Massive extranglandular aromatization of plasma androstenedione resulting in feminization of a prepubertal boy

This report describes the mechanism of origin and the quantity of estrogen produced in a prepubertal boy who developed severe feminization at 8 yr of age as the result of a heretofore undescribed metabolic abnormality. The clinical findings were gynecomastia and accelerated linear growth and bone maturation. At the time feminization developed, there were no signs of growth or development of the otherwise normal prepubertal male external genitalia or any increase of muscle mass that normally accompanies male puberty. The hyperestrogenism was found to be the consequence of massive extraglandular conversion of plasma androstenedione to estrone. During a 6-mo period of study, the plasma production rate of androstenedione ranged from 1.2 to 1.6 mg/day. More than 55% of plasma androstenedione was metabolized by aromatization to estrone which, in turn, was extensively sulfurylated in the tissue sites of aromatization before its entry into the blood. Thus, estrone sulfate was the final product in the aromatizing sites, and the plasma production rate of estrone sulfate derived from plasma androstenedione was 782 mug/24 h. The extent of extraglandular conversion of plasma androstenedione to estrone measured in this boy was 50 times that observed in two normal prepubertal boys. Moreover, 94% of the extraglandular aromatization occurred in extrahepatic sites. The metabolic clearance rate of plasma androstenedione, 2,380 liters/day per m(2), was markedly increased in this boy. Approximately 1,500 liters of plasma androstenedione clearance was accounted for by extrahepatic, extraglandular aromatization. The fractional conversion of testosterone to estradiol, 0.16, was 50 times greater in this boy than that observed in normal young adult men. The total extent of aromatization of plasma prehormones was even greater in this boy inasmuch as evidence was obtained that aromatization of 16-hydroxysteroids, e.g. 16alpha-hydroxy androstenedione and 16alpha-hydroxy dehydroisoandrosterone (sulfate), resulted in estriol formation independent of estrone formation. Thus, extensive extrahepatic, extraglandular aromatization resulted in advanced feminization in this prepubertal boy by a previously undescribed metabolic abnormality.     

The origin of androgen and estrogen in a virilized postmenopausal woman with bilateral benign cystic teratomas

Clitoromegaly was observed in a 73-year-old woman who had bilateral ovarian benign cystic teratomas that contained only epithelial derivatives. Scattered in the stroma of these ovaries were clusters of hyperplastic cells. The plasma testosterone production rate was 2.4 mg/day, a value that is ten times greater than that of normal postmenopausal women. The elevated plasma concentrations of testosterone and androstenedione decreased to normal by the second day after bilateral oophorectomy. Ovarian venous concentrations of testosterone and androstenedione were also increased and were considerably greater than those in peripheral blood. Estrone production was 68 microng/day, most of which could be accounted fo by the extraglandular formation from androstenedione. Estradiol production was 71 microng/day, of which 75% was computed to arise from extraglandular formation and the remainder likely arose by ovarian secretion. The modest increase in estrogen production was associated with slight cystic hyperplasia of the endometrium. From this study we conclude that the hyperplastic ovarian stroma was the source of the excessive androgen production.     

Profound suppression of plasma estrogens by megestrol acetate in postmenopausal breast cancer patients

Twelve postmenopausal women suffering from advanced breast cancer had plasma estrogens, androgens, cortisol, and gonadotropins determined before therapy and during treatment with megestrol acetate (MA) in oral doses escalated from 40 to 160 mg. The plasma clearance and production rate of estrone and estrone sulfate were determined before treatment and after 4 weeks of therapy with 160 mg MA. Treatment with MA suppressed plasma levels of dehydroepiandrosterone sulfate, androstenedione, and cortisol in a dose-dependent manner to <10% of pretreatment values. Plasma testosterone, estradiol, estrone, and estrone sulfate were suppressed to 18-29% of pretreatment values, whereas the gonadotropins were suppressed to 35-52%. The plasma clearance rates of estrone and estrone sulfate were increased by a mean value of 23.7% (P < 0.01) and 23.5% (P < 0.025), whereas the production rates were reduced by 76.7% (P < 0.0005) and 76.1% (P < 0.0005), respectively. Our findings indicate that MA causes profound suppression of adrenal steroid production but in addition suppresses ovarian secretion of androgens in postmenopausal breast cancer patients. The reduction in plasma estrogens is comparable to values obtained with commonly used aromatase inhibitors and may be responsible for its antitumor effects in breast cancer.     

