Synthesis of A-ring halogenated 13α-estrone derivatives as potential 17β-HSD1 inhibitors

Recent advances in the development of 17beta-hydroxysteroid dehydrogenase inhibitors

The family of 17β-hydroxysteroid dehydrogenases (17β-HSDs) occupies a prominent place due to its number of isoforms, which carry out a bidirectional transformation (reduction of a steroid carbonyl to alcohol and oxidation of a steroid alcohol to ketone) depending on the nature of the cofactor present. Involved in the activation or inactivation of key estrogens and androgens, 17β-HSDs are therefore therapeutic targets whose selective inhibition would make it possible to be considered for the treatment of several diseases, such as breast cancer, prostate cancer, endometriosis, Alzheimer's disease and osteoporosis. This review article is a continuation of those having reported the great diversity of inhibitors developed over the last years but focusses on inhibitors recently developed. Work to obtain more effective inhibitors that target the first known isoforms (types 1, 2, 3, 5 and 7) has continued, among others, but new inhibitors that target the isoforms more recently reported in the literature (types 10, 12, 13 and 14) are now being reported. Dual inhibitors of two enzymes (17β-HSD1 and steroid sulfatase) were also reported. These inhibitors were grouped according to the 17β-HSD type inhibited and their backbone (steroidal or non-steroidal) when necessary. They were also reported in chronological order and according to the research group.

Synthesis of 13β- and 13α-epimers of 3-hydroxy-17-hydroxymethylestra-1,3,5(10)-triene and considerations on their hormonal and antiproliferative potency

Starting from 3-methoxyestra-1,3,5(10),16-tetraene-17-carbaldehydes of natural (13β) and epimeric (13α) series, a series of isomeric 3-hydroxy-17-hydroxymethylestra-1,3,5(10)-trienes, including those containing 16α,17α-annulated cyclopropane and cyclohexane ring D', were prepared using the Corey-Chaykovsky and Diels-Alder reactions followed by reduction-demethylation with diisobutylaluminum hydride and hydrogenation. Target compounds showed antiproliferative effects on MCF-7 breast cancer cells to varying degrees superior to that on MCF-10A cells, in low micromolar concentrations. The ERα-mediated luciferase reporter gene assay demonstrated that obtained steroids without an additional carbocycle or with a cyclopropane 16α,17α-annulated carbocycle are effective ERα activators. In this test, steroids of the natural configuration showed high activity at both 10 nM and 100 nM concentrations, whereas 13α-steroids showed a strong dose-dependent effect, surpassing their natural counterparts at a concentration of 100 nM. The 13β-steroid bearing additional 16α,17α-cyclohexane ring had low activity in the test. A simple docking approach using AutoDock Vina was used as a test for a preliminary assessment of the estrogenicity of the compounds. The scope of its applicability and limitations were shown using examples of synthesized molecules.

Cytotoxic effect of 13α-estrane derivatives on breast, endometrial and ovarian cancer cell lines

Hormone-dependent cancers such as breast, uterine, and ovarian cancers account for more than 35% of all cancers in women. Worldwide, these cancers occur in more than 2.7 million women/year and account for 22% of cancer-related deaths/year. The generally accepted mechanism for the pathophysiology of estrogen-dependent cancers is estrogen receptor-mediated cell proliferation associated with an increased number of mutations. Therefore, drugs that can interfere with either local estrogen formation or estrogen action via estrogen receptors are needed. Estrane derivatives that have low or minimal estrogenic activity can affect both pathways. In this study, we investigated the effect of 36 different estrane derivatives on the proliferation of eight breast, endometrial, and ovarian cancer cell lines and the corresponding three control cell lines. Estrane derivatives 3 and 4_2Cl showed a stronger effect on the endometrial cancer cell lines KLE and Ishikawa, respectively, compared with the control cell line HIEEC, with IC50 values of 32.6 microM and 17.9 microM, respectively. Estrane derivative 4_2Cl was most active in the ovarian cancer cell line COV362 compared to the control cell line HIO80 with an IC50 value of 3.6 microM. In addition, estrane derivative 2_4I showed a strong antiproliferative effect on endometrial and ovarian cancer cell lines, while the effect on the control cell line was slight or absent. The addition of halogen at carbon 2 and/or 4 in estrane derivatives 1 and 2 increased the selectivity for endometrial cancer cells. Overall, these results suggest that single estrane derivatives are efficient cytotoxic agents for endometrial and ovarian cancer cell lines, and thus potential lead compounds for drug development.

