Metabolism of the aromatase inhibitor 4-hydroxyandrostenedione in vivo. Identification of the glucuronide as a major urinary metabolite in patients and biliary metabolite in the rat

History of aromatase: saga of an important biological mediator and therapeutic target

Aromatase is the enzyme that catalyzes the conversion of androgens to estrogens. Initial studies of its enzymatic activity and function took place in an environment focused on estrogen as a component of the birth control pill. At an early stage, investigators recognized that inhibition of this enzyme could have major practical applications for treatment of hormone-dependent breast cancer, alterations of ovarian and endometrial function, and treatment of benign disorders such as gynecomastia. Two general approaches ultimately led to the development of potent and selective aromatase inhibitors. One targeted the enzyme using analogs of natural steroidal substrates to work out the relationships between structure and function. The other approach initially sought to block adrenal function as a treatment for breast cancer but led to the serendipitous finding that a nonsteroidal P450 steroidogenesis inhibitor, aminoglutethimide, served as a potent but nonselective aromatase inhibitor. Proof of the therapeutic concept of aromatase inhibition involved a variety of studies with aminoglutethimide and the selective steroidal inhibitor, formestane. The requirement for even more potent and selective inhibitors led to intensive molecular studies to identify the structure of aromatase, to development of high-sensitivity estrogen assays, and to "mega" clinical trials of the third-generation aromatase inhibitors, letrozole, anastrozole, and exemestane, which are now in clinical use in breast cancer. During these studies, unexpected findings led investigators to appreciate the important role of estrogens in males as well as in females and in multiple organs, particularly the bone and brain. These studies identified the important regulatory properties of aromatase acting in an autocrine, paracrine, intracrine, neurocrine, and juxtacrine fashion and the organ-specific enhancers and promoters controlling its transcription. The saga of these studies of aromatase and the ultimate utilization of inhibitors as highly effective treatments of breast cancer and for use in reproductive disorders serves as the basis for this first Endocrine Reviews history manuscript.

Update on the use of aromatase inhibitors in breast cancer

Estrogens are biosynthesised from androgens by the CYP450 enzyme complex called aromatase. Aromatase is expressed in the ovary, placenta, brain, bone, adipose tissue and breast tissue. In breast cancer, intratumoural aromatase is the source for local estrogen production in the tissue. Inhibition of aromatase is an important approach for reducing growth stimulatory effects of estrogens in estrogen-dependent breast cancer. The potent and selective third-generation aromatase inhibitors anastrozole, letrozole and exemestane were introduced to the market as endocrine therapy in postmenopausal patients failing anti-estrogen therapy alone, or multiple hormonal therapies. Anastrozole and letrozole are both non-steroidal aromatase inhibitors that compete with the substrate for binding to the enzyme active site. Exemestane is a mechanism-based steroidal inhibitor that mimics the substrate, is converted by the enzyme to a reactive intermediate, and results in inactivation of aromatase. These third-generation aromatase inhibitors are currently approved as first-line therapy for the treatment of postmenopausal women with metastatic estrogen-dependent breast cancer. The use of an aromatase inhibitor as initial therapy, or after treatment with tamoxifen, is now recommended as adjuvant hormonal therapy for postmenopausal women with hormone-dependent breast cancer. Several clinical studies of aromatase inhibitors focus on the use of these agents in the adjuvant setting, for the treatment of early breast cancer. Recently published results show improved responses with these agents compared with tamoxifen.

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.

Hormonal therapy in postmenopausal women with breast cancer

The treatment of postmenopausal women with breast cancer presents a number of challenges. Tamoxifen has had a substantial impact on mortality in women with early-stage, estrogen-receptor-positive tumors. Despite the improvement in the treatment of breast cancer, many patients will ultimately experience a recurrence. Present treatment approaches can provide effective palliation in the advanced disease setting, but, at best, there has been a modest impact on survival. Numerous options are now available to provide effective palliation for patients with advanced disease. These options include antiestrogens, pure antiestrogens, aromatase inhibitors and progestins and LHRH agonists. Recently, several studies have reviewed the efficacy of aromatase inhibitors as first-line agents in postmenopausal women. Anastrozole and letrozole have recently been approved as first-line agents in women with metastatic breast cancer. In addition to their role in metastatic breast, trials investigating the potential of aromatase inhibitors in the early breast cancer, both in the adjuvant and neoadjuvant setting are underway.

