Estrogen Action, Selective Estrogen Receptor Modulators and Women's Health

The following sections are included:

• CONTENTS

• ABOUT THE AUTHOR AND EDITOR

• CONTRIBUTORS

• ACKNOWLEDGEMENTS

• FOREWORDS

The story of deciphering the mechanisms that control the growth of sex-hormone-dependent cancers started more than a hundred years ago. Clinical observations of the apparently random responsiveness of breast cancer to endocrine ablation (hormonal withdrawal) provoked scientific enquiry in the laboratory that resulted in the development of effective strategies for targeting therapy to the estrogen receptor (ER) (or the androgen receptor in the case of prostate cancer), the development of antihormonal treatments that dramatically enhanced patient survival, and the fi rst successful testing of agents to reduce the risk of developing any cancer. Most importantly, elucidating the receptor-mediated mechanisms of sex-steroid-dependent growth and the clinical success of antihormones has had broad implications for medicinal chemistry with the synthesis of new selective hormone receptor modulators for numerous clinical applications. Indeed, the successful translational research on the ER was the catalyst for the current strategy for developing targeted therapies for the tumor and the start of “individualized medicine.” Over the past 50 years, ideas about the value of antihormones have translated effectively from the laboratory to improvement of clinical care, increase in national survival rates and signifi cant reduction in the burden of cancer.

Diethylstilbestrol (DES) is a synthetic nonsteroidal estrogen that was used by millions of pregnant women in the United States to prevent miscarriages and other pregnancy-related complications between 1938 and 1971. In 1971, the U. S. Food and Drug Administration issued a warning against the use of DES during pregnancy after a relationship was found between exposure to DES during the first trimester of pregnancy and the development of clear cell adenocarcinoma of the vagina and cervix in young women whose mothers had taken DES during pregnancy. Additional side effects associated with DES included an increased risk for breast cancer in women who took DES during pregnancy and structural reproductive tract anomalies, increased infertility, and poor pregnancy outcomes for women who were exposed to DES in utero. Compilation of the various sites of DES toxicity in humans and experimental animals indicates that lesions appear predominantly in estrogen-responsive target tissues suggesting that the presence of the estrogen receptor (ER) in such target tissues may help to govern the toxic effects of DES. Interestingly, it should be noted that DES, despite its adverse effects in reproductive endocrinology, was the fi rst successful “chemical therapy” for cancer. Specifi cally, DES was available for the treatment of advanced prostate and breast cancer from the 1940s up until the 1990s. This review discusses the long-term health implications of DES for mothers, DES daughters and DES sons, and the possible side effects of DES on the third generation. We also propose possible mechanism(s) of actions of DES as a carcinogen and its paradoxical action as the first effective hormonal therapy for prostate and breast cancer.

The initiation of studies starting the train of development of the hormonal contraceptives occurred over 100 years ago with animal studies which demonstrated that ovarian transplants and ovarian extracts were able to inhibit ovulation. Subsequent studies defi ned the endocrinology of the ovarian cycle and factors involved in ovulation. This was followed by a dramatic era of pharmaceutical chemistry during which the orally active estrogens and progestins were discovered. Early studies demonstrated that a combination of the steroids could produce an effective inhibition of ovulation and would serve as an acceptable contraceptive for women. Over the years dramatic changes have been made to these products such that the levels of both the estrogen and progestin components have been greatly lowered and a variety of new steroidal compounds (progestins) have been produced along the way to reduce adverse effects, probably imparting some pharmaceutical patent protection for the manufacturers. Most recently, the utilization of these same products formulated for nonoral administration has turned out to be a fine-tuning of the use of estrogen–progestin contraception for better utility and compliance. Hormonal contraceptives have been an important sociological discovery having a great impact on the sexual liberation of women, allowing reasonable family planning and initiating controversies along the way in politics, religion and clinical medicine. They have been the most thoroughly studied pharmaceutical agents in history and have brought tremendous benefi ts to mankind in their effective and, by all standards, relatively safe usage in many different clinical situations. They are currently used by millions of women around the world as the major form of contraception. Perhaps the most surprising aspect of such pharmaceutical agents to inhibit ovulation is that we now recognize a broad spectrum of noncontraceptive benefits which have extended the utility of these agents into many areas of women's health. This chapter briefl y discusses the development, utility and relationship of estrogen–progestin contraception to women's health.

Hormone replacement therapy (HRT) is the most effi cient treatment for the relief of menopausal symptoms. Prophylactic administration of HRT has been used in clinical practice in postmenopausal women with increased risk for osteoporosis, cardiovascular disease or Alzheimer's disease. However, potential risks of cancer, thromboembolism and stroke have created some insecurity in the prolonged use of therapy. In this chapter, an extensive review of clinical trials will be made to assess the potential benefi ts and risks of HRT.