Solvent- and Wavelength-Dependent Photolysis of Estrone

The direct photolysis of estrone in solvents ranging from water to cyclohexane is reported. The photodegradation is dominated by lumiestrone, an epimer of estrone resulting from the inversion of the methyl group at carbon 13, regardless of solvent and photolysis wavelength in the range 254-320 nm. Solvent addition products are also observed in lesser amounts. The photodegradation rate in water is an order of magnitude slower than in nonaqueous solvents. Short wavelength excitation enhances photodegradation. Together, these results suggest complicated photophysics underlie the photochemistry with implications for the remediation of environmental estrogens.

Selective antiproliferative effect of C-2 halogenated 13α-estrones on cells expressing Organic anion-transporting polypeptide 2B1 (OATP2B1)

Organic anion-transporting polypeptide 2B1 (OATP2B1) is a multispecific transporter mediating the cellular uptake of steroids and numerous drugs. OATP2B1 is abundantly expressed in the intestine and is also present in various tumors. Increased steroid hormone uptake by OATP2B1 has been suggested to promote progression of hormone dependent tumors. 13α-estrones are effective inhibitors of endogenous estrogen formation and are potential candidates to inhibit proliferation of hormone dependent cancers. Recently, we have identified a variety of 13α/β-estrone-based inhibitors of OATP2B1. However, the nature of this interaction, whether these inhibitors are potential transported substrates of OATP2B1 and hence may be enriched in OATP2B1-overexpressing cells, has not yet been investigated. In the current study we explored the antiproliferative effect of the most effective OATP2B1 inhibitor 13α/β-estrones in control and OATP2B1-overexpressing A431 carcinoma cells. We found an increased antiproliferative effect of 3-O-benzyl 13α/β-estrones in both mock transfected and OATP2B1-overexpressing cells. However, C-2 halogenated 13α-estrones had a selective OATP2B1-mediated cell growth inhibitory effect. In order to demonstrate that increased sensitization can be attributed to OATP2B1-mediated cellular uptake, tritium labeled 2-bromo-13α-estrone was synthesized for direct transport measurements. These experiments revealed increased accumulation of [³H]2-bromo-13α-estrone due to OATP2B1 function. Our results indicate that C-2 halogenated 13α-estrones are good candidates in the design of anti-cancer drugs targeting OATP2B1.

Synthesis and evaluation of AKR1C inhibitory properties of A-ring halogenated oestrone derivatives

Enzymes AKR1C regulate the action of oestrogens, androgens, and progesterone at the pre-receptor level and are also associated with chemo-resistance. The activities of these oestrone halides were investigated on recombinant AKR1C enzymes. The oestrone halides with halogen atoms at both C-2 and C-4 positions (13β-, 13α-methyl-17-keto halogen derivatives) were the most potent inhibitors of AKR1C1. The lowest IC₅₀ values were for the 13α-epimers 2_2I,4Br and 2_2I,4Cl (IC₅₀, 0.7 μM, 0.8 μM, respectively), both of which selectively inhibited the AKR1C1 isoform. The 13α-methyl-17-keto halogen derivatives 2_2Br and 2_4Cl were the most potent inhibitors of AKR1C2 (IC₅₀, 1.5 μM, 1.8 μM, respectively), with high selectivity for the AKR1C2 isoform. Compound 1_2Cl,4Cl showed the best AKR1C3 inhibition, and it also inhibited AKR1C1 (Ki: AKR1C1, 0.69 μM; AKR1C3, 1.43 μM). These data show that halogenated derivatives of oestrone represent a new class of potent and selective AKR1C inhibitors as lead compounds for further optimisations.

Synthesis and conversion of primary and secondary 2-aminoestradiols into A-ring-integrated benzoxazolone hybrids and their in vitro anticancer activity

Hybrid systems are often endowed with completely different and improved properties compared to their parent compounds. In order to extend the chemical space toward sterane-based molecular hybrids, a number of estradiol-derived benzoxazol-2-ones with combined aromatic rings were synthesized via the corresponding 2-aminophenol intermediates. 2-Aminoestradiol was first prepared from estrone by a two-step nitration/reduction sequence under mild reaction conditions. Subsequent reductive aminations with different arylaldehydes furnished secondary 2-aminoestradiol derivatives in good yields. The proton dissociation processes of the aminoestradiols were investigated in aqueous solution by UV-visible spectrophotometric titrations to reveal their actual chemical forms at physiological pH. The determined pK₁ and pK₂ values are attributed to the ⁺NH₃ or ⁺NH₂R and OH moieties, and both varied by the different R substituents of the amino group. Primary and secondary 2-aminoestradiols were next reacted with carbonyldiimidazole as a phosgene equivalent to introduce a carbonyl group with simultaneous ring-closure to give A-ring-fused oxazolone derivatives in high yields. The novel aminoestradiols and benzoxazolones were subjected to in vitro cytotoxicity analysis and were found to exert cancer cell specific activity.

Estradiol–benzoxazolone hybrids with a common aromatic moiety were efficiently synthesized via primary and secondary aminophenol intermediates, and their anticancer activities were investigated.

