Role of the TATA box in transcription of the mouse mammary tumor virus long terminal repeat.

An in vitro transcription system from mammary cells was established to study transcription of the long terminal repeat (LTR) of the mouse mammary tumor virus (MMTV). Experiments with progressive 5'-deletion constructs of the MMTV LTR revealed that a 19-base pair (bp) region from -41 to -23 bp, encompassing the TATA box and flanking DNA sequence, was as transcriptionally active as larger promoter constructs, both in nuclear extracts from human mammary cell lines (T47D and MCF7) and a nonmammary cell line (HeLa). The cell-free system was capable of supporting transcriptional induction by factors binding upstream of the TATA box, however, since purified glucocorticoid receptor-induced transcription in larger promoter constructs encompassing the MMTV hormone-responsive elements. Transcription from two other promoters, the adenovirus major late promoter and the human immunodeficiency virus LTR, also revealed a significant transcriptional contribution of upstream elements. The 19-bp TATA region from the MMTV LTR was shown to have considerably more activity in this transcription system than comparable TATA regions from other promoters. Sequences critical to the MMTV TATA region were evaluated by single base pair mutagenesis and found to comprise a consensus TATA box sequence, TATAAAA, as well as a single A just upstream of the TATAAAA sequence. Thus, the high level of basal transcription observed with the TATA region from MMTV is due to a perfect consensus TATA box sequence and a single base immediately 5' adjacent. It is likely that the high basal rate of transcription observed with this TATA box region on histone-free templates represents an inappropriate level of basal expression and that a complete evaluation of transactivation mechanisms in this system will require the recapitulation in vitro of the chromatin-mediated repressive state that exists in vivo.

Studies using fluorescent tetrahydrochrysene estrogens for in situ visualization of the estrogen receptor in living cells.

We have analyzed four fluorescent nonsteroidal estrogens for their potential to serve as vital cytological stains to visualize the estrogen receptor (ER) in a model receptor expression system. The novel estrogen fluorophores are based on the rigidified stilbene-like structure of 5,6,11,12-tetrahydrochrysene (THC), and they embody electron-donor (hydroxyl) and electron-acceptor groups (nitrile, amide, ester, or ketone) that afford efficient, long wavelength, and environment-sensitive fluorescence. These probes bind with high affinity to human ER (relative binding affinity, 22-85 vs. estradiol, 100), and they stimulate the transcriptional activity of this receptor. The strong fluorescence of the estrogenic THCs permits visualization, using conventional epifluorescence microscopy, of ER in transfected Cos-7 cells that express elevated levels of receptor. Cell staining by the donor-acceptor THCs characteristically displays a nonuniform pattern of nuclear fluorescence that can be fully inhibited by nonfluorescent estrogens such as estradiol or diethylstilbestrol. Additionally, this staining appears to be specific for ER, since it coincides with the distribution of receptor as determined by indirect immunofluorescence analysis using an ER-specific monoclonal antibody. Using these probes, we have analyzed the intracellular distribution of ER mutants containing a variety of deletions. Evidence is presented to show that removal of amino-terminal sequences within the ER polypeptide results in an altered pattern of intranuclear distribution with preferential accumulation of receptor protein within the nucleolus. These THC fluorophores therefore represent excellent probes for cytological studies involving ER expressed in cultured cells and represent an important advance toward the goal of exploiting fluorescence technology to analyze the expression and distribution of ER within tissue samples.

Molecular characterization by mass spectrometry of the human estrogen receptor ligand-binding domain expressed in Escherichia coli.

