Aromatase inhibitors: cellular and molecular effects

Marked cellular and molecular changes may occur in breast cancers following treatment of postmenopausal breast cancer patients with aromatase inhibitors. Neoadjuvant protocols, in which treatment is given with the primary tumour still within the breast, are particularly illuminating. In Edinburgh, we have shown that 3 months treatment with either anastrozole, exemestane or letrozole produces pathological responses in the majority of oestrogen receptor (ER)-rich tumours (39/59) as manifested by reduced cellularity/increased fibrosis. Changes in histological grading may also take place, most notably a reduction in mitotic figures. This probably reflects an influence on proliferation as most tumours (82%) show a marked decrease in the proliferation marker, Ki67. These effects are generally more dramatic than seen with tamoxifen given in the same setting. Differences between aromatase inhibitors and tamoxifen are also apparent in changes in steroid hormone expression. Thus, immuno-staining for progesterone receptor (PgR) is reduced in almost all cases by aromatase inhibitors, becoming undetectable in many. This contrasts with effects of tamoxifen in which the most common change on PgR is to increase expression. Changes in proliferation occur rapidly following the onset of exposure to aromatase inhibitors. Thus, neoadjuvant studies with letrozole in which tumour was sampled before and after 14 days and 3 months treatment show that decreased expression of Ki67 occur at 14 days and, in many cases, the effect is greater at 14 days than 3 months. These early changes precede evidence of clinical response but do not predict for it. However, this study design has allowed RNA analysis of sequential biopsies taken during the neoadjuvant therapy. Based on clustering techniques, it has been possible to subdivide tumours into groups showing distinct patterns of molecular changes. These changes in tumour gene expression may allow definition of tumour cohorts with differing sensitivity to aromatase inhibitors and permit early recognition of response and resistance.     

The control and biological importance of intratumoural aromatase in breast cancer

The existence of aromatase activity in human breast carcinomas has been established for about 20 years but the clinical and biological importance of this remains unclear. A number of studies in clinical material suggest that aromatase activity may be a prerequisite of response to aromatase inhibitors and that aromatase activity may be enhanced in those tumours relapsing during treatment with one such inhibitor, aminoglutethimide. These results would carry more significance, however, if it was demonstrable that the growth of breast carcinomas is affected by the conversion of androgens to oestrogens by intratumoural aromatase. We have tried to address this by establishing model systems with aromatase-transfected MCF7 breast cancer cells. We have demonstrated that these cells can be stimulated mitogenically with androgen and that this proliferation is suppressible with aromatase inhibitors. Similarly the growth of aromatase transfected cells but not wild type cells as xenografts is supported by androstenedione and inhibitable by both the steroidal aromatase inhibitor, 4-hydroxyandrostenedione and the non-steroidal inhibitor, CGS 20267. Work with the former of these, which is a suicide inhibitor allowed us to demonstrate that growth can proceed with aromatase activity approximating to the highest level seen in breast carcinomas indicating that at least at this extreme level the intratumoural conversion of androgens to oestrogens may indeed be able to support tumour growth. Further work with this model system should allow us to define the minimal amount of intratumoural activity which can support tumour growth.     

Local endocrine effects of aromatase inhibitors within the breast

To determine the effects of aromatase inhibitors on oestrogen uptake, in situ aromatase activity and endogenous oestrogens in the breast, postmenopausal women with large primary ER-rich breast cancers have been treated neoadjuvantly for 3 months with either letrozole (2.5 or 10 mg daily) or anastrozole (1 or 10 mg daily) or exemestane (25 mg daily). Patients were given an infusion of ³H-androstenedione and ¹⁴C-oestrone for 18 h before and at the end of the study period. Blood, tumour and non-malignant breast were taken immediately after each infusion; oestrogens were extracted and purified. Tumour volume was measured before and during treatment at monthly intervals so that endocrinological changes could be related to clinical response. Treatment with each of the aromatase inhibitors was associated with a profound reduction in peripheral aromatase (as monitored by the level of plasma ³H-oestrone). There was no consistent effect on uptake of radioactively labelled oestrogen into breast tumours but a tendency for levels to increase after treatment in non-malignant breast. Conversely, therapy was associated with a marked inhibition of in situ oestrogen synthesis in both tumour and non-malignant breast (in occasional tissues, inhibitors appeared to be less effective but the effect was not related to clinical or pathological responses). Similar decreases were apparent in endogenous levels of oestrone and oestradiol. The absence of in situ aromatase activity tended to be associated with lack of clinical response to aromatase inhibition but the relationship was not absolute, limiting the utility of measurements of tumour aromatase as a predictive indices. Ex vivo studies of tissue aromatase indicated that such measurements consistently underestimate the inhibitory potential of reversible non-steroidal agents (and occasionally paradoxical in vitro increases in aromatase activity were seen with treatment). However, in situ assays demonstrate that new aromatase inhibitors such as anastrozole, exemestane and letrozole have profound effects on the local endocrinology within the postmenopausal breast, these being compatible with the clinico-pathological changes which occur with treatment.     