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.

Pharmacokinetics and metabolism of formestane in breast cancer patients

Formestane (Lentaron(R), 4-hydroxyandrostenedione) is a steroidal aromatase inhibitor used for treatment of advanced breast cancer. Clinically, it is administered as a depot form once fortnightly by intramuscular (i.m.) injection. To investigate the pharmacokinetics, bioavailability and metabolism of the drug, seven patients received single 250 mg i.m. doses of commercial formestane on Days 0, 21, 35, 49 and 63 of this trial. On Day 63, three of the patients received an additional single intravenous (i.v.) pulse dose of 1 mg of 14C-labelled formestane. The plasma kinetics after i.m. dosing confirmed a sustained release of formestane from the site of injection. Within 24-48 h of the first dose, the circulating drug reached a C(max) of 48.0+/-20.9 nmol/l (mean+/-S.D.; N=7). At the end of the dosing interval, after 14 days, the plasma concentration was still at 2.3+/-1.8 nmol/l. The kinetic variables did not significantly change during prolonged treatment. Intramuscular doses appear to be fully bioavailable. Following i.v. injection of 14C-formestane, the unchanged drug disappeared rapidly from plasma, the terminal elimination half-life being 18+/-2 min (N=3). Plasma clearance, CL was 4.2+/-1.3 l/(h kg) and the terminal distribution volume V(z) was 1.8+/-0.5 l/kg. The drug is mainly eliminated by metabolism, renal excretion of metabolites accounting for 95% of dose. The excretory balance of 14C-compounds in urine and faeces totals up to 98.9+/-0.8% of the i.v. dose after 168 h. The 14C-compounds in plasma and urine were separated by HPLC, and three major metabolites were submitted to structural analysis by MS, NMR and UV spectroscopy. One of the metabolites is the direct 4-O-glucuronide of formestane. The other two represent 3-O-sulfates of the exocons 3beta,4beta-dihydroxy-5alpha-androstane-17-one and 3alpha,4beta-dihydroxy-5alpha-androstane-17-one, their ratio being 7:3. These exocons are formed by stereoselective 3-keto reduction, accompanied by reduction of the 4,5-enol function. The exocons do not inhibit human placental aromatase activity in vitro.

Pharmacological profiles of exemestane and formestane, steroidal aromatase inhibitors used for treatment of postmenopausal breast cancer

Steroidal aromatase inhibitors like formestane and exemestane are useful drugs for endocrine treatment of postmenopausal breast cancer. In addition, these drugs should be considered valuable probes to explore the biology of breast cancer with particular emphasis on possible relations between the degree of estrogen suppression and clinical efficacy and the possible role of intratumor estrogen synthesis. The fact that steroidal and non-steroidal aromatase inhibitors bind to different parts of the aromatase enzyme suggests these drugs may act in concert aggravating plasma estrogen suppression. Thus, use of a steroidal and a non-steroidal aromatase inhibitors in concert may be one way to improve breast cancer treatment and may also provide important information to a better understanding of the dose-response relationship between estrogen suppression and clinical effects. Further, the finding that patients progressing on non-steroidal aromatase inhibitors may respond to formestane as well as exemestane suggests these drugs may have differential effects, probably on the aromatization in the tumor tissue. Further studies are warranted to explore the influence of steroidal and non-steroidal aromatase inhibitors on intratumor aromatase activity and intratumor estrogen concentrations and to correlate these findings to intratumor drug concentrations. The findings that steroidal aromatase inhibitors may have clinical effects in patients progressing on treatment with the non-steroidal aromatase inhibitor aminoglutethimide is challenging, and suggest further studies to evaluate possible benefits of using different novel aromatase inhibitors in concert or sequence.

