A rationally designed genotoxin that selectively destroys estrogen receptor-positive breast cancer cells

Structure-guided identification of novel dual-targeting estrogen receptor α degraders with aromatase inhibitory activity for the treatment of endocrine-resistant breast cancer

Drug resistance is a major challenge in conventional endocrine therapy for estrogen receptor (ER) positive breast cancer (BC). BC is a multifactorial disease, in which simultaneous aromatase (ARO) inhibition and ERα degradation may effectively inhibit the signal transduction of both proteins, thus potentially overcoming drug resistance caused by overexpression or mutation of target proteins. In this study, guided by the X-ray structure of a hit compound 30a in complex with ER-Y537S, a structure-based optimization was performed to get a series of multiacting inhibitors targeting both ERα and ARO, and finally a novel class of potent selective estrogen receptor degraders (SERDs) based on a three-dimensional oxabicycloheptene sulfonamide (OBHSA) scaffold equipped with aromatase inhibitor (AI) activity were identified. Of these dual-targeting SERD-AI hybrids, compound 31q incorporating a 1H-1,2,4-triazole moiety showed excellent ERα degradation activity, ARO inhibitory activity and remarkable antiproliferative activity against BC resistant cells. Furthermore, 31q manifested efficient tumor suppression in MCF-7 tumor xenograft models. Taken together, our study reported for the first time the highly efficient dual-targeting SERD-AI hybrid compounds, which may lay the foundation of translational research for improved treatment of endocrine-resistant BC.

(4-Picolylamino)-17β-Estradiol derivative and analogues induce apoptosis with death receptor trail R2/DR5 in MCF-7

In order to discover more effective and less toxic drugs in the field of anti-tumor, the backbone structure of 17β-estradiol was modified, and 11 target compounds were synthesized. Compounds 5 and 10, which exhibited better anti-tumor activity and higher selectivity (more than 10-fold), were chosen for further biological investigation. Flow cytometry results indicated that 5 and 10 could arrest MCF-7 cells in the G2 phase and induce apoptosis. Immunohistochemical analysis revealed that 5 and 10 could bind to the estradiol receptor alpha in MCF-7 cells. Western blotting and real-time PCR assays were performed to detect the effects of compounds on apoptosis-related targets at the protein and gene levels. These results showed that both 5 and 10 could dosed-dependently increase the expression of Apaf-1, Bax, caspase-3,8,9 and reduce the expression levels of the anti-apoptotic factors Bcl-2 and Bcl-xL. Besides, the Human apoptosis array assay demonstrated the expression level of death receptor Trail R2/DR5 was upregulated obviously while the expression of TNF R1, IAPs and Hsp27/60/70 were downregulated. On the whole, 5 induced MCF-7 cell death through the endogenous pathway in mitochondria and the exogenous pathway with death receptor Trail R2/DR5.

A Comprehensive Review on Steroidal Bioconjugates as Promising Leads in Drug Discovery

Ever increasing unmet medical requirements of the human race and the continuous fight for survival against variety of diseases give birth to novel molecules through research. As diseases evolve, different strategies are employed to counter the new challenges and to discover safer, more effective, and target-specific therapeutic agents. Among several novel approaches, bioconjugation, in which two chemical moieties are joined together to achieve noticeable results, has emerged as a simple and convenient technique for a medicinal chemist to obtain potent molecules. The steroid system has been extensively used as a privileged scaffold gifted with significantly diversified medicinal properties in the drug discovery and development process. Steroidal molecules are preferred for their rigidness and good ability to penetrate biological membranes. Slight alteration in the basic ring structure results in the formation of steroidal derivatives with a wide range of therapeutic activities. Steroids are not only active as such, conjugating them with various biologically active moieties results in increased lipophilicity, stability, and target specificity with decreased adverse effects. Thus, the steroid nucleus prominently behaves as a biological carrier for small molecules. The steroid bioconjugates offer several advantages such as synergistic activity with fewer side effects due to reduced dose and selective therapy. The steroidal bioconjugates have been widely explored for their usefulness against various disorders and have shown significant utility as anticancer, anti-inflammatory, anticoagulant, antimicrobial, insecticidal/pesticidal, antioxidant, and antiviral agents along with several other miscellaneous activities. This work provides a comprehensive review on the therapeutic progression of steroidal bioconjugates as medicinally active molecules. The review covers potential biological applications of steroidal bioconjugates and would benefit the wider scientific community in their drug discovery endeavors.

