Aromatase inhibitors and breast cancer

Oestrogens are involved in risk to, and progression of, breast cancer. Drugs that inhibit the production of oestrogens (aromatase inhibitors, AIs), are therefore extremely attractive strategies both to prevent the disease and to treat established tumours. AIs now have a central role in the treatment of established breast cancer and are being considered for prevention. Third generation agents have been derived from rational drug design. They are able to block oestrogen production and reduce oestrogen levels to a degree that has not been observed previously and without affecting levels of other hormones. Such selective reduction of endogenous oestrogen provides targeted therapy for hormone-dependent breast cancer. This has led to improved clinical benefits in patients with these tumours. Anastrozole, letrozole and exemestane all have impressive antitumour effects in postmenopausal women with breast cancer and they are at least as beneficial as or better than other established endocrine agents when used to treat hormone-sensitive cancer in the advanced setting or as an adjuvant to surgery in earlier stages of the disease; ongoing trials are exploring the use of AIs in the preventative setting. Third generation inhibitors are well tolerated, having no greater side effects than might be expected from oestrogen suppression. Important differences in endocrinological and molecular effects exist between AIs and SERMs. These have implications for the preferred drug sequence and setting in which AIs are used. Since the major obstacle to more widespread use is primary/acquired resistance, discovery of the mechanisms by which resistance occurs offers hope for the future. More detailed study of AIs will yield important information about the involvement of oestrogen on the development and progression of breast cancer. Consequently AIs offer major clinical benefits to patients with breast cancer and the promise of relatively nontoxic intervention in women at high risk of the disease.     

Aromatase and breast cancer susceptibility

Based on experimental and epidemiological evidence it is hypothesized that estrogen increases breast cancer risk by increasing mitotic activity in breast epithelial cells. Aromatase is crucial to the biosynthesis of estrogens and may therefore play a role in breast cancer development. Supporting data for an etiological role of aromatase in breast tumor biology are several-fold. First, the association between weight and postmenopausal breast cancer risk may be mediated by aromatase. Secondly, a pilot study found a higher aromatase expression in normal breast adipose tissue from breast cancer cases as opposed to healthy women. Thirdly, experimental data in animals suggest that aromatase activity predisposes mammary tissue to preneoplastic and neoplastic changes. In a multiethnic cohort study conducted in Los Angeles and on Hawaii we investigated (i) whether the plasma estrone to androstenedione (E1/A) ratio in different ethnic groups was associated with ethnic differences in breast cancer incidence, and (ii) whether genetic variation in the CYP19 gene encoding the P450 aromatase protein was associated with breast cancer risk. The age- and weight-adjusted ethnic specific E1/A ratios x 100 among women without oophorectomy were 7.92 in African-Americans, 8.22 in Japanese, 10.73 in Latinas and 9.29 in non-Latina Whites (P=0.09). The high E1/A ratio in Latina women was not associated with a high breast cancer incidence; in fact Latina women had the lowest breast cancer incidence in the cohort observed so far. We found no consistent association of an intronic (TTTA)n repeat polymorphism with breast cancer risk in different ethnic groups. This polymorphism was not associated with differences in the plasma E1/A ratio in a way that would predict its functional relevance. We describe a newly identified TTC deletion in intron 5 of the CYP19 gene that is associated with the (TTTA)n repeat polymorphism. Neither this polymorphism, nor a polymorphism at codon 264 in exon VII of the CYP19 gene, was associated with breast cancer. We did not identify any genetic variation in exon VIII in 54 African-American subjects. We identified rare genetic variants of unknown functional relevance in the promoter 1.4 of the CYP19 gene in 3 out of 24 Latina women. Further investigation into the role of aromatase in breast cancer etiology is important, given that the potential use of aromatase inhibitors as breast cancer chemopreventives depends on these results.     

