Discovery of a Non-Estrogenic Irreversible Inhibitor of 17β-Hydroxysteroid Dehydrogenase Type 1 from 3-Substituted-16β-(m-carbamoylbenzyl)-estradiol Derivatives

Emerging and Re-emerging Warheads for Targeted Covalent Inhibitors: An Update

Covalent inhibitors and other types of covalent modalities have seen a revival in the past two decades, with a variety of new targeted covalent drugs having been approved in recent years. A key feature of such molecules is an intrinsically reactive group, typically a weak electrophile, which enables the irreversible or reversible formation of a covalent bond with a specific amino acid of the target protein. This reactive group, often called the "warhead", is a critical determinant of the ligand's activity, selectivity, and general biological properties. In 2019, we summarized emerging and re-emerging warhead chemistries to target cysteine and other amino acids (Gehringer, M.; Laufer, S. A. J. Med. Chem. 2019, 62, 5673-5724; DOI: 10.1021/acs.jmedchem.8b01153). Since then, the field has rapidly evolved. Here we discuss the progress on covalent warheads made since our last Perspective and their application in medicinal chemistry and chemical biology.

Description of Chemical Synthesis, Nuclear Magnetic Resonance Characterization and Biological Activity of Estrane-Based Inhibitors/Activators of Steroidogenesis

Steroid hormones play a crucial role in several aspects of human life, and steroidogenesis is the process by which hormones are produced from cholesterol using several enzymes that work in concert to obtain the appropriate levels of each hormone at the right time. Unfortunately, many diseases, such as cancer, endometriosis, and osteoporosis as examples, are caused by an increase in the production of certain hormones. For these diseases, the use of an inhibitor to block the activity of an enzyme and, in doing so, the production of a key hormone is a proven therapeutic strategy whose development continues. This account-type article focuses on seven inhibitors (compounds 1-7) and an activator (compound 8) of six enzymes involved in steroidogenesis, namely steroid sulfatase, aldo-keto reductase 1C3, types 1, 2, 3, and 12 of the 17β-hydroxysteroid dehydrogenases. For these steroid derivatives, three topics will be addressed: (1) Their chemical synthesis from the same starting material, estrone, (2) their structural characterization using nuclear magnetic resonance, and (3) their in vitro or in vivo biological activities. These bioactive molecules constitute potential therapeutic or mechanistic tools that could be used to better understand the role of certain hormones in steroidogenesis.

Chemical Synthesis and Biological Evaluation of 3-Substituted Estrone/Estradiol Derivatives as 17β-Hydroxysteroid Dehydrogenase Type 1 Inhibitors Acting via a Reverse Orientation of the Natural Substrate Estrone

Estradiol (E2) plays an important role in the progression of diseases such as breast cancer and endometriosis. Inhibition of 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1), the enzyme that catalyzes the last step in the biosynthesis of the estrogenic hormone E2, therefore constitutes an interesting approach for the treatment of these two estrogen-dependent diseases. In order to obtain new inhibitors of 17β-HSD1, the impact of a m-carbamoylphenyloxy group at position three of an estrane nucleus was evaluated by preparing three derivatives of estrone (E1) and E2 using a microwave-assisted synthesis of diaryl ethers. Their inhibitory activity was addressed on two cell lines (T-47D and Z-12) representative of breast cancer and endometriosis, respectively, but unlike T-47D cells, Z-12 cells were not found suitable for testing potential 17β-HSD1 inhibitors. Thus, the addition of the m-carbamoylphenyl group at C3 of E1 (compound 5) did not increase the inhibition of E1 to E2 transformation by 17β-HSD1 present in T-47D cells (IC₅₀ = 0.31 and 0.21 μM for 5 and E1, respectively), and this negative effect was more obvious for E2 derivatives 6 and 10 (IC₅₀ = 1.2 and 1.3 μM, respectively). Molecular docking allowed us to identify key interactions with 17β-HSD1 and to highlight these new inhibitors' actions through an opposite orientation than natural enzyme substrate E1's classical one. Furthermore, molecular modeling experiments explain the better inhibitory activity of E1-ether derivative 5, as opposed to the E2-ether derivatives 6 and 10. Finally, when tested on T-47D and Z-12 cells, compounds 5, 6 and 10 did not stimulate the proliferation of these two estrogen-dependent cell lines. In fact, they reduced it.