Drug conjugates-an emerging approach to treat breast cancer

Breast cancer treatment using a single drug is associated with a high failure rate due, in part, to the heterogeneity of drug response within individuals, nonspecific target action, drug toxicity, and/or development of resistance. Use of dual-drug therapies, including drug conjugates, may help overcome some of these roadblocks by more selective targeting of the cancer cell and by acting at multiple drug targets rather than one. Drug-conjugate approaches include linking drugs to antibodies (antibody-drug conjugates), radionuclides (radioimmunoconjugates), nanoparticles (nanoparticle-drug conjugates), or to other drugs (drug-drug conjugates). Although all of these conjugates might be designed as effective treatments against breast cancer, the focus of this review will be on drug-drug conjugates because of the increase in versatility of these types of drugs with respect to mode of action at the level of the cancer cell either by creating a novel pharmacophore or by increasing the potency and/or efficacy of the drugs' effects at their respective molecular targets. The development, synthesis, and pharmacological characteristics of drug-drug conjugates will be discussed in the context of breast cancer with the hope of enhancing drug efficacy and reducing toxicities to improve patient quality of life.

Efficacy of hybrid vitamin D receptor agonist/histone deacetylase inhibitors in vitamin D-resistant triple-negative 4T1 breast cancer

Hormonal 1,25-dihydroxyvitamin D (1,25D) and its analogues have shown efficacy in some preclinical models of cancer. However, many models are resistant to the antiproliferative effects of 1,25D or its analogues in vitro or in vivo, and such compounds have failed in the clinic as monotherapies because of tumor resistance. Given the observed synergism between 1,25D analogues and histone deacetylase inhibitors (HDACi) in 1,25D-resistant cells, we previously developed a series of hybrid secosteroidal and easily assembled non-secosteroidal analogues that combined agonism for the vitamin D receptor and HDACi in a single backbone. These compounds displayed enhanced efficacy against 1,25D-resistant malignant cells in vitro. Structure/function studies led to synthesis of several non-secosteroidal variants in which HDACi potency was optimized without substantially sacrificing VDR agonism. Here, we present the first studies of efficacy in vivo of two of these compounds, DK-366 and DK-406, in the aggressive mouse 4T1 model of triple-negative breast cancer, a form of the disease for which treatment options are limited. 4T1 cells are resistant in vitro to the cytostatic and cytotoxic effects of 1,25D and the potent HDACi SAHA individually up to concentrations of 1μM and 50μM, respectively, whereas combinations of the two are efficacious. In vitro, DK-366 or -406 induced dose-dependent arrest of cell proliferation and cytotoxicity at 10-20μM. In vivo, the maximum tolerated dose (MTD) of DK-366 and DK-406 were 2.5 and 5.0mg/kg, respectively. Although the compounds induced hypercalcemia at elevated doses, consistent with VDR agonism in vivo, they both reduced tumor burden at doses below their MTD's. Moreover, in a separate experiment, DK-406 at 5mg/kg reduced 4T1 lung metastases by at least 50%. Under the same conditions, 1,25D (0.25μg/kg) and SAHA (25mg/kg) combined had no effect on tumor burden or on lung metastases. These experiments show that hybrid compounds are bioavailable and efficacious against a particularly aggressive model of metastatic breast cancer, providing strong support for the therapeutic potential of the hybrid concept.

The potential of combi-molecules with DNA-damaging function as anticancer agents

DNA-damaging agents, such as methylating agents, chloroethylating agents and platinum-based agents, have been extensively used as anticancer drugs. However, the side effects, high toxicity, lack of selectivity and resistance severely limit their clinical applications. In recent years, a strategy combining a DNA-damaging agent with a bioactive molecule (e.g., enzyme inhibitors) or carrier (e.g., steroid hormone and DNA intercalators) to produce a new 'combi-molecule' with improved efficacy or selectivity has been attempted to overcome these drawbacks. The combi-molecule simultaneously acts on two targets and is expected to possess better potency than the parent compounds. Many studies have shown DNA-damaging combi-molecules exhibiting excellent anticancer activity in vitro and in vivo. This review focuses on the development of combi-molecules, which possess increased DNA-damaging potency, anticancer efficacy and tumor selectivity and reduced side reactions than the parent compounds. The future opportunities and challenges in the discovery of combi-molecules were also discussed.

Glycoconjugated Site-Selective DNA-Methylating Agent Targeting Glucose Transporters on Glioma Cells

DNA-alkylating drugs continue to remain an important weapon in the arsenal against cancers. However, they typically suffer from several shortcomings because of the indiscriminate DNA damage that they cause and their inability to specifically target cancer cells. We have developed a strategy for overcoming the deficiencies in current DNA-alkylating chemotherapy drugs by designing a site-specific DNA-methylating agent that can target cancer cells because of its selective uptake via glucose transporters, which are overexpressed in most cancers. The design features of the molecule, its synthesis, its reactivity with DNA, and its toxicity in human glioblastoma cells are reported here. In this molecule, a glucosamine unit, which can facilitate uptake via glucose transporters, is conjugated to one end of a bispyrrole triamide unit, which is known to bind to the minor groove of DNA at A/T-rich regions. A methyl sulfonate moiety is tethered to the other end of the bispyrrole unit to serve as a DNA-methylating agent. This molecule produces exclusively N3-methyladenine adducts upon reaction with DNA and is an order of magnitude more toxic to treatment resistant human glioblastoma cells than streptozotocin is, a Food and Drug Administration-approved, glycoconjugated DNA-methylating drug. Cellular uptake studies using a fluorescent analogue of our molecule provide evidence of uptake via glucose transporters and localization within the nucleus of cells. These results demonstrate the feasibility of our strategy for developing more potent anticancer chemotherapeutics, while minimizing common side effects resulting from off-target damage.