The minimal effective exemestane dose for endocrine activity in advanced breast cancer

Phase I studies have demonstrated that exemestane, an irreversible oral aromatase inhibitor, is able to suppress circulating oestrogen levels. In our previous experience, doses ranging from 2.5 to 25 mg induced a similar suppression of oestrogens. The aim of this study was to identify the minimum effective exemestane dose on the basis of endocrine activity. 20 evaluable postmenopausal advanced breast cancer patients were randomly given exemestane 0.5, 1, 2.5 or 5 mg, in double-blind conditions. Oestrone (E1), oestradiol (E2), oestrone sulphate (E1S), gonadotrophins, sex-hormone binding globulin and dehydroepiandrosterone sulphate serum levels were evaluated from the first day of treatment to the 7th, 14th, 28th and 56th day. Serum E1, E2 and E1S levels were suppressed by all doses starting from day 7; the degree of inhibition versus baseline was 25 up to 72% for E1, 30 up to 62% for E2 and 16 up to 52% for E1S, with higher doses achieving greater suppression; these changes were maintained over time. A significant increase in FSH and LH levels was observed for all doses. Treatment tolerability was satisfactory. The endocrine effects of exemestane appear to be dose related and 0.5 and 1 mg are ineffective for adequately suppressing circulating oestrogens.

Formestane. A review of its pharmacological properties and clinical efficacy in the treatment of postmenopausal breast cancer

Formestane (4-hydroxyandrostenedione) is an aromatase inhibitor which significantly reduces plasma levels of estrogen and has shown antitumour activity in postmenopausal women with breast cancer. Objective response rates in heavily pretreated patients with advanced breast cancer generally range between 20 and 30% during treatment with intramuscular formestone 250 or 500mg once every 2 weeks, and a further 20 to 30% of patients experience disease stabilisation. The median duration of response is between 8 and 14 months. Highest response rates are observed in soft tissue metastases, in patients with estrogen-responsive tumours and in those showing a response to previous endocrine therapy. Furthermore, there is some evidence to suggest that higher response rates are achieved with formestane 500 versus 250mg once every 2 weeks. In comparative studies, the clinical efficacy of intramuscular formestane 250mg did not differ significantly from that of oral megestrol when administered as second-line endocrine therapy to patients with advanced disease in whom previous tamoxifen therapy had failed. In addition, formestane produced a response rate, duration of response and overall survival rate that was not significantly different from that of oral tamoxifen when administered as first-line endocrine therapy to patients with advanced disease, but tamoxifen was superior in some measures. Further investigation of these 2 agents, including the higher dosage of formestane (500mg), is necessary to confirm their relative efficacies. Formestane is well tolerated by the majority of patients; adverse events rarely necessitate cessation of therapy. The most common adverse events are local reactions at the injection site and systemic events usually related to the effect of the drug on the hormonal milieu. The systemic tolerability of formestane is similar to that of tamoxifen but better than that of megestrol. Thus, formestane is effective and well tolerated as first-line endocrine therapy for advanced disease. However, at present, it is unlikely to challenge tamoxifen in this indication, based on recent findings from a large comparative study and the fact that formestane requires intramuscular administration. Nonetheless, formestane, which appears to have a better tolerability profile than other currently available second-line agents (including megestrol and the aromatase inhibitor aminoglutethimide), is a valuable drug for the second-line treatment of postmenopausal women with advanced breast cancer.

