Novel high-affinity steroidal estrogenic ligands: synthesis and receptor binding of 11 beta-vinyl-17 alpha-E/Z-phenylselenovinyl estradiols

Convergent synthesis of a steroidal antiestrogen-mitomycin C hybrid using "click" chemistry

A convergent synthesis of a novel estrogen receptor-targeted drug hybrid was developed based on structures of the potent anti-proliferative mitomycin C and the steroidal anti-estrogen RU 39411. The steroidal antiestrogen was prepared with an azido-triethylene glycoloxy linker while the mitomycin C derivative (porfirimycin) incorporated a complementary 7-N-terminal alkyne. The two components were ligated using the Huisgen [3 + 2] cycloaddition ("click") reaction. Preliminary biological assays demonstrated that the final hybrid compound retained both potent anti-estrogenic and anti-proliferative activities.

Synthesis and evaluation of 11β-(4-substituted phenyl) estradiol analogs: transition from estrogen receptor agonists to antagonists

INTRODUCTION

As part of our program to develop estrogen receptor (ER) targeted imaging and therapeutic agents we chose to evaluate 11β-substituted estradiol analogs as a representative scaffold. Previous synthetic studies provided an entry into this class of compounds and other work indicated that 11β-(substituted aryl) estradiol analogs were potent antagonists of the ER. Little information existed about the specific structural features involved in the transition from agonism to antagonism for the 11β-aryl estradiol analogs or their potential as scaffolds for drug conjugation.

METHODS

We prepared and characterized a series of 11β-(4-Substituted phenyl) estradiol analogs using modifications of existing synthetic methods. The new compounds, as well as standard steroidal agonists and antagonists, were evaluated as competitive ligands for the ERβ-LBD. Functional assays used the induction of alkaline phosphatase in Ishikawa cells to determine potency of the compounds as ER agonists or antagonists.

RESULTS

The synthetic strategy successfully generated a series of compounds in which the 4-substituent was sequentially modified from hydroxyl to methoxy to azidoethoxy/N,N-dimethylaminoethoxy and eventually to a prototypical 1,4-naphthoquinone-containing moiety. The new compounds all retained high relative binding affinity (RBA) for the ERα-LBD, ranging from 13-83% that of estradiol. No subtype selectivity was observed. More importantly, the transition from agonist to antagonist activity occurs at the 4-methoxy stage where the compound is a mixed antagonist. More notably, antagonism appeared to be more dependent upon the size of the 11β-substituent than upon the nature of the terminal group

CONCLUSIONS

We have developed a synthetic strategy that provides facile access to potent 11β-(4-substituted phenyl) estradiol analogs. The resultant compounds retain high affinity for the ERα-LBD and, more importantly, demonstrate potent antagonist activity in cells. Large functionalities distal to the 11β-phenyl ring had little additional effect on either affinity or efficacy, suggesting the incorporation of diverse imaging or biologically active groups can be attached without significantly compromising the ER-binding capacity. Future studies are in progress to exploit the 11β-aryl estradiol analogs as potential drug delivery systems and imaging agents.

Design, synthesis, and initial biological evaluation of a steroidal anti-estrogen-doxorubicin bioconjugate for targeting estrogen receptor-positive breast cancer cells

As part of our program to develop breast cancer specific therapeutic agents, we have synthesized a conjugate agent that is a conjugate of the steroidal anti-estrogen and the potent cytotoxin doxorubicin. In this effort, we employed a modular assembly approach to prepare a novel 11β-substituted steroidal anti-estrogen functionalized with an azido-tetraethylene glycol moiety, which could be coupled to a complementary doxorubicin benzoyl hydrazone functionalized with a propargyl tetraethylene glycol moiety. Huisgen [3 + 2] cycloaddition chemistry gave the final hybrid that was evaluated for selective uptake and cytotoxicity in ER(+)-MCF-7 and ER(-)-MDA-MB-231 breast cancer cell lines. The results demonstrated that the presence of the anti-estrogenic component in the hybrid compound was critical for selectivity and cytotoxicity in ER(+)-MCF-7 human breast cancer cells as the hybrid was ~70-fold more potent than doxorubicin in inhibition of cell proliferation and promoting cell death.

Synthesis and evaluation of 17α-(dimethylphenyl)vinyl estradiols as probes of the estrogen receptor-α ligand binding domain

As part of our program to explore the influence of small structural modifications on the biological response of the estrogen receptor-α (ERα), we prepared and evaluated a series of mono-and di-substituted phenyl vinyl estradiols. The target compounds were prepared in 45-80% yields using the Stille coupling reaction and evaluated using competitive binding analysis with the ERα-ligand binding domain (hERα-LBD) and estrogenic activity (induction of alkaline phosphatase in Ishikawa cells). Results indicated that the 2,4- and 2,5-dimethyl derivatives, 5b and 5c, had the highest relative binding affinity (RBA=20.5 and 37.3%) and relative stimulatory activity (RSA=101.0% and 12.3%) of the di-methyl series.

