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Selective Estrogen-Receptor Modulators for Primary Prevention of Breast Cancer

Carol J. Fabian, Bruce F. Kimler

From the Departments of Internal Medicine and Radiation Oncology, University of Kansas Medical Center, Kansas City, KS

Address reprint requests to Carol J. Fabian, MD, University of Kansas Medical Center, 3901 Rainbow Blvd, Kansas City, KS 66160; e-mail: cfabian{at}kumc.edu.

	INTRODUCTION

TOP INTRODUCTION BIOLOGY BEHIND THE MULTIFACETED... TAMOXIFEN Authors' Disclosures of... REFERENCES

 
Selective estrogen receptor modulators (SERMs) impact a variety of biologic processes regulated by activated estrogen receptor (ER). Depending on the target tissue, physiologic conditions, and their structure, SERMs may exhibit either estrogen antagonist or estrogen agonist effects. SERMs have a long history in the treatment of breast cancer, based on estrogen antagonist activity in ER-positive breast cancer cells. Beginning with tamoxifen, SERMs have moved to the prevention arena, where their partial estrogen effects may provide other organ benefits, particularly for postmenopausal women. It is because of these partial estrogen agonist effects on organs such as the bone, vagina, CNS, and cardiovascular system that SERMs are likely to remain preferable to pure antiestrogens as primary breast cancer prevention agents in the context of total women's health.¹ The ideal SERM should function as an antiestrogen in the breast and uterus and a partial estrogen agonist in skeletal, cardiovascular, CNS, gastrointestinal tract, and vagina. Further, this ideal SERM should be devoid of procoagulant effects and should not be associated with an increase in perimenopausal symptoms. Although several generations of SERMs have been developed, the ideal SERM for prevention remains elusive. We will review progress to date for SERMs as single agents and potential for combination therapy in the future.

	BIOLOGY BEHIND THE MULTIFACETED ACTION OF SERMs

TOP INTRODUCTION BIOLOGY BEHIND THE MULTIFACETED... TAMOXIFEN Authors' Disclosures of... REFERENCES

 
Depending on their structure and context, SERMs differentially modify expression of multiple genes whose transcription is regulated by activated ER.^2,3 A SERM's gene- and tissue-specific estrogen agonist or antagonist activity is determined primarily by whether coactivators or corepressors are preferentially bound to the SERM/ER nuclear receptor transcription complex.^4-7

The ability of a SERM to block coactivator recruitment into the ER-ligand binding pocket is dependent on its three-dimensional structure.⁸ When estrogen binds to ER, a conformational change occurs such that helix 12 is repositioned, exposing multiple coactivator sites. Subsequent to coactivator binding, full estrogen agonist transcription is initiated at both ER-activating function domains (AF-1 and -2).^9,10 SERMs block full closure of helix 12, reducing the number of coactivation binding sites so that one or both activation domains are inhibited.^9,10 SERMs such as tamoxifen, which block activation of AF-2 but not AF-1, might be expected to have partial estrogen agonist activity for a wider spectrum of target genes/tissue than SERMs such as raloxifene, arzoxifene, or acolbifene, which also block AF-1 activation.^9,11 Frasor et al,³ using DNA microarrays, have recently shown that tamoxifen indeed appears to have estrogen agonist effects on more genes than does raloxifene.

Estrogen agonist or antagonist effects are also dependent on the organ-specific type and amount of ER (Table 1) available for ligand binding as well as the type of target gene promoter, estrogen response element (ERE), or AP-1 (Table 2).^15-36 For example, estrogen, when combined with ERß and acting at genes with an AP-1 type promoter, inhibits rather than facilitates gene transcription.^16,17 Tamoxifen, when combined with ERß and a target gene promoter that uses an ERE, will function as an estrogen antagonist similar to the situation for ER.^13,15,18 However, when tamoxifen is combined with ERß and the target gene has an AP-1 type promoter, tamoxifen becomes a partial estrogen agonist.^15,18,19 Generally, the amount of ER increases and ERß decreases during the promotion and progression phases of breast cancer^20,21; however, breast cancers which have a substantial amount of ERß may exhibit tamoxifen resistance.^22,23 Whether the same principles apply in the prevention setting is unclear.