Exemestane (FCE 24304), a new steroidal aromatase inhibitor

Exemestane (FCE 24304; 6-methylenandrosta-1,4-diene-3,17-dione) is a novel orally active irreversible aromatase inhibitor. Its in vitro and in vivo pharmacological properties have been compared to 4-hydroxyandrostenedione (4-OHA). In preincubation studies with human placental aromatase, exemestane, like 4-OHA, showed enzyme inactivating properties with a similar affinity (Ki 26 vs 29 nM) and a lower rate of inactivation (t1/2 13.9 vs 2.1 min). Conversely, when tested in pregnant mares' serum gonadotropin-treated rats, exemestane was more potent in reducing microsomal ovarian aromatase activity than 4-OHA, after both subcutaneous (ED50 1.8 vs 3.1 mg/kg) and oral dosing (ED50 3.7 vs greater than 100 mg/kg). No interference of exemestane on desmolase or 5 alpha-reductase activity was found. The compound did not show any relevant binding affinity to steroidal receptors, but slight binding to the androgen receptor (approximately 0.2% of dihydrotestosterone), like 4-OHA. In the first phase I trial, healthy postmenopausal volunteers were given single oral doses of exemestane, ranging from 0.5 to 800 mg, and plasma [estrone (E1), estradiol (E2) and estrone sulphate (E1S)] and urinary estrogens (E1 and E2) were measured up to 5-8 days. The minimal effective dose in decreasing estrogens was 5 mg. At 25 mg the maximal suppression was observed at day 3: plasma estrogens fell to 35 (E1), 39 (E2) and 28% (E1S), and urinary estrogens fell to 20 (E1) and 25% (E2) of basal values, these effects still persisting on day 5. No effects on plasma levels of cortisol, aldosterone, 17-hydroxyprogesterone, DHEAS, LH and FSH, and no significant adverse events were observed up to the highest tested dose of 800 mg exemestane.

Identification of 4-hydroxyandrost-4-ene-3,17-dione metabolites in prostatic cancer patients by liquid chromatography-mass spectrometry

Liquid chromatography with thermospray mass spectrometry has proved to be an invaluable technique for the study of metabolic degradation of xenobiotics in complex biological fluids. This paper describes the detection of 4-hydroxyandrost-4-ene-3,17-dione and its metabolites in urinary extracts from prostatic cancer patients. Several metabolites were detected including 4 beta,5 alpha-dihydroxyandrostan-3,17-dione, 3,17-dihydroxyandrostan-4-ones and 3 alpha-hydroxy-5 beta-androstan-4,17-dione.

Determination of 4-hydroxyandrostenedione in plasma and urine by extractive alkylation and electron-capture gas chromatography

A sensitive and specific quantitative assay has been developed for the determination of 4-hydroxyandrostenedione (4-OHA), a potent aromatase inhibitor used in the treatment of estrogen-dependent breast cancer. This steroid has a high first-pass metabolism and is extensively metabolized, mainly by glucuronidation. Plasma levels of unchanged 4-OHA are very low, even after high peroral doses. The analytical method is based on the addition of 17 alpha-ethinylestradiol (internal standard), liquid-liquid extraction from biological material followed by extractive alkylation with pentafluorobenzyl bromide and quantitation by gas chromatography. The method has been validated for sensitivity, accuracy and precision and was found to be suitable for application to pharmacokinetic and bioavailability studies of peroral formulations of 4-OHA.

Determination of 4-hydroxyandrost-4-ene-3,17-dione metabolism in breast cancer patients using high-performance liquid chromatography-mass spectrometry

A sensitive procedure for studying the metabolism of the steroidal aromatase inhibitor 4-hydroxy-androst-4-ene-3,17-dione (4OHA) was developed based on enzyme hydrolysis, liquid-liquid extraction and reversed-phase liquid chromatography coupled with a mass spectrometer (LC-MS) using a thermospray interface. Seven metabolites were identified from the hydrolysed urine samples together with the parent drug. The major routes of metabolism were via dehydrogenation, reduction of the ketone functional groups, reduction at the C-4-C-5 double bond and hydroxylation at the C-5 position. Confirmation of the identity of 4OHA and its metabolites isolated from female patients' urine samples was accomplished by comparison of the retention times of their corresponding synthetic standards on LC-MS. We have demonstrated that this technique is particularly suitable for studying the metabolism of steroidal drugs.

