Pharmacology and clinical experience with exemestane

Bone health in breast cancer

Early breast cancer is among the most common cancers worldwide. Recent advances continue to improve outcomes and increase long-term survivorship. However, therapeutic modalities are deleterious for patients' bone health. While antiresorptive therapy may partially negate this, consequent reduction in rates of fragility fractures remains unproven. Selective prescription of bisphosphonates or denosumab may be an amicable middle ground. Recent evidence also suggests a possible role of osteoclast inhibitors as adjuvant therapy, but the evidence is modest at best. In this narrative clinical review, we explore the impact of various adjuvant modalities on bone mineral density and fragility fracture rates of early breast cancer survivors. We also review optimal patient selection for antiresorptive agents, their impact on rates of fragility fractures, and the possible role of these agents as adjuvant therapy.

Ruxolitinib and exemestane for estrogen receptor positive, aromatase inhibitor resistant advanced breast cancer

Circulating IL-6, an activator of JAK/STAT signaling, is associated with poor prognosis and aromatase inhibitor (AI) resistance in hormone-receptor positive (HR+) breast cancer. Here we report the results of a phase 2 single-arm Simon 2-stage trial combining Ruxolitinib, an oral selective inhibitor of JAK1/2, with exemestane, a steroidal AI, in patients with HR+ metastatic breast cancer (MBC) after progression on non-steroidal AI (NSAI). Safety and efficacy were primary objectives, and analysis of inflammatory markers as predictors of response was a key secondary objective. Twenty-five subjects enrolled. The combination of ruxolitinib and exemestane was safe, though anemia requiring transfusion in 5/15 (33%) at the 25 mg dose in stage 1 led to a reduction to 15 mg twice daily in stage 2 (with no additional transfusions). Clinical benefit rate (CBR) in the overall study population was 24% (95% CI 9.4-45.1); 6/25 patients demonstrated stable disease for ≥6 months. Median progression-free survival was 2.8 months (95% CI 2.6-3.9). Exploratory biomarkers revealed high levels of systemic inflammation and 60% harbored a high-risk IL-6 genotype. Pharmacodynamics demonstrated modest on-target inhibition of phosphorylated-STAT3 by ruxolitinib at a tolerable dose. Thus, ruxolitinib combined with exemestane at a tolerable dose was safe but minimally active in AI-resistant tumors of patients with high levels of systemic inflammation. These findings highlight the need for more potent and specific therapies targeting inflammation in MBC.

An overview on Estrogen receptors signaling and its ligands in breast cancer

Estrogen governs the regulations of various pathological and physiological actions throughout the body in both males and females. Generally, 17β-estradiol an endogenous estrogen is responsible for different health problems in pre and postmenopausal women. The major activities of endogenous estrogen are executed by nuclear estrogen receptors (ERs) ERα and ERβ while non-genomic cytoplasmic pathways also govern cell growth and apoptosis. Estrogen accomplished a fundamental role in the formation and progression of breast cancer. In this review, we have hyphenated different studies regarding ERs and a thorough and detailed study of estrogen receptors is presented. This review highlights different aspects of estrogens ranging from receptor types, their isoforms, structures, signaling pathways of ERα, ERβ and GPER along with their crystal structures, pathological roles of ER, ER ligands, and therapeutic strategies to overcome the resistance.

Preclinical evaluation of exemestane as a novel chemotherapy for gastric cancer

CYP19A1/aromatase (Ar) is a prognostic biomarker of gastric cancer (GCa). Ar is a critical enzyme for converting androstenedione to oestradiol in the steroidogenesis cascade. For decades, Ar has been targeted with Ar inhibitors (ARIs) in gynaecologic malignancies; however, it is unexplored in GCa. A single‐cohort tissue microarray examination was conducted to study the association between Ar expression and disease outcome in Asian patients with GCa. The results revealed that Ar was a prognostic promoter. Bioinformatics analyses conducted on a Caucasian‐based cDNA microarray databank showed Ar to be positively associated with GCa prognosis for multiple clinical modalities, including surgery, 5‐Fluorouracil (5‐FU) for adjuvant chemotherapy, or HER2 positivity. These findings imply that targeting Ar expression exhibits a potential for fulfilling unmet medical needs. Hence, Ar‐targeting compounds were tested, and the results showed that exemestane exhibited superior cancer‐suppressing efficacy to other ARIs. In addition, exemestane down‐regulated Ar expression. Ablating Ar abundance with short hairpin (sh)Ar could also suppress GCa cell growth, and adding 5‐FU could facilitate this effect. Notably, adding oestradiol could not prevent exemestane or shAr effects, implicating a nonenzymatic mechanism of Ar in cancer growth. Regarding translational research, treatment with exemestane alone exhibited tumour suppression efficacy in a dose‐dependent manner. Combining subminimal doses of 5‐FU and exemestane exerted an excellent tumour suppression effect without influencing bodyweight. This study validated the therapeutic potentials of exemestane in GCa. Combination of metronomic 5‐FU and exemestane for GCa therapy is recommended.

