Nonsteroidal and Steroidal Aromatase Inhibitors in Breast Cancer

August 1, 2001

Anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin) are members of the third generation of aromatase inhibitors that has now replaced aminoglutethimide (Cytadren), the progestins, and tamoxifen

Anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin) are members of the third generation of aromatase inhibitors that has now replaced aminoglutethimide (Cytadren), the progestins, and tamoxifen (Nolvadex) as the hormonal therapy of choice in estrogen-receptor-positive, postmenopausal, metastatic breast cancer. This article will review the role of aromatase in the pathogenesis of breast cancer and the results of recent studies that have established the role of its inhibitors in estrogen-receptor-positive breast cancer. We will also briefly outline the rationale and design of ongoing studies. [ONCOLOGY 15(8):965-979, 2001]


The hormonal dependency of  breast cancer was first recognized more than a century ago.[1] Although it has yet to be proven that estrogen is directly responsible for the initiation of breast tumors, it is clear from epidemiologic evidence,[2] from "prevention" studies using the antiestrogen tamoxifen (Nolvadex),[3] and from the clinical impact of hormonal manipulation[4,5] that estrogen is a significant factor in the maintenance and progression of established tumors.

Estrogen is produced by aromatization of androgens. In premenopausal women, androgens are synthesized from cholesterol by the adrenals and the ovaries in roughly equivalent proportions. Approximately 60% of estrogens in premenopausal women are synthesized in the aromatase-rich cytoplasm of the granulosa cells of the ovaries. Aromatization by the cycling ovary is regulated by follicle-stimulating hormone that is regulated, in turn, by estrogen in a negative feedback loop. The remaining 40% of estrogens in premenopausal women are synthesized in the peripheral tissues, particularly in fat.

At menopause, ovarian production of both estrogen and the estrogen precursor androstenedione ceases, so that most of the circulating estrogen in postmenopausal women derives from the peripheral conversion of adrenal androgens. Circulating estrogen levels in postmenopausal women are approximately 20% of those of premenopausal women, and they achieve a steady-state concentration in the absence of cyclical ovarian function.[6]

Intratumoral Aromatase

Although circulating levels of estrogens are relatively low in postmenopausal women, aromatase expression is maintained in breast tissue after menopause. Estrogen levels in the breast tissue of postmenopausal women are thus significantly higher than those detected in plasma, and may be as high as the plasma levels in premenopausal women.[7,8]

Although the exact site of aromatase production in breast cancer tissues has not yet been determined, both immunocytochemistry and in situ hybridization techniques have demonstrated aromatase enzyme and mRNA expression in the epithelial cells of the terminal ductal lobular units and the surrounding stromal cells of the normal human breast.[9] Tumor cells may produce aromatase themselves or they may produce cytokines that induce tumor-stromal-cell expression of aromatase.[10] Importantly, breast cancer tissues that retain aromatase expression may be able to function in an autocrine fashion by producing their own growth factor.[11-13]

The functional significance of tumor aromatase has not been well defined but is suggested by several lines of evidence. Aromatase activity is frequently found to be much higher in tumor tissue than in surrounding benign tissue from the same breast, supporting a role for aromatase activity in the emergence of the malignant phenotype.[14,15] Studies of tumor aromatase levels and known prognostic factors, such as tumor cell proliferative activity or lymph node involvement, have yielded conflicting results. No clear correlation between the level of tumor aroma-tase activity and the biological behavior of the tumor has yet been demonstrated.[14,16,17]

Studies examining the relationship between aromatase expression and estrogen- and progesterone-receptor positivity have also been inconsistent.[16,18,19] Notably, two small studies have suggested a correlation between tumor aromatase activity and response to aromatase inhibition therapy with aminoglutethimide (Cytadren).[20,21]

The Aromatase Inhibitors

There are two general categories of aromatase inhibitors: (1) the nonsteroidal inhibitors, which bind competitively with aromatase, and (2) the steroidal inhibitors, which bind irreversibly (see Table 1).

First- and Second-Generation Aromatase Inhibitors

The first aromatase inhibitor with documented antitumor efficacy was the nonsteroidal agent aminoglutethimide. Although its use as second- or third-line endocrine therapy achieved response rates of 20% to 40%, the drug was associated with problematic effects. Aminoglutethimide inhibits the production of other adrenal steroids, including cortisol, and therefore must be taken with hydrocortisone. A high incidence of skin rash and fatigue also made the drug difficult for many patients to tolerate. Other early aromatase inhibitors, such as fadrozole (CGS 16949A) and the parenterally administered formestane (4-OHA), demonstrated antitumor activity and fewer adverse effects than aminoglutethimide, but they have now been supplanted by the third-generation inhibitors described below.[22]

Third-Generation Aromatase Inhibitors

The current generation of nonsteroidal inhibitors includes anastrozole (Arimidex), letrozole (Femara), and vorozole (Rivizor), all of which are administered orally as a once-daily dose. The development of vorozole has been terminated, so it will not be discussed below. The only registered steroidal inhibitor of the current generation is exemestane (Aromasin).