Aromatase inhibitors and hormone-dependent cancers

Aromatase (estrogen synthetase) occurs in a variety of tissues. Using immunocytochemistry, we have recently located this enzyme in cellular compartments of several types of human tissue. Furthermore, we found the mRNA was located in the same structures where tested. As both gonadal and peripherally formed estrogen contribute to growth of hormone sensitive cancers, we have developed aromatase inhibitors to block synthesis of this hormone. We have determined that 4-hydroxyandrostenedione (4-OHA) selectively inhibits aromatase activity in ovarian and peripheral tissues and reduces plasma estrogen levels in rat and non-human primate species. 4-OHA was also found to inhibit gonadotropin levels and reduce estrogen and progesterone receptor levels in treated animals. The mechanism of these effects appear to be associated with the weak androgenic activity of the compound. These effects together with aromatase inhibition may result in a synergistic response reducing estrogen production and action. In postmenopausal women, estrogens are mainly of peripheral origin. When postmenopausal breast cancer patients were administered either daily oral or parenteral weekly treatment with 4-OHA at doses that did not affect their gonadotropin levels, plasma estrogen concentrations were significantly reduced. Complete or partial response to treatment occurred in 34% of 100 patients with advanced breast cancer, while the disease was stabilized in 12%. These results indicate that 4-OHA is of benefit in postmenopausal patients with advanced disease who have relapsed from prior hormonal therapies, and that steroidal inhibitors may be of value in premenopausal patients.

4-Aminoandrostenedione derivatives: a novel class of irreversible aromatase inhibitors. Comparison with FCE 24304 and 4-hydroxyandrostenedione

FCE 24928 (4-aminoandrosta-1,4,6-triene-3,17-dione) was selected among a series of 4-aminoandrostenedione derivatives as a novel irreversible aromatase inhibitor. Its in vitro and in vivo properties have been studied and compared to FCE 24304 (6-methylenandrosta-1,4-diene-3,17-dione) and 4-OHA (4-hydroxyandrostenedione). FCE 24928 caused time-dependent inhibition of human placental aromatase with a t1/2 of 4 min and Ki of 59 nM. Enzyme inactivation by FCE 24928 was faster than by FCE 24304 (t1/2 13.9 min). In PMSG-treated rats, microsomal ovarian aromatase activity was reduced 24 h after FCE 24928 dosing by both the s.c. (ED50 1.2 mg/kg) and the oral (ED50 14.1 mg/kg) routes. The compound was more potent than FCE 24304 and 4-OHA (ED50 1.8 and 3.1 mg/kg s.c.). FCE 24928 did not show any interference with 5 alpha-reductase and desmolase activity nor any significant binding affinity for androgen and estrogen receptors. Slight binding affinity for androgen receptor was observed with FCE 24304 and 4-OHA (0.21 and 0.25% of DHT). In immature, castrated rats, FCE 24928 did not show any intrinsic androgenic activity, up to 100 mg/kg/day s.c., in contrast to a slight androgenic activity observed with FCE 24304 at 10 mg/kg s.c.

Serum kinetics of the anti-cancer agent 4-hydroxyandrostenedione in the rat

A previously described radioimmunoassay (RIA) method for the measurement of 4-hydroxyandrostenedione (4-OHA) was used to investigate the serum drug levels attained after a single oral dose in male and female rats. Marked variability of serum drug concentrations and their time course were evident in male animals at all dose levels. In the female rat, in contrast, serum 4-OHA showed fewer individual differences, rose more rapidly and was sustained at substantially higher concentrations. In all animals, 4-OHA appeared in the serum within 0.5 h following the oral dose and persisted for at least 48 h. Doubling the dose from 8 mg/kg produced a disproportionately large elevation in serum drug levels, but a further increase to 32 mg/kg did not further increase serum levels.