Changes in expression of oestrogen regulated and proliferation genes with neoadjuvant treatment highlight heterogeneity of clinical resistance to the aromatase inhibitor, letrozole

Introduction

Clinical resistance is a major factor limiting benefits to endocrine therapy. Causes of resistance may be diverse and the mechanism of resistance in individual breast cancers is usually unknown. The present study illustrates how changes in the expression of proliferation and oestrogen-regulated genes occurring during neoadjuvant treatment with the aromatase inhibitor, letrozole, may define distinctive tumour subgroups and suggest different mechanisms of resistance in clinically endocrine resistant breast cancers.

Methods

Postmenopausal women with large primary oestrogen-receptor (ER)-rich breast cancers were treated neoadjuvantly with letrozole (2.5 mg daily) for three months. Clinical response was determined by ultrasound changes in tumour volume. Tumour ribonucleic acid (RNA) from biopsies taken before, after 14 days and after three months of treatment was hybridized on Affymetrix U133A chips. Changes in expression of KIAA0101, TFF3, SERPINA3, IRS-1 and TFF1 were taken as markers of oestrogen regulation and those in CDC2, CKS-2, Cyclin B1, Thymidine Synthetase and PCNA as markers of proliferation.

Results

Fifteen tumours with < 50% volume reduction over three months of treatment were classified as being clinically non-responsive. Gene expression changes after 14 days of treatment with letrozole revealed different patterns of change in oestrogen regulated and proliferation genes in individual resistant tumours. Tumours could be separated into three different subgroups as follows: i) nine cases in which both proliferation and oestrogen signalling signatures were generally reduced on treatment (ii) four cases in which both signatures were generally unaffected or increased with treatment and (iii) two cases in which expression of the majority of oestrogen-regulated genes decreased whereas proliferation genes remained unchanged or increased. In 14 out of 15 tumours, RNA profiles were also available after three months of treatment. Patterns of change observed after 14 days were maintained or accentuated at three months in nine tumours but changes in patterns were apparent in the remaining five cancers.

Conclusions

Different dynamic patterns of expression of oestrogen-regulated and proliferation genes were observed in tumours clinically resistant to neoadjuvant letrozole, thus illustrating heterogeneity of resistance and discriminating molecular sub-classes of resistant tumours. Molecular phenotyping might help to direct circumventing therapy suggesting the targeting of specific pathways in different tumour subtypes.

Changes in breast cancer transcriptional profiles after treatment with the aromatase inhibitor, letrozole

OBJECTIVE

The aim of the study was to identify changes in tumour expression profiling associated with short-term therapy of breast cancer patients with letrozole.

EXPERIMENTAL DESIGN

Microarray analysis was performed on RNA extracted from paired tumour core biopsies taken before and after 14 days of treatment with letrozole (2.5 mg/daily) in 58 patients. Changes in expression profile were identified by three different approaches on the basis of frequency of changes, magnitude of changes and significance analysis of microarray.

RESULTS

No single gene was consistently changed by therapy in all cases. Fifty-two genes, however, were downregulated and 36 upregulated in at least 45 of the 58 cases. In terms of quantitative change, 46 genes showed at least a median 1.5-fold change in expression. Significance analysis of microarray identified 62 genes that were significantly changed by therapy (P<0.0001, 56 downregulated and six upregulated). All three approaches showed that greater numbers of genes were downregulated rather than upregulated. Merging data produced a total of 143 genes, which were subject to gene ontology and cluster analysis. The ontology of the 91 downregulated genes showed that they were functionally associated with cell cycle progression, particularly mitosis. In contrast, upregulated genes were associated with organ development, connective tissue extracellular matrix regulation and inflammatory response. Cluster analysis segregated the patients into four groups differing in patterns of gene expression.

CONCLUSION

Genes have been identified which either change markedly or consistently in breast cancer after 14 days treatment with letrozole. These are new important data in understanding letrozole's molecular mechanism of action in breast cancers.