The clinical fi nding that the rodent antifertility agent nonsteroidal antiestrogen clomiphene (a mixture of geometric isomers) was actually a profertility agent in subfertile women created the first practical method for enhancing fertility in women. A related compound, ICI 46,474, the pure trans isomer of a substituted triphenylethylene, was also a product of an industry fertility control program in the 1960s and it too was tested and then marketed in the United Kingdom as an inducer of ovulation in subfertile women, at the same time as an orphan drug for the treatment of metastatic breast cancer in postmenopausal women. Clomiphene has, however, remained the clinical gold standard for the induction of ovulation worldwide for 40 years. However, the fact that tamoxifen is a potent inducer of ovulation in premenopausal women remains an important consideration in breast cancer patients made infertile by combination cytotoxic chemotherapy. This chapter will trace the genesis of agents for the induction of ovulation and the current potential applications of tamoxifen for women with breast cancer who choose to preserve fertility.

During the 1980s the structure–function relationship of nonsteroidal estrogens and antiestrogens using an a prolactin gene target in primary cultures of rat pituitary cells created basic models for the estrogen receptor (ER)–mediated modulation of estrogen-induced prolactin synthesis. However, the cloning and sequencing of the human ER, made possible by the creation of monoclonal antibodies to the ER, created a new dimension in our understanding of the mechanics of estrogen and antiestrogen action. Although the whole human ER has not been crystallized with ligands that binds to the ligand-binding domain (LBD), the crystallization and resolution of estrogen and antiestrogen LBD complexes are consistent and informative. Simply stated, an array of different complexes are formed with selective ER modulators (SERMs) and a pure antiestrogen causes considerable structural disruption of the LBD. This chapter tells the story of how the shape of the ligand–ER complex programs the interaction with coactivators for estrogen actions or becomes the signal for the destruction of the complex.

The majority of breast cancer cases rely on estrogen receptor signal ing and hence treatment strategies have focused in large part on blocking signaling through the estrogen receptor. Direct antagonists have undesirable side effects since they have antagonist effects in all tissues of the body. The discovery and development of selective estrogen receptor modulators (SERMs) has revolutionized the treatment of breast cancer as they have antagonist properties in the breast and yet agonist properties in other tissues lessening the severity of side effects of estrogen signaling blockade. Resistance can occur in patients treated with SERMs and this is thought to be due to changes in the balance of recruitment of coregulator molecules to estrogen-regulated transcriptional complexes. The effects of this shift may alter SERM function in the breast, moving it from antagonist to agonist. In this chapter, we will present the evidence that coregulator recruitment can alter the response to SERMs.

Breast cancers that occur in women who carry mutations of the breast cancer susceptibility gene 1 (BRCA1) are mostly estrogen receptor (ER)–negative; while those that occur in BRCA2 mutation carriers are usually ER-positive, similar to sporadic breast cancers. Despite these phenotypic differences, the evidence suggests that hormonal factors affect the risk of breast cancer in both BRCA1 and BRCA2 mutation carriers. In particular, prophylactic oophorectomy reduces breast cancer risk and the antiestrogen tamoxifen reduces the occurrence of contralateral breast cancer in both BRCA1 and BRCA2 carriers. Interestingly, while both BRCA1 and BRCA2 exert important functions in DNA repair and the maintenance of genomic integrity, cell-biologic and animal studies indicate that BRCA1 exerts an additional function in the regulation of ER activity that may, in part, explain why BRCA1 mutation carriers develop hormonally infl uenced tumor types. Here, BRCA1 binds directly to the ER and inhibits its transcriptional activity, through a specifi c posttranslational modifi cation (monoubiquitination). BRCA1 also binds to the progesterone receptor (PR) and inhibits its activity; and, in mouse models, BRCA1 loss-of-function mutations confer enhanced proliferative responses to estrogen and progesterone and an increased incidence of ER-driven mammary cancers. Finally, recent studies suggest that BRCA1 may function in mammary epithelial cell differentiation by promoting the conversion of ER-negative stem cells to ER-positive cells. Together, these experimental fi ndings are beginning to shed light on the puzzling clinical observations that hormonal manipulations can alter breast cancer risk in BRCA1 mutation carriers even though a large majority of BRCA1-related breast cancers are ER-negative.

