Synthesis of analogs of 2-methoxyestradiol with enhanced inhibitory effects on tubulin polymerization and cancer cell growth

The Structural Diversity and Biological Activity of Steroid Oximes

Steroids and their derivatives have been the subject of extensive research among investigators due to their wide range of pharmacological properties, in which steroidal oximes are included. Oximes are a chemical group with the general formula R1R2C=N-OH and they exist as colorless crystals and are poorly soluble in water. Oximes can be easily obtained through the condensation of aldehydes or ketones with various amine derivatives, making them a very interesting chemical group in medicinal chemistry for the design of drugs as potential treatments for several diseases. In this review, we will focus on the different biological activities displayed by steroidal oximes such as anticancer, anti-inflammatory, antibacterial, antifungal and antiviral, among others, as well as their respective mechanisms of action. An overview of the chemistry of oximes will also be reported, and several steroidal oximes that are in clinical trials or already used as drugs are described. An extensive literature search was performed on three main databases-PubMed, Web of Science, and Google Scholar.

Triazole-based estradiol dimers prepared via CuAAC from 17α-ethinyl estradiol with five-atom linkers causing G2/M arrest and tubulin inhibition

Microtubule dynamic is exceptionally sensitive to modulation by small-molecule ligands. Our previous work presented the preparation of microtubule-targeting estradiol dimer (ED) with anticancer activity. In the present study, we explore the effect of selected linkers on the biological activity of the dimer. The linkers were designed as five-atom chains with carbon, nitrogen or oxygen in their centre. In addition, the central nitrogen was modified by a benzyl group with hydroxy or methoxy substituents and one derivative possessed an extended linker length. Thirteen new dimers were subjected to cytotoxicity assay and cell cycle profiling. Dimers containing linker with benzyl moiety substituted with one or more methoxy groups and longer branched ones were found inactive, whereas other structures had comparable efficacy as the original ED (e.g. D1 with IC₅₀ = 1.53 µM). Cell cycle analysis and immunofluorescence proved the interference of dimers with microtubule assembly and mitosis. The proposed in silico model and calculated binding free energy by the MM-PBSA method were closely correlated with in vitro tubulin assembly assay.

Cell Fate following Irradiation of MDA-MB-231 and MCF-7 Breast Cancer Cells Pre-Exposed to the Tetrahydroisoquinoline Sulfamate Microtubule Disruptor STX3451

A tetrahydroisoquinoline (THIQ) core is able to mimic the A and B rings of 2-methoxyestradiol (2ME2), an endogenous estrogen metabolite that demonstrates promising anticancer properties primarily by disrupting microtubule dynamic instability parameters, but has very poor pharmaceutical properties that can be improved by sulfamoylation. The non-steroidal THIQ-based microtubule disruptor 2-(3-bromo-4,5-dimethoxybenzyl)-7-methoxy-6-sulfamoyloxy-1,2,3,4-tetrahydroisoquinoline (STX3451), with enhanced pharmacokinetic and pharmacodynamic profiles, was explored for the first time in radiation biology. We investigated whether 24 h pre-treatment with STX3451 could pre-sensitize MCF-7 and MDA-MB-231 breast cancer cells to radiation. This regimen showed a clear increase in cytotoxicity compared to the individual modalities, results that were contiguous in spectrophotometric analysis, flow cytometric quantification of apoptosis induction, clonogenic studies and microscopy techniques. Drug pre-treatment increased radiation-induced DNA damage, with statistically more double-strand (ds) DNA breaks demonstrated. The latter could be due to the induction of a radiation-sensitive metaphase block or the increased levels of reactive oxygen species, both evident after compound exposure. STX3451 pre-exposure may also delay DNA repair mechanisms, as the DNA damage response element ataxia telangiectasia mutated (ATM) was depressed. These in vitro findings may translate into in vivo models, with the ultimate aim of reducing both radiation and drug doses for maximal clinical effect with minimal adverse effects.

Rh-Catalyzed Oxidation and Trifluoroethoxylation of N-Aryl-pyrrolidin-2-ones : A Domino Approach for the Synthesis of N-Aryl-5-(2,2,2-trifluoroethoxy)-1,5- dihydro-2H-pyrrol-2-ones

The synthesis of trifluoroethoxylated dihydropyrrolidones via rhodium-catalyzed oxidation and trifluoroethoxylation of pyrrolidones is presented in this paper. This process realized trifluoroethoxylation of non-activated sp3 C-H by domino approach for the...

New Estrone Oxime Derivatives: Synthesis, Cytotoxic Evaluation and Docking Studies

The interest in the introduction of the oxime group in molecules aiming to improve their biological effects is increasing. This work aimed to develop new steroidal oximes of the estrane series with potential antitumor interest. For this, several oximes were synthesized by reaction of hydroxylamine with the 17-ketone of estrone derivatives. Then, their cytotoxicity was evaluated in six cell lines. An estrogenicity assay, a cell cycle distribution analysis and a fluorescence microscopy study with Hoechst 3358 staining were performed with the most promising compound. In addition, molecular docking studies against estrogen receptor α, steroid sulfatase, 17β-hydroxysteroid dehydrogenase type 1 and β-tubulin were also accomplished. The 2-nitroestrone oxime showed higher cytotoxicity than the parent compound on MCF-7 cancer cells. Furthermore, the oximes bearing halogen groups in A-ring evidenced selectivity for HepaRG cells. Remarkably, the Δ^9,11-estrone oxime was the most cytotoxic and arrested LNCaP cells in the G₂/M phase. Fluorescence microscopy studies showed the presence of condensed DNA typical of prophase and condensed and fragmented nuclei characteristic of apoptosis. However, this oxime promoted the proliferation of T47-D cells. Interestingly, molecular docking studies estimated a strong interaction between Δ^9,11-estrone oxime and estrogen receptor α and β-tubulin, which may account for the described effects.