Surgical issues surrounding use of aromatase inhibitors

There are important surgical issues related to the use of the third generation aromatase inhibitors in both the neoadjuvant and adjuvant settings. Neoadjuvant hormone therapy is effective at downstaging tumours, particularly large tumours initially thought to be inoperable or requiring mastectomy. Randomised trials have shown that the newer aromatase inhibitors letrozole and anastrozole increase the numbers of women who are suitable for breast-conservation compared with tamoxifen, and that letrozole is superior to tamoxifen in terms of clinical response.

Aromatase inhibitors are most effective in ER-rich tumours and are clinically and biologically effective in both HER2 positive and negative tumours, whereas HER2 positive tumours show a level of resistance to tamoxifen.

In neoadjuvant studies comparing aromatase inhibitors with tamoxifen, the duration of use has been 3–4 months, by which time any response is usually evident but longer treatment periods produce continued shrinkage and response. The re-excision rate following breast conservation surgery after neoadjuvant hormone therapy is favourable compared with the rates following immediate wide local excision. Local recurrence rates are acceptable in patients undergoing neoadjuvant therapy and breast-conserving surgery providing post-operative radiotherapy is given.

Adjuvant aromatase inhibitors, as well as having an effect on metastatic disease and survival, reduce local and regional recurrence.     

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.     

An integrated view of aromatase and its inhibition

Aromatase inhibition has become a major treatment strategy for postmenopausal women with oestrogen-dependent breast cancer. Its optimal application is, however, dependent upon (i) the accurate identification of cancers which are ultimately dependent upon the activity of the aromatase enzyme, (ii) the use of the best method/inhibitor by which to blockade aromatase activity.

The single best predictor of response to aromatase inhibitors is the presence of tumour oestrogen receptors; receptor-negative cancers rarely respond whereas those with high levels seem particularly likely to benefit. However, there is a need for additional discriminatory markers. The use of microarray technology coupled with neoadjuvant therapy is likely to yield promising candidate genes. The finding that, amongst peripheral tissues, the tumour itself may have high activity has led to the suggestion that the tumour aromatase measurements may be predictive; however, in situ studies and the lack of robust assays for tumour aromatase suggest that tumour aromatase may not be an influential marker.

Whilst drugs such as anastrozole, exemestane, formestane and letrozole are all effective and specific inhibitors of aromatase, they differ in structure, potency and mechanism of action. Thus, differential sensitivity of tissues/tumours and non-cross resistance mean inhibitors are not equivalent and individual agents may have differing roles according to the setting in which they will be used. Aromatase inhibitors have evolved as key endocrine agents in the treatment of breast cancer. They offer the promise of rational treatment management based on the accurate identification of individual cohorts of tumours responsive to specific drugs.     

Relationship between tumour aromatase activity, tumour characteristics and response to therapy

Aromatase activity has been measured in human breast cancers by incubating tumour minces with [7-³H]testosterone and characterizing purified oestradiol (E₂) fractions by chemical derivative formation. Of 247 primary tumours, 178 showed evidence of oestrogen biosynthesis, levels varying between 0.5 and 12.5 fmol E₂ produced/h/g tissue. These values were quantitatively small but at least comparable with those in other peripheral tissues. There was no correlation between presence or level of aromatase activity and the histopathology of the tumours although oestrogen biosynthesis was more likely to be present in more cellular tumours. Aromatase activity was also unrelated to age, menopausal status, lymph node status and T stage of the patient from which the tumour was derived. In a subgroup of patients presenting without clinical evidence of distant metastatic disease, no significant relation was detected between tumour aromatase and disease-free interval, but tumours without aromatase activity were associated with increased survival at 36 months after primary treatment. A statistically significant correlation was also detected between the presence of tumour aromatase and oestrogen receptors. Furthermore, in small subgroups of patients with “advanced” breast cancer tumour aromatase was related to response to aminoglutethimide but not tamoxifen therapy. Whilst these results do not conclusively define a role for local synthesis of oestrogen in the progression of breast cancer, this possibility still exists and further studies on tumour aromatase are warranted.     

