Steroidal inhibitors as chemical probes of the active site of aromatase

Anticancer steroids: linking natural and semi-synthetic compounds

Steroids, a widespread class of natural organic compounds occurring in animals, plants and fungi, have shown great therapeutic value for a broad array of pathologies. The present overview is focused on the anticancer activity of steroids, which is very representative of a rich structural molecular diversity and ability to interact with various biological targets and pathways. This review encompasses the most relevant discoveries on steroid anticancer drugs and leads through the last decade and comprises 668 references.

Aromatase Inhibitors: Structural Features and Biochemical Characterization

Aromatase is the enzyme synthesizing estrogens from androgens. In estrogen-dependent breast tumors, estrogens induce the expression of growth factors responsible for cancer cell proliferation. In situ estrogen synthesis by aromatase "is thought to play a key role in the promotion of breast cancer growth. Aromatase inhibitors (AIs) provide new approaches for the prevention and treatment of breast cancer by inhibiting estrogen biosynthesis. Through reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemical techniques, aromatase has been found to be expressed in many endocrine tissues and tumors originating from these tissues. Unexpectedly, this enzyme is now known to also be expressed in liver, lung, and colon cancers. Such findings suggest a potential role for endocrine manipulation of these types of cancer using AIs. Three Food and Drug Administration (FDA)-approved AIs, anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin), effectively challenging tamoxifen, have been used as first-line drugs in the treatment of hormone-dependent breast cancer, and possibly other aromatase-expressing cancers. In addition, natural anti-aromatase chemicals, such as flavones and coumarins, have been identified. Efforts to develop new lines of AIs derived from these phytochemicals have been initiated in several laboratories. Finally, significant progress has been made in the understanding of the structure-function relationship of aromatase. Such information has helped the examination of binding characteristics of AIs, the evaluation of reaction mechanism of aromatase, and the explanation of the molecular basis for a low catalytic activity of the natural variant, M364T.

Design and synthesis of new steroidal inhibitors of estrogen synthase (aromatase)

The estrogen synthase (aromatase) enzyme system is responsible for the biosynthesis of estrogen hormones in human females. Estrogens are vital for normal growth and development, but will promote the growth of certain breast cancers. Approximately 30-50% of breast cancers are considered to be hormone-dependent. Consequently regulation of estrogen biosynthesis has advanced as a potential therapeutic strategy. This has led to the development of active-site inhibitors, which may have potential for the control of breast cancer. We have recently prepared a number of new steroidal inhibitors that have been evaluated as aromatase inhibitors. These include steroidal A/B-ring isoxazoles and a series of A/B-ring pyrazoles with alkyl- and aryl-substituted nitrogen. In addition, we have developed new chemical procedures for the synthesis of 6beta-hydroxy steroids, which could be key intermediates in the preparation of C-19 inhibitors of aromatase activity.

Testosterone and its metabolites affect afterdischarge thresholds and the development of amygdala kindled seizures

In boys with epilepsy, pubertal increases in seizure frequency may be associated with rising androgen levels. The present study tested the hypothesis that testosterone (T) and/or its metabolites might affect amygdala seizure thresholds and the development of secondary generalization from amygdala foci (kindling). Afterdischarge thresholds and kindling rate were measured in gonadectomized (GDX) male rats, with or without T replacement therapy. Drugs that block either androgen or estradiol (E(2)) receptor-mediated responses were also tested.

METHODS

Kindling electrodes were implanted in the basolateral amygdala of adult male Wistar rats. In Experiment 1, subjects were GDX and implanted with a silastic capsule containing either: cholesterol (control); T; 5% E(2) in cholesterol; or 5alpha-dihydrotestosterone (DHT). In Experiment 2, intact subjects were treated with daily injections of vehicle (control); daily injections of flutamide (an androgen receptor antagonist); or Silastic implants containing 1,4,9-androstatriene 3,17-dione (ATD; an aromatase inhibitor).

RESULTS

In Experiment 1, initial afterdischarge (AD) thresholds were significantly lowered by E(2) treatment, as compared to cholesterol controls, and remained low throughout the kindling paradigm. In T replaced males, AD threshold significantly decreased over the kindling period, a response that was not observed in DHT treated rats. Rates of kindling were significantly faster as a result of T, E(2) and DHT treatment, as compared to cholesterol controls. E(2) treated males kindled the fastest of all 3 groups. In Experiment 2, initial AD thresholds were significantly lowered by flutamide treatment, as compared to cholesterol controls, and remained low throughout the kindling paradigm. AD threshold significantly decreased over the kindling period in intact males, a response that was blocked by ATD treatment. Both flutamide and ATD significantly slowed the rate of kindling, as compared to intact controls. ATD had the most dramatic inhibitory effect on kindling rate.

CONCLUSIONS

In males, T and its two metabolites, E(2) and DHT, all appear to enhance the development of amygdala-kindled seizures. E(2) has the most potent epileptogenic effect. Antagonism of E(2) mediated effects in the brain may have potential therapeutic value for males with epilepsy.

