Anastrozole shows evidence of activity in postmenopausal patients who have responded or stabilised on formestane therapy

Review of hormone-based treatments in postmenopausal patients with advanced breast cancer focusing on aromatase inhibitors and fulvestrant

Background

Endocrine therapy constitutes a central modality in the treatment of oestrogen receptor (ER)-positive advanced breast cancer.

Purpose

To evaluate the evidence for endocrine treatment in postmenopausal patients with advanced breast cancer focusing on the aromatase inhibitors, letrozole, anastrozole, exemestane and fulvestrant.

Methods

A review was carried out using PubMed. Randomised phase II and III trials reporting on ≥100 patients were included.

Results

35 trials met the inclusion criteria. If not used in the adjuvant setting, a non-steroid aromatase inhibitor was the optimal first-line option. In general, the efficacy of the different aromatase inhibitors and fulvestrant was similar in tamoxifen-refractory patients. A randomised phase II trial of palbociclib plus letrozole versus letrozole alone showed significantly increased progression-free survival (PFS) when compared with endocrine therapy alone in the first-line setting (20.2 vs 10.2 months). Furthermore, the addition of everolimus to exemestane in the Breast Cancer Trials of OraL EveROlimus-2 (BOLERO-2) study resulted in an extension of median PFS by 4.5 months after recurrence/progression on a non-steroid aromatase inhibitor. However, overall survival was not significantly increased.

Conclusion

Conventional treatment with an aromatase inhibitor or fulvestrant may be an adequate treatment option for most patients with hormone receptor-positive advanced breast cancer. Mammalian target of rapamycin (mTOR) inhibition and cyclin-dependent kinase 4/6 (CDK4/6) inhibition might represent substantial advances for selected patients in some specific settings. However, there is an urgent need for prospective biomarker-driven trials to identify patients for whom these treatments are cost-effective.

Effects of aromatase inhibitors and body mass index on steroid hormone levels in women with early and advanced breast cancer

BACKGROUND

Aromatase inhibitors (AIs) are central to the management of oestrogen receptor-positive breast cancer in the adjuvant and metastatic setting. Levels of circulating steroid hormones (SHs) were measured in patients established on AIs to investigate: the influence of body mass index (BMI) in both the adjuvant and metastatic setting; the class of AI utilized in the adjuvant setting (steroidal versus non-steroidal); and differences in SH levels between women treated adjuvantly and those receiving a second-line AI for locally advanced/metastatic disease.

METHODS

Plasma levels of androstenedione, 5-androstene-3β,17β-diol, dehydroepiandrosterone, oestradiol and testosterone were measured by radioimmunoassay in women with breast cancer who were receiving AIs in either an adjuvant or a metastatic setting. Differences between mean SH levels by class of AI, BMI, and second-line versus adjuvant therapy were assessed.

RESULTS

Sixty-four women were receiving AI therapy, 45 (70 per cent) in an adjuvant setting and 19 (30 per cent) were taking a second-line AI. There was no significant correlation between BMI and SH levels. However, BMI was significantly higher in the second-line AI cohort compared with the adjuvant cohort (29.8 versus 26.2 kg/m2 respectively; P = 0.026). In the adjuvant setting, patients receiving a steroidal AI had significantly higher levels of all five hormones (P < 0.050). In the second-line AI cohort, oestradiol levels were significantly higher than in the adjuvant cohort (4.5 versus 3.3 pg/ml respectively; P = 0.022). Multivariable analysis adjusted for BMI confirmed the higher residual oestradiol level in the second-line AI group (P = 0.063) and a significantly higher androstenedione level (P = 0.022).

CONCLUSION

Residual levels of SH were not significantly influenced by BMI. However, the significant differences in residual SH levels between the second-line and adjuvant AI cohort is of relevance in the context of resistance to AI therapy, and warrants further investigation.

Aromatase inhibitors in the treatment of breast cancer in post-menopausal female patients: an update

Estrogen and its metabolites play a significant role in the proliferation of hormone receptor-positive breast cancer. In postmenopausal women, aromatase inhibitors can significantly reduce estrogen levels by blocking enzyme-mediated estrogen synthesis within tissues. Third-generation aromatase inhibitors have now surpassed tamoxifen as first-line therapy for postmenopausal women with metastatic, hormone receptor-positive, breast cancer, showing improved response rates and time to progression. Aromatase inhibitors have shown incremental improvements in disease-free survival, lower local recurrence rates, lower metastatic recurrence rates, and a lower incidence of contralateral breast cancer over tamoxifen when used in the adjuvant setting. Aromatase inhibitors are recommended to be used as adjuvant therapy within the first 5 years of hormonal therapy and may be used either upfront for 5 years or sequenced with tamoxifen. No superiority of one aromatase inhibitor over another has yet been shown. The side effect profiles of aromatase inhibitors have some key differences compared with tamoxifen. These differences may influence treatment choices as well as impact compliance.

Evaluation of plasma and tissue estrogen suppression with third-generation aromatase inhibitors: of relevance to clinical understanding?

