Effect of estradiol and tamoxifen on brain membranes: investigation by infrared and fluorescence spectroscopy

Nongenomic effects of steroids on rat brain neurotransmitter transporters and receptors have been reported in several laboratories. In the present study, we have investigated possible membrane effects of 17alpha- and 17beta-estradiol, as well as tamoxifen, by studying their interactions with synthetic phospholipid membranes using Fourier transform infrared spectroscopy. We have also used the fluidity of rat striatal and frontal cortex membranes, as determined by fluorescence depolarization of the probe 1,6-diphenyl-1,3,5-hexatriene (DPH), to probe the effects of these drugs on membranes. Our results show that tamoxifen induces conformational disorder along the acyl chains of deuterated dimirystoylphosphatidylcholine and decreases the gel to liquid-crystalline phase transition temperature by approximately 10 degrees C. Similar effects, although less pronounced, were observed with 17beta-estradiol, whereas 17alpha-estradiol had no significant effect. The DPH fluorescence anisotropy of striatum and frontal cortex membranes was decreased in vitro with 17beta-estradiol or tamoxifen and also with 17alpha-estradiol, but to a lesser extent. These results suggest a stereospecific estradiol effect on membranes and that the effects of these compounds are not related to their activity on estrogen receptors. These observations support a different mechanism of action of steroids that could be implicated in their neuroprotective activity.

Tamoxifen-DNA adducts detected in the endometrium of women treated with tamoxifen

Women treated for breast cancer with tamoxifen are at increased risk of developing endometrial cancer. This carcinogenic effect has been attributed to estrogenic stimulation and/or to a genotoxic effect of this drug. To examine genotoxicity, we developed a (32)P-postlabeling TLCL/HPLC procedure for quantitative analysis of tamoxifen-DNA adducts in endometrial tissue. This assay is several orders of magnitude more sensitive than those previously used for this purpose; with it, we can detect five tamoxifen-DNA adducts in 10(11) bases. Endometrial tissue was obtained from women undergoing tamoxifen therapy and from untreated control subjects. DNA adducts, identified as trans and cis epimers of alpha-(N(2)-deoxyguanosinyl)tamoxifen, were detected in six of thirteen patients in the tamoxifen-treated group. Levels of trans and cis adducts ranged from 0.5 to 8.3 and from 0.4 to 4.8 adducts/10(8) nucleotides, respectively. Tamoxifen-DNA adducts were not detected in endometrial tissue obtained from the control subjects. We conclude from this study that one or more tamoxifen metabolites react with endometrial DNA to form covalent adducts, establishing the potential genotoxicity of this drug for women and suggesting the use of TAM-DNA adducts as biomarkers for investigations of tamoxifen-induced endometrial cancer.

Cutaneously applied 4-hydroxytamoxifen is not carcinogenic in female rats

Tamoxifen is widely used to treat oestrogen-dependent carcinoma of the breast. Previous long-term studies have shown that oral administration of tamoxifen induces hepatoproliferative lesions and hepatocellular tumours in rats. 4-hydroxytamoxifen is an active metabolite of tamoxifen undergoing clinical evaluation for the treatment of various non-malignant breast diseases by topical application. In the present study, 4-hydroxytamoxifen was administered daily by cutaneous application for 101 weeks to groups of 50 female Sprague-Dawley rats at 20, 140 or 1000 microg/kg/day. The product was applied with no occlusive bandage and oral ingestion was avoided by application of an Elizabethan collar for 6 h after administration. Treatment with 4-hydroxytamoxifen was clinically well tolerated and induced changes such as decreased food consumption and body weight gain, uterine and ovarian atrophy, mucification of vaginal epithelium and reduced mammary development, all of which were attributed to its pharmacological action. Mortality was significantly lower in the treated animals. The number of animals with palpable masses was similarly reduced. The incidence of mammary tumours and hypophyseal tumours was markedly lower in 4-hydroxytamoxifen-treated animals. The incidence of chronic tubulo-interstitial nephropathies, a common cause of mortality, was also lowered. There was no evidence of a carcinogenic action of 4-hydroxytamoxifen on the liver, genital organs or skin. Plasma levels of 4-hydroxytamoxifen were stable over the duration of the study and were proportional to the administered dose, exceeding clinical plasma levels by 60-fold at the high dose-level. In conclusion, 4-hydroxytamoxifen is not carcinogenic in the rat and reduces the incidence of spontaneous mammary and hypophyseal tumours.

Mutagenic potential of alpha-(N2-deoxyguanosinyl)tamoxifen lesions, the major DNA adducts detected in endometrial tissues of patients treated with tamoxifen

Breast cancer patients treated with the antiestrogen tamoxifen (TAM) show an increased risk of developing endometrial cancer. We have recently detected TAM-DNA adducts in endometrium obtained from patients treated with TAM and identified them as trans- and cis-forms of alpha-(N2-deoxyguanosinyl)tamoxifen (dG-N2-TAM). To explore the mutagenic properties of these TAM-DNA adducts, we prepared site-specifically modified oligodeoxynucleotides containing a single isomer of dG-N2-TAM by reacting a 15-mer oligodeoxynucleotide containing a single dG (5'-TCCTCCTCGCCTCTC) with tamoxifen alpha-sulfate. These modified oligodeoxynucleotides were inserted into a single-stranded shuttle vector to investigate mutagenic specificities of the adducts in simian kidney (COS-7) cells. An epimer of dG-N2-trans-TAM showed targeted mutations ranging from 0.7 to 1.5%. The other dG-N2-trans-TAM adduct showed 9.6% G-->T transversions, accompanied by 2.8% G-->A transitions. Both dG-N2-cis-TAM adducts showed similar mutation spectra, where G-->T transversions (11-12%) predominated, along with a small number of G-->A transitions and G-->C transversions. Thus, dG-N2-TAMs are mutagenic lesions in mammalian cells. The tamoxifen-DNA adducts detected in patient endometrium may cause mutations and initiate endometrial cancer.

Identification of the major tamoxifen-DNA adducts in rat liver by mass spectroscopy

We present here the first mass spectroscopic (MS) identification of the main tamoxifen-induced DNA adducts in rat liver. The two main adducts were isolated by high-performance liquid chromatography (HPLC) and identified by MS, MS-MS and ultraviolet spectroscopy. Adduct 1 was the N-desmethyltamoxifen-deoxyguanosine adduct in which the alpha-position of the metabolite N-desmethyltamoxifen is linked covalently to the amino group of deoxyguanosine. Adduct 2 was confirmed to be the trans isomer of alpha-(N2-deoxyguanosinyl)tamoxifen, as previously suggested by co-chromatography.

4-Hydroxytamoxifen gives DNA adducts by chemical activation, but not in rat liver cells

The drug tamoxifen shows evidence of genotoxicity, and induces liver tumors in rats. Covalent DNA adducts have been detected in the liver of rats treated with tamoxifen, and in rat hepatocytes in culture. These arise primarily from its metabolite alpha-hydroxytamoxifen, and may also arise, in part, from another metabolite, 4-hydroxytamoxifen. We have prepared two model compounds for the potential reactive metabolite formed from 4-hydroxytamoxifen in rat liver. One of these was its alpha-acetoxy ester. This was much more reactive than that from tamoxifen, and could not be isolated in pure form. It reacted with DNA in the same way that alpha-acetoxytamoxifen did, to give adducts which were isolated by hydrolysis and chromatography, and identified as alkyldeoxyguanosines. The second derivative was alpha, beta-dehydro-4-hydroxytamoxifen. This also reacts with DNA in vitro, to give the same products as those from alpha-acetoxy-4-hydroxytamoxifen. Reaction probably proceeds through the same resonance-stabilized carbocation in either case. However, when primary cultures of rat hepatocytes were treated with either 4-hydroxytamoxifen, 4,alpha-dihydroxytamoxifen, or alpha, beta-dehydro-4-hydroxytamoxifen at a concentration of 10 microM, no adducts could be detected in their DNA by the 32P-postlabeling technique. Similarly, no adducts could be found in the liver DNA of female Fischer F344 rats treated orally (at 0.12 mmol kg-1) with the same substances. If 4-hydroxytamoxifen is metabolized to 4, alpha-dihydroxytamoxifen in rat liver, then either this substance is not converted to reactive esters or they are rapidly detoxified.

