7-substituted 1,4,6-androstatriene-3,17-diones as enzyme-activated irreversible inhibitors of aromatase

Steroid conversion with CYP106A2 - production of pharmaceutically interesting DHEA metabolites

Background

Steroids are lipophilic compounds with a gonane skeleton and play an important role in higher organisms. Due to different functionalizations - mainly hydroxylations - at the steroid molecule, they vary highly in their mode of action. The pharmaceutical industry is, therefore, interested in hydroxysteroids as therapeutic agents. The insertion of hydroxyl groups into a steroid core, however, is hardly accomplishable by classical chemical means; that is because microbial steroid hydroxylations are investigated and applied since decades. CYP106A2 is a cytochrome P450 monooxygenase from Bacillus megaterium ATCC 13368, which was first described in the late 1970s and which is capable to hydroxylate a variety of 3-oxo-delta4 steroids at position 15beta. CYP106A2 is a soluble protein, easy to express and to purify in high amounts, which makes this enzyme an interesting target for biotechnological purposes.

Results

In this work a focused steroid library was screened in vitro for new CYP106A2 substrates using a reconstituted enzyme assay. Five new substrates were identified, including dehydroepiandrosterone and pregnenolone. NMR-spectroscopy revealed that both steroids are mainly hydroxylated at position 7beta. In order to establish a biotechnological system for the preparative scale production of 7beta-hydroxylated dehydroepiandrosterone, whole-cell conversions with growing and resting cells of B. megaterium ATCC1336 the native host of CYP1062 and also with resting cells of a recombinant B. megaterium MS941 strain overexpressing CYP106A2 have been conducted and conversion rates of 400 muM/h (115 mg/l/h) were obtained. Using the B. megaterium MS941 overexpression strain, the selectivity of the reaction was improved from 0.7 to 0.9 for 7beta-OH-DHEA.

Conclusions

In this work we describe CYP106A2 for the first time as a regio-selective hydroxylase for 3-hydroxy-delta5 steroids. DHEA was shown to be converted to 7beta-OH-DHEA which is a highly interesting human metabolite, supposed to act as neuroprotective, anti-inflammatory and immune-modulatory agent. Optimization of the whole-cell system using different B. megaterium strains lead to a conversion of DHEA with B. megaterium showing high selectivity and conversion rates and displaying a volumetric yield of 103 mg/l/h 7beta-OH-DHEA.

Aromatase inhibitors in the treatment of breast cancer

Estradiol, the most potent endogenous estrogen, is biosynthesized from androgens by the cytochrome P450 enzyme complex called aromatase. Aromatase is present in breast tissue, and intratumoral aromatase is the source of local estrogen production in breast cancer tissues. Inhibition of aromatase is an important approach for reducing growth-stimulatory effects of estrogens in estrogen-dependent breast cancer. Steroidal inhibitors that have been developed to date build upon the basic androstenedione nucleus and incorporate chemical substituents at varying positions on the steroid. Nonsteroidal aromatase inhibitors can be divided into three classes: aminoglutethimide-like molecules, imidazole/triazole derivatives, and flavonoid analogs. Mechanism-based aromatase inhibitors are steroidal inhibitors that mimic the substrate, are converted by the enzyme to a reactive intermediate, and result in the inactivation of aromatase. Both steroidal and nonsteroidal aromatase inhibitors have shown clinical efficacy in the treatment of breast cancer. The potent and selective third-generation aromatase inhibitors, anastrozole, letrozole, and exemestane, were introduced into the market as endocrine therapy in postmenopausal patients failing antiestrogen therapy alone or multiple hormonal therapies. These agents are currently approved as first-line therapy for the treatment of postmenopausal women with metastatic estrogen-dependent breast cancer. Several clinical studies of aromatase inhibitors are currently focusing on the use of these agents in the adjuvant setting for the treatment of early breast cancer. Use of an aromatase inhibitor as initial therapy or after treatment with tamoxifen is now recommended as adjuvant hormonal therapy for a postmenopausal woman with hormone-dependent breast cancer.