An irreversible inhibitor of 17β-hydroxysteroid dehydrogenase type 1 inhibits estradiol synthesis in human endometriosis lesions and induces regression of the non-human primate endometriosis

Endometriosis is a gynecological disorder affecting about 10% of women and can lead to invalidating painful symptoms and infertility. Since there is no current definitive cure for this disease, new therapeutic options are necessary. 17β-Hydroxysteroid dehydrogenase type 1 (17β-HSD1) is involved in the production of estradiol (E2), the most potent estrogen in women, and of 5-androstene-3β,17β-diol (5-diol), a weaker estrogen than E2, but whose importance increases after menopause. 17β-HSD1 is therefore a pharmacological target of choice for the treatment of estrogen-dependent diseases such as endometriosis. We developed a targeted-covalent (irreversible) and non-estrogenic inhibitor of 17β-HSD1, a molecule named PBRM, and herein evaluated its efficiency for the treatment of endometriosis. In a cell-free assay containing estrone (E1), the natural substrate of 17β-HSD1, PBRM was able to block the formation of E2 in a collection of 50 human endometriosis lesions from a different clinical feature type, location, and phase. When given orally by gavage at 15 mg/kg to baboons, the resulting plasmatic concentration of PBRM was found to be sufficiently high (up to 125 ng/mL) for an efficacy study in a non-human primate (baboon) endometriosis model. After 2 months of treatment, the number of lesions/adhesions decreased in 60% of animals (3/5) in the PBRM-treated group, compared to the placebo group which showed an increase in the number of lesion/adhesions in 60% (3/5) of animals. Indeed, the total number of lesions/adhesions decreased in treated group (-6.5 or -19% when excluding one animal) while it increased in the control group receiving a placebo (+11%). Analysis of specific endometriotic lesions revealed that PBRM decreased the number of red lesions (-67%; 8/12) and white lesions (-35%; 11/31), but not of blue-black lesions. Similarly, PBRM decreased the surface area of dense adhesions and filmy adhesions, as compared to placebo. Also, PBRM treatment did not significantly affect the number of menstrual days. Finally, this targeted covalent inhibitor showed no adverse effects and no apparent toxicity for the duration of the treatment. These data indicate that 17β-HSD1 inhibitor PBRM is a promising candidate for therapy targeting endometriosis and supports the need of additional efforts toward clinical trials.

C-Ring Oxidized Estrone Acetate Derivatives: Assessment of Antiproliferative Activities and Docking Studies

C-Ring oxidized estrone acetate derivatives as antiproliferative agents were prepared and tested against five cancer cell lines by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Flow cytometry assays to evaluate cell viability and modifications in cell cycle phases and molecular docking research against estrogen receptor α, steroid sulfatase, and 17β-hydroxysteroid dehydrogenase type 1 were performed. 9α-Hydroxy,11β-nitrooxyestrone acetate was the most cytotoxic molecule against hormone-dependent cancer cells. Furthermore, flow cytometry experiments revealed that this 9α-hydroxy,11β-nitrooxy derivative markedly reduced HepaRG cells viability (~92%) after 24 h of treatment. However, 9α-hydroxyestrone acetate led to selective inhibition of HepaRG cells growth, inducing a G0/G1 cycle arrest, and did not originate a proliferation effect on T47-D cancer cells. Docking studies estimated a generally lower affinity of these compounds to estrogen receptor α than predicted for estrone and 17β-estradiol. Therefore, this structural modification can be of interest to develop new anticancer estrane derivatives devoid of estrogenic action.

