Design, Synthesis, and Biological Evaluation of (Hydroxyphenyl)naphthalene and -quinoline Derivatives: Potent and Selective Nonsteroidal Inhibitors of 17β-Hydroxysteroid Dehydrogenase Type 1 (17β-HSD1) for the Treatment of Estrogen-Dependent Diseases

Brønsted Acid-Catalyzed Cascade Ring-Opening/Cyclization of 3-Ethoxy Cyclobutanones to Access 2,8-Dioxabicyclo[3.3.1]nonane Derivatives

A cascade ring opening of 3-ethoxy cyclobutanones followed by a double cyclization strategy has been developed via Brønsted acid catalysis. A range of 2,8-dioxabicyclo[3.3.1]nonanes are obtained from various substituted naphthols in a one-pot and open flux manner. Additionally, a 15-membered macrocycle has been synthesized by ring closing metathesis as a synthetic application.

Back in Person: Frontiers in Medicinal Chemistry 2023

The Frontiers in Medicinal Chemistry (FiMC) is the largest international Medicinal Chemistry conference in the German speaking area and took place from April 3rd to 5th 2023 in Vienna (Austria). Fortunately, after being cancelled in 2020 and two years (2021-2022) of entirely virtual meetings, due to the COVID-19 pandemic, the FiMC could be held in a face-to-face format again. Organized by the Division of Medicinal Chemistry of the German Chemical Society (GDCh), the Division of Pharmaceutical and Medicinal Chemistry of the German Pharmaceutical Society (DPhG), together with the Division of Medicinal Chemistry of the Austrian Chemical Society (GÖCH), the Austrian Pharmaceutical Society (ÖPhG), and a local organization committee from the University of Vienna headed by Thierry Langer, the meeting brought together 260 participants from 21 countries. The program included 38 lectures by leading scientists from industry and academia as well as early career investigators. Moreover, 102 posters were presented in two highly interactive poster sessions.

MeOTf/KI-catalyzed efficient synthesis of 2-arylnaphthalenes via cyclodimerization of styrene oxides

The MeOTf/KI-catalyzed synthesis of 2-arylnaphthalene derivatives from aryl ethylene oxides in alcohol under ambient conditions is described. The present protocol has a higher atom efficiency and wider substrate applicability with excellent yields. The reaction proceeded using the aryl ethylene oxides to give 2-arylnaphthalenes either in homo-coupling or in cross-coupling. The reaction could also be carried out at the gram scale in minutes.

Inhibitors of 17β-hydroxysteroid dehydrogenase type 1, 2 and 14: Structures, biological activities and future challenges

During the past 25 years, the modulation of estrogen action by inhibition of 17β-hydroxysteroid dehydrogenase types 1 and 2 (17β-HSD1 and 17β-HSD2), respectively, has been pursued intensively. In the search for novel treatment options for estrogen-dependent diseases (EDD) and in order to explore estrogenic signaling pathways, a large number of steroidal and nonsteroidal inhibitors of these enzymes has been described in the literature. The present review gives a survey on the development of inhibitor classes as well as the structural formulas and biological properties of their most interesting representatives. In addition, rationally designed dual inhibitors of both 17β-HSD1 and steroid sulfatase (STS) as well as the first inhibitors of 17β-HSD14 are covered.

Estrogen signaling: An emanating therapeutic target for breast cancer treatment

Breast cancer, a most common malignancy in women, was known to be associated with steroid hormone estrogen. The discovery of estrogen receptor (ER) gave us not only a powerful predictive and prognostic marker, but also an efficient target for the treatment of hormone-dependent breast cancer with various estrogen ligands. ER consists of two subtypes i.e. ERα and ERβ, that are mostly G-protein-coupled receptors and activated by estrogen, specially 17β-estradiol. The activation is followed by translocation into the nucleus and binding with DNA to modulate activities of different genes. ERs can manage synthesis of RNA through genomic actions without directly binding to DNA. Receptors are tethered by protein-protein interactions to a transcription factor complex to communicate with DNA. Estrogens also exhibit nongenomic actions, a characteristic feature of steroid hormones, which are so rapid to be considered by the activation of RNA and translation. These are habitually related to stimulation of different protein kinase cascades. Majority of post-menopausal breast cancer is estrogen dependent, mostly potent biological estrogen (E2) for continuous growth and proliferation. Estrogen helps in regulating the differentiation and proliferation of normal breast epithelial cells. In this review we have investigated the important role of ER in development and progression of breast cancer, which is complicated by receptor's interaction with co-regulatory proteins, cross-talk with other signal transduction pathways and development of treatment strategies viz. selective estrogen receptor modulators (SERMs), selective estrogen receptor down regulators (SERDs), aromatase and sulphatase inhibitors.