Aromatase inhibitors in breast cancer

The development of aromatase inhibitors for breast cancer therapy is a result of successful translational research exploring the biochemical effects of different compounds in vivo. Studies assessing plasma oestrogen levels as well as in vivo aromatase inhibition have revealed a consistent difference with respect to biochemical efficacy between the third generation compounds (anastrozole, letrozole and exemestane) and the previous, first and second generation drugs, corresponding to the improved clinical effects of these compounds as outlined in large phase III studies. Thus, endocrine evaluation has been found to be a valid surrogate parameter for clinical efficacy. Moreover, the results from these studies have added important biological information to our understanding of endocrine regulation of breast cancer. Based on the clinical results so far, aromatase inhibitors are believed to play a key role in future adjuvant therapy of postmenopausal breast cancer patients and potentially also for breast cancer prevention. Interesting findings such as the lack of cross-resistance between steroidal and non-steroidal compounds should be further explored, as this may add additional information to our understanding of breast cancer biology.     

Aromatase inhibitors for breast cancer

Endocrine therapy is a mainstay for the many women who develop in situ or invasive steroid receptor-positive breast cancer. The use of tamoxifen has reduced mortality in such women. Recently estrogen deprivation strategies have come under scrutiny. Here the use of aromatase inhibitors for treatment of postmenopausal endocrine-responsive breast cancer in the metastatic, adjuvant, and preoperative settings is reviewed. Supported in part by CA88843 and UDI-GM06137-06 from the National Institutes of Health     

Aromatase inhibitors in breast cancer.

Several compounds that selectively inhibit estrogen synthesis via aromatase have been developed. Steroidal substrate analogs, such as formestane and exemestane, inactivate aromatase by binding irreversibly to it. Non-steroidal inhibitors, such as the triazole compounds letrozole and anastrozole, are highly potent, reversible inhibitors with good specificity for aromatase. The intratumoral aromatase model for postmenopausal breast cancer has been used to investigate the efficacy of letrozole, anastrozole and exemestane in combination and sequentially. Combining letrozole or arimidex with tamoxifen or faslodex was not more effective than the aromatase inhibitors alone, but was more effective than tamoxifen alone. Letrozole was superior to and longer lasting than the other agents, suggesting that aromatase inhibitors control tumor growth effectively by inducing greater tumor response and extending treatment time. In addition, aromatase inhibitors can be effective in patients relapsing from tamoxifen. Because two types of aromatase inhibitors are available, steroidal enzyme inactivators and reversible non-steroidal inhibitors in sequential therapy could be useful if resistance to one type develops.     

Aromatase inhibitors as adjuvant treatment of breast cancer

The development of the novel, third-generation aromatase inhibitors and inactivators represents a major improvement of endocrine therapy in breast cancer. Subsequent to phase III studies revealing clinical superiority of these drugs compared to conventional therapy as second- and first-line treatment for advanced disease, several randomised studies have found these compounds, administered either as monotherapy or in sequence with tamoxifen, to improve relapse-free survival compared to tamoxifen monotherapy in the adjuvant setting. Notably, we lack data on long-term follow-up with respect to efficacy as well as side effects, and there are currently no data on head to head comparisons between the different aromatase inhibitors. Several critical issues, like influences of treatment on bone and blood lipid profiles underline the need for long-term follow-up with respect to clinical events like bone fractures and cardiovascular events. Finally, we need data on cost-utility with respect to the different strategies as well as with respect to patient age and risk profile.     

Aromatase inhibitors in the treatment of breast cancer

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

Aromatase and gynecomastia

An imbalance between estrogen action relative to androgen action at the breast tissue level results in gynecomastia. Enhancement of aromatization of androgens to estrogens is important in the pathogenesis of gynecomastia associated with obesity, aging, puberty, liver disease, thyrotoxicosis, 17-oxosteroid reductase deficiency. Klinefelter's syndrome, and neoplasms of the testes, adrenals and liver. A primary aromatase excess syndrome with exuberant gynecomastia had been found both sporadically and in a familial setting. Although aromatase inhibition would appear to be an important class of drugs to treat gynecomastia, relatively little published data with these drugs exist and most concern the use of delta1-testolactone, which reduces the size of the breast glandular tissue, but does not completely ameliorate the problem. Studies with the newer generation of more potent aromatase inhibitors need to be carried out.     