Metabolism of tamoxifen by isolated rat hepatocytes. Identification of the glucuronide of 4-hydroxytamoxifen

Metabolism of 4-hydroxytamoxifen by hepatocytes isolated from rats administered with phenobarbital and examination by TLC of the components not extractable into ethyl acetate revealed 4-hydroxytamoxifen beta-glucuronide; its identity was confirmed by comparison of its 1H NMR spectrum with that of synthetic material. This conjugate was also formed on metabolism of tamoxifen. It bound to cytosolic oestrogen receptors with only one thousandth the affinity of 4-hydroxytamoxifen and gave a correspondingly very weak inhibition of growth of the MCF-7 human breast cancer cell line. Therefore, in contrast to reported observations on the 3-glucuronide of oestradiol, the MCF-7 cells were unable to hydrolyse 4-hydroxytamoxifen glucuronide and on this evidence, formation of this metabolite is solely a deactivation pathway.

Treatment of advanced breast cancer in postmenopausal women with 4-hydroxyandrostenedione

4-Hydroxyandrostenedione (4-OHA), a new specific aromatase inhibitor, was used to treat 57 postmenopausal women with advanced breast cancer at a dose of 250 mg by i.m. injection every 2 weeks; 55 women were assessable for response. In all, 18 patients (33%) had objective evidence of a response to treatment, with a median duration of 12 months; the disease stabilised in 8 (14%) patients. Serum oestradiol levels, which were measured weekly in nine of the patients, were found to be suppressed to a mean of between 36% and 51% of pretreatment levels during the first 6 weeks of treatment. Three patients were withdrawn from treatment because of toxicity (pain at injection site, sterile abscess and rash). One patient had an isolated episode of anaphylaxis after 6 months of treatment. In comparison with our previous reports of 4-OHA treatment, a dose of 250 mg given i.m. fortnightly appears to be the optimal dose regimen. The efficacy of the drug seems to be similar to that of tamoxifen and aminoglutethimide.

Radioimmunoassay of the anti-cancer agent 4-hydroxyandrostenedione in body fluids

Antibodies were produced in sheep against a new anti-breast cancer drug 4-hydroxyandrostenedione (4-OHA) using two hapten-ovalbumin conjugates. One of these conjugates (4-hydroxytestosterone-ovalbumin) produced an antiserum suitable for the development of a radioimmunoassay that would allow direct measurement of 4-OHA in plasma at concentrations down to 82 pmol/l, with adequate accuracy, precision and scope for further sensitivity. Although this assay would measure 4-hydroxytestosterone (4-OHT) in addition to 4-OHA, the present data suggest that the magnitude of any interference from endogenous steroids and those derived from 4-OHA could only be minimal. A comparison of solvent-extracted and unextracted samples showed that only unconjugated drug was analysed by this radioimmunoassay. A study of plasma protein binding of 4-OHA showed that at therapeutic concentrations, between 13.5 and 16.5% of plasma 4-OHA was not bound to proteins. This assay system could be a useful adjunct to the future development of 4-OHA as an anti-cancer drug.

Comparison of the pharmacokinetics and pharmacodynamics of unformulated and formulated 4-hydroxyandrostenedione taken orally by healthy men

A study of the aromatase inhibitor 4-hydroxy-androstenedione (4-OHA) was conducted in normal healthy men to compare the oral administration of two preparations of the drug: an unformulated, micronized powder and a formulated microcrystalline material (CGP 32349). The formulated material achieved a significantly higher mean peak concentration (88% greater than that obtained using the unformulated powder) and a higher mean AUC (not significant). The median time to peak was 1.5 h for both preparations and the elimination rate constants were similar (0.31 for micronized 4-OHA and 0.36 h-1 for formulated 4-OHA). Plasma concentrations of 4-OHA in this group were markedly lower than those previously observed in postmenopausal breast cancer patients. Significant biological activity was demonstrated with the formulated material in its suppression of plasma oestradiol levels, whereas no significant suppression was obtained using the micronized powder. An increase in androgen levels was observed that may have been due to competitive inhibition of enzymes involved in metabolic clearance of androgens and/or to decreased feedback inhibition of gonadotrophin secretion by oestradiol.