Letrozole-, anastrozole-, and tamoxifen-responsive genes in MCF-7aro cells: a microarray approach

Antiestrogens and aromatase inhibitors are important drugs in the treatment of estrogen-dependent breast cancer. To investigate the effects of these drugs on gene expression in breast cancer cells, we treated estrogen receptor-positive MCF-7 cells stably transfected with the aromatase gene (known as MCF-7aro cells) with testosterone, 17 beta-estradiol, two aromatase inhibitors (letrozole and anastrozole), and an antiestrogen (tamoxifen). We found that testosterone or 17 beta-estradiol induced the proliferation of MCF-7aro cells at a rate six times faster than the untreated cells. In addition, the testosterone-induced proliferation of MCF-7aro cells was effectively suppressed by letrozole, anastrozole, or tamoxifen. Microarray analyses on Affymetrix Human Genome U133A GeneChips (Affymetrix, Santa Clara, CA) were carried out using total RNA isolated from the control and treated cells. At the false discovery rate of 0.05 and a minimum fold-change criteria of 1.5, 104 genes were identified that were up-regulated and 109 genes were identified that were down-regulated by both androgen and estrogen. More than 50% of these hormone-regulated genes were counter-regulated by all three inhibitors and >90% were counter-regulated by at least one of the inhibitors. Comparing the effect of each inhibitor on gene expression, we observed that letrozole and anastrozole are more similar in terms of the genes they affect compared with treatment with tamoxifen. To validate the gene expression profiles identified from microarray analyses, the expression patterns of 13 representative genes were examined by Northern analysis. Finally, the genes identified as statistically significant were classified based on their expression patterns and biological function/pathways. The results of this study provide us with a better understanding of the actions of both aromatase inhibitors and antiestrogens at the molecular level. We believe that the results of this study serve as the first step in identifying unique expression patterns following drug treatment, and that this will ultimately be useful in customizing patient treatment strategies for hormone-dependent breast cancer.     

Predictors of response to aromatase inhibitors

Aromatase inhibitors are now considered to be part of the endocrine treatment for most hormone receptor-positive breast cancer in post-menopausal women for both early and advanced disease. Despite the impressive efficacy of these agents, up to 50% of treated patients exhibit de novo or intrinsic resistance to aromatase inhibitors and hence identification of response predictors is essential to allow treatment to be directed towards responsive populations and for alternative or additional therapies to be offered to resistant patients. Emerging data seem to suggest a role for the conventional tumour markers of oestrogen receptor and progesterone receptor as possible predictors of response but, particularly in the adjuvant setting, the extent to which these are useful has not been fully elucidated. Data from both the neo-adjuvant and advanced disease settings suggest that response to aromatase inhibitors does not appear to be adversely affected by HER-2 overexpression. Within neo-adjuvant aromatase inhibitor studies, the proliferation marker Ki67 has shown a significant correlation with relapse-free survival, suggesting a role in prediction for measurement of Ki67 and other dynamic markers of response. Analysis of multiple gene expression changes over a short treatment period may also have potential clinical utility for prediction of response.     

Shaping the BRCAness mutational landscape by alternative double-strand break repair,replication stress and mitotic aberrancies

Tumours with mutations in the BRCA1/BRCA2 genes have impaired double-stranded DNA break repair, compromised replication fork protection and increased sensitivity to replication blocking agents, a phenotype collectively known as ‘BRCAness’. Tumours with a BRCAness phenotype become dependent on alternative repair pathways that are error-prone and introduce specific patterns of somatic mutations across the genome. The increasing availability of next-generation sequencing data of tumour samples has enabled identification of distinct mutational signatures associated with BRCAness. These signatures reveal that alternative repair pathways, including Polymerase θ-mediated alternative end-joining and RAD52-mediated single strand annealing are active in BRCA1/2-deficient tumours, pointing towards potential therapeutic targets in these tumours. Additionally, insight into the mutations and consequences of unrepaired DNA lesions may also aid in the identification of BRCA-like tumours lacking BRCA1/BRCA2 gene inactivation. This is clinically relevant, as these tumours respond favourably to treatment with DNA-damaging agents, including PARP inhibitors or cisplatin, which have been successfully used to treat patients with BRCA1/2-defective tumours. In this review, we aim to provide insight in the origins of the mutational landscape associated with BRCAness by exploring the molecular biology of alternative DNA repair pathways, which may represent actionable therapeutic targets in in these cells.     