Recent Applications of Pd-Catalyzed Suzuki–Miyaura and Buchwald–Hartwig Couplings in Pharmaceutical Process Chemistry

Cross-coupling reactions have changed the way complex molecules are synthesized. In particular, Suzuki–Miyaura and Buchwald–Hartwig amination reactions have given opportunities to elegantly make pharmaceutical ingredients. Indeed, these reactions are at the forefront of both the stages of drug development, medicinal chemistry, and process chemistry. On the one hand, these reactions have given medicinal chemists a resource to derivatize the core compound to arrive at scaffold rapidly. On the other hand, these cross couplings have offered the process chemists a smart tool to synthesize the development candidates safely, quickly, and efficiently. Generally, the application of cross-coupling reactions is broad. This review will specifically focus on their real (pharma) world applications in large-scale synthesis appearing in the last three years.

Synthesis of 16β-derivatives of 3-(2-bromoethyl)-estra-1,3,5(10)-trien-17β-ol as inhibitors of 17β-HSD1 and/or steroid sulfatase for the treatment of estrogen-dependent diseases

17β-Hydroxysteroid dehydrogenase type 1 (17β-HSD1) and steroid sulfatase (STS) are involved in the synthesis of the most potent estrogen in the human body, estradiol (E2). These enzymes are known to play a pivotal role in the progression of estrogen-dependent diseases, such as breast cancer and endometriosis. Therefore, the inhibition of 17β-HSD1 and/or STS represents a promising avenue to modulate the growth of estrogen-dependent tumors or lesions. We recently established the key role of a bromoethyl side chain added at the C3-position of a 16β-carbamoyl-benzyl-E2 nucleus to covalently inhibit 17β-HSD1. To extend the structure-activity relationship study to the C16β-position of this new selective irreversible inhibitor (PBRM), we synthesized a series of analog compounds by changing the nature of the C16β-side chain but keeping the 2-bromoethyl group at position C3. We determined their 17β-HSD1 inhibitions in T-47D cells (transformation of E1 into E2), but we did not obtain a stronger 17β-HSD1 inhibitor than PBRM. Compounds 16 and 17 were found to be more likely to bind to the catalytic site and showed a promising but moderate inhibitory activity with estimated IC₅₀ values of 0.5 and 0.7 µM, respectively (about 10 times higher than PBRM). Interestingly, adding one or two sulfamate groups in the D-ring's surroundings did not significantly decrease compounds' potential to inhibit 17β-HSD1, but clearly improved their potential to inhibit STS. These results open the door to the development of a new family of steroid derivatives with dual (17β-HSD1 and STS) inhibiting actions.

A Targeted-Covalent Inhibitor of 17β-HSD1 Blocks Two Estrogen-Biosynthesis Pathways: In Vitro (Metabolism) and In Vivo (Xenograft) Studies in T-47D Breast Cancer Models

Simple Summary

17β-Hydroxysteroid dehydrogenase type 1 (17β-HSD1) is responsible for the production of estrogens estradiol (E2) and 5-androsten-3β,17β-diol (5-diol). This enzyme is therefore a target of choice for the treatment of estrogen-dependent diseases such as breast cancer and endometriosis, by blocking estrogen biosynthesis. After we developed the first irreversible and non-estrogenic 17β-HSD1 inhibitor, a molecule named PBRM, our goal was to demonstrate its therapeutic potential. PBRM was able to block the formation of E2 and 5-diol in T-47D human breast cancer cells. When given orally to mice, PBRM was also able to block the tumor growth without any observed toxic effects. Thanks to its irreversible type of inhibition, PBRM retained its anti-tumor growth effect, even after reducing its frequency of administration to only once a week, a clear advantage over reversible inhibitors. These results strongly support the use of PBRM as a new approach in the treatment of breast cancer.