Development of aromatase inhibition and aromatase inhibitors as a therapeutic strategy was initiated through two different pathways. The one pathway went through systematic exploration of aromatase substrate analogues for enzyme inhibitions, subsequently leading to the development of steroidal agents for clinical use. The second involved clinical observation with an unsuccessful anti-epileptic compound named aminoglutethimide, attempting to achieve a "medical adrenalectomy". Endocrine studies on patients treated with aminoglutethimide lead to direct assessment of in vivo aromatase inhibition in patients on treatment, thus identifying a novel therapeutic strategy. As such, both research programs represent different examples of pioneering translational work leading towards a successful therapeutic strategy. Subsequent studies with respect to total aromatase inhibition have led to successful development of more potent strategies. Most importantly, these studies have revealed a correlation between aromatase inhibition and clinical outcome. Ongoing studies exploring tissue estrogen levels as well as gene expression profiles on therapy may further improve this important therapeutic area.

Lack of complete cross-resistance between different aromatase inhibitors; a real finding in search for an explanation?

While third-generation aromatase inhibitors (anastrozole, letrozole and exemestane) are successfully implemented as adjuvant and first-line therapy for hormone-sensitive breast cancer in postmenopausal women, important questions remain to be addressed. An issue of particular interest is the question about lack of complete cross-resistance between steroidal and non-steroidal compounds. Although the studies reporting this phenomenon in general contain a small number of patients, the findings across the different reports seem consistent. While several potential mechanisms have been suggested, so far we lack scientific proof what mechanisms may be responsible for this finding. Finally, we do not know whether lack of cross-resistance actually signals an improved efficacy for certain compounds or may be due to alternative mechanisms of action. Neither do we know whether some tumours are more sensitive to particular drugs. This paper summarizes clinical findings up to now with respect to lack of cross-resistance and discuss potential mechanisms involved.

Exploring the lack of cross-resistance between aromatase inhibitors: evidence for a difference?

A lack of cross-resistance between the aromatase inhibitors (AIs) provides evidence to suggest that there are clinical differences between these agents. Available data from clinical trials indicate that patients exposed to nonsteroidal AIs may benefit from a steroidal compound of similar biochemical potency, and durable stable disease can be achieved in a significant proportion of patients. To date, there is little evidence suggesting specific pharmacokinetic/pharmacodynamic resistance for individual tumours to particular compounds. To clarify fully this issue, a head-to-head comparative trial in the adjuvant setting is needed and the results of the MA.27 trial randomizing patients to the steroidal AI exemestane vs. the nonsteroidal AI anastrozole will be invaluable in this regard.

Postmenopausal advanced breast cancer: Options for therapy after tamoxifen and aromatase inhibitors

All patients receiving endocrine treatment for advanced breast cancer (ABC) eventually progress, resulting in a need for new therapies that lack cross-resistance with existing agents. Oestrogen receptor (ER) modulators such as toremifene and raloxifene have poor efficacy following tamoxifen failure, whereas the non-steroidal aromatase inhibitors (AIs), anastrozole and letrozole and the steroidal AI exemestane are effective. Fulvestrant is a new ER antagonist with no agonist effects that is as effective as anastrozole in treating patients who have progressed on tamoxifen. AIs are replacing tamoxifen as first-line treatments for ABC and in the adjuvant setting, necessitating a re-evaluation of optimal sequencing. Preliminary data suggest that tamoxifen, exemestane and fulvestrant have activity in patients who have progressed on non-steroidal AIs and hence could be considered for use in this setting. Due to the apparent lack of cross-resistance between non-steroidal and steroidal AIs, non-steroidal AIs could also be effective following steroidal AI failure. Clinical trials are underway to assess the most appropriate treatment sequence following non-steroidal AI failure, with comparisons of fulvestrant and exemestane of major interest.

Sequential Treatment with Exemestane and Non-Steroidal Aromatase Inhibitors in Advanced Breast Cancer

BACKGROUND

The steroidal aromatase inactivator exemestane has demonstrated activity after prior failure of non-steroidal aromatase inhibitors (including third-generation inhibitors letrozole and anastrozole) in postmenopausal women with advanced breast cancer. If exemestane is used as first anti-aromatase agent, however, it is unclear whether patients can still benefit from letrozole or anastrozole after progression.

PATIENTS AND METHODS

Postmenopausal patients with advanced, hormone receptor-positive or -unknown breast cancer were eligible for this study. Patients with no prior exposure to anti-aromatase drugs received exemestane, 25 mg daily, as first anti-aromatase agent. At the time of progression, patients were crossed-over to anastrozole or letrozole if further endocrine therapy was considered appropriate. Patients with prior exposure to anti-aromatase agents were also included in the study, and were given anastrozole or letrozole if they had previously received exemestane, or exemestane if they had previously received anastrozole or letrozole. The primary endpoint of the study was the clinical benefit rate (complete response + partial response + stabilization of disease for >or=24 weeks).