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

Tamoxifen has been shown to be a potent liver carcinogen in rats, and generates covalent DNA adducts. On-line high performance liquid chromatography/electrospray ionisation mass spectrometry (HPLC/ESI-MS) has been used to further study the metabolites of tamoxifen formed by rat liver microsomes in the presence of NADPH with a view to identifying potential reactive metabolites which may be responsible for the formation of DNA adducts, and liver carcinogenesis. A metabolite has been detected with a protonated molecule at m/z 773. The mass of this compound is consistent with a dimer of hydroxylated tamoxifen (m/z 388). Analysis of 4-hydroxytamoxifen incubated with a rat liver microsomal preparation showed the formation of a similar metabolite with an apparent MH+ ion at m/z 773, believed to be a dimer of 4-hydroxytamoxifen formed by a free radical reaction. The retention time for this metabolite from 4-hydroxytamoxifen is identical to that of the tamoxifen metabolite, suggesting that these two compounds are the same. The levels of the dimer were higher when 4-hydroxytamoxifen was used as substrate and, in addition, two isomers were detected. It is proposed that tamoxifen was first converted to arene oxides which react with DNA or to 4-hydroxytamoxifen, either directly or via 3,4-epoxytamoxifen, which then undergoes activation via a free radical reaction to give reactive intermediates which can then react with DNA and protein, or with themselves, to give the dimers (m/z 773).

Intracellular distribution of tamoxifen in resistant human breast adenocarcinoma cells using tamoxifen-eosin association

A tamoxifen-resistant cell line (MCF7TAM) was established from tamoxifen-sensitive MCF-7 human breast cancer cells expressing estrogen receptors. Though the resistant cell line grows in the presence of tamoxifen, estrogen receptors continue to be expressed at similar levels as in the parental cell line. However, estrogen receptors appeared to be altered in the resistant cell line since important discrepancies are observed between results obtained with ligand binding assays and immunoenzymatic assays, tending to show modifications of estrogen receptor ligand binding capacity. The intracellular distribution of tamoxifen in sensitive and resistant cell lines was investigated using fluorescence of eosin-tamoxifen ionic association. Fluorescence emission spectra of eosin, tamoxifen and eosin-tamoxifen complex (lambda(ex)=480 nm) were analyzed and the maximal fluorescence intensity found for the complex (lambda(em)=540 nm) was four times higher than that of eosin alone, while tamoxifen alone did not emit any fluorescence in this spectral range. In MCF-7 cells, tamoxifen was found to be mainly located surrounding the nucleus, although nuclear fluorescence intensity was significantly lower. No highly fluorescent granules were observed in the resistant cell lines as opposed to sensitive cells. Improvement of this fluorescence microscopy methodology could appear of interest, taking into account the complexity of tamoxifen resistance molecular pathways.

4-iodotamoxifen aziridine, a new affinity labeling agent for the rapid detection of estrogen receptor isoforms

We describe the simple and fast preparation of a new radioiodinated probe for the detection of the estrogen receptor (ER) and its isoforms. Iodotamoxifen aziridine was labeled with iodine 125 ([125I]TAZ) in position 4 of the alpha aromatic ring. The yield was high (>75%), the label was stable and the specific activity was near optimal (1900-2170 Ci/mmol). The apparent relative binding affinity of the probe to a recombinant human ER (hER) was high (RBA = 35 vs estradiol = 100). Electrophoretic studies (SDS-PAGE) with this hER indicated the high potency of [125I]TAZ at very low concentration (<1 nM) to reveal ER bands after a short exposure time (1-4 days). Competition between this probe and various compounds as well as chemical treatments of the ER with SH-reactive chemicals, demonstrated the labeling specificity. Analysis of cytosols from a panel of cell lines and various rat reproductive organs displayed characteristic ER bands (67, 50 and 37 kDa) suppressed by unlabeled E2. Detection in nonreproductive organs of 43 kDa E2-nondisplaceable peptide raised the question upon the presence of altered and/or variant ERs in many tissues. Data concerning human breast cancer cytosols were in complete accordance with those established with [3H]TAZ: high ER polymorphism in most ER-positive samples and peculiar forms (mainly 43 kDa) in ER-negative samples. Hence, [125I]TAZ appears especially useful for the detection of altered ER or related peptides in breast cancers.

Determination of DNA damage in F344 rats induced by geometric isomers of tamoxifen and analogues

To investigate the activation mechanisms involved in tamoxifen carcinogenicity, analogues of tamoxifen isomers modified at the ethyl group were synthesized and assessed for their ability to induce hepatic DNA damage following their administration to female F344 rats. The cis isomer was prepared by acid-catalyzed isomerization of tamoxifen and isolated by preparative HPLC. The active metabolite alpha-hydroxytamoxifen and geometric isomers of bromotamoxifen and C-desmethylenetamoxifen, analogues in which the ethyl group has been replaced by a bromine atom and methyl group, respectively, were synthesized according to published procedures. The levels of hepatic DNA adducts induced were determined by 32P-postlabeling. Bromotamoxifen and tamoxifen 1,2-epoxide caused no detectable DNA damage relative to controls. Trans isomers of tamoxifen, C-desmethylenetamoxifen, and alpha-hydroxytamoxifen all produced DNA adducts at a 5-90-fold higher level than the corresponding cis isomers. In contrast, both the cis and trans isomers of alpha-hydroxytamoxifen showed similar reactivity toward calf thymus DNA in vitro. Molecular models of alpha-hydroxytamoxifen isomers suggest this difference in DNA adduct-forming ability is due to steric hindrance of the enzymes involved in the activation of this metabolite. There were high adduct levels in the liver, but no uterine DNA adducts were detected in rats treated with alpha-hydroxytamoxifen. This suggests that in contrast to the liver, alpha-hydroxytamoxifen is not further activated in rat uterus. This may help to explain the absence of uterine tumors in rats following long-term tamoxifen treatment.

Synthesis and DNA reactivity of alpha-hydroxylated metabolites of nonsteroidal antiestrogens

Tamoxifen [(E)-1-(4-(2-(N,N-dimethylamino)ethoxy)phenyl)-1, 2-diphenylbut-1-ene], a nonsteroidal antiestrogen, induces liver tumors in rats by a genotoxic mechanism. The mechanism of DNA adduct formation is believed to proceed via the formation of a reactive carbocation at the alpha-position from the alpha-hydroxylated metabolite. Molecular mechanics calculations [Kuramochi, H. (1996) J. Med. Chem. 39, 2877-2886] have predicted that 4-substitution will affect the stability of the carbocation and thus will alter its reactivity toward DNA. We have synthesized the putative alpha-hydroxylated metabolites of 4-hydroxytamoxifen [(E)-1-(4-(2-(N, N-dimethylamino)ethoxy)phenyl)-1-(4-hydroxyphenyl)-3-hydroxy-2-phenyl but-1-ene] and idoxifene [(Z)-1-(4-iodophenyl)-3-hydroxy-2-phenyl-1-(4-(2-(N-pyrrolidino) ethoxy)phenyl)but-1-ene] and compared their reactivities with DNA with that of alpha-hydroxytamoxifen [(E)-1-(4-(2-(N, N-dimethylamino)ethoxy)phenyl)-3-hydroxy-1,2-diphenylbut-1-ene]. As predicted, the bis-hydroxylated compound reacted with DNA in aqueous solution at pH 5 to give 12-fold greater levels of adducts than alpha-hydroxytamoxifen, whereas alpha-hydroxyidoxifene gave one-half the number of adducts. The results demonstrate that idoxifene presents a significantly lower genotoxic hazard than tamoxifen for the treatment and prophylaxis of breast cancer.