Abstract

17β-Hydroxysteroid dehydrogenase type 1 (17β-HSD1) plays an important role in estrogen-dependent breast tumor growth. In addition to being involved in the production of estradiol (E2), the most potent estrogen in women, 17β-HSD1 is also responsible for the production of 5-androsten-3β,17β-diol (5-diol), a weaker estrogen than E2, but whose importance increases after menopause. 17β-HSD1 is therefore a target of choice for the treatment of estrogen-dependent diseases such as breast cancer and endometriosis. After we developed the first targeted-covalent (irreversible) and non-estrogenic inhibitor of 17β-HSD1, a molecule named PBRM, our goal was to demonstrate its therapeutic potential. Enzymatic assays demonstrated that estrone (E1) and dehydroepiandrosterone (DHEA) were transformed into E2 and 5-diol in T-47D human breast cancer cells, and that PBRM was able to block these transformations. Thereafter, we tested PBRM in a mouse tumor model (cell-derived T-47D xenografts). After treatment of ovariectomized (OVX) mice receiving E1 or DHEA, PBRM given orally was able to reduce the tumor growth at the control (OVX) level without any observed toxic effects. Thanks to its irreversible type of inhibition, PBRM retained its anti-tumor growth effect, even after reducing its frequency of administration to only once a week, a clear advantage over reversible inhibitors.

Application of Various Molecular Modelling Methods in the Study of Estrogens and Xenoestrogens

In this review, applications of various molecular modelling methods in the study of estrogens and xenoestrogens are summarized. Selected biomolecules that are the most commonly chosen as molecular modelling objects in this field are presented. In most of the reviewed works, ligand docking using solely force field methods was performed, employing various molecular targets involved in metabolism and action of estrogens. Other molecular modelling methods such as molecular dynamics and combined quantum mechanics with molecular mechanics have also been successfully used to predict the properties of estrogens and xenoestrogens. Among published works, a great number also focused on the application of different types of quantitative structure-activity relationship (QSAR) analyses to examine estrogen's structures and activities. Although the interactions between estrogens and xenoestrogens with various proteins are the most commonly studied, other aspects such as penetration of estrogens through lipid bilayers or their ability to adsorb on different materials are also explored using theoretical calculations. Apart from molecular mechanics and statistical methods, quantum mechanics calculations are also employed in the studies of estrogens and xenoestrogens. Their applications include computation of spectroscopic properties, both vibrational and Nuclear Magnetic Resonance (NMR), and also in quantum molecular dynamics simulations and crystal structure prediction. The main aim of this review is to present the great potential and versatility of various molecular modelling methods in the studies on estrogens and xenoestrogens.

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.

Chemical synthesis of C3-oxiranyl/oxiranylmethyl-estrane derivatives targeted by molecular modeling and tested as potential inhibitors of 17β-hydroxysteroid dehydrogenase type 1

17β-Hydroxysteroid dehydrogenase type 1 (17β-HSD1) is a promising therapeutic target known to play a pivotal role in the progression of estrogen-dependent diseases such as breast cancer, and endometriosis. This enzyme is responsible for the last step in the biosynthesis of the most potent estrogen, estradiol (E2) and its inhibition would prevent the growth of estrogen-sensitive tumors. Based on molecular modeling with docking experiments, we identified two promising C3-oxiranyl/oxiranylmethyl-estrane derivatives that would bind competitively and irreversibly in the catalytic site of 17β-HSD1. They have been synthesized in a short and efficient route and their inhibitory activities over 17β-HSD1 have been assessed by an enzymatic assay. Compound 15, with an oxiranylmethyl group at position C3, was more likely to bind the catalytic site and showed an interesting, but weak, inhibitory activity with an IC₅₀ value of 1.3 µM (for the reduction of estrone into E2 in T-47D cells). Compound 11, with an oxiranyl at position C3, produced a lower inhibition rate, and the IC₅₀ value cannot be determined. When tested in estrogen-sensitive T-47D cells, both compounds were also slightly estrogenic, although much less than the estrogenic hormone E2.

