Combined treatment with 4-hydroxyandrostenedione and aminoglutethimide: effects on aromatase inhibition and oestrogen suppression

Estradiol measurement in translational studies of breast cancer

Plasma estrogen measurement with use of radioimmunoassays has been instrumental in the development of aromatase inhibitors for endocrine therapy of postmenopausal breast cancer. However, due to low plasma estrogen concentrations in postmenopausal women, direct radioimmunoassays lack the sensitivity required. While certain laboratories have developed highly sensitive assays for research purposes revealing plasma estrogen suppression consistent with results from tracer studies, such assays are time and labor-consuming due to need for pre-analytical chromatographic purification, sample concentration and sometimes conversion of precursors to products. While novel chromatographic methods involving mass spectrometry analysis are likely to replace such radioimmunoassays in the future, so far a limited number of laboratories have developed suitable assays with a detection limit (around 1 pM) that is required for analyzing plasma estrogen levels in patients during treatment with potent aromatase inhibitors.

Aromatase inhibition 2013: clinical state of the art and questions that remain to be solved

Following their successful implementation for the treatment of metastatic breast cancer, the 'third-generation' aromatase inhibitors (anastrozole, letrozole, and exemestane) have now become standard adjuvant endocrine treatment for postmenopausal estrogen receptor-positive breast cancers. These drugs are characterized by potent aromatase inhibition, causing >98% inhibition of estrogen synthesis in vivo. A recent meta-analysis found no difference in anti-tumor efficacy between these three compounds. As of today, aromatase inhibitor monotherapy and sequential treatment using tamoxifen followed by an aromatase inhibitor for a total of 5 years are considered equipotent treatment options. However, current trials are addressing the potential benefit of extending treatment duration beyond 5 years. Regarding side effects, aromatase inhibitors are not found associated with enhanced risk of cardiovascular disease, and enhanced bone loss is prevented by adding bisphosphonates in concert for those at danger of developing osteoporosis. However, arthralgia and carpal tunnel syndrome preclude drug administration among a few patients. While recent findings have questioned the use of aromatase inhibitors among overweight and, in particular, obese patients, this problem seems to focus on premenopausal patients treated with an aromatase inhibitor and an LH-RH analog in concert, questioning the efficacy of LH-RH analogs rather than aromatase inhibitors among overweight patients. Finally, recent findings revealing a benefit from adding the mTOR inhibitor everolimus to endocrine treatment indicate targeted therapy against defined growth factor pathways to be a way forward, by reversing acquired resistance to endocrine therapy.

The potency and clinical efficacy of aromatase inhibitors across the breast cancer continuum

The strategy of using estrogen suppression to treat breast cancer led to the development of aromatase inhibitors, including the third-generation nonsteroidal compounds anastrozole and letrozole, and the steroidal compound exemestane. Aromatase inhibitors potently inhibit aromatase activity and also suppress estrogen levels in plasma and tissue. In clinical studies in postmenopausal women with breast cancer, third-generation aromatase inhibitors were shown superior to tamoxifen for the treatment of metastatic disease. Studies of adjuvant therapy with aromatase inhibitors include (i) head-to-head studies of 5 years of the aromatase inhibitor versus 5 years of tamoxifen monotherapy; (ii) sequential therapy of 2-3 years of tamoxifen followed by an aromatase inhibitor (or the opposite sequence) versus 5 years of tamoxifen monotherapy; (iii) extended therapy with an aromatase inhibitor after 5 years of tamoxifen; and (iv) sequential therapy with an aromatase inhibitor versus aromatase inhibitor monotherapy. Recent results from the Arimidex, Tamoxifen, Alone or in Combination and Breast International Group 1-98 trials advocate using an aromatase inhibitor upfront. This article examines the clinical data with aromatase inhibitors, following a brief summary of their pharmacology.

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.

Aromatase inhibitors: assessment of biochemical efficacy measured by total body aromatase inhibition and tissue estrogen suppression