RESULTS

Forty patients received exemestane 25 mg daily as first anti-aromatase agent, with a CB rate of 67.5% (95% CI 52.9-82.0%) and a median time to progression (TTP) of 9.6 months. In 18 patients, letrozole (n = 17) or anastrozole (n = 1) were used after failure of exemestane: the CB rate was 55.6% (95% CI 32.6-78.5%) with a median TTP of 9.3 months. In 23 patients, exemestane was used after failure of letrozole or anastrozole: the CB rate was 43.5% (95% CI 23.2-63.7%) with a median TTP of 5.1 months.

CONCLUSIONS

Our study confirms that exemestane is active after prior failure of letrozole or anastrozole. We have also shown that patients can receive exemestane as their first anti-aromatase agent and still benefit from letrozole or anastrozole after progression. This suggests that the partial non-cross resistance between steroidal and non-steroidal anti-aromatase agents is independent of the sequence employed.

Sequencing of aromatase inhibitors

Since the development of the third-generation aromatase inhibitors (AIs), anastrozole, letrozole and exemestane, these agents have been the subject of intensive research to determine their optimal use in advanced breast cancer. Not only have they replaced progestins in second-line therapy and challenged the role of tamoxifen in first-line, but there is also evidence for a lack of cross-resistance between the steroidal and nonsteroidal AIs, meaning that they may be used in sequence to obtain prolonged clinical benefit. Many questions remain, however, as to the best sequence of the two types of AIs and of the other available agents, including tamoxifen and fulvestrant, in different patient groups.

Aromatase inhibitors: rationale and use in breast cancer

Considerable data implicate estrogens in breast cancer carcinogenesis and progression. In the postmenopausal woman, estrogens are produced in breast tissues and many other sites throughout the body when androgen precursors are converted into estrogens via the enzyme aromatase. Inhibition of this enzyme with aromatase inhibitors (AIs) has demonstrated reductions in systemic as well as intratumoral estrogens. These drugs have now been utilized in large phase 3 randomized trials and have led to greater improved clinical benefit than the "gold standard," tamoxifen. Questions remain about the long-term side effects and safety profile of AIs. They are associated with increasing incidence of osteoporosis and bone fractures. Nevertheless, AIs add to our armamentarium for therapy and possible prevention of breast cancer.

Exemestane for Breast Cancer Prevention: A Feasible Strategy?

Third-generation aromatase inhibitors and inactivators have been successfully implemented in therapy of metastatic breast cancer, and three large phase III trials have revealed superiority compared with tamoxifen monotherapy in the adjuvant setting. Notably, each of these trials recorded a substantial reduction in contralateral breast cancer among patients exposed to the aromatase inhibitor/inactivator. A major concern in implementing use of these compounds in the preventive setting relates to potential detrimental effects of estrogen suppression on bone and lipid metabolism. Recent data from a placebo-controlled study now reveal 2 years of treatment with exemestane compared with placebo to have moderate effects on bone metabolism and plasma lipid profile, supporting further evaluation of exemestane as a potential preventive agent for breast cancer in postmenopausal women.

Oestrogen suppression--lessons from clinical studies

In few, if any, areas of cancer research has "translational research" played a role such as the one it has played in the development of endocrine therapy. Thus, much of the understanding of the mechanisms of action of different therapies has been achieved by developing novel hypotheses based on clinical observations. Ovarian ablation was developed as an empirical therapy long before characterization of oestrogen disposition or detection of the oestrogen receptor. The first-generation aromatase inhibitor, aminoglutethimide, was implemented as a strategy to achieve a 'medical adrenalectomy' and was discovered as an aromatase inhibitor subsequent to clinical observations. Currently, observations such as the lack of cross-resistance between aromatase inhibitors and inactivators and the diverse effects from combined therapy using tamoxifen in concert with hormonal suppression in pre- and post-menopausals are provoking interesting questions for further research.

Efficacy of tamoxifen following anastrozole (‘Arimidex’) compared with anastrozole following tamoxifen as first-line treatment for advanced breast cancer in postmenopausal women

Anastrozole ('Arimidex') is indicated for the treatment of advanced breast cancer in postmenopausal women. Combined analysis of two international randomised, double-blind trials (n=1021) showed that in patients with hormone receptor-positive tumours, first-line treatment with anastrozole significantly prolonged the time to progression (TTP) compared with tamoxifen (median TTP: 10.7 versus 6.4 months, respectively; P=0.022). Second-line tamoxifen following anastrozole, or vice versa, in this trial population was unblinded. The treatments were crossed over and then efficacy was assessed using a questionnaire. Of 511 patients randomised to anastrozole, 137 (26.8%) received second-line tamoxifen. Questionnaire data were available for 119 patients; 58 (48.7%) gained clinical benefit (CB=complete+partial response (CR+PR)+(stable disease (SD) >/=24 weeks)), while 12 (10.1%) had an objective response (OR=CR+PR). Of 510 patients randomised to tamoxifen, 134 (26.3%) received second-line anastrozole. Questionnaire data from 95 patients showed that 54 (56.8%) gained CB and 7 of the patients gaining CB (7.4%) had an OR. Previous studies showed anastrozole is effective after first-line tamoxifen. These data show that the sequential administration of first-line anastrozole followed by tamoxifen provides effective use of these drugs in the treatment of postmenopausal women with advanced breast cancer.