Influence of dexaminoglutethimide, an optical isomer of aminoglutethimide, on the disposition of estrone sulfate in postmenopausal breast cancer patients

Aminoglutethimide (AG) has been the most widely used aromatase inhibitor in breast cancer patients to date. Commercially, AG (Orimeten) is available as a racemate (DL-AG). Previous studies suggested the stereoisomers of AG (D-AG and L-AG) to differ considerably in their affinities and potencies to inhibit different cytochrome P-450-dependent enzymes, with D-AG being the potent aromatase inhibitor. DL-AG, apart from being an aromatase inhibitor, is known to enhance the metabolism of plasma estrone sulfate (E1S). In the present study we compared the effects of D-AG (500 mg daily) and DL-AG (1000 mg daily) on plasma estrogen levels and estrone (E1) and E1S clearance rates, determined after the injection of [14C]E1 and [3H]E1S, in a cross-over study involving 12 postmenopausal breast cancer patients. Treatment with DL-AG and D-AG suppressed plasma E1S to 18.6% and 15.0% of pretreatment levels, whereas E1 and estradiol E2 levels fell to 18.6% and 23.4% of their pretreatment levels during treatment with DL-AG and to 17.7% and 23.4% during treatment with D-AG, respectively. Thus, both treatment options suppressed all estrogens measured to a similar extent. The clearance rate of E1S increased from a mean pretreatment value of 5.9 to 14.0 and 10.0 L/h during treatment with DL-AG and D-AG, respectively (P < 0.05, comparing the two on-treatment situations), whereas the production rate of E1S decreased from a pretreatment value of 1.44 to 0.64 nmol/h with DL-AG and 0.36 nmol/h with D-AG (P < 0.05, comparing on-treatment values). These findings are consistent with the hypothesis that the D- as well as the L-form of AG may enhance the clearance rate of E1S. The finding of a higher estrogen production rate during treatment with DL-AG compared to D-AG probably reflects an increased plasma level of the estrogen precursor androstenedione (mean levels of androstenedione of 2.54 and 1.27 nmol/L during treatment with D-AG and DL-AG, respectively; P < 0.05).     

Aromatase inhibitors--new possibilities in treatment of breast carcinoma

Aromatase inhibition is now an acknowledged second line treatment modality for advanced breast cancer in postmenopausal women. Aminoglutethimide is an inhibitor of adrenal steroid biosynthesis and blocks the conversion of cholesterol to pregnenolone, and therefore reduces levels of adrenal androgens, which are a source of estrogens in both premenopausal and postmenopausal women. Aminoglutethimide has produced antitumor response rates of 35% in unselected patients, most of whom have undergone prior therapy with either chemotherapy or hormonal manipulation. As is true of other hormonal responses, high response rates of up to 70% are observed in patients who are ER and/or PR positive. The reason why these drugs are currently used after tamoxifen is mainly due to the side effects of aminoglutethimide, which impairs the mineralocorticoid and glucocorticoid synthesis. New, less toxic compounds appear, which block the conversion of androstenedione to estrone and efficiently suppress plasma estrogen levels., e.g. formestane, anastrozole and letrozole. Aromatase inhibitors are now being compared to tamoxifen as first-line endocrine treatment in relapsing patients. If these trials confirm a similar or better response rate to new aromatase inhibitors compared to tamoxifen, the time will come to study them as the first line adjuvant treatment in non-metastatic disease.     

Anastrozole: a new selective nonsteroidal aromatase inhibitor

Aromatase (estrogen synthetase) is the enzyme complex responsible for the final step in estrogen synthesis--the conversion of androstenedione and testosterone to estrone and estradiol, respectively. Inhibitors of this enzyme have been shown to be clinically effective in the treatment of advanced breast cancer in postmenopausal women, in whom the major source of estrogen production derives from aromatization of adrenal androgens in peripheral tissues, such as muscle, liver, and fat. The most widely used aromatase inhibitor has been aminoglutethimide; however, it is nonselective and also inhibits adrenocorticosteroid synthesis, necessitating hydrocortisone supplementation. Aminoglutethimide is also associated with frequent and troublesome side effects. Formestane, the first selective aromatase inhibitor to be developed, has an improved safety profile and selectivity, but its use has been limited somewhat by its inconvenient administration via intramuscular injection. In this article, the preclinical and clinical data published to date on the new third-generation aromatase inhibitor anastrozole (Arimidex) are presented in the context of current endocrine therapies. Future applications of aromatase inhibitors, both as monotherapy and in combination with other endocrine therapies, are discussed. The use of aromatase inhibitors in advanced disease, the adjuvant setting, and as possible chemopreventive agents are examined.     