The ligand binding domain of the human estrogen receptor (hER-LBD), encompassing the sequence MDPS282AG...V595, has been expressed at high levels in Escherichia coli from a pET-23d vector, and a purified preparation has been characterized both by mass spectrometry and biochemical methods. Inclusion bodies from the bacterial expression were solubilized by sonication and the hER-LBD was purified to near homogeneity by affinity chromatography over an estradiol-Sepharose column in urea-containing buffer. This material ran as a single peak on reversed-phase HPLC, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed a band with apparent molecular mass of 31-32 kilodaltons (kDa), somewhat smaller than that expected from the construct (35.6 kDa). Edman degradation revealed a single sequence of MDPSAGDMRA, consistent with an intact N terminus. Further characterization of this material using low resolution matrix-assisted laser desorption ionization mass spectrometry indicated an apparent single protein species of average mass 33,143 daltons (Da). Detailed molecular analysis by electrospray ionization mass spectrometry, however, revealed that this purified hER-LBD preparation was actually composed of a number of both modified and unmodified LBD protein fragments between 32,900-33,400 Da. The bulk of this 33-kDa protein mixture consisted of three LBD protein fragments with C termini at Ala571 (70%), Ala569 (23%), and Ser575 (4%), with only a trace amount (3%) of the full length expressed LBD (MDPS282...V595; 35, 602 Da). These four protein species also appear to be partially chemically modified by carbamylation. The functional integrity of this hER-LBD preparation was investigated by a ligand-exchange assay, which showed that the hER-LBD retained full estradiol-binding capacity; specific, covalent labeling was also observed using either the electrophilic affinity-labeling ligand tamoxifen aziridine (TAZ) or the photoaffinity-labeling ligand hexestrol diazirine. Thus, this expressed hER-LBD preparation, while appearing nominally pure by conventional biochemical techniques and having the expected ligand-binding capacity, was shown by sensitive high resolution electrospray ionization mass spectrometry techniques to be largely truncated 20-26 amino acids from the expected C terminus and to have a substantial level of covalent modification arising from the urea.(ABSTRACT TRUNCATED AT 400 WORDS)

Coactivator peptides have a differential stabilizing effect on the binding of estrogens and antiestrogens with the estrogen receptor.

The effectiveness of estrogens in stimulating gene transcription mediated by the estrogen receptor (ER) appears to depend on ER interactions with coactivator proteins. These coactivators bind to ER when it is liganded with an estrogen agonist, but not when it is liganded with an estrogen antagonist. Because estrogen agonists are known to induce a conformation in ER that stabilizes coactivator binding, we asked whether coactivator binding to ER causes a reciprocal stabilization of agonist ligand binding. We used a fluorescent ligand for ER, tetrahydrochrysene-ketone, to monitor the rates of ligand dissociation from ERalpha and ERbeta, and to see how this process is affected by the p160-class coactivator, steroid receptor coactivator-1 (SRC-1). We used a 15-amino acid peptide corresponding to the second nuclear receptor box LXXLL motif in SRC-1 (NR-2 peptide), which is known to interact with the ER ligand-binding domain, a mutant peptide with an LXXAL sequence (NR-2A peptide), and a 203-amino acid fragment of SRC-1, termed the nuclear receptor domain (SRC1-NRD), embodying all three of the internal NR boxes of this protein. Both the NR-2 peptide and the SRC1-NRD fragment markedly slow the rate of dissociation of the agonist ligands tetrahydrochrysene-ketone, estradiol, and diethylstilbestrol, increasing the half-life of the ER-agonist complex by up to 50- to 60-fold. The SRC1-NRD has much higher potency in retarding ligand dissociation than does the NR-2 peptide; it is maximally effective at 30 nM, and it appears to bind with the stoichiometry of one SRC1-NRD per ER dimer. The peptides had little effect on the dissociation rate of antagonist ligands. Consistent with these results, we find that increasing the concentration of SRC-1 in cells by transfection of an expression plasmid encoding SRC-1 causes a 17-fold increase in the potency of estradiol in an estrogen-responsive reporter gene transcription assay. Thus, there is multifactorial control over receptor-coactivator interaction, its strength being determined by the agonist vs. antagonist nature of the ligand and the particular structure of the agonist ligand, and by the receptor subtype and the NR box sequence. The stabilizing effect of coactivator on ER-agonist ligand complexes may be important in determining the potency of estrogen agonists in a cell and may also underlie the tissue-selective pharmacology of certain synthetic estrogens.

Tamoxifen aziridines: effective inactivators of the estrogen receptor.