Comparison between aromatase inhibitors and sequential use

The biochemical efficacy of aromatase inhibitors and inactivators in vivo may be determined by two types of methods; by measuring plasma or tissue estrogen levels, or assessment of the conversion of the androgen substrate (in practice, androstenedione) into estrogens (estrone) by the use of tracer methods. While methods to determine plasma and tissue estrogens are limited through lack of sensitivity required to measure the very low concentrations recorded in postmenopausal women on treatment with these compounds, measurement of in vivo aromatization is an extensive procedure, applicable to a limited number of patients only. While we may correlate the mean level of aromatase inhibition achieved with different compounds to clinical efficacy, data correlating individual estrogen suppression to clinical outcome among patients treated with a specific compound is limited. The now well-characterized phenomenon of lack of cross-resistance between non-steroidal aromatase inhibitors and steroidal aromatase inactivators are likely due to biochemical effects not related to differences in total body aromatase inhibition.

Aromatase inhibitors as adjuvant therapies in patients with breast cancer

There is increasing evidence that endocrine therapy has an important role in patients with oestrogen receptor positive breast cancer. Several large meta-analyses have reinforced the value of both ovarian ablation and tamoxifen in improving survival. Over the past decade, aromatase inhibitors have become the treatment of choice for second-line therapy of metastatic breast cancer, and the third generation inhibitors have now an established reputation for good patient tolerability. Early studies indicated that aminoglutethimide/hydrocortisone could benefit postmenopausal patients with primary breast cancer, and in 2001, the ATAC study showed that the third generation aromatase inhibitor, anastrozole, seemed superior to tamoxifen in that anastrozole-treated patients had a longer disease-free survival. Other studies will report on the relative merits of the steroidal inhibitor exemestane as well as non-steroidal letrozole. The exact duration and sequencing of treatment, together with the long-term effects on bone are at present, unknown.

Is there a benefit by the sequence anastrozole-formestane for postmenopausal metastatic breast cancer women?

To explore the different sequence interactions between reversible non-steroidal (anastrozole, ANZ and letrozole, LTZ) and non-reversible steroidal aromatase inhibitors (formestane, FOR and exemestane, EXE), we evaluated the clinical benefit (CB) in postmenopausal breast cancer patients, who had previously received anastrozole and subsequently formestane. In 19 out of 21 patients (90.5%), a clinical benefit response was achieved by anastrozole, with a median duration of 12 months. Out of the 21 women progressing on anastrozole, 12 achieved stable disease (SD)>/=6 months by formestane only. The overall clinical benefit was 66.5%. The median duration of clinical benefit was 11 months with a time to progression of 6.5 months. The median duration of clinical benefit in our series is similar to that reported in two phase II trials with the sequence aminogluthetimide-->formestane and aminogluthetimide-->exemestane as third-line hormonal therapy, suggesting a non-cross-resistance between the two classes of inhibitors.

Aromatase inhibitors and inactivators for breast cancer therapy

Aromatase inhibitors and inactivators are increasingly important to the therapy of advanced breast cancer in postmenopausal women. These compounds are also currently being evaluated in the adjuvant setting and may have potential in breast cancer prevention. In addition to the recent clinical results, experimental research with development of aromatase 'knockout' mice as well as certain clinical observations in individuals lacking this enzyme have deepened our understanding of estrogens outside of the field of reproduction. Such information should help us to further develop this type of therapy in breast cancer and, in particular, extend our understanding of the lack of complete cross-resistance between aromatase inhibitors and inactivators. Clinically, third-generation aromatase inhibitors and inactivators have shown superiority compared with conventional treatment in advanced postmenopausal breast cancer with respect to second-line (tamoxifen failures) as well as first-line therapy. The fact that tamoxifen is noncurative in metastatic disease but improves long-term survival in the adjuvant setting suggests that even modest improvements in therapy of advanced disease may be translated into survival benefits in patients with early disease. In addition, these novel compounds with lack of complete cross-resistance extend the scope of using sequential treatment options to maximise the duration of optimal endocrine therapy in metastatic breast cancer disease.

Short-term effects of anastrozole treatment on insulin-like growth factor system in postmenopausal advanced breast cancer patients

Insulin-like growth factors (IGFs) play a fundamental role in cancer development by acting in both an endocrinal and paracrinal manner, and hormone breast cancer treatments affect the IGF system by modifying circulating growth factor levels. We evaluated total IGF-1, IGF-2, IGF binding protein (IGFBP)-1 and IGFBP-3 in the blood of 34 postmenopausal advanced breast cancer patients (median age 63 years, range 41-85) treated with anastrozole, a non-steroidal structure aromatase inhibitor (NSS-AI). The plasma samples were obtained at baseline, and after 2, 4, 8 and 12 weeks of treatment. The IGFs were quantitated by means of sensitive radioimmunoassays (RIAs). IGF-1 significantly increased during anastrozole treatment (baseline versus 12 weeks, P=0.031), IGF-2 showed a trend towards an increase, and IGFBP-1 constantly but not significantly decreased; IGFBP-3 did not seem to be affected at all. The anastrozole-induced changes in IGFs and IGFBP-1 appeared to be different in the patients receiving a clinical benefit from those observed in non-responders. We have previously shown that letrozole (a different type of NSS-AI) modifies blood IGF-1 levels, and the results of this study of the biological effects of anastrozole on the components of the IGF system confirm our previous observations.