Interaction of the anticancer drug tamoxifen with the human erythrocyte membrane and molecular models

Tamoxifen is a non steroidal antiestrogen drug extensively used in the prevention and treatment of hormone-dependent breast cancer. To evaluate its perturbing effect upon cell membranes it was made to interact with human erythrocytes and molecular models. These consisted of bilayers of dimyristoylphosphatidylcholine (DMPC) and of dimyristoylphosphatidylethanolamine (DMPE), representative of phospholipids classes located in the outer and inner leaflets of the erythrocyte membrane, respectively. Experiments by fluorescence spectroscopy showed that tamoxifen interacted with DMPC vesicles fluidizing both its polar head and acyl chain regions. These results were confirmed by X-ray diffraction which indicated that tamoxifen perturbed the same regions of the lipid. However, it did not cause any significant structural perturbation to DMPE bilayers. The examination by electron microscopy of human erythrocytes incubated with tamoxifen revealed that they changed their normal discoid shape to stomatocytes. According to the bilayer couple hypothesis, this result means that the drug is inserted in the inner leaflet of the erythrocyte membrane. Given the fact that tamoxifen did not interact with DMPE, it is concluded that it interacted with a protein located in the cytoplasmic moiety of the erythrocyte membrane.

Alpha-hydroxytamoxifen is a substrate of hydroxysteroid (alcohol) sulfotransferase, resulting in tamoxifen DNA adducts

When alpha-hydroxytamoxifen (alpha-OHTAM) was incubated with rat liver hydroxysteroid (alcohol) sulfotransferase a (STa) and 3'-phosphoadenosine 5'-phosphosulfate, (E)-alpha-OHTAM was found to be a better substrate for STa than (Z)-alpha-OHTAM. To explore the formation of tamoxifen (TAM)-derived DNA adducts, DNA was incubated with STa and either (E)-alpha-OHTAM or (Z)-alpha-OHTAM in the presence of 3'-phosphoadenosine 5'-phosphosulfate. Using 32P-postlabeling analysis, the amount of TAM-DNA adducts resulting from (E)-alpha-OHTAM was 29 times higher than that observed with (E)-alpha-OHTAM alone. Using (Z)-alpha-OHTAM and STa, some TAM-DNA adducts were also detected but at levels 6.5 times lower than that observed with (E)-alpha-OHTAM and STa. When compared with standards of stereoisomers of 2'-deoxyguanosine 3'-monophosphate-N2-tamoxifen, the major tamoxifen adduct was identified chromatographically as an epimer of the trans form of alpha-(N2-deoxyguanosinyl)tamoxifen, and the minor adduct was identified as an epimer of the cis form. In the reaction mixture, a conversion from (E)-alpha-OHTAM to (Z)-alpha-OHTAM through the carbocation intermediate was also detected. These results show that sulfation of alpha-OHTAM catalyzed by STa results in the formation of TAM-DNA adducts.

Optimised separation of E- and Z- isomers of tamoxifen, and its principal metabolites using reversed-phase high performance liquid chromatography

A reversed phase isocratic high-performance liquid chromatographic method is reported in which a formal structured procedure, the solvent selectivity triangle, was applied to predict the mobile phase composition giving baseline resolution of the clinically important triphenylethylene antioestrogenic agent (Z)-tamoxifen, its principal (Z)-metabolites, and also the clinically relevant (E)-geometric isomers of tamoxifen and 4-hydroxytamoxifen. The technique of solvent selectivity triangle was used to select the optimal organic modifier parameter for use with a Hichrom ODS 1 column, to achieve baseline separation of six triphenylethylene solutes. The detection system utilised post-column ultraviolet irradiation to convert solutes into their respective photocyclisation products, followed by fluorescence detection (lambda[ex] = 254 nm, lambda[em] = 360 nm), and the low detection limit for tamoxifen in serum of 0.1 microM. The optimal mobile phase composition was determined to be methanol-acetonitrile-water-trichloroacetic acid (50:31:18.9:0.1, v/v, pH 2.9). A single stage liquid-liquid extraction method for determination of triphenylethylene drugs in serum was developed. Reproducible recoveries for the (Z)-geometric isomers of tamoxifen (84 +/- 3%) and its principal metabolites including Metabolite Y (94 +/- 3%), N-desmethyltamoxifen (94 +/- 3%) and 4-hydroxytamoxifen (92 +/- 3%) were achieved, though more variable results were obtained for their corresponding (E)-geometric isomers (71 +/- 7% and 70 +/- 10%, respectively).

Molecular determinants of tissue selectivity in estrogen receptor modulators

Interaction of the estrogen receptor/ligand complex with a DNA estrogen response element is known to regulate gene transcription. In turn, specific conformations of the receptor-ligand complex have been postulated to influence unique subsets of estrogen-responsive genes resulting in differential modulation and, ultimately, tissue-selective outcomes. The estrogen receptor ligands raloxifene and tamoxifen have demonstrated such tissue-specific estrogen agonist/antagonist effects. Both agents antagonize the effects of estrogen on mammary tissue while mimicking the actions of estrogen on bone. However, tamoxifen induces significant stimulation of uterine tissue whereas raloxifene does not. We postulate that structural differences between raloxifene and tamoxifen may influence the conformations of their respective receptor/ligand complexes, thereby affecting which estrogen-responsive genes are modulated in various tissues. These structural differences are 4-fold: (A) the presence of phenolic hydroxyls, (B) different substituents on the basic amine, (C) incorporation of the stilbene moiety into a cyclic benzothiophene framework, and (D) the imposition of a carbonyl "hinge" between the basic amine-containing side chain and the olefin. A series of raloxifene analogs that separately exemplify each of these differences have been prepared and evaluated in a series of in vitro and in vivo assays. This strategy has resulted in the development of a pharmacophore model that attributes the differences in effects on the uterus between raloxifene and tamoxifen to a low-energy conformational preference imparting an orthogonal orientation of the basic side chain with respect to the stilbene plane. This three-dimensional array is dictated by a single carbon atom in the hinge region of raloxifene. These data indicate that differences in tissue selective actions among benzothiophene and triarylethylene estrogen receptor modulators can be ascribed to discrete ligand conformations.

Species differences in metabolism of panomifene, an analogue of tamoxifen

In vitro metabolism of panomifene (E-1,2-diphenyl-1--4-(2-(2-hydroxyethyl-amino)-ethoxy)-phenyl--3,3,3- trifluoropropene), a novel antiestrogen against hormone dependent tumors, has been investigated using liver microsomes from mouse, rat, dog, and human. Hydroxylation and side chain modifications were the routes of panomifene metabolism. Microsomal biotransformation showed some qualitative similarities, but several differences were observed in the metabolic profiles of the four species tested. Seven metabolites were detected in the incubation mixtures analyzed by thin layer chromatography and autoradiography, although there was only one produced by all species that had lost the side chain. Among the side chain shortened metabolites, the compound that had lost the hydroxyethyl-amino group was formed by the microsomal system of rodents, whereas the one that had lost the hydroxyethyl group was detected in the incubation mixtures with rat, dog, and human microsomes. Three metabolites (M1, M3, and M4) were produced exclusively by the dog. The structure of M3 was identified by mass spectroscopy as 4-hydroxy-panomifene. Furthermore, human liver microsomes formed a metabolite (M8) that was not detectable in the mixtures with mouse, rat, or dog microsomes. Its structure is suspected to be an oxidized form of panomifene with a double bound in the side chain. The structure of panomifene is analogous to tamoxifen, an antiestrogen currently used as a therapeutic agent against breast cancer, and there are some similar routes in their metabolism. The main difference is that the rate of tamoxifen biotransformation seems faster than that of panomifene. On the other hand, 4-hydroxy-panomifene is produced by only dog, while 4-hydroxylated derivative is one of the main metabolites of tamoxifen that has potent antiestrogenic activity and is considered to be responsible for the formation of DNA-adducts.