Two analogs of the antiestrogen tamoxifen, which bear a chemically reactive aziridine function in place of the dimethylamino group, bind to the estrogen receptor from rat uterus and from MCF-7 human breast cancer cells and appear to react irreversibly with the receptor at the estrogen binding site, in a time-and concentration-dependent fashion. BEcause these compounds are effective receptor inactivators in uterus and breast cancer cells, they should prove to be useful probes for investigating the role of receptor in regulating cellular responses to estrogen and in studying the dynamics of estrogen receptor synthesis and turnover.

Synthesis of high affinity fluorine-substituted ligands for the androgen receptor. Potential agents for imaging prostatic cancer by positron emission tomography.

We have prepared nine androgens substituted with fluorine at C-16 or C-20 to evaluate their potential, as positron emission tomographic (PET) imaging agents for prostatic cancer when labeled with the positron emitting radionuclide fluorine-18 (t1/2 = 110 min). These compounds represent members from the following classes of androgens: testosterone (T), 5 alpha-dihydrotestosterone (DHT), 7 alpha-methyl-19-nortestosterone (MNT), mibolerone (Mib), and metribolone (R1881). All of these compounds were prepared by functionalization of suitable androgen precursors, and the synthetic routes were developed to allow the introduction of fluorine by a fluoride ion displacement reaction late in the synthesis, as is required for the preparation of these compounds in fluorine-18 labeled form. We have also prepared four androgens in which the C-3 carbonyl or 17 beta-hydroxyl groups are replaced by fluorine. Most of the fluorine-substituted androgens show high affinity for the androgen receptor (AR), although fluorine substitution lowers their affinity by a small factor. None of the androgens where fluorine replaces oxygen functions at C-3 or C-17 have substantial affinity for AR. Derivatives of the natural androgens (T and DHT) as well as MNT have little affinity for other steroid hormone receptors (progesterone and mineralocorticoid receptors), whereas the Mib and R1881 derivatives have somewhat greater heterologous binding. With sex steroid binding protein, a human serum binding protein, the pattern of binding affinities is nearly the reverse, with derivatives of Mib, R1881 and MNT having low affinity, and DHT and T, high affinity. From these fluorine-substituted compounds, we can select several whose preparation in fluorine-18 labeled form for further tissue distribution studies is merited.

11 beta-substituted estradiol derivatives. 2. Potential carbon-11- and iodine-labeled probes for the estrogen receptor.

Four new classes of 11 beta-substituted estradiol and estriol derivatives (cyanoalkyl, ethynyl, propynyl, and iodovinyl) have been synthesized, and their binding affinity for the estrogen receptor has been evaluated. The binding affinity values indicate that the estrogen receptor has tolerance for estradiol derivatives bearing 11 beta-groups whose size, rigidity, and polarity are limited. The estradiol derivatives have higher affinity than the estriol derivatives. The potential of these agents as imaging agent for estrogen receptor-positive breast tumors is discussed. On the basis of the results of this and a previously reported study (Napolitano, E.; Fiaschi, R.; Carlson, K. E.; Katzenellenbogen, J. A. 11 beta-Substituted Estradiol Derivatives, Potential High-Affinity Carbon-11-Labeled Probes for the Estrogen Receptor: A Structure-Affinity Relationship Study. J. Med. Chem. 1995, 38, 429-434), a general strategy for designing high-affinity probes for the estrogen receptor is proposed.

Progestin 16 alpha, 17 alpha-dioxolane ketals as molecular probes for the progesterone receptor: synthesis, binding affinity, and photochemical evaluation.

Chemical probes for steroid receptors have proven useful in providing molecular details about important hormone-receptor interactions. A series of progestin 16 alpha, 17 alpha-dioxolane ketals of acetophenone or substituted acetophenones that bind to the progesterone receptor (PgR) with comparable or higher affinities than the natural ligand, progesterone, have been prepared and evaluated as potential in vitro and in vivo probes for the progesterone receptor. p-Azidoacetophenone ketal 6, the tetrafluoro analog 8, and the p-(benzoyl)acetophenone ketal 9 demonstrate the required combination of high relative binding affinity (RBA) (6 = 15%, 8 = 14%, 9 = 6.6%, progesterone = 13%, R5020 = 100%) and photoinactivation efficiency (6 = 80%, 8 = 77%, 9 = 29% at 30 min) required for potential photoaffinity labeling reagents for the PgR. The synthesis of azide 6 has been modified to accommodate a palladium-catalyzed tritium gas hydrogenolysis of an iodoaryl precursor in the final stage of the synthetic sequence; this procedure has been verified by hydrogenation. In addition, the progestin p-fluoroacetophenone ketal 10 was selected for preparation in fluorine-18-labeled form, on the basis of its high affinity for the PgR (RBA = 53%). Fluorine-18-labeled progestins may be evaluated as potential diagnostic imaging agents for PgR-positive breast tumors. The radiochemical syntheses and further biochemical results with the fluorine-18-labeled ketal 10 and the tritium-labeled aryl azide 6 will be presented in an accompanying paper and elsewhere.