In summary, the organ-specific response to an individual SERM depends not only on the drug's structure and pharmacologic properties, but also upon the target tissue's ER:ERß ratio, whether target genes are activated by ERE or an AP-1 transcription complex, and the level of MAP kinase pathway activation.^32-36

	TAMOXIFEN

TOP INTRODUCTION BIOLOGY BEHIND THE MULTIFACETED... TAMOXIFEN Authors' Disclosures of... REFERENCES

 
Tamoxifen is a SERM with predominate estrogen antagonist effects in the breast, partial estrogen agonist activity in the bone, cardiovascular system and CNS, and predominant estrogen agonist effects in the uterus, liver, and vagina.^37-45 The estrogen agonist effects of tamoxifen on the uterus and liver, which result in an increased incidence of uterine cancer and thromboembolic phenomena, keep it from being considered an ideal SERM. Nonetheless, tamoxifen is the only US Food and Drug Administration–approved drug in the United States for breast cancer risk reduction.⁴⁶

In an overview of four completed primary prevention trials in which tamoxifen was to be given for 5 or more years (National Surgical Adjuvant Breast and Bowel Project [NSABP] P-1,⁴⁷ International Breast Cancer Intervention Study-1,⁴⁸ Royal Marsden,⁴⁹ and Italian⁵⁰), women randomly assigned to tamoxifen had a 36% reduction in the incidence of ductal carcinoma-in-situ (DCIS) and a 46% reduction in invasive breast cancer.⁵¹ ER-positive tumors were reduced by 48% in the overview, but no significant reduction was observed in the incidence of ER-negative tumors.⁵¹ The excess of serious side effects in women taking tamoxifen (116 events) almost offset the reduction in cancer incidence (176 cases).⁵¹ The incidence of endometrial cancer was increased 2.4-fold, deep venous thrombosis 1.9-fold, and cerebral vascular accident 1.5-fold.⁵¹ The increase in uterine epithelial hyperplasia, as well as uterine cancer, may be the result of tamoxifen-induced increases in the steroid receptor coactivator (SRC) and increases in uterine IGF-1 and IGFR activation.⁵² In the overview analysis, a nonsignificant increase in death from any cause was observed in women randomly assigned to tamoxifen.⁵¹ Despite the favorable effect of tamoxifen on several risk biomarkers for coronary artery disease, there was no significant reduction in coronary artery disease–related events.⁵¹ However, another meta-analysis, which included 25 breast cancer treatment trials in addition to the four prevention trials, indicated that tamoxifen significantly decreased myocardial infarction deaths.⁵³ Because of major side effects and incomplete efficacy, it has been estimated that only one in four women meeting the National Surgical Adjuvant Breast and Bowel Project P-1 minimum eligibility requirement (5-year Gail risk 1.67%) for tamoxifen would have a favorable benefit:risk ratio. Fear of both major and minor side effects and uncertainty over who will benefit often make tamoxifen unattractive to healthy women.^47,48

Despite endorsement by the American Society of Clinical Oncology and the US Food and Drug Administration, only 5% to 30% of eligible high-risk women agree to take tamoxifen for primary prevention following a recommendation by their health care provider.^54-56 By selecting women with high short-term risk, and those most likely to respond to SERMs, the benefit:risk ratio can be maximized.^57,58 Women with a deleterious mutation in BRCA1 or BRCA2, or with a prior diagnosis of DCIS, lobular carcinoma-in-situ (LCIS), and/or atypical hyperplasia, have 5% 5-year risk of breast cancer.^59-62 This level of risk is generally associated with clinical benefit from prevention tamoxifen regardless of a woman's age or whether or not her uterus is intact.⁵⁷ A limited number of studies in BRCA1/2 mutation carriers appear to indicate benefit for tamoxifen despite the observation that the majority of BRCA1-associated cancers are ER-negative.^63-65 Study results have been mixed, however. Randomization to tamoxifen was correlated with a reduction in breast cancers in BRCA2-, but not BRCA1-associated cancers in the National Surgival Adjuvant Breast and Bowel Project (NSABP) P-1 trial.⁶⁶ Atypical hyperplasia, LCIS, and the majority of non–high-grade DCIS lesions are characterized by a high proportion of ER positive cells,²⁰ which in turn predict favorable outcome to tamoxifen in adjuvant trials.^67-69 The finding of any of these precancerous lesions would then most likely result in a benefit:risk ratio which would favor tamoxifen use.⁴⁷ Hershman et al⁷⁰ have determined that 5 years of tamoxifen is likely to be cost effective for women with prior atypical ductal hyperplasia, LCIS, and/or 5-year Gail risk >5%, particularly if the drug is started before the age of 60 years. However, the majority of women at increased risk on the basis of family history have never had a diagnostic biopsy.