Miscoding potential of tamoxifen-derived DNA adducts: alpha-(N2-deoxyguanosinyl)tamoxifen

The treatment of tamoxifen, widely used as adjuvant chemotherapy for breast cancer, increases significantly the risk of developing endometrial cancer. The miscoding properties of tamoxifen-derived DNA adducts, alpha-(N2-deoxyguanosinyl)tamoxifens (dG-N2-tamoxifen), have been explored, using an in vitro experimental system to quantify base substitutions and deletions. Site-specifically modified oligodeoxynucleotides containing an epimer of trans- and cis-forms of dG-N2-tamoxifens were prepared postsynthetically and used as templates in primer extension reactions catalyzed by mammalian DNA polymerases alpha, beta, and delta. Pol alpha catalyzed incorporation of dCMP and dAMP opposite all four stereoisomers of dG-N2-tamoxifen, accompanied by lesser amounts of dGMP. In contrast, pol delta catalyzed preferential incorporation of dCMP, a correct base, opposite the lesions; one of the trans-forms of dG-N2-tamoxifens only promoted incorporation of dTMP. Using pol beta, preferential incorporation of dCMP, along with small amounts of incorporation of dAMP and dGMP, was detected. One- and two base deletions were also observed with pol alpha and pol beta. The miscoding specificities and frequencies of dG-N2-tamoxifens varied depending on the DNA polymerase used. In addition, with pol alpha and pol beta, large amounts of 5-base deletions were preferentially formed at the cis-forms of dG-N2-tamoxifen, but not at the trans-forms of dG-N2-tamoxifen. We conclude that dG-N2-tamoxifen adducts have high miscoding potentials.

FLP recombinase/estrogen receptor fusion proteins require the receptor D domain for responsiveness to antagonists, but not agonists

The ligand-binding domains of steroid receptors convey ligand-dependent regulation to certain proteins to which they are fused. Here we characterize fusion proteins between a site-specific recombinase, FLP, and steroid receptor ligand-binding domains. These proteins convert ligand binding into DNA recombination. Thus, ligand binding is directly coupled to an enzyme activity that is easily measured by DNA rearrangements or heritable genetic changes in marker gene expression, as opposed to the multiple events leading to transcription. Recombination by a FLP-estrogen receptor (FLP-EBD) fusion is activated by all tested estrogens, whether agonists or antagonists, indicating that all induce EBD release from the 90-kDa heat shock protein complex. Altering the distance between FLP and the EBD domain in the fusion proteins, by reducing the included length of the estrogen receptor D domain, affects ligand efficacy. A FLP-EBD with no D domain shows reduced inducibility by agonists and, unexpectedly, complete insensitivity to induction by all antagonists tested. A FLP-EBD including some D domain shows a ligand-inducible phenotype intermediate to those displayed by FLP-EBDs containing all or none of the D domain. Thus, we observed a tethered interference between FLP and the EBD domains that differs depending on the distance between the two domains, the conformations induced by agonists or antagonists, and which presents a previously undetectable distinction between estrogen agonists and antagonists in yeast.

Synthesis and biological evaluation of 7-hydroxy-3,4-diphenyl-1,2-dihydroisoquinolines as new 4-hydroxytamoxifen analogues

A phenolic 3,4-diphenyl-1,2-dihydroisoquinoline derivative (4a) as a new 4-hydroxytamoxifen analogue and a related compound (4c) were synthesized from 3,4-diphenyl-1,2,3,4-tetrahydroisoquinolin-4-ols (5a, c), which were prepared by intramolecular Barbier reaction of N-(2-iodobenzyl)phenacylamines. Anti-proliferative activities of 4a,c and 5a,c, as well as 4b and 5b prepared previously, against human mammary carcinoma MCF-7 cell line and human nasopharyngeal carcinoma KB cell line were evaluated. The 3,4-diphenyl-1,2-dihydroisoquinoline derivatives (4a,c) and isoquinolin-4-ols (5a,b) were active against MCF-7 cells and were nearly equipotent to the corresponding nonphenolic compound (1a). The mechanism of the anti-proliferative activity of 4a-c against MCF-7 cells is discussed.

Safety assessment of tamoxifen and toremifene

Nonclinical and clinical safety studies on the two antiestrogens, tamoxifen (Nolvadex) and toremifene (Fareston), are reviewed. Tamoxifen is genotoxic, carcinogenic in experimental animals, and carcinogenic in humans. Toremifene has yielded some positive findings for genotoxicity, but was not an initiating carcinogen in experimental animals. Thus, toremifene has a superior nonclinical safety profile, although information on its long-term effects in humans is needed to ascertain whether its use results in improved safety.

Identification of tamoxifen-DNA adducts formed by alpha-sulfate tamoxifen and alpha-acetoxytamoxifen

alpha-Sulfate trans-tamoxifen and alpha-sulfate cis-tamoxifen were synthesized as proposed active metabolites of tamoxifen that react with DNA. alpha-Acetoxytamoxifen was prepared as a model-activated form to produce a reactive carbocation. Calf thymus DNA was reacted with alpha-hydroxytamoxifen or the activated forms of tamoxifen, and tamoxifen-DNA adducts were analyzed by a 32P-postlabeling method. The reactivity of alpha-sulfate trans-tamoxifen to DNA was much higher than that of alpha-hydroxytamoxifen. The formation of tamoxifen-DNA adducts induced by alpha-acetoxytamoxifen and alpha-sulfate cis-tamoxifen was 1100- and 1600-fold, respectively, higher than that of alpha-hydroxytamoxifen. Both alpha-sulfate tamoxifens and alpha-acetoxytamoxifen were highly reactive to 2'-deoxyguanosine. Four reaction products of dG-tamoxifen were isolated by HPLC and characterized by mass- and proton magnetic resonance spectroscopy. Fractions 1 and 2 that eluted first were identified as the epimers of trans form of dG-N2-tamoxifen. Fractions 3 and 4 were identified as the epimers of cis form of dG-N2-tamoxifen. When DNA was reacted with alpha-acetoxytamoxifen in vitro, three isomers of dG-N2-tamoxifen were detected: fraction 2 was the major adduct while fractions 1 and 3 were minor adducts.

Antiestrogens: future prospects

Tamoxifen is currently the endocrine therapy of choice for early and advanced breast cancer. Attempts to improve the therapeutic efficacy have included altering the triphenylethylene ring structure of tamoxifen, forming new nonsteroidal ring structures or creating steroidal estradiol analogs with greater antiestrogenic activity. There are now six nonsteroidal compounds either on the market or in clinical development and one steroidal "pure" antiestrogen has entered clinical trials. A number of these agents show improved estrogen-receptor binding affinity, antiestrogenic activity, and/or antitumor activity compared with tamoxifen. Preclinical and clinical data on these compounds are discussed and compared with tamoxifen when possible.

[Molecular aspects of different mechanisms of tamoxifen resistance]

Tamoxifen is the most currently used antiestrogen in the endocrine treatment of breast cancer. However, despite a small proportion of estrogen receptor positive tumors presenting de novo resistance to treatment, numerous tumors develop acquired resistance after a first phase of response. Many mechanisms have been proposed, but none could be identified as a real explanation of these phenomena of resistance. The hypotheses suggested are related to the series of events implied in the transduction of the signal following the ligand binding to estrogen receptor and concerning several levels: (1) loss or mutation of the estrogen receptor; (2) modification in estrogen receptor associated parameters; (3) alteration in the estrogen response element; (4) high levels of antiestrogen binding sites; (5) alteration of metabolism or availability of tamoxifen. The tamoxifen resistance certainly concerns several of these mechanism. Therefore, it is necessary to go on studying these mechanisms and to elucidate the connections existing between all of them.

A permanently charged tamoxifen derivative displays anticancer activity and improved tissue selectivity in rodents

A quaternized form of tamoxifen (TAM), tamoxifen methiodide (TMI), was shown to demonstrate very low brain uptake compared to TAM and, unexpectedly, was considerably less estrogenic than TAM in the uterus. The agonist activity of TMI in the bone was similar to that of TAM. TMI manifested significant dose-dependent tumoricidal activity with a rapid onset of action against MCF-7 human breast cancer implants in nude mice and a mean reduction in tumor size of 60% over six weeks.

