Metabolism of toremifene in the rat

Synthesis of a novel antiestrogen radioligand (99mTc-TOR-DTPA)

This study was aimed at developing a hydrophilic radioligand as an antiestrogen drug derivative to be used for imaging breast tumors. Toremifene [TOR; 4-chloro-1,2-diphenyl-1-(4-(2-(N,N-di-methylamino)ethoxy)phenyl)-1-butene, as citrate salt] was selected as the starting material to be derived, since it has been used extensively as an antiestrogen drug for treatment and prevention of human breast cancer. An antiestrogen drug derivative, TOR attached to diethylenetriamine pentaacetic acid (DTPA), was synthesized by two experimental treatments, including a purification and a reaction step. We described the synthesis of this TOR derivative, (3Z)-4-{4-[2-(dimethylamino) ethoxy] phenyl}-3,4-diphenylbut-3-en-1-ylN,N-bis[2-(2,6-dioxomorpholin-4-yl)ethyl]glycinate (TOR-DTPA), in detail. Mass spectroscopy confirmed the expected structures. TOR-DTPA was labeled with technetium-99m ((99m)Tc), using stannous chloride (SnCl(2)) as the reducing agent. Biodistribution studies were performed on female Albino Wistar rats. Quality controls, radiochemical yield, and stability studies were done utilizing high-performance liquid chromatography, radioelectrophoresis, thin-layer chromatography, and thin-layer radiochromatography methods. The synthesized compound was found to be hydrophilic and anionic, with high stability for the duration of the testing period in vitro. The results indicated that the radiolabeled compound has estrogen-receptor specificity, especially for the breast tissue. It is highly possible that this compound could be used for imaging breast tumors as a novel technetium-labeled hydrophilic estrogen derivative radioligand.

Histopathology and histomorphometry of the urogenital tract in 15-month old male and female rats treated neonatally with SERMs and estrogens

In this study, two selective estrogen receptor modulators (SERMs), tamoxifen (TAM) and toremifene (TOR) or two estrogens, ethinylestradiol (EE) and diethylstilbestrol (DES) were administered to newborn male and female Sprague-Dawley rats (days 1-5) to investigate the occurrence of developmental abnormalities in the adulthood. The compounds were dosed (s.c.) at an equimolar dose of 24.9 micromol/kg. During the follow-up period, mortality occurred mainly in DES-treated male rats (3/4), associated with obstructive urinary calculi and suppurative renal inflammation in 2/3 rats. Similar lesions were not evident in other groups. At the age of 15 months, the animals were necropsied and organs were collected for histopathology and histomorphometry. Treatment-related abnormalities were restricted to the reproductive organs. Chronic prostatitis and epithelial abnormalities in the vas deferens were observed in all treatment groups. The columnar epithelium of vas deferens showed hyperplasia and development of subepithelial glandular structures resembling epididymal cysts reported in humans exposed in utero to DES. Testicular atrophy was observed especially in estrogen-treated rats. Mainly in SERM-treated female rats, the uterus showed luminal dilation or obstruction, loss of endometrial glands and myometrium disorganization including foci of muscular disruption. TOR-treated female rats showed polyp-like nodules (incidence 4/15) and a high incidence (9/15) of a simple cuboidal epithelium in cervical regions normally occupied by multilayered epithelia. In conclusion, the vas deferens is a main target organ following neonatal administration of SERMs and estrogens. In addition, female rats were significantly more susceptible to SERM treatment than to treatment with estrogens.

DNA damage and altered gene expression of enzymes for metabolism and DNA repair by tamoxifen and toremifene in the female rat liver

Effects of hepatocarcinogenic TAM and non-hepatocarcinogenic TOR on the formation of hepatic DNA adducts and on the gene expression of hepatic drug-metabolizing enzymes and DNA repair enzymes/proteins were comparatively examined in female Sprague-Dawley rats treated with TAM (20 or 40 mg/kg/day, i.g.) or TOR (40 mg/kg/day, i.g.) for 1, 2 or 8 weeks. Hepatic TAM-DNA adducts were formed even after 1 week of treatment with TAM at either dose, and the adduct levels increased in a dose- and treatment period-dependent manner, whereas no DNA adducts were detected in any of the TOR-treated rats. Conversely, TAM and TOR showed almost the same capacity for increasing the gene expression of drug-metabolizing enzymes responsible for metabolic activation and detoxification, at least up to the 2-week treatment mark. Accordingly, differences in DNA adduct formation between TAM- and TOR-treated rats would not be primarily dependent on the capacity for inducing hepatic drug-metabolizing enzymes. In addition, a drastic increase in the gene expression of cytochrome P4503A2 (CYP3A2), an activation enzyme of TAM, by the 8-week treatment with TAM might have contributed to the increased formation of DNA adducts. Gene expressions of DNA repair enzymes/proteins responsible for a nucleotide excision repair system were not significantly changed in any of the rats treated with either drug. The present findings suggest that the difference between TAM and TOR in hepatocarcinogenic potency is dependent on the capacity to form DNA adducts rather than modulating the expression of drug-metabolizing enzymes and DNA repair enzymes/proteins.

