17Alpha-alkan (or alkyn) amide derivatives of estradiol as inhibitors of steroid-sulfatase activity

Steroid derivatives as inhibitors of steroid sulfatase

Sulfated steroids function as a storage reservoir of biologically active steroid hormones. The sulfated steroids themselves are biologically inactive and only become active in vivo when they are converted into their desulfated (unconjugated) form by the enzyme steroid sulfatase (STS). Inhibitors of STS are considered to be potential therapeutics for the treatment of steroid-dependent cancers such as breast, prostate and endometrial cancer. The present review summarizes steroid derivatives as inhibitors of STS covering the literature from the early years of STS inhibitor development to October of 2012. A brief discussion of the function, structure and mechanism of STS and its role in estrogen receptor-positive (ER+) hormone-dependent breast cancer is also presented. This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Development of steroid sulfatase inhibitors

Hydrolysis of biologically inactive steroid sulfates to unconjugated steroids by steroid sulfatase (STS) is strongly implicated in rendering estrogenic stimulation to hormone-dependent cancers such as those of the breast. Considerable progress has been made in the past two decades with regard to the discovery, design and development of STS inhibitors. We outline historical aspects of their development, cumulating in the discovery of the first clinical trial candidate STX64 (BN83495, Irosustat) and other sulfamate-based inhibitors. The development of reversible STS inhibitors and the design of dual inhibitors of both aromatase and STS is also discussed.

Inhibitors of type 1 17β-hydroxysteroid dehydrogenase with reduced estrogenic activity: Modifications of the positions 3 and 6 of estradiol

Breast cancer is the second most frequent cancer affecting women. Among all endocrine therapies for the treatment of breast cancer, inhibition of estrogen biosynthesis is becoming an interesting complementary approach to the use of antiestrogens. The enzyme type 1 17beta-hydroxysteroid dehydrogenase (17beta-HSD) plays a critical role in the biosynthesis of estradiol catalyzing preferentially the reduction of estrone into estradiol, the most active estrogen. Consequently, this enzyme is an interesting biological target for designing drugs for the treatment of estrogen-sensitive diseases such as breast cancer. Our group has reported the synthesis and the biological evaluation of N-methyl, N-butyl 6beta-(thiaheptamamide)estradiol as a potent reversible inhibitor of type 1 17beta-HSD. Unfortunately, this inhibitor has shown an estrogen effect, thus reducing its possible therapeutic interest. Herein three strategies to modify the biological profile (estrogenicity and inhibitory potency) of the initial lead compound were reported. In a first approach, the thioether bond was replaced with a more stable ether bond. Secondly, the hydroxyl group at position 3, which is responsible for a tight binding with the estrogen receptor, was removed. Finally, the amide group of the side-chain was changed to a methyl group. Moreover, the relationship between the inhibitory potency and the configuration of the side-chain at position 6 was investigated. The present study confirmed that the 6beta-configuration of the side chain led to a much better inhibition than the 6alpha-configuration. The replacement of the 3-OH by a hydrogen atom as well as that of the amide group by a methyl was clearly unfavorable for the inhibition of type 1 17beta-HSD. Changing the thioether for an ether bond decreased by 10-fold the estrogenic profile of the lead compound while the inhibitory potency on type 1 17beta-HSD was only decreased by 5-fold. This study contributes to the knowledge required for the development of compounds with the desired profile, that is, a potent inhibitor of type 1 17beta-HSD without estrogen-like effects.

Steroid sulfatase: molecular biology, regulation, and inhibition

Steroid sulfatase (STS) is responsible for the hydrolysis of aryl and alkyl steroid sulfates and therefore has a pivotal role in regulating the formation of biologically active steroids. The enzyme is widely distributed throughout the body, and its action is implicated in physiological processes and pathological conditions. The crystal structure of the enzyme has been resolved, but relatively little is known about what regulates its expression or activity. Research into the control and inhibition of this enzyme has been stimulated by its important role in supporting the growth of hormone-dependent tumors of the breast and prostate. STS is responsible for the hydrolysis of estrone sulfate and dehydroepiandrosterone sulfate to estrone and dehydroepiandrosterone, respectively, both of which can be converted to steroids with estrogenic properties (i.e., estradiol and androstenediol) that can stimulate tumor growth. STS expression is increased in breast tumors and has prognostic significance. The role of STS in supporting tumor growth prompted the development of potent STS inhibitors. Several steroidal and nonsteroidal STS inhibitors are now available, with the irreversible type of inhibitor having a phenol sulfamate ester as its active pharmacophore. One such inhibitor, 667 COUMATE, has now entered a phase I trial in postmenopausal women with breast cancer. The skin is also an important site of STS activity, and deficiency of this enzyme is associated with X-linked ichthyosis. STS may also be involved in regulating part of the immune response and some aspects of cognitive function. The development of potent STS inhibitors will allow investigation of the role of this enzyme in physiological and pathological processes.