Plasma sex steroid hormone levels and risk of breast cancer in postmenopausal women

BACKGROUND: A positive relationship has generally been observed between plasma estrogen levels and breast cancer risk in postmenopausal women, but most of these studies have been small and few have evaluated specific estrogen fractions (such as percent bioavailable estradiol). In addition, few studies have evaluated plasma androgen levels in relation to breast cancer risk, and their results have been inconsistent. We prospectively evaluated relationships between sex steroid hormone levels in plasma and risk of breast cancer in postmenopausal women by use of a case-control study nested within the Nurses' Health Study. METHODS: Blood samples were collected during the period from 1989 through 1990. Among postmenopausal women not using hormone replacement therapy at blood collection (n = 11,169 women), 156 women were diagnosed with breast cancer after blood collection but before June 1, 1994. Two control subjects were selected per case subject and matched with respect to age, menopausal status, month and time of day of blood collection, and fasting status at the time of blood collection. RESULTS: From comparisons of highest and lowest (reference) quartiles, we observed statistically significant positive associations with risk of breast cancer for circulating levels of estradiol (multivariate relative risk [RR] = 1.91; 95% confidence interval [CI] = 1.06-3.46), estrone (multivariate RR = 1.96; 95% CI = 1.05-3.65), estrone sulfate (multivariate RR = 2.25; 95% CI = 1.23-4.12), and dehydroepiandrosterone sulfate (multivariate RR = 2.15; 95% CI = 1.11-4.17). We found no substantial associations with percent free or percent bioavailable estradiol, androstenedione, testosterone, or dehydroepiandrosterone. The positive relationships were substantially stronger among women with no previous hormone replacement therapy. CONCLUSION: Our data, in conjunction with past epidemiologic and animal studies, provide strong evidence for a causal relationship between postmenopausal estrogen levels and the risk of breast cancer.     

Editorial: Sex and alcohol: a second peek

The effects of alcohol on male sexual function is discussed. Various phenomena affecting sexual function in males have been reported: 1) increased conversion of androgens to estrogens in the liver, 2) interconversion of weak and potent estrogens, 3) increased rate of conversion of adrenocortical steroid precursor to estrogenic substances, 4) decreased plasma levels of luteinizing hormone(LH) and follicle stimulating hormone (FSH), and 5) diminished uptake of androstenedione by the liver and increased peripheral conversion to estrone. It is proposed that the pathogenesis of feminization in alcoholic males may be due partly to the derivative effects of liver damage and to a direct action of alcohol on the testes. Rats fed alcohol have shown atrophy of the gonads and accessory organs, and impaired spermtogenesis. In normal men ingesting alcohol for short periods of time, the frequency and intensity of bursts of testosterone secretion by the testes is dampened, and testosterone concentrations and production rates are increased while the metabolic clearance of testosterone is diminished. Deranged hypothalamic-pituitary function has also been observed. Further research is needed to elucidate the mechanism(s) by which alcohol influences sexual function.     

Inhibition of breast cancer tissue aromatase activity and estrogen concentrations by the third-generation aromatase inhibitor vorozole

In about one-third of advanced breast cancers, estrogen deprivation causes tumor regression. Estrogen concentrations in tumor tissue seem to depend largely on local production. The aromatase enzyme complex is thought to be the key enzyme in this respect. In the present study, the effect of the new third-generation nonsteroidal aromatase inhibitor vorozole (Rivizor) on tumor tissue aromatase activity and estrogen concentrations was evaluated. During 7 days preceding mastectomy, 11 postmenopausal breast cancer patients were treated with 2.5 mg of vorozole once daily. Eight patients could be evaluated. Intratumoral aromatase activity and estrone and estradiol levels were measured and compared to the values of nine untreated postmenopausal breast cancer patients. In treated patients, median tissue aromatase activity was 89% lower than that in controls (P < 0.001). Similarly, median tissue estrone and estradiol concentrations were 64 and 80% lower, respectively, in treated patients (P = 0.001 and P < 0.05, respectively). These results support the hypothesis that depleting the tumor of estrogens, thus impairing estrogenic stimulation, is an important mechanism in the antitumor activity of aromatase inhibitors.     