Toremifene--a promising therapy for the prevention of prostate cancer and complications of androgen deprivation therapy

Deregulation of the estrogen axis in humans prompts a series of tissue-specific events. In the breast and prostate, alterations in estrogen signalling lead to genotypic and phenotypic molecular alterations that result in dysplastic cellular appearance, deregulated cell growth and carcinoma. In bone, decreased estrogen leads to increased osteoclastogenesis and bone resorption, decreased bone mineral density and a significant fracture risk. Toremifene is a selective estrogen receptor modulator that exerts pharmacological activity in the breast, bone and prostate. An intense interest in developing this agent for prostate cancer chemoprevention is based on the reduction of premalignant and malignant prostate lesions in a transgenic model of prostate cancer. Biological and clinical activity was demonstrated in Phase II trials by the prevention of progression to prostate cancer in men with high-grade prostate intraepithelial neoplasia and through suppression of bone turnover biomarkers and increased bone mineral density in men on androgen deprivation therapy for prostate cancer.

Evaluation of rat intestinal absorption data and correlation with human intestinal absorption

The absorption of 111 drug and drug-like compounds was evaluated from 111 references based on the ratio of urinary excretion of drugs following oral and intravenous administration to intact rats and biliary excretion of bile duct-cannulated rats. Ninety-eight drug compounds for which both human and rat absorption data were available were selected for correlation analysis between the human and rat absorption. The result shows that the extent of absorption in these two species is similar. For 94% of the drugs the absorption difference between humans and rats is less than 20% and for 98% of drugs the difference is less than 30%. There is only one drug for which human absorption is significantly different from rat absorption. The standard deviation is 11% between human and rat absorption. The linear relationship between human and rat absorption forced through the origin, as determined by least squares regression, is %Absorption (human)=0.997%Absorption (rat) (n=98, SD=11). It is suggested that the absorption in rats could be used as an alternative method to human absorption in pre-clinical oral absorption studies.

Cone voltage and collision cell collision-induced dissociation study of triphenylethylenes of pharmaceutical interest

Fragmentations of three triphenylethylene compounds (toremifene and its two metabolites) with different functional side-chain groups (alcohol, acid and amine) were studied. The compounds were dissociated by collision-induced dissociation (CID) in the interface region of an electrospray ionization source (ESI(+)) and in the collision cell of a triple quadrupole mass spectrometer. Fragmentation pathways for these molecules are proposed, based on accurate mass measurements of in-source fragment ions and MS/MS experiments using product and precursor ion scanning. The side-chain functional groups were found to strongly affect the fragmentations of the molecular ions. The fragmentation pathways of the protonated molecule and sodium ion adduct were quite similar, but the subsequent stabilities of certain common fragments were surprisingly different.

Selective oestrogen receptor modifiers (SERMs) and breast cancer therapy

Antioestrogen therapy is currently receiving renewed interest for several reasons. Tamoxifen was introduced in the treatment of metastatic breast cancer more than three decades ago. The drug significantly reduces long term mortality and also reduces the risk of contralateral tumours when administered in early breast cancer. Five years of tamoxifen is now standard in adjuvant endocrine therapy, and the drug is currently being evaluated for breast cancer prevention. Despite this, several aspects regarding the pharmacology of the drug are still unclear, and the scientific rationale for dose selection has recently been challenged. Several novel antioestrogen compounds, called selective oestrogen receptor modifiers (SERMs), express selective oestrogen agonistic or antagonistic properties depending on the organ or test system evaluated. Some of these drugs, like raloxifene, do not seem to promote the development of endometrial cancer, although they still have selected oestrogen-like beneficial effects. This paper reviews the pharmacologic and the pharmacokinetic aspects of the different SERMs with particular emphasis on their potential use in therapy and prevention of breast cancer.