Aromatase, aromatase inhibitors, and breast cancer

Estrogens are involved in numerous physiologic processes and have crucial roles in particular disease states, such as mammary carcinomas. Estradiol, the most potent endogenous estrogen, is biosynthesized from androgens by the cytochrome P-450 enzyme complex called aromatase. Aromatase is found in breast tissue, and the importance of intratumoral aromatase and local estrogen production is being unraveled. Inhibition of aromatase is an important approach for reducing growth stimulatory effects of estrogens in hormone-dependent breast cancer. Effective aromatase inhibitors have been developed as therapeutic agents for controlling estrogen-dependent breast cancer. Investigations into the development of aromatase inhibitors began in the 1970s and have expanded greatly in the past three decades. Competitive aromatase inhibitors are molecules that compete with the substrate androstenedione for noncovalent binding to the active site of the enzyme to decrease the amount of product formed. Steroidal inhibitors that have been developed to date build on the basic androstenedione nucleus and incorporate chemical substituents at varying positions on the steroid. The structure-activity relationships for steroidal inhibitors have become more refined in the past decade, and only some modifications can be made to the steroid and still keep its affinity for aromatase. Nonsteroidal aromatase inhibitors can be divided into three classes: aminoglutethimide-like molecules, imidazole/triazole derivatives, and flavonoid analogs. Mechanism-based aromatase inhibitors are inhibitors that mimic the substrate, are converted by the enzyme to a reactive intermediate, and result in the inactivation of aromatase. Aromatase inhibitors, both steroidal and nonsteroidal, have shown clinical efficacy for the treatment of breast cancer. The initial nonselective nature of nonsteroidal inhibitors such as aminoglutethimide has been greatly reduced in the later generations of inhibitors, anastrozole and letrozole. Mechanism-based steroidal inhibitors such as 4-hydroxyandrostenedione and exemestane produce prolonged aromatase inhibition in patients. The potent and selective third-generation aromatase inhibitors anastrozole, letrozole, and exemestane are approved for clinical use as second-line endocrine therapy in postmenopausal patients failing antiestrogen therapy alone or multiple hormonal therapies.

Aromatase inhibitors: past, present and future

For the cellular physiology of sex steroid sensitive cells, the androgen/estrogen ratio may be more important than only one hormone action per se, in both sexes. This ratio is controlled in vertebrates by aromatase; its gene expression can be inhibited in different ways, and this is crucial for the treatment of estrogen-dependent diseases such as breast cancer, or gynecomastia in males for instance. To reach this goal, new steroidal and non-steroidal inhibitors are continuously being developed, and some of them are used as first or second line agents. Aromatase inhibition is also an essential tool for studying the role of estrogens in the adult, or during development. Aromatase inhibitors have shown in particular that estrogens are essential also in males for skeletal maturation and bone mineralization, development of masculine dendritic morphology in male brain linked to mating behaviour, and testicular function. Testosterone is often the prohormone converted in situ in active estrogens, at these levels. Several strategies can be used for aromatase inhibition. The first ones employed were blind screening or deductions from in vivo observations, which led for instance to the discovery of the role of aminoglutethimide in aromatase inhibition. Subsequently, in the years 1975-1990, the molecular modeling of compounds to mimic the substrate shape of the enzyme constituted the major idea. Hundreds of chemicals were synthesized by numerous authors, ranging from the well-known and very efficient 4-OHA to complicated imidazole or indane derivatives tested by sophisticated comparative molecular field analyses. Reticulum-bound active aromatase has not as yet been X-ray analyzed. Thus, aromatase inhibitors were also used more recently to probe and understand the active site conformation of the enzyme and its modelization was obtained from comparisons with bacterial-related cytochromes. We developed a mammalian model considerably closer to human aromatase in order to study the active site shape with new potent aromatase non-steroidal inhibitors. This model is equine aromatase. This enzyme was biochemically characterized, purified, and cloned by our group. It allowed testing, by site-directed mutagenesis, predictive hypotheses in human aromatase which contributed to designing of new inhibitors. The understanding of the functioning of an essential member of the cytochrome P450 family, which is necessary for cellular detoxification, was also facilitated. Inhibition of aromatase activity has also been carried out with antibodies directed to the catalytic site and at the gene level by knock-out or by control of factor-specific promoters. This may result in different mRNA synthesized by alternative splicing. We have also obtained specific inhibition of aromatase activity in human cells with antisense stable phosphorothioate oligodeoxynucleotides directed against aromatase mRNA tertiary structures. Besides known steroidal and non-steroidal inhibitors, the antiaromatase effects of compounds found in our daily environment such as dietary flavonoids or xenobiotic pollutants have also been described. Finally, we underline that all these aromatase inhibitors, or methods of aromatase inhibition, can modulate the estrogenic balance essential not only for female, but also for male physiology, including gonadal function.