The high incidence and multicentric distribution of atypical ductal and lobular hyperplasia in prophylactic mastectomy specimens from women of hereditary breast cancer families suggests that atypical hyperplasia should be amenable to discovery by random tissue sampling in high-risk women.⁷¹ Approximately 20% of women with a median age of 44 years at increased risk by virtue of a first-degree relative or multiple second-degree relatives with breast cancer, prior precancerous biopsy, or contralateral breast cancer can be expected to exhibit atypia in their cytology specimens when cells are harvested by random periareolar fine needle aspiration.⁷² Similar figures are reported for ductal lavage⁷³. Detection of atypia by nipple aspirate fluid harvest or random periareolar fine needle aspiration has been associated in prospective studies with a substantial increase in relative risk in average- and high-risk populations, respectively.^72,74 In addition to breast tissue morphology, other biomarkers have also been associated with increase in risk for breast cancer including serum IGF-1 and its binding protein 3 (IGFBP-3) in premenopausal women; serum bioavailable estradiol, free testosterone and sex hormone binding globulin (SHBG) in postmenopausal women; and mammographic breast density in both pre- and postmenopausal women.^59,75-77 It is unknown at present whether detection of atypia or any other risk biomarker will increase the likelihood that eligible women will take prevention tamoxifen following a recommendation for its use. However, Vogel et al⁵⁶ reported that eligible women with a prior diagnostic biopsy exhibiting atypical hyperplasia were more likely than women without atypical hyperplasia to enter the NSABP P-1 or the Study of Tamoxifen and Raloxifene (STAR) trials.

Tamoxifen use has been observed to have a favorable effect on many risk markers including serum IGF-1, IGFBP-3, SHBG, mammographic breast density, and subsequent prevalence of breast biopsies with benign breast disease with and without atypical hyperplasia.^78-80 Consequently, modulation of many of these risk biomarkers is used in early clinical trials to evaluate promising new prevention strategies.^81,82 It is not yet clear that favorable risk biomarker modulation accurately predicts prevention of breast cancer for an individual woman.

In summary, the most serious adverse events attributable to tamoxifen are the increased risk of uterine cancer and thromboembolic phenomena. Furthermore, up to one half of ER-positive breast cancers and the majority of ER-negative cancers are not prevented with tamoxifen.^47,48,51,83 Despite these drawbacks, the use of tamoxifen has been judged to be cost effective as a primary breast cancer prevention agent in women with prior atypical hyperplasia carcinoma-in-situ, and/or 5-year Gail risk >5%.

Current SERM prevention research aims to enhance the benefit:risk ratio observed with tamoxifen through: (1) use of low-dose tamoxifen to reduce side effects; (2) development of new SERMs with a more favorable estrogen antagonist/agonist profile; (3) development of combination therapy including SERMs for prevention of ER-positive tamoxifen resistant and ER-negative breast cancer.