Effects of grinding and tableting on physicochemical stability of an anticancer drug, TAT-59

The effects of grinding and tableting on the physicochemical stability of TAT-59, (E)-4-[1-[4-[2-(dimethylamino)ethoxy]phenyl]-2-(4-isopropyl) phenyl]-1-butenyl]phenyl monophosphate, were studied. The crystallinity of TAT-59 ground in a planetary ball mill for 0-120 min or compressed at 0-4500 kg/cm2 was evaluated by X-ray diffraction analysis and differential scanning calorimetry (DSC). The surface of TAT-59 was measured under a scanning electron microscope (SEM). The physicochemical stability of TAT-59, ground or compressed, was determined by measurements of water content, crystallinity and the amount of hydrolysis product, DP-TAT-59, formed. The crystallinity of ground TAT-59 decreased with increasing grinding time, and the amount of DP-TAT-59 increased with decrease in the crystallinity. Similar to ground TAT-59, the crystallinity of TAT-59 tablet gradually decreased with increasing compression pressure, and the amount of DP-TAT-59 tended to increase with decreasing crystallinity. These findings suggested that the decrease of the crystallinity of TAT-59 by mechanical force, such as grinding and tableting, raised the drug's reactivity and affected its stability.

Conformational studies and electronic structures of tamoxifen and toremifene and their allylic carbocations proposed as reactive intermediates leading to DNA adduct formation

Toremifene, a compound which differs from tamoxifen by the substitution of a chlorine atom for a hydrogen atom in the ethyl group, is significantly less potent than tamoxifen in causing DNA adduct formation in rats. To examine the relationship of the DNA adduct-forming ability of these compounds with their physicochemical properties such as stable conformation and chemical reactivity, we carried out molecular mechanics, molecular dynamics, and quantum mechanics calculations for the two compounds. For tamoxifen, six stable conformers were identified by conformational search with CFF91 force field. Molecular dynamics simulations showed that these were often interconverted within 1.0 ns. On the other hand, although the conformation of stable conformers and dynamical behavior of toremifene were almost the same as those of tamoxifen, a few conformations were slightly different from those of tamoxifen owing to the effect of the chlorine atom at chloroethyl group. In addition, the stability of the allylic carbocation, which had been proposed as the reactive intermediate leading to DNA adduct formation, was calculated with both semiempirical and density functional methods. Results showed that the carbocation intermediate of toremifene was less stable than that of tamoxifen by 4-5 kcal/mol, suggesting that toremifene was less frequently activated to the intermediate than tamoxifen. Furthermore, the carbocation intermediates of two other tamoxifen derivatives, 4-iodotamoxifen and droxifene, which show no DNA adduct-forming ability, were also less stable compared with that of tamoxifen. These calculated results suggest a close relation between the stability of the proposed carbocation intermediate and DNA adduct-forming ability.

A systematic review and critical comparison of internal standards for the routine liquid chromatographic assay of amiodarone and desethylamiodarone

Despite potential adverse effects, clinical use of amiodarone is increasing because of its efficacy in treating arrhythmias. Thus there is a continued need for a rapid, practical amiodarone assay to better study the relationship between serum concentrations and clinical effects and to guide safer dosing. Because the most widely used internal standard, L8040, is no longer available, a systematic comparison of potential alternatives was undertaken based on physicochemical and chromatographic characteristics. All amiodarone assays indexed on Medline were reviewed to produce a list of alternatives and five other potential substances considered based on previous experiences. An isocratic high-performance liquid chromatographic method was modified to allow simultaneous resolution of multiple compounds. The internal standard was expected to perform well in the solid-phase extraction of small sample volumes. No commercially available substances were able to duplicate all the advantages of L8040. Tamoxifen, the most acceptable alternative, was used to develop an assay to measure amiodarone and desethylamiodarone at concentrations as low as 0.25 mg/L in 100 microliters of serum (5 ng detected in a 20 microliters injection). Standard curves were linear over the range of concentrations found in our patients (0.25 to 8 mg/L), within-run coefficients of variation (CVs) averaged 5.3% for amiodarone and 2.9% for desethylamiodarone, and between-run CVs were 4.5% for amiodarone and 1.6% for desethylamiodarone.

Age-related difference in tamoxifen disposition

PURPOSE

To investigate the pharmacokinetic aspects of tamoxifen, such as the pharmacokinetic-pharmacodynamic (toxicity and clinical response) relationship and the influence of hepatic dysfunction, age, treatment duration, and associated chemotherapy on tamoxifen pharmacokinetics.

PATIENTS AND METHODS

Three hundred sixteen patients with breast cancer (247 postmenopausal women) were investigated. Mean age was 58 years (age range, 29 to 85 years). One hundred seventeen patients received tamoxifen as single therapy (adjuvant, 60; neoadjuvant, 17; metastatic, 40); 292 of 316 received 30 mg daily. We obtained 794 blood samples at steady state. Tamoxifen and metabolites, N-desmethyltamoxifen, N-desdimethyltamoxifen, primary alcohol, and 4-hydroxytamoxifen were measured by HPLC.

RESULTS

Serum concentrations of tamoxifen and metabolites showed a wide asymmetrical distribution. Median and extremes were 347 nmol/L (not detectable [ND] to 1677) for tamoxifen, 572 nmol/L (ND to 3132) for N-desmethyltamoxifen, 109 nmol/L (ND to 795) for N-desdimethyltamoxifen, and 59 nmol/L (ND to 390) for primary alcohol. 4-Hydroxytamoxifen was detectable in 9.5% of the samples (ND to 162 nmol/L). Neither the absolute nor the relative concentrations of each compound showed significant variations during treatment. Chemotherapy concomitant with tamoxifen slightly increased the tamoxifen blood concentration. Hepatic dysfunction had no obvious effect on drug concentrations, an exception being a slight reduction in the relative proportion of tamoxifen. The influence of age revealed that concentrations of tamoxifen and metabolites increased significantly with age: women younger than 40 years had a tamoxifen plus metabolite median concentration of 802 nmol/L compared with 2428 nmol/L for women older than 80 years. In the 28 patients in whom tamoxifen-related side effects developed, the proportion of demethylated metabolites was higher than that in patients in whom toxicity did not develop. There was no difference in drug concentrations between responding and nonresponding patients.

CONCLUSION

Despite the tremendous interpatient variability in drug concentrations, the present data show that tamoxifen and metabolite concentrations significantly increase with age.

Homologs of idoxifene: variation of estrogen receptor binding and calmodulin antagonism with chain length

A series of homologs of idoxifene [1a, (E)-1-[4-(N-pyrrolidinoethoxy)phenyl]-1-(4-iodophenyl)-2-phenyl-1-butene ] and selected homologs of 4-iodotamoxifen [2a,(E)-1-[4-(N-dimethylamino)-ethoxy]phenyl]-1-(4-iodophenyl)-2-phenyl -1-butene] with the side chain (CH(2))(n) varying in length from n=3 (1b,2b) to n=10(1i,2i) have been synthesized and tested for antagonism of of the calmodulin-dependent activity of cAMP phosphodiesterase and for binding affinity to rat uterine estrogen receptor. Compared with 1a (IC(50) =1.5 microM), the homologs showed a progressive increase in calmodulin antagonism with a maximum inhibition at n=7-9 (1f-h)(IC(50)=0.2 microM), declining at n=10 (1i) to IC(50) =1.6 microM. In the pyrrolidino series, estrogen receptor binding affinity peaked at n=3 (1b, RBA= 23; estradiol = 100), declining by n=10 (1i) to RBA = 0.4, but the homolog n=8 (1g, RBA = 3.5) was still comparable to tamoxifen (RBA = 3.9). A similar pattern of activity was seen for the dimethylamino counterparts. These compounds represent a new class of antiestrogens with potent calmodulin antagonism.

