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Selective Estrogen Receptor Modulators: Structure, Function, and Clinical Use

From the Breast Center and Departments of Medicine and Molecular and Cellular Biology, Baylor College of Medicine, Houston, TX.

Address reprint requests to C. Kent Osborne, MD, Breast Center, Baylor College of Medicine, One Baylor Plaza–MS600, Houston, TX 77030; email kosborne{at}bmc.tmc.edu

ABSTRACT

ABSTRACT: The sex hormone estrogen is important for many physiologic processes. Prolonged stimulation of breast ductal epithelium by estrogen, however, can contribute to the development and progression of breast cancer, and treatments designed to block estrogen’s effects are important options in the clinic. Tamoxifen and other similar drugs are effective in breast cancer prevention and treatment by inhibiting the proliferative effects of estrogen that are mediated through the estrogen receptor (ER). However, these drugs also have many estrogenic effects depending on the tissue and gene, and they are more appropriately called selective estrogen receptor modulators (SERMs). SERMs bind ER, alter receptor conformation, and facilitate binding of coregulatory proteins that activate or repress transcriptional activation of estrogen target genes. Theoretically, SERMs could be synthesized that would exhibit nearly complete agonist activity on the one hand or pure antiestrogenic activity on the other. Depending on their functional activities, SERMs could then be developed for a variety of clinical uses, including prevention and treatment of osteoporosis, treatment and prevention of estrogen-regulated malignancies, and even for hormone replacement therapy. Tamoxifen is effective in patients with ER-positive metastatic breast cancer and in the adjuvant setting. The promising role for tamoxifen in ductal carcinoma-in-situ or for breast cancer prevention is evolving, and its use can be considered in certain patient groups. Other SERMs are in development, with the goal of reducing toxicity and/or improving efficacy, and future agents have the potential of providing a new paradigm for maintaining the health of women.

THE SEX STEROID hormone estrogen is important in both men and women for a variety of physiologic processes. Estrogen affects growth, differentiation, and function of tissues of the reproductive system, including the mammary glands, uterus, vagina, and ovaries in females, and the testis, epididymis, and prostate in males. It also plays important roles in maintaining bone density and protecting against osteoporosis, and it is thought to be cardioprotective, largely through its effects on blood lipids. In the brain, estrogen regulates reproductive behavior, gonadotrophin production and release from the pituitary gland, and mood and behavior, and it may also have a role in slowing the destructive processes that lead to dementias.

Nearly all of the effects of estrogens are mediated through their binding to nuclear proteins called estrogen receptors (ER), transcription factors that regulate expression of estrogen-responsive genes.¹ Other natural compounds and synthetic drugs are also capable of binding to the ER. Some of these compounds mimic the effects of estrogen; others have more antiestrogenic activity. In the 1960s, the triphenylethylene tamoxifen was synthesized, and it demonstrated antiproliferative effects in the breast.² Shortly thereafter, in trials in patients with metastatic breast cancer, it was shown to be an effective therapy in those patients whose tumors expressed ER.^3,4 Tamoxifen thus became widely known as an antiestrogen, a misnomer that persists today.

It was soon discovered that tamoxifen, paradoxically, had many estrogenic qualities, including agonist effects on bone, blood lipids, and the endometrium.³ The mixed estrogenic and antiestrogenic effects of tamoxifen are species-, tissue-, and cell-dependent. Perhaps more surprising, however, tamoxifen has estrogenic effects on certain genes while having antiestrogenic effects on others, even in the same cell. These dual activities provide several fortuitous advantages for women receiving tamoxifen treatment. For instance, not only is there a beneficial effect through inhibition of proliferation of breast ductal epithelium and breast cancer, but women may also benefit by both maintenance of bone density and reduction in cholesterol.³ On the other hand, the estrogenic effects in the endometrium in postmenopausal women can result in an increased incidence of endometrial cancer coincident with prolonged tamoxifen use.⁵

We are only now beginning to understand how drugs like tamoxifen can differentially influence gene expression in specific tissues. The recognition that certain ligands can modulate ER in different ways has led to an explosion in the development of new drugs tailored to have specific and selective effects on ER function. These drugs, of which tamoxifen is the prototype, are now collectively known as selective estrogen receptor modulators (SERMs). It is now clear that SERMs with activities ranging from nearly full estrogenic activity to almost pure antiestrogenic activity can be developed for specific therapeutic uses ranging from treatment and prevention of osteoporosis to the prevention and/or treatment of breast cancer. These drugs offer great promise for the development of optimal hormone replacement therapy for women, an approach that might avoid stimulation of the breast while, at the same time, would effectively treat menopausal symptoms, reduce postmenopausal bone loss. and, perhaps, reduce the risk of cardiac disease. In this article, we will briefly review new information on the function of the ER that might help explain the complex activities of the various SERMs, and, in addition, we will also discuss the preclinical activity and available clinical uses for these exciting agents.

