Pyrazole ligands: structure-affinity/activity relationships and estrogen receptor-alpha-selective agonists

(4R,5S)/(4S,5R)-4,5-Bis(4-hydroxyphenyl)-2-imidazolines: ligands for the estrogen receptor with a novel binding mode

(4R,5S)/(4S,5R)-4,5-Bis(4-hydroxyphenyl)-2-imidazolines 1-7 were synthesized by the reaction of the methoxy-substituted (1R,2S)/(1S,2R)-1,2-diarylethylenediamines 1b-7b with triethyl orthoformate and subsequent ether cleavage with BBr(3). All compounds were tested for estrogen receptor (ER) binding in a competition experiment with [(3)H]-estradiol and for gene activation in a luciferase assay using ER positive MCF-7-2a breast cancer cells stably transfected with the plasmid ERE(wtc)luc. The relative binding affinities of the 2-imidazolines were very low (RBA < 0.1%). Nevertheless, 4-7 possessed full agonistic activity in the luciferase assay. The relative transcription potency increased in the order 5 (2,2'-I) < 6 (2,6-Cl(2), 2'-F) < 4 (2,2'-Cl) < 7 (2,6-Cl(2), 2'-Cl). These data together with spectroscopic and molecular modeling studies were used to investigate the preferred binding mode adopted by the imidazoline ligands. The 1,2-diarylethane pharmacophor takes a Z-stilbene-like structure with pseudoaxially oriented phenyl rings at the planar heterocyclic ring. Because of this unusual spatial structure, the (4R,5S)/(4S,5R)-4,5-bis(4-hydroxyphenyl)-2-imidazolines have to be assigned to a second class of estrogenically active compounds (type II estrogens). In contrast to type I estrogens, e.g., estradiol (E2), diethylstilbestrol (DES), and meso-hexestrol (HES), which are connected to His 524 in the binding site, type II estrogens are very likely H-bonded to Asp 351 in a hydrophobic side pocket.

A validation study on the practical use of automated de novo design

The de novo design program Skelgen has been used to design inhibitor structures for four targets of pharmaceutical interest. The designed structures are compared to modeled binding modes of known inhibitors (i) visually and (ii) by means of a novel similarity measure considering the size and spatial proximity of the maximum common substructure of two small molecules. It is shown that the Skelgen algorithm generates representatives of many inhibitor classes within a very short time and that the new similarity measure is useful for comparing and clustering designed structures. The results demonstrate the necessity of properly defining search constraints in practical applications of de novo design.

Differential interactions of estrogens and antiestrogens at the 17 beta-hydroxyl or counterpart hydroxyl with histidine 524 of the human estrogen receptor alpha

We investigated the role of H524 of the human estrogen receptor alpha (ERalpha) for the binding of various estrogens [estradiol (E(2)), 3-deoxyestradiol (3-dE(2)), and 17beta-deoxyestradiol (17beta-dE(2))] and antiestrogens [4-hydroxytamoxifen (OHT), RU 39 411 (RU), and raloxifene (Ral)], which possess the 17beta-hydroxyl or counterpart hydroxyl (designated: 17beta/c-OH), with the exception of 17beta-dE(2) and OHT. The work involved a comparison of the binding affinities of these ligands for wild-type and H524 mutant ERs, modified or not with diethyl pyrocarbonate (DEPC), a selective histidine reagent. Alanine substitution of H524 did not significantly change the association affinity constant (relative to OHT) of 17beta-dE(2), whereas those of RU, Ral, E(2), and 3-dE(2) were decreased 3-fold, 14-fold, 24-fold, and 49-fold, respectively. Values of the two ligands available in radiolabeled form (E(2) and OHT) were correlated with the dissociation rate constants, which were increased 250-fold and 2-fold, respectively. The action of DEPC on wild-type ER led to a homogeneous ER population which still bound antiestrogens and 17beta-dE(2) with practically unchanged affinities (less than 4-fold decreases in relative affinity constants), while E(2) and 3-dE(2) displayed markedly decreased affinities (56-fold decrease for E(2)). Conversely, DEPC treatment of H524A mutant ER did not induce marked decreases in the relative affinities of any of the checked compounds (decreases </=3-fold). All of these effects appeared to involve H524 as the H516A mutant behaved as wild-type ER. These combined data relative to mutated or DEPC-modified ER converged to support that the interaction of 17beta/c-OH of ER ligands with H524 is strong for estrogens and weaker for antiestrogens, with quantitative or qualitative differences between the binding modes of the latter, as illustrated by RU and Ral. The abilities of E(2) and OHT to protect the various ER types against inactivation by DEPC were strikingly different: OHT totally prevented the effect of DEPC on wild-type, H516A, and H524A ERs, while E(2) only partially protected wild-type and H516A ERs (H516A ER > wild-type ER) and very weakly protected H524A ER. Molecular modeling was tentatively used to interpret the biochemical results.