Intrinsic reactivity of tamoxifen and toremifene metabolites with DNA

The antiestrogen tamoxifen is known to cause liver cancer in rats. This may be due to the formation of abundant DNA adducts in rat liver. A likely precursor to some of the tamoxifen adducts in rats is alpha-hydroxytamoxifen. It is not clear whether the rat data are relevant to human exposure. In the present study, we show that one of the major metabolites in humans reacts with double-stranded DNA in vitro in the absence of any metabolizing enzymes or activating chemicals. At least two distinct adduct spots resulting from 4-hydroxy-N-desmethyltamoxifen (metabolite Bx) were detected by 32P postlabeling and thin layer chromatography. The adduct level increases dramatically when metabolite Bx is irradiated with UV light to fuse into a phenanthrene ring system. 4-hydroxy-N-desmethyltoremifene, which differs from Bx by a single chlorine atom, forms fewer DNA adducts without irradiation but similar amounts after irradiation. These results suggest that the chlorine atom may interfere with drug-DNA interactions which facilitate adduct formation.

Chronic exposure to secoisolariciresinol diglycoside alters lignan disposition in rats

Mammalian lignans from colonic bacterial action on secoisolariciresinol diglycoside (SDG) may mediate the anticarcinogenic effect of prolonged SDG feeding in rats. To elucidate lignan bioactivity, our objective was to determine 3H-SDG disposition in rats with acute or chronic SDG treatment over 48 h. After food deprivation overnight, female Sprague-Dawley rats (70-72 d old) were given a single gavage of 3H-SDG (3.7 kBq/g body weight) either immediately (acute, n = 12) or after 10 d of gavage with 1.5 mg unlabeled SDG/d (chronic, n = 12). Rats were killed at 12, 24, 36 and 48 h after gavage, and samples collected and analyzed for radioactivity by liquid scintillation counting. Radioactivity was 1- to 16-fold higher at 12 vs. 48 h for tissues, blood and gastrointestinal contents (P < 0.05). By 48 h, >80% of the recovered dose was excreted in both groups (feces > 50%, urine = 28-32%). Tissue radioactivity was highest (by 0.5- to 176-fold) in the cecum (P < 0.05). Levels in the liver, kidney and uterus (12 h) were 0.2- to 7.5-fold higher than in other nongastrointestinal tissues. At 12 h, fecal radioactivity was negligible, and cecal content, liver and adipose radioactivity were one- to threefold greater in rats with chronic SDG exposure than in those acutely exposed (P < 0.05). Blood radioactivity, present mostly in the plasma fraction (0.4% of dose), suggested that lignan concentrations could be 3000 times higher than peak estrogen levels in rats. Thus, SDG metabolites accumulated in the liver, kidney, intestinal tissues and uterus. Chronic SDG exposure delayed fecal excretion while increasing liver and adipose lignan levels.

A two-year dietary carcinogenicity study of the antiestrogen toremifene in Sprague-Dawley rats

The carcinogenic potential of the nonsteroidal triphenylethylene antiestrogen toremifene (Fareston) was evaluated in a standard 104-week rat dietary carcinogenicity study. The doses were 0, 0.12, 1.2, 5.0 and 12 mg/kg/day and the number of animals 50/sex/dose group. The body weight gain and food consumption were monitored once weekly (study weeks 1-16) or once every four weeks thereafter (study weeks 17-104). Blood samples were taken at weeks 34, 52 and 104 and the plasma concentrations of toremifene, as well as the two main metabolites (deaminohydroxy)toremifene and N-demethyltoremifene, were measured. All doses of toremifene reduced food intake and body weight gain. Toremifene caused a significant reduction in mortality, which was mainly due to reduced incidences of pituitary tumors. This was evident in all dose groups. Drug-related decrease of mammary tumors in females (at all doses) and testicular tumors in male rats (doses > or = 1.2 mg/kg/day) were also evident. The incidence of the preneoplastic foci of basophilic hepatocytes were significantly decreased in treated female groups. Toremifene induced no preneoplastic or neoplastic lesions. Based on histopathology, no obvious toxicity could be observed. Drug-related changes were observed in the genital organs, thyroid, spleen, mammary gland, adrenal, kidney, stomach and lung. These changes were due to hormonal disturbances or as a result of reduced food consumption or reduced incidences of pituitary, mammary or testicular tumors. This study indicates that toremifene is an efficient antiestrogen in long-term treatment, is well tolerated and has no tumorigenic potential in rats.