Relative Potency of Aromatase Inhibitors

The in vivo potency of aromatase inhibitors is defined by their ability to suppress both aromatase activity and plasma estrogen levels. In vivo aromatase activity is assessed by radioimmunoassay of urinary estrogens following administration of radiolabeled androstenedione.[23] Plasma endogenous estrogens are usually measured with highly sensitive radioimmunoassays after separation with high-performance liquid chromatography.[24]

While the early aromatase inhibitors inhibited aromatization by approximately 90% in postmenopausal women, the third-generation aromatase inhibitors are far more potent, suppressing aromatization by approximately 98%.[25] When radioimmunoassays are used to assess estrogen suppression, they generally correlate with the degree of aromatization suppression observed (see Table 2).

The randomized clinical studies of letrozole[26] and vorozole[27] vs aminoglutethimide have demonstrated that the improvement in aromatase inhibition provided by the third-generation inhibitors is clinically meaningful, but the clinical relevance of any differences between members of the third generation is less clear. While most aromatization studies are not randomized studies-so that any comparison of their results must be interpreted with caution-one small (n = 12) randomized, crossover study has compared anastrozole to letrozole.[28] This study demonstrated that letrozole is a more potent aromatase inhibitor than anastrozole (aromatization suppression rates were > 99.1% vs 97%, P = .003, with confirmatory estrogen suppression data).

The clinical relevance of this small difference, demonstrated at a level of inhibition that is so close to complete, remains uncertain. Equally uncertain is the clinical relevance of exemestane’s irreversible binding to aromatase, compared with the competitive, reversible binding of the nonsteroidal agents.

Aromatase Inhibitors in Metastatic Breast Cancer

Treatment Following Tamoxifen Failure

Anastrozole,[29] letrozole,[30] and exemestane[31] have all been compared with megestrol acetate, and letrozole has been compared with aminoglutethimide,[29] in multicenter, randomized phase III trials in postmenopausal women with estrogen-receptor-positive or estrogen-receptor-unknown metastatic breast cancer following tamoxifen failure. The results of these studies placed the new generation of aromatase inhibitors ahead of progestins as the hormonal treatments of choice following tamoxifen failure, and rendered the use of aminoglutethimide obsolete.[32]

Anastrozole vs Megestrol Acetate: Anastrozole was compared with megestrol acetate in two large randomized trials, the results of which were pooled for publication.[29] A total of 764 patients were randomized to receive megestrol acetate (40 mg qid) or one of two doses of anastrozole (1 mg daily or 10 mg daily). Although no difference was noted between the two agents in response rate, time to progression, or time to treatment failure, median survival was 27 months in patients receiving anastrozole at a dose of 1 mg daily and 23 months in patients receiving megestrol acetate (P = .02). There was no significant survival advantage to the 10-mg dose of anastrozole over either 1 mg anastrozole or megestrol acetate.

Anastrozole produced significantly less weight gain than megestrol acetate (2% vs 12%). The toxicity profiles of the two agents were otherwise comparable.

The survival benefit in this study was interpreted cautiously, as it was evident only in patients who received the lower, 1 mg dose. Moreover, no significant improvements in any other efficacy end point were noted, and the survival advantage was evident in only one of the two studies that were combined for publication.

Letrozole vs Megestrol Acetate or Aminoglutethimide: Letrozole has been compared with both megestrol acetate[30] and aminoglutethimide[26] in randomized trials. The first trial was double-blind and randomly assigned 363 patients to receive either megestrol acetate or letrozole at a daily dose of either 0.5 or 2.5 mg. It convincingly demonstrated an advantage for letrozole at the daily dose of 2.5 mg. Letrozole was superior to megestrol acetate in terms of response rate (24% vs 16%, P = .04), response duration (not reached vs 18 months, P = .01), time to treatment failure (5.1 vs 3.9 months, P = .04), quality of life/deterioration in performance status (39% vs 52%), and drug-related serious adverse events (0% vs 12%, P < .05). In addition, overall survival favored letrozole, but this did not reach statistical significance (25 vs 22 months, P = .15). Weight gain and thromboembolic events occurred less frequently in patients receiving letrozole. The results of this study substantially reinforced the weaker results of the earlier anastrozole vs megestrol acetate trials.

In the aminoglutethimide study, which again tested the two doses of letrozole, 555 patients were randomized. Letrozole (2.5 mg daily) was shown to be superior to aminoglutethimide in terms of time to progression, time to treatment failure, and overall survival (28 vs 20 months, P = .002). Response rates also favored letrozole (20% vs 12%) but did not reach statistical significance. Toxicity, particularly rash, was less common in the letrozole treatment arms.

Exemestane vs Megestrol Acetate: Exemestane was compared with megestrol acetate in a study of 769 patients.[31] Like the nonsteroidal agents, exemestane was shown to be superior to the progestin. At a dose of 25 mg daily, it displayed a longer time to progression (20 vs 17 weeks, P = .037), time to treatment failure, and overall survival at a median follow-up of 11 months (not reached vs 28 months, P = .039). Response rates favored exemestane (15% vs 12%) in both visceral and nonvisceral disease, although the difference did not reach statistical significance. Like anastrozole and letrozole, exemestane produced significantly less weight gain than megestrol acetate.