MOLECULAR MECHANISM OF ER ACTION

Estrogen and other ligands influence gene expression and cellular phenotypic changes by diffusing into the cell and binding to ERs in the nucleus. This binding activates the dimerization of these receptors, which further facilitates the direct interaction of receptor dimers with promoter regions in the DNA of target genes to activate or repress transcription. The two isoforms of ER belong to a super family of nuclear hormone receptors that function as transcription factors when they are bound by their respective ligands. The nuclear receptors share common structural and functional features; their functional domains have been designated A-F (Fig 1). The classical ER (now called ER) contains 595 amino acids with a central DNA-binding domain (DBD), along with a carboxy-terminal hormone-binding domain (HBD). Recently, another ER subtype, called ERß, has been identified (Fig 1).^6,7 ERß is somewhat shorter than ER, containing 530 amino acids.⁸ The region of highest homology between ER and ERß is in the DBD (95%). There is less conservation in the A/B, D, and E domains.

View larger version (12K): [in this window] [in a new window]   Fig 1. Structure and functional domains of ER and ERß.

ER-mediated gene transcription is stimulated through at least two distinct transactivation domains located in the amino-terminal A/B region (called AF-1) and the carboxy-terminal E region of the receptor (called AF-2).^12-14 The AF-1 domain is hormone-independent, whereas the AF-2 domain is hormone-dependent.¹⁵ Both AF-1 and AF-2 are required for maximal ER transcriptional activity.¹⁶ However, with certain promoters, AF-1 and AF-2 can function independently.¹⁶ Recently, it was demonstrated that the activity of the ERß AF-1 region is negligible compared with the AF-1 of ER, but that their AF-2 activities are comparable.¹⁷ Thus the activity of ER may exceed that of ERß on estrogen responsive element–containing genes that require both transactivation domains. It is also known that antiestrogens such as tamoxifen can function as pure antagonists on genes that require only the AF-2 domain for ER-mediated transcriptional activity. In contrast, in genes for which the ER AF-2 domain is not required, then transcription is driven only by AF-1 and tamoxifen can function as a partial agonist.^16,18 It is this capacity of many antiestrogens to act differently upon the two transactivation domains that, in part, makes them clinically useful antagonists in the breast but agonists in the uterus, in the bone, and in the liver.

After hormone binding and dimerization, ERs bind to DNA with high affinity through their DBD (Fig 1, C region) at specific sites, termed estrogen responsive elements.^19,20 The binding of ER to other response elements in the promoter region of target genes may also be triggered by the binding of certain ligands. A raloxifene response element, for example, has been identified that could explain some of the modest functional differences between tamoxifen and raloxifene.²¹ In addition, ER and ERß can form heterodimers that could also alter gene transactivation.

Besides this classical mechanism of direct DNA binding, the two ER subtypes can also activate other pathways.^22,23 AP-1 response elements, for instance, are regulated indirectly through interactions between ER and the AP-1 transcription factors c-fos and c-jun.²⁴ These transcription factors regulate genes involved in many cellular processes, including proliferation, differentiation, cell motility, and apoptosis. Thus the ER–AP-1 interaction could be important clinically. It has been shown that tamoxifen can act as an agonist on genes under the control of an AP-1 response element when ER or ERß are expressed. In striking contrast, however, estradiol does not activate these genes when ERß alone is expressed.²⁵ This finding could also have important implications for the differential effects of SERMs.

Cross-talk with other growth factor signaling pathways represents still another way in which ER can affect important cellular processes. For instance, members of the epidermal growth factor family of tyrosine kinase receptors can activate ER by directly phosphorylating crucial residues.²⁶ There is also considerable cross-talk between ER and insulin-like growth factor (IGF) signal transduction pathways. ER functions to increase levels of several of the key IGF signaling molecules, and IGFs, in turn, may activate ER.^27,28 This cross-talk between signaling pathways could conceivably contribute to the development of estrogen independence and/or clinical resistance to hormonal therapy.

ER undergoes extensive conformational changes after ligand binding as revealed by recent crystal structures of ER bound to various ligands.^29,30 The conformation of the ER ligand-binding domain is determined by the nature of the particular ligand that is bound. In the estrogen-liganded complex, helix 12 within the ER ligand-binding domain is repositioned over the ligand-binding cavity to form a secure lid over the ligand. This ER conformation exposes several amino acids that are critical for binding to specific coactivator proteins. In contrast, the repositioning of helix 12 over the ligand-binding cavity is prevented by SERMs such as tamoxifen or raloxifene. At the molecular level, we know that the precise positioning of helix 12 is very important for ER transcriptional activation.