Virtual screening applications in short-chain dehydrogenase/reductase research

Several members of the short-chain dehydrogenase/reductase (SDR) enzyme family play fundamental roles in adrenal and gonadal steroidogenesis as well as in the metabolism of steroids, oxysterols, bile acids, and retinoids in peripheral tissues, thereby controlling the local activation of their cognate receptors. Some of these SDRs are considered as promising therapeutic targets, for example to treat estrogen-/androgen-dependent and corticosteroid-related diseases, whereas others are considered as anti-targets as their inhibition may lead to disturbances of endocrine functions, thereby contributing to the development and progression of diseases. Nevertheless, the physiological functions of about half of all SDR members are still unknown. In this respect, in silico tools are highly valuable in drug discovery for lead molecule identification, in toxicology screenings to facilitate the identification of hazardous chemicals, and in fundamental research for substrate identification and enzyme characterization. Regarding SDRs, computational methods have been employed for a variety of applications including drug discovery, enzyme characterization and substrate identification, as well as identification of potential endocrine disrupting chemicals (EDC). This review provides an overview of the efforts undertaken in the field of virtual screening supported identification of bioactive molecules in SDR research. In addition, it presents an outlook and addresses the opportunities and limitations of computational modeling and in vitro validation methods.

Type 2 17-β hydroxysteroid dehydrogenase as a novel target for the treatment of osteoporosis

Low estradiol level in postmenopausal women is implicated in osteoporosis, which occurs because of the high bone resorption rate. Estrogen formation is controlled by 17-β hydroxysteroid dehydrogenase 17-β HSD enzymes, where 17-β HSD type 1 contributes in the formation of estradiol, while type 2 catalyzes its catabolism. Inhibiting 17-β HSD2 can help in increasing estradiol concentration. Several promising 17-β HSD2 inhibitors that can act at low nanomolar range have been identified. However, there are some specific challenges associated with the application of these compounds. Our review provides an up-to-date summary of the current status and recent progress in the production of 17-β HSD2 inhibitors as well as the future challenges in their clinical application.

Current knowledge of the multifunctional 17β-hydroxysteroid dehydrogenase type 1 (HSD17B1)

At the late 1940s, 17β-HSD1 was discovered as the first member of the 17β-HSD family with its gene cloned. The three-dimensional structure of human 17β-HSD1 is the first example of any human steroid converting enzyme. The human enzyme's structure and biological function have thus been studied extensively in the last two decades. In humans, the enzyme is expressed in placenta, ovary, endometrium and breast. The high activity of estrogen activation provides the basis of 17β-HSD1's implication in estrogen-dependent diseases, such as breast cancer, endometriosis and non-small cell lung carcinomas. Its dual function in estrogen activation and androgen inactivation has been revealed in molecular and breast cancer cell levels, significantly stimulating the proliferation of such cells. The enzyme's overexpression in breast cancer was demonstrated by clinical samples. Inhibition of human 17β-HSD1 led to xenograft tumor shrinkage. Unfortunately, through decades of studies, there is still no drug using the enzyme's inhibitors available. This is due to the difficulty to get rid of the estrogenic activity of its inhibitors, which are mostly estrogen analogues. New non-steroid inhibitors for the enzyme provide new hope for non-estrogenic inhibitors of the enzyme.

Synthesis of 2-phenylnaphthalenes from styryl-2-methoxybenzenes

A new simple and efficient method for the synthesis of 2-phenylnaphthalenes from electron-rich 1-styryl-2-methoxybenzenes has been described. The reaction proceeds via TFA catalyzed C-C bond cleavage followed by intermolecular [4+2]-Diels-Alder cycloaddition of an in situ formed styrenyl trifluoroacetate intermediate. The quantum chemical calculations identified the transition state for the cycloaddition reaction and helped in tracing the reaction mechanism. The method has been efficiently utilized for synthesis of the phenanthrene skeleton and a naphthalene-based potent and selective ER-β agonist.