Combined Biophysical Chemistry Reveals a New Covalent Inhibitor with a Low-Reactivity Alkyl Halide

17β-Hydroxysteroid dehydrogenase type 1 (17β-HSD1) plays a pivotal role in the progression of estrogen-related diseases because of its involvement in the biosynthesis of estradiol (E2), constituting a valuable therapeutic target for endocrine treatment. In the present study, we successfully cocrystallized the enzyme with the reversible inhibitor 2-methoxy-16β-( m-carbamoylbenzyl)-E2 (2-MeO-CC-156) as well as the enzyme with the irreversible inhibitor 3-(2-bromoethyl)-16β-( m-carbamoylbenzyl)-17β-hydroxy-1,3,5(10)-estratriene (PBRM). The structures of ternary complexes of 17β-HSD1-2-MeO-CC-156-NADP⁺ and 17β-HSD1-PBRM-NADP⁺ comparatively show the formation of a covalent bond between His²²¹ and the bromoethyl side chain of the inhibitor in the PBRM structure. A dynamic process including beneficial molecular interactions that favor the specific binding of a low-reactivity inhibitor and subsequent N-alkylation event through the participation of His²²¹ in the enzyme catalytic site clearly demonstrates the covalent bond formation. This finding opens the door to a new design of alkyl halide-based specific covalent inhibitors as potential therapeutic agents for different enzymes, contributing to the development of highly efficient inhibitors.

Structure Confirmation and Evaluation of a Nonsteroidal Inhibitor of 17β-Hydroxysteroid Dehydrogenase Type 10

17β-Hydroxysteroid dehydrogenase type 10 (17β-HSD10) is a steroidogenesis enzyme known for its potential role in Alzheimer’s disease. For comparison purposes between steroidal and nonsteroidal 17β-HSD10 inhibitors 1 and 2, respectively, we attempted the chemical synthesis of benzothiazole phosphonate derivative 2. Instead of a one-pot synthesis, we report a two-step synthesis with characterization of both imine intermediate 5 and final compound 2. Furthermore, complete assignation of 1H and 13C nuclear magnetic resonance (NMR) signals of 2 is provided, as we observed a divergence of NMR data with those published previously. Finally, biological assays showed that 1 and 2 inhibited the oxidation of estradiol (E2) into estrone (E1) by the 17β-HSD10 recombinant protein. However, in human embryonic kidney (HEK)-293 intact cells transfected with 17β-HSD10, only the steroidal inhibitor 1 induced a dose-dependent inhibition of E2 to E1 transformation.

Blocking oestradiol synthesis pathways with potent and selective coumarin derivatives

A comprehensive set of 3-phenylcoumarin analogues with polar substituents was synthesised for blocking oestradiol synthesis by 17-β-hydroxysteroid dehydrogenase 1 (HSD1) in the latter part of the sulphatase pathway. Five analogues produced ≥62% HSD1 inhibition at 5 µM and, furthermore, three of them produced ≥68% inhibition at 1 µM. A docking-based structure-activity relationship analysis was done to determine the molecular basis of the inhibition and the cross-reactivity of the analogues was tested against oestrogen receptor, aromatase, cytochrome P450 1A2, and monoamine oxidases. Most of the analogues are only modestly active with 17-β-hydroxysteroid dehydrogenase 2 – a requirement for lowering effective oestradiol levels in vivo. Moreover, the analysis led to the synthesis and discovery of 3-imidazolecoumarin as a potent aromatase inhibitor. In short, coumarin core can be tailored with specific ring and polar moiety substitutions to block either the sulphatase pathway or the aromatase pathway for treating breast cancer and endometriosis.