A phase II study also addressed the activity of exemestane after failure of a nonsteroidal aromatase inhibitor.[33] In a study of 242 patients, 44% had received aminoglutethimide and 56%, another aromatase inhibitor. An objective response was seen in 7% of patients, and stabilization of disease for at least 6 months occurred in another 17%. The median duration of response was 14 months, and the median time to progression was 15 weeks.

Aromatase Inhibitor vs Tamoxifen

All three third-generation aromatase inhibitors have also been compared with tamoxifen as first-line therapy for estrogen-receptor-positive or estrogen-receptor-unknown metastatic breast cancer in postmenopausal women. Preliminary data from these investigations have established the aromatase inhibitors as the therapy of choice for estrogen-receptor-positive metastatic breast cancer in menopausal patients.

Anastrozole: Anastrozole[34] was compared with tamoxifen in two double-blind, placebo-controlled studies that enrolled a total of 1,021 patients. In a combined analysis, 40% of patients had unknown estrogen-receptor status, 60% were estrogen-receptor-positive and/or progesterone-receptor-positive, and 9% had received adjuvant hormonal therapy. Anastrozole exhibited a slightly longer time to progression than tamoxifen (8.5 vs 7.0 months), although this difference did not achieve statistical significance in the intention-to-treat analysis (P = .103). In the hormone-receptor-positive subgroup (n = 611), however, there was a statistically significant advantage to the aromatase inhibitor (10.7 vs 6.4 months, P = .022). Importantly, the superiority of anastrozole was more evident in patients who had not received prior hormonal therapy (ie, tamoxifen) than in those who had, so the advantage cannot be dismissed as being the result of preexisting tamoxifen resistance. Response rates and adverse events were comparable in the two treatment arms.

Letrozole: Letrozole was compared with tamoxifen in a randomized, double-blind, crossover phase III study of 907 patients with estrogen-receptor-positive (65%) or estrogen-receptor-unknown (35%) tumors who had received no prior hormonal therapy for metastatic disease.[35] The treatment arms were well balanced. As first-line therapy, letrozole was shown to be superior to tamoxifen in terms of response rate (30% vs 20%, P = .001), clinical benefit (49% vs 38%, P = .001), time to progression (41 vs 26 weeks, P = .0001), and time to treatment failure (40 vs 25 weeks, P = .0001). Adverse events, thromboembolic events, and duration of response were similar in the two arms.

Exemestane: Exemestane was compared with tamoxifen in a randomized phase II study.[36] To enter the study, patients who had received adjuvant hormonal therapy were required to have had a disease-free interval of at least 6 months if estrogen receptors were positive, and at least 2 years if the estrogen-receptor status was unknown. In an analysis of the first 63 patients, 14 had received adjuvant tamoxifen and 56% had visceral disease. Response rate (42% vs 16%), complete response rate (10% vs 3%), "clinical benefit" (58% vs 31%), and time to progression (8.9 vs 5.2 months) all favored exemestane therapy. Toxicity was similar in the two treatment arms.

Aromatase Inhibitors in Early Breast Cancer

Adjuvant Aromatase Inhibition

Tamoxifen is the gold standard for hormonal therapy in the adjuvant setting. It effectively antagonizes estrogen in the tumor, reducing relapse by 47% and death by 26%, regardless of menopausal status,[4] and has positive effects on bone mineral density and lipid profiles.[37] While the aromatase inhibitors have been shown to be highly effective in postmenopausal women with estrogen-receptor-positive metastatic disease, their role in the adjuvant setting is not yet established. This is because the long-term effects on bone mineralization and cardiovascular function have not yet been adequately assessed.

The current phase III adjuvant studies are therefore designed to compare the aromatase inhibitors with tamoxifen, primarily in terms of prevention of breast cancer relapse, but effects on bone density and cardiovascular morbidity are major secondary end points. Three basic study designs have been used in the current phase III studies of an aromatase inhibitor vs tamoxifen in the adjuvant setting.

Substitution for Tamoxifen: The first design substitutes the aromatase inhibitor for the standard 5 years of tamoxifen. The hypothesis behind this design is that the advantage to the aromatase inhibitors seen in metastatic disease will translate directly into the adjuvant setting, and that any deleterious effects on bone or cardiovascular function will be minimal. The ATAC (Arimidex, Tamoxifen Alone and Combination) trial, coordinated by the British Cancer Research Campaign and AstraZeneca, has accrued a total of 9,100 node-positive and node-negative patients to receive anastrozole or tamoxifen, or anastrozole plus tamoxifen, each for 5 years. The study has completed accrual, and is now in follow-up.

Partial Substitution for Tamoxifen: The second design substitutes the aromatase inhibitor for a portion of the standard 5-year period-based on the hypothesis that the benefits of tamoxifen are greatest in the first few years of treatment, after which some tumor cells may develop tamoxifen dependence. If one accepts the premise that relapse is partially due to the emergence of tamoxifen dependence, tamoxifen withdrawal and estrogen deprivation by aromatase inhibition may indeed provide a survival advantage. This strategy assumes that any deleterious effects of aromatase inhibition will be less evident if the period of aromatase inhibition is kept to a minimum.