Scope of the two-step, one-pot palladium-catalyzed borylation/Suzuki cross-coupling reaction utilizing bis-boronic acid

The use of bis-boronic acid for the direct synthesis of boronic acids has greatly facilitated the two-step, one-pot borylation/Suzuki cross-coupling reaction between aryl and heteroaryl halides. With use of Buchwald's second-generation XPhos preformed catalyst, high yields of cross-coupled products were obtained for most substrates. The method also allows an efficient two-step, one-pot synthesis, providing access to three distinct cross-coupled products after column chromatography. The method also provides a rapid and convenient route to teraryl compounds.

Development of potent and selective inhibitors of aldo-keto reductase 1C3 (type 5 17β-hydroxysteroid dehydrogenase) based on N-phenyl-aminobenzoates and their structure-activity relationships

Aldo-keto reductase 1C3 (AKR1C3; type 5 17β-hydroxysteroid dehydrogenase) is overexpressed in castration resistant prostate cancer (CRPC) and is implicated in the intratumoral biosynthesis of testosterone and 5α-dihydrotestosterone. Selective AKR1C3 inhibitors are required because compounds should not inhibit the highly related AKR1C1 and AKR1C2 isoforms which are involved in the inactivation of 5α-dihydrotestosterone. NSAIDs, N-phenylanthranilates in particular, are potent but nonselective AKR1C3 inhibitors. Using flufenamic acid, 2-{[3-(trifluoromethyl)phenyl]amino}benzoic acid, as lead compound, five classes of structural analogues were synthesized and evaluated for AKR1C3 inhibitory potency and selectivity. Structure-activity relationship (SAR) studies revealed that a meta-carboxylic acid group relative to the amine conferred pronounced AKR1C3 selectivity without loss of potency, while electron withdrawing groups on the phenylamino B-ring were optimal for AKR1C3 inhibition. Lead compounds did not inhibit COX-1 or COX-2 but blocked the AKR1C3 mediated production of testosterone in LNCaP-AKR1C3 cells. These compounds offer promising leads toward new therapeutics for CRPC.

Hydroxybenzothiazoles as new nonsteroidal inhibitors of 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1)

17β-estradiol (E2), the most potent estrogen in humans, known to be involved in the development and progession of estrogen-dependent diseases (EDD) like breast cancer and endometriosis. 17β-HSD1, which catalyses the reduction of the weak estrogen estrone (E1) to E2, is often overexpressed in breast cancer and endometriotic tissues. An inhibition of 17β-HSD1 could selectively reduce the local E2-level thus allowing for a novel, targeted approach in the treatment of EDD. Continuing our search for new nonsteroidal 17β-HSD1 inhibitors, a novel pharmacophore model was derived from crystallographic data and used for the virtual screening of a small library of compounds. Subsequent experimental verification of the virtual hits led to the identification of the moderately active compound 5. Rigidification and further structure modifications resulted in the discovery of a novel class of 17β-HSD1 inhibitors bearing a benzothiazole-scaffold linked to a phenyl ring via keto- or amide-bridge. Their putative binding modes were investigated by correlating their biological data with features of the pharmacophore model. The most active keto-derivative 6 shows IC₅₀-values in the nanomolar range for the transformation of E1 to E2 by 17β-HSD1, reasonable selectivity against 17β-HSD2 but pronounced affinity to the estrogen receptors (ERs). On the other hand, the best amide-derivative 21 shows only medium 17β-HSD1 inhibitory activity at the target enzyme as well as fair selectivity against 17β-HSD2 and ERs. The compounds 6 and 21 can be regarded as first benzothiazole-type 17β-HSD1 inhibitors for the development of potential therapeutics.

Triazole ring-opening leads to the discovery of potent nonsteroidal 17β-hydroxysteroid dehydrogenase type 2 inhibitors

17β-Hydroxysteroid dehydrogenase type 2 (17β-HSD2) catalyzes the oxidation of the highly potent steroids: the estrogen estradiol (E2) and the androgen testosterone (T) to the less active estrone and androstenedione, respectively. Inhibition of this enzyme may help maintain the local E2 level in bone tissue when the circulating E2 level drops and is therefore a novel and promising approach for the treatment of osteoporosis. In this work, a series of new nonsteroidal and achiral 17β-HSD2 inhibitors, namely N-benzyl-diphenyl-3(or 4)-carboxamide and N-benzyl-5-phenyl-thiophene-2-carboxamide was designed and the compounds were synthesized in a two to three steps reaction. A small library was built applying parallel synthesis. Highly potent 17β-HSD2 inhibitors could be identified in the thiophene-2-carboxamide class with IC(50) in the low nanomolar range. These compounds also showed a good selectivity profile toward 17β-HSD1 and toward the estrogen receptors α and β. The most interesting 17β-HSD2 inhibitor identified in this study is the 5-(2-fluoro-3-methoxyphenyl)-N-(3-hydroxybenzyl)-N-methylthiophene-2-carboxamide 6w displaying an IC(50) of 61 nM and a selectivity factor of 73 toward 17β-HSD1.