Targeting the androgen receptor with steroid conjugates

The androgen receptor (AR) is a major therapeutic target in prostate cancer pharmacology. Progression of prostate cancer has been linked to elevated expression of AR in malignant tissue, suggesting that AR plays a central role in prostate cancer cell biology. Potent therapeutic agents can be precisely crafted to specifically target AR, potentially averting systemic toxicities associated with nonspecific chemotherapies. In this review, we describe various strategies to generate steroid conjugates that can selectively engage AR with high potency. Analogies to recent developments in nonsteroidal conjugates targeting AR are also evaluated. Particular focus is placed on potential applications in AR pharmacology. The review culminates with a description of future prospects for targeting AR.

Steroid-linked nitrogen mustards as potential anticancer therapeutics: a review

Nitrogen mustards, an important class of drugs for cancer therapy, are known as DNA alkylating agents. The nitrogen mustards are highly reactive and, as a consequence, lack of selectivity and produce several adverse side effects. In order to minimize these undesirable effects, the attachment of nitrogen mustards to a steroidal hormone with affinity for its receptor can lead to highly selective and less toxic antineoplastic therapeutics. This review will focus on the design, synthesis and evaluation of such steroid-nitrogen mustard hybrids as antineoplastic agents. Among these compounds, modified steroids with aromatic nitrogen mustards linked by an ester function were found to have better DNA alkylating properties, improved selectivity as well as low toxicity. This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Design of a platinum-acridine-endoxifen conjugate targeted at hormone-dependent breast cancer

The synthesis of a novel pharmacophore comprising a DNA-targeted platinum-acridine hybrid agent and estrogen receptor-targeted 4-hydroxy-N-desmethyltamoxifen (endoxifen) using carbamate coupling chemistry and its evaluation in breast cancer cell lines are described.

Synthesis and preliminary evaluation steroidal antiestrogen-geldanamycin conjugates

Three novel steroidal antiestrogen-geldanamycin conjugates were prepared using a convergent strategy. The antiestrogenic component utilized the 11β-(4-functionalized-oxyphenyl) estradiol scaffold, while the geldanamycin component was derived by replacement of the 17-methoxy group with an appropriately functionalized amine. Ligation was achieved in high yield using azide alkyne cyclization reactions. Evaluation of the products against two breast cancer cell lines indicated that the conjugates retained significant antiproliferative activity.

Promise and challenges in drug discovery and development of hybrid anticancer drugs

BACKGROUND

Because cancer is a complex disease, it is unlikely that a single mono functional 'targeted' drug will be effective for treating this most advanced disease. Combined drugs that impact multiple targets simultaneously are better at controlling complex disease systems, are less prone to drug resistance and are the standard of care in cancer treatment. In order to improve the efficiency of using a two-drug cocktail, one approach involves the use of the so-called hybrid drugs, which comprises the incorporation of two drugs in a single molecule with the intention of exerting dual drug action.

OBJECTIVE

In the present article, we discuss the design, synthesis and various applications of anticancer hybrid agents and the developments in this field during the last few decades. Additionally, we describe different types of linkers and their role in contributing towards biological effects and the in vivo mechanism of drug release. We also depict some challenges from scientific and regulatory perspectives in the hybrid drug development process.

CONCLUSION

In the era of increasing drug resistance in cancer patients, the discovery of hybrid drugs could provide an effective strategy to create chemical entities likely to be more efficacious and less prone to resistance. However, some technical and regulatory challenges will have to be surmounted before hybrid drugs succeed in the clinical settings and justify the considerable promise of this novel concept.

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.

A lipid-modified estrogen derivative that treats breast cancer independent of estrogen receptor expression through simultaneous induction of autophagy and apoptosis

It is a challenge to develop a universal single drug that can treat breast cancer at single- or multiple-stage complications, yet remains nontoxic to normal cells. The challenge is even greater when breast cancer-specific, estrogen-based drugs are being developed that cannot act against multistaged breast cancer complications owing to the cells differential estrogen receptor (ER) expression status and their possession of drug-resistant and metastatic phenotypes. We report here the development of a first cationic lipid-conjugated estrogenic derivative (ESC8) that kills breast cancer cells independent of their ER expression status. This ESC8 molecule apparently is nontoxic to normal breast epithelial cells, as well as to other noncancer cells. ESC8 induces apoptosis through an intrinsic pathway in ER-negative MDA-MB-231 cells. In addition, ESC8 treatment induces autophagy in these cells by interfering with the mTOR activity. This is the first example of an estrogen structure-based molecule that coinduces apoptosis and autophagy in breast cancer cells. Further in vivo study confirms the role of this molecule in tumor regression. Together, our results open new perspective of breast cancer chemotherapy through a single agent, which could provide the therapeutic benefit across all stages of breast cancer.