11 beta-Substituted estradiol derivatives, potential high-affinity carbon-11-labeled probes for the estrogen receptor: a structure-affinity relationship study.

In view of their possible development as carbon-11-labeled receptor-based radiotracers for imaging estrogen-responsive breast tumors, we have synthesized a series of estradiols (1), estriols (2), 11 beta-ethylestradiols (3), 11 beta-ethylestriols (4), 11 beta-methoxyestradiols (5), and 11 beta-methoxyestriols (6), differing in the type of substituent R present at the 17 alpha-position (a, -H; b, -CH3; c, -C identical to CH; d, -C identical to CCH3; e, -Ph; f, -CH = CHMe cis), and measured their binding affinity for the estrogen receptor relative to estradiol (RBA). As expected, all the derivatives having an 11 beta-ethyl substituent have good binding properties (3a-d, 4a-d, RBA (25 degrees C): 109-3000%), and among them there are several promising candidates for carbon-11 labeling. Moxestrol (RBA (25 degrees C) = 185%) and its corresponding estriol derivative (4c, RBA (25 degrees C) = 20%) were the analogs having the highest affinity in the 11 beta-methoxyestradiol (5a-f) and 11 beta-methoxyestriol (6a-e) series, respectively; other analogs (R = Me, C identical to CMe, Ph, or cis-CH = CHMe) had uniformly lower RBA values.

16 beta-([18F]fluoro)estrogens: systematic investigation of a new series of fluorine-18-labeled estrogens as potential imaging agents for estrogen-receptor-positive breast tumors.

In order to understand the structural features that might lead to an estrogen receptor (ER) based breast tumor imaging agent with improved uptake characteristics, we have synthesized several new analogs of 16 beta-fluoroestradiol (beta FES) and studied their tissue distribution in immature rats. The compounds we prepared were 11 beta-methoxy-beta FES (7a), 11 beta-ethyl-beta FES (7b), 17 alpha-ethynyl-beta FES (8c), 17 alpha-ethynyl-11 beta-methoxy-beta FES (8a), and 11 beta-ethyl-17 alpha-ethynyl-beta FES (8b). All of the analogs exhibit good affinity for ER, ranging at 25 degrees C from 10 to 460, with estradiol equal to 100. Measurement of their octanol/water partition coefficients by an HPLC method allowed us to estimate their level of nonspecific binding and thereby to predict their binding selectivity indices (BSI, i.e., the ratio of their ER-specific to nonspecific binding); the BSI values of three fluorine-substituted analogs exceed that of estradiol. These ligands have been labeled in the 16 beta position with fluorine-18 by the nucleophilic displacement of an alpha-disposed trifluoromethanesulfonate by [18F]fluoride ion. Reduction with lithium aluminum hydride produced the estradiol series ([18F]-7a-c), while treatment with lithium trimethylsilylacetylide afforded the ethynylated series ([18F]-8a-c). The synthesis time was 85 min for [18F]-7a-c and 120 min for [18F]-8a-c, with radiochemical yields ranging from 16 to 43%, and effective specific activities being 90-2900 Ci/mmol (3.3-107 TBq/mmol). In tissue distribution studies in immature female rats, all of the labeled analogs demonstrated ER-selective uptake in the principal target tissues, the uterus and the ovaries, and also in organs with lower titers of ER, the secondary target sites kidney, thymus, fat, and muscle. Although factors other than specific and nonspecific binding obviously affect the tissue distribution of these 16 beta-fluoroestrogens, we find that their ER-specific uptake by both the principal and the secondary target tissues correlates with their BSI values at a high level of statistical significance in most cases. The ethynylated-11 beta-methoxy analog [18F]-8a had high selectivity (uterus to blood ratio) after 3 h and exhibited the highest uterine uptake (percent injected dose/gram) of any fluorine-substituted estradiol ligand we have studied to date. This compound has been chosen for more detailed studies (to be described elsewhere), including clinical trials in human patients diagnosed with primary breast cancer.