Sequential functionalization of pyrazole 1-oxides via regioselective metalation: synthesis of 3,4,5-trisubstituted 1-hydroxypyrazoles

A range of 3,5-diarylated and 3,4,5-triarylated 2-(4-methoxybenzyl)pyrazole 1-oxides have been prepared by regioselective deprotonation at C-5 or bromine-magnesium exchange at C-3 or C-4 followed by transmetalation with ZnCl(2) and palladium(0)-catalyzed cross-coupling. Furthermore, the metalated pyrazole 1-oxides could be trapped with electrophiles. The sequential metalation/functionalization of the pyrazole 1-oxides may follow the order C-5, C-3, C-4, or alternatively the order C-3, C-5, C-4. The 4-methoxybenzyl group of the functionalized 2-(4-methoxybenzyl)pyrazole 1-oxides could be removed by treatment with TFA and i-Pr(3)SiH in CH(2)Cl(2), providing the corresponding functionalized 1-hydroxypyrazoles.

Synthesis, structural evaluation, and estrogen receptor interaction of 2,3-diarylpiperazines

To develop novel estrogen receptor (ER) ligands, ring-fused derivatives of the hormonally active (1R,2S)/(1S,2R)-1-(2-chloro-4-hydroxyphenyl)-2-(2,6-dichloro-4-hydroxyphenyl)ethylenediamine 4b were synthesized. (2R,3S)/(2S,3R)-2-(2-Chloro-4-hydroxyphenyl)-3-(2,6-dichloro-4-hydroxyphenyl)piperazine 4 induced ligand-dependent gene expression in MCF-7-2a cells, stably transfected with the plasmid ERE(wtc)luc and was therefore used as a lead structure. The influence of the substitution pattern in the aromatic rings (4-OH (1), 2-F,4-OH (2), 2-Cl,4-OH (3), 2,6-Cl2,3-OH (5), and 2,6-Cl2,4-OH (6)) and the effect of N-ethyl chains on the ER binding and activation of gene expression were studied. The synthesis started from the respective methoxy-substituted (1R,2S)/(1S,2R)-configurated 1,2-diarylethylenediamines 6b to 4b, which were reacted with dimethyl oxalate in order to get 5,6-diarylpiperazine-2,3-diones. Reduction with BH3*tetrahydrofuran and ether cleavage with BBr3 yielded the piperazines 1-6. The N-alkylation of the piperazines 1a-3a, which was employed for obtaining compounds 7-11, was succeeded by acetic anhydride followed by reduction and ether cleavage. Nuclear magnetic resonance (NMR) spectroscopical studies revealed a synclinal conformation of the 1,2-diarylethane pharmacophore and a preference of the substituents at the heterocyclic ring for an equatorial position. This spatial structure prevents an interaction with the ER analogously to that of estradiol (E2). Therefore, the piperazines can displace E2 from its binding site only to a very small extent. Only the N-ethyl (8) and N,N'-diethyl (11) derivatives of piperazine 3 showed relative binding affinity values > 0.1% (8, 0.42%, and 11, 0.17%). Nevertheless, ER-mediated gene activation was verified for the piperazines 4 (20%), 6 (73%), 7 (34%), 8 (74%), and 11 (37%) (concentration, 1 microM; E2, 100% activation) on the MCF-7-2a cell line. O-methylation led to completely inactive compounds and showed the necessity of H bridges from the piperazines to the ER for activating gene expression.