A study of the structural basis of the carcinogenicity of tamoxifen, toremifene and their metabolites

An analysis of the chemical structure of tamoxifen, toremifene and their metabolites indicates that metabolism to a DNA-reactive hydroxylamine intermediate is possible. The parent compounds and many of their metabolites are predicted to be rodent carcinogens. Moreover, many of these metabolites contain a 6 A or 8.4 A distance descriptor biphore. These geometric descriptors may be related to an ability of these chemicals to bind to an estrogen receptor. The prediction of the carcinogenicity of toremifene is not in accord with studies published thus far. However, the reports available have not excluded this possibility, since the protocols used have not addressed it systematically.

Dissociation of 4-hydroxytamoxifen, but not estradiol or tamoxifen aziridine, from the estrogen receptor as the receptor binds estrogen response element DNA

Estradiol-liganded estrogen receptor (E2-ER) binds EREs with a stoichiometry of one E2-ER dimer per estrogen response element (ERE). In contrast, although 4-hydroxytamoxifen (4-OHT)-liganded ER (4-OHT-ER) binds EREs with high affinity, its saturation ERE binding capacity is consistently half that of E2-ER, giving an apparent stoichiometry of one 4-OHT-ER monomer per ERE. Here we show that one molecule of 4-OHT ligand dissociates from the ER dimer apparently during the process of binding to DNA. Under equilibrium conditions, the type I antiestrogen tamoxifen aziridine (TAz), covalently attached to ER (TAz-ER), binds a single ERE with high affinity (Kd = 0.27 nM), comparable to that of E2-ER and 4-OHT-ER. In contrast to 4-OHT-ER, the ERE binding stoichiometry of TAz-ER was identical to that of E2-ER: one dimeric receptor per ERE. By measuring [3H]ligand that was initially bound to ER, a significant loss of [3H]4-OHT from ER was detected after ERE binding, whereas all [3H]E2 or [3H]TAz remained ER-bound. These results confirm that one molecule of 4-OHT ligand dissociates from the ER dimer as a consequence of ERE binding. Binding of 4-OHT and TAz are likely to induce a conformation in ER dimers that alters their capacity for gene activation. Upon ER binding to DNA, this conformation reveals itself by allowing 4-OHT dissociation, and predictably would allow TAz dissociation were it not bound covalently.

Comparison between inhibition of protein kinase C and antagonism of calmodulin by tamoxifen analogues

A variety of analogues of tamoxifen were tested for inhibition of protein kinase C (PKC) activity in MCF-7 breast cancer cells. These results were compared with the calmodulin antagonism exhibited by the analogues as measured by inhibition of calmodulin-dependent cyclic AMP phosphodiesterase. The same structural features that enhanced PKC inhibition also led to an increase in calmodulin antagonism, namely 4-iodination and elongation of the basic side-chain. The most potent analogue has a 4-iodine substituent and eight carbon atoms in its basic side-chain with IC50 values of 38 microM for PKC inhibition and 0.3 microM for calmodulin antagonism, which compares with 92 and 6.8 microM, respectively, for tamoxifen. Some selectivity was achieved with a ring-fused analogue that retained the potency of tamoxifen as a PKC inhibitor, but lacked calmodulin antagonism.

Synthesis and biological evaluation of basic side chain derivatives of Analog II as pure antiestrogens and antitumor agents

In an effort to prepare effective non-steroidal antiestrogens without intrinsic estrogenicity and with greater antagonism than those of the triarylethylenes (tamoxifen; TAM) four N-substituted (Z)-1,1-dichloro-2-[4-(2-aminoethoxy)phenyl]-3-phenylcyclopropa ne derivatives of the antiestrogen, Analog II, in which the basic side chains contain cyclic (piperidino and piperazino) and non-cyclic (dimethyl amino and diethyl amino) moieties, were synthesized. These compounds were prepared from an intermediate methanesulfonyloxyethoxy side chain ester of 1,1-dichloro-2,3-cis-diphenylcyclopropane using their respective side chain bases in triethylamine and acetonitrile. The gem-dichloro-cis-diarylcyclopropane derivatives were tested for their ability to inhibit the growth-stimulating effect of estradiol on immature mouse uteri and the growth of estrogen receptor (ER)-positive MCF-7E3, ER-negative MDA-MB-231 and the ER-positive MCF-7LY2 antiestrogen-resistant breast cancer cells in culture. The introduction of the various aminoethoxy side chains into Analog II did not improve its ER-binding affinity. Like Analog II, the derivatives did not exhibit any intrinsic estrogenicity, and compounds 9 and 10 antagonized estradiol action more completely than the parent compound. None of the compounds potentiated the uterine weight gain from the stimulating dose of estradiol (0.03 micrograms). Derivatives 9 (150 micrograms), 10 (150 micrograms) and 11 (150 micrograms) had uterine mean weights significantly below the estradiol-treated group, and were better antagonists than Analog II and MER25 at the same concentrations. All compounds exhibited a statistically significant (P < 0.01) reduction in control growth (antitumor activity) from 0.01 to 10 nM concentration in the MCF-7E3 cells. At 10 nM concentration, 8 (66%) and 9 (64%) had the greater antitumor activity over 10 (58%) and 11 (58%). No activity in this cell line was observed for Analog II, TAM and ICI 182,780. Antitumor action was also demonstrated in the MDA-MB-231 cells for all derivatives at 1.0 microM dose, with 9 having the greatest (27%) inhibition of control growth, followed by 8 (20%), 10 (18%) and 11 (12%). Analog II and ICI 182,780 had no antitumor activity in this cell line, while TAM exhibited only 8% inhibition. In the MCF-7E3 cell line at 1.0 microM, 9 exhibited 86% inhibition of the estradiol-stimulated growth (antiestrogenic activity), followed by 8 (64%), 10 (52%) and 11 (21%).(ABSTRACT TRUNCATED AT 400 WORDS)

Rationally designed analogues of tamoxifen with improved calmodulin antagonism

Computerized molecular modeling studies on the interactions of the antiestrogen tamoxifen (1) and its analogues bound to the calcium-binding protein calmodulin have guided the rational design of more potent antagonists. Compounds with either three or four methylene units in the basic side chain or slim lipophilic 4-substituents were expected to be more potent. All compounds were tested for antagonism of the calmodulin-dependent activity of cAMP phosphodiesterase and for binding affinity to the estrogen receptor from rat uteri. Some compounds were assayed for cytotoxicity against MCF-7 breast tumor cells in vitro. Introduction of lipophilic 4-substituents was accomplished by using palladium(0)-catalyzed coupling reactions with a 4-iodinated precursor. Both the 4-ethynyl (16 and 17) and 4-butyl (18 and 19) compounds were more potent calmodulin antagonists than tamoxifen. Extension of the basic aminoethoxy side chain of 4-iodotamoxifen (3) and idoxifene (2) ((E)-1-[4-[2-(N-pyrrolidino)ethoxy]phenyl]-1-(4-iodophenyl)-2-phen yl-1- butene) by one or two methylene units resulted in modest gains in calmodulin antagonism (10-13). All the compounds assayed retained estrogen receptor binding characteristics. The compound possessing the optimal combination of calmodulin antagonism and estrogen receptor binding was 12 ((E)-1-[4-[3-(N-pyrrolidino)propoxy]phenyl]-1-(4-iodophenyl)-2-phe nyl-1 - butene) (IC50 = 1.1 microM, RBA = 23). Correlation between calmodulin antagonism and cytotoxicity was demonstrated for selected compounds.

Molecular modelling of the human estrogen receptor and ligand interactions based on site-directed mutagenesis and amino acid sequence homology

A molecular model of the human estrogen receptor is reported based on a new alignment with the alpha 1-antitrypsin sequence, a homologous protein of known crystal structure. The putative ligand binding site is situated roughly equidistant between the DNA binding and dimerization regions. This binding site contains a number of amino acid residues shown by site-directed mutagenesis to be associated with the binding of agonists and antagonists. This putative ligand binding pocket is well-defined within a loop of peptide, containing complementary amino acids for binding interactions with agonists and antagonists. A leucine-rich region, common to most steroid-binding proteins, is in an optimum position for dimerization leading to DNA interaction. It is likely that ligand binding influences dimerization and DNA interaction by a conformational change in the receptor via the transcriptional activation residues. This model suggests that ligand binding may affect the hydrogen bonding pattern such that transpeptide signalling is initiated. The model accommodates steroidal estrogens and antiestrogens as well as the non-steroidal partial antagonist, hydroxytamoxifen.