Aromatase inhibitors--gene discovery

Microarray analysis of tumour RNA is an extremely powerful tool which allows global gene expression to be measured. When used in combination with neoadjuvant treatment protocols in which therapy is given with the primary tumour within the breast, sequential biopsies may be analysed and results correlated with clinical and pathological response. In the present study, a neoadjuvant protocol has been used, administering the third generation inhibitor, letrozole, for 3 months and subjecting RNA extracted from biopsies taken before and after 10-14 days of treatment to microarray analysis. The objectives were to discover: (i) genes that change with estrogen deprivation (the only known biological effect of letrozole is to inhibit aromatase activity and reduce endogenous estrogens in postmenopausal women) and (ii) genes whose basal, on treatment or change in expression differ between tumours which are either responsive or resistant to treatment (so that predictive indices of response/resistance may be developed). Early changes in gene expression were identified by comparing paired tumour core biopsies taken before and after 14 days treatment in 58 patients using three different approaches based on frequency of changes, magnitude of changes and SAM analysis. All three approaches showed a greater number of genes were down-regulated than up-regulated. Merging of the data produced a total of 143 genes which were subject to gene ontology and cluster analysis. The ontology of the 91 down-regulated genes showed that they were functionally associated with cell cycle progression, particularly mitosis. In contrast, up-regulated genes were associated with organ development and extra-cellular matrix turnover and regulation. Clinical response was assessable in 52 patients; 37 (71%) tumours were classified as clinical responders (>50% reduction in volume at 3 months). Microarray analysis of pre- and 14-day biopsies identified 291 covariates (84 baselines, 72 14-day and 135 changes) highly predictive of response status. A similarity matrix using the covariates showed responding tumours have a similar genetic profile which was dissimilar to non-responding cancers whereas non-responsive cases were distinctive from each other. Changed genes predicting for response showed no concordance with those changed significantly by treatment in the overall group.

Hormonal therapy for advanced breast cancer

Hormonal therapy for advanced breast cancer has evolved significantly in the more than 100 years since the first publications documenting the effect of ovarian ablation on advanced breast cancer in premenopausal women. Since that time, not only have we developed the methods to measure estrogen and progesterone receptors in cancer cells, but more recently we have understood that expression of these receptors determines response to hormone therapy. The availability of more selective antiestrogen therapies has changed and significantly improved the treatment options for women who have advanced hormone-responsive breast cancer. Current research is focusing on reversing resistance to hormone therapy with the addition of targeted biologic agents to standard hormonal treatment.

Update on the use of aromatase inhibitors in breast cancer

Estrogens are biosynthesised from androgens by the CYP450 enzyme complex called aromatase. Aromatase is expressed in the ovary, placenta, brain, bone, adipose tissue and breast tissue. In breast cancer, intratumoural aromatase is the source for local estrogen production in the tissue. Inhibition of aromatase is an important approach for reducing growth stimulatory effects of estrogens in estrogen-dependent breast cancer. The potent and selective third-generation aromatase inhibitors anastrozole, letrozole and exemestane were introduced to the market as endocrine therapy in postmenopausal patients failing anti-estrogen therapy alone, or multiple hormonal therapies. Anastrozole and letrozole are both non-steroidal aromatase inhibitors that compete with the substrate for binding to the enzyme active site. Exemestane is a mechanism-based steroidal inhibitor that mimics the substrate, is converted by the enzyme to a reactive intermediate, and results in inactivation of aromatase. These third-generation aromatase inhibitors are currently approved as first-line therapy for the treatment of postmenopausal women with metastatic estrogen-dependent breast cancer. The use of an aromatase inhibitor as initial therapy, or after treatment with tamoxifen, is now recommended as adjuvant hormonal therapy for postmenopausal women with hormone-dependent breast cancer. Several clinical studies of aromatase inhibitors focus on the use of these agents in the adjuvant setting, for the treatment of early breast cancer. Recently published results show improved responses with these agents compared with tamoxifen.

Aromatase and the breast: regulation and clinical aspects

The aromatase enzyme is unique to the pathway of oestrogen biosynthesis and converts androgen precursors into oestrogens, major stimulatory factors for breast cancer proliferation. Although there is only a single gene for aromatase and a single protein for the enzyme, transcriptional control is complex using different promoters which are in part tissue-specific. These generate different mRNA transcripts that vary in the presence/absence of individual untranslated exon 1s. In breast cancers, species vary between individual tumours, types I.3 and I.4 being the major species in some tumours but type II predominates in the majority. Since the type II promoter is regulated by prostaglandins/cyclic AMP, agents signalling through these systems seem largely responsible for local regulation of intratumoural oestrogen biosynthesis. Autocrine production of these factors would account for the high activity in breast cancers and paracrine secretion for the raised activity in breast fat associated with the local presence of cancer. Given the central role of oestrogen in normal development and pathological processes, there has been great interest in controlling aromatase activity by the use of specific inhibitors. Clinically, this is particularly evident in the management of postmenopausal women with breast cancer.