Estrogenic affinity labels: synthesis, irreversible receptor binding, and bioactivity of aziridine-substituted hexestrol derivatives.

To develop an affinity label for the estrogen receptor that would be an estrogen agonist, rather than antagonist, we prepared several aziridine derivatives of the potent nonsteroidal estrogen hexestrol [3R,4S)-3,4-bis(4-hydroxyphenyl)hexane) bearing an aziridine function on the side chain. Three functional groups link the hexestrol ligand and the aziridine: a carbonyl group (ketone or ester), a thioether, or a methylene chain. The apparent competitive binding affinity of these derivatives for the estrogen receptor ranges from 1.8% to 25% that of estradiol, and most of them bind in a time-dependent, irreversible manner with the receptor, although the rate and efficiency of this binding vary widely, often with relatively small changes in structure. This is consistent with the irreversible attachment requiring a precise alignment of activating and reacting residues in the binding site of the receptor. The estrogenic and antiestrogenic activity of these aziridine derivatives was investigated in MCF-7 human breast cancer cells. Most of the compounds are agonists, with one being an antagonist. The derivative (6R,7S)-1-N-aziridinyl-6,7-bis(4-hydroxyphenyl)-5-nonanone (keto-nonestrol aziridine 3) appears to have the most ideal behavior of the estrogenic affinity labeling agents prepared: It is an agonist, and it binds to receptor irreversibly, efficiently, and quite rapidly.

Estrogen receptor subtype-selective ligands: asymmetric synthesis and biological evaluation of cis- and trans-5,11-dialkyl- 5,6,11, 12-tetrahydrochrysenes.

We have recently reported that racemic 5,11-cis-diethyl-5,6,11, 12-tetrahydrochrysene-2,8-diol (THC, rac-2b) acts as an agonist on estrogen receptor alpha (ERalpha) and as a complete antagonist on estrogen receptor beta (ERbeta) (Sun et al. Endocrinology 1999, 140, 800-804). To further investigate this novel ER subtype-selective estrogenic activity, we have synthesized a series of cis- and trans-dialkyl THCs. cis-Dimethyl, -diethyl, and -dipropyl THCs 2a-c were prepared in a highly enantio- and diastereoselective manner by the acyloin condensation of enantiomerically pure alpha-alkyl-beta-arylpropionic esters, followed by a Lewis acid-mediated double cyclization under conditions of minimal epimerization. ERalpha and ERbeta binding affinity of both cis and trans isomers of dimethyl, diethyl, and dipropyl THCs was determined in competitive binding assays, and their transcriptional activity was determined in reporter gene assays in mammalian cells. Nearly all THCs examined were found to be affinity-selective for ERbeta. All these THCs are agonists on ERalpha, and THCs with small substituents are agonists on both ERalpha and ERbeta. As substituent size was increased, ERbeta-selective antagonism developed first in the (R,R)-cis enantiomer series and finally in the trans diastereomer and (S,S)-cis enantiomer series. The most potent and selective ligand was identified as (R,R)-cis-diethyl THC 2b, which mimicked the ERbeta-selective antagonist character of racemic cis-diethyl THC 2b. This study illustrates that the antagonist character in THC ligands for ERbeta depends in a progressive way on the size and geometric disposition of substituent groups and suggests that the induction of an antagonist conformation in ERbeta can be achieved with these ligands with less steric perturbation than in ERalpha. Furthermore, antagonists that are selectively effective on ERbeta can have structures that are very different from the typical antiestrogens tamoxifen and raloxifene, which are antagonists on both ERalpha and ERbeta.