Probing the binding pocket of the active site of aromatase with 6-ether or 6-ester substituted androst-4-ene-3,17-diones and their diene and triene analogs

A series of 6-ester- (3 and 4) and 6-ether- (7 and 8) substituted androst-4-ene-3,17-diones (androstenediones) and their 1,4-diene analogs (5 and 6, and 9 and 10) as well as C6-substituted 4,6-diene and 1,4,6-triene steroids 11 and 12 were synthesized as aromatase inhibitors to gain insight into the structure-activity relationship between various substituents and inhibitory activity. All of the inhibitors synthesized blocked aromatase in a competitive manner. The inhibitory activities of all of the steroids, except for the 6beta-benzoates 4g and 6h and the 6beta-acetate 6a, were fairly effective to very powerful (K(i): 7.0-320 nM). The 6alpha-n-hexanoyloxy- and 6alpha-benzyloxyandrostenediones (3e and 7e) were the most potent inhibitors (K(i): 7.0 nM each). In the series of 4-ene and 1,4-diene steroids, the 6alpha-substituted steroids had higher affinity for the enzyme than the corresponding 6beta-isomers. In the 1,4-diene steroid series, 6beta-substituted steroids 6a, e, g, and 10a, b, e caused a time-dependent inactivation of aromatase, whereas their 6alpha-isomers 5 and 9 essentially did not. The ether-substituted 1,4,6-trienes 12 inactivated the enzyme in a time-dependent manner; in contrast, their 4,6-diene analogs 11 did not. The substrate androstenedione blocked the inactivation, but no significant effect of L-cysteine was observed. Based on molecular modeling with the PM3 method, along with the present inhibition and inactivation results, it is thought that both the steric effects of the 6-substituents as well as the electronic effects of the C-6 oxygen functions play a critical role in the binding of inhibitors to the active site of aromatase.

Biological aromatization of delta4,6- and delta1,4,6-androgens and their 6-alkyl analogs, potent inhibitors of aromatase

Enzymic aromatization of delta6- and delta1,6-derivatives of the natural substrate androstenedione with human placental aromatase was first studied using gas-chromatography-mass spectrometry. The two steroids were aromatized with apparent Km and Vmax values of 62 nM and 32 pmol/min/mg protein for the delta6-steroid and 167 nM and 10 pmol/min/mg protein for the delta1,6-steroid, respectively. We next explored the aromatization of a series of 6-alkyl (methyl, ethyl, n-propyl, and n-pentyl)-substituted delta6-androstenediones and their delta1,6-analogs, potent competitive inhibitors of aromatase, to gain insight into the relationships between the inhibitory activity of the 6-alkyl-C19 steroids and their ability to serve as a substrate of aromatase. In a series of the delta1,6-androstenediones, all the 6-alkyl steroids were more efficient substrates than the parent delta1,6-steroid in which the aromatization rates of the alkyl steroids were about 2-fold that of the parent steroid, in contrast, all of the 6-alkyl-substituted delta6-androstenediones were converted into the corresponding 6-alkyl-delta6-estrogens with the rates of less than about a half that of the parent steroid. These results indicate that the 6-alkyl function decreases the aromatization rate of the delta6-steroid but enhances that of the delta1,6-steroid. The relative apparent Km values for the C19 steroids obtained in this study are different from the relative Ki values obtained previously, indicating that a good inhibitor is not essentially a good substrate in the 6-alkyl-substituted delta6- and delta1,6-androstenedione series.

Synthesis and structure-activity relationships of 6-phenylaliphatic-substituted C19 steroids having a 1,4-diene, 4,6-diene, or 1,4,6-triene structure as aromatase inhibitors