Pharmacokinetic profile of PBRM in rodents, a first selective covalent inhibitor of 17β-HSD1 for breast cancer and endometriosis treatments

The development of a covalent inhibitor of 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1) is a promising approach for the treatment of hormone-dependent breast cancer and endometriosis. After reporting the steroid derivative PBRM as a first potent covalent inhibitor of 17β-HSD1 without estrogenic activity, we are now interested in studying its pharmaceutical behavior. The metabolism study in a human liver microsomal preparation showed a gradual transformation of PBRM into PBRM-O, an oxidized ketonic form of PBRM at position C17. Interestingly, PBRM-O also inhibits 17β-HSD1 and is not estrogenic in estrogen-sensitive T-47D cells. However, when PBRM was injected subcutaneously (sc) in mice, a very small proportion of PBRM-O was measured in a 24 h-time course experiment. A pharmacokinetic study in mice revealed suitable values for half-life (T_1/2 = 3.4 h), clearance (CL = 2088 mL/h kg), distribution volume (Vz = 10.3 L/kg) and absolute bioavailability (F = 65%) when PBRM was injected sc at 14.7 mg/kg. A good F value of 33% was also obtained when PBRM was given orally. A tritiated version of PBRM, ³H-PBRM, was synthesized and used for an in vivo biodistribution study that showed its gradual accumulation in various mouse tissues (peak at 6 h) followed by elimination until complete disappearance after 72 h. Elimination was found to occur in feces (93%) and urine (7%) as revealed by a mass balance experiment. PBRM was also evaluated for its toxicity in mice and it was found to be very well tolerated after weekly sc administration (30-405 mg/kg for 8 weeks) or by po administration (300-900 mg/kg for 4 weeks). Overall, these experiments represent important steps in the preclinical characterization of the pharmaceutical behavior of PBRM, as well as for its translation to clinical trials.

Structure-Based Design and Synthesis of New Estrane-Pyridine Derivatives as Cytochrome P450 (CYP) 1B1 Inhibitors

Inhibition of cytochrome P450 (CYP) 1B1 is a promising therapeutic strategy, as such an inhibitor could modulate the bioactivation of procarcinogens while reducing drug resistance. Based on docking studies, the synthesis of 12 estra-1,3,5(10)-triene derivatives containing a pyridin-3-/4-yl moiety at position C2, C3, or C4 was performed, and we measured their inhibitory activity on CYP1B1 using the ethoxyresorufin-O-deethylase (EROD) assay. The position of the nitrogen atom in the aromatic ring has little influence on their inhibition potency, but compounds with a pyridinyl at C2 of the steroid nucleus are more potent CYP1B1 inhibitors than those with a pyridinyl at C3 or C4. Estradiol derivatives (OH at C17β) are also 10-fold more potent inhibitors than estrone derivatives (carbonyl at C17). Thus, 2-(pyridin-3-yl)-estradiol (4a) is the best CYP1B1 inhibitor (IC₅₀ = 0.011 μM) from this series of compounds, and the best steroid inhibitor reported until now. It is also 7.5-fold more potent than the well-known nonsteroidal CYP1B1 inhibitor α-naphthoflavone (IC₅₀ = 0.083 μM).

Design of a Mestranol 2-N-Piperazino-Substituted Derivative Showing Potent and Selective in vitro and in vivo Activities in MCF-7 Breast Cancer Models

Anticancer structure-activity relationship studies on aminosteroid (5α-androstane) derivatives have emerged with a promising lead candidate: RM-133 (2β-[1-(quinoline-2-carbonyl)pyrrolidine-2-carbonyl]-N-piperazine-5α-androstane-3α,17β-diol), which possesses high in vitro and in vivo activities against several cancer cells, and selectivity over normal cells. However, the relatively weak metabolic stability of RM-133 has been a drawback to its progression toward clinical trials. We investigated the replacement of the androstane backbone by a more stable mestranol moiety. The resulting compound, called RM-581 ({4-[17α-ethynyl-17β-hydroxy-3-methoxyestra-1,3,5(10)-trien-2-yl]piperazin-1-yl}[(2S)-1-(quinolin-2-ylcarbonyl)pyrrolidin-2-yl]methanone), was synthesized efficiently in only five steps from commercially available estrone. In comparison with RM-133, RM-581 was found to be twice as metabolically stable, retains potent cytotoxic activity in breast cancer MCF-7 cell culture, and fully blocks tumor growth in a mouse xenograft model of breast cancer. Advantageously, the selectivity over normal cells has been increased with this estrane version of RM-133. In fact, RM-581 showed a better selectivity index (15.3 vs. 3.0) for breast cancer MCF-7 cells over normal breast MCF-10A cells, and was found to be nontoxic toward primary human kidney proximal tubule cells at doses reaching 50 μm.