Thus ER does not act alone to influence gene expression. Many of the tissue-specific effects of the ER may be dependent on the cellular pool of other factors that influence its transcriptional activity. These receptor coregulatory factors were first suggested from studies in which the activity of one nuclear receptor could interfere with or squelch the activity of another receptor by stealing other coregulatory proteins important for both receptors. There is an ever-growing list of coregulatory proteins that have been identified to date (reviewed in Horwitz et al³¹ and McKenna et al³² ). These proteins function as signaling intermediates between the receptor and the general transcriptional machinery and can either enhance ER transcriptional activity (coactivators), decrease its activity (corepressors), or function as cellular cointegraters for other diverse transcriptional signaling proteins.³³ Some of these proteins possess intrinsic histone acetylase or deacetylase activities. One simplistic model of coactivator/corepressor action could be envisioned whereby the receptor is kept in an inactive state by the recruitment of corepressors to the receptor complex (Fig 2). Generally speaking, the corepressors possess histone deacetylase activity, and this activity silences transcription by allowing DNA to wrap more tightly around the core histone proteins. Ligand binding then releases the corepressors from the transcription complex, enabling the receptor to recruit coactivators and cointegrators with their associated histone acetylase activity. These acetyl transferases add acetyl groups to histones, thereby loosening their interaction with DNA, which then exposes important residues to the basal transcriptional machinery. However, there also seems to be a diversity of enzymatic activities associated with many of the receptor coregulatory proteins; these include protease, ubiquitin ligase, ATPase, and kinase activities.^34-37

View larger version (19K): [in this window] [in a new window]   Fig 2. Role of ER coregulators in gene transactivation.

Of potential clinical importance to breast cancer is the observation that corepressors, at least in experimental systems, can block the partial agonist activity of tamoxifen by direct interaction with antagonist-bound receptor.³⁹ This suggests that some SERMs may induce active repression of the ER by promoting the association of the receptor with specific transcriptional corepressors. The net response of SERMs in a cell may depend on the particular balance of activators and repressors present in cells. With a relative abundance of coactivator proteins in a particular breast cancer, specific SERMs like tamoxifen may have more estrogen agonist activity and thus be ineffective or even deleterious in breast cancer patients. In support of this concept, overexpression of SRC-1 coactivator can indeed enhance the agonist activity of tamoxifen, although it cannot totally overcome the repression of tamoxifen’s agonist activity by the corepressor SMRT protein.⁴⁰ Thus full agonists like estradiol, but not mixed agonists/antagonists like tamoxifen, may enable the ER to completely overcome corepressor function. The agonist activity of SERMs may manifest itself when corepressor expression is low, when coactivator levels are high, or, alternatively, when the relative balance of bound coregulators favors an agonistic ER conformation. Lending support to this hypothesis is the correlation of reduced N-CoR corepressor levels with the acquisition of tamoxifen resistance in a xenograft model of breast cancer.⁴¹ In addition, there are other specific coactivators, such as L7/SPA, which are preferentially bound to tamoxifen-occupied ER, thereby enhancing its agonist activity in cells.³⁹ It is hoped that the targeted application of these recent advances in understanding nuclear receptor action and the identification of key coregulators to use as potentially predictive biomarkers will be rapidly translated to the clinic for the improved diagnosis and treatment of breast cancer.

To summarize, the net agonist/antagonist activity of ER ligands depends on ligand-induced conformational changes of the receptor and the receptor isoform as well as the particular ensemble of coregulatory proteins and promoter sequences that give functional specificity of the receptor down to the gene level (Fig 3). Among the consequences of this functional complexity of the ER is the possible development of specific SERMs with the most desirable mix of specific functional activities but with minimal side effects.

View larger version (21K): [in this window] [in a new window]   Fig 3. Mechanism of gene transactivation by ER.

SERMs can be conveniently divided into three major categories: (1) triphenylethylene derivatives like tamoxifen, (2) other nonsteroidal compounds, and (3) steroidal compounds that have more complete antiestrogenic activity. The chemical structure of several different SERMs as compared with estradiol is shown in Fig 4.

View larger version (36K): [in this window] [in a new window]   Fig 4. Chemical structures of 17ß estradiol and various SERMs.

Tamoxifen, like other SERMs, binds to ER and, in breast cancer cells, antagonizes the effect of estrogen on a variety of growth-regulatory genes.⁴⁵ The predominant effect of tamoxifen and many other SERMs is cytostatic with the induction of a G₁ cell cycle block, thereby slowing cell proliferation.³ Tamoxifen may also induce apoptosis, although this process has not always been easy to demonstrate in tumors.^3,46

In the CNS and vaginal mucosa, the predominant clinical effects of tamoxifen are also antiestrogenic. This accounts for the most common side effects attributed to tamoxifen, eg, menopausal symptoms. Although symptoms can vary considerably from patient to patient, women receiving tamoxifen frequently report hot flashes and/or vaginal discharge or dryness.^47,48 These side effects become more troublesome when tamoxifen is used for breast cancer prevention than when it is prescribed for the treatment of breast cancer, a potentially life-threatening illness.