Probing the topological tolerance of multimeric protein interactions: evaluation of an estrogen/synthetic ligand for FK506 binding protein conjugate

Bivalent small molecules composed of a targeting element and an element that recruits endogenous proteins have been shown to block protein-protein interactions in some systems. We have attempted to apply such an approach to disrupt the interaction of the estrogen receptor α with either its associated coactivators or its dimerization partner (i.e., another estrogen receptor). We show here that a conjugate capable of simultaneously binding both the estrogen receptor and a recruited protein (FK506 Binding Protein 12 kDa) is, however, incapable of disrupting the multimeric estrogen receptor dimer/coactivator complex both in vitro and in cell-based reporter gene assays. We postulate why it may not be possible to disrupt this particular protein-protein complex-as well as other systems having high topological tolerance-with such bivalent inhibitors.

Selective estrogen receptor modulator delivery of quinone warheads to DNA triggering apoptosis in breast cancer cells

Estrogen exposure is a risk factor for breast cancer, and estrogen oxidative metabolites have been implicated in chemical carcinogenesis. Oxidation of the catechol metabolite of estrone (4-OHE) and the beta-naphthohydroquinone metabolite of equilenin (4-OHEN) gives o-quinones that produce ROS and damage DNA by adduction and oxidation. To differentiate hormonal and chemical carcinogensis pathways in estrogen receptor positive ER(+) cells, catechol or beta-naphthohydroquinone warheads were conjugated to the selective estrogen receptor modulator (SERM) desmethylarzoxifene (DMA). ER binding was retained in the DMA conjugates; both were antiestrogens with submicromolar potency in mammary and endometrial cells. Cytotoxicity, apoptosis, and caspase-3/7 activation were compared in ER(+) and ER(-)MDA-MB-231 cells, and production of ROS was detected using a fluorescent reporter. Comparison was made to DMA, isolated warheads, and a DMA-mustard. Conjugation of warheads to DMA increased cytotoxicity accompanied by induction of apoptosis and activation of caspase-3/7. Activation of caspase-3/7, induction of apoptosis, and cytotoxicity were all increased significantly in ER(+) cells for the DMA conjugates. ROS production was localized in the nucleus for conjugates in ER(+) cells. Observations are compatible with beta-naphthohydroquinone and catechol groups being concentrated in the nucleus by ER binding, where oxidation and ROS production result, concomitant with caspase-dependent apoptosis. The results suggest that DNA damage induced by catechol estrogen metabolites can be amplified in ER(+) cells independent of hormonal activity. The novel conjugation of quinone warheads to an ER-targeting SERM gives ER-dependent, enhanced apoptosis in mammary cancer cells of potential application in cancer therapy.

Estrogen Receptor {alpha} Enhances the Rate of Oxidative DNA Damage by Targeting an Equine Estrogen Catechol Metabolite to the Nucleus

Exposure to estrogens increases the risk of breast and endometrial cancer. It is proposed that the estrogen receptor (ER) may contribute to estrogen carcinogenesis by transduction of the hormonal signal and as a "Trojan horse" concentrating genotoxic estrogen metabolites in the nucleus to complex with DNA, enhancing DNA damage. 4-Hydroxyequilenin (4-OHEN), the major catechol metabolite of equine estrogens present in estrogen replacement formulations, autoxidizes to a redox-cycling quinone that has been shown to cause DNA damage. 4-OHEN was found to be an estrogen of nanomolar potency in cell culture using a luciferase reporter assay and, using a chromatin immunoprecipitation assay, was found to activate ERalpha binding to estrogen-responsive genes in MCF-7 cells. DNA damage was measured in cells by comparing ERalpha(+) versus ERalpha(-) cells and 4-OHEN versus menadione, a reactive oxygen species (ROS)-generating, but non-estrogenic, quinone. 4-OHEN selectively induced DNA damage in ERalpha(+) cells, whereas menadione-induced damage was not dependent on cellular ER status. The rate of 4-OHEN-induced DNA damage was significantly enhanced in ERalpha(+) cells, whereas ER status had no effect on the rate of menadione-induced damage. Imaging of ROS induced by 4-OHEN showed accumulation selective for the nucleus of ERalpha(+) cells within 5 min, whereas in ERalpha(-) or menadione-treated cells, no selectivity was observed. These data support ERalpha acting as a Trojan horse concentrating 4-OHEN in the nucleus to accelerate the rate of ROS generation and thereby amplify DNA damage. The Trojan horse mechanism may be of general importance beyond estrogen genotoxins.