Low-Dose Tamoxifen
Reduction in the proliferation marker Ki67 after several weeks of tamoxifen has been associated with subsequent clinical response in cancer treatment trials.⁸⁴ Consequently, reduction in proliferation between baseline core biopsy and re-excision 4 to 6 weeks later in women with DCIS or a small invasive cancer is often used to define a potentially effective dose range for a prevention agent in the so-called presurgical clinical model.^81,82 Using this model, Decensi et al⁸⁵ recently reported the results of a trial in which women were randomly assigned to one of two low doses (1 or 5 mg) or a standard dose (20 mg) of tamoxifen administered daily for 4 weeks between biopsy and definitive surgical resection of their breast cancer. Women with ER-negative and ER-positive tumors not undergoing preoperative treatment served as nonrandomized controls. Despite a >10-fold difference in tamoxifen serum concentration, there were no significant differences in change in breast tissue Ki-67 (Fig 1)⁸⁵ in the two low-dose (1 or 5 mg/d) versus standard dose (20 mg/d) arms.^85,86 However, there was a significant difference in change in Ki-67 between the tamoxifen treatment groups (relative median decrease of 15%) and controls (relative median increase of 13%). A dose response relationship was observed for levels of several blood protein biomarkers reflecting tamoxifen's estrogen agonist effects on the liver. These include increased levels of SHBG and decreased levels of C-reactive protein, fibrogen, antithrombin III, and low-density lipoprotein (LDL) cholesterol. These observations added further support to the authors' hypothesis that low-dose tamoxifen might be an effective estrogen antagonist in the breast, but the reduced dosage would result in a less potent estrogen agonist effect on the liver and uterus and fewer serious side effects. In support of this hypothesis, de Lima et al⁸⁷ observed that tamoxifen in doses of 5, 10, or 20 mg was able to significantly suppress proliferation in fibroadenomas in premenopausal women.

View larger version (17K): [in this window] [in a new window]   Fig 1. Dose-dependent changes in serum levels of sex hormone binding globulin (SHBG) and insulin-like growth factor-1 (IGF-1) versus dose-independent changes in Ki-67 expression observed in breast cancer patients treated with low doses of tamoxifen.85

View larger version (19K): [in this window] [in a new window]   Fig 2. Chemical structure of four generations of selective estrogen receptor modulators (SERMs) representative of the four generations of SERM development.

A secondary analysis of the MORE trial also indicated a significant reduction in breast cancer incidence for women randomly assigned to raloxifene.^96,97 Raloxifene reduced the incidence of all breast cancers by 62%, invasive breast cancer by 72%, and invasive ER-positive breast cancer by 84%. Similar to findings for tamoxifen in the NSABP P-1 trial, there was no reduction of ER-negative tumors.⁹⁷ Unlike tamoxifen in the NSABP P-1 trial, no reduction in DCIS was observed for women randomly assigned to raloxifene. Importantly, randomization to raloxifene was not associated with an excess of uterine cancers or episodes of postmenopausal uterine bleeding.⁹⁷

Raloxifene, like tamoxifen, is associated with an increase in procoagulant activity, probably as a result of its estrogen agonist–like effects on the liver.⁹⁸ Women randomly assigned to raloxifene in the MORE trial had a three-fold excess of thromboembolic events compared with the placebo group, similar to that observed for women randomly assigned to tamoxifen in the NSABP P-1 trial.⁹⁶ Raloxifene's effects on the neuorendocrine system are also similar to tamoxifen,⁹⁹ leading to elevations in follicle stimulating hormone (FSH) and estradiol in premenopausal women¹⁰⁰ and reductions in FSH and lutenizing hormone (LH) in postmenopausal women¹⁰¹ and an increase in hot flashes.^102,103 In an integrated analysis of several trials, hot flashes were reported in 28% of women randomly assigned to raloxifene versus 21% of those randomly assigned to placebo.¹⁰⁴ Similar to tamoxifen,^105,106 prolonged administration of raloxifene in elderly women does not appear to impair cognition.¹⁰⁷

Raloxifene's partial estrogen agonist effects on the liver results in reduction of total and LDL cholesterol, C-reactive protein, and homocysteine.^102,108,109 However, little change in triglycerides or high-density lipoproteins has been observed.¹⁰⁸ With the exception of triglycerides, the effects on lipids are similar to those observed for tamoxifen.^39,110-114 Raloxifene administration is associated with an increase in leptin in pre- and postmenopausal women,^115,116 similar to that observed for tamoxifen.^117,118 Increases in leptin may be associated with increased risk of breast and colorectal cancer.^119-122 In the MORE trial, women at increased risk for coronary disease who were randomly assigned to raloxifene had a 40% reduction in cardiovascular related events (P = .03) but there was no reduction in cardiovascular events for the group as a whole.¹²³