Structural basis for species specific inhibition of 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1): computational study and biological validation

17β-Hydroxysteroid dehydrogenase type 1 (17β-HSD1) catalyzes the reduction of estrone to estradiol, which is the most potent estrogen in humans. Inhibition of 17β-HSD1 and thereby reducing the intracellular estradiol concentration is thus a promising approach for the treatment of estrogen dependent diseases. In the past, several steroidal and non-steroidal inhibitors of 17β-HSD1 have been described but so far there is no cocrystal structure of the latter in complex with 17β-HSD1. However, a distinct knowledge of active site topologies and protein-ligand interactions is a prerequisite for structure-based drug design and optimization. An elegant strategy to enhance this knowledge is to compare inhibition values obtained for one compound toward ortholog proteins from various species, which are highly conserved in sequence and differ only in few residues. In this study the inhibitory potencies of selected members of different non-steroidal inhibitor classes toward marmoset 17β-HSD1 were determined and the data were compared with the values obtained for the human enzyme. A species specific inhibition profile was observed in the class of the (hydroxyphenyl)naphthols. Using a combination of computational methods, including homology modelling, molecular docking, MD simulation, and binding energy calculation, a reasonable model of the three-dimensional structure of marmoset 17β-HSD1 was developed and inhibition data were rationalized on the structural basis. In marmoset 17β-HSD1, residues 190 to 196 form a small α-helix, which induces conformational changes compared to the human enzyme. The docking poses suggest these conformational changes as determinants for species specificity and energy decomposition analysis highlighted the outstanding role of Asn152 as interaction partner for inhibitor binding. In summary, this strategy of comparing the biological activities of inhibitors toward highly conserved ortholog proteins might be an alternative to laborious x-ray or site-directed mutagenesis experiments in certain cases. Additionally, it facilitates inhibitor design and optimization by offering new information on protein-ligand interactions.

A succinct synthesis of the vaulted biaryl ligand vanol via a dienone-phenol rearrangement

Vanol is a member of the vaulted biaryl family of ligands and it has been proven to be very effective in a number of asymmetric catalytic reactions. The previous synthesis of vanol, while effective, is limited by the cost of reagents involved. The present work evaluates three different approaches to the synthesis of 3-phenyl-1-naphthol, a key intermediate in the synthesis of vanol. The first approach has its key step as the Michael addition of a benzyl Grignard to methyl cinnamate. In the second approach the key step is the first step, a Reformatsky reaction of ethyl bromoacetate and deoxybenzoin. The final and most-efficient approach involves a dienone-phenol rearrangement of a 4-aryl-1-tetralenone generated in-situ from the reaction of 4-chloro-1-naphthol with AlCl(3) and benzene, and preliminary results are reported on the extension of this method to substituted vanol derivatives.

Steroid sulfatase inhibitors: promising new tools for breast cancer therapy?

Inhibition of aromatase is currently well-established as the major treatment option of hormone-dependent breast cancer in postmenopausal women. However, despite the effects of aromatase inhibitors in both early and metastatic breast cancer, endocrine resistance may cause relapses of the disease and progression of metastasis. Thus, driven by the success of manipulating the steroidogenic enzyme aromatase, several alternative enzymes involved in steroid synthesis and metabolism have recently been investigated as possible drug targets. One of the most promising targets is the steroid sulfatase (STS) which converts steroid sulfates like estrone sulfate (E1S) and dehydroepiandrosterone sulfate (DHEAS) to estrone (E1) and dehydroepiandrosterone (DHEA), respectively. Estrone and DHEA may thereafter be used for the synthesis of more potent estrogens and androgens that may eventually fuel hormone-sensitive breast cancer cells. The present review summarizes the biology behind steroid sulfatase and its inhibition, the currently available information derived from basic and early clinical trials in breast cancer patients, as well as ongoing research. Article from the Special Issue on Targeted Inhibitors.