Targeting cytochrome P450 (CYP) 1B1 with steroid derivatives

Inhibition of cytochrome P450 1B1 (CYP1B1) represents a promising therapeutic strategy, because it would enable action at three different levels: (1) by inhibiting the formation of mutagenic 4-hydroxy-estradiol, (2) by inhibiting the bioactivation of procarcinogens, and (3) by reducing drug-resistance. Surprisingly, few steroids were reported as inhibitors of CYP1B1. From a screening performed with 90 steroid derivatives, we identified thioestrone (B19) as an inhibitor (IC₅₀=3.4μM) of CYP1B1. Molecular modeling studies showed that the 3-SH group of B19 is closer (3.36Å) to the iron atom of the heme system than the 3-OH group of enzyme substrates estrone and estradiol (4.26Å and 3.58Å, respectively). B19 also produced a better docking GOLD score that correlated with the inhibitory results obtained. The estrane derivative B19 represents an interesting lead compound that can be easily modified to extend the structure-activity relationship study and to provide a next generation of more powerful CYP1B1 inhibitors.

Impact of structural modifications at positions 13, 16 and 17 of 16β-(m-carbamoylbenzyl)-estradiol on 17β-hydroxysteroid dehydrogenase type 1 inhibition and estrogenic activity

The chemical synthesis of four stereoisomers (compounds 5a-d) of 16β-(m-carbamoylbenzyl)-estradiol, a potent reversible inhibitor of 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1), and two intermediates (compounds 3a and b) was performed. Assignment of all nuclear magnetic resonance signals confirmed the stereochemistry at positions 13, 16 and 17. Nuclear overhauser effects showed clear correlations supporting a C-ring chair conformation for 5a and b and a C-ring boat conformation for 5c and d. These compounds were tested as 17β-HSD1 inhibitors and to assess their proliferative activity on estrogen-sensitive breast cancer cells (T-47D) and androgen-sensitive prostate cancer cells (LAPC-4). Steroid derivative 5a showed the best inhibitory activity for the transformation of estrone to estradiol (95, 82 and 27%, at 10, 1 and 0.1μM, respectively), but like the other isomers 5c and d, it was found to be estrogenic. The intermediate 3a, however, was weakly estrogenic at 1μM, not at all at 0.1μM, and showed an interesting inhibitory potency on 17β-HSD1 (90, 59 and 22%, at 10, 1 and 0.1μM, respectively). As expected, no compound showed an androgenic activity. The binding modes for compounds 3a and b, 5a-d and CC-156 were evaluated from molecular modeling. While the non-polar interactions were conserved for all the inhibitors in their binding to 17β-HSD1, differences in polar interactions and in binding conformational energies correlated to the inhibitory potencies.

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.

Identification of a first enzymatic activator of a 17β-hydroxysteroid dehydrogenase

Small molecule activators that directly modulate the activity of an enzyme are uncommon entities, and such activators had never yet been identified for any 17β-hydroxysteroid dehydrogenase (17β-HSD). We hereby report the fortuitous discovery of a steroid derivative that caused an up to 3-fold increase in the activity of 17β-HSD12. The stimulation of estrone to estradiol conversion has been characterized in intact and homogenized stably transfected HEK-293 cells and has also been observed in T47D breast cancer cells. Structure-activity relationships closely linked to the nature of the substituent on the [1,3]oxazinan-2-one ring of an estradiol derivative emerged from this study and may help in the identification of a previously unsuspected endogenous activation of 17β-HSD12. This activator will therefore be a useful tool to study this relatively unknown enzyme as well as the possible activation of other 17β-HSD family members.