Endocrine treatment for breast cancers: biological rationale and current progress

Endocrine therapy is a major treatment modality for the systemic management of breast cancer. In comparison with alternatives such as chemotherapy, hormone manipulations have the advantage of lower toxicity but suffer from the disadvantages of producing responses in only 30-40% of patients with metastatic disease and seldom being curative. Nevertheless in recent years there have been significant advances in the endocrine treatment of breast cancer which have stemmed from a better understanding of the sources from which breast tumours may be supplied with hormones, the mechanism by which hormones regulate tumour proliferation and the more accurate identification of hormone sensitive tumours. As a result agents such as antioestrogens, aromatase inhibitors. LHRH agonists have largely superseded surgical and radiological ablation of endocrine organs. The major reduction in morbidity associated with these medical regimes means that they are much more acceptable to patients and may be used as adjuvants to local treatment of the breast in patients with "earlier" stages of the disease. At the same time patients can now be offered rational treatment selected on the basis of tumour biology rather than on more empirical criteria. The aims of this review are to provide details of the research which has led to this progress in endocrine treatment of breast cancer and to put into perspective the prospects for further advances.     

Molecular endocrinology and breast cancer--a reed before the wind lives on (a tribute to Professor Mike Reed)

Oestrogens in breast cancers are derived from both uptake from the circulation and in situ synthesis. Third generation aromatase inhibitors (AIs) effectively block aromatase activity within the breast. The effects of AIs on the molecular biology of breast cancers may be monitored in patients given neoadjuvant therapy. Changes in tumour gene expression associated with AIs is influenced by time of drug exposure and gene expression profiles may provide important information on tumour response/ resistance to AIs.     

Aromatase and COX-2 expression in human breast cancers

We have investigated aromatase and the inducible cyclooxygenase COX-2 expression using immunocytochemistry in tumors of a series of patients with advanced breast cancer treated with aromatase inhibitors. Aromatase was expressed in 58/102 breast cancers. This is similar to the percentage previously reported for aromatase activity. Interestingly, aromatase was expressed in a variety of cell types, including tumor, stromal, adipose, and endothelial cells. Since prostaglandin E₂ is known to regulate aromatase gene expression and is the product of COX-2, an enzyme frequently overexpressed in tumors, immunocytochemistry was performed on the tissue sections using a polyclonal antibody to COX-2. Aromatase was strongly correlated (P<0.001) with COX-2 expression. These results suggest that PGE₂ produced by COX-2 in the tumor may be important in stimulating estrogen synthesis in the tumor and surrounding tissue. No correlation was observed between aromatase or COX-2 expression and the response of the patients to aromatase inhibitor treatment. However, only 13 patients responded. Nine of these patients were aromatase positive. Although similar to responses in other studies, this low response rate to second line treatment suggests that tumors of most patients were no longer sensitive to the effects of estrogen. Recent clinical studies suggest that greater responses occur when aromatase inhibitors are used as first line treatment. In the intratumoral aromatase mouse model, expression of aromatase in tumors is highly correlated with increased tumor growth. First line treatment with letrozole was effective in all animals treated and was more effective than tamoxifen in suppressing tumor growth. Letrozole was also effective in tumors failing to respond to tamoxifen, consistent with clinical findings. In addition, the duration of response was significantly longer with the aromatase inhibitor than with tamoxifen, suggesting that aromatase inhibitors may offer better control of tumor growth than this antiestrogen.     

Mechanisms of resistance to aromatase inhibitors

Aromatase inhibitors are rapidly becoming the first choice for hormonal treatment of steroid receptor positive breast cancer in postmenopausal women. An understanding of the resistance mechanisms to these agents is, therefore, important for the appropriate delivery of treatment to responsive patients and the rational development of new agents targeted at the resistance pathways. De novo resistance appears to be a quantitative rather than qualitative phenomenon with virtually all oestrogen receptor positive tumours showing an anti-proliferative response to the aromatase inhibitor anastrozole. While the expression of type 1 growth factor receptors reduces response to tamoxifen this appears to have little detrimental effect on response to aromatase inhibitors. Studies of acquired resistance in vitro have indicated that acquisition of hypersensitivity to oestrogenic stimulation is a key mechanism that is dependent on enhanced cross-talk of growth factor and oestrogen signaling pathways. Collection of resistant biopsy tissues from patients is important to determine if this mechanism is clinically relevant.     