A series of 6alpha- and 6beta-phenylaliphatic-substituted androsta-1,4-diene-3,17-diones [9b-f and 10b-f; (CH2)nPh, n = 1-5] and their 4,6-diene and 1,4,6-triene analogs (11b-f and 12b-f) along with their respective phenyl analogs 9a-12a were synthesized and tested as aromatase inhibitors. All of the steroids examined were very powerful competitive inhibitors of aromatase in human placental microsomes with apparent Ki values ranging from 8.5 to 80 nM. The inhibitory activities of the benzyl- and phenethyl-4,6-dienes 11b and 11c (Ki, 9.0 and 10 nM) as well as the 6-phenethyl-1,4,6-triene 12c (Ki, 8.5 nM) were extremely high among them. All of the phenylaliphatic steroids, except for the 6beta-phenethyl compound 10c, and the 6-phenyl-4,6-diene 11a had higher affinity for aromatase than the corresponding parent 1,4-diene, 4,6-diene, and 1,4,6-triene steroids 9g, 11g, and 12g. All of the 6alpha-substituted 1,4-dienes (9a-9g) and the 6-substituted 1,4,6-trienes (12a-12g) caused a time-dependent inactivation of aromatase. On the other hand, only the 6beta-substituted 1,4-dienes (10a-10d) having no or less than four carbon atoms between the steroid nucleus and the phenyl group also caused a time-dependent inactivation of aromatase. Their inactivation rates (k(inact) 0.076-0.156 min(-1)) were higher than the respective parent steroids, 9g and 12g. In contrast, in the 4,6-diene series, only the 6-phenpropyl steroids 11d inactivated aromatase in a time-dependent manner with 0.155 min(-1) of k(inact) value. The inactivation was prevented by the substrate androstenedione, and no significant effect of L-cysteine on the inactivation was observed in each case. These results indicate that length and/or stereochemistry of the C-6 substituent of steroids 9-12 as well as a terminal phenyl group incorporated in the C-6 substituent play a critical role not only in tight binding to the active site of aromatase but also in the cause of a time-dependent inactivation of the enzyme.

Inhibition and inactivation of equine aromatase by steroidal and non-steroidal compounds. A comparison with human aromatase inhibition

In order to approach the detailed structure-function relationships of aromatase, we studied the inhibitory and inactivatory potencies of several steroidal androstenedione analogues (1: 4-hydroxyandrostenedione, 2: 4-acetoxyandrostenedione and 3: 7 alpha-(4'-amino)phenylthio-4-androstene-3, 17-dione) and non-steroidal imidazole derivatives (4: ketoconazole, 5: miconazole and 6: fadrozole) on equine aromatase in placental microsomes, a well established mammalian model. Human placental microsomes and the purified enzyme from equine testis were also used to compare inhibition by 1 and 2. In equine microsomes, all compounds tested exhibited a competitive inhibition, with Ki values of 4.1, 26 and 1.8 nM for 1, 2 and 3, and of 2400, 1.4 and 4 nM for 4, 5, and 6, respectively. The Km for androstenedione, the substrate mainly used in these studies, was 1.8 +/- 0.13 nM. The three non-steroidal derivatives did not inactivate equine aromatase, but 1 and 2 acted as comparable inactivators to a much higher degree than 3. Compound 1 inhibited in a similar manner (89-94%) purified or equine and human microsomal aromatases, whereas 2 inhibited microsomal aromatase more efficiently in the horse than in man (92% and 33% inhibition, respectively). There was only a 40% inhibition with 2 on the purified equine enzyme, which is no more in the natural membrane environment. The comparisons between equine and human microsomal aromatases allow precise functional and structural differences to be observed with these enzymes.

6-Alkylandrosta-4,6-diene-3,17-diones and their 1,4,6-triene analogs as aromatase inhibitors. Structure-activity relationships

Two series of 6-alkylandrosta-4,6-diene-3,17-diones (5) and their 1,4,6-triene analogs 6 were synthesized as aromatase inhibitors to gain insight into the structure-activity relationship between varying the 6-n-alkyl substituents (C1-C7) and inhibitory activity. All of the steroids synthesized were extremely powerful competitive inhibitors of aromatase in human placental microsomes, with apparent Ki values for the 6-alkyl-4,6-diene steroids 5 ranging from 17 to 36 nM and with those for the 1,4,6-triene steroids 6 ranging from 2.5 to 58 nM. The 6-ethyl-1,4,6-triene compound 6b (Ki = 2.5 nM) was the most potent inhibitor among them. The 6-alkyl-1,4,6-triene steroids 6, except for the 6-methyl analog 6a, and higher affinity for aromatase than the natural substrate androstenedione (K(m) = 24 nM), and their inhibitory activities were more potent than the corresponding 4,6-diene steroids 5. In a series of the 4,6-diene steroids 5, compounds 5c-f with the n-alkyl chain substituents (C3 to C6) also had slightly higher affinity than androstenedione for dromatase. All of the 1,4,6-triene steroids 6 inactivated aromatase in a time-dependent manner, with k(inact) values ranging from 0.021 to 0.074 min-1; in contrast, the 4,6-diene analogs 5 did not. The inactivation was prevented by androstenedione, and no significant effect of L-cysteine on the inactivation was observed in each case. These results indicate that the length of the n-alkyl substituent at C-6 of androsta-1,4,6-triene-3,17-dione (6h), rather than its 4,6-diene analog 5h, plays a critical role in tight binding to the active site of aromatase. No significant correlation was observed between affinity for the enzyme and the inactivation ability of the 6-alkyl-1,4,6-trienes.