Targeting the estrogen receptor with metal-carbonyl derivatives of estradiol

As part of our program to develop new probes for the estrogen receptor binding domain, we prepared and evaluated a novel 17α-(rhenium tricarbonyl bipyridyl) vinyl estradiol complex. Preparation of the final compound was achieved using the Stille coupling between the preformed brominated rhenium tricarbonyl bipyridine complex and the tributylstannyl vinyl estradiol. Competitive receptor binding assays and stimulatory assays demonstrated that the final complex retained affinity and efficacy comparable to the corresponding pyridyl vinyl estradiol analog, but lower than that of the phenyl vinyl estradiol analog.

Synthesis and Evaluation of Isomeric (17α,20E)-11β-Methoxy-21-(trifluoromethylphenyl)-19-norpregna-1,3,5(10),20-tetraene-3,17β-diols as ERα-Hormone Binding Domain Ligands: Effect of the Methoxy Group on Receptor Binding and Uterotrophic Growth

In this study we have introduced the 11beta-methoxy group, a substituent known to increase in vivo potency in other steroidal estrogens, into the (17alpha,20E)-21-(trifluoromethylphenyl)-19-norpregna-1,3,5(10),20-tetraene-3,17beta-diols: (trifluoromethylphenyl)vinyl estradiols. Receptor binding, using the ERalpha-HBD, indicated that the 11beta-methoxy group had little effect on the relative binding affinity of the target compounds compared to the corresponding 11beta-unsubstituted analogs, however, the 11beta-methoxy derivatives were significantly more potent in stimulating uterotrophic growth in immature female rats. Molecular modeling studies suggest that while the 11beta-methoxy group does not contribute significantly to the overall binding energy of the ligand-ERalpha-HBD complex, it stabilizes residues associated with the coregulator protein binding site. Such effects would influence the dynamics of subsequent events, such as transcription and biological responses.

Synthesis and Evaluation of (17α,20Z)-21-(4-Substituted-phenyl)-19-norpregna-1,3,5(10),20-tetraene-3,17β-diols as Ligands for the Estrogen Receptor-α Hormone Binding Domain: Comparison with 20E-Isomers

As part of our ongoing program to develop probes for the hormone binding domain of the estrogen receptor-alpha (ERalpha), we prepared and evaluated a series of 17alpha,Z-(4-substituted-phenyl)vinyl estradiol derivatives. The results indicated that the relative binding affinities (RBAs) at 25 degrees C for the new compounds were significant (RBA = 9-57) although less than that of estradiol (RBA = 100) or of the parent unsubstituted phenylvinyl estradiol (RBA = 66). All of the Z-compounds were full agonists in the uterotrophic assay, indicating that the ligands formed estrogen-like complexes with the estrogen receptor-alpha hormone binding domain (ERalpha-HBD). Comparison of corresponding Z- and E-4-substituted phenylvinyl ligands complexed with the ERalpha-HBD indicated small but significant differences in binding modes that may account for the differing trends seen in the structure-activity relationships for the two series.

The novel estrogen 17α-20Z-21-[(4-amino)phenyl]-19-norpregna-1,3,5(10),20-tetraene-3,17β-diol induces apoptosis in prostate cancer cell lines at nanomolar concentrations in vitro

Prostate cancer remains the number one cause of noncutaneous cancer, with 220,900 new cases predicted for the year 2003 alone. Of the more promising classes of compounds studied thus far for the treatment of prostate cancer, estrogens of various types have consistently exhibited antitumor activities both in vitro and in vivo. For this reason, we have synthesized and screened a library of unique 17α/11β modified 17β-estradiol (E2) analogues designed for estrogen receptor β (ER-β) specificity and a potential for cytotoxic activity directed toward prostate cancer cells. From this library, the novel compound 17α-20Z-21-[(4-amino)phenyl]-19-norpregna-1,3,5(10),20-tetraene-3,17β-diol (APVE2) was identified as the primary lead, found to induce a high level (>90%) of cell death through an apoptotic mechanism, with an EC50 of 1.4, 2.7, and 16 nm in the LNCaP, PC3, and DU145 cell lines, respectively. APVE2 was found to bind to ER-β, albeit weakly, with an EC50 of 250 nm and a binding activity of 6.2% relative to E2, nearly two orders of magnitude less than the concentration required to induce apoptosis. APVE2 bound preferentially to ER-β by 7-fold over ER-α, and did not induce growth in the MCF-7 cell line, thus indicating that it is not a classical ER agonist. Furthermore, the cytotoxic actions of APVE2 were not reversed by co-treatment with a 50-fold excess E2. In summary, a novel 17 modified estrogen APVE2 was identified as a lead compound, capable of inducing apoptosis in three prostate cancer cell lines at low nanomolar concentrations, through a mechanism inconsistent with an ER-mediated mechanism.