Aromatase and its inhibitors

Inhibitors of aromatase (estrogen synthetase) have been developed as treatment for postmenopausal breast cancer. Both steroidal substrate analogs, type I inhibitors, which inactivate the enzyme and non-steroidal competitive reversible, type II inhibitors, are now available. 4-hydroxyandrostenedione (4-OHA), the first selective aromatase inhibitor, has been shown to reduce serum estrogen concentrations and cause complete and partial responses in approximately 25% of patients with hormone responsive disease who have relapsed from previous endocrine treatment. Letrozole (CGS 20, 269) and anastrozole (ZN 1033) have been recently approved for treatment. Both suppress serum estrogen levels to the limit of assay detection. Letrozole has been shown to be significantly superior to megace in overall response rates and time to treatment failure, whereas anastrozole was found to improve survival in comparison to megace. Both were better tolerated than the latter. The potential of aromatase within the breast as a significant source of estrogen mediating tumor proliferation and which might determine the outcome of inhibitor treatment was explored. Using immunocytochemistry and in situ hybridization, aromatase and mRNA_arom was detected mainly in the epithelial cells of the terminal ductal lobular units (TDLU) of the normal breast and also in breast tumor epithelial cells as well as some stromal cells. Increase in proliferation, measured by increased thymidine incorporation into DNA and by PCNA immunostaining in response to testosterone was observed in histocultures of breast cancer samples. This effect could be inhibited by 4-OHA and implies that intratumoral aromatase has functional significance. An intratumoral aromatase model in the ovariectomized nude mouse was developed which simulated the hormone responsive postmenopausal breast cancer patient. This model also allows evaluation of the efficacy of aromatase inhibitors and antiestrogens in tumors of estrogen receptor positive, human breast carcinoma cells transfected with the human aromatase gene. Thus, the cells synthesized estrogen which stimulated tumor formation. Both aromatase inhibitors and antiestrogens were effective in suppressing tumor growth in this model. However, letrozole was more effective than tamoxifen. When the aromatase inhibitors were combined with tamoxifen, tumor growth was suppressed to about the same extent as with the aromatase inhibitors alone. Thus, there was no additive or synergistic effects of combining tamoxifen with aromatase inhibitors. This suggests that sequential treatment with these agents is likely to be more beneficial to the patient in terms of longer response to treatment.     

Transforming growth factor-β signalling in tumour resistance to the anti-PD-(L)1 therapy: Updated

Low frequency of durable responses in patients treated with immune checkpoint inhibitors (ICIs) demands for taking complementary strategies in order to boost immune responses against cancer. Transforming growth factor-β (TGF-β) is a multi-tasking cytokine that is frequently expressed in tumours and acts as a critical promoter of tumour hallmarks. TGF-β promotes an immunosuppressive tumour microenvironment (TME) and defines a bypass mechanism to the ICI therapy. A number of cells within the stroma of tumour are influenced from TGF-β activity. There is also evidence of a relation between TGF-β with programmed death-ligand 1 (PD-L1) expression within TME, and it influences the efficacy of anti-programmed death-1 receptor (PD-1) or anti-PD-L1 therapy. Combination of TGF-β inhibitors with anti-PD(L)1 has come to the promising outcomes, and clinical trials are under way in order to use agents with bifunctional capacity and fusion proteins for bonding TGF-β traps with anti-PD-L1 antibodies aiming at reinvigorating immune responses and promoting persistent responses against advanced stage cancers, especially tumours with immunologically cold ecosystem.     

Development of molecular agents for IGF receptor targeting.

The type 1 insulin-like growth factor receptor (IGF1R) is a promising anticancer treatment target, being frequently overexpressed by tumours, and mediating proliferation, motility and apoptosis protection. Design of specific kinase inhibitors is problematic because of homology between the IGF1R and insulin receptor. This obstacle can be circumvented using sequence-specific molecular agents including antisense, triplex and ribozymes. Recent studies indicate that profound sequence-specific IGF1R gene silencing can be induced by small interfering RNAs that mediate RNA interference in mammalian cells. IGF1R downregulation blocks tumour growth and metastasis, and enhances sensitivity to cytotoxic drugs and irradiation. In murine melanoma cells, radiosensitisation is associated with impaired activation of Atm, which is required for initiation of cell cycle checkpoints and DNA repair pathways after double-strand DNA breaks. Furthermore, tumour cells killed in vivo following IGF1R downregulation can provoke an immune response, protecting against tumour rechallenge. After years of studying the role of the IGF system in tumour biology, novel agents for IGF1R targeting will soon be available for clinical testing. This review summarises the development of molecular agents, and considers factors that will influence clinical activity, including the requirement of established tumours for IGF signalling, and the efficacy and toxicity of IGF1R inhibitors.     