Third annual William L. McGuire Memorial Lecture. "Studies on the estrogen receptor in breast cancer"--20 years as a target for the treatment and prevention of cancer

In 1973, McGuire and Chamness (In: O'Malley BW and Means AR (ed) Receptors for Reproductive Hormones, Plenum Press) summarized their work on the estrogen receptor in animal and human breast tumors, and in so doing described a target for therapeutic intervention. At that time there were no clinically useful antiestrogens, but the subsequent development of tamoxifen for breast cancer therapy has revolutionized the approach to treatment. Long-term adjuvant tamoxifen adjuvant therapy (i.e., greater than one year) has proven efficacy to enhance the survival of breast cancer patients. In addition, because there is an associated 40% decrease in contralateral breast cancer during adjuvant tamoxifen therapy and tamoxifen maintains bone density and reduces fatal myocardial infarction, clinical trials to test the worth of tamoxifen as a preventive for breast cancer in high risk women have started in the United States, United Kingdom, and Italy. Initial concerns that long-term tamoxifen causes endometrial cancer have been placed in perspective and analyzed by a review of the literature. Tamoxifen only doubles the normal risk of detecting endometrial cancer (i.e., to 2 per 1,000 tamoxifen-treated women per year), and 80% of these cases are early stage, good prognosis disease. Annual gynecological examinations and education are essential to provide reassurance for patients. The success of tamoxifen has encouraged the development of new antiestrogens to exploit the estrogen receptor as a therapeutic target. Droloxifene and TAT-59 mimic the metabolite 4-hydroxytamoxifen in having a high affinity for the estrogen receptor (Jordan et al, J Endocrinol 75:305, 1977). These drugs appear to have a pharmacological profile similar to tamoxifen. In contrast, the new pure antiestrogens have a distinct mechanism of action and will be valuable either as a first line therapy for advanced breast cancer or as a second line endocrine therapy after the failure of long-term adjuvant tamoxifen therapy. Finally, a new strategy is being developed to exploit the target site specific action of antiestrogens. Raloxifene, an antiestrogen with high affinity for the estrogen receptor but only weak estrogenicity for the uterus, prevents rat mammary tumorigenesis and maintains bone density. The drug is to be evaluated as a treatment for osteoporosis, but may also prevent the development of breast and endometrial cancer in a broad group of treated subjects. The identification of the estrogen receptor as a target for therapeutic opportunities has proved to be extremely beneficial for the control of breast cancer and has the added potential to control osteoporosis and coronary heart disease in women.

Effects of compression pressure on physical and chemical stability of tablets containing an anticancer drug TAT-59

(E)-4-[1-[4-[2-(Dimethylamino)ethoxy]phenyl]-2-(4-isopropyl) phenyl]-1-butenyl]phenyl monophosphate (TAT-59) is a new drug for the treatment of breast cancer. Physical and chemical stability of mixtures of TAT-59 and microcrystalline cellulose (1:9) compressed at 0, 300, 600 and 1400 kg/cm2 was evaluated by determination of water content, porosity and the amount of hydrolysis product, DP-TAT-59, formed. The water contents and porosity of these tablets scarcely changed during 57 d at 25-60 degrees C under 50% relative humidity (RH). The degradation rate of TAT-59 increased with increasing compression pressure as well as temperature. The apparent activation energy and frequency factor were determined from an Arrhenius plot of the degradation rate. Activation energy of these tablets was almost the same, while the frequency factor tended to increase with increasing compression pressure. The porosity and pore sizes in TAT-59-containing tablets decreased with increasing compressive force. We speculated from these observations that the increase in compression pressure decreased the distance and increased the contact area between TAT-59 and microcrystalline cellulose. The proximity between TAT-59 and moisture presented at the surface of microcrystalline cellulose by compression was considered to enhance the degradation of TAT-59.

Evidence that tamoxifen binds to calmodulin in a conformation different to that when binding to estrogen receptors, through structure-activity study on ring-fused analogues

A ring-fused analogue of tamoxifen, which had previously been shown to have practically identical estrogen receptor (ER) affinity and antitumour potency against estrogen responsive cells as tamoxifen, failed to inhibit calmodulin-dependent cyclic AMP phosphodiesterase. The substitution of an extra methyl group into the ring-fused analogue, at a position which the ethyl group of tamoxifen can occupy in one of its conformations, restored the calmodulin inhibition. Also, the replacement of the tamoxifen ethyl group by methyl diminishes calmodulin inhibition. Direct interaction of these tamoxifen analogues with calmodulin was demonstrated through the use of the fluorescent probe, 2-p-toluidinyl-naphthalene-6-sulfonic acid (TNS). These findings lead to the conclusion that tamoxifen binds to calmodulin in a conformation not accessible to the fused analogue and therefore likely to be different to that which binds to the ER. Also, the results on the ring-fused analogues indicate that the calmodulin binding cannot be essential for antitumour activity.

Molecular structures and conformational studies of triarylcyclopropyl and related nonsteroidal antiestrogens

Molecular structures and conformational characteristics of a series of 1,1-dichloro-2,2,3-triarylcyclopropanes (DTACs), which were reported previously to be distinctly antiestrogenic and inhibitors of the estrogen-receptor-positive MCF-7 human breast cancer cells in culture, are reported. In addition, structural and conformational features of the DTACs were compared to the first-known nonsteroidal antiestrogen, MER25, and the clinically useful antiestrogen Tamoxifen. The molecular structures of four DTAC compounds were determined by X-ray diffraction. Crystallographic structures show that the DTAC molecules have nearly the same relative conformation for the three aryl rings which is designated as a "nonpropeller" conformation in contrast to the observed "propeller" conformation for the three rings in all known triarylethylenes. Systematic conformational searches were performed to find the conformational preferences of DTACs, MER25, and Tamoxifen using idealized model compounds built from their respective crystal structure. Energy-minimization and conformational-search studies demonstrated that all DTAC molecules have a common, single global minimum energy conformer for their central core containing the dichlorotriarylcyclopropyl system, which is similar to that found in their crystal structures. Conformational search of MER25 showed that the molecule can assume a number of low-energy conformers of which two, one anti (A1) and one gauche (G1A), have about the same energy. The anti conformation is similar to the one observed in its crystal structure and resembles the estrogenic E-isomer of Tamoxifen, while the lowest energy gauche conformer of MER25 resembles more closely the antiestrogenic Z-isomer of Tamoxifen. NMR spectroscopic analysis of MER25 showed that the molecule exists predominantly in the anti conformation in solution. A comparative review of the structural features and bioactivities of Tamoxifen, DTACs, and MER25 provides a possible explanation for their low estrogen receptor binding affinity which is common to these compounds together with their antiestrogenic activity.

Effects of a non-steroidal pure antioestrogen, ZM 189,154, on oestrogen target organs of the rat including bones

ZM 189,154 ([1RS,2RS]-2-(4-hydroxyphenyl)-2-methyl- 1-[9-(4,4,5,5,5-penta-fluoropentyl)sulphinylnonyl]-1,2,3,4- tetrahydronaphth-6-ol) is a non-steroidal pure antioestrogen. It has a high relative affinity for the oestrogen receptor, completely blocks the trophic action of oestradiol (OE2) on the uterus in immature and ovariectomized (OVX) adult rats and, in the latter, also completely blocks the trophic action of OE2 on vagina, bone and growth rate. ZM 189,154 displays no intrinsic oestrogen-agonist activity on uterus, vagina, bone, LH secretion or growth rate in OVX rats. Differential sensitivity of OE2-regulated processes was more apparent in intact rats. Daily doses of 0.6 mg/kg per day of ZM 189,154 blocked ovulation; 2 mg/kg per day achieved maximal uterine atrophy but did not affect bone density or growth rate; 10 mg/kg per day produced a broader spectrum of effects (reduced bone density, increased basal LH, slightly increased growth rate), but the magnitude of these was smaller than after ovariectomy; the 10 mg/kg dose also produced multiple ovarian follicular cysts. The failure of ZM 189,154 to achieve complete ovariectomy-like effects in intact rats may be due to the action of ovarian factors other than OE2, or to the circulating OE2 levels resulting from the disturbance to ovarian function posing too strong a challenge to the antagonist.