Access to new Steroids via a (1,2) Wittig Rearrangement

Novel steroids are obtained by reaction of 9,11-dehydro-dibenzylestradiol and dibenzylestradiol with phenyllithium in THF at room temperature.

Design, Synthesis, Anticancer Evaluation and Molecular Modeling of Novel Estrogen Derivatives

A series of estrone derivatives 3–8 was designed and synthesized using estrone arylmethylenes 2a,b as starting materials and their structures were confirmed by different spectral data and elemental analyses. All the newly synthesized compounds exhibited potent in vitro and in vivo cytotoxic activities against breast cancer cell lines. In addition, all compounds were subjected to in vitro and in vivo inhibition assays for EGFR and VEGFR-2 kinases as well as p53 ubiquitination activity to obtain more details about their mechanism of action. Based on the promising results, a molecular docking study was investigated for the most representative compound 5a against the two targets, EGFR and VEGFR-2 kinases, to assess its binding affinity, hoping to rationalize and obtain potent anticancer agents in the future.

Antiproliferative and antimetastatic properties of 3-benzyloxy-16-hydroxymethylene-estradiol analogs against breast cancer cell lines

Despite emerging new therapeutic opportunities, cancer is still a major health problem and a leading cause of death worldwide. Breast tumors are the most frequently diagnosed female malignancies, and the triple-negative subtype is associated with poorer prognosis and lower survival rates than other breast cancer types. The aims of the present study were to determine the anticancer potency of a set of C-3 and C-16 modified estradiol-derivatives against a panel of breast cancer cell lines, and to characterize the mechanism of action of two selected compounds (1 and 5) against the MDA-MB-231 triple-negative breast cancer cell line. Growth-inhibitory properties were investigated by an MTT-assay. Cell cycle analysis by flow cytometry has revealed G1 phase accumulation and indicated the proapoptotic effect of 1 and 5 through the elevation of the apoptotic subG1 phase on MDA-MB-231 cells after 24 h treatment. The antimetastatic activities of these compounds were examined by wound healing and Boyden chamber assays, and both compounds were shown to significantly inhibit the migration and invasion of MDA-MB-231 cells at sub-antiproliferative concentrations. Gelatin zymography assay has indicated that matrix metalloproteinase-2 and -9 are not involved in the antimetastatic action of the molecules. Western blot analysis was performed with 24 h incubation to examine the possible changes in the level of focal adhesion kinase (FAK), and both compounds were found to inhibit the phosphorylation of FAK in a concentration-dependent manner in MDA-MB-231 cells. The results of this study demonstrate that C-3 and C-16 modified estradiol derivatives are potent antiproliferative and antimetastatic compounds against a triple-negative breast cancer cell line with a mechanism of action involving the inhibition of FAK, a novel anticancer therapeutic target. Therefore, these findings can be utilized in the development of promising anticancer agents with steroid skeleton.

Design, synthesis and antiproliferative effect of 17β-amide derivatives of 2-methoxyestradiol and their studies on pharmacokinetics

A series of 17β-amide-2-methoxyestradiol compounds were synthesized with an aim to enhance the antiproliferative effect of 2-methoxyestradiol. The antiproliferative activity of 2-methoxyestradiol analogs against human cancer cells was investigated. 2-methoxy-3-benzyloxy-17β-chloroacetamide-1,3,5(10)-triene (5e) and 2-methoxy-3-hydroxy-17β-butyramide-1,3,5(10)-triene (6c) had comparable or better antitumor activity than 2-methoxyestradiol. The elimination half-life of 6c (t_1/2β=240.93min) is ten times longer than 2-ME and the area under the curve was seven times (AUC_0-tmin=2068.20±315.74μgmL^-1min) higher than 2-ME, respectively. Whereas 5e had similar pharmacokinetic behavior with 2-ME (t_1/2β=22.28min) with a t_1/2β of 29.5 min. 6c had higher blood concentration, longer actuation duration and better suppression rate against S180 mouse ascites tumor than 2-methoxyestradiol.

Synthesis and Biological Evaluation of Triazolyl 13α-Estrone-Nucleoside Bioconjugates

2′-Deoxynucleoside conjugates of 13α-estrone were synthesized by applying the copper-catalyzed alkyne–azide click reaction (CuAAC). For the introduction of the azido group the 5′-position of the nucleosides and a propargyl ether functional group on the 3-hydroxy group of 13α-estrone were chosen. The best yields were realized in our hands when the 3′-hydroxy groups of the nucleosides were protected by acetyl groups and the 5′-hydroxy groups were modified by the tosyl–azide exchange method. The commonly used conditions for click reaction between the protected-5′-azidonucleosides and the steroid alkyne was slightly modified by using 1.5 equivalent of Cu(I) catalyst. All the prepared conjugates were evaluated in vitro by means of MTT assays for antiproliferative activity against a panel of human adherent cell lines (HeLa, MCF-7 and A2780) and the potential inhibitory activity of the new conjugates on human 17β-hydroxysteroid dehydrogenase 1 (17β-HSD1) was investigated via in vitro radiosubstrate incubation. Some protected conjugates displayed moderate antiproliferative properties against a panel of human adherent cancer cell lines (the protected cytidine conjugate proved to be the most potent with IC₅₀ value of 9 μM). The thymidine conjugate displayed considerable 17β-HSD1 inhibitory activity (IC₅₀ = 19 μM).

Recent Advances in chemistry and pharmacology of 2-methoxyestradiol: An anticancer investigational drug

2-Methoxyestradiol (2ME2), an estrogen hormone metabolite is a potential cancer chemotherapeutic agent. Presently, it is an investigational drug under various phases of clinical trials alone or in combination therapy. Its anticancer activity has been attributed to its antitubulin, antiangiogenic, pro-apoptotic and ROS induction properties. This anticancer drug candidate has been explored extensively in last twenty years for its detailed chemistry and pharmacology. Present review is an update of its chemistry and biological activity. It also extends an assessment of potential of 2ME2 and its analogues as possible anticancer drug in future.