Involvement of cytochrome P450 3A enzyme family in the major metabolic pathways of toremifene in human liver microsomes

The anti-estrogen toremifen-Fc-1157a or 4-chloro-1,2-diphenyl-1-[4-[2(N,N-dimethylamino)ethoxy]-phenyl]-1- butene is now used for the treatment of breast cancer. This drug is extensively metabolized by cytochrome P450 dependent hepatic mixed function oxidase in man, yielding mainly the N-demethyl-(DMTOR), 4-hydroxy-(4OH-TOR) and deamino-hydroxy-(TOR III) toremifene metabolites. The specific forms of cytochrome P450 involved in these oxidation reactions were examined in 32 human liver microsomal preparations previously characterized with respect to their contents of several known P450 enzymes. Toremifene was demethylated with an apparent Km of 124 microM while it was hydroxylated with an apparent Km of 139 microM. The metabolic rates were 71 +/- 56, 13 +/- 9 and 15 +/- 4 pmol/min/mg microsomal protein, respectively, for DMTOR, 4-OH-TOR and TOR III. The N-demethylation activity was strongly correlated with estradiol 2-hydroxylation (r = 0.75), nifedipine oxidation (r = 0.86), tamoxifen N-demethylation (r = 0.73), testosterone 6 beta-hydroxylation (r = 0.78) and erythromycin N-demethylation (r = 0.84), all these monooxygenase activities known to be supported by CYP3A4 isoform. Furthermore, the CYP3A content of liver microsomal samples, measured by western blot analysis using a monoclonal anti-human CYP3A4 antibody, was strongly correlated with DMTOR formation (r = 0.80). Compounds such as cyclosporin, triacetyl-oleandomycin and testosterone inhibited the N-demethylation of toremifene metabolism at 80, 89 and 56% vs control, respectively, while the formation of TOR III was inhibited at 78, 82 and 73% vs control and the 4-hydroxylation pathway was inhibited no more than about 50% vs control. Prior incubation of microsomes with 100 microM gestodene, known to be a selective mechanism-based inhibitor of CYP3A4 in the presence of NADPH, led to 76 +/- 6 and 76 +/- 5% (N = 5 samples) reductions in the N-demethylation and formation of TOR III, respectively. Polyclonal antibody directed against human CYP3A enzymes inhibited formation of DMTOR and TOR III by 60 and 46%, respectively. The metabolism of toremifene was not activated by alpha-naphthoflavone. Finally, the use of yeasts genetically engineered for expression of human P4501A1, 1A2, 2C9 and 3A4 allowed us to demonstrate that DMTOR and TOR III formations are mediated by P4501A and 3A4 enzymes and by contrast these enzymes are not involved in the 4-hydroxylation pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of toremifene on antipyrine elimination in the isolated perfused rat liver

Toremifene is a triphenylethylene antioestrogen with significant antitumor activity. It is structurally very similar to tamoxifen. Both drugs undergo extensive hepatic metabolism, and tamoxifen is known to inhibit hepatic mixed-function oxidases (MFO). Using the isolated perfused rat-liver model, we investigated the effect of toremifene on the elimination of antipyrine, a standard marker of MFO activity. Perfusate consisted of 20% red cells in a modified Krebs-Henseleit buffer, and 80 ml was recirculated at 14 ml/min for 3 h. High but clinically relevant steady-state toremifene levels of 3 and 10 micrograms/ml were achieved using bolus plus constant infusion into the reservoir. Elimination of 2.5 mg antipyrine was not inhibited by steady-state toremifene, but methanol (maximal perfusate concentration, 1.29%), the vehicle used for toremifene administration, caused a statistically significant increase in the antipyrine elimination half-life (mean, 1.4 +/- 0.2 h for controls vs 2.2 +/- 0.3 h for methanol; P < 0.05, n = 4). Whereas the methanol had no apparent effect on liver viability as assessed by bile flow and perfusate back-pressure, toremifene at a steady-state concentration of 10 micrograms/ml caused a statistically significant decrease in bile flow (value at 180 min, 0.22 +/- 0.05 ml/h as compared with 0.52 +/- 0.06 ml/h in the methanol control; P < 0.05) and a statistically significant increase in perfusate back-pressure (value at 180 min, 17.5 +/- 1.8 cm vs 11.0 +/- 2.6 cm in the methanol control; P < 0.05). Therefore, toremifene used at high doses can impair liver function in the isolated perfused rat liver, but it does not have any effect on antipyrine elimination.

High-performance liquid chromatographic method for the determination of toremifene and its major human metabolites

A high-performance liquid chromatographic method has been developed for the measurement of toremifene and its major human metabolites in plasma and urine. We have simplified other published methods, such that our assay uses protein precipitation in place of organic extraction, and ultraviolet detection instead of photochemical activation followed by fluorescence detection. In a stability study toremifene and metabolites remained unchanged for up to seven weeks at -70 degrees C. This simple and specific assay allowed toremifene and three metabolites to be quantitated for pharmacokinetic analyses in a high-dose Phase I trial.