Design and Applications of Bifunctional Small Molecules: Why Two Heads Are Better Than One

Induction of protein--protein interactions is a daunting challenge, but recent studies show promise for small molecules that specifically bring two or more protein molecules together for enhanced or novel biological effect. The first such bifunctional molecules were the rapamycin- and FK506-based "chemical inducers of dimerization", but the field has since expanded with new molecules and new applications in chemical genetics and cell biology. Examples include coumermycin-mediated gyrase B dimerization, proteolysis targeting chimeric molecules (PROTACs), drug hybrids, and strategies for exploiting multivalency in toxin binding and antibody recruitment. This Review discusses these and other advances in the design and use of bifunctional small molecules and potential strategies for future systems.

A bifunctional platinum(II) antitumor agent that forms DNA adducts with affinity for the estrogen receptor

A strategy is described for the re-design of DNA damaging platinum(II) complexes to afford elevated toxicity towards cancer cells expressing the estrogen receptor (ER). Two platinum-based toxicants are described in which a DNA damaging warhead, [Pt(en)Cl(2)] (en, ethylenediamine), is tethered to either of two functional groups. The first agent, [6-(2-amino-ethylamino)-hexyl]-carbamic acid 2-[6-(7alpha-estra-1,3,5,(10)-triene)-hexylamino]-ethyl ester platinum(II) dichloride ((Est-en)PtCl(2)), terminates in a ligand for the ER. The second agent is a control compound lacking the steroid; this compound, N-[6-(2-amino-ethylamino)-hexyl]-benzamide platinum(II) dichloride ((Bz-en)PtCl(2))), terminates in a benzamide moiety, which lacks affinity for the ER. Using a competitive binding assay, Est-en had 28% relative binding affinity (RBA) for the ER as compared to 17beta-estradiol. After covalent binding to a synthetic DNA duplex 16-mer, the compound retained its affinity for the ER; specificity of the binding event was demonstrated by the ability of free 17beta-estradiol as a competitor to disrupt the DNA adduct-ER complex. The (Est-en)PtCl(2) compound showed higher toxicity against the ER positive ovarian cancer cell line CAOV3 than did the control compound. (Est-en)PtCl(2) was also more toxic to the ER positive breast cancer line, MCF-7, than to an ER negative line, MDA-MB231.

Potential mechanisms of estrogen quinone carcinogenesis

There is a clear association between the excessive exposure to estrogens and the development of cancer in hormone-sensitive tissues (breast, endometrium). It has become clear that there are likely multiple overlapping mechanisms of estrogen carcinogenesis. One major pathway is the extensively studied hormonal pathway, by which estrogen stimulates cell proliferation through nuclear estrogen receptor (ER)-mediated signaling, thus resulting in an increased risk of genomic mutations during DNA replication. A similar "nongenomic pathway", potentially involving newly discovered membrane-associated ERs, also appears to regulate extranuclear estrogen signaling pathways. This perspective is focused on a third pathway involving the metabolism of estrogens to catechols mediated by cytochrome P450 and further oxidation of these catechols to estrogen o-quinones. Oxidative enzymes, metal ions, and in some cases molecular oxygen can catalyze o-quinone formation, so that these electrophilic/redox-active quinones can cause damage within cells by alkylation and/or oxidation of cellular proteins and DNA in many tissues. It appears that the endogenous estrogen quinones primarily form unstable N3-adenine or N7-guanine DNA adducts, ultimately resulting in mutagenic apurinic sites. In contrast, equine estrogen quinones, formed from estrogens present in popular hormone replacement therapy prescriptions, generate a variety of DNA lesions, including bulky stable adducts, apurinic sites, DNA strand cleavage, and oxidation of DNA bases. DNA damage induced by these equine quinones is significantly increased in cells containing ERs, leading us to hypothesize a mechanism involving ER binding/alkylation by the catchol/quinone, resulting in a "Trojan horse". The "Trojan horse" carries the highly redox-active catechol to estrogen -sensitive genes, where high amounts of reactive oxygen species are generated, causing selective DNA damage. Our data further suggest that other key protein targets for estrogen o-quinones could be redox-sensitive enzymes (i.e, GST P1-1, QR). These proteins are involved in stress response cascades that are known to contribute to the regulation of cell proliferation and apoptosis. Finally, it has been shown that catechol estrogens can transform breast epithelial cells into a tumorigenic phenotype and that these transformed cells had differential gene expression of several genes involved in oxidative stress. Given the direct link between excessive exposure to estrogens, metabolism of estrogens, and increased risk of breast cancer, it is crucial that factors that affect the formation, reactivity, and cellular targets of estrogen quinoids be thoroughly explored.