Aryl cyclopentadienyl tricarbonyl rhenium complexes: novel ligands for the estrogen receptor with potential use as estrogen radiopharmaceuticals

The need for imaging agents for estrogen receptor positive (ER+) tumors that are both cost effective and widely available, as well as the need for novel radiotherapeutic agents for the treatment of breast cancer, has prompted us to investigate cyclopentadienyl tricarbonyl metal [CpMet(CO)(3), Met=Re, Tc-99m] complexes that bind well to the ER. Thus, we have prepared a series of p-hydroxyphenyl-substituted CpRe(CO)(3) complexes and evaluated them (and, in some cases, their cyclopentadiene precursors) for binding to ER. These compounds constitute a new class of structurally integrated organometallic ligands for ER in which the CpMet(CO)(3 )organometallic unit forms the very structural core of these molecules and thus is necessarily intimately involved in their interaction with the receptor. The CpRe(CO)(3) compounds were prepared by reaction of the lithium salt of the arene-substituted cyclopentadiene with a suitable Re(CO)(3)(+) precursor, followed by deprotection of the methyl ether. The X-ray crystal structure of one of these analogues shows that it has the classical 'piano stool'-like geometry, with the alkyl groups directed upward, away from the tripodyl metal carbonyl base. The aryl-substituted CpRe(CO)(3) complexes that we have prepared all bind to the ER, some with affinity as great as 20% that of the native ligand, estradiol. In general, at least two p-hydroxyphenyl substituents and one to two alkyl groups attached to the organometallic cyclopentadienyl core are needed for high ER affinity. Where we have been able to make comparisons, the metal complexes bind to ER with an affinity greater than their cyclopentadiene precursors. The high affinity of some of these complexes indicates that the bulky Re(CO)(3) unit is able to exploit the considerable volume in the center of the ER ligand binding pocket that is not occupied by most ligands, a consideration that is supported by molecular modeling. The preparation of the best of these agents in technetium-99m labeled form is currently being investigated.

Recognition of resveratrol by the human estrogen receptor-alpha: a molecular modeling approach to understand its biological actions

OBJECTIVES

Resveratrol (RSVL) is an edible phytoestrogen with multifaceted health benefits that may originate from binding to the estrogen receptors. Despite its structural similarity to the estrogen receptor-alpha (ER alpha) agonist diethylstilbestrol (DES), RSVL showed distinct biological profiles in estrogen-responsive biological systems. The molecular basis of such biological profiles has been undefined.

METHODS

We considered possible orientations for RSVL in ER alpha binding pocket. These conformations have been analyzed based on: (i) alignment with the key pharmacophoric elements of DES; (ii) computational energy of interaction, and (iii) pattern of accommodation at the ER alpha binding pocket. The characteristics of the most favored RSVL orientation have been compared with those of DES.

RESULTS

Both RSVL and DES interacted with the catalytic amino acid triad of the ER alpha pocket (His524, Arg394 and Glu 353). However, unlike the Er alpha agonists DES and estradiol (E2), RSVL formed three additional hydrogen bonds with Gly521 and Leu525, two paramount ligand recognition residues, and with Met343 at the ER alpha binding cavity. Lastly, RSVL displayed a more favorable energy of interaction with the ER alpha binding cavity.