Review of the pharmacological properties of toremifene

New compounds were synthesized with the aim to develop new anti-estrogenic antitumor drugs. The biological properties of the molecules were screened by (1) estrogen receptor (ER) binding, (2) effect on MCF-7 cells, (3) uterotrophic effect and inhibition of estradiol induced uterotropic effect and (4) antitumor effect in DMBA induced rat mammary cancer. One of the molecules, Fc-1157a = toremifene, exhibited the following characteristics: competitive inhibition of [3H]estradiol binding to ER (IC50 = 0.3 mumol/l), inhibition of MCF-7 cell growth in a concentration-dependent manner and cell-killing effect at higher than 3 mumol/l concentrations. Minimal estrogenic dose of toremifene on rat uterus weight was about 40 times higher than that of tamoxifen. Toremifene had statistically significant effect against DMBA-induced rat mammary cancer. Further screening consisted of antitumor, pharmacokinetic and safety studies. Toremifene inhibited the growth of ER-negative, glucocorticoid sensitive, mouse uterine sarcoma in a dose-dependent manner. Pharmacokinetics and metabolism of toremifene resembled closely those of tamoxifen, but since the chlorine atom of the toremifene molecule was not metabolically cleaved tamoxifen and toremifene did not have chemically similar metabolites. Toremifene was well tolerated in animal toxicity studies. No hyperplastic or neoplastic nodules, which were seen in almost all high-dose (48 mg/kg for 24 weeks) tamoxifen-treated rats, were found in toremifene-treated rats (dose 48 mg/kg). In clinical phase I studies in healthy voluntary postmenopausal women, no side effects were reported, at doses less than or equal to 460 mg, neither after a single dose nor after five daily doses. At the dose of 680 mg two out of five persons experienced vertigo and headache. Toremifene, at the dose of 68 mg daily, had antiestrogenic effect on estradiol-induced human vaginal epithelial cells. Clinical phase II studies have confirmed that toremifene has a promising antitumor effect.

Biochemical and pharmacological effects of toremifene metabolites

Toremifene, a new antiestrogenic antitumor compound, has several biologically active metabolites. The hormonal effects of the main metabolites resemble those of unchanged toremifene. The main metabolite in humans, N-demethyltoremifene, is bound to estrogen receptors (ER), inhibits the growth of MCF-7 cells, and exerts an antiestrogenic effect similar to that of toremifene. However, its antitumor effect in vivo against dimethylbenz(a)anthracene (DMBA)-induced rat mammary cancers is weaker than that of toremifene. Didemethyltoremifene has antiestrogenic actions in mouse and rat uterus at high doses. 4-Hydroxytoremifene is bound to ER with higher affinity and inhibits MCF-7 growth at concentrations lower than those of toremifene. It has a weaker intrinsic estrogenic effect than does toremifene. The efficacy of 4-hydroxytoremifene against DMBA-induced cancers is weak except at very high doses. Oxidations of N-demethylated metabolites to (deamino)hydroxylated compounds and carboxylic acids are the detoxification routes of toremifene. (deaminohydroxy)Toremifene has only weak hormonal actions at high doses and carboxylated metabolites have no estrogenic/antiestrogenic effects. The antitumor effect of toremifene in vivo is mainly due to unchanged toremifene, but hormonal effects (which may have a role in antitumor actions) are partly attributable to metabolites N-demethyltoremifene, didemethyltoremifene, (deaminohydroxy)toremifene, 4-hydroxy-N-demethyltoremifene, and 4-hydroxytoremifene, which have pharmacological properties similar to those of toremifene.

Binding of toremifene to human serum proteins

The in vitro protein binding of toremifene in human serum was measured by ultracentrifugation using 3H-toremifene together with unlabeled toremifene, 50, 500, and 5000 ng/ml. Of the total radioactivity 99.7 per cent was bound to the proteins independent of the concentration of the unlabeled drug. Binding of toremifene to different protein fractions was studied by adding 3H-toremifene and 500 ng/ml of cold toremifene to normal serum. The serum samples were exposed to agarose gel electrophoresis to fractionate different proteins. The radioactivity was localized using a position-sensitive proportional counter. After that the proteins were visualized by staining. Of the total protein bound radioactivity 92 per cent was bound to albumin, about 6 per cent to beta 1 globulin fraction and about 2 per cent to a fraction between albumin and alpha 1 globulins, part of this probably to alpha 1 acid glycoprotein.