Structure–antileukemic activity relationship study of B- and D-ring modified and nonmodified steroidal esters of 4-methyl-3-N,N-bis(2-chloroethyl)amino benzoic acid: a comparative study

This study was designed as a rational continuation of our research regarding the functional requirements essential for the antileukemic activity of compounds comprising an alkylating moiety and a modified steroid. The steroidal esteric derivatives of 4-methyl-3-N,N-bis(2-chloroethyl)amino benzoic acid were tested on leukemias P388 and L1210 in vivo and in normal human lymphocytes in vitro. Among them the B-lactamic steroidal esters proved more potent antileukemic agents than the 7-oxidized and those with a simple B-ring, but not more effective inducers of DNA damage and cell cycle arrest in vitro. We speculate that these results indicate a different mechanism of action induced by the lactamized B steroidal ring, in comparison to the 7-keto or the D-lactamic groups, which involves the interaction of the -NHCO- moiety with cellularcomponents essential for tumor growth. 4-Methyl-3-N,N-bis(2-chloroethyl)amino benzoic acid proved a more proper module for the B-lactams than chlorambucil and phenyl acetic acid's nitrogen mustard probably because the esteric bond is less cleaved by the esterases, resulting in an increased concentration of the drug in the vinicity of the target site essential for an antineoplasmatic response.

Design, synthesis, and estrogenic activity of a novel estrogen receptor modulator--a hybrid structure of 17beta-estradiol and vitamin E in hippocampal neurons

We recently discovered that ICI 182,780 (1), an antagonist of estrogen receptor (ER)-dependent proliferation in reproductive tissues, functions as an estrogenic agonist in primary neurons. The present study investigated whether the agonist properties of 1 in neurons could be translated into structural analogs. 7alpha-[(4R,8R)-4,8,12-trimethyltridecyl]estra-1,3,5-trien-3,17beta-diol (2), a hybrid structure of 17beta-estradiol and vitamin E, was synthesized and found to bind to both ERalpha and ERbeta. In vitro analyses demonstrated that 2 was neuroprotective and effective in activating molecular mechanisms associated with estrogenic agonist activity in rat primary hippocampal neurons. Collectively, the data support an estrogenic agonist profile of 2 action comparable to 1 in primary neurons, confirming that estrogenic activity of 1 in neurons is not a unique phenomenon. These results provide support for the development of a brain-selective ER modulator, with potential as an efficacious and safe estrogen alternative to prevent Alzheimer's disease and cognitive decline in postmenopausal women.

Benzothiophene Selective Estrogen Receptor Modulators with Modulated Oxidative Activity and Receptor Affinity

The regulation of estrogenic and antiestrogenic effects of selective estrogen receptor modulators (SERMs) is thought to underlie their clinical use. Most SERMs are polyaromatic phenols susceptible to oxidative metabolism to quinoids, which are proposed to be genotoxic. Conversely, the redox reactivity of SERMs may contribute to antioxidant and chemopreventive mechanisms, providing a new approach to improve the therapeutic properties of SERMs. An improved synthetic strategy was developed to generate a family of benzothiophene SERMs. Using computational modeling methods and measurements of antioxidant activity and estrogen receptor (ER) ligand binding, this SERM family was shown to provide both a range of ERalpha/ERbeta selectivity from 1.2- to 67-fold and a range of redox activity. Antioxidant activity was successfully modulated by varying a substituent remote from the OH group; the source of the antioxidant capacity. An efficient synthetic procedure is reported yielding benzothiophene SERMs wherein redox activity and ER affinity are modulated.

DNA adducts formed by a novel antitumor agent 11beta-dichloro in vitro and in vivo

The multifunctional molecule 11beta-dichloro consists of a ligand for the androgen receptor linked to a bifunctional alkylating group, permitting it to create DNA adducts that bind the androgen receptor. We propose that binding of the androgen receptor to 11beta-DNA adducts acts to both shield damaged sites from repair and disrupt the expression of genes essential for growth and survival. We investigated the formation 11beta-DNA adducts in tumor xenograft and nontumor tissues in mice. Using [14C]-11beta-dichloro, we show that the molecule remains intact in blood and is widely distributed in mouse tissues after i.p. injection. Covalent 11beta-guanine adducts identified in DNA that had been allowed to react with 11beta-dichloro in vitro were also found in DNA isolated from cells in culture treated with 11beta-dichloro as well as in DNA isolated from liver and tumor tissues of mice treated with the compound. We used accelerator mass spectrometry to determine the levels of [14C]-11beta-DNA adducts in LNCaP cells treated in culture as well as in liver tissue and LNCaP xenograft tumors in treated mice. The level of DNA adducts in tumor tissue was found to be similar to that found in LNCaP cells in culture treated with 2.5 micromol/L 11beta-dichloro. Our results indicate that 11beta-dichloro has sufficient stability to enter the circulation, penetrate tissues, and form DNA adducts that are capable of binding the androgen receptor in target tissues in vivo. These data suggest the involvement of our novel mechanisms in the antitumor effects of 11beta-dichloro.