This strategy looks promising, as evidenced by the preliminary results of an Italian study of tamoxifen for 5 years vs tamoxifen for 3 years followed by aminoglutethimide for 2 years. The study randomized 381 patients; the median age was 66 years, and 70% of patients had positive axillary nodes. At a median follow-up of 45 months, there was no significant difference in disease-free survival (76% vs 74%), but survival was significantly longer in patients who had received aminoglutethimide (95% vs 87%, P = .006).[38]

The German Adjuvant Breast Cancer Group (GABG) and the Austrian Breast Cancer Study Group (ABCSG) are both comparing 5 years of tamoxifen with 2 years of tamoxifen followed by 3 years of anastrozole in patients with node-positive or node-negative, low- to moderate-grade tumors. A total of 1,300 patients will be accrued to the German trial GABG IV-C (also known as ARNO), and 1,200 patients will be accrued to ABCSG Study 8.

Exemestane is being studied in BIG 02-97 (also known as Study 96 OEXE 031-C/13/96), coordinated by the British-based International Collaboration Cancer Group (ICCG). This study randomizes patients who have already received 2 to 3 years of tamoxifen treatment to either continue tamoxifen or to receive exemestane for a total of 5 years. The study will accrue 2,200 patients.

Four-Way Treatment Design: The Breast International Group (BIG) has combined these first two study designs into BIG 01-98 (or IBCSG 18-98), coordinated by the International Breast Cancer Study Group (IBCSG). This study has a four-way treatment design that randomly assigns patients to 5 years of tamoxifen, 5 years of letrozole, 2 years of tamoxifen followed by 3 years of letrozole, or 2 years of letrozole followed by 3 years of tamoxifen. It will accrue 3,500 patients.

Adjuvant Aromatase Inhibition After 5 Years of Tamoxifen: Another study design addresses whether the introduction of an aromatase inhibitor following 5 years of tamoxifen treatment can further improve survival. The premise here is that 5 years is the optimal duration of tamoxifen therapy, but that late relapse occurs because dormant cells have retained estrogen dependence and subsequently reactivate following tamoxifen withdrawal.

ABCSG study 6A rerandomizes patients, who were treated on ABCSG study 6 (tamoxifen for 5 years vs tamoxifen for 5 years plus aminoglutethimide for 2 years) and who are disease-free at 5 years, to either anastrozole or observation for an additional 3 years. A total of 1,700 patients are expected to be rerandomized.

Protocol B-33 of the National Surgical Adjuvant Breast and Bowel Project (NSABP) is randomizing patients who are disease-free after 5 years of tamoxifen to 2 years of either exemestane or placebo. The study plans to accrue 3,000 patients.

BIG 01-97 (or NCIC CTG MA-17) will randomize 2,400 patients in Canada, Europe, the United States, and Australasia, who remain disease-free after 5 years of adjuvant tamoxifen, to another 5 years of treatment with either letrozole or placebo. Importantly, this study has formal lipid and bone mineral density companion studies.

Neoadjuvant Aromatase Inhibitors

The use of aromatase inhibitors in the neoadjuvant (preoperative) setting has not been widely investigated. Most studies of preoperative therapy have used chemotherapy, and those that have investigated hormonal therapy have used tamoxifen. In the case of aromatase inhibitors, Dixon et al have reported on a series of hormone-receptor-positive patients treated with a 3-month course of letrozole, anastrozole, or tamoxifen.[39] These patients were not part of a randomized study. The response rates for letrozole and anastrozole were high (88% and 94%, respectively), and the median reduction in tumor volume was greatest for letrozole (81%), followed by anastrozole (64%) and tamoxifen (48%).

In premenopausal women, there is evidence that the hormonal environment at the time of surgery may influence the likelihood of relapse. Specifically, some investigators have found that women who undergo surgery during the proliferative phase of the menstrual cycle, a time when circulating estrogens are at their highest levels, are at greater risk of metastases.[40-43] Although the validity of these findings is debated in the literature, they suggest a role for estrogen in the growth of viable metastases from tumor cells disseminated at the time of surgery. It is possible that, at the time of surgery, the inhibition of estrogen with an aromatase inhibitor, alone or in combination with a luteinizing hormone-releasing hormone (LHRH) agonist, could improve the outcome for women undergoing breast cancer surgery.