Influence of letrozole and anastrozole on total body aromatization and plasma estrogen levels in postmenopausal breast cancer patients evaluated in a randomized, cross-over study

PURPOSE

To compare the effects of the two novel, potent, nonsteroidal aromatase inhibitors anastrozole and letrozole on total-body aromatization and plasma estrogen levels.

PATIENTS AND METHODS

Twelve postmenopausal women with estrogen receptor-positive, metastatic breast cancer were treated with anastrozole 1 mg orally (PO) and letrozole 2.5 mg PO once daily, each given for a time interval of 6 weeks in a randomized sequence. Total-body aromatization was determined before treatment and at the end of each treatment period using a dual-label isotopic technique involving isolation of the metabolites with high-performance liquid chromatography. Plasma levels of estrone (E(1)), estradiol (E(2)), and estrone sulfate (E(1)S) were determined in samples obtained before each injection using highly sensitive radioimmunoassays.

RESULTS

Pretreatment aromatase levels ranged from 1.68% to 4.27%. On-treatment levels of aromatase were detectable in 11 of 12 patients during treatment with anastrozole (mean percentage inhibition in the whole group, 97.3%) but in none of the 12 patients during treatment with letrozole (> 99.1% suppression in all patients; Wilcoxon, P =.0022, comparing the two drug regimens). Treatment with anastrozole suppressed plasma levels of E(1), E(2), and E(1)S by a mean of 81.0%, 84.9%, and 93.5%, respectively, whereas treatment with letrozole caused a corresponding decrease of 84.3%, 87.8% and 98.0%, respectively. The suppression of E(1) and E(1)S was found to be significantly better during treatment with letrozole compared with anastrozole (P =.019 and.0037, respectively).

CONCLUSION

This study revealed letrozole (2.5 mg once daily) to be a more potent suppressor of total-body aromatization and plasma estrogen levels compared with anastrozole (1 mg once daily) in postmenopausal women with metastatic breast cancer.

Where do selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) now fit into breast cancer treatment algorithms?

The agents used for endocrine therapy in patients with breast cancer have changed markedly over the past decade. Tamoxifen remains the anti-oestrogen of choice, but could be replaced by the oestrogen receptor down-regulator ICI 182780 or by the fixed ring triphenylethylene arzoxifene (previously SERM III) soon. Whilst aminoglutethimide and 4-OH androstenedione were the aromatase inhibitors of choice, they have been replaced by non-steroidal (anastrozole and letrozole) and steroidal (exemestane) inhibitors of high potency and low side effect profile. Previously, often used treatments such as progestogens (megestrol acetate and medroxyprogesterone acetate) and androgens are now rarely used or confined to fourth or fifth line treatments. The LHRH agonist, goserelin, remains the treatment of choice for pre-menopausal patients with advanced breast cancer although recent randomised trials indicate a response, time to progression and survival advantage for the combination of goserelin and tamoxifen compared with goserelin alone. The newer treatments have led to questions concerning the optimum sequence of agents to use in advanced breast cancer and as neo-adjuvant and adjuvant therapy in relation to surgery. Two trials of anastrozole compared with tamoxifen and one trial of letrozole compared with tamoxifen indicate that the new triazole aromatase inhibitors have a significant advantage over the anti-oestrogen with respect to time to progression and survival. Similarly, triazole aromatase inhibitors give faster and more complete responses compared with tamoxifen when used in post-menopausal women before surgery. Major research questions remain with respect to the aromatase inhibitors used as adjuvant therapy. Anastrozole is being tested alone or in combination with tamoxifen compared with tamoxifen in the 'so-called' ATAC trial. Over 9000 patients have been randomised to this important study: the results will be available late-2001. A similar study comparing letrozole and tamoxifen started recently under the auspices of the Breast International Group. Importantly, this trial is also comparing the sequence of tamoxifen followed by letrozole (or vice versa). A similar trial of exemestane given after 2-3 years of tamoxifen compared with 5 years of tamoxifen is recruiting well as is a study comparing letrozole (or placebo) for 5 years after 5 years of adjuvant tamoxifen. These studies may show that aromatase inhibitors are superior to tamoxifen or that a sequence is preferable.ICI 182780 causes complete oestrogen receptor down-regulation leading to a the lack of agonist activity of the drug. Two trials of ICI 182780 compared with anastrozole for advanced disease will report later this year and a comparison with tamoxifen next year. Arzoxifene (SERM III) is being tested against tamoxifen. These studies are likely to result in new anti-oestrogens being introduced into the clinic. Most of our endocrine treatments deprived the tumour cell of oestradiol. In vitro experiments with MCF-7 cells indicate that tumour cells can adapt and then grow in response to low oestrogen concentrations in the tissue--culture medium. Importantly, the cells were shown to apoptose in response to high oestrogen concentrations. A recent clinical trial has demonstrated a high response rate to stilboestrol given after a median of four previous oestrogen depriving endocrine therapies. These data and the newer treatments available indicate a need to re-think our general approach to endocrine therapy and endocrine prevention.