Synthesis of trans-16-triazolyl-13α-methyl-17-estradiol diastereomers and the effects of structural modifications on their in vitro antiproliferative activities

Novel 16-triazoles in the 13α-estrone series were synthesized via Cu(I)-catalyzed azide-alkyne cycloaddition of the two diastereomeric (on C-16 and on C-17) 16-azido-13α-estra-1,3,5(10)-trien-17-ol 3-benzyl ethers with substituted phenylacetylenes. The new heterocyclic derivatives were evaluated in vitro by means of MTT assays for antiproliferative activity against a panel of human adherent cancer cell lines (HeLa, MCF-7, A431, A2780, T47D, MDA-MB-231 and MDA-MB-361). The inversion of the configurations at C-16 and C-17 selectively affected the growth-inhibitory properties of the tested compounds. The 16β,17α isomers generally proved to be potent on all cell lines, with IC50 values comparable to those of the reference agent cisplatin. Change of the substitution pattern of the phenyl group of the acetylene led to great differences in antiproliferative properties. Exclusively the p-phenyl-substituted triazoles exerted high cytostatic effects. One of the most potent compounds activated caspase-3 and caspase-9 without influencing caspase-8, confirming the induction of apoptosis via the intrinsic pathway.

Synthesis and in vitro antiproliferative evaluation of d-secooxime derivatives of 13β- and 13α-estrone

d-Secooximes were synthesized from the d-secoaldehydes in the 13β- and 13α-estrone series. The oximes were modified at three sites in the molecule: the oxime function was transformed into an oxime ether, oxime ester or nitrile group, the propenyl side-chain was saturated and the 3-benzyl ether was removed in order to obtain a phenolic hydroxy function. Triazoles were formed via Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC) from 3-(prop-2-yniloxy)-d-secooximes and benzyl azides. All the products were evaluated in vitro by means of MTT assays for antiproliferative activity against a panel of human adherent cell lines (HeLa, MCF-7, A2780 and A431). Some of them exhibited activities with submicromolar IC50 values, better than that of the reference agent cisplatin. The structural modifications led to significant differences in the cytostatic properties. Flow cytometry indicated that one of the most potent agents resulted in a cell cycle blockade.

Syntheses and antiproliferative effects of D-homo- and D-secoestrones

Substituted and/or heterocyclic d-homoestrone derivatives were synthetized via the intramolecular cyclization of a δ-alkenyl-d-secoaldehyde, -d-secoalcohol or -d-secocarboxylic acid of estrone 3-benzyl ether. The d-secoalcohol was modified at three sites in the molecule. The in vitro antiproliferative activities of the new d-homo- and d-secoestrone derivatives were determined on HeLa, MCF-7, A431 and A2780 cells through use of MTT assay. d-Homoalcohols 3 and 5 displayed cell line-selective cytostatic effects against ovarian and cervical cell lines, respectively. Two d-secoestrones (6 and 12c) proved to be effective, with IC50 values comparable with those of the reference agent cisplatin. A selected compound (6) was tested by tubulin polymerization assay and its cancer specificity was additionally determined by using noncancerous human fibroblast cells.

Tetrahydroisoquinolinone-based steroidomimetic and chimeric microtubule disruptors

A structure-activity relationship (SAR) translation strategy was used for the discovery of tetrahydroisoquinoline (THIQ)-based steroidomimetic and chimeric microtubule disruptors based upon a steroidal starting point. A steroid A,B-ring-mimicking THIQ core was connected to methoxyaryl D-ring ring mimics through methylene, carbonyl and sulfonyl linkers to afford a number of steroidomimetic hits (e.g., 7-methoxy-2-(3- methoxybenzyl)-6-sulfamoyloxy-1,2,3,4-tetrahydroisoquinoline (20 c) GI₅₀=2.1 μM). Optimisation and control experiments demonstrate the complementary SAR of this series and the steroid derivatives that inspired its design. Linkage of the THIQ-based A,B-mimic with the trimethoxyaryl motif prevalent in colchicine site binding microtubule disruptors delivered a series of chimeric molecules whose activity (GI₅₀=40 nM) surpasses that of the parent steroid derivatives. Validation of this strategy was obtained from the excellent oral activity of 7-methoxy-6-sulfamoyloxy-2-(3,4,5-trimethoxybenzyl)-1,2,3,4-tetrahydroisoquinoline relative to a benchmark steroidal bis- sulfamate in an in vivo model of multiple myeloma.

Synthesis of novel estrone analogs by incorporation of thiophenols via conjugate addition to an enone side chain

Functionalized estrogen analogs have received interest due to their unique and differing biological activity compared to their parent compounds. The synthesis of a new class of 3-methoxyestrone analogs functionalized at the C17 position possessing both alkyl and aryl substituted α,β-unsaturated ketones is described, along with their thiophenol conjugate addition products.

Current status on development of steroids as anticancer agents

Steroids are important biodynamic agents. Their affinities for various nuclear receptors have been an interesting feature to utilize them for drug development particularly for receptor mediated diseases. Steroid biochemistry and its crucial role in human physiology, has attained importance among the researchers. Recent years have seen an extensive focus on modification of steroids. The rational modifications of perhydrocyclopentanophenanthrene nucleus of steroids have yielded several important anticancer lead molecules. Exemestane, SR16157, fulvestrant and 2-methoxyestradiol are some of the successful leads emerged on steroidal pharmacophores. The present review is an update on some of the steroidal leads obtained during past 25 years. Various steroid based enzyme inhibitors, antiestrogens, cytotoxic conjugates and steroidal cytotoxic molecules of natural as well as synthetic origin have been highlighted. This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

An overview of tubulin inhibitors that interact with the colchicine binding site

Tubulin dynamics is a promising target for new chemotherapeutic agents. The colchicine binding site is one of the most important pockets for potential tubulin polymerization destabilizers. Colchicine binding site inhibitors (CBSI) exert their biological effects by inhibiting tubulin assembly and suppressing microtubule formation. A large number of molecules interacting with the colchicine binding site have been designed and synthesized with significant structural diversity. CBSIs have been modified as to chemical structure as well as pharmacokinetic properties, and tested in order to find a highly potent, low toxicity agent for treatment of cancers. CBSIs are believed to act by a common mechanism via binding to the colchicine site on tubulin. The present review is a synopsis of compounds that have been reported in the past decade that have provided an increase in our understanding of the actions of CBSIs.