The implementation of aromatase inhibitors for treatment of early and metastatic breast cancer has been one of the major improvements in endocrine therapy of breast cancer. Measurement of endocrine effects of aromatase inhibition in vivo has been a major tool in the process of evaluating novel compounds. Biochemical efficacy of aromatase inhibitors in vivo may be determined from their effects on "total body aromatization" as well changes in plasma and tissue estrogen levels. Due to high sensitivity, tracer methods allowing calculation of whole body aromatase inhibition are still considered the gold standard. The method developed by our group in collaboration with the Royal Marsden Hospital and the results of this joint program are summarized and discussed. These studies allowed classification of the different aromatase inhibitors and their optimal dosage, selecting the best compounds for clinical evaluation. In vivo total body aromatase assessment is a work-consuming method, allowing such studies to be conducted in a limited number of patients only. In contrast, plasma estrogen measurement is a cruder but simpler method, allowing screening of larger groups of patients. As plasma estrogens arise through passive diffusion of estrogens synthesized in different body compartments, plasma estrogens, as well as total body aromatase assessment, present a rough estimate of total body tissue estrogen production, and changes associated with treatment with aromatase inhibitors reflect the effects on tissue estrogen production in general. However, plasma estrogen levels do not correlate to breast cancer tissue estrogen levels. This is due to the endocrine autonomy of breast cancer tissue with significant local estrogen production in some tumors. Thus, direct measurement of intratumor estrogens is demanded to evaluate the effects of aromatase inhibitors in malignant target tissues. Our group has developed a highly sensitive HPLC-RIA for the simultaneous measurement of estrone, estradiol, and estrone sulfate in malignant breast tissue samples, and we are currently using this method to assess alterations in intratumor estrogen levels during treatment with different aromatase inhibitors.

Aromatase inhibition: translation into a successful therapeutic approach

The development of the novel third-generation aromatase inhibitors and inactivators for breast cancer treatment is one of the most successful contemporary achievements in cancer therapy. Parallel to studies evaluating toxicity and clinical efficacy in metastatic disease, the endocrine effects of multiple compounds were evaluated, leading to the identification of the highly potent third-generation aromatase inhibitors based on estrogen deprivation and aromatase inhibition in vivo. Thus, translational studies have been of vital importance identifying the unique characteristics of these compounds. Whereas first- and second-generation aromatase inhibitors inhibit estrogen synthesis in vivo by up to 90%, the third-generation compounds anastrozole, exemestane, and letrozole were found to cause > or =98% aromatase inhibition. This article summarizes and discusses the "translational research" that provided the background for the implementation of the third-generation aromatase inhibitors and inactivators into large clinical trials. The need for future translational research exploiting the mechanisms of resistance to these compounds for future improvement of endocrine therapy is emphasized.

Endocrine effects of aromatase inhibitors and inactivators in vivo: review of data and method limitations

The so-called "third-generation" aromatase inhibitors/inactivators have become standard first-line endocrine therapy for postmenopausal women in the metastatic setting. In addition, these compounds, administered as monotherapy or in sequence with tamoxifen, are likely to become standard adjuvant therapy in most countries in the near future. In contrast to the SERMs, aromatase inhibitors may be assessed for their biochemical efficacy in vivo either by measuring their ability to suppress plasma and tissue estrogen levels or, alternatively, by measuring their ability to inhibit the conversion of tracer-labelled androstenedione into estrone. While contemporary methods for estrogen measurement (with the exception of estrone sulphate) lack the sensitivity to measure plasma estrogen levels during treatment with the most potent compounds, in vivo aromatase inhibition can be determined with a much better sensitivity. Thus, in a joint program conducted by the Royal Marsden Hospital, London and our team in Bergen, we were able to reveal profound differences between first- and second-generation aromatase inhibitors, causing 50-90% aromatase inhibition, and the three third-generation compounds, causing >98% inhibition of total body aromatization.

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.

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.

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.

Aromatase inhibitors in the treatment of postmenopausal breast cancer

Anastrozole, letrozole and vorozole are new aromatase inhibitors with a nonsteroidal structure (NSS), and have been demonstrated to be highly effective and better tolerated than standard endocrine therapy with megestrol (megestrol acetate) and aminoglutethimide (AG). These agents are very potent and selective: all of them are capable of suppressing estrone (E1) and estradiol (E2) to the limit of sensitivity methods, and plasma estrone sulfate (E1S) levels are also suppressed. However, the fact that this potency has not led to any greater clinical efficacy, and that there is no relationship between estrogen suppression and clinical response, suggests that aromatase inhibitors may have additional mechanisms of action. A number of international, multicentre clinical trials have compared anastrozole, letrozole and vorozole with megestrol 160 mg/day or AG 500 mg/day plus hydrocortisone in patients with advanced breast cancer. Letrozole proved to be significantly more effective than megestrol but anastrozole had a greater effect on survival than either agent. However, letrozole therapy led to longer survival than that observed in patients treated with AG. The activity of vorozole was similar to that of megestrol and AG. These results have raised a number of questions. The first is how should the clinical results be evaluated, given that 'disease stabilisation lasting > or =6 months' has been considered a response? The second is how should these drugs be used, and whether there is a rationale for using them in combination or sequentially in the treatment of patients with advanced breast cancer? Finally, is the possible effect of formestane and vorozole on intratumoral aromatase an alternative or concomitant mechanism of action? Anastrozole, letrozole and vorozole will be compared with tamoxifen in postmenopausal patients with breast cancer in adjuvant and primary settings. However, we feel that concomitant biological and clinical studies should also be carried out in order to clarify the properties of these drugs and avoid possible risks for patients over time.