In most other tissues, tamoxifen is predominantly an estrogen. Due to its estrogenic activity in the liver, serum concentrations of total cholesterol and low-density lipoprotein cholesterol are reduced by tamoxifen, although unlike estrogen, there is no elevation of high-density lipoprotein cholesterol.⁴⁹ These effects on lipid profiles may account for the reduction in cardiac events such as myocardial infarction reported in some studies.^50,51 Tamoxifen is also estrogenic in bone, preserving bone density in postmenopausal women.^52,53 However, bone density may fall in premenopausal women on tamoxifen, perhaps because it antagonizes the more potent activity of endogenous estrogen.⁵⁴ The slightly increased risk for thromboembolic phenomena may also be related to its estrogen agonist activity.^47,55 The rare ocular toxicity of tamoxifen is probably not related to its endocrine effects.⁵⁶ Women receiving tamoxifen who have preexisting cataracts more frequently require corrective surgery than women receiving placebo.

The most serious adverse effect of tamoxifen is a consequence of its estrogenic activity in the endometrium, which results in endometrial hyperplasia and low-grade endometrial cancers.^57,58 Although electrophilic metabolites of tamoxifen can form DNA adducts, there is no clear evidence that they play a clinically important role in the development of endometrial or other malignancies.⁵⁹ Other SERMs have been developed with the intent to either enhance clinical efficacy, reduce side effects, or both.

Toremifene (Fareston; Schering Corp, Kenilworth, NJ). Toremifene differs from tamoxifen only by the presence of one chlorine atom at the 4 position, and its preclinical and clinical activities are very similar to those of tamoxifen. Toremifene has antitumor activity in rodent mammary carcinoma and in human breast cancer growing in athymic mice.^60,61 Unlike tamoxifen, toremifene does not produce DNA adducts and is not a hepatocellular carcinogen in rats.^62,63 This potential advantage is of doubtful clinical significance. Toremifene has estrogenic effects in the uterus similar to those of tamoxifen.^64,65 Its estrogenic effects in bone may be somewhat less than those of tamoxifen.⁶⁶ Toremifene is completely cross-resistant with tamoxifen, and in a model of tamoxifen resistance in which breast tumors become tamoxifen-stimulated, toremifene has an identical effect.^67-69

Droloxifene. Droloxifene (3-hydroxytamoxifen) has a spectrum of activities similar to that of both tamoxifen and toremifene. It has a higher affinity for ER and is also a somewhat more potent growth inhibitor of cultured human breast cancer cells and rodent breast cancer.^70,71 It can preserve bone density in ovariectomized rats, and it exhibits less estrogenic activity in rat uterus compared with tamoxifen.^72,73 Like toremifene, droloxifene does not cause DNA adduct formation and is not a hepatocellular carcinogen in rats.⁷⁰ Although droloxifene seemed to have some potential advantages over tamoxifen and although it demonstrated activity in phase II clinical trials, it was found to be less active than tamoxifen in phase III trials and is no longer in development.^45,74

Idoxifene. Idoxifene is a metabolically stable analog of tamoxifen with the substitution of an iodine atom at the 4 position and a pyrrolidino side chain.⁷⁵ It binds to ER with higher affinity than tamoxifen and is a more potent inhibitor of rat mammary tumor growth and growth of human breast cancer cells in xenograft models.^76,77 Idoxifene produces fewer DNA adducts than tamoxifen. However, like tamoxifen, it has estrogen agonist effects in bone and liver.⁷⁸ Furthermore, it was less uterotropic in preclinical models.^76,78 Finally, idoxifene was associated with a reduced frequency of acquired antiestrogen resistance in a human breast cancer xenograft model, and responses were observed in a phase II trial of patients with tamoxifen-resistant disease.^79,80 With this favorable profile, idoxifene generated excitement for its potential use as both a breast cancer preventive agent as well as a treatment for breast cancer itself. Unfortunately, the unexpected and unexplained increased incidence of uterine prolapse and polyps with this agent precludes its study in breast cancer prevention and dampens enthusiasm for its further evaluation in breast cancer.

TAT-59. This phosphorylated derivative of 4-hydroxytamoxifen is being developed for the treatment of breast cancer. It has a higher binding affinity for ER compared with tamoxifen, and it is a more potent inhibitor of dimethylbenzanthracene-induced rat mammary carcinoma as well as human breast cancer xenografts growing in athymic mice.^81-83

GW5638. This carboxylic acid derivative of tamoxifen was developed with the primary goal of achieving more complete antiestrogenic activity in the breast and the uterus while maintaining estrogenic activity in bones and in the cardiovascular system.⁸⁴ In preclinical studies, this analog displayed full estrogen agonist activity in maintaining bone density and in lowering cholesterol, but it had little or no agonist activity in the uterus of ovariectomized rats.⁸⁵ Like other SERMs, this drug possesses antitumor activity in human breast cancers transplanted into athymic mice.⁸⁶ The carboxylic acid moiety predicts that GW5638 may have low penetration into the CNS and, as such, may prove to be active systemically while avoiding the induction of postmenopausal symptoms such as hot flashes.