Synthesis of 2-substituted 17β-hydroxy/17-methylene estratrienes and their in vitro cytotoxicity in human cancer cell cultures

Synthesis of various types of 2-(alkylaminomethyl) and 2-(aroyl) 17β-estradiol analogs are reported. The synthesis of similar types of 2-substituted 17-methylene estratriene analogs was also achieved. Synthesis of chalcone derivatives of 17β-estradiol and 17-methylene estratriene were also realized. All these 2-substituted estratrienes were tested for their antiproliferative activity by using four different cell lines from colon, lung, glioma and breast cancers. Among the various 2-substituted estratrienes, the compounds 10d, 14a-h and 17e were found to have in vitro antiproliferative activity comparable to that of parent analogs 1-4. Comparison of the SAR pattern of these 2-susbtituted estratriene derivatives confirmed that relatively, 17-methylene estratrienes are more active than that of 17β-estradiol analogs.

Therapeutic promises of 2-methoxyestradiol and its drug disposition challenges

2-Methoxyestradiol (2MeO-E2) is an endogenous metabolite of estrogen which was initially considered to be inactive. During the last few decades it has been shown that 2MeO-E2 is a promising anticancer drug. In vitro experiments have demonstrated that it has several anticancer activities, and potential to alleviate hypertension, glomerulosclerosis, hypercholesterolemia, and other disorders. However, due to its low solubility and extensive glucuronidation, to achieve effective concentrations large doses of 2MeO-E2 would be required. Clinical studies reflected very high inter- and intrapatient variability and oral bioavailability of 1 to 2%. Thus, this review paper highlights the origin of this compound, its therapeutic promises, and possible mechanisms of action. It also discusses the pharmacokinetic properties of 2MeO-E2 as well as current developments to overcome low drug solubility and its extensive first pass metabolism.

Synthesis and antitumor activities of 3-modified 2-methoxyestradiol analogs

The syntheses of 2-methoxyestradiol analogs with modifications at the 3-position are described. The analogs were assessed for their antiproliferative, antiangiogenic, and estrogenic activities. Several lead substituents were identified with similar or improved antitumor activities and reduced metabolic liability compared to 2-methoxyestradiol.

Synthesis of 2- and 17-substituted estrone analogs and their antiproliferative structure-activity relationships compared to 2-methoxyestradiol

A novel series of 17-modified and 2,17-modified analogs of 2-methoxyestradiol (2ME2) were synthesized and characterized. These analogs were designed to retain or potentiate the biological activities of 2ME2 and have diminished metabolic liability. The analogs were evaluated for antiproliferative activity against MDA-MB-231 breast tumor cells, antiangiogenic activity in HUVEC, and estrogenic activity on MCF-7 cell proliferation. Several analogs were evaluated for metabolic stability in human liver microsomes and in vivo in a rat cassette dosing model. This study lead to several 17-modified analogs of 2ME2 that have similar or improved antiproliferative and antiangiogenic activity, lack estrogenic properties and have improved metabolic stability compared to 2ME2.

Syntheses and biological activities of novel 2-methoxyestradiol analogs, 2-fluoroethoxyestradiol and 2-fluoropropanoxyestradiol, and a radiosynthesis of 2-[(18)F]fluoroethoxyestradiol for positron emission tomography

INTRODUCTION

2-Methoxyestradiol (2ME2) is an endogenous metabolite of the human hormone, estrogen, which has been shown to possess anti-tumor activity. 2-Fluoroethoxyestradiol (2FEE2) and 2-fluoropropanoxyestradiol (2FPE2), novel analogs of 2-methoxyestradiol, were designed and synthesized to be utilized as F-18 radiotracers for positron emission tomography (PET), with which the bio-distribution and intratumoral accumulations of 2ME2 could be measured in vivo for potential translation to human use.

METHODS

2FEE2 and 2FPE2 were synthesized from 3,17beta-estradiol in five steps respectively. Drug-induced microtubule depolymerization, antiproliferative activity against human cancer cell lines and HIF-1alpha down-regulation by 2FEE2 and 2FPE2 were investigated to examine whether these molecules possess similar anti-tumor activities as 2-methoxyestradiol. 2-[(18)F]Fluoroethoxyestradiol was synthesized for PET.

RESULTS

Novel 2ME2 analogs, 2FEE2 and 2FPE2, were synthesized in 29% and 22% overall yield, respectively. 2FEE2 and 2FPE2 showed microtubule depolymerization and cytotoxicities against the human ovarian carcinoma cell line, 1A9, and the human glioma cell line, LN229. HIF-1alpha was down-regulated by 2FEE2 and 2FPE2 under hypoxic conditions. 2FEE2 was chosen as an F-18 radiotracer candidate, since it showed stronger antiproliferative activity than 2ME2 and 2FPE2. 2-[(18)F]Fluoroethoxyestradiol (2[(18)F]FEE2) was prepared in 8.3% decay-corrected yield in 90 min, based on a production of H[(18)F]F with more than 98% radiochemical purity.

CONCLUSIONS

2FEE2 and 2FPE2 showed similar activity as 2ME2. 2[(18)F]FEE2 was synthesized to be utilized as a PET radiotracer to measure the biological efficacy of 2ME2 and its analogs in vivo.