2-phenylindole sulfamates: inhibitors of steroid sulfatase with antiproliferative activity in MCF-7 breast cancer cells

A number of 2-phenylindole sulfamates with lipophilic side chains in 1- or 5-position of the indole were synthesized and evaluated as steroid sulfatase (estrone sulfatase) inhibitors. Most of the new sulfamates inhibited the enzymatic hydrolysis of estrone sulfate in MDA-MB 231 breast cancer cells with IC(50) values between 2 nM and 1 microM. A favorable position for a long side chain is the nitrogen of a carbamoyl group at C-5 of the indole when the phenyl ring carries the sulfamate function. These derivatives inhibit gene activation in estrogen receptor (ER)-positive MCF-7 breast cancer cells in submicromolar concentrations and reduce cell proliferation with IC(50) values of ca. 1 microM. All of the potent inhibitors were devoid of estrogenic activity and have the potential for in vivo application as steroid sulfatase inhibitors.

Solid-phase parallel synthesis of 17alpha-substituted estradiol sulfamate and phenol libraries using the multidetachable sulfamate linker

We report an application of the multidetachable sulfamate linker in the synthesis of two model libraries of N-derivatized 17alpha-piperazinomethyl estradiols (phenols and sulfamates) by solid-phase parallel chemistry. The solid-phase precursor, a 3-sulfamoyl-17alpha-(N-trifluoroacetyl-piperazinomethyl) estradiol, was synthesized in solution from estrone and loaded efficiently onto trityl chloride resin as polymeric support. After cleavage of the trifluoroacetyl protecting group, sequential acylation reactions with five Fmoc-protected amino acids and five carboxylic acids were performed to introduce two levels of molecular diversity. Finally, the resins were split into two parts, and acidic (5% trifluoroacetic acid in dichloromethane) and nucleophilic (piperazine in tetrahydrofuran) cleavages were used to generate libraries A (5 x 5 sulfamates) and B (5 x 5 phenols) members in overall yields of 18-66% and high HPLC purities (87-96%) without purification steps. A preliminary screening test for inhibition of steroid sulfatase showed that the phenols were clearly weaker inhibitors, as compared to their sulfamate analogues. The most potent inhibitors were those with suitable hydrophobic amino acid and carboxylic acid substituents. Thus, compounds with a phenylalanine residue as the first element of diversity inhibited over 90% of steroid sulfatase activity at a concentration of 1 nM in homogenates of HEK-293 transfected cells, being as potent as the leading inhibitor 17alpha-tert-butylbenzyl estradiol 3-O-sulfamate previously reported. These results suggest that the steroid sulfatase inhibitory potency of estradiol derivatives, sulfamoylated or not, can be increased by the hydrophobic effect of a suitable substituent introduced in the proximity of the D ring of the steroid. The present work also demonstrated the efficiency and the cleavage versatility of the sulfamate linker to generate libraries of compounds with relevant biological importance, phenols and sulfamates.

A multidetachable sulfamate linker successfully used in a solid-Phase strategy to generate libraries of sulfamate and phenol derivatives

The sulfamates and phenols constitute two families of compounds with numerous interesting biological properties. Using the ability of a new multidetachable sulfamate linker to generate these two families of compounds from the same resin, we designed and synthesized libraries of estradiol derivatives, sulfamoylated or not. A C-16beta side chain was then judiciously diversified to target two key steroidogenic enzymes, the steroid sulfates and the type 1 17beta-HSD. Four libraries of sulfamate and phenol derivatives were easily obtained by solid-phase parallel synthesis in good crude overall yields (13-62%) and HPLC purities (85-96%). Such strategy using the new two-in-line sulfamate linker could be also extended to other therapeutic targets than steroidogenic enzymes, thus adding to its potential.

2-Substituted 4-(thio)chromenone 6-O-sulfamates: potent inhibitors of human steroid sulfatase

Steroid sulfatase (STS) has emerged as a highly attractive target for the therapy of a number of disorders. Starting with the known inhibitor estrone sulfamate (1) as lead compound and with the finding that steroid sulfamates containing a nonaromatic A-ring are inactive, chromen-4-one sulfamates were designed, prepared, and tested for their ability to block human STS. This new class of nonsteroidal inhibitors shows high potency when the sulfamate group and the side chain are situated in diagonally opposite positions (i.e., 2,6- and 3,7-substitution pattern). The highest activity is achieved with fully branched, bulky aliphatic side chains and with thiochromen-4-one as the core element. 2-(1-Adamantyl)-4H-thiochromen-4-on-6-O-sulfamate (6c) is the most potent STS inhibitor discovered so far, and it is about 170-fold superior to 1. As with 1, all chromenone sulfamates are irreversible inhibitors of STS with a biphasic time course of inactivation.