Aromatase Inhibitors and Breast Cancer Prevention

The approval of the selective estrogen-receptor modifier tamoxifen for the prevention of breast cancer in high-risk women was a recent milestone in the battle against breast cancer. Aromatase inhibitors are candidates for future preventive agents. Many of the epidemiologic factors associated with an increased risk of breast cancer (eg, early menarche, late menopause, increased age at first full-term pregnancy) point to the importance of estrogen exposure, regardless of whether the tumor expresses hormone receptors.[44,45] Several researchers have therefore proposed preventive strategies that decrease breast exposure to estrogen by inhibiting aromatase.[44,46,47]

Total suppression of estrogen production would likely have the adverse effects commonly associated with menopause: increased osteoporosis, cardiovascular disease, and urogenital atrophy. Preclinical models suggest that it may be possible to obtain chemopreventive effects without total suppression of aromatase and circulating estrogen levels.[46] Selectively suppressing local estrogen production in the breast might some day be possible, since researchers have discovered a unique transcriptional promoter of aromatase gene expression found only in breast adipose tissue.[47]

Combined Hormonal Therapy

Aromatase Inhibitors Plus Tamoxifen

In the 1980s, four studies were published that compared tamoxifen alone with tamoxifen plus amino- glutethimide in metastatic disease.[48-51] None of the studies displayed any advantage to the combination, and sequential single-agent hormonal therapy was subsequently established as the standard of care. In the adjuvant setting, a recent Austrian study comparing tamoxifen with tamoxifen plus aminoglutethimide in 2,021 patients also failed to demonstrate any advantage to the combination regimen.[52] A pharmacokinetic interaction between these agents has been demonstrated, however, with the concentration of tamoxifen and its metabolites decreased by coadministration of aminoglutethimide.[53] This may partially explain the clinical findings.

As neither anastrozole nor letrozole influences tamoxifen pharmacokinetics,[54,55] the issue of combined therapy has been revisited in the previously mentioned ATAC trial. Studies in nude mice, on the other hand, would predict that there is unlikely to be any clinical benefit from combining tamoxifen with either anastrozole or letrozole.[56] Likewise, there is no rationale for combining letrozole with tamoxifen, as coadministration of these agents results in a significant (38%) reduction of plasma letrozole levels.[57]

Aromatase Inhibitors Plus LHRH Agonists in Premenopausal Women

There are few clinical data on aromatase inhibitors in premenopausal women, since early studies showed that these agents were unable to effectively inhibit estrogen synthesis in the presence of an intact premenopausal estrogen-follicle-stimulating hormone feedback loop.[58-60] The role of aromatase inhibitors in premenopausal women is now being revisited, however, for a number of reasons.

First, the clinical studies discussed in the preceding paragraphs have conclusively demonstrated that the new aromatase inhibitors are more potent than aminoglutethimide. Second, current data have not established any significant clinical differences among the members of the current generation of aromatase inhibitors. The irreversible nature of the binding between exemestane and aromatase may realize some advantage in this setting, particularly if the drug is administered at higher doses than those used in postmenopausal disease. Third, the addition of LHRH agonists to tamoxifen in premenopausal women has been shown to be an effective treatment strategy in both metastatic disease[61] and the adjuvant setting.[62,63] Long-acting LHRH agonists, such as goserelin (Zoladex) or buserelin (Suprefact), may be used to inhibit ovarian cycling, thereby suppressing ovarian estrogen production to postmenopausal levels.

Combinations of the new aromatase inhibitors with LHRH agonists are therefore now being prospectively studied. British investigators have made a preliminary report of their study of premenopausal women who received anastrozole with goserelin for locally advanced or metastatic breast cancer. Eight of nine evaluable patients were progression-free after 6 months of treatment.[64] The ABCSG is conducting an adjuvant study of goserelin plus anastrozole vs goserelin plus tamoxifen (ABCSG Study 12).

Aromatase Inhibitors in Male Breast Cancer

Male breast cancer comprises approximately 1% of all breast cancer cases. Since the disease arises in a hormonal environment of low levels of circulating estrogens and high levels of circulating androgens, intratumoral aromatase may well be important in its pathogenesis. In fact, aromatase overexpression in intratumoral stromal cells appears to be much more frequent in men than in women.[17]

While the small number of patients precludes meaningful clinical trials, breast cancer in men is generally treated according to the same principles as in women, and tamoxifen therapy appears to be effective in hormone-receptor-positive tumors. Aromatase inhibitors decrease levels of serum estrogen in volunteer male subjects,[65] and they are likely to be useful in the treatment of male breast cancer.


The third generation of aromatase inhibitors-comprising anastrozole, letrozole, and exemestane-is now the standard of care for postmenopausal patients with estrogen-receptor-positive metastatic breast cancer. Numerous large randomized studies are being conducted to address the value of these agents in the adjuvant setting. Additional information regarding the value of new aromatase inhibitors in combination with LHRH agonists will be available from studies in premenopausal women.


1. Beatson GT: On the treatment of inoperable cases of carcinoma mamma: Suggestions for a new method of treatment with illustrative cases. Lancet 2:104-107, 1896.

2. Gail MH, Brinton LA, Byar DP, et al: Projecting individualized probabilities of developing breast cancer for white females who are being examined annually (see comments in J Natl Cancer Inst 81:1845-1855, 1989; 82:879-882, 1990). J Natl Cancer Inst 81:1879-1886, 1989.

3. Fisher B, Costantino JP, Wickerham DL, et al: Tamoxifen for prevention of breast cancer: report of the National Surgical Adjuvant Breast and Bowel Project P-1 Study (see comments in J Nat Cancer Inst 91:188, 1999; 91:730, 1999; 92:659-660, 2000; 92:757-758, 2000; 92:943-944, 2000). J Natl Cancer Inst 90:1371-1388, 1998.