Significant antitumor activity in vivo following treatment with the microtubule agent ENMD-1198

Clinical studies using the microtubule-targeting agent 2-methoxyestradiol (2ME2; Panzem) in cancer patients show that treatment is associated with clinical benefit, including prolonged stable disease, complete and partial responses, and an excellent safety profile. Studies have shown that 2ME2 is metabolized by conjugation at positions 3 and 17 and oxidation at position 17. To define structure-activity relationships for these positions of 2ME2 and to generate metabolically stable analogues with improved anti-tubulin properties, a series of analogues was generated and three lead analogues were selected, ENMD-1198, ENMD-1200, and ENMD-1237. These molecules showed improved metabolic stability with >65% remaining after 2-h incubation with hepatocytes. Pharmacokinetic studies showed that oral administration of the compounds resulted in increased plasma levels compared with 2ME2. All three analogues bind the colchicine binding site of tubulin, induce G(2)-M cell cycle arrest and apoptosis, and reduce hypoxia-inducible factor-1alpha levels. ENMD-1198 and ENMD-1200 showed improved in vitro antiproliferative activities. Significant reductions in tumor volumes compared with vehicle-treated mice were observed in an orthotopic breast carcinoma (MDA-MB-231) xenograft model following daily oral treatment with all compounds (ANOVA, P < 0.05). Significantly improved median survival time was observed with ENMD-1198 and ENMD-1237 (200 mg/kg/d) in a Lewis lung carcinoma metastatic model (P < 0.05). In both tumor models, the high-dose group of ENMD-1198 showed antitumor activity equivalent to that of cyclophosphamide. ENMD-1198 was selected as the lead molecule in this analogue series and is currently in a phase I clinical trial in patients with refractory solid tumors.

Structure-activity relationships of C-17 cyano-substituted estratrienes as anticancer agents

The synthesis, SAR, and preclinical evaluation of 17-cyanated 2-substituted estra-1,3,5(10)-trienes as anticancer agents are discussed. 2-Methoxy-17beta-cyanomethylestra-1,3,5(10)-trien-3-ol ( 14), but not the related 2-ethyl derivative 7, and the related 3- O-sulfamates 8 and 15 display potent antiproliferative effects (MCF-7 GI 50 300, 60 and 70 nM, respectively) against human cancer cells in vitro. Investigation of the SAR reveals that a sterically unhindered hydrogen bond acceptor attached to C-17 is most likely key to the enhanced activity. Compound 8 displayed significant in vitro antiangiogenic activity, and its ability to act as a microtubule disruptor was confirmed. Inhibitory activity of the sulfamate derivatives against steroid sulfatase and carbonic anhydrase II (hCAII) was also observed, and the interaction between 15 and hCAII was investigated by protein crystallography. The potential of these multimechanism anticancer agents was confirmed in vivo, with promising activity observed for both 14 and 15 in an athymic nude mouse MDA-MB-231 human breast cancer xenograft model.

Synthesis and structure-activity relationships of 16-modified analogs of 2-methoxyestradiol

A series of 16-modified 2-methoxyestradiol analogs were synthesized and evaluated for antiproliferative activity toward HUVEC and MDA-MB-231 cells, and for susceptibility to conjugation. In addition, the estrogenicity of these analogs was accessed by measuring cell proliferation of the estrogen-dependent cell line MCF7 in response to compound treatment. It was observed that antiproliferative activity dropped as the size of the 16 substituent increased. Selected analogs tested in glucuronidation assays had similar rates of clearance to 2-methoxyestradiol, but had enhanced clearance in sulfonate conjugation assays.

3,17-disubstituted 2-alkylestra-1,3,5(10)-trien-3-ol derivatives: synthesis, in vitro and in vivo anticancer activity

Estradiol-3,17-O,O-bis-sulfamates inhibit steroid sulfatase (STS), carbonic anhydrase (CA), and, when substituted at C-2, cancer cell proliferation and angiogenesis. C-2 Substitution and 17-sulfamate replacement of the estradiol-3,17-O,O-bis-sulfamates were explored with efficient and practical syntheses developed. Evaluation against human cancer cell lines revealed the 2-methyl derivative 27 (DU145 GI(50) = 0.38 microM) as the most active novel bis-sulfamate, while 2-ethyl-17-carbamate derivative 52 (GI(50) = 0.22 microM) proved most active of its series (cf. 2-ethylestradiol-3,17-O,O-bis-sulfamate 4 GI(50) = 0.21 microM). Larger C-2 substituents were deleterious to activity. 2-Methoxy-17-carbamate 50 was studied by X-ray crystallography and was surprisingly 13-fold weaker as an STS inhibitor compared to parent bis-sulfamate 3. The potential of 4 as an orally dosed anti-tumor agent is confirmed using breast and prostate cancer xenografts. In the MDA-MB-231 model, dramatic reduction in tumor growth or regression was observed, with effects sustained after cessation of treatment. 3-O-Sulfamoylated 2-alkylestradiol-17-O-carbamates and sulfamates have considerable potential as anticancer agents.

Endoreduplication of human smooth muscle cells induced by 2-methoxyestradiol: a role for cyclin-dependent kinase 2

Endoreduplication has been suggested to contribute to the development of hypertrophy of smooth muscle cells (SMCs) in hypertension. However, endoreduplication in vascular SMCs and the underlying molecular mechanisms are not clear. Treatment of human SMCs with 10 μM 2-methoxyestradiol (2-ME) for 24 h induces accumulation of cells with ≥4N DNA content, and some polyploid/aneuploid cells actively synthesize their DNA, suggesting the occurrence of endoreduplication. In addition, 2-ME treatment upregulates the expression of cyclin-dependent kinase 2 (Cdk2). The present study was designed to characterize endoreduplication of human SMCs and explore the potential roles of Cdk2 in endoreduplication induced by 2-ME. Treatment with 2-ME (10 μM) for 2–4 days not only caused increases in >4N cells and their reentry into S phase but also induced overduplication of chromosomes. Furthermore, 2-ME increased the kinase activity of Cdk2 and its interaction with cyclin E. Inducible overexpression of dominant-negative Cdk2 in human SMCs inhibited both DNA synthesis of >4N cells and the accumulation of >4N cells induced by 2-ME. We conclude that 2-ME induces endoreduplication of human SMCs and Cdk2 plays an important role in endoreduplication in response to 2-ME.