Obesity and prostate cancer

The relationship between obesity and prostate cancer is currently a hotly debated topic, but despite the number of publications devoted to the topic, the actual nature of the relationship remains uncertain. Obesity has been shown to have a direct relationship with the incidence of prostate cancer in a number of studies but an equal number of studies have shown no association. The relationship is further obscured with recent findings that obesity in younger obese men may actually be protective against prostate cancer. Confounding factors include the lack of correlation of body mass index (BMI) as a measure of central obesity and the lack of consistency in timing of BMI measurements, i.e. before or after diagnosis and in young or advanced adulthood. Evidence for increased BMI as a risk factor for prostate cancer is unclear, but less ambiguous is the mounting substantiation that obesity is associated with prognostically worse disease, poorer post-surgical outcomes and increased prostate cancer mortality, irregardless of margin status. From a biologic perspective, one can put forth a number of potential mechanisms by which obesity might promote prostate cancer and/or prostate cancer progression including; low levels of testosterone, increased levels of estrogen, co-existing diabetes or metabolic syndrome, increased circulating insulin-growth factor-one (IGF-1), increased levels of leptin, decreased levels of adiponectin and increased dietary saturated fats. Evidence for the association of these factors with prostate cancer are examined herein. The timing of serum measurements is crucial in elucidating whether these factors have causative influence on prostate cancer or rather are produced by the prostate cancer cells and are better understood as markers of disease. The interaction between obesity and prostate cancer is important to clarify because it will have impact on the prevention, prognostication and treatment of prostate cancer. Future study with careful attention to avoid the methodological pitfalls of the past need be accomplished to bear out the nature of the interaction of obesity and prostate cancer.     

Oestrogens and prostate cancer

Androgens are essential for stimulating normal development, growth and secretory activities of the prostate whereas oestrogens are generally regarded as inhibitors of growth. Evidence for the local synthesis of oestrogens includes the detection of aromatase mRNA and protein in the stroma of human non-malignant tissues and in malignant tissue, where it is detected in epithelial tumour cells. As well, aromatase activity was measured by biochemical assay and protein was detected in prostatic non-malignant and tumour cell lines. Taken together with the identification of direct oestrogenic actions on the prostate, these results suggest that alterations in local oestrogen synthesis may have significant consequences in malignancy of these organs. Genetically modified mouse models were studied in order to evaluate the action of oestrogens alone or in combination with androgens on the prostate gland. Hypogonadal (hpg) mice are deficient in gonadotrophins and androgens but showed direct proliferative responses to oestradiol. The responses were characterised by discrete lobe-specific changes including smooth-muscle regression, fibroblast proliferation, inflammation, and basal epithelial cell proliferation and metaplasia. The aromatase knockout (ArKO) mouse, deficient in oestrogens due to a non-functional aromatase enzyme, developed prostatic hyperplasia during the lifelong exposure to elevated androgens, however, no malignant changes were detected in the prostate at any time. In contrast, combined androgen and oestrogen treatment has been shown to induce prostatic dysplasia and adenocarcinoma. These results demonstrate that malignant changes to the prostate gland are dependent upon both androgenic and oestrogenic responses and that neither hormone alone is sufficient to evoke aberrant patterns of growth, resulting in malignancy.     

Inflammation and prostate cancer

There is emerging evidence that prostate inflammation may contribute to prostatic carcinogenesis. Chronic inflammation has been associated with the development of malignancy in several other organs such as esophagus, stomach, colon, liver and urinary bladder. Inflammation is thought to incite carcinogenesis by causing cell and genome damage, promoting cellular turnover, and creating a tissue microenvironment that can enhance cell replication, angiogenesis and tissue repair. Epidemiological data have correlated prostatitis and sexually transmitted diseases with an increased risk of prostate cancer and intake of anti-inflammatory drugs and antioxidants with a decreased risk. Evidence from genetic and molecular studies also support the hypothesis that prostate inflammation and/or infection may be a cause of prostate cancer. In 1999 De Marzo et al proposed that proliferative inflammatory atrophy (PIA) is a precursor to PIN and cancer. Further research will provide opportunities for the discovery and development of strategies for treatment and prevention of prostate cancer.     