Other Nonsteroidal Compounds
EM-800 and EM-652. EM-800 is the orally active prodrug of the active compound EM-652 (benzopyrene derivative). The latter shows higher affinity for the ER than estradiol, 4-hydroxytamoxifen, or the steroidal antiestrogen ICI 182780, giving it the highest affinity for the ER relative to other SERMs.⁸⁷ The drug is a potent inhibitor of ER-positive breast cancer cells in vitro and of breast cancer xenografts in athymic mice.^88-90 In addition to its potent inhibition of breast cancer growth in preclinical models, EM-800 has estrogen agonist activity in bone and liver, while demonstrating more antagonistic effects in the uterus.⁹¹ Whether these potential advantages translate to improved management of patients with breast cancer awaits the results of ongoing studies.

Raloxifene (Evista; Eli Lilly and Co, Indianapolis, IN). Raloxifene, a benzothiophene derivative, is the most widely studied of the newer SERMs. It binds to ER with an affinity equal to that of estradiol.⁹² Development of this drug as an antiestrogen for breast cancer was discontinued in the late 1980s, but because it was found to maintain bone density, to prevent rodent breast cancer, and to inhibit tamoxifen-stimulated endometrial cancer growth, it was developed for osteoporosis, for which it is now an approved drug.^93-96

The activity of raloxifene is similar to that of tamoxifen, with the exception of the endometrium, where raloxifene possesses less estrogen agonist activity.⁹⁷ As described earlier, the raloxifene response element that was identified in the promoter region of certain genes could contribute to some of its functional differences.^21,93 Raloxifene is an inhibitor of cultured breast cancer cells, and it possesses antitumor activity in rat mammary tumor models similar to that of tamoxifen.^98,99 It also has similar effects in bone, although it is somewhat less effective than estradiol.⁹⁷ Like tamoxifen, raloxifene reduces total cholesterol but does not increase high-density lipoprotein cholesterol, a feature that may lessen any cardioprotective effects.^97,100 Although activity in the CNS is more difficult to ascertain, raloxifene—like tamoxifen—fails to relieve hot flashes, suggesting that it is more antiestrogenic. DNA adduct formation has not been reported for raloxifene.

LY353381 (SERM 3) and LY357489. These drugs are newer, more potent raloxifene analogs.^101,102 They are more potent inhibitors of in vitro breast cancer cell growth than raloxifene or tamoxifen and are devoid of intrinsic estrogen agonist effects on cultured breast cancer cells.⁴⁵ SERM 3 is also a more potent agonist than raloxifene in bone and liver, and it completely inhibits estrogen-induced uterine weight gain in immature rats.^45,103 SERM 3 is targeted for breast cancer prevention and treatment given its promising preclinical profile. CP336,156 is a derivative of tetrahydronaphthalene with a similar preclinical profile.¹⁰⁴

Steroidal Antiestrogens
ICI182,780 (Faslodex; AstraZeneca Pharmaceuticals). A derivative of estradiol with a long, hydrophobic side chain at the 7 alpha position, ICI182,780 demonstrates a pure antiestrogenic profile on all genes and in all tissues studied to date.¹⁰⁵ The mechanism of action of this steroidal antiestrogen differs significantly from other SERMs with mixed agonist/antagonist properties. In contrast to other SERMs, ICI182,780 blocks ER transactivation coming from both the AF-1 and AF-2 domains.¹⁰⁶ The drug may also impair ER dimerization, but most importantly, ICI182,780 induces ER degradation, with a marked reduction in the cellular concentration of ER.^107-109 Because ER is the major target for all antiestrogens, depletion of ER from the cell would theoretically constitute optimal therapy.

As a consequence of these cellular effects, ICI182,780 is a potent inhibitor of transcription of estrogen-regulated genes. In rat uterus, both estradiol and tamoxifen can stimulate expression of several genes, but in each case, ICI182,780 has no activity on its own and it completely blocks estrogen or tamoxifen induction of these genes.^110-113 In an in vivo model, ICI182,780 was also much more potent than tamoxifen or than estrogen withdrawal in blocking transcription of estrogen-regulated genes in human breast cancer cells.⁶⁸

ICI182,780 binds to ER with an affinity similar to that of estradiol, and it exhibits 100 times greater binding affinity than tamoxifen. Although it is clearly antiestrogenic in the breast and uterus, studies of its effects on bone density in preclinical models are conflicting, with one study showing no effect and the other suggesting an increase in bone resorption in rats.^114,115 Given its mechanism of action, however, it seems likely that the effect of the drug in bone and in liver will be antiestrogenic.