2-substituted estradiol bis-sulfamates, multitargeted antitumor agents: synthesis, in vitro SAR, protein crystallography, and in vivo activity

The anticancer activities and SARs of estradiol-17-O-sulfamates and estradiol 3,17-O,O-bis-sulfamates (E2bisMATEs) as steroid sulfatase (STS) inhibitors and antiproliferative agents are discussed. Estradiol 3,17-O,O-bis-sulfamates 20 and 21, in contrast to the 17-O-monosulfamate 11, proved to be excellent STS inhibitors. 2-Substituted E2bisMATEs 21 and 23 additionally exhibited potent antiproliferative activity with mean graph midpoint values of 18-87 nM in the NCI 60-cell-line panel. 21 Exhibited antiangiogenic in vitro and in vivo activity in an early-stage Lewis lung model, and 23 dosed p.o. caused marked growth inhibition in a nude mouse xenograft tumor model. Modeling studies suggest that the E2bisMATEs and 2-MeOE2 share a common mode of binding to tubulin, though COMPARE analysis of activity profiles was negative. 21 was cocrystallized with carbonic anhydrase II, and X-ray crystallography revealed unexpected coordination of the 17-O-sulfamate of 21 to the active site zinc and a probable additional lower affinity binding site. 2-Substituted E2bisMATEs are attractive candidates for further development as multitargeted anticancer agents.

C6-(N,N-butyl-methyl-heptanamide) derivatives of estrone and estradiol as inhibitors of type 1 17β-hydroxysteroid dehydrogenase: Chemical synthesis and biological evaluation

A series of estrone and estradiol derivatives having an N-butyl,methyl heptanamide side chain at C6-position were synthesized, tested as inhibitors of type 1 17beta-HSD and assessed for their possible estrogenic activity. A better type 1 17beta-HSD inhibition was obtained for the 6beta-side chain orientation over 6alpha; the C17-alcohols are more potent inhibitors than the corresponding ketones; introducing a 2-methoxy group decreased the inhibitory potency; and the replacement of a C-S bond by a C-C bond in the C6beta-side chain is not detrimental to inhibition. Interestingly, the new inhibitors were also found less estrogenic than the lead compound in two breast cancer cell lines, T-47D and MCF-7.

SAR studies of 2-methoxyestradiol and development of its analogs as probes of anti-tumor mechanisms

The major estrogen metabolite 2-methoxyestradiol (2ME) has been shown to target tumor cells without severe side effects and is currently being evaluated in clinical trials for several types of cancer. Despite its promise for use in clinical setting, the mechanism(s) by which 2ME exerts its anti-tumor activity is not clearly defined at this time. Employing organic chemistry tools, we synthesized 2ME analogs with which 2ME affinity column was prepared, enabling us to detect a protein that selectively interacts with 2ME. This 2ME analog will be useful as a probe to identify the biological target(s) of 2ME and study their functions in tumor cells.

A-ring-substituted estrogen-3-O-sulfamates: potent multitargeted anticancer agents

Efficient and flexible syntheses of 2-substituted estrone, estradiol and their 3-O-sulfamate (EMATE) derivatives have been developed using directed ortho-lithiation methodology. 2-Substituted EMATEs display a similar antiproliferative activity profile to the corresponding estradiols against a range of human cancer cell lines. 2-Methoxy (3, 4), 2-methylsulfanyl (20, 21) and 2-ethyl EMATEs (32, 33) proved the most active compounds with 2-ethylestradiol-3-O-sulfamate (33), displaying a mean activity over the NCI 55 cell line panel 80-fold greater than the established anticancer agent 2-methoxyestradiol (2). 2-Ethylestradiol-3-O-sulfamate (33) was also an effective inhibitor of angiogenesis using three in vitro markers, and various 2-substituted EMATEs also proved to be inhibitors of steroid sulfatase (STS), a therapeutic target for the treatment of hormone-dependent breast cancer. The potential of this novel class of multimechanism anticancer agents was confirmed in vivo with good activity observed in the NCI hollow fiber assay and in a MDA-MB-435 xenograft mouse model.

A common pharmacophore for a diverse set of colchicine site inhibitors using a structure-based approach

Modulating the structure and function of tubulin and microtubules is an important route to anticancer therapeutics, and therefore, small molecules that bind to tubulin and cause mitotic arrest are of immense interest. A large number of synthetic and natural compounds with diverse structures have been shown to bind at the colchicine site, one of the major binding sites on tubulin, and inhibit tubulin assembly. Using the recently determined X-ray structure of the tubulin:colchicinoid complex as the template, we employed docking studies to determine the binding modes of a set of structurally diverse colchicine site inhibitors. These binding models were subsequently used to construct a comprehensive, structure-based pharmacophore that in combination with molecular dynamics simulations confirms and extends our understanding of binding interactions at the colchicine site.