DCIS and aromatase inhibitors

In patients with hormone receptor positive DCIS tamoxifen reduces recurrence rates by almost 50%. Few data are available with aromatase inhibitors from randomised studies. In the ATAC study there were three DCIS lesions in the anastrozole arm and four in the tamoxifen arm in the women with ER positive invasive cancer. In the MA17 study which randomised patients to up to 5 years of letrozole or placebo there was only one DCIS event in the contralateral breast in patients taking letrozole and five on placebo. There were also four patients in this study who had DCIS in the conserved breast on placebo and none in the letrozole treated group. The few clinical data that are available therefore suggest the aromatase inhibitors are likely to be effective in DCIS. A histological review of a study of 206 postmenopausal women with invasive oestrogen receptor positive breast cancer who were randomised as part of a 14 day preoperative study to receive 2.5 mg of letrozole or 1 mg of anastrozole identified 27 patients with 28 pairs of tumours in whom there was sufficient ER positive DCIS in invasive cancer in the initial core biopsy and in the subsequent surgery specimen, to evaluate for PgR activity and proliferation. Within the DCIS both aromatase inhibitors significantly reduced PgR expression and both drugs also produced a significant fall in proliferation. There was a moderate degree of agreement between the fall in PgR in both the invasive cancer and DCIS (Kappa = 0.5; p = 0.0013) and between the fall in proliferation and between the invasive and in situ components (correlation coefficient = 0.68; p < 0.001). This study has shown significant effects of aromatase inhibitors on DCIS indicating that these agents are therapeutically active in this condition.     

Role of steroid sulfatase in local formation of estrogen in post-menopausal breast cancer patients

More than two-thirds of breast cancers occur in post-menopausal women, and depend on the estrogens for their proliferation and survival. For the treatment of estrogen-dependent breast cancers, two major treatment options are now available. One is selective estrogen receptor modulator (SERM) such as Tamoxifen and another is aromatase inhibitor such as Anastrozole, Letrozole and Exemestane, which reduce local in situ formation of estrogens. Although these therapies are clinically active for advanced and early breast cancers, de novo and/or acquired resistance to SERM and/or aromatase inhibitors are also clinical problem. Recent studies suggest that local formation of estrogens in the breast tumors is more important than circulating estrogen in plasma for the growth and survival of estrogen-dependent breast cancer in post-menopausal women. The rationale for the importance of local formation of estrogens is based on the following evidences. Estradiol (E2) levels in breast tumors are equivalent to those of pre-menopausal patients, although plasma E2 levels are 50-fold lower after menopause. E2 concentrations in breast tumors of post-menopausal women are 10–40 times higher than serum level. Biosynthesis of estrogens in breast tumors tissues occurs via two major different routes, one is aromatase pathway and another is steroid-sulfatase (STS) pathway. Whereas many studies has been reported about aromatase inhibitor and its clinical trial results in breast cancer patients, limited information are available regarding to other estrogen regulating enzymes including STS, its role in breast tumors and STS inhibitors. STS is the enzyme that hydrolyses estrone 3-sulfate (E₁S) and dehydroepiandrosterone-sulfate (DHEA-S) to their active un-sulfoconjugated forms, thereby stimulating the growth and survival of estrogen-dependent breast tumors. It has been well known that E₁S level are much higher than E2 level both in plasma and tumor of post-menopausal patients. Recent reports show that more than 80% of breast tumors are stained with anti-STS antibody and the expression of STS is an independent prognostic factor in breast cancer. Taking these findings into consideration, local formation of estrogens could be partially synthesized from large amount of E₁S by STS, which exist in breast cancer. On the other hand, aromatase localizes in stroma and adipocyte surrounding breast cancer. Furthermore, since estrogen formation from E₁S and DHEA-S (STS pathway) cannot be blocked by aromatase inhibitors, STS is thought to be a new molecular target for the treatment of estrogen-dependent tumor post-SERM and/or aromatase inhibitors. In this symposium, these recent rationale for the importance of STS in post-menopausal breast cancer patients is reviewed as well as STS inhibitor.