ICI182,780 demonstrates exciting antitumor activity in preclinical models. For instance, tamoxifen-resistant cell lines selected in vitro may remain sensitive to growth inhibition by ICI182,780.^115,116 When MCF-7 human breast cancer cells are grown in an athymic mouse model, ICI182,780 is a much more potent inhibitor of tumorigenesis, and it is also more potent in inducing tumor regressions in mice with established tumors.⁶⁸ In some mice, the tumor is completely eradicated and the duration of tumor suppression in others is twice as long as treatment with either tamoxifen or estrogen withdrawal. In this same model, tumors become resistant to tamoxifen as a consequence of the development of tamoxifen-stimulated growth after an initial period of growth suppression. ICI182,780 is a potent inhibitor of these tamoxifen-stimulated tumors, which suggests that it might have activity in patients who are resistant to tamoxifen, a prediction that has now been confirmed in the clinic. Serial biopsies from tumors in patients treated with either tamoxifen or ICI182,780 show that the latter is a more potent inducer of apoptosis.⁴⁶ ICI182,780 may not cross the blood-brain barrier and may, therefore, not cause hot flashes, a clinically important problem with other SERMs.¹¹⁷ The cumulative data suggest that although ICI182,780 may not be the most desirable SERM for breast cancer prevention in normal women because of its antiestrogenic profile, it might be a superior antitumor agent.

CLINICAL USE OF SERMs

Given their broad array of activities, SERMs have several potential clinical uses. These include the prevention and treatment of osteoporosis, the prevention and treatment of estrogen-responsive malignancies such as breast cancer, and even hormone replacement therapy. It is likely that specific SERMs will need to be developed for each clinical use. Drugs used in normal women to correct menopausal symptoms and/or for prevention of disease have a narrow therapeutic index and must have minimal side effects. On the other hand, a somewhat greater side-effect profile would be acceptable for treatment of patients with a potentially life-threatening disease like breast cancer if the drug is very effective.

Treatment and Prevention of Osteoporosis
Raloxifene is currently the only SERM approved for osteoporosis. Several trials, including the large randomized placebo-controlled Multiple Outcomes of Raloxifene Evaluation (MORE) trial, show improvement in bone density and reduced fracture rates in post menopausal women with osteoporosis.¹¹⁸ A secondary end point in this study was breast cancer incidence, and raloxifene resulted in a 75% reduction in invasive breast cancer and a 50% reduction of in situ breast cancer in these elderly patients with an average age of 67 years. Side effects included menopausal symptoms and an increased rate of thromboembolic problems. There was a reduction in hypertension, but for unknown reasons, an increased incidence of diabetes mellitus was reported. So far, in this study, there is no difference between raloxifene and placebo in the incidence of endometrial cancer, although patients on raloxifene do have an increase in endometrial thickness that may be related more to fluid accumulation than to hyperplastic change. A reduction in myocardial infarction has not been seen in women receiving raloxifene. Raloxifene is an acceptable alternative for treatment of osteoporosis in women who do not wish to take estrogens or bisphosphonates. The failure of raloxifene and other SERMs to treat menopausal symptoms such as hot flashes may limit their use in some women.

Breast Cancer Prevention
The ancillary benefits of tamoxifen in adjuvant trials of breast cancer demonstrating favorable effects in bone and on blood lipids and the reduced incidence of contralateral breast cancer provided the rationale for trials of this agent in breast cancer prevention. Three breast cancer prevention trials using tamoxifen have completed accrual, two in Europe and one in the United States.^119-121 The raloxifene trial in women with osteoporosis had a secondary end point to evaluate breast cancer incidence.¹¹⁸

The National Surgical Adjuvant Breast and Bowel Project (NSABP) tamoxifen trial in women at high risk for breast cancer and the raloxifene trial of postmenopausal women with osteoporosis are the largest of the four trials. Because the raloxifene trial is primarily designed to evaluate bone fractures, its conclusions on the secondary end point of breast cancer incidence are less definitive than are those of the NSABP trial, although the results are very similar.^118,119 Both of these studies, after 3 to 4 years of follow-up, show a significant reduction in the incidence of invasive and in situ breast cancer and a reduction in the incidence of fractures. Both studies also show an increase in thromboembolic phenomenon and a slight, but not statistically significant, increase in stroke. Neither study shows an impact on cardiovascular mortality at this early follow-up time. As predicted, tamoxifen increases the risk of endometrial cancer, although all of the cancers to date are International Federation of Gynecology and Obstetrics stage I, and there have been no deaths among women receiving tamoxifen. The incidence of endometrial cancer in the raloxifene study so far is similar in patients on raloxifene and placebo. Menopausal symptoms are the most common side effects with both agents.

At the present time with only short follow-up, the major effect of both drugs is to reduce the incidence of ER-positive breast tumors. One interpretation is that the predominant effect now seen in these early data is inhibition of the progression of subclinical ER-positive cancers to clinically evident cancers. If these drugs do have a true prevention effect to inhibit the evolution of premalignant disease to invasive cancer, then one might expect a reduction in both ER-negative and ER-positive cancers. Nearly all premalignant lesions are strongly ER-positive and would be expected to be sensitive to the antiproliferative effects of these SERMs. The striking reduction in breast cancer incidence (both ER-positive and ER-negative) in the tamoxifen trial in women with a prior breast biopsy showing atypical hyperplasia further supports this argument. On the other hand, some ER-negative cancers may not evolve through an ER-positive, premalignant stage and thus might not be influenced at all by SERM therapy.