Similar to tamoxifen, raloxifene use has been observed to have a favorable effect on serum hormones and growth factors that have been identified as risk markers. Specifically, raloxifene causes an increase in SHBG^124,125 and IGFBP-3^115,126 in pre- and postmenopausal women, and a decrease in IGF-1 in postmenopausal women.^126,127

The same effects on bone mineral density were observed with raloxifene as had been reported for tamoxifen; density was reduced in premenopausal women and increased in postmenopausal women.^37,108,124

Investigators and study participants in the MORE trial were given the option of further participation in a 4-year extension trial of raloxifene versus placebo. The extension was entitled the "Continuing Outcomes Relevant to Evista (CORE)" trial. Breast cancer incidence was a primary end point in CORE. Since there were no obvious differences between the 60-mg and the 120-mg dose levels in MORE, those women randomly assigned to either dose in MORE were given 60 mg of raloxifene per day, and those originally randomly assigned to placebo were continued on placebo. Approximately two thirds of the women who participated in MORE contributed data to CORE. For women participating in the 4 years of CORE, total breast cancers were reduced by 50%, invasive breast cancer was reduced by 59%, and ER-positive invasive breast cancer was reduced by 66%. Similar to MORE, there was no reduction in DCIS or ER-negative invasive breast cancer for women randomly assigned to raloxifene.¹²⁸ There was no significant difference in overall or breast cancer–specific survival. The CORE study is important, as reduction in breast cancers in the raloxifene group continued over the entire 8-year period (an overall relative reduction of 58% in MORE plus CORE). Whether this is a direct result of continuing raloxifene for another 4 years or a carry-over effect from the initial 4 years is difficult to determine. Participants in the MORE and CORE study were of relatively average risk for their age group. Approximately half the subjects had a 5-year Gail risk 1.67%. Thus, prolonged raloxifene administration appears safe and beneficial for average-risk, postmenopausal, osteopenic women for both osteoporosis and breast cancer prevention.

It is important to note that in the MORE trial women with detectable estradiol levels ( 2.7 pg/mL) had a higher cancer incidence rate than the group at large and were the only ones who appeared to derive significant benefit from raloxifene. Women with undetectable estradiol levels had a similar low risk with or without raloxifene administration.¹²⁹

Is raloxifene equivalent to tamoxifen for breast cancer prevention in high-risk, postmenopausal women, with perhaps fewer side effects? This question is being addressed in the National Cancer Institute–sponsored STAR trial in which postmenopausal women with a 5-year Gail risk of 1.67% and/or LCIS were randomly assigned to 5 years of tamoxifen or raloxifene.⁵⁶ If equivalence in breast cancer risk reduction with fewer serious adverse events is observed with raloxifene, it would likely become the new standard of care for primary breast cancer prevention in postmenopausal women. However, tamoxifen would remain the standard of care for premenopausal women.

The potential cardioprotective effects of raloxifene for postmenopausal women are being examined in the Raloxifene Use for the Heart (RUTH) trial in which 10,000 women were randomly assigned to 60 mg of raloxifene or placebo. The primary end point is the incidence of myocardial infarction, but the incidence of breast cancer and osteoporotic fractures will also be examined as secondary outcomes.¹³⁰

Raloxifene After Tamoxifen
Raloxifene does not appear effective for tamoxifen-resistant tumor cells, thus there would seem to be little rationale at present for treating women who have received 5 years of therapeutic or prevention tamoxifen with raloxifene for the purpose of breast cancer risk reduction.^91,92 Preclinical studies indicate that raloxifene may stimulate endometrial cancer cells previously exposed to tamoxifen, and it has been suggested that administration of raloxifene to women with an intact uterus previously exposed to 5 years of tamoxifen may further increase the risk of endometrial cancer over that observed with tamoxifen alone.⁹¹