Aromatase inhibition in JAr choriocarcinoma cells by 7alpha-arylaliphatic androgens

The JAr choriocarcinoma cell cultures have demonstrated high levels of aromatase activity and have been useful for assaying a wide variety of aromatase inhibitors for aromatase inhibition in intact cells. Recently, several 7alpha-arylaliphatic androgens have shown effective inhibition of human placental microsomal aromatase in vitro, with apparent Ki values ranging from 10 to 20 nM. A series of 7alpha-arylaliphatic androst-4-ene-3,17-dione compounds demonstrated potent competitive inhibition, and 7alpha-arylaliphatic androsta-1,4-diene-3,17-diones were enzyme-activated irreversible inhibitors. Both series of these potent inhibitors were investigated for the ability to inhibit aromatase activity in JAr cells by measuring the conversion of [1beta-3H]-androstenedione to 3H2O and unlabelled estrone. JAr cell cultures were incubated for 2 h at 37 degrees C with the aromatase inhibitors at concentrations of 10 pM to 10 microM, the percentage of enzyme inhibition was determined, and IC50 values for inhibitors were calculated. Both series of synthetic compounds demonstrated good to excellent aromatase inhibition, and the most effective inhibitors in both series were those compounds with a phenylpropyl substituent at the 7alpha-position of the steroid nucleus. The 7alpha-arylaliphatic androst-4-ene-3,17-diones exhibited inhibition of JAr aromatase activity with IC50 values from 300 to 434 nM. More potent aromatase inhibition was observed with the 7alpha-arylaliphatic androsta-1,4-diene-3,17-diones, which exhibited IC50 values from 64 to 232 nM. Enhanced efficacy of steroidal enzyme-activated irreversible inhibitors compared to competitive inhibitors was observed in these studies and is consistent with previous reports. These results suggest that JAr choriocarcinoma cells with high levels of aromatase activity may be useful in differentiating steroidal aromatase inhibitors exhibiting different mechanisms of enzyme inhibition. In summary, the 7alpha-phenylpropyl androsta-1,4-diene-3,17-dione analogs, which are enzyme-activated irreversible inhibitors, demonstrated the most effective inhibition of aromatase activity present in the JAr cell cultures among the various 7alpha-arylaliphatic androgens.

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.

Aromatase inhibitors in the treatment of breast cancer

A number of inhibitors of estrogen synthesis are now becoming available which could be of value in the treatment of breast cancer. 4-Hydroxyandrostenedione (4-OHA), the first of these compounds to enter the clinic has been found to be effective in postmenopausal patients who have relapsed from tamoxifen. Thus, in studies of 240 patients, 26% patients experienced partial or complete response to treatment. An additional 25% patients had disease stabilization. 4-OHA is a potent selective, steroidal inhibitor which causes inactivation of aromatase in vitro. It is effective in reducing concentrations of ovarian estrogens in rats and of ovarian and peripheral estrogens in non-human primate species. The compound has been shown to lower serum estrogen levels in postmenopausal breast cancer patients. However, not all of these patients experienced disease remission, suggesting that their tumors were hormone insensitive rather than that the dose of 4-OHA was suboptimal. In trials of patients who had not received prior tamoxifen treatment, 4-OHA (250 mg i.m. every 2 weeks) was found to induce complete or partial tumor regression in 33% of patients. The response of patients was not significantly different from that observed in patients treated with tamoxifen (30 mg o.d) of 37%. No significant difference between treatments was observed for disease stabilization, the duration of response or median survival. Several other steroidal aromatase inhibitors have been studied, such as 7 alpha-substituted androstenedione derivatives. MDL 18962 [10-(2-propynyl)estr-4-ene-3,17-dione] and FCE 24304 (6-methylen-androsta-1,4-diene-3,17-dione) are currently in clinical trials. Non-steroidal inhibitors of cytochrome P-450 enzymes, such as imidazole and triazole derivatives have been developed which are highly selective for aromatase. Three triazoles which are very potent and selective inhibitors are vorazole (6-[(4-chlorophenyl)(1H-1,2,4-triazol-1-yl)-methyl]1-methyl-1H- benzotriazole R 76713, arimidex 2,2'[5-(1H-1,2,4-triazol-1-yl methyl)-1,3-phenylene]bis(2-methylpropiononitrile) (ZD1033) and letrozole 4-[1-(cyanophenyl)-1-(1,2,4-triazolyl)methyl]benzonitril (CGS 20267). These compounds reduce serum estradiol concentration to undetectable levels in breast cancer patients. These highly potent inhibitors provide the opportunity to determine whether a further degree of estrogen suppression will be important in producing greater clinical response.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Aromatase, its inhibitors and their use in breast cancer treatment

Aromatase, a cytochrome P450 enzyme, catalyses the rate-limiting step in the biosynthesis of estrogens. Many processes in male and female development and reproduction and especially in the growth of hormone-dependent cancers, are dependent on estrogens. Therefore, controlling estrogen production by inhibition of aromatase is a logical treatment strategy. Two classes of aromatase inhibitors, steroidal and non-steroidal compounds, are now coming into use. Among the steroid substrate analogs, 4-hydroxyandrostenedione has been shown to be effective in breast cancer patients with advanced disease and was recently approved for treatment in the United Kingdom. Several highly potent and selective non-steroidal inhibitors are now in clinical trials. The variety of compounds that act as aromatase inhibitors should provide breast cancer patients with a number of new treatment options.