Plasma hormones in clomiphene citrate therapy

Daily plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrone (E1), estradiol-17beta (E2), progesterone (P), androstenedione (A), and testosterone (T) were measured in six clomiphene citrate (Clomid) treated cycles. Three patients ovulated and 1 of them conceived during the study cycle. Three other patients failed to ovulate in spite of some evidence of ovarian response to clomiphene treatment in 2 of them. Plasma gonadotropin levels, of LH in particular, rose during the clomiphene therapy and reached a peak during Day 5 to Day 7 of therapy. Levels of plasma estrogens, both E1 and E2, gradually rose, reflecting follicular maturation in the ovary. When E2 reached a critical level as in the normal ovulatory cycle, it triggered an LH surge which consequently initiated ovulation. When the E2 level was inadequate or excessive, ovulation failed in spite of an LH surge. Following ovulation, plasma P rose and fell in a manner similar to the normal ovulatory cycle, with occasional values that exceeded the normal range. Levels of androgens, both A and T, rose during clomiphene therapy in some cases and T seemed to fluctuate in correlation with LH level. The possible local inhibitory influence of high E2 and T levels on follicular maturation in the ovary during clomiphene therapy is suspected in some cycles in which ovarian response was evident, but ovulation failed to occur.     

Fetal and maternal endocrine responses to experimental intrauterine infection in rhesus monkeys

OBJECTIVE: Our purpose was to describe the temporal and quantitative relationships among intrauterine infection, fetal-placental steroid biosynthesis, and preterm labor in a nonhuman primate model. STUDY DESIGN: On approximately day 130 of gestation (term 167 days) chronically instrumented rhesus monkeys (Macaca mulatta) were infected with 10(6) colony-forming units of group B streptococci either by intraamniotic (n = 4) or choriodecidual (n = 2) inoculation. As controls, four additionally chronically instrumented noninfected monkeys were followed up to spontaneous parturition. Amniotic fluid and maternal and fetal arterial blood were serially sampled in all monkeys (both before and after infection) for progesterone, estrone, estradiol, dehydroepiandrosterone, dehydroepiandrosterone sulfate, androstenedione, and cortisol by specific radioimmunoassays, and uterine activity was continuously recorded. RESULTS: Spontaneous parturition was preceded by gradual and significant increases in the plasma concentrations of fetal dehydroepiandrosterone, dehydroepiandrosterone sulfate, and androstenedione and fetal and maternal levels of estrone, estradiol, and progesterone but not by changes in cortisol. In contrast, infection-associated parturition (either intraamniotic or choriodecidual) was characterized by abrupt increases in fetal dehydroepiandrosterone, dehydroepiandrosterone sulfate, androstenedione, progesterone, and cortisol but not by increases in maternal or fetal estrone or estradiol. Infection-associated steroid changes occurred concurrently with or after increases in uterine activity. CONCLUSION: Infection-associated preterm parturition is associated with dramatic increases in fetal adrenal steroid biosynthesis but not by corresponding increases in placental estrogen biosynthesis. This suggests that fetal stress in accompanied by placental dysfunction and that infection-associated parturition is not dependent on the increased estrogen biosynthesis observed in spontaneous parturition.     

In vivo inhibition of aromatization by exemestane, a novel irreversible aromatase inhibitor, in postmenopausal breast cancer patients

The effect of exemestane (6-methylenandrosta-1,4-diene-3,17-dione) 25 mg p.o. once daily on in vivo aromatization was studied in 10 postmenopausal women with advanced breast cancer. Aromatization was determined before treatment and after 6-8 weeks on therapy by administering a bolus injection of [3H]androstenedione (500 microCi) and [14C]estrone (5 microCi) followed by measurement of the isotope ratio of urinary estrogens after high-performance liquid chromatography purification. In addition, plasma endogenous estrogens were measured with highly sensitive radioimmunoassays after separation with high-performance liquid chromatography. Treatment with exemestane suppressed whole body aromatization from a mean pretreatment value of 2.059% to 0.042% (mean suppression of 97.9%). Plasma levels of estrone, estradiol, and estrone sulfate were found to be suppressed by 94.5%, 92.2%, and 93.2%, respectively. This is the first study revealing near total aromatase inhibition in vivo with the use of a steroidal aromatase inhibitor. The observation that exemestane is a highly potent aromatase inhibitor, together with the fact that the drug is administered p.o. and causes limited side effects, suggests that exemestane is a promising new drug for the treatment of hormone sensitive breast cancer.     