Selective insensitivity of ZR-75-1 human breast cancer cells to 2-methoxyestradiol: evidence for type II 17beta-hydroxysteroid dehydrogenase as the underlying cause

2-Methoxyestradiol (2-MeO-E2), a nonpolar endogenous metabolite of 17beta-estradiol, has strong antiproliferative, apoptotic, and antiangiogenic actions. Among the four human breast cancer cell lines tested (MCF-7, T-47D, ZR-75-1, and MDA-MB-435s), the ZR-75-1 cells were selectively insensitive to the antiproliferative actions of 2-MeO-E2, although these cells had a similar sensitivity as other cell lines to several other anticancer agents (5-fluorouracil, mitomycin C, doxorubicin, colchicine, vinorelbine, and paclitaxel). Mechanistically, this insensitivity is largely attributable to the presence of high levels of a steroid-selective metabolizing enzyme, the type II 17beta-hydroxysteroid dehydrogenase (17beta-HSD), in the ZR-75-1 cells, which rapidly converts 2-MeO-E2 to the inactive 2-methoxyestrone, but this enzyme does not metabolically inactivate other nonsteroidal anticancer agents. The type II 17beta-HSD-mediated conversion of 2-MeO-E2 to 2-methoxyestrone in ZR-75-1 cells followed the first-order kinetics, with a very short half-life (approximately 2 hours). In comparison, the T-47D, MCF-7, and MDA-MB-435s human breast cancer cells, which were highly sensitive to 2-MeO-E2, had very low or undetectable catalytic activity for the conversion of 2-MeO-E2 to 2-methoxyestrone. Reverse transcription-PCR analysis of the mRNA levels of three known oxidative 17beta-HSD isozymes (types II, IV, and VIII) revealed that only the type II isozyme was selectively expressed in the ZR-75-1 cells, whereas the other two isozymes were expressed in all four cell lines. Taken together, our results showed, for the first time, that the high levels of type II 17beta-HSD present in ZR-75-1 cells were largely responsible for the facile conversion of 2-MeO-E2 to 2-methoxyestrone and also for the selective insensitivity to the antiproliferative actions of 2-MeO-E2.

2-methoxyestradiol is an estrogen receptor agonist that supports tumor growth in murine xenograft models of breast cancer

PURPOSE

2-Methoxyestradiol (2MEO) is being developed as a novel antitumor agent based on its antiangiogenic activity, tumor cell cytotoxicity, and apparent lack of toxicity. However, pharmacologic concentrations of 2MEO bind to estrogen receptors (ER). We have therefore examined the ER activity of 2MEO.

EXPERIMENTAL DESIGN

Estrogenic actions of 2MEO were evaluated by changes in gene expression of the ER-positive (MCF7) breast tumor cell line and, in vivo, estrogenicity was assessed in breast tumor xenograft models and by measuring endocrine responses in uterus and liver.

RESULTS

In the ER-positive breast tumor cell line (MCF7), microarray experiments revealed that 269 of 279 changes in gene expression common to 2MEO and estradiol were prevented by the ER antagonist, ICI 182,780. Changes in the expression of selected genes and their sensitivity to inhibition by ICI 182,780 were confirmed by quantitative reverse transcription-PCR measurement. Activation of ER in MCF7 cells by 2MEO was further confirmed by stimulation of an estrogen response element-dependent reporter gene that was blocked by ICI 182,780 (1 micromol/L). Doses of 2MEO (15-150 mg/kg) that had no antitumor efficacy in either nu/nu BALB/c or severe combined immunodeficient mice bearing ER-negative MDA-MB-435 tumors had uterotropic and hepatic estrogen-like actions. In female nu/nu BALB/c mice inoculated with the estrogen-dependent MCF7 tumor cells, 2MEO (50 mg/kg/d) supported tumor growth.

CONCLUSIONS

Tumor growth enhancement by 2MEO at doses generating serum levels (100-500 nmol/L) that have estrogenic activity suggests that a conservative approach to the further clinical evaluation of this agent should be adopted and that its evaluation in breast cancer is inappropriate.

Anti-cancer activities of novel D-ring modified 2-substituted estrogen-3-O-sulfamates

Sulfamoylated derivatives of the endogenous estrogen metabolite 2-methoxyestradiol (2-MeOE2 (7)), such as 2-methoxy-3-O-sulfamoyl estrone (2-MeOEMATE (1)), display greatly enhanced activity against the proliferation of human cancer cells and inhibit steroid sulphatase (STS), another current oncology target. We explore here the effects of steroidal D-ring modification on the activity of such 2-substituted estrogen-3-O-sulfamates in respect of inhibition of tumour cell proliferation and steroid sulphatase. The novel 17-deoxy analogues of 2-MeOEMATE and the related 2-ethyl and 2-methylsulfanyl compounds showed greatly reduced inhibition of MCF-7 proliferation. Introduction of a 17alpha-benzyl substituent to such 2-substituted estrogen sulfamates also proved deleterious to anti-proliferative activity but could, in one case, enhance STS inhibition with respect to the parent substituted estrone sulfamate. In contrast, selected 17-oxime derivatives of 2-MeOEMATE displayed an enhanced anti-proliferative activity. These results illustrate that enhanced in vitro anti-cancer activity can be achieved in the 2-substituted estrogen sulfamate series and highlight, in particular, the importance of potential hydrogen bonding effects around the steroidal D-ring in the activity of these molecules. The SAR parameters established herein will assist the future design of anti-proliferative and anti-endocrine agents as potential therapeutics for both hormone dependent and independent cancers.

Synergism between the Anticancer Actions of 2-Methoxyestradiol and Microtubule-Disrupting Agents in Human Breast Cancer

2-Methoxyestradiol (2-MeO-E2), a well-known nonpolar endogenous metabolite of 17β-estradiol, has strong antiproliferative, apoptotic, and antiangiogenic actions in vitro and in vivo at pharmacologic concentrations. We determined in the present study whether 2-MeO-E2 can enhance the anticancer actions of paclitaxel or vinorelbine (two commonly used microtubule-disrupting agents) in several human breast cancer cell lines, including the estrogen receptor–positive MCF-7 and T-47D cells and the receptor-negative MDA-MB-435s and MDA-MB-231 cells. 2-MeO-E2 in combination with paclitaxel or vinorelbine exhibited a synergistic anticancer effect in these human breast cancer cells in vitro, and this synergistic effect was more pronounced when each of the drugs was used at relatively low concentrations. Additional experiments using female athymic BALB/c nu/nu mice showed that p.o. administration of 2-MeO-E2 at 30 mg/kg body weight, once a week for 6 weeks, markedly enhanced the activity of paclitaxel or vinorelbine against the growth of the estrogen receptor–negative MDA-MB-231 human breast cancer xenografts in these animals. By contrast, combination of 2-MeO-E2 with 5-fluorouracil only had a partial additive effect against the growth of these cell lines in culture, and no synergistic effect was observed. Interestingly, when doxorubicin was used in combination with 2-MeO-E2, the antiproliferative effect of 2-MeO-E2 was somewhat antagonized by doxorubicin when it was present at high concentrations. Our results showed that 2-MeO-E2 at nontoxic or subtoxic doses selectively enhanced the effects of certain microtubule-disrupting agents (such as paclitaxel and vinorelbine) against the growth of the receptor-negative human breast cancer cells in culture and also in athymic nude mice.