Pd-Catalyzed microwave-assisted synthesis of phosphonated 13α-estrones as potential OATP2B1, 17β-HSD1 and/or STS inhibitors

Novel 2- or 4-phosphonated 13α-estrone derivatives were synthesized via the Hirao reaction. Bromo regioisomers (2- or 4-) of 13α-estrone and its 3-benzyl or 3-methyl ether were reacted with diethyl phosphite or diphenylphosphine oxide using Pd(PPh₃)₄ as catalyst under microwave irradiation. The influence of the new compounds on the transport function of the organic anion transporting polypeptide OATP2B1 was investigated by measuring Cascade Blue uptake. Derivatives bearing a 3-benzyl ether function displayed substantial submicromolar OATP2B1 inhibitory activity. The inhibitory effects of the compounds on human placental steroid sulfatase (STS) and 17β-hydroxysteroid dehydrogenase type 1 isozyme (17β-HSD1) were investigated by in vitro radiosubstrate incubation methods. None of the test compounds inhibited the STS markedly. The structure-activity relationship evaluation revealed that 2-substituted 3-hydroxy derivatives are able to inhibit the 17β-HSD1 enzyme with submicromolar IC₅₀ values. Dual OATP2B1 and 17β-HSD1 inhibitors have been identified.

Synthesis and structure-activity relationships of 2- and/or 4-halogenated 13β- and 13α-estrone derivatives as enzyme inhibitors of estrogen biosynthesis

Ring A halogenated 13α-, 13β-, and 17-deoxy-13α-estrone derivatives were synthesised with N-halosuccinimides as electrophile triggers. Substitutions occurred at positions C-2 and/or C-4. The potential inhibitory action of the halogenated estrones on human aromatase, steroid sulfatase, or 17β-hydroxysteroid dehydrogenase 1 activity was investigated via in vitro radiosubstrate incubation. Potent submicromolar or low micromolar inhibitors were identified with occasional dual or multiple inhibitory properties. Valuable structure–activity relationships were established from the comparison of the inhibitory data obtained. Kinetic experiments performed with selected compounds revealed competitive reversible inhibition mechanisms against 17β-hydroxysteroid dehydrogenase 1 and competitive irreversible manner in the inhibition of the steroid sulfatase enzyme.

The first Pd-catalyzed Buchwald-Hartwig aminations at C-2 or C-4 in the estrone series

A facile Pd-catalyzed C(sp²)–N coupling to provide a range of 2- or 4-[(subst.)phenyl]amino-13α-estrone derivatives has been achieved under microwave irradiation. The reactions were mediated with the use of Pd(OAc)₂ as a catalyst and KOt-Bu as a base in the presence of X-Phos as a ligand. The desired products have been obtained in good to excellent yields. The nature and the position of the aniline substituent at the aromatic ring influenced the outcome of the couplings. 2-Amino-13α-estrone was also synthesized in a two-step protocol including an amination of 2-bromo-13α-estrone 3-benzyl ether with benzophenone imine and subsequent hydrogenolysis.

Metabolic Epoxidation Is a Critical Step for the Development of Benzbromarone-Induced Hepatotoxicity

Benzbromarone (BBR) is effective in the treatment of gout; however, clinical findings have shown it can also cause fatal hepatic failure. Our early studies demonstrated that CYP3A catalyzed the biotransformation of BBR to epoxide intermediate(s) that reacted with sulfur nucleophiles of protein to form protein covalent binding both in vitro and in vivo. The present study attempted to define the correlation between metabolic epoxidation and hepatotoxicity of BBR by manipulating the structure of BBR. We rationally designed and synthesized three halogenated BBR derivatives, fluorinated BBR (6-F-BBR), chlorinated BBR (6-Cl-BBR), and brominated BBR (6-Br-BBR), to decrease the potential for cytochrome P450-mediated metabolic activation. Both in vitro and in vivo uricosuric activity assays showed that 6-F-BBR achieved favorable uricosuric effect, while 6-Cl-BBR and 6-Br-BBR showed weak uricosuric efficacy. Additionally, 6-F-BBR elicited much lower hepatotoxicity in mice. Fluorination of BBR offered advantage to metabolic stability in liver microsomes, almost completely blocked the formation of epoxide metabolite(s) and protein covalent binding, and attenuated hepatic and plasma glutathione depletion. Moreover, the structural manipulation did not alter the efficacy of BBR. This work provided solid evidence that the formation of the epoxide(s) is a key step in the development of BBR-induced hepatotoxicity.