Aromatization of androstenedione by cultured human fibroblasts

The conversion of (1, 2, 6, 7-3H)androstenedione to (3H)estrone was measured in fibroblast monolayers grown from biopsies of genital and nongenital skins obtained from 15 control subjects, 9 males with developmental defects of the urogenital tract, and 8 patients with hereditary male pseudohermaphroditism. Under the standardized conditions utilized in this study, the rate of estrone formation in the fibroblasts from normal controls varied from less than 0.2 to 5.5 pmol/100 mg protein/h, and these rates were enhanced by incubation of intact monolayers with choleragen, theophylline, or dexamethasone. Rates of estrone formation were higher in some foreskin strains grown from subjects with developmental defects of the urogenital tract and in strains from scrotum and foreskin of patients with familial incomplete male pseudohermaphroditism, types 1 and 2 than in normal strains or strains from patients with testicular feminization. The meaning of these apparent high rates of estrone formation is unclear.     

Role of 17 beta-hydroxysteroid dehydrogenase type 1 in endocrine and intracrine estradiol biosynthesis

Enzymes with 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) activity catalyse reactions between the low-active female sex steroid, estrone, and the more potent estradiol, for example. 17 beta-HSD activity is essential for glandular (endocrine) sex hormone biosynthesis, but it is also present in several extra-gonadal tissues. Hence, 17 beta-HSD enzymes also take part in local (intracrine) estradiol production in the target tissues of estrogen action. Four distinct 17 beta-HSD isozymes have been characterized so far, and the data strongly suggests that different 17 beta-HSD isozymes have distinct roles in endocrine and intracrine metabolism of sex steroids. Current data suggest that 17 beta-HSD type 1 is the principal isoenzyme involved in glandular estradiol production both in humans and rodents. During ovarian follicular development and luteinization, rat 17 beta-HSD type 1 is regulated by gonadotropins, and the effects of gonadotropins are modulated by steroid hormones and paracrine growth factors. Human 17 beta-HSD type 1 favors the reduction reaction, thereby converting estrone to estradiol both in vitro and in cultured cells. Hence, the enzymatic properties of the enzyme are also in line with its suggested role in estradiol biosynthesis. Interestingly, 17 beta-HSD type 1 is also expressed in certain target tissues of estrogen action such as normal and malignant human breast and endometrium. Hence, 17 beta-HSD type 1 could be one of the factors leading to a relatively high tissue/plasma ratio of estradiol in breast cancer tissues of postmenopausal women. We conclude that 17 beta-HSD type 1 has a central role in regulating the circulating estradiol concentration as well as its local production in estrogen target cells.     

Follicular fluid hormone concentrations after ovarian stimulation using gonadotropin preparations with different FSH/LH ratios. I. Comparison of an FSH-dominant and a purified FSH preparation

OBJECTIVE: A small amount of LH is necessary for 17beta-estradiol production in the ovarian follicle. Human menopausal gonadotropin (hMG) contains equal amounts of FSH and LH activity, whereas recombinant FSH is a gonadotropin preparation without LH. The aim of the present randomized study was to investigate whether ovarian stimulation treatment with recombinant FSH or hMG resulted in different steroidal composition of follicular fluid. METHODS: Antral fluid from mature follicles was collected in in vitro fertilization cycles and concentrations of testosterone, androstenedione, estrone, estradiol, progesterone, FSH, and LH were determined. Seven patients (27 samples) were treated with hMG, 6 patients (22 samples) with recombinant FSH. RESULTS: Androgen, estrogen, progesterone, and FSH concentrations in follicular fluid tended to be lower in the group treated with recombinant FSH, but the variation was large and differences were statistically not significant. CONCLUSION: Treatment with a gonadotropin preparation containing no LH resulted in adequate androgen and estrogen levels in antral fluid of the ovarian follicle in women with normal endocrine profiles, even during pituitary suppression by a GnRH agonist. Apparently, the amount of endogenous LH was sufficient for steroid production within the follicle.     