Nonsteroidal compounds designed to mimic potent steroid sulfatase inhibitors

Chemical synthesis and enzyme inhibition results are reported for a series of nonsteroidal sulfatase inhibitors, 1-(p-sulfamoyloxyphenyl)-5-(p-t-butylbenzyl)-5-alkanols and the lower active phenolic analogues. These compounds conserve some structural elements from the previously reported potent steroidal inhibitor 3-O-sulfamate-17alpha-(p-t-butylbenzyl)-17beta-hydroxy-estra-1,3,5(10)-triene, while the C18-methyl group and the hydrocarbon backbone represented by the steroid rings B, C, and D were replaced with a free conformational chain. Using estrone sulfate (100 microM) as substrate and homogenate of transfected HEK-293 cells as source of steroid sulfatase activity, the IC(50) values of the best inhibitors, the undecanol derivatives, were 0.4+/-0.1 and >300 nM, respectively, in the sulfamate and phenolic series. Although these sulfamoylated nonsteroidal inhibitors appear a bit less active than their steroidal analogues, they are however more potent than known inhibitors estrone-3-O-sulfamate and p-(O-sulfamoyl)-N-tetradecanoyl tyramine. The optimal side-chain length for the inhibition of steroid sulfatase activity was found to be six carbons, which corresponds to the number of carbons that mimic the B, C and D steroid rings, between C6 and C17. Furthermore, compounds with only the t-butylbenzyl group or the alkyl chain of six carbons are less potent inhibitors compared to the one that include both of these hydrophobic substituents. Such results suggest that compound from this later category better mimic the steroidal inhibitor.

Structure-activity relationships of 17alpha-derivatives of estradiol as inhibitors of steroid sulfatase

The steroid sulfatase or steryl sulfatase is a microsomal enzyme widely distributed in human tissues that catalyzes the hydrolysis of sulfated 3-hydroxy steroids to the corresponding free active 3-hydroxy steroids. Since androgens and estrogens may be synthesized inside the cancerous cells starting from dehydroepiandrosterone sulfate (DHEAS) and estrone sulfate (E(1)S) available in blood circulation, the use of therapeutic agents that inhibit steroid sulfatase activity may be a rewarding approach to the treatment of androgeno-sensitive and estrogeno-sensitive diseases. In the present study, we report the chemical synthesis and biological evaluation of a new family of steroid sulfatase inhibitors. The inhibitors were designed by adding an alkyl, a phenyl, a benzyl, or a benzyl substituted at position 17alpha of estradiol (E(2)), a C18-steroid, and enzymatic assays were performed using the steroid sulfatase of homogenized JEG-3 cells or transfected in HEK-293 cells. We observed that a hydrophobic substituent induces powerful inhibition of steroid sulfatase while a hydrophilic one was weak. Although a hydrophobic group at the 17alpha-position increased the inhibitory activity, the steric factors contribute to the opposite effect. As exemplified by 17alpha-decyl-E(2) and 17alpha-dodecyl-E(2), a long flexible side chain prevents adequate fitting into the enzyme catalytic site, thus decreasing capacity to inhibit the steroid sulfatase activity. In the alkyl series, the best compromise between hydrophobicity and steric hindrance was obtained with the octyl group (IC(50) = 440 nM), but judicious branching of side chain could improve this further. Benzyl substituted derivatives of estradiol were better inhibitors than alkyl analogues. Among the series of 17alpha-(benzyl substituted)-E(2) derivatives studied, the 3'-bromobenzyl, 4'-tert-butylbenzyl, 4'-butylbenzyl, and 4'-benzyloxybenzyl groups provided the most potent inhibition of steroid sulfatase transformation of E(1)S into E(1) (IC(50) = 24, 28, 25, and 22 nM, respectively). As an example, the tert-butylbenzyl group increases the ability of the E(2) nucleus to inhibit the steroid sulfatase by 3000-fold, and it also inhibits similarly the steroid sulfatase transformations of both natural substrates, E(1)S and DHEAS. Interestingly, the newly reported family of steroid sulfatase inhibitors acts by a reversible mechanism of action that is different from the irreversible mechanism of the known inhibitor estrone sulfamate (EMATE).

Solid-phase synthesis of phenolic steroids: from optimization studies to a convenient procedure for combinatorial synthesis of biologically relevant estradiol derivatives

During the course of our studies on therapeutic agents for the treatment of breast cancer, we became interested in the solid-phase combinatorial synthesis of estradiol derivatives that contain a functionalized side chain at either position 16 beta or 7 alpha. Both types of compounds have already demonstrated inhibitory activity toward both biosynthesis and action of estradiol. As a first step, two versatile precursors bearing an azidoalkyl side chain at either position 16 beta or 7 alpha of estradiol were synthesized using standard solution-phase methods. Afterward, the effectiveness of five linkers to attach the phenolic function of these estradiol derivatives to a polystyrene resin was investigated; they were benzylic ether (Merrifield), 4-alkoxy-benzylic ethers (Wang, Sheppard), tetrahydropyranyl ether (Ellman), benzoic ester, and o-nitrobenzyl ether. To test the linker in a synthetic context, a short sequence of reactions, including reduction of the azide and acylation of the corresponding amine, was performed on the polymer-bound estradiol derivative. While all of the tested linkers proved effective in attaching the phenol functionality of the precursor, only the o-nitrobenzyl ether photolabile linker enabled the release of the final products in acceptable purities. Consequently, this linker was used to perform successfully the solid-phase synthesis of four different classes of estradiol derivatives in acceptable yields and excellent purities. This study was preliminary to the combinatorial synthesis of larger libraries of biologically relevant estradiol derivatives.