Overall, the British and Italian tamoxifen trials do not show a statistically significant reduction in the incidence of breast cancer.^120,121 However, the British trial was initiated as a pilot feasibility trial and its accrual was less than 20% of that on the NSABP tamoxifen trial.¹²⁰ The trial was also confounded by the use of estrogen replacement therapy in many patients who were randomized to tamoxifen. Finally, women on this trial have different characteristics from those on the other trials. Many more women on this trial would be predicted to have a hereditary breast cancer susceptibility based on family history information. Although clear data are not yet available, this raises the question of whether women with a breast cancer susceptibility gene benefit as much from SERM therapy as other women, a hypothesis that needs additional testing. A larger, more definitive British trial is ongoing. The Italian trial in previously hysterectomized women with an average breast cancer risk was confounded by a high noncompliance rate as well as by the concomitant use of estrogen replacement therapy in some patients.¹²¹

Although many questions remain to be answered through longer follow-up and by additional study, it seems reasonable to consider tamoxifen for breast cancer prevention in women with the characteristics of those entered onto the NSABP trial or to use raloxifene in postmenopausal women with osteoporosis who may also be concerned about their breast cancer risk. These two drugs are now being directly compared in the NSABP P-2 prevention trial. A review of the indications for SERMs in breast cancer prevention can be found in the American Society of Clinical Oncology’s technology assessment position paper.¹²²

Treatment of Breast Cancer
Metastatic disease. Metastatic breast cancer is not curable with today’s treatment, and the clinical goal is to provide the best quality of life and to control the disease for as long as possible. Because of the lack of side effects, endocrine therapy is the primary treatment approach for patients with ER- and/or progesterone receptor (PR)–positive tumors who do not have immediately life-threatening disease. Two SERMs, tamoxifen and toremifene, are important alternative primary therapies for such patients, and they are the only SERMs now approved for this indication. Overall, approximately one third of women with metastatic breast cancer who are treated with either of these drugs have objective tumor regression for an average of 12 to 18 months. Another 20% have prolonged stable disease lasting for at least 6 months.^4,123-126 Both pre- and postmenopausal women benefit. In a large prospective trial, patients who benefited most were postmenopausal and had tumors strongly positive for both ER and PR.¹²³ Patients with ER-positive, PR-negative tumors have a lesser but still significant response. Patients with truly ER-negative tumors uncommonly benefit from endocrine therapy. Patients with metastatic disease who relapsed 6 months or more after the discontinuation of adjuvant therapy with tamoxifen may respond to a rechallenge with tamoxifen or toremifene.¹²⁶ Although recent studies suggest that aromatase inhibitors may be equivalent to or even somewhat superior to tamoxifen in postmenopausal women, these SERMs are still reasonable choices for front-line therapy for metastatic disease.¹²⁷ Tamoxifen is also effective treatment in men with metastatic breast cancer whose tumors express ER.¹²⁸

All patients with metastatic breast cancer eventually develop resistance to treatment. Mechanisms for this resistance have not been fully explained but are probably multifactorial.³ Loss of ER from the tumor is not common but could explain resistance in some patients. Variant forms of ER or cross-talk with growth factor signaling pathways may also contribute. Disease progresses during tamoxifen therapy in some patients because the drug begins to stimulate tumor growth.^68,129 This explains the withdrawal response that occurs in some patients when tamoxifen is stopped. Withdrawal responses are hard to quantify given the need to watch patients for 2 months or longer after the drug is stopped because of tamoxifen’s prolonged half-life, but they have been documented by many studies and are in the range of 20% to 25% if patients with stable disease for longer than 6 months are included.¹³⁰ Tamoxifen-stimulated growth may also explain the lack of response to second-line ovarian ablation in premenopausal women if tamoxifen is not discontinued at the time of tumor progression.¹²⁹ The tamoxifen flare in which clinical evidence of progressive disease in the form of bone pain, hypercalcemia, or growth of visible tumor nodules occurs within the first week or two after starting treatment may also result from its estrogenic qualities. It seems reasonable to hypothesize that the intrinsic estrogen agonist activity of tamoxifen and other similar SERMs may contribute to the development of tamoxifen-stimulated growth as a form of acquired clinical drug resistance, and furthermore, that SERMs demonstrating a more complete antiestrogenic profile may be active in tamoxifen-resistant patients. In any event, it is prudent to discontinue tamoxifen or toremifene when evidence of tumor progression is clear.