Metabolism of catechol estrogen by human erythrocytes

In a study designed to evaluate the kinetics of catechol estrogen formation from plasma estrone in vivo, we obtained evidence that the red blood cell (RBC) enzyme, catechol-O-methyl transferase, catalyzes the transformation of 2-hydroxyestrone to 2-methoxyestrone. Under in vitro conditions, the rate of conversion of 2-hydroxyestrone to 2-methoxyestrone by human RBC's was such that 7 nmol of 2-methoxyestrone were formed per h per ml of RBC.     

The pathogenesis of polycystic ovary syndrome: lessons from ovarian stimulation studies

In polycystic ovary syndrome (PCOS) the ovary produces markedly increased amounts of both androgens and estrogens in response to gonadotropin stimulation. Distinctive responses of 17-hydroxyprogesterone and androstenedione to ovarian stimulation testing suggest that ovarian hyperandrogenism is a result of dysregulation of theca cell androgen production which is intrinsic to the ovary. The occurrence of hyperestrogenism together with hyperandrogenism in PCOS suggests that whatever the abnormality of local regulatory factors of steroidogenesis, it affects granulosa as well as theca cells. Dysregulation is often associated with an increase in the number of follicles which evade atresia and reach the 2-8 mm stage of development. Autocrine/paracrine factors, especially those which are FSH-dependent, likely play an important role in the pathogenesis of the ovarian abnormality. Both LH and insulin hypersecretion probably play a secondary role in PCOS by amplifying the preexisting ovarian dysregulation. Because FSH secretion is under tight long-loop negative-feedback control and LH is not, hyperandrogenism is the primary clinical manifestation of dysregulation of steroid production in PCOS. However, anovulation in PCOS is most likely a result of excessive estrogen and inhibin production by multiple, small follicles which inhibit FSH secretory dynamics sufficiently to prevent selection of a dominant follicle.     

Migration and international health policies

Parity, age at first birth, age at menarche, and a family history of breast cancer have each been associated consistently with breast cancer risk. Whether this increase in risk is mediated, at least in part, through changes in endogenous hormone levels is unclear. We conducted a cross-sectional study of the relationships between these factors and plasma hormone levels in 216 healthy postmenopausal women in the Nurses' Health Study (United States). The hormones evaluated were estradiol, percent and total free estradiol, percent and total bioavailable estradiol, estrone, estrone sulfate, and prolactin. After controlling for age, body mass index (weight/height2), and alcohol use, we observed inverse associations between estrone sulfate and parity (r = -0.15, P = 0.03) and between percent bioavailable estradiol and age at first birth (r = -0.17, P = 0.02). Although women with a family history of breast cancer tended to have higher estrogen levels compared with women without such history, the differences were not statistically significant. Age at menarche was not related significantly to any of the hormones. These data provide some additional evidence that the inverse relationship observed between parity and breast cancer risk may be mediated, at least in part, through decreased estrogen levels. Our data do not support a substantial influence of either family history of breast cancer or age at menarche on postmenopausal estrogen or prolactin levels.     

In vivo measurement of aromatase inhibition by letrozole (CGS 20267) in postmenopausal patients with breast cancer

Thirteen postmenopausal women with advanced breast cancer were enrolled in an open randomized Phase I trial of a new p.o. active aromatase inhibitor, CGS 20267 (letrozole). The primary aim of the trial was to assess the impact of two doses of letrozole (0.5 and 2. 5 mg/day) on the peripheral aromatization of androstenedione to estrone. An in vivo isotopic technique was used to measure peripheral aromatization in each patient before treatment. The patients were then randomly assigned to one of the two doses, and measurements of aromatization were repeated after 6 weeks. At 0.5 mg and 2.5 mg/day, letrozole inhibited aromatization by 98.4% (97.3 to >99.1) and >98.9% (98.5 to >99.1; geometric means and ranges), respectively. Plasma estrogen levels were also measured before and during treatment. At the dose of 0.5 mg/day estrone and estradiol levels fell by 82.0% and 84.1% (geometric means), respectively. At the dose of 2.5 mg/ day, the estrogens fell by 80.8% and 68.1%, respectively. There were no significant differences between the doses in aromatase inhibition. No formal statistical analysis was performed on the estrogen data. Letrozole is therefore a highly effective inhibitor of aromatase, causing near complete inhibition of the enzyme in peripheral tissues at the doses investigated. The falls in estrogen levels were greater than those seen with earlier generation aromatase inhibitors.