CONCLUSIONS

The present study suggests, for the first time, that RSVL is well recognized by the human ER alpha but in a manner distinct from the pure agonists DES and E2. These variations may well entail the unique biological responses of RSVL in ER-responsive systems.

Diaryl-dialkyl-substituted pyrazoles: regioselective synthesis and binding affinity for the estrogen receptor

We have developed two novel series of tetrasubstituted pyrazoles, embodying 1,3-diaryl-4,5-dialkyl or 3,5-diaryl-1,4-dialkyl substitution patterns. The scope of a regioselective method, developed by us earlier, was expanded to allow the synthesis of the first series of these tetrasubstituted pyrazoles directly from alpha,beta-unsaturated ketones. The binding affinity of some of these pyrazoles for the estrogen receptor (ER) subtypes ERalpha and ERbeta is very high, and the overall affinity pattern suggests the importance of three phenol substituents for high affinity, ERalpha-selective binding.

SKF-82958 is a subtype-selective estrogen receptor-alpha (ERalpha ) agonist that induces functional interactions between ERalpha and AP-1

The transcriptional activity of estrogen receptors (ERs) can be regulated by ligands as well as agents such as dopamine, which stimulate intracellular signaling pathways able to communicate with these receptors. We examined the ability of SKF-82958 (SKF), a previously characterized full dopamine D1 receptor agonist, to stimulate the transcriptional activity of ERalpha and ERbeta. Treatment of HeLa cells with SKF-82958 stimulated robust ERalpha-dependent transcription from an estrogen-response element-E1b-CAT reporter in the absence of estrogen, and this was accompanied by increased receptor phosphorylation. However, induction of ERbeta-directed gene expression under the same conditions was negligible. In our cell model, SKF treatment did not elevate cAMP levels nor enhance transcription from a cAMP-response element-linked reporter. Control studies revealed that SKF-82958, but not dopamine, competes with 17beta-estradiol for binding to ERalpha or ERbeta with comparable relative binding affinities. Therefore, SKF-82958 is an ERalpha-selective agonist. Transcriptional activation of ERalpha by SKF was more potent than expected from its relative binding activity, and further examination revealed that this synthetic compound induced expression of an AP-1 target gene in a tetradecanoylphorbol-13-acetate-response element (TRE)-dependent manner. A putative TRE site upstream of the estrogen-response element and the amino-terminal domain of the receptor contributed to, but were not required for, SKF-induced expression of an ERalpha-dependent reporter gene. Overexpression of the AP-1 protein c-Jun, but not c-Fos, strongly enhanced SKF-induced ERalpha target gene expression but only when the TRE was present. These studies provide information on the ability of a ligand that weakly stimulates ERalpha to yield strong stimulation of ERalpha-dependent gene expression through cross-talk with other intracellular signaling pathways producing a robust combinatorial response within the cell.

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).

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.

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.

Furans with basic side chains: synthesis and biological evaluation of a novel series of antagonists with selectivity for the estrogen receptor alpha

3-alkyl-2,4,5-triarylfurans with basic side-chain substituents were prepared as ligands for the estrogen receptor. Those analogues having the basic side chain on the C(4) phenol were high-affinity, ERalpha-selective antagonists.

Biphenyls as surrogates of the steroidal backbone. Part 1: synthesis and estrogen receptor affinity of an original series of polysubstituted biphenyls

In the course of a programme aimed at discovering new ligands of the estrogen receptor, we explored a series of substituted biphenyls. Their synthesis and binding affinity are described.

Synthesis and evaluation of hexahydrochrysene and tetrahydrobenzofluorene ligands for the estrogen receptor

To prepare novel estrogen receptor (ER) ligands, we have developed a facile approach to substituted hexahydrochrysene and tetrahydrobenzo[a]fluorene systems. Substituents, including basic side chains, were added to these systems, and their binding affinity to ERalpha and ERbeta, and in some cases their transcriptional activity were evaluated.