Aromatase is the key enzyme in the biosynthesis of estrogen from precursor androgens via the process termed aromatization. Aromatase is highly expressed in the placenta and the granulosa cells of the ovary. Other tissues including adipose tissue, muscle the normal breast and breast cancer also express the enzyme but at much lower levels. In postmenopausal women these tissues serve as the main sources of estrogen production when the ovaries no longer function. Nevertheless the concentration of estrogens in the breast tissue is similar to levels in premenopausal women or as high as 4–6 times the serum concentration and up to 10 times in breast cancer tissue. Aromatase inhibitors reduce estrogen levels in postmenopausal patients by inhibiting aromataization. The clinical experience with the three FDA approved Als, letrozole, anastrozole and exemestane is reviewed according to disease stage including metastatic disease and early stage breast cancer as well as the role of Als in chemoprevention. Although Als have proved to be the most effective treatment for hormone responsive breast cancer, the emergence of resistance to them can be a limitation on their sustained clinical benefit. Cross talk between ER and HER2 or MAPK as well as PI3/K signaling pathways appear to be key mechanisms which cause previously hormone-responsive tumors to become resistance to Als. Phosphorylation of ER via activation of MAPK or PI3K/Akt pathways can lead to transcription of genes that regulate and promote tumor proliferation despite estrogen deprivation. Cotargeting the ER signaling pathway with Als and other novel agents to disrupt cross talk appears to be a promising approach to prolong the benefi ts of Als. Multiple therapies are currently under clinical development to target resistance and to individualize treatment for breast cancer patients.

The development of effective, life-saving strategies for the adjuvant antihormone treatment of estrogen receptor (ER)–positive breast cancer is a major advance in medicine. The effectiveness of the long-term adjuvant tamoxifen therapy has saved perhaps millions of lives worldwide during the past 30 years. However, tamoxifen therapy for the postmenopausal patient has the concern of a small but significant increase in endometrial cancer and thromboembolic events. The innovation of blocking endogenous estrogen synthesis with aromatase inhibitors has not only reduced the side effects of tamoxifen with endometrial cancer and thromboembolic disorders but also enhanced survivorship. Tamoxifen remains the endocrine treatment of choice for the premenopausal patient.

The discovery of tamoxifen as an anticancer drug more than four decades ago and the constant gain of knowledge about its metabolism and mechanistic action together with long term clinical experience could finally yield strategies for individualized endocrine treatment of breast cancer. This notion evolves from tamoxifen efficacy being subject to genetic predisposition, which explains the interindividual variability of outcome in adjuvant postmenopausal breast cancer treatment. This chapter introduces the evolution of knowledge that tamoxifen's efficacy depends on the formation of clinically active metabolites 4-hydroxytamoxifen and endoxifen, which have a greater affinity to the estrogen receptor and ability to control cell proliferation as compared to the parent drug. It explains why the cytochrome P450 (CYP) 2D6 enzyme plays a key role in this biotransformation and why lack of tamoxifen efficacy can be linked to low CYP2D6 activity. There is now considerable mechanistic, pharmacologic and clinical pharmacogenetic evidence in support of the notion that CYP2D6 genetic variants and phenocopying effects through drug interaction by CYP2D6 inhibitors influence plasma concentrations of active tamoxifen metabolites and negatively impact tamoxifen outcome. These interrelations are particularly critical for patients with nonfunctional (poor metabolizer) and severely impaired (intermediate metabolizer) CYP2D6 variants, and also for patients in need of comedications such as serotonin reuptake inhibitors to control adverse effects like hot flashes and other menopausal symptoms. A personalized approach for an optimal tamoxifen benefit should therefore consider a CYP2D6 genotype-guided adjuvant endocrine treatment strategy and avoid nonadherence as well as strong CYP2D6 inhibitors as comedications.

By introducing the evolution of knowledge about tamoxifen metabolism and the key role of the drug metabolizing enzyme CYP2D6, the chapter explains the current understanding of the metabolic activation of tamoxifen to its putative active agent, endoxifen. It follows that tamoxifen resistance can be explained at the level of genetic predisposition of variable drug metabolism, a rationale underlying the pharmacogenomic principle that explains the interindividual variability of outcome in adjuvant postmenopausal breast cancer tamoxifen treatment. The focus is on the veracity of the hypothesis of CYP2D6 defi ciency affecting the pharmacokinetics of tamoxifen and the interpretation of the available clinical evidence of the effects of CYP2D6 polymorphisms on tamoxifen outcome and the relevance of phenocopying effects through comedications such as selective serotonin reuptake inhibitors (SSRIs). Finally, we summarize the actions necessary for selecting appropriate long-term endocrine treatment for postmenopausal breast cancer patients with ER positive disease, and for improving the value of tamoxifen as a “personalized targeted treatment” within the context of improving women's health.

The estrogen receptor (ER) has proved to be an excellent target for the treatment and prevention of breast cancer. Although the majority (80%) of breast cancer is ER positive, not all ER positive tumors respond to antihormone therapy. When an ER positive tumor does not respond at all to antihormone therapy it is described to have intrinsic resistance. This contrasts with acquired antihormone resistance where the tumor initially responds with regression but then cell populations expand that grow because of tamoxifen or despite estrogen deprivation. Based on laboratory studies using ER positive cell lines, the evolution of acquired antihormone resistance has been documented. Growth factor pathways expand and subvert the action of the ER at the genome. The new knowledge about molecular mechanisms of resistance has created new opportunities for combinations of antihormone therapies and inhibitors of growth factor pathways.

