, which has recently been suggested in sub-human primates, also in the human remains unclear. Studies on the effect of anti-estrogens on the endometrium during the secretory phase of the cycle are ongoing.

Tumor and serum tamoxifen concentrations in the athymic nude mouse.

The athymic nude mouse has been used as an in vivo model for pharmacologic studies of the antiestrogen, tamoxifen. Serum and tumor tamoxifen and metabolite concentrations were examined during and after 100 and 1000 micrograms/day doses injected s.c. Tamoxifen and tamoxifen metabolites were quantitated by high-performance liquid chromatography. Tamoxifen was detected in tumors after a dose of 100 micrograms/day, although serum concentrations were not detected. At a dose of 1000 micrograms/day, tumor tamoxifen and its active metabolites were detected in high concentrations ranging up to greater than 6 mmol/g tissue. Serum tamoxifen metabolites were not detected at either dose. In summary, high doses of tamoxifen were required in the nude mouse to obtain clinically relevant serum concentrations, and significant tumor levels were achieved at doses that resulted in undetectable serum levels. The relationship between serum tamoxifen concentrations, tumor tamoxifen levels, and the biologic activity of the drug requires further study.