Synthesis of 7alpha-substituted derivatives of 17beta-estradiol

Estrogen receptor (ER) pure antagonists such as ICI-182,780 (fulvestrant) are effective alternatives to tamoxifen (an ER antagonist/weak partial agonist) in the treatment of postmenopausal, receptor-positive human breast cancers. Structurally, these pure antagonists contain the basic core structure of 17beta-estradiol (E(2)) with a long side chain attached to its C-7alpha position. We explored and compared in this study various synthetic routes for preparing a number of C-7alpha-substituted derivatives of E(2), which are highly useful for the design and synthesis of high-affinity ER antagonists, ER-based imaging ligands, and other ER-based multi-functional agents. Using E(2) as the starting material and 1-iodo-6-benzyloxyhexane as a precursor for the C-7alpha side chain, a seven-step synthetic procedure afforded 3,17beta-bis(acetoxy)-7alpha-(6-hydroxyhexanyl)-estra-1,3,5(10)-triene (one of the derivatives prepared) in an overall yield of approximately 45% as compared to other known procedures that afforded substantially lower overall yield (8-27%). The synthetic steps for this representative compound include: (1) protection of the C-3 and C-17beta hydroxyls of E(2) using methoxymethyl groups; (2) hydroxylation of the C-6 position of the bismethoxymethyl ether of E(2); (3) Swern oxidation of the C-6 hydroxy to the ketone group; (4) C-7alpha alkylation of the C-6 ketone derivative of E(2); (5) deprotection of the two methoxymethyl groups; (6) reprotection of the C-3 and C-6 free hydroxyls with acetyl groups; (7) removal of the C-6 ketone and the benzyl group on the side chain by catalytic hydrogenation in acetic acid. As predicted, two of the representative C-7alpha-substituted derivatives of E(2) synthesized in the present study retained strong binding affinities (close to those of E(2) and ICI-182,780) for the human ERalpha and ERbeta subtypes as determined using the radioligand-receptor binding assays.

Analysis of protein covalent modification by xenobiotics using a covert oxidatively activated tag: raloxifene proof-of-principle study

Numerous xenobiotics, including therapeutics agents, are substrates for bioactivation to electrophilic reactive intermediates that may covalently modify biomolecules. Selective estrogen receptor modulators (SERMs) are in clinical use for long-term therapy of postmenopausal syndromes and chemoprevention and provide a potential alternative for hormone replacement therapy (HRT). Raloxifene, in common with many SERMs and other xenobiotics, is a polyaromatic phenol that has been shown to be metabolically bioactivated to electrophilic and redox active quinoids. Nucleic acid and glutathione adduct formation have been reported, but little is known about protein covalent modification. A novel COATag (covert oxidatively activated tag) was synthesized in which raloxifene was linked to biotin. The COATag was reactive toward a model protein, human glutathione-S-transferase P1-1, in the presence but not the absence of monooxygenase. The covalent modification of proteins in rat liver microsomal incubations was NADPH-dependent implicating cytochrome P450 oxidase. The COATag facilitated isolation and identification of covalently modified microsomal proteins: cytosolic glucose regulated protein (GRP78/BiP), three protein disulfide isomerases, and microsomal glutathione S-transferase 1. Oxidative metabolism of raloxifene produces reactive intermediates of sufficient lifetimes to covalently modify proteins in tissue microsomes, behavior anticipated for other polyaromatic phenol xenobiotics that can be tested by the COATag methodology. The combined use of a COATag with a simple biotin-linked electrophile (such as an iodoacetamide tag) is a new technique that allows quantification of protein covalent modification via alkylation vs oxidation in response to xenobiotic reactive intermediates. The identification of modified proteins is important for defining pathways that might lead alternatively to either cytotoxicity or cytoprotection.

Interstrand and Intrastrand DNA−DNA Cross-Linking by 1,2,3,4-Diepoxybutane: Role of Stereochemistry

1,2,3,4-Diepoxybutane (DEB) is a bifunctional electrophile capable of forming DNA-DNA and DNA-protein cross-links. DNA alkylation by DEB produces N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-guanine monoadducts, which can then form 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) lesions. All three optical isomers of DEB are produced metabolically from 1,3-butadiene, but S,S-DEB is the most cytotoxic and genotoxic. In the present work, interstrand and intrastrand DNA-DNA cross-linking by individual DEB stereoisomers was investigated by PAGE, mass spectrometry, and stable isotope labeling. S,S-, R,R-, and meso-diepoxides were synthesized from l-dimethyl-2,3-O-isopropylidene-tartrate, d-dimethyl-2,3-O-isopropylidene-tartrate, and meso-erythritol, respectively. Total numbers of bis-N7G-BD lesions (intrastrand and interstrand) in calf thymus DNA treated separately with S,S-, R,R-, or meso-DEB (0.01-0.5 mM) were similar as determined by capillary HPLC-ESI(+)-MS/MS of DNA hydrolysates. However, denaturing PAGE has revealed that S,S-DEB produced the highest number of interchain cross-links in 5'-GGC-3'/3'-CCG-5' sequences. Intrastrand adduct formation by DEB was investigated by a novel methodology based on stable isotope labeling HPLC-ESI(+)-MS/MS. Meso DEB treatment of DNA duplexes containing 5'-[1,7, NH(2)-(15)N(3),2-(13)C-G]GC-3'/3'-CCG-5' and 5'-GGC-3'/3'-CC[(15)N(3),2-(13)C-G]-5' trinucleotides gave rise to comparable numbers of 1,2-intrastrand and 1,3-interstrand bis-N7G-BD cross-links, while S,S DEB produced few intrastrand lesions. R,R-DEB treated DNA contained mostly 1,3-interstrand bis-N7G-BD, along with smaller amounts of 1,2-interstrand and 1,2-intrastrand adducts. The effects of DEB stereochemistry on its ability to form DNA-DNA cross-links may be rationalized by the spatial relationships between the epoxy alcohol side chains in stereoisomeric N7-(2'-hydroxy-3',4'-epoxybut-1'-yl)-guanine adducts and their DNA environment. Different cross-linking specificities of DEB stereoisomers provide a likely structural basis for their distinct biological activities.