Effects of altering the electronics of 2-methoxyestradiol on cell proliferation, on cytotoxicity in human cancer cell cultures, and on tubulin polymerization

A series of new analogues of 2-methoxyestradiol (1) were synthesized to further elucidate the relationships between structure and activity. The compounds were designed to diminish the potential for metabolic deactivation at positions 2 and 17 and were analyzed as inhibitors of tubulin polymerization and for cytotoxicity. 17alpha-methyl-beta-estradiol (30), 2-propynyl-17alpha-methylestradiol (39), 2-ethoxy-17-(1'-methylene)estra-1,3,5(10)-triene-3-ol (50) and 2-ethoxy-17alpha-methylestradiol (51) showed similar or greater tubulin polymerization inhibition than 2-methoxyestradiol (1) and contained moieties that are expected to inhibit deactivating metabolic processes. All of the compounds tested were cytotoxic in the panel of 55 human cancer cell cultures, and generally, the derivatives that displayed the most activity against tubulin were also the most cytotoxic.

Concentration-dependent mitogenic and antiproliferative actions of 2-methoxyestradiol in estrogen receptor-positive human breast cancer cells

We compared in this study the effects of 2-methoxyestradiol (2-MeO-E(2)) on the growth of two estrogen receptor (ER)-negative human breast cancer cell lines (MDA-MB-231 and MDA-MB-435s) and two ER-positive human breast cancer cell lines (MCF-7 and T-47D). 2-MeO-E(2) exerted a concentration-dependent antiproliferative action in the ER-negative MDA-MB-231 and MDA-MB-435s cells. The presence or absence of exogenous 17beta-estradiol (E(2)) in the culture medium did not affect the potency and efficacy of 2-MeO-E(2)'s antiproliferative action in these ER-negative cells. When the ER-positive MCF-7 and T-47D cells were cultured in a medium supplemented with 10nM of exogenous E(2), 2-MeO-E(2) at 750 nM to 2 microM concentrations exerted a similar antiproliferative effect. However, when the ER-positive cell lines were cultured in the absence of exogenous E(2), 2-MeO-E(2) at relatively low concentrations (10-750 nM) had a moderate mitogenic effect, with its apparent efficacy 75-80% of that of E(2). This mitogenic effect of 2-MeO-E(2) was ER-mediated and largely attributable to 2-MeO-E(2)'s residual estrogenic activity on the basis of our following findings: (i) its effect was only manifested in the ER-positive cells but not in the ER-negative cells; (ii) its effect in the ER-positive cells was partially or fully abolished when exogenous E(2) was concomitantly present in the culture medium; (iii) 2-MeO-E(2) retained 1-2% of E(2)'s binding affinity for the human ERalpha and ERbeta, and its mitogenic effect was inhibited in a concentration-dependent manner by ICI-182,780, a pure ER antagonist; and (iv) its effect was not due to its metabolic conversion to 2-hydroxyestradiol. Our timely findings are of importance to the on-going clinical trials designed to evaluate 2-MeO-E(2)'s effectiveness for the treatment of different types (ER-positive or ER-negative) of human breast cancer. This knowledge will improve the design of clinical trials as well as the interpretation of clinical outcomes when 2-MeO-E(2) is used as a single agent therapy or as part of a combination therapy for human breast cancer.

Novel benzylidene-9(10H)-anthracenones as highly active antimicrotubule agents. Synthesis, antiproliferative activity, and inhibition of tubulin polymerization

A novel series of 10-benzylidene-9(10H)-anthracenones and 10-(phenylmethyl)-9(10H)-anthracenones were synthesized and evaluated for antiproliferative activity in an assay based on K562 leukemia cells. The 3-hydroxy-4-methoxybenzylidene analogue 9h was found to be the most active compound (IC(50) K562: 20 nM). Structure-activity relationships are also considered. The highly active compound 9h and the 2,4-dimethoxy-3-hydroxybenzylidene analogue 9l were tested against five tumor cell lines using the XTT assay, including multidrug resistant phenotypes. Induction of cell death in a variety of tumor cell lines was determined in a monolayer assay using propidium iodide. Noteworthy, all compounds within the series induced elongations in K562 cells similar to vinblastine-treated cells. The effect of the lead compound 9h on K562 cell growth was associated with cell cycle arrest in G2/M. Concentrations for 50% KB/HeLa cells arrested in G2/M after treatment with 9h and 9l were determined and found to be in the range of 0.2 microM. Additionally, we monitored the dose dependent caspase-3-like protease activity in K562 cells and MCF-7/Casp-3 cells treated with 9h, indicating induction of apoptosis. Western blotting analysis demonstrated that 9h caused a shift in tubulin concentration from the polymerized state found in the cell pellet to the unpolymerized state found in the cell supernatant. Seven compounds strongly inhibited tubulin polymerization with activities higher or comparable to those of the reference compounds such as colchicine, podophyllotoxin, and nocodazole. In general, the antiproliferative activity correlated with inhibition of tubulin polymerization. The most active compounds strongly displaced [(3)H]colchicine from its binding site in the tubulin, yielding IC(50) values 3- to 4-fold lower than that of colchicine. The novel benzylidene-9(10H)-anthracenones described in the present study constitute an interesting group of highly active and easily accessible antimitotic agents that inhibit tubulin polymerization.