A multicentre, randomized, pharmacokinetic, endocrine and clinical study to evaluate formestane in breast cancer patients at first relapse: endocrine and clinical results. The Italian Trials in Medical Oncology (I.T.M.O.) group

BACKGROUND

In postmenopausal breast cancer (BC) patients, tamoxifen (TAM) is frequently used in first-line therapy, and for those relapsing under TAM, aromatase inhibitors would be the drug of choice. Formestane, a new aromatase inhibitor, has been demonstrated to be as effective as TAM in first-line therapy. This trial was carried out to investigate the pharmacokinetics and antitumor activity of two formestane doses in BC patients at first relapse, as well as their effects on estrogen levels, evaluated by means of a new analytical method.

PATIENTS AND METHODS

One hundred fifty-two postmenopausal BC patients were randomly given formestane 250 mg or 500 mg intramuscularly every two weeks. The blood samples for estrogen measurements were taken on the first day of therapy, at 4 and 10 weeks, and every 12 weeks thereafter. Tumor response was first evaluated after 2.5 months, and then every three months.

RESULTS

Seventy-three patients received formestane 250 mg and 79 received 500 mg. After four weeks, plasma estrone, estradiol and estrone sulphate levels were significantly (P < 0.001) suppressed in both groups. The overall response rates were 30% and 40% on 250 mg and 500 mg, respectively.

CONCLUSIONS

Both of the formestane doses are effective in reducing plasma estrogen levels in BC patients at first relapse, and the new analytical method improved the quality of results. The antitumor response was highly satisfactory.

Aromatase inhibition for breast cancer treatment

While the first generation aromatase inhibitor aminoglutethimide and second- and third generation inhibitors like formestane and fadrozole all have been found to inhibit in vivo aromatization by 85-93%, the novel aromatase inhibitors letrozole and arimidex inhibit in vivo aromatization by 97-99%. However, we do not know whether these drugs cause a higher response rate or a longer duration of remission compared with less potent aromatase inhibitors. Lack of cross-resistance to steroidal and non-steroidal aromatase inhibitors suggests that these drugs may have partially different mechanisms of action, probably by influencing the intratumour aromatase enzyme. Recent studies have shown that breast cancer cells may adapt to alterations in oestrogen concentration in vitro by increasing their sensitivity. The observation that patients suffering relapse following castration, hypophysectomy or adrenalectomy may respond to treatment with aromatase inhibitors suggests that similar mechanisms could be responsible for acquired resistance to oestrogen deprivation.

A sensitive assay for measurement of plasma estrone sulphate in patients on treatment with aromatase inhibitors

A major obstacle to the understanding of the mechanisms of action of aromatase inhibitors in breast cancer is the observation that plasma estrogens are sustained at about 30-50% of their control levels despite 85-95% inhibition of the conversion of tracer androstenedione (A) to estrone (E1). The discrepancy could be due to lack of sensitivity of current RIAs. Due to low levels of plasma estradiol (E2) (mean about 20 pM) and E1 (mean about 75 pM) in postmenopausal women, it is difficult to develop RIA methods with the sensitivity required to detect > 90% suppression from baseline. In contrast, the plasma level of the estrogen conjugate estrone sulphate (E1S) is substantially higher (mean level about 400 pM). This paper describes a new assay to measure plasma E1S in the low range aiming to detect > 95% suppression of E1S from baseline values in patients treated with aromatase inhibitors. E1S was separated from unconjugated estrogens, hydrolysed and purified as unconjugated E1. E1 was subsequently reduced to E2, purified, and measured by a highly sensitive RIA using oestradiol-6-(O-carboxymethyl) oximino-(2(-)[125I]iodohistamine as ligand. The sensitivity limit of the method was 2.7 pM. Patients on treatment with the aromatase inhibitors formestane or aminoglutethimide or both drugs in concert were found to have plasma levels of E1S ranging from 3 to 274 pM with a mean suppression of 78, 86 and 95%, respectively, compared to baseline, a lower suppression than that reported in previous trials with these drugs.

Mechanisms of action of endocrine treatment in breast cancer

Endocrine treatment plays an important role in the therapy of breast cancer. While the basic mechanisms are understood, additional mechanisms may be of importance to their action and they may also contribute to the mechanism(s) of acquired resistance. Currently, several novel drugs are entering into clinical trials. Observations of the absence or presence of cross resistance to novel 'pure' steroidal antiestrogens and the non-steroidal tamoxifen may add important information to our understanding of the mechanisms of action of both classes of drugs. Similarly, exploration of different aromatase inhibitors in sequence or concert, as well as the combining of different endocrine treatment options may be warranted. Additionally, alterations in different biochemical parameters such as growth factors should not only be carefully explored in relation to treatment options but should also be followed during the course of treatment to asess alterations over time and in relation to the development of drug resistance.