Quantitative determination of 4-hydroxy-4-androstene-3,17-dione (4-OHA), a potent aromatase inhibitor, in human plasma, using isotope dilution mass spectrometry

An original method is described for the determination in human plasma of 4-hydroxy-4-androstene-3,17-dione (4-OHA), a potent aromatase inhibitor, by isotope dilution mass-spectrometry using 7,7-[2H2]-4-OHA as internal standard. This compound was synthesized starting from 7,7-[2H2]-4-androstene-3,17-dione. The procedure includes an extraction step using an Extrelut 1 column and a derivatization with N,o-bis(trimethylsilyl)trifluoroacetamide (BSTFA). The minimum detection level of the method is 0.650 pg and the coefficients of variation for the 0.5 ng/ml (plasma) and 5 ng/ml (plasma) concentrations are 3.2% (within assay) and 6.7% (between assay) and 1.86% (within assay) and 2.3% (between assay) respectively.

Metabolism of steroid-modifying anticancer agents

The application of steroid-modifying drugs as a strategy for the treatment of hormone-dependent cancers has gained increasing popularity during the past decade. However, it is important to point out and emphasize that very few of the agents were originally designed for their current application. Most were designed for other purposes, predominantly fertility control (e.g. LHRH agonists and the antiestrogens). Nevertheless, now it is possible to integrate their actions to design rational therapies. There are many reasons for the current interest in antisteroidal drugs. The initial euphoria over the potential ability of combination chemotherapy to cure breast and prostatic carcinoma has proved to be premature. Combination chemotherapy has many severe side-effects which limits patient acceptability, especially if the patient realizes that the likelihood of a cure is remote. In the main, antisteroidal therapies do not have many side-effects and those that do, e.g. aminoglutethimide, are the focus of increased efforts in drug design to produce increased drug specificity. Finally, there is a growing realization that hormone-dependent cancer control with a nontoxic, antisteroidal therapy may be the most acceptable approach currently available for early disease management. Chemotherapy would then be reserved as the final option for treatment. The description of drug metabolism has been central to the development of synthetic LHRH analogs and an understanding of the mode of action of nonsteroidal antiestrogens and antiandrogens. The discovery of steroid synthetic pathways has been essential for the development of the aromatase inhibitors. This whole area of endeavor has now become a major focus of attention for the medicinal chemist. A new generation of agents is entering clinical evaluation which will provide a wealth of valuable information about the successful (or unsuccessful?) methods to control hormone-dependent disease. Since the success or failure of a drug can often depend upon formulation, pharmacokinetics, bioavailability or metabolism, it is our hope that this overview might help solve some of the future problems.

Aromatase inhibitors: their biochemistry and clinical potential

It has been proposed that one of the endocrinological factors in the pathogenesis of benign prostatic hyperplasia is estrogen stimulation of stromal growth. Current clinical experience with anti-estrogenic compounds indicates that, in the case of mammary carcinoma, aromatase inhibitors provide a viable alternative to estrogen receptor antagonists for treatment of the disease. It is proposed that inhibitors of estrogen biosynthesis could likewise provide a non-invasive therapy for benign prostate disease. Some aspects of the activity of known aromatase inhibitors as substrates for enzymes of steroid metabolism and their potential relevance to the pharmacology of the compounds are discussed.

Metabolism of 4-hydroxyandrost-4-ene-3,17-dione by rat hepatocytes

4-[14C]HAD was rapidly metabolized (99% after 5 min) by hepatocytes from phenobarbital-treated rats. An array of phase I metabolites was formed, variously involving one and two reductions, hydroxylation, hydration and hydroxylation plus one or two reductions. Some of the metabolites were identified by synthesis and others tentatively by mass spectrometry. After 10 min, approximately 30% of the original radioactivity was present in HAD glucuronide and, after 15 min, approximately 60% was present in the total glucuronide fraction which contained several components. Only one of the phase I metabolites (2-hydroxy-HAD) exhibited significant aromatase inhibitory activity (45% of that of HAD).