Contribution to the development of inhibitors of 17β-hydroxysteroid dehydrogenase types 1 and 7: key tools for studying and treating estrogen-dependent diseases

17β-Hydroxysteroid dehydrogenases (17β-HSDs) belong to a group of key enzymes involved in the biosynthesis of steroidal hormones by catalyzing the reduction of 17-ketosteroids or the oxidation of 17β-hydroxysteroids. From three members known in the early nineties, the 17β-HSD functional family has grown to 15 members over the last 20 years. This growing number of 17β-HSD isoforms questioned the importance of each member, especially in their implication in estrogen- and androgen-dependent diseases, such as breast and prostate cancers. One of the strategies used to address the physiological importance of 17β-HSDs is to use potent and selective inhibitors. Furthermore, enzyme inhibitors could also be of therapeutic interest by reducing the level of estradiol (E2). Focusing on estrogens, we targeted 17β-HSD types 1 and 7, two enzymes able to transform the weak estrogen estrone (E1) into the potent estrogen E2. The present review article gives a description of different classes of inhibitors of 17β-HSD1 (C6-derivatives of E2, C16-derivatives of E2 as alkylating and dual action compounds, E2-adenosine hybrids, E2-simplified adenosine hybrids, and C16-derivatives of E1 or E2) and of inhibitors of 17β-HSD7, all these inhibitors developed in our laboratory. The chemical structures and inhibitory activity of these steroidal inhibitors, their potential as therapeutic agents, and their use as tools to elucidate the role of these enzymes in particular biological systems will be discussed. Article from the Special issue on Targeted Inhibitors.

Human hydroxysteroid dehydrogenases and pre-receptor regulation: insights into inhibitor design and evaluation

Hydroxysteroid dehydrogenases (HSDs) represent a major class of NAD(P)(H) dependent steroid hormone oxidoreductases involved in the pre-receptor regulation of hormone action. This is achieved by HSDs working in pairs so that they can interconvert ketosteroids with hydroxysteroids resulting in a change in ligand potency for nuclear receptors. HSDs belong to two protein superfamilies the aldo-keto reductases and the short-chain dehydrogenase/reductases. In humans, many of the important enzymes have been thoroughly characterized including the elucidation of their three-dimensional structures. Because these enzymes play fundamental roles in steroid hormone action they can be considered to be drug targets for a variety of steroid driven diseases, e.g. metabolic syndrome and obesity, inflammation, and hormone dependent malignancies of the endometrium, prostate and breast. This article will review how fundamental knowledge of these enzymes can be exploited in the development of isoform specific HSD inhibitors from both protein superfamilies. Article from the Special issue on Targeted Inhibitors.

17β-Hydroxysteroid dehydrogenases (17β-HSDs) as therapeutic targets: protein structures, functions, and recent progress in inhibitor development

17β-Hydroxysteroid dehydrogenases (17β-HSDs) are oxidoreductases, which play a key role in estrogen and androgen steroid metabolism by catalyzing final steps of the steroid biosynthesis. Up to now, 14 different subtypes have been identified in mammals, which catalyze NAD(P)H or NAD(P)(+) dependent reductions/oxidations at the 17-position of the steroid. Depending on their reductive or oxidative activities, they modulate the intracellular concentration of inactive and active steroids. As the genomic mechanism of steroid action involves binding to a steroid nuclear receptor, 17β-HSDs act like pre-receptor molecular switches. 17β-HSDs are thus key enzymes implicated in the different functions of the reproductive tissues in both males and females. The crucial role of estrogens and androgens in the genesis and development of hormone dependent diseases is well recognized. Considering the pivotal role of 17β-HSDs in steroid hormone modulation and their substrate specificity, these proteins are promising therapeutic targets for diseases like breast cancer, endometriosis, osteoporosis, and prostate cancer. The selective inhibition of the concerned enzymes might provide an effective treatment and a good alternative to the existing endocrine therapies. Herein, we give an overview of functional and structural aspects for the different 17β-HSDs. We focus on steroidal and non-steroidal inhibitors recently published for each subtype and report on existing animal models for the different 17β-HSDs and the respective diseases. Article from the Special issue on Targeted Inhibitors.

Design and synthesis of substrate mimetics based on an indole scaffold: potential inhibitors of 17β-HSD type 1

BACKGROUND

Human 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1) acts at a pre-receptor level. It catalyzes NADPH-dependent reduction of the weak estrogen estrone into the most potent estrogen estradiol, which exerts its proliferative effects via estrogen receptors. Overexpression of 17β-HSD1 in estrogen-responsive tissues is related to the development of hormone-dependent diseases, such as breast cancer and endometriosis. 17β-HSD1 thus represents an attractive target for development of new drugs.