Synthesis and evaluation of 17alpha-20E-21-(4-substituted phenyl)-19-norpregna-1,3,5(10),20-tetraene-3,17beta-diols as probes for the estrogen receptor alpha hormone binding domain

As part of our program to develop probes for the hormone binding domain of the estrogen receptor alpha (ERalpha), we prepared a series of 4-para-substituted phenylvinyl estradiol derivatives using a combination of solution and solid-phase Pd(0)-catalyzed methods. The compounds 5a-j were evaluated for their binding affinity using the ERalpha hormone binding domain (HDB) isolated from transfected BL21 cells. The results indicated that although the new compounds were somewhat lower in relative binding affinity (RBA at 25 degrees C is 1-60%) than estradiol (100%), most had higher affinity than the unsubstituted parent phenylvinyl estradiol (RBA = 9%). Because the substituents did not generate a structure-activity relationship directly based on physicochemical properties, the series was evaluated using molecular modeling and molecular dynamics to determine key interactions between the ligand, especially the para substituent, and the protein. The results suggest that the observed relative binding affinities are directly related to the calculated binding energies and that amino acids juxtaposed to the para position play a significant but not dominant role in binding. In conclusion, we have identified the 17alpha-E-(4-substituted phenyl)vinyl estradiols as a class of ligands that retain significant affinity for the ERalpha-HBD. In particular, 4-substitution tends to increase receptor affinity compared to the unsubstituted analogue, as exemplified by 5e (4-COCH(3)), which had the highest RBA value (60%) of the series. Palladium(0)-catalyzed coupling reactions on solid support or in solution using suitably substituted iodo arenes and 17alpha-E-tributylstannylvinyl estradiols offer a flexible approach to their preparation. Molecular modeling studies of the receptor suggest that there exists additional ligand accessible regions within the ERalpha-HBD to generate interactions that may enhance receptor affinity or modify efficacy in developing new therapeutic agents. Studies to undertake modification in the properties and/or position of the aryl substituents in subsequent series to further define that role are in progress.

Evaluation of 17alpha-E-(trifluoromethylphenyl)vinyl estradiols as novel estrogen receptor ligands

As part of our program to develop novel ligands for the estrogen receptor, we synthesized the series of isomeric 17alpha-(trifluoromethyl)phenylvinyl estradiols using our solid-phase organic synthesis methodology. The compounds were evaluated for their relative binding affinity (RBA) using the ERalpha-LBD and in vivo potency using the immature rat uterotrophic growth assay. The ortho-isomer had the highest RBA values, 48-223, and the highest estrogenicity in vivo. The other isomers had significantly lower affinities and were weaker agonists in the uterotrophic assay. The results suggest that introduction of substituents at the 17alpha-position of estradiol is tolerated by the ER-LBD and permit agonist responses in the intact animal, however, the effect is sensitive to the position of groups on the phenyl ring. This study demonstrates that the 17alpha-position of estradiol is a reasonable site for modification but the position and physicochemical properties of such modifications may significantly affect the affinity and efficacy of the ligand.

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.

Conformational studies on (17alpha,20Z)-21-(X-Phenyl)-19-norpregna-1, 3,5(10),20-tetraene-3,17beta-diols using 1D and 2D NMR spectroscopy and GIAO calculations of (13)C shieldings

Differences in agonist responses of the novel estrogen receptor ligands (17alpha,20Z)-(p-methoxyphenyl)vinyl estradiol (1), (17alpha, 20Z)-(o-alpha,alpha,alpha-trifluoromethylphenyl)vinyl estradiol (2), and (17alpha,20Z)-(o-hydroxymethylphenyl)vinyl estradiol (3) led us to investigate their solution conformation. In competitive binding assay studies, we observed that several phenyl-substituted (17alpha, 20E/Z)-(X-phenyl)vinyl estradiols exhibited significant estrogen receptor binding, but with variation (RBA (1) = 20; RBA (2) = 23; RBA (3) = 140 where estradiol RBA = 100) depending on the phenyl substitution pattern. Because the 17alpha-phenylvinyl substituent interacts with the key helix-12 of the ligand binding domain, we considered that differences in the preferred conformation of 1-3 could account for their varying binding affinity. 2D NMR experiments at 500 MHz allowed the complete assignment of the (13)C and (1)H spectra of 1-3. The conformations of these compounds in solution were established by 2D and 1D NOESY spectroscopy. A statistical approach of evaluating contributing conformers of 1-3 from predicted (13)C shifts correlated quite well with the NOE data. The 17alpha substituents of 1 and 2 exist in similar conformational equilibria with some differences in relative populations of conformers. In contrast, the 17alpha substituent of 3 exists in a different conformational equilibrium. The similarity in solution conformations of 1 and 2 suggests they occupy a similar receptor volume, consistent with similar RBA values of 20 and 23. Conversely, the different conformational equilibria of 3 may contribute to the significant binding affinity (RBA = 140) of this ligand.