Stepwise estrogen suppression manipulating the estrostat

Estrogen suppression is an effective endocrine treatment option in pre- as well as postmenopausal breast cancer patients. The fact that it produces clinical benefits not only in these two groups of patients that differ significantly with respect to plasma estrogen levels but also among patients with very low plasma estrogen levels due to previous hypophysectomy, adrenalectomy or treatment with first/second generation aromatase inhibitors, suggests estrogen deprivation to work independent of pretreatment plasma estrogen levels. Interestingly, in vitro studies have revealed MCF-7 cells to respond to estrogen deprivation by sensitization, causing maximum estradiol stimulation at a concentration 10(-5) to 10(-4) the concentration needed in wild-type cells. While results from recent phase III studies comparing novel aromatase inhibitors and inactivators to conventional therapy have suggested that a more effective hormone ablation may be translated into an improved clinical efficacy, the biochemical rationale for lack of complete cross-resistance between aromatase inhibitors and inactivators or aromatase inhibitors and megestrol acetate remains to be explained. Interestingly, patients becoming resistant to estrogen deprivation may still respond to estrogens administered in pharmacological doses. Future studies are warranted to explore alterations in gene expression and signaling mechanisms in response to different therapies in tumor tissue in vivo.

Formestane, a steroidal aromatase inhibitor after failure of non-steroidal aromatase inhibitors (anastrozole and letrozole): is a clinical benefit still achievable?

BACKGROUND

There are few clinical data on the sequential use of aromatase inhibitors (AI). This paper focuses on the relevance of clinical benefit CB (CR + PR + SD > or = 6 months) in postmenopausal metastatic breast cancer (MBC) patients treated with the steroidal aromatase inhibitor (SAI) formestane (FOR). who had already received non-steroidal aromatase inhibitor (nSAI): letrozole (LTZ) or anastrozole (ANZ).

PATIENTS AND METHODS

Twenty postmenopausal women with MBC were analysed in this retrospective two-centre study with the sequence nSAI-FOR. When receiving ANZ, 1 of 11 achieved a complete response and 9 of 11 a stable disease > or = 6 months, and receiving LTZ 1 of 9 achieved a partial response and 4 of 9 a stable disease > or = 6 months. The analysis of the entire population treated with FOR showed an overall CB of 55% (11 of 20) with a median duration of 15 months and median time to progression (TTP) of 6 months.

CONCLUSIONS

Formestane 250 mg once bi-weekly seems to be an attractive alternative third-line hormonal therapy for the treatment of patients with MBC, previously treated with nSAI.

Anastrozole: pharmacological and clinical profile in postmenopausal women with breast cancer

A significant proportion of breast cancers are estrogen-dependent and are therefore amenable to endocrine therapy. Although tamoxifen has been the mainstream of endocrine treatment for over 20 years, new agents have entered the clinic, which have potentially superior activity and an improved safety profile. The development of orally-active, potent and selective third-line aromatase inhibitors represents a major advantage in the management of hormone-sensitive breast cancer. Anastrozole (Arimidex) was the first of these agents to become available and is currently widely indicated for both first- and second-line treatment for postmenopausal women with breast cancer. This review focuses on the biochemical properties and clinical efficacy of anastrozole, providing an overview of the current clinical status and possible future applications.

Aromatase inhibitors: past, present and future

For the cellular physiology of sex steroid sensitive cells, the androgen/estrogen ratio may be more important than only one hormone action per se, in both sexes. This ratio is controlled in vertebrates by aromatase; its gene expression can be inhibited in different ways, and this is crucial for the treatment of estrogen-dependent diseases such as breast cancer, or gynecomastia in males for instance. To reach this goal, new steroidal and non-steroidal inhibitors are continuously being developed, and some of them are used as first or second line agents. Aromatase inhibition is also an essential tool for studying the role of estrogens in the adult, or during development. Aromatase inhibitors have shown in particular that estrogens are essential also in males for skeletal maturation and bone mineralization, development of masculine dendritic morphology in male brain linked to mating behaviour, and testicular function. Testosterone is often the prohormone converted in situ in active estrogens, at these levels. Several strategies can be used for aromatase inhibition. The first ones employed were blind screening or deductions from in vivo observations, which led for instance to the discovery of the role of aminoglutethimide in aromatase inhibition. Subsequently, in the years 1975-1990, the molecular modeling of compounds to mimic the substrate shape of the enzyme constituted the major idea. Hundreds of chemicals were synthesized by numerous authors, ranging from the well-known and very efficient 4-OHA to complicated imidazole or indane derivatives tested by sophisticated comparative molecular field analyses. Reticulum-bound active aromatase has not as yet been X-ray analyzed. Thus, aromatase inhibitors were also used more recently to probe and understand the active site conformation of the enzyme and its modelization was obtained from comparisons with bacterial-related cytochromes. We developed a mammalian model considerably closer to human aromatase in order to study the active site shape with new potent aromatase non-steroidal inhibitors. This model is equine aromatase. This enzyme was biochemically characterized, purified, and cloned by our group. It allowed testing, by site-directed mutagenesis, predictive hypotheses in human aromatase which contributed to designing of new inhibitors. The understanding of the functioning of an essential member of the cytochrome P450 family, which is necessary for cellular detoxification, was also facilitated. Inhibition of aromatase activity has also been carried out with antibodies directed to the catalytic site and at the gene level by knock-out or by control of factor-specific promoters. This may result in different mRNA synthesized by alternative splicing. We have also obtained specific inhibition of aromatase activity in human cells with antisense stable phosphorothioate oligodeoxynucleotides directed against aromatase mRNA tertiary structures. Besides known steroidal and non-steroidal inhibitors, the antiaromatase effects of compounds found in our daily environment such as dietary flavonoids or xenobiotic pollutants have also been described. Finally, we underline that all these aromatase inhibitors, or methods of aromatase inhibition, can modulate the estrogenic balance essential not only for female, but also for male physiology, including gonadal function.