METHODS

We designed and synthesized a series of 3-, 5- and 6-phenyl indole derivatives as mimetics of the steroid substrate estrone. All of these compounds were evaluated for inhibition of recombinant human 17β-HSD1 from Escherichia coli, at concentrations of 0.6 μM and 6.0 μM.

RESULTS

Among 14 indole derivatives, compound 9 was an initial hit inhibitor of 17β-HSD1, with moderate inhibition (64% at 6 μM). Molecular docking into the crystal structure of 17β-HSD1 (1A27) revealed that this 5-phenyl indole derivative binds to 17β-HSD1 similarly to co-crystalized E2. Compound 9 forms two H-bonds with 17β-HSD1: one between the indole nitrogen and His222, and the second between the phenolic OH group and catalytic Tyr155.

CONCLUSIONS

The indole scaffold is one of the possible starting points for the design of substrate mimetics of the steroid substrate estrone. Our study shows that these 6- and, especially, 5-phenol indole derivatives can act as moderate inhibitors of 17β-HSD1. Based on inhibition assays and docking simulations, we can infer further improvements of the 5-phenol indole derivatives that might result in better inhibition profiles.

First Selective CYP11B1 Inhibitors for the Treatment of Cortisol-Dependent Diseases

Outgoing from an etomidate-based design concept, we succeeded in the development of a series of highly active and selective inhibitors of CYP11B1, the key enzyme of cortisol biosynthesis, as potential drugs for the treatment of Cushing's syndrome and related diseases. Thus, compound 33 (IC50 = 152 nM) is the first CYP11B1 inhibitor showing a rather good selectivity toward the most important steroidogenic CYP enzymes aldosterone synthase (CYP11B2), the androgen-forming CYP17, and aromatase (estrogen synthase, CYP19).

Bicyclic substituted hydroxyphenylmethanones as novel inhibitors of 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1) for the treatment of estrogen-dependent diseases

Estradiol (E2), the most important estrogen in humans, is involved in the initiation and progression of estrogen-dependent diseases such as breast cancer and endometriosis. Its local production in the target cell is regulated by 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1), which catalyzes E2-formation by reduction of the weak estrogen estrone (E1). Because the enzyme is expressed in the diseased tissues, inhibition of 17β-HSD1 is considered as a promising therapy for the treatment of estrogen-dependent diseases. For the development of novel inhibitors, a structure- and ligand-based design strategy was applied, resulting in bicyclic substituted hydroxyphenylmethanones. In vitro testing revealed high inhibitory potencies toward human placental 17β-HSD1. Compounds were further evaluated with regard to selectivity (17β-HSD2, estrogen receptors ERα and ERβ), intracellular activity (T47D cells), and metabolic stability. The most promising compounds, 14 and 15, showed IC(50) values in the low nanomolar range in the cell-free and cellular assays (8-27 nM), more than 30-fold selectivity toward 17β-HSD2 and no affinity toward the ERs. The data obtained make these inhibitors interesting candidates for further preclinical evaluation.

Molecular therapy of breast cancer: progress and future directions

Breast cancer is a major cause of death in Western women, with a 10% lifetime risk of the disease. Most breast cancers are estrogen-dependent. Molecular therapies for breast cancer have developed rapidly in the past few decades and future treatment strategies are being investigated. The selective estrogen receptor (ER) modulator tamoxifen, which until now has served as a standard therapy, functions not only as an estrogen antagonist but also as an estrogen agonist in terms of bone maintenance. Aromatase inhibitors have performed well in international trials and have become a new standard therapy for estrogen-dependent breast cancer. The systematic study of estrogen activation pathways suggests that the enzymes steroid sulfatase and 17beta-hydroxysteroid dehydrogenase type 1, which both have pivotal roles in estrogen biosynthesis, are promising targets; the results of a phase I trial of steroid sulfatase inhibitors are encouraging. The activity of the human epidermal growth factor receptor (HER) pathway correlates negatively with that of the ER. HER2 is overexpressed in 22% of all breast cancers. In the decade since HER2 began being targeted, the monoclonal antibody trastuzumab has been used as well as pertuzumab and HER2 vaccines. Among the estrogen-independent breast cancers, the basal-like subtype has low survival, and therapeutic improvement is a priority. Crosstalk between ER and HER2 signaling pathways means that combinatory therapies may hold the key to enhancement of treatment responses. Other molecular therapies involving functional genomics and RNA interference studies also hold promise.