Three-dimensional quantitative structure-activity relationships of steroid aromatase inhibitors

Inhibition of aromatase, a cytochrome P450 that converts androgens to estrogens, is relevant in the therapeutic control of breast cancer. We investigate this inhibition using a three-dimensional quantitative structure-activity relationship (3D QSAR) method known as Comparative Molecular Field Analysis, CoMFA [Cramer III, R.D. et al., J. Am. Chem. Soc., 110 (1988) 5959]. We analyzed the data for 50 steroid inhibitors [Numazawa, M. et al., J. Med. Chem., 37 (1994) 2198, and references cited therein] assayed against androstenedione on human placental microsomes. An initial CoMFA resulted in a three-component model for log(l/Ki), with an explained variance r2 of 0.885, and a cross-validated q2 of 0.673. Chemometric studies were performed using GOLPE [Baroni, M. et al., Quant. Struct.-Act. Relatsh., 12 (1993) 9]. The CoMFA/GOLPE model is discussed in terms of robustness, predictivity, explanatory power and simplicity. After randomized exclusion of 25 or 10 compounds (repeated 25 times), the q2 for one component was 0.62 and 0.61, respectively, while r2 was 0.674. We demonstrate that the predictive r2 based on the mean activity (Ym) of the training set is misleading, while the test set Ym-based predictive r2 index gives a more accurate estimate of external predictivity. Using CoMFA, the observed differences in aromatase inhibition among C6-substituted steroids are rationalized at the atomic level. The CoMFA fields are consistent with known, potent inhibitors of aromatase, not included in the model. When positioned in the same alignment, these compounds have distinct features that overlap with the steric and electrostatic fields obtained in the CoMFA model. The presence of two hydrophobic binding pockets near the aromatase active site is discussed: a steric bulk tolerant one, common for C4, C6-alpha and C7-alpha substituents, and a smaller one at the C6-beta region.

An approach towards the development of progesterone antagonists: synthesis of 7 alpha/7 beta-aryl androstene derivatives

Synthesis of 3 beta,17 beta-dihydroxy-7 alpha/7 beta-(4-hydroxyphenyl)-androst-5-ene 3,17-diacetate (4 and 5, R = H) and 3 beta,17 beta-dihydroxy-7 alpha/7 beta-(4-methoxyphenyl)-androst-5-ene 3,17-diacetate (4 and 5, R = Me) have been carried out by Friedel-Crafts reaction on 3 beta,7,17 beta-trihydroxy-androst-5-ene 3,17-diacetate (3, R = H) with phenol and anisole, respectively. Compounds 4 (R = H) and 5 (R = H) have been separated and their stereochemistry assigned on the basis of COSY and NOE experiments.

Aromatase inhibitors--mechanisms of steroidal inhibitors

Inhibition of aromatase has been an attractive approach for examining the roles of estrogen biosynthesis in various physiological or pathological processes. Effective aromatase inhibitors can serve as potential therapeutic agents for controlling estrogen-dependent diseases such as hormone-dependent breast cancer. Investigations on the development of aromatase inhibitors have therefore expanded greatly in the past two decades. Numerous steroidal agents have been developed that have high affinities for the aromatase enzyme complex and exhibit either competitive inhibition, irreversible inhibition, or mechanism-based (enzyme-activated) inhibition. Mechanism-based inhibitors have distinct advantages in drug design, since these inhibitors are highly enzyme specific, produce prolonged inhibition, and exhibit minimal toxicities. Examination of the structure-activity relationships of the numerous steroidal aromatase inhibitors suggest that the spacial requirements for interaction of agents with the active site of aromatase are very restrictive, permitting only small structural changes to be made on the A-ring and at C-19. Incorporation of small polar substituents at the C-4 position, such as a hydroxyl group, or addition of aryl functionalities at the 7 alpha-position of the steroid, are the exceptions, and inhibitors with such modifications exhibit enhanced affinity for the enzyme. Future investigations of steroidal aromatase inhibitors as probes of the active site of purified aromatase will provide valuable information on enzyme structure at the molecular level, will permit a more detailed examination of the mechanisms of inhibition, and will enhance the development of more specific and effective inhibitors for the treatment of estrogen-dependent breast cancer.