Aromatase inhibitors in the treatment and prevention of breast cancer

PURPOSE

The purpose of this article is to provide an overview of the current clinical status and possible future applications of aromatase inhibitors in breast cancer.

METHODS

A review of the literature on the third-generation aromatase inhibitors was conducted. Some data that have been presented but not published are included. In addition, the designs of ongoing trials with aromatase inhibitors are outlined and the implications of possible results discussed.

RESULTS

All of the third-generation oral aromatase inhibitors--letrozole, anastrozole, and vorozole (nonsteroidal, type II) and exemestane (steroidal, type I)--have now been tested in phase III trials as second-line treatment of postmenopausal hormone-dependent breast cancer. They have shown clear superiority compared with the conventional therapies and are therefore considered established second-line hormonal agents. Currently, they are being tested as first-line therapy in the metastatic, adjuvant, and neoadjuvant settings. Preliminary results suggest that the inhibitors might displace tamoxifen as first-line treatment, but further studies are needed to determine this.

CONCLUSION

The role of aromatase inhibitors in premenopausal breast cancer and in combination with chemotherapy and other anticancer treatments are areas of future exploration. The ongoing adjuvant trials will provide important data on the long-term safety of aromatase inhibitors, which will help to determine their suitability for use as chemopreventives in healthy women at risk of developing breast cancer.

Recent advances in the clinical application of aromatase inhibitors

Aromatase inhibitors have evolved over a period of 20 years to well tolerated agents that can effectively obliterate aromatase activity in postmenopausal women. Breast cancer is the predominant clinical application and here the newer agents have established themselves as the preferred second-line agent after tamoxifen in the treatment of advanced disease. Recent data indicate that they be more efficacious than tamoxifen and, therefore, may replace it as the first-line agent of choice in the near future. On-going clinical trials in the adjuvant setting and prospective prevention studies will elucidate whether these drugs have a yet greater role in breast cancer.

Resistance to endocrine therapy of breast cancer: recent advances and tomorrow's challenges

The role of endocrine therapy in early as well as advanced breast cancer cannot be overrated. Long-term tamoxifen exposure (5 years) in the adjuvant setting has been shown to be effective not only in improving relapse-free and overall survival but also in reducing the incidence of contralateral cancers. Promising results have been achieved in breast cancer prevention with use of antiestrogens. Novel aromatase inhibitors and inactivators have been found superior to conventional treatment in metastatic disease and are currently being evaluated in the adjuvant setting to improve relapse-free and overall survival. If potential health hazards from estrogen deprivation with regard to cardiovascular disease as well as bone metabolism can be addressed, adjuvant endocrine therapy may include such drugs in the future. However, while endocrine therapy of breast cancer has become more and more important in the clinic, the major problems in hormonal therapy are primary and acquired resistance to endocrine manipulations. The causes for endocrine resistance and possible ways to delay or avoid this phenomenon are only allusively understood. Elucidation of the mechanisms underlying endocrine resistance in vivo represents the key to improve our treatment strategies. Due to intense use of in vitro models and animal systems, many potential mechanisms of endocrine resistance have been described; however, our understanding of the problem of drug resistance in vivo remains limited. Hopefully, ongoing programs on translational research in the neoadjuvant, adjuvant, and palliative settings will provide information that will improve our understanding of the biology of endocrine resistance in vivo and, thus, provide us with a better rationale to improve early as well as late endocrine therapy in breast cancer patients. The present publication summarizes the state of the art with respect to endocrine resistance.