Synthesis of novel 13α-estrone derivatives by Sonogashira coupling as potential 17β-HSD1 inhibitors

Novel 13α-estrone derivatives were synthesized by Sonogashira coupling. Transformations of 2- or 4-iodo regioisomers of 13α-estrone and its 3-methyl ether were carried out under different conditions in a microwave reactor. The 2-iodo isomers were reacted with para-substituted phenylacetylenes using Pd(PPh₃)₄ as catalyst and CuI as a cocatalyst. Coupling reactions of 4-iodo derivatives could be achieved by changing the catalyst to Pd(PPh₃)₂Cl₂. The product phenethynyl derivatives were partially or fully saturated. Compounds bearing a phenolic OH group furnished benzofurans under the conditions used for the partial saturation. The inhibitory effects of the compounds on human placental 17β-hydroxysteroid dehydrogenase type 1 isozyme (17β-HSD1) were investigated by an in vitro radiosubstrate incubation method. Certain 3-hydroxy-2-phenethynyl or -phenethyl derivatives proved to be potent 17β-HSD1 inhibitors, displaying submicromolar IC₅₀ values.

Impact of androstane A- and D-ring inversion on 17β-hydroxysteroid dehydrogenase type 3 inhibitory activity, androgenic effect and metabolic stability

17β-Hydroxysteroid dehydrogenase type 3 (17β-HSD3) is a major player in human endocrinology, being one of the most important enzymes involved in testosterone production. To capitalize on the discovery of RM-532-105, a steroidal 17β-HSD3 inhibitor, we explored the effect of its backbone configuration on inhibitory activity, androgenic profile, and metabolic stability. Two modifications that greatly alter the natural shape of steroids, i.e. inversion of the methyl on carbon 13 (13α-CH₃ instead of 13β-CH₃) and inversion of the hydrogen on carbon 5 (5β-H instead of 5α-H), were tested after the syntheses in 6 steps of 2 isomeric forms (5α/13α-RM-532-105 (6a) and 5β/13β-RM-532-105 (6b), respectively) of the 17β-HSD3 inhibitor RM-532-105 (5α/13β-configurations). For compound 6b, a cis/trans junction of the A/B rings did not significantly alter the inhibitory activity on 17β-HSD3 (IC₅₀=0.15μM) as well as the liver microsomal stability (16.6% of 6b remaining after 1h incubation) compared to RM-532-105 (IC₅₀=0.11μM and 14.1% remaining). In contrast, a trans/cis junction of C/D rings reduced the inhibitory activity on 17β-HSD3 (IC₅₀=1.09μM) but increased the metabolic stability with 29.4% of compound 6a remaining after incubation. The structural modifications represented by compounds 6a and 6b did not change the non-androgenicity profile of an androsterone derivative such as RM-532-105, but slightly increased its cytotoxic activity.

Synthesis, functionalization and biological activity of arylated derivatives of (+)-estrone

Various novel arylated estrone derivatives, such as 2-aryl-, 4-aryl- and 2,4-diaryl-estrones, by Suzuki-Miyaura reactions. While the synthesis of 4-arylestrones could be carried out under standard conditions, the synthesis of 2-arylestrones and 2,4-diarylestrones required a thorough optimization of the conditions and it proved to be important to use sterically encumbered biaryl ligands. The best results were obtained by the use of RuPhos. Combination of developed Suzuki coupling reactions with subsequent cyclization reactions afforded more complex hybrid structures, containing dibenzofuran, benzocoumarin and steroid moieties. These derivatives were tested as pancreatic lipase inhibitors and it was found that most of the compounds exhibited inhibition of pancreatic lipase but the maximum inhibitory potential was shown by 4-arylestrones. All of the synthesized derivatives showed inhibitory values in the range of 0.82±0.01-59.7±3.12μM. The biological activity was also rationalized on the bases of docking studies.

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

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

Comparative investigation of the in vitro inhibitory potencies of 13-epimeric estrones and D-secoestrones towards 17β-hydroxysteroid dehydrogenase type 1

The inhibitory effects of 13-epimeric estrones, D-secooxime and D-secoalcohol estrone compounds on human placental 17β-hydroxysteroid dehydrogenase type 1 isozyme (17β-HSD1) were investigated. The transformation of estrone to 17β-estradiol was studied by an in vitro radiosubstrate incubation method. 13α-Estrone inhibited the enzyme activity effectively with an IC₅₀ value of 1.2 μM, which indicates that enzyme affinity is similar to that of the natural estrone substrate. The 13β derivatives and the compounds bearing a 3-hydroxy group generally exerted stronger inhibition than the 13α and 3-ether counterparts. The 3-hydroxy-13β-D-secoalcohol and the 3-hydroxy-13α-D-secooxime displayed an outstanding cofactor dependence, i.e. more efficient inhibition in the presence of NADH than NADPH. The 3-hydroxy-13β-D-secooxime has an IC₅₀ value of 0.070 μM and is one of the most effective 17β-HSD1 inhibitors reported to date in the literature.