Early results indicate that several newer SERMs have activity in patients with metastatic disease. Whether they are superior to currently available SERMs or other endocrine therapies remains to be defined. A phase II trial of raloxifene using doses significantly higher than those used for osteoporosis showed activity in postmenopausal women with advanced breast cancer not known to be resistant to tamoxifen.¹³¹ Little activity was seen in an earlier study in patients with tamoxifen-resistant tumors.¹³² A phase I study of SERM 3, an analog of raloxifene, showed modest activity in a group of patients with advanced treatment-refractory breast cancer, but ongoing phase II and III studies should provide more important information on its activity in breast cancer.¹³³

Phase I and phase II studies of idoxifene have also been completed.^80,134 The drug has clinical activity even in some patients with acquired tamoxifen resistance, but its uterine toxicity may limit development. EM-800 was also active in a group of postmenopausal women with metastatic breast cancer, many of whom had demonstrated acquired tamoxifen resistance.⁹¹ Phase III trials of these agents are in progress.

As described earlier, ICI182,780 (Faslodex) has a different mechanism of action than other SERMs, and preclinical data suggest that it might be more potent than tamoxifen and effective in patients with documented tamoxifen resistance.⁶⁸ A completed phase I-II study of this drug in 19 postmenopausal women with tamoxifen-resistant tumors demonstrated significant benefit in more than 60% of patients.^135,136 The results of phase III trials comparing ICI182,780 with anastrozole (Arimidex; AstraZeneca Pharmaceuticals) should be available in the coming months. A phase III trial comparing tamoxifen with ICI182,780 is still accruing patients. Thus within the next year, we should know whether this or other SERMs have a defined role in metastatic breast cancer, results that may lead to their incorporation into studies of adjuvant therapy.

Adjuvant therapy of breast cancer. Tamoxifen is currently the only SERM approved for use in the adjuvant treatment of breast cancer. Nearly 40,000 women worldwide have participated in randomized trials of adjuvant tamoxifen, some with more than 20 years of follow-up, and definitive conclusions can be drawn.^3,137 Tamoxifen adjuvant therapy is effective in both pre- and postmenopausal patients. Recurrence and death are substantially reduced, but only in patients whose tumors express ER and/or PR. With the data available now, a 5-year treatment duration is recommended. The two largest trials evaluating treatment durations greater than 5 years reported inferior risks for recurrence with longer treatment duration; the results in one trial reached statistical significance.^138,139 Whether these results are real, and whether they can be explained by tamoxifen-stimulated growth developing after an initial period of growth suppression due to increasing agonist activity, remains to be clarified. Meanwhile, durations of tamoxifen treatment greater than 5 years or the use of other SERMs with intrinsic agonist activity in a patient who has completed tamoxifen adjuvant therapy should be avoided outside of controlled clinical trials.

The reduction in the annual odds of recurrence and death associated with 5 years of tamoxifen in women with ER-positive tumors is listed in Table 1. All age groups—including very young patients—benefit, with an approximately 50% reduction in the odds of recurrence.¹³⁷ Even greater benefits are observed in the subset of women with tumors expressing high levels of ER. Additional benefits in recurrence and survival are achieved in some patients who are treated with combinations of chemotherapy and tamoxifen.^3,135

Tamoxifen in ductal carcinoma-in-situ. Standard treatment for ductal carcinoma-in-situ is evolving. Many patients can be treated safely by lumpectomy plus breast irradiation, whereas mastectomy or lumpectomy alone may be optimal in others depending on tumor size, breast size, margin status, age, and other factors. Tamoxifen may also have a role in some patients. Several studies are evaluating tamoxifen in this setting and one completed study, NSABP B-24, showed that tamoxifen, when added to lumpectomy and radiation therapy, reduced the incidence of ipsilateral and contralateral invasive breast cancer.¹⁴⁵ The role of tamoxifen in patients treated with mastectomy is less clear, although one might consider such patients in the same manner as women with high breast cancer risk being evaluated for tamoxifen prevention.

In conclusion, although SERMs were initially developed as antiestrogens for the treatment of breast cancer, their unusual properties have led to their use in the prevention and treatment of other diseases as well. New information on the mechanisms of ER function provide clues that may explain the unusual properties of these drugs and provide the basis for the development of other "ideal" SERMs for clinical practice. Perhaps in the near future, SERMs will be used as agents for hormone replacement therapy useful for most women. Drugs that reduce menopausal symptoms, preserve bone, reduce cholesterol, and yet block the effects of estrogens on epithelial cells in the breast and uterus are theoretically possible to develop, and, when available, will provide a dramatic new paradigm for maintaining the health of women.

ACKNOWLEDGMENTS

Supported in part by grant nos. P01 CA 30195 and P50 CA 58183 from the National Cancer Institute and a Postdoctoral Fellowship in Breast Cancer grant from the Susan G. Komen Breast Cancer Foundation.

NOTES

C.K.O. has been a consultant for AstraZeneca (Wilmington, DE), Novartis (East Hanover, NJ), Eli Lilly (Indianapolis, IN), and Schering-Plough (Kennilworth, NJ).

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