Designing effective hybrid toxins

A study of a well-designed androgen-mustard conjugate provides evidence supporting a novel mechanism for its selective toxicity in androgen-receptor-positive cancer cells. This represents a solid step forward on the path toward effective hybrid toxins for targeted cancer therapy.

Disruption of Gene Expression and Induction of Apoptosis in Prostate Cancer Cells by a DNA-Damaging Agent Tethered to an Androgen Receptor Ligand

The goal of our work was the design of DNA-damaging agents that disrupt both DNA repair and signaling pathways in prostate tumor cells. A DNA alkylator (N,N-bis-2-chloroethyl aniline) was linked to a steroid ligand (17beta-hyroxy-estra-Delta(4(5),9(10))-3-one) to produce a complex molecule (11beta-dichloro) that forms DNA adducts that bind the androgen receptor (AR). We speculated that DNA adducts in an AR-DNA adduct complex would be camouflaged from DNA repair proteins that would otherwise remove the adducts in prostate cancer cells. Furthermore, transcription dependent on the AR would be antagonized by AR redistribution to sites distant from AR-driven promoters. The anticancer potential of 11beta-dichloro was demonstrated against prostate cancer cells in vitro and in vivo. 11beta-dichloro induces a unique pattern of gene disruption, induces apoptosis in apoptosis-resistant cells, and shows promising anticancer activity in animals.

Adenine-containing DNA-DNA cross-links of antitumor nitrogen mustards

Nitrogen mustards (NMs) are useful chemotherapeutic agents in the treatment of lymphoma, leukemia, multiple myeloma, and ovarian carcinoma. The antitumor activity of NMs has been attributed to their ability to cross-link the twin strands of DNA. The resulting bifunctional lesions, if not repaired, can inhibit DNA replication and transcription, eventually leading to cell cycle arrest, apoptosis, and the inhibition of tumor growth. The predominant bifunctional DNA lesions of NM have been reported to involve the distal guanine bases in the opposite strands of 5'-GNC sequences. In the present work, the formation of guanine-adenine and adenine-adenine adducts of N,N-bis(2-chloroethyl)methylamine (mechlorethamine) in double-stranded DNA is demonstrated. Guanine-adenine cross-links of mechlorethamine were identified as N-(2-[N3-adenyl]ethyl)-N-(2-[N7-guanyl]ethyl)methylamine (N3A-N7G-EMA), N-(2-[N1-adenyl]ethyl)-N-(2-[N7-guanyl]ethyl)methylamine, and N-(2-[N(6)-adenyl]ethyl)-N-(2-[N7-guanyl]ethyl)methylamine. All three adducts were produced interstrand, while N3A-N7G-EMA was the dominant intrastrand G-A cross-link. The prevalent adenine-adenine mechlorethamine lesions have the structure of N,N-bis(2-[N3-adenyl]ethyl)methylamine (bis-N3A-EMA). DNA-derived lesions have the same HPLC retention times, UV spectra, and MS/MS fragmentation patterns as the authentic standards prepared independently. bis-N3A-EMA lesions were produced in a concentration-dependent manner in calf thymus DNA treated with increasing amounts of mechlorethamine. Furthermore, HPLC-ESI-MS/MS analysis was used to demonstrate the formation of analogous N3-N3 adenine lesions in DNA treated with aromatic nitrogen mustards, N,N-bis(2-chloroethyl)-p-aminophenylbutyric acid and L-phenylalanine mustard. The presence of cross-linked adenine-adenine lesions may explain the enhanced cytotoxicity and mutagenicity of NMs in cells deficient in N3-alkyladenine glycosylase.

Design, synthesis, and evaluation of estradiol-linked genotoxicants as anti-cancer agents

A series of bifunctional compounds was prepared consisting of 17beta estradiol linked to a DNA damaging N,N-bis-(2-chloroethyl)aniline. The objective of our studies was to determine the characteristics of the linker that permitted both reaction with DNA and binding of the resultant covalent adducts to the estrogen receptor. Linker characteristics were pivotal determinants underlying the ability of the compounds to kill selectively breast cancer cells that express the estrogen receptor.