4. Early Breast Cancer Trialists’ Collaborative Group: Tamoxifen for early breast cancer: an overview of the randomized trials. Lancet 351:1451-1467, 1998.

5. Early Breast Cancer Trialists’ Collaborative Group: Ovarian ablation in early breast cancer: an overview of the randomized trials. Lancet 348:1189-1196, 1996.

6. Carr BR, Bradshaw KD: Disorders of the overy and female reproductive tract, in Fauci AS, Braunwald E, Isselbacher KJ, et al (eds): Harrison’s Principles of Internal Medicine, 14th ed, pp 2097-2102. New York, McGraw-Hill, 1998.

7. Thorsen T, Tangen M, Stoa K: Concentrations of endogenous estradiol as related to estradiol receptor sites in breast tumor cytosol. Eur J Cancer 18:333-337, 1982.

8. van Landeghem AA, Poortman J, Nabuurs M, et al: Endogenous concentration and subcellular distribution of estrogens in normal and malignant human breast tissue. Cancer Res 45:2900-2906, 1985.

9. Brodie A, Long B, Lu Q: Aromatase expression in the human breast. Breast Cancer Res Treat 49:85-91; [incl discussion 109-119], 1998.

10. Santen RJ, Santner SJ, Pauley RJ, et al: Estrogen production via the aromatase enzyme in breast carcinoma: Which cell type is responsible? J Steroid Biochem Mol Biol 61:267-271, 1997.

11. Lipton A, Santner SJ, Santen RJ, et al: Aromatase activity in primary and metastatic human breast cancer. Cancer 59:779-782, 1987.

12. Miller W, Forrest A: Oestradiol synthesis from C19 steroids by human breast cancer. Br J Cancer 33:16-18, 1974.

13. Lu Q, Nakmura J, Savinov A, et al: Expression of aromatase protein and messenger ribonucleic acid in tumor epithelial cells and evidence of functional significance of locally produced estrogen in human breast cancers. Endocrinology 137:3061-3068, 1996.

14. Sasano H, Nagura H, Harada N, et al: Immunolocalization of aromatase and other steroidogenic enzymes in human breast disorders. Hum Pathol 25:530-535, 1994.

15. Bulun SE, Price TM, Aitken J, et al: A link between breast cancer and local estrogen biosynthesis suggested by quantification of breast adipose tissue aromatase cytochrome P450 transcripts using competitive polymerase chain reaction after reverse transcription. J Clin Endocrinol Metab 77:1622-1628, 1993.

16. Miller WR, Anderson TJ, Jack WJ: Relationship between tumour aromatase activity, tumour characteristics, and response to therapy. J Steroid Biochem Mol Biol 37:1055-1059, 1990.

17. Sasano H, Ozaki M: Aromatase expression and its localization in human breast cancer. J Steroid Biochem Mol Biol 61:293-298, 1997.

18. Lipton A, Santen RJ, Santner SJ, et al: Prognostic value of breast cancer aromatase. Cancer 70:1951-1955, 1992.

19. Esteban JM, Warsi Z, Haniu M, et al: Detection of intratumoral aromatase in breast carcinomas. An immunohistochemical study with clinicopathologic correlation. Am J Pathol 140:337-343, 1992.

20. Miller WR, O’Neill J: The importance of local synthesis of estrogen within the breast. Steroids 50:537-548, 1987.

21. Bezwoda WR, Mansoor N, Dansey R: Correlation of breast tumour aromatase activity and response to aromatase inhibition with aminoglutethimide. Oncology (Basel) 44:345-9, 1987.

22. Goss PE, Gwyn KM: Current perspectives on aromatase inhibitors in breast cancer. J Clin Oncol 12:2460-2470, 1994.

23. Jacobs S, Lonning PE, Haynes B, et al: Measurement of aromatisation by a urine technique suitable for the evaluation of aromatase inhibitors in vivo. J Enzyme Inhib 4:315-325, 1991.

24. Klein KO, Demers LM, Santner SJ, et al: Use of ultrasensitive recombinant cell bioassay to measure estrogen levels in women with breast cancer receiving the aromatase inhibitor, letrozole. J Clin Endocrinol Metab 80:2658-2660, 1995.

25. Lonning PE: Aromatase inhibition for breast cancer treatment. Acta Oncol 35:38-43, 1996.

26. Gershanovich M, Chaudri HA, Campos D, et al: Letrozole, a new oral aromatase inhibitor: Randomized trial comparing 2.5 mg daily, 0.5 mg daily and aminoglutethimide in postmenopausal women with advanced breast cancer. Letrozole International Trial Group (AR/BC3). Ann Oncol 19:639-645, 1998.

27. Bergh J, Bonneterre J, Illiger HJ: Vorozole versus aminoglutethimide in the treatment of postmenopausal breast cancer relapsing after tamoxifen (abstract 543). Proc Am Soc Clin Oncol 16:155a, 1997.

28. Dowsett M, Geisler J, Haynes BP, et al: Letrozole achieves more complete inhibition of whole body aromatisation than anastrozole (abstract 221). Eur J Cancer 36(suppl 5):88, 2000.