Estrogen plays a central role in women's health and diseases. It mediates its diverse biological effects in the estrogen target tissues through estrogen receptors (ERs) alpha (α) and beta (β). Besides the several benefi cial effects of estrogen in female health, it also fuels the proliferation of ERα-positive breast cancers and uterine cancers in women. As a result, targeting and selectively modulating the activity of the ERs remains the approach of choice to block the harmful effects of estrogen action in tissues involved in diseases while retaining the beneficial effects in other tissues. Selective estrogen receptor modulators (SERMs) are molecules used to achieve selective modulation of the ERs such that it functions as an estrogen antagonist at breast level by blocking the proliferation of the cancer cells while retaining the antiosteoporotic effects of estrogen and thus functioning as an estrogen agonist at the bone level. Tamoxifen, the prototypical SERM, is the first molecule to be therapeutically used for treatment as well as prevention of breast cancers. This advance in therapeutics was followed by a second SERM, raloxifene (formerly known as keoxifene), which is currently used to treat and prevent postmenopausal osteoporosis and can also prevent breast cancers as effi ciently as tamoxifen. The advantage of raloxifene is that, unlike tamoxifen treatment, there is no increase in endometrial cancer in postmenopausal patients. However, tamoxifen and raloxifene treatment is also associated with some undesirable side effects. The clinical success of these two SERMs has encouraged directing the efforts toward developing new SERM molecules with better efficacy and fewer side effects. In addition, the discovery of ERβ has naturally enhanced a quest for subtype selective receptor modulators. Academic institutions as well as the pharmaceutical industry have directed their efforts toward designing novel ERβ selective modulators which can be used therapeutically to treat various diseases. This chapter summarizes the preclinical findings on novel SERMs and the current status of research and development in this field.

During the past four decades, the prospects of developing medicines to reduce the risk of breast cancer in healthy women have gone from a laboratory concept to a clinical reality. Tamoxifen, the pioneering selective estrogen receptor modulator (SERM), was initially shown to prevent rodent mammary carcinogenesis and the data were used to suggest prospective placebo-controlled clinical trials. Tamoxifen reduced the incidence of breast cancer by 50%; but, most importantly, this reduction versus placebo was maintained for a decade after stopping the drug. A small but significant increase in endometrial cancer in postmenopausal women taking tamoxifen, but not premenopausal women, makes tamoxifen the chemopreventive of choice for the high risk premenopausal woman. The finding that the SERM raloxifene maintains bone density in postmenopausal women but decreases breast cancer incidence without an increase in endometrial cancer makes raloxifene the chemopreventive of choice in postmenopausal women. A range of new and novel SERMs has completed clinical testing.

Selective androgen receptor modulators (SARMs) are a new class of anabolic agents that can increase muscle mass and improve physical function without unwanted effects on the prostate, skin, or hair that are commonly associated with steroidal androgens. These agents may have therapeutic utility in both women and men with muscle wasting.

Ligands for the nuclear receptor superfamily control many aspects of biology, including development, reproduction, and homeostasis, through regulation of the transcriptional activity of their cognate nuclear receptors (NRs). These receptors now represent one of the most important targets for therapeutic drug development. Selective nuclear receptor modulators (SNRMs) are receptor ligands that exhibit agonistic or antagonistic characterization in a cell and tissue context–dependent manner. The prototypical SNRM is tamoxifen, which, as a selective estrogen receptor modulator (SERM), can activate or inhibit estrogen receptor action with tissue selectivity. The success of SERMs led to the emergence of the concept to develop tissue-selective drugs for all members of the NR superfamily. We will review the current progress in developing NR modulators from designing synthetic compounds that mimic the full function of cognate ligands to developing compounds with tissue selectivity that modulate the functional activity of an NR in a manner that is distinct from that of cognate ligands. The goal is to reduce undesirable side effects.

The following sections are included:

• Appendices
  • APPENDIX 1: SCIENTIFIC SURVIVAL SUGGESTIONS
  • APPENDIX 2: DECADES OF DISCOVERY: THE SERM STORY — THE ST. GALLEN PRIZE
  • APPENDIX 3: AN ACCOUNT OF STUDENTS OBTAINING A PH.D. DEGREE (OR AN M.D. FOR PHYSICIANS IN THE BRITISH SYSTEM) WHILE IN THE TAMOXIFEN TEAM OVER THE LAST 30 YEARS

• INDEX