Salicylaldoxime moiety as a phenolic "A-Ring" substitute in estrogen receptor ligands.

The phenolic "A-ring" of natural and synthetic estrogen receptor (ER) ligands was effectively replaced by a planar six-member ring formed through an intramolecular hydrogen bond within a salicylaldoxime. Thus, oxime 1, a structural analogue of a triarylethylene estrogen, showed a significant binding affinity for the ER. The OH of the oxime function appears to mimic the phenolic OH present in more "classical" ER ligands because the binding reduced when the oxime OH is methylated (2) or absent (3).

Antiestrogens and antiestrogen metabolites: preparation of tritium-labeled (+/-)-cis-3-[p-(1,2,3,4-tetrahydro-6-methoxy-2-phenyl-1-naphthyl)phenoxyl]-1,2-propanediol (U-23469) and characterization and synthesis of a biologically important metabolite.

The Upjohn antiestrogen (+/-)-cis-3-[p-(1,2,3,4-tetrahydro-6-methoxy-2-phenyl-1-naphthyl)phenoxy]-1,2-propanediol (2b, U 23469) has been prepared in tritium-labeled form by reduction of an unsaturated dihydronaphthalene precursor with carrier-free tritium gas over a palladium catalyst followed by alkylation with 3-iodo-1,2-propanediol. After extensive chromatographic purification, the final material was obtained with a specific activity of 13 Ci/mmol and a radiochemical purity of 94%. In vivo studies with immature rats show that [3H]2b is slowly converted to a more polar metabolite that is selectively accumulated in the nuclear fraction of the uterus where it is bound to the estrogen receptor. Chromatographic comparisons indicate that this metabolite is the demethylated analogue 2c, a compound that has an affinity for estrogen receptor more than 300 times greater than that of 2b. These studies suggest that the demethylated analogue 2c may be a biologically important metabolite of 2b that is involved in the action of this antiestrogen.

Chemically reactive estrogens: synthesis and estrogen receptor interactions of hexestrol ether derivatives and 4-substituted deoxyhexestrol derivatives bearing alkylating functions.

A series of chemically reactive derivatives of the nonsteroidal estrogen hexestrol have been synthesized as potential affinity labels for the estrogen receptor or as cytotoxic agents with selective activity against receptor-containing cells. These compounds are hexestrol ethers with halo ketone, halohydrin, or epoxide functions or 4-substituted deoxyhexestrols with halo ketone, benzyl halide, nitro, azide, sulfonyl fluoride, or sulfonyl azide groups. The alkylating activity of the electrophilic derivatives was measured using the colorimetric reagent nitrobenzylpyridine, the bromo derivatives being considerably more reactive than the chloro ones. Their reversible binding to the lamb uterine estrogen receptor was measured by competitive binding assays, and their irreversible reaction with receptor was measured by exchange assays that determine the rate and extent of receptor inactivation. In general, monoetherification of hexestrol or substitution of deoxyhexestrol produces compounds with relatively low affinity for the estrogen receptor (0.3-10% that of estradiol). Most of the electrophilic derivatives are rapid and effective inactivators of receptor (24-70% inactivation within 0.5-5 h at 25 degrees C). Of the photosensitive derivatives, 4-azidodeoxyhexestrol appears to be the most efficient receptor inactivator (49%). The high reactivity of these compounds toward the estrogen receptor and the lack of interference by their reaction with other cellular nucleophiles suggest that these compounds may be useful as affinity-labeling agents or as selective cytotoxic agents in intact systems.

Synthesis and biological evaluation of a novel series of furans: ligands selective for estrogen receptor alpha.