Steroidal inhibitors as chemical probes of the active site of aromatase

Androstenedione analogs containing 7 alpha-substituents have proven to be potent inhibitors of aromatase in human placental microsomes, in MCF-7 mammary cell cultures, and in JAr choriocarcinoma cells. Recent investigations have focused on the use of mechanism-based inhibitors, such as 7 alpha-substituted 1,4-androstadienediones, to biochemically probe the active site of aromatase. Inhibition kinetics were determined under initial velocity conditions using purified human placental cytochrome P450arom protein in a reconstituted system. Derivatives of 1,4-androstadiene-3,17-dione and 1,4,6-androstatriene-3,17-dione exhibited high affinity in the purified enzyme system. 7 alpha-(4'-Amino)phenylthio-1,4-androstadiene-3,17-dione, abbreviated 7 alpha-APTADD, demonstrated rapid time-dependent, first-order inactivation of reconstituted aromatase activity only in the presence of NADPH. The apparent Kinact for 7 alpha-APTADD is 11.8 nM, the first-order rate of inactivation is 2.72 x 10(-3) sec-1, and the half-time of inactivation at infinite inhibitor concentration is 4.25 min. The values for the rate constant and half-time of inactivation are similar to those observed in the placental microsomal assay system. Further studies were performed with radioiodinated 7 alpha-(4'-iodo)phenylthio-1,4-androstadienedione, 7 alpha-IPTADD, and the reconstituted aromatase system. Incubations with [125I] 7 alpha-IPTADD were followed by protein precipitation, solvent extraction, and column chromatography. Analysis of the isolated cytochrome P450arom by gel electrophoresis and autoradiography demonstrated the presence of only one radioactive band, which corresponded to the protein staining band for cytochrome P450arom. HPLC radiochromatographic analysis of the isolated cytochrome P450aroM confirmed the presence of only one radioactive peak coeluting with the u.v. peak for cytochrome P450arom. Peptide mapping analysis by reverse-phase HPLC of digested inhibitor-cytochrome P450arom complex demonstrates that the radioactive inhibitor is covalently bound to a lipophilic fragment. In summary, these inhibitors produced enzyme-catalyzed inactivation of reconstituted aromatase activity, and radioiodinated 7 alpha-IP-TADD binds covalently to the cytochrome P450arom.

Aromatase inhibition by 7-substituted steroids in human choriocarcinoma cell culture

Androstenedione analogs containing 7 alpha-substituents have proven to be potent inhibitors of aromatase both in vitro and in vivo. Several of these agents have exhibited higher affinity for the enzyme complex than the substrate. In order to examine further the interaction(s) of 7-substituted steroids with aromatase, 7-substituted 4,6-androstadiene-3,17-diones were synthesized and demonstrated competitive inhibition of aromatase activity in human placental microsomes. 7-Substituted 1,4,6-androstatriene-3,17-diones demonstrated mechanism-based inhibition of placental aromatase activity. These agents were evaluated for inhibition of aromatase activity in the JAr human choriocarcinoma line. The 7-substituted 4,6-androstadiene-3,17-diones produced dose dependent inhibition of aromatase activity in the cell cultures, with IC50 values ranging from 490 nM to 4.5 microM. However, these agents are less effective when compared to other steroidal inhibitors, such as 7 alpha-thiosubstituted androstenediones. These results on the 7-substituted 4,6-androstadiene-3,17-diones are consistent with the data from biochemical enzyme inhibition studies using human placental aromatase. On the other hand, 7-phenethyl-1,4,6-androstatriene-3,17-dione exhibits greater inhibitory activity, with an IC50 value of 80 nM. Other mechanism-based inhibitors, 7 alpha-(4'-amino)phenylthio-1,4-androstadiene-3,17-dione and 4-hydroxyandrostenedione, also exhibited potent inhibition of aromatase activity in JAr cells. In summary, the most effective B-ring modified steroidal aromatase inhibitors are those derivatives that can project the 7-aryl substituent into the 7 alpha-position.