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 inhibitors in advanced breast cancer

The third-generation aromatase inhibitors suppress whole-body estrogen production in postmenopausal women with high specificity and potency. In women with hormone-sensitive breast cancer, three of these agents, letrozole, anastrozole, and exemestane, provide an important alternative endocrine therapy to the antiestrogen tamoxifen, which blocks estrogen activation of the estrogen receptor. For treatment of advanced or metastatic breast cancer that has progressed on first-line tamoxifen, all three agents are active. On that basis, they have each been compared with tamoxifen as first-line therapy of advanced breast cancer, in phase III trials. Letrozole was significantly superior to tamoxifen in the primary end point, median time to progression, as well as in response rate and clinical benefit rate, and treatment was well tolerated. Although there was no significant difference in median overall survival, an advantage seen with letrozole for the first 2 years may have been lost because of crossover to the alternate agent at disease progression. Anastrozole was evaluated in two separate trials designed for combined analysis. Overall, anastrozole was at least equivalent to tamoxifen in activity, but clearly superior only for median time to progression in the subgroup of patients with hormone receptor-positive disease. Treatment was generally as well tolerated as tamoxifen. In an early report, exemestane was significantly better than tamoxifen in response rate and median time to progression, with overall survival data not yet available. To date, letrozole appears to be the most effective aromatase inhibitor in the first-line advanced breast cancer setting.

Biological rationale for endocrine therapy in breast cancer

Oestrogens are heavily implicated in the risk to, and progression of, breast cancer. Therapeutic strategies targeted at the oestrogenic stimulus to the breast and hormone-sensitive breast cancers are extremely attractive measures both to prevent the disease and to treat established tumours. The present review outlines the biological rationale for such endocrine therapy and traces the evolution whereby irreversible surgical procedures have been replaced by potent and specific drugs. In particular, the development of the latest generation of agents which inhibit oestrogen biosynthesis (aromatase inhibitors) is considered by defining the central role of the aromatase enzyme, its regulation and contribution to circulating and tumour endogenous oestrogens. The nature of response and resistance which may be elicited following the use of endocrine therapy is also described as this may determine the optimal use of aromatase inhibitors and, more generally, anti-hormone therapy in the management of women at high risk to, or with, breast cancer.

Update on the current use of hormonals as therapy in advanced breast cancer

Hormonal agents have a confirmed role in the management of postmenopausal women with receptor-positive advanced breast cancer. Until recently, tamoxifen has been the accepted agent for treating these patients. However, accumulating evidence suggests that the new antiaromatase agents will replace the antiestrogens as the preferable option in hormone-naive patients. Comparative trials indicate that the aromatase inhibitors, anastrozole and letrozole, and the aromatase inactivator, exemestane, have at least equivalent efficacy to tamoxifen with similar or superior tolerability. These agents are also more effective than the progestin, megestrol acetate, when studied in patients progressing on tamoxifen. The improved aromatase selectivity and high potency of these antiaromatase agents when compared with earlier agents have resulted in improved efficacy and tolerability. Additionally, no cross-resistance has been reported between the antiaromatase agents and tamoxifen or, in some instances, among the antiaromatase agents themselves. The role of antiaromatase agents will certainly expand in the near future to include not only treatment of metastatic breast cancer, but use in the adjuvant and neoadjuvant settings as well, and, ultimately, breast cancer prevention. The results of ongoing investigations are awaited with interest.

Aromatase inhibitors in breast cancer therapy

Estrogens are involved in numerous physiological processes and have crucial roles in certain disease states, such as mammary carcinomas. Estradiol, the most potent endogenous estrogen, is biosynthesized from androgens by the cytochrome P450 enzyme complex called aromatase. Aromatase is found in breast tissue and the importance of intratumoral aromatase and local estrogen production is being unraveled. Inhibition of aromatase is an important approach for reducing growth stimulatory effects of estrogens in estrogen-dependent breast cancer. Steroidal and nonsteroidal aromatase inhibitors have shown clinical efficacy for the treatment of breast cancer. The initial nonselective nature of nonsteroidal inhibitors, such as aminoglutethimide, has been greatly reduced in the later generations of inhibitors, anastrozole and letrozole. Mechanism-based steroidal inhibitors, such as 4-hydroxyandrostenedione and exemestane produce potent aromatase inhibition in patients. The potent and selective third-generation aromatase inhibitors, anastrozole, letrozole and exemestane, are approved for clinical use as first-line endocrine therapy in postmenopausal women with metastatic hormone-dependent breast cancer and as second-line endocrine therapy in postmenopausal patients failing antiestrogen therapy alone or multiple hormonal therapies.