A variety of nonsteroidal systems can function as ligands for the estrogen receptor (ER), in some cases showing selectivity for one of the two ER subtypes, ER alpha or ER beta. We have prepared a series of heterocycle-based (furans, thiophenes, and pyrroles) ligands for the estrogen receptor and assessed their behavior as ER ligands. An aldehyde enone conjugate addition approach and an enolate alkylation approach were developed to prepare the 1,4-dione systems that were precursors to the trisubstituted and tetrasubstituted systems, respectively. All of the diones were easily converted into the corresponding furans, but formation of the thiophenes and pyrroles from the more highly substituted 1,4-diones was problematical. Of the systems investigated, the tetrasubstituted furans proved to be most interesting. They were ER alpha binding- and potency-selective agents, with the triphenolic 3-alkyl-2,4,5-tris(4-hydroxyphenyl)furans (15a-d) displaying generally higher subtype binding selectivity than the bisphenolic analogues (15f-i). Binding selectivity for ER alpha was as high as 50-70-fold, and transcriptional activation studies showed that several members of this series were ER alpha selective agonists, with the best compound [3-ethyl-2,4,5-tris(4-hydroxyphenyl)furan, 15b] having full transcriptional activity on ER alpha while being inactive on ER beta. Comparative binding affinity analysis and molecular modeling were used to investigate the preferred binding mode adopted by the furan ligands, which appears to have the C(2) phenol mimicking the important role of the A-ring of estradiol. These ligands should be useful in studying the biological roles of both ER alpha and ER beta, and they might form the basis for the development of novel estrogen pharmaceuticals.

Estrogen receptor-beta potency-selective ligands: structure-activity relationship studies of diarylpropionitriles and their acetylene and polar analogues.

Through an effort to develop novel ligands that have subtype selectivity for the estrogen receptors alpha (ERalpha) and beta (ERbeta), we have found that 2,3-bis(4-hydroxyphenyl)propionitrile (DPN) acts as an agonist on both ER subtypes, but has a 70-fold higher relative binding affinity and 170-fold higher relative potency in transcription assays with ERbeta than with ERalpha. To investigate the ERbeta affinity- and potency-selective character of this DPN further, we prepared a series of DPN analogues in which both the ligand core and the aromatic rings were modified by the repositioning of phenolic hydroxy groups and by the addition of alkyl substituents and nitrile groups. We also prepared other series of DPN analogues in which the nitrile functionality was replaced with acetylene groups or polar functions, to mimic the linear geometry or polarity of the nitrile, respectively. To varying degrees, all of the analogues show preferential binding affinity for ERbeta (i.e., they are ERbeta affinity-selective), and many, but not all of them, are also more potent in activating transcription through ERbeta than through ERalpha (i.e., they are ERbeta potency-selective). meso-2,3-Bis(4-hydroxyphenyl)succinonitrile and dl-2,3-bis(4-hydroxyphenyl)succinonitrile are among the highest ERbeta affinity-selective ligands, and they have an ERbeta potency selectivity that is equivalent to that of DPN. The acetylene analogues have higher binding affinities but somewhat lower selectivities than their nitrile counterparts. The polar analogues have lower affinities, and only the fluorinated polar analogues have substantial affinity selectivities. This study suggests that, in this series of ligands, the nitrile functionality is critical to ERbeta selectivity because it provides the optimal combination of linear geometry and polarity. Furthermore, the addition of a second nitrile group beta to the nitrile in DPN or the addition of a methyl substitutent at an ortho position on the beta-aromatic ring increases the affinity and selectivity of these compounds for ERbeta. These ERbeta-selective compounds may prove to be valuable tools in understanding the differences in structure and biological function of ERalpha and ERbeta.

Fluorine-18-labeled progestin ketals: synthesis and target tissue uptake selectivity of potential imaging agents for receptor-positive breast tumors.

We have studied two new fluorine-substituted progestins as potential imaging agents for progesterone-receptor-positive human breast tumors. The steroids are 16 alpha, 17 alpha-fluoroacetophenone ketals of 16 alpha, 17 alpha-dihydroxyprogesterone and 16 alpha, 17 alpha, 21-trihydroxy-19-norprogesterone. Synthesis of the latter compound in seven steps from 19-norandrost-4-ene-3,17-dione is reported. Both compounds demonstrate high affinity for the progesterone receptor (PgR) (52.5 and 240%, respectively, relative to R5020 = 100). The syntheses were adapted to 18F-labeling with 4'-[18F]-fluoroacetophenone, prepared from 4'-nitroacetophenone by nucleophilic substitution with K18F/Kryptofix. Considerable adjustment of reaction conditions was required to effect ketalization using tracer quantities of the ketone. In tissue distribution studies in estrogen-primed immature female rats, both ketals showed selective uterine uptake, which was blocked by coinjection of a saturating dose of the unlabeled progestin ORG 2058. Additionally, metabolic stability of the radiolabel was indicated by the low radioactivity levels seen in bone. Both compounds showed relatively high uptake in fat, in accord with their relative lipophilicities demonstrated by HPLC-derived octanol-water partition coefficients. The selective uterine uptake and metabolic stability of these compounds suggests that this class of PgR ligands might be promising for the selective imaging of receptor-positive tumors if derivatives of reduced lipophilicity can be prepared.