An X-ray crystallographic study of the nonsteroidal contraceptive agent centchroman

We have determined an X-ray crystal structure for the N-methyl iodide derivative of the nonsteroidal contraceptive centchroman. The pendant aromatic substituents on C-3 and C-4 of the chroman system are nearly perpendicular to the plane of the chroman system, an orientation expected in such a chroman, but perturbed to some degree by the gem dimethyl substituents at C-2. Structural superposition with other nonsteroidal antiestrogens, tamoxifen and nafoxidine, shows a similar disposition of the tertiary amine side chains responsible for antagonist activity. The aryl rings also show good superposition, but in contrast to tamoxifen and nafoxidine, which have the potential for ring double bond conjugation, the centchroman aryl rings show a larger dihedral twist. While different superpositions between the enantiomers of centchroman and the bioactive enantiomer of estradiol (d-estradiol, 8 beta,9 alpha,13 beta,14 alpha,17 beta) are possible, when the chroman ring system is positioned over the AB rings of estradiol, then (3R,4R)-centchroman makes the best fit. The aryl substituents in both enantiomers make comparable overlays with the steroidal skeleton, but the axial methyl group at C-2 in (3R,4R)-centchroman is directed downward along the C-7 alpha axis of estradiol, a site where many substituents are known to be well tolerated by the estrogen receptor, while in the 3S,4S-enantiomer, this methyl group is projected upward. Thus, we suggest that the bioactive l-enantiomer of centchroman will have the 3R,4R absolute configuration.

A mechanistic hypothesis for DNA adduct formation by tamoxifen following hepatic oxidative metabolism

A mechanism for the generation of electrophilic alkylating agents from tamoxifen by hepatic alpha-oxidation of its ethyl group is presented which accounts for the formation of covalent DNA adducts associated with its hepatic carcinogenicity. The key process leading to the generation of electrophilic alkylating agents from tamoxifen is postulated to be alpha-hydroxylation in conjunction with 4-hydroxylation or secondary metabolic conjugation. The mechanism explains the lack of DNA adducts observed for the tamoxifen analogues toremifene, droloxifene and idoxifene and accounts for the species variation in hepatic carcinogenicity of tamoxifen.

Tamoxifen: new membrane-mediated mechanisms of action and therapeutic advances

Tamoxifen protects membranes and lipoprotein particles against oxidative damage. This antioxidant action is likely to contribute to the observed cardioprotective action of tamoxifen and supports the use of this compound in treating and even preventing breast cancer. Membrane-mediated mechanisms of tamoxifen action, through a putative modulation of membrane fluidity, are likely to play an important role in its anticancer action and its ability to reverse multidrug resistance, and could also lead to clinical uses as an anti-Candida and anti-viral agent. In this review, Helen Wiseman discusses the interaction of tamoxifen with membranes and lipoprotein particles, and considers the possible clinical implications.

Estradiol derivatives bearing the side-chain of tamoxifen antagonize the association between the estrogen receptor and calmodulin

Calmodulin (CaM) is known to associate with the estrogen receptor (ER). The antiestrogen tamoxifen impedes this association suggesting that the latter would play an important role in CaM-dependent enzymatic catalyses. The ethoxyaminoalkyl side-chain of tamoxifen which confers antiestrogenicity appears to be involved in this antagonism. Antiestrogenic estradiol derivatives bearing the side-chain of tamoxifen in position 11 beta (RU 39,411) or 7 alpha (RU 45,144) were tested for their potential antagonism towards the association between CaM and ER. According to molecular modelling studies, such graftings position the chain in an orientation corresponding to that found in tamoxifen. Both compounds impeded the binding of ER to CaM-Sepharose at the same concentrations as found with tamoxifen indicating similar effectiveness. Steroidal analogs with or without a side-chain in a non-appropriate orientation failed to show this property. On the contrary, a non-conjugated side-chain analog antagonized the binding of the receptor indicating that the steroidal backbone of RU 39,411 and RU 45,144 did not play a major role in this regard. Since this free side-chain had been reported to be totally devoid of antiestrogenicity, one may consider that the steroidal backbone of these two antiestrogens participate to their antiproliferative activity. One may speculate that within the cell, ER should convey such compounds to CaM leading to a blockade of CaM-dependent catalyses. This hypothesis would also be relevant to the stilbene backbone of tamoxifen.

Reduced tamoxifen accumulation is not associated with stimulated growth in tamoxifen resistance

To study tamoxifen resistance-stimulated growth, 30 female ovariectomized nude mice were implanted with tamoxifen-resistant tumors and treated with 10-1000 micrograms/day of tamoxifen citrate subcutaneously. Tamoxifen stimulated MCF-7 tumor growth in a dose-dependent manner, with tumoral tamoxifen concentrations increasing proportionally to the dose (1-13 nmol/g), as measured by high-performance liquid chromatography (HPLC). Flow-cytometric analysis revealed that tamoxifen-resistant tumors had a different DNA content as compared with wild-type MCF-7 cells. In contrast to earlier results, these data suggest that tamoxifen resistance-stimulated growth is associated with increasing rather than decreasing tumoral tamoxifen concentrations. Furthermore, the observed ploidy changes in the tamoxifen-resistant tumors imply that a genetic basis may exist for the development of tamoxifen resistance.

Molecular mechanisms of resistance to tamoxifen therapy in breast cancer

Clinical data suggest that the use of adjuvant tamoxifen citrate (Nolvadex) for a minimum of 5 years, and possibly indefinitely, will result in maximal antitumor benefit. There is concern that long-term tamoxifen maintenance therapy may result in the induction of drug resistance. This article reviews the potential molecular mechanisms of resistance to tamoxifen and explores the possibility of tamoxifen-stimulated tumor growth.

The anticancer drug tamoxifen induces changes in the physical properties of model and native membranes

The interactions of tamoxifen with lipid bilayers of model and native membranes were investigated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and by intramolecular excimer formation of 1,3-di(1-pyrenyl)propane (Py(3)Py). The effects of TAM of liposomes of DMPC, DPPC and DSPC are temperature dependent. In the fluid phase, TAM reduces dynamics of the upper bilayer region as observed by Py(3)Py and has no effect on the hydrophobic region as detected by DPH. In the gel phase, the effects of TAM evaluated by Py(3)Py are not discernible for DMPC and DPPC bilayers, whereas DSPC bilayers become more fluid. However, DPH detects a strong fluidizing effect of TAM in the hydrophobic region of the above membrane systems, where DPH distributes, as compared with the small effects detected by Py(3)Py. TAM decreases the main phase transition temperature but does not extensively broaden the transition thermotropic profile of pure lipids, except for bilayers of DMPC where TAM induces a significant broadening detected with the two probes. In fluid liposomes of sarcoplasmic reticulum lipids and native membranes, TAM induces an ordering effect, as evidenced by Py(3)Py, failing DPH to detect any apparent effect as observed for the fluid phase of liposomes of pure lipid bilayers. These findings confirm the hydrophobic nature of tamoxifen and suggest that the localization and effects of TAM are modulated by the order and fluidity of the bilayer. These changes in the dynamic properties of lipids and the non-specific interactions with membrane lipids, depending on the order or fluidity of the biomembrane, may be important for the multiple cellular effects and action mechanisms of tamoxifen.