Effect of the aromatase inhibitor, MEN 11066, on growth of two different MCF-7 sublines

The racemate compound MEN 11066 (1-[(benzofuran-2-yl)(4'-cyanophenyl)methyl]-1H-1,2,4-triazole) and its enantiomers, (+)-MEN 11623 and (-)-MEN 11622, showed potent and selective aromatase activity on human placental microsomes. In addition, to better evaluate their potency as anticancer drugs, the compounds were assayed on testosterone-induced cell proliferation to measure their ability in inhibiting oestrogen-dependent tumour growth. Two different sublines originated from the human breast carcinoma MCF-7 were used. One, named MCF-7(tumour aromatase) (TA), that had maintained its intrinsic aromatase activity, was more sensitive to estradiol or testosterone-induced growth than the second subline named MCF-7(human placental aromatase) (hPA). The latter had been transfected with the human placental aromatase cDNA, after recognizing that the parental cells had aromatase activity reduced to undetectable levels. The MEN compounds completely reverted the testosterone-induced proliferation in both MCF-7(TA) and MCF-7(hPA) cells, while they did not affect the estradiol-triggered proliferation as a proof of their specificity for aromatase enzyme. Interestingly, MCF-7(TA) cells were more susceptible to the effects of aromatase inhibitors than the MCF-7(hPA) cell. These data suggest the efficacy of aromatase inhibitors in breast cancer when the growth dependency from oestrogen is high and a relatively low aromatase activity may be extremely important for tumour development.

Pharmacology and clinical experience with exemestane

Since the introduction of the first generation aromatase inhibitor, aminoglutethimide, for breast cancer treatment 30 years ago, we now have at hand novel, potent and well-tolerated steroidal and non-steroidal compounds, allowing near complete inhibition of oestrogen synthesis. The third-generation aromatase inhibitor, or more accurately termed inactivator, exemestane, is a potent suppressor of oestrogen synthesis and is shown to be an effective antitumour agent in postmenopausal breast cancer patients. Exemestane has been shown to be effective in patients failing multiple endocrine regimens. A large randomised study has revealed that exemestane improves time-to-disease progression as well as overall survival compared with megestrol acetate as second-line therapy in patients failing tamoxifen. In current studies, exemestane is compared with tamoxifen as first-line therapy for metastatic disease. Sequential therapy with tamoxifen followed by exemestane is also being compared with tamoxifen monotherapy in the adjuvant setting. In addition, the drug may have potential for breast cancer prevention.

Estrogen receptor alpha in human breast cancer: occurrence and significance

Estrogens have long been recognized as being important for stimulating the growth of a large proportion of breast cancers. Now it is recognized that estrogen action is mediated by two receptors, and the presence of estrogen receptor alpha (ER alpha) correlates with better prognosis and the likelihood of response to hormonal therapy. Over half of all breast cancers overexpress ER alpha and around 70% of these respond to anti-estrogen (for example tamoxifen) therapy. In addition, the presence of elevated levels of ER alpha in benign breast epithelium appears to indicate an increased risk of breast cancer, suggesting a role for ER alpha in breast cancer initiation, as well as progression. However, a proportion of ER alpha-positive tumors does not respond to endocrine therapy and the majority of those that do respond eventually become resistant. Most resistant tumors remain ER alpha-positive and frequently respond to alternative endocrine treatment, indicative of a continued role for ER alpha in breast cancer cell proliferation. The problem of resistance has resulted in the search for and the development of diverse hormonal therapies designed to inhibit ER alpha action, while research on the mechanisms which underlie resistance has shed light on the cellular mechanisms, other than ligand binding, which control ER alpha function.

Is there a growing role for endocrine therapy in the treatment of breast cancer?

Novel biochemical findings on the molecular mechanisms of estrogen actions may help us to understand some of the unexplained observations seen in breast cancer treatment and suggest new therapeutic opportunities. Thus, apart from the challenge of improving the clinical treatment of patients with advanced disease, results from trials in this setting may reveal new therapeutic principles that may be evaluated in the adjuvant setting. The role of endocrine therapy in metastatic as well as early breast cancer is increasing, and the possibility of improving cure rates for breast cancer by implementing therapy with novel aromatase inhibitors in the adjuvant setting is exciting. While the results from prevention trials are most interesting, suggesting the possibility of reducing breast cancer incidence in high-risk groups, more data are needed before we can decide whether such interventions are warranted in women at high risk of developing breast cancer.

New generation aromatase inhibitors in breast cancer. Weighing out potential costs and benefits

Endocrine therapy is the oldest and still one of the most effective forms of systemic therapy for breast cancer. Unfortunately, only one-third of all breast carcinomas respond to a strategy that modifies the activity of estrogen at the level of the tumour. Therefore, it is important that patients with cancer likely to respond are reliably identified. Substantial evidence indicates that tumour estrogen receptor level is the best predictor of response to hormonal therapy. Although antiestrogen therapy is still considered the endocrine modality of choice for all stages of breast cancer, there is renewed interest in finding new agents with improved therapeutic indices. The development of agents which selectively suppress aromatase, a key enzyme in estrogen biosynthesis, can be attributed not only to the importance of extraglandular aromatase activity, but also to the unparalleled success of tamoxifen. The present status, emerging roles and concerns of the new aromatase inhibitors are discussed in order to assess their potential costs and therapeutic merit.