11 beta-methoxy-, 11 beta-ethyl- and 17 alpha-ethynyl-substituted 16 alpha-fluoroestradiols: receptor-based imaging agents with enhanced uptake efficiency and selectivity.

We have prepared three analogues of 16 alpha-fluoroestradiol (FES) substituted either with an 11 beta-methoxy group (1, 11 beta-MeO-FES), an 11 beta-ethyl group (2, 11 beta-Et-FES), or a 17 alpha-ethynyl group (3, 17 alpha-ethynyl-FES). These substituents all lower the binding of FES to the serum proteins alphafetoprotein and sex steroid binding protein, but their effect on estrogen receptor binding varies: Receptor binding is increased by the 11 beta-ethyl and 17 alpha-ethynyl groups, but decreased by the 11 beta-methoxy group. These substituents also have a parallel effect on the lipophilicity, and hence the nonspecific binding estimated for these compounds. All three compounds were prepared in fluorine-18 labeled form, at effective specific activities of 90-1600 Ci/mmol, by fluoride ion displacement reactions as done previously with FES. Tissue distribution studies in immature rats show high uptake selectivity by target tissue (uterus) and effective competition by an excess of unlabeled estradiol. Percent injected dose per gram values (% ID/g) at 1 h are 6% for 11 beta-MeO-FES and 11-13% for 11 beta-Et-FES and 17 alpha-ethynyl-FES (FES itself has a % ID/g of 9%). Uptake selectivity in terms of uterus to blood or muscle ratios at 1 h is highest for 11 beta-MeO-FES and 17 alpha-ethynyl-FES (43-149). Metabolic consumption studies show that most activity in uterus is unmetabolized and in blood is rapidly and nearly completely metabolized. In muscle, FES and the substituted estrogens show intermediate levels of metabolic consumption; in some cases activity in muscle extracts is nearly unmetabolized. Thus, the substituents on FES cause major alterations in receptor and nonreceptor binding affinity, uptake efficiency and selectivity, and extent of metabolism. It is not readily clear, however, whether the alterations in uptake efficiency and selectivity are the result of differences in receptor or nonreceptor binding or lipophilicity, or altered patterns of metabolism. Nevertheless, these compounds should be useful in providing a spectrum of uptake properties that could be used for imaging different estrogen-receptor-containing structures.

Receptor-binding radiopharmaceuticals for imaging breast tumors: estrogen-receptor interactions and selectivity of tissue uptake of halogenated estrogen analogs.

Four halogenated estrogen analogs--o-fluorohexestrol, and 1-fluoro-, 1-bromo-, and 1-iodohexestrol--have been prepared and tritium-labeled in high specific activity, to investigate their potential as estrogen-receptor-based agents for imaging breast tumors. These compounds bind with high affinity in vitro to the cytoplasmic uterine estrogen receptor from rat and lamb and sediment as 8S receptor complexes on sucrose gradients. After 1 hr in immature rats, these compounds show high uptake into the uterus, but low uptakes (10--25% of the uterine levels) into most nontarget tissues. The uterine uptake is estrogen specific since it is depressed by excess nonradioactive estradiol. Uptake selectivity is greatest for the fluorohexestrols and decreases for the bromo and iodo compounds. In mature rats bearing DMBA-induced mammary tumors, selective uptake by the uterus and tumors is seen with 1-fluoro[3H4]hexestrol and o-fluoro[3H3]hexestrol. The studies indicate that these four halogenated hexestrols are promising candidates as estrogen-receptor-based agents for the imaging of human breast tumors.