29. Buzdar AU, Jonat W, Howell A, et al: Anastrozole versus megestrol acetate in the treatment of postmenopausal women with advanced breast carcinoma: Results of a survival update based on a combined analysis of data from two mature phase III trials. Arimidex Study Group. Cancer 83:1142-1152, 1998.

30. Dombernowsky P, Smith I, Falkson G, et al: Letrozole, a new oral aromatase inhibitor for advanced breast cancer: Double-blind randomized trial showing a dose effect and improved efficacy and tolerability compared with megestrol acetate. J Clin Oncol 16:453-461, 1998.

31. Kaufmann M, Bajetta E, Dirix LY, et al: Exemestane is superior to megestrol acetate after tamoxifen failure in postmenopausal women with advanced breast cancer: Results of a phase III randomized double-blind trial. The Exemestane Study Group. J Clin Oncol 18:1399-1411, 2000.

32. Hamilton A, Piccart M: The third-generation non-steroidal aromatase inhibitors: A review of their clinical benefits in the second-line hormonal treatment of advanced breast cancer. Ann Oncol 10:377-384, 1999.

33. Lonning PE, Bajetta E, Murray R, et al: Activity of exemestane in metastatic breast cancer after failure of nonsteroidal aromatase inhibitors: A phase II trial. J Clin Oncol 18:2234-2244, 2000.

34. Buzdar A, Nabholtz JM, Robertson JF, et al: Anastrozole versus tamoxifen as first-line therapy for advanced breast cancer in postmenopausal women-combined analysis from two identically designed multicenter trials (abstract 609D). Proc Am Soc Clin Oncol 19:154a, 2000.

35. Mouridsen H, Gershanovich M, Sun Y, et al: Superior efficacy of letrozole versus tamoxifen as first-line therapy for postmenopausal women with advanced breast cancer: Results of a phase III study of the International Letrozole Breast Cancer Group. J Clin Oncol 19:2596-2606, 2001.

36. Paridaens R, Dirix LY, Beex L, et al: Exemestane is active and well tolerated as first-line hormonal therapy of metastatic breast cancer patients: Results of a randomized phase II trial (abstract 316). Proc Am Soc Clin Oncol 19:83a, 2000.

37. Assikis VJ, Jordan VC: Risks and benefits of tamoxifen therapy. Oncology 11:21-23, 1997.

38. Boccardo F, Rubagotti A, Amoroso D, et al: Tamoxifen vs aminoglutethimide in breast cancer patients previously treated with adjuvant tamoxifen: Preliminary results of a multicentric comparative study (abstract 273). Proc Am Soc Clin Oncol 19:71a, 2000.

39. Dixon JM, Love CD, Renshaw L, et al: Lessons from the use of aromatase inhibitors in the neoadjuvant setting. Endocr Relat Cancer 6:227-230, 1999.

40. Jatoi I: Timing of surgery for primary breast cancer with regard to the menstrual phase and prognosis. Breast Cancer Res Treat 52:217-225, 1998.

41. Hagen AA, Hrushesky WJ: Menstrual timing of breast cancer surgery. Am J Surg 175:245-261, 1998.

42. Senie RT, Tenser SM: The timing of breast cancer surgery during the menstrual cycle. Oncology 11:1509-1517 (incl discussion, 1518-1522, 1524), 1997.

43. Goldhirsch A, Gelber RD, Castiglione M, et al: Menstrual cycle and timing of breast surgery in premenopausal node-positive breast cancer: Results of the International Breast Cancer Study Group (IBCSG) Trial VI. Ann Oncol 8:751-756, 1997.

44. Dowsett M: Use of risk determinants for different breast cancer prevention strategies. Eur J Cancer 36:1283-1287, 2000.

45. Hulka BS, Liu ET, Lininger RA: Steroid hormones and risk of breast cancer. Cancer 74:1111-1124, 1994.

46. Kelloff GJ, Lubet RA, Lieberman R, et al: Aromatase inhibitors as potential cancer chemopreventives. Cancer Epidemiol Biomarkers Prev 7:65-78, 1998.

47. Santen RJ, Yue W, Naftolin F, et al: The potential of aromatase inhibitors in breast cancer prevention. Endocr Relat Cancer 6:235-243, 1999.

48. Rose C, Kamby C, Mouridsen HT, et al: Combined endocrine treatment of postmenopausal patients with advanced breast cancer: A randomized trial of tamoxifen vs. tamoxifen plus aminoglutethimide and hydrocortisone. Breast Cancer Res Treat 7:45-50, 1986.

49. Smith IE, Harris AL, Morgan M, et al: Tamoxifen versus aminoglutethimide versus combined tamoxifen and aminoglutethimide in the treatment of advanced breast carcinoma. Cancer Res 42:3430-3433, 1982.

50. Corkery J, Leonard RC, Henderson IC, et al: Tamoxifen and aminoglutethimide in advanced breast cancer. Cancer Res 42:3409-3414, 1982.

51. Ingle JN, Green SJ, Ahmann DL, et al: Randomized trial of tamoxifen alone or combined with aminoglutethimide and hydrocortisone in women with metastatic breast cancer. J Clin Oncol 4:958-964, 1986.