Aromatase in the context of breast and endometrial cancer. A review

Generally, estrogens are considered to be involved in the neoplastic transformation of endometrium. After the menopause these estrogens mainly originate from conversion of adrenal androgens by aromatization in body fat. However, in case of stromal hyperplasia of the ovaries, it cannot be excluded that production of aromatizable androgens by postmenopausal ovaries leads to increased availability of androgen precursors for intratumoral estrogen synthesis in the endometrial tissue as well. The local presence of androgens and the local expression and activity of aromatase is considered important for this steroidogenesis. In this review, we will discuss the available evidence that androgens, produced in hyperplastic ovarian stroma or body fat tissues, play a role in the development of endometrial cancer through conversion into estrogens, a reaction catalyzed in the endometrium by the enzyme aromatase cytochrome P450. As the presence of aromatase appeared to be a pathophysiological factor in the formation of breast cancer, the latter will be evaluated in relation to the development of endometrioid endometrial cancer as well, since both disorders appear partly estrogen dependent. As treatment with aromatase inhibitors appeared feasible in breast cancer, current knowledge of comparable treatment modalities in hormone dependent endometrial cancer will be reviewed.     

Aromatase and breast cancer: W39R, an inactive protein

BACKGROUND: Aromatase (CYP19) catalyzes the conversion of androgens into estrogens. It is in particular involved in development, reproduction and breast cancer. One of its polymorphisms, W39R localized in the N-terminal region of CYP19, significantly decreases breast cancer risk among Japanese women and was chosen for this study. In this work, we studied the structure-function relationships between W39R polymorphism and CYP19 enzyme activity. OBJECTIVE: To examine the kinetic properties of the mutant W39R recombinant protein in transfected human cells devoid of steroidogenic activity. METHODS: Expression vectors for the wild-type or the mutated R39 aromatase were transiently transfected into E293 human embryonal kidney cells. The conversions of androstenedione to estrone and of testosterone and nortestosterone to 17beta-estradiol were assayed by RIA. Expression of recombinant cDNAs was analyzed by semi-quantitative RT-PCR and immunoblotting. RESULTS: W39R recombinant protein was devoid of aromatase activity whatever the substrate used. This absence of activity was not due to the lack of expression of the recombinant enzyme since the mRNA and protein were detected. CONCLUSION: Our present in vitro study shows that the R39 mutant is unable to synthesize estrogens. This work provides a novel observation, being consistent with the fact that Japanese women with the variant allele (arg) have significantly lower risk of developing a breast tumor.     

Aromatase in breast cancer and the role of aminoglutethimide and other aromatase inhibitors

Approximately one third of human breast carcinomas are hormone dependent and regress upon reduction of circulating estrogen levels. Traditional treatment strategies utilized surgical ablative methods to lower estrogen concentrations as treatment of breast cancer. Currently, investigative emphasis is focused upon development of highly specific antiestrogens and inhibitors of estrogen production. The enzyme, aromatase, as the terminal step in estrogen biosynthesis, is a logical target for blockade with potent and specific inhibitors. The earliest available aromatase antagonist, aminoglutethimide, suppresses estrogen production to the same extent as surgical ablation and is an effective treatment for breast cancer. Aminoglutethimide, however, blocks other cytochrome P-450-mediated steroid hydroxylations, requires concomitant glucocorticoid administration, and is associated with initial side effects. Several more specific inhibitors by destroying aromatase irreversibly as well as by competitive inhibition. One of these, 4-hydroxy-androstenedione, has been intensively studied in animals and is undergoing clinical trial. New data regarding these inhibitors further emphasize the key role of aromatase in estrogen production and the practical utility of blocking this enzyme.