52. Samonigg H, Jakesz R, Hausmaninger H, et al: Tamoxifen vs tamoxifen plus aminoglutethimide for stage I and II, receptor-positive, postmenopausal breast cancer patients: Four-year results of a randomized trial of the Austrian Breast Cancer Study Group (abstract 253). Proc Am Soc Clin Oncol 18:68a, 1999.

53. Lien EA, Anker G, Lonning PE, et al: Decreased serum concentrations of tamoxifen and its metabolites induced by aminoglutethimide. Cancer Res 50:5851-5857, 1990.

54. Dowsett M, Tobias JS, Howell A, et al: The effect of anastrozole on the pharmacokinetics of tamoxifen in post-menopausal women with early breast cancer. Br J Cancer 79:311-315, 1999.

55. Ingle JN, Suman VJ, Johnson PA, et al: Evaluation of tamoxifen plus letrozole with assessment of pharmacokinetic interaction in postmenopausal women with metastatic breast cancer. Clin Cancer Res 5:1642-1649, 1999.

56. Brodie A, Lu Q, Liu Y, et al: Preclinical studies using the intratumoral aromatase model for postmenopausal breast cancer. Oncology 12:36-40, 1998.

57. Dowsett M, Pfister C, Johnston SR, et al: Impact of tamoxifen on the pharmacokinetics and endocrine effects of the aromatase inhibitor letrozole in postmenopausal women with breast cancer. Clin Cancer Res 5:2338-2343, 1999.

58. Santen RJ, Samojlik E, Wells SA: Resistance of the ovary to blockade of aromatization with aminoglutethimide. J Clin Endocrinol Metab 51:473-477, 1980.

59. Harris AL, Dowsett M, Jeffcoate SL, et al: Endocrine and therapeutic effects of aminoglutethimide in premenopausal patients with breast cancer. J Clin Endocrinol Metab 55:718-722, 1982.

60. Stein RC, Dowsett M, Hedley A, et al: The clinical and endocrine effects of 4-hydroxyandrostenedione alone and in combination with goserelin in premenopausal women with advanced breast cancer. Br J Cancer 62:679-683, 1990.

61. Klijn JG, Beex LV, Mauriac L, et al: Combined treatment with buserelin and tamoxifen in premenopausal metastatic breast cancer: A randomized study (see comments in J Natl Cancer Inst 92:859-860, 2000). J Natl Cancer Inst 92:903-11, 2000.

62. Davidson N, O’Neill A, Vukov A, et al: Effect of chemohormonal therapy in premenopausal node-positive, receptor-positive breast cancer: An Eastern Cooperative Oncology Group phase III intergroup trial (E5188, INT-0101) (abstract 249). Proc Am Soc Clin Oncol 18:67a, 1999.

63. Rutqvist LE: Zoladex and tamoxifen as adjuvant therapy in premenopausal breast cancer: A randomised trial by the Cancer Research Campaign Breast Cancer Trials Group, the Stockholm Breast Cancer Group, the Southeast Sweden Breast Cancer Group and the Gruppo Interdisciplinare Valutazione Interventi in Oncologia (abstract 251). Proc Am Soc Clin Oncol 18:67a, 1999.

64. Forward D, Cheung KL, Jackson L, et al: Combined use of goserelin and anastrozole in premenopausal women with metastatic breast cancer, (abstract 582). Proc Am Soc Clin Oncol 19:147a, 2000.

65. Lonning PE, Geisler J, Dowsett M: Pharmacological and clinical profile of anastrozole. Breast Cancer Res Treat 49:53-7; (incl discussion, 73-77), 1998.

66. Geisler J, Lien EA, Ekse D, et al: Influence of aminoglutethimide on plasma levels of estrone sulphate and dehydroepiandrosterone sulphate in postmenopausal breast cancer patients. J Steroid Biochem Mol Biol 63:53-58, 1997.

67. Vorobiof DA, Kleeberg UR, Perez-Carrion R, et al: A randomized, open, parallel-group trial to compare the endocrine effects of oral anastrozole (Arimidex) with intramuscular formestane in postmenopausal women with advanced breast cancer. Ann Oncol 10:1219-1225, 1999.

68. Geisler J, Johannessen DC, Anker G, et al: Treatment with formestane alone and in combination with aminoglutethimide in heavily pretreated breast cancer patients: Clinical and endocrine effects. Eur J Cancer 32:789-792, 1996.

69. Dowsett M, Donaldson K, Tsuboi M, et al: Effects of the aromatase inhibitor anastrozole on serum oestrogens in Japanese and Caucasian women. Cancer Chemother Pharmacol 46:35-39, 2000.

70. Geisler J, King N, Dowsett M, et al: Influence of anastrozole (Arimidex), a selective, non-steroidal aromatase inhibitor, on in vivo aromatisation and plasma oestrogen levels in postmenopausal women with breast cancer. Br J Cancer 74:1286-1291, 1996.

71. Geisler J, King N, Anker G, et al: In vivo inhibition of aromatization by exemestane, a novel irreversible aromatase inhibitor, in postmenopausal breast cancer patients. Clin Cancer Res 4:2089-2093, 1998.