Third- and Fourth-Generation SERMs
Arzoxifene is a third-generation SERM of the benzothiophene class (Fig 2), similar in structure to raloxifene, but modified to improve bioavailability and potency.¹³¹ Acolbifene is a fourth-generation SERM of the benzopyrans class. In preclinical models, both drugs are more potent than tamoxifen or raloxifene for inhibiting growth of tamoxifen sensitive tumors.^131-135 Both agents are able to suppress growth of tumor cells in some tamoxifen-resistant xenograft models. Arzoxifene is able to inhibit growth of tamoxifen resistant MCF-7 cells but not tamoxifen resistant T47-D cells; acolbifene suppresses growth in the tamoxifen resistant ZR75-1 cell line.^136-138 Unlike tamoxifen, acolbifene blocks the stimulatory effect of growth factors on the AF-1 activation site of ER; like tamoxifen, acolbifene also blocks activation of the AF-2 site.^139,140

Both arzoxifene and acolbifene have demonstrated activity in metastatic breast cancer. Arzoxifene in doses ranging from 10 to 200 mg/d produced a 19% clinical benefit rate in subjects previously treated with tamoxifen and chemotherapy in a phase I trial.¹⁴¹ Significant reductions in bone turnover markers, LDL and total cholesterol, FSH, and LH were seen beginning at the 10 mg/d dose.¹⁴¹

Response rates of 26% to 43% in women with tamoxifen-sensitive metastatic breast cancer have been reported for arzoxifene at 20 mg/d,^142,143 with a 10% objective response rate in tamoxifen-resistant patients.¹⁴² Acolbifene in doses of 20 to 40 mg has also undergone phase II testing in 43 women who had previously been exposed to tamoxifen, of whom 22 could be classified as tamoxifen sensitive and 21 as resistant. The overall objective response rate was 18% in tamoxifen-sensitive and 5% in tamoxifen-resistant patients.¹⁴⁴ Both drugs were well tolerated in phase I and II trials with no evidence of uterine hypertrophy. Hot flashes, muscle and bone pain, fatigue, asthenia, and nausea were the most frequent complaints.^141,142,144

Preclinical studies of both drugs in ovarectomized rats indicate favorable effects on cholesterol, bone mineral density, and uterine weight, supporting their use as potential prevention agents.^{131,134,145,146} Similar to the situation for raloxifene, preclinical models indicate that arzoxifene may stimulate growth of endometrial cancers arising after prolonged tamoxifen treatment.¹⁴⁷ An increase in uterine thickness was observed in five of 25 previously tamoxifen-exposed women at the 50-mg dose level.¹⁴² Both drugs may be more effective than raloxifene in preventing bone loss.^146,148 In preclinical models, acolbifene prevented diet and oopherectomy-induced insulin resistance,^149,150 which has been implicated as a risk factor for postmenopausal breast cancer.¹⁵¹

In the nitrosomethylurea model for breast cancer prevention, arzoxifene was a highly potent chemoprevention agent. It was superior to raloxifene and at least equal to tamoxifen.¹³² Acolbifene has been shown to be effective in preventing breast cancer in the dimethylbenz(a)anthracene rat model.¹³⁴

Arzoxifene has undergone preliminary clinical evaluation as a potential breast cancer chemoprevention drug utilizing the presurgical model.¹⁵² In both phase IA and phase IB trials, decreases in the molar ratio of serum IGF-1:IGFBP-3 and increases in serum SHBG were observed, consistent with favorable modulation of these two risk biomarkers.^75,76 Reduction in proliferation was noted in the phase IA dose-finding trial at 20 mg/d; however, a significant difference in reduction in proliferation between placebo versus arzoxifene was not observed in the IB study, perhaps due to the confounding effects of the profound reduction in proliferation associated with stopping hormone replacement therapy between biopsy and re-excision for a sizable proportion of both the arzoxifene and placebo groups.¹⁵² A phase II, placebo-controlled, biomarker modulation study has recently been completed in women at high risk for development of breast cancer. The primary end point is change in breast tissue morphology as assessed by random periareolar fine needle aspiration. A randomized phase III trial of arzoxifene versus placebo has been initiated in postmenopausal women 60 years of age with prevention of vertebral fractures and breast cancer as two primary end points. Acolbifene has not yet entered clinical prevention trial testing, but reduction in the amount of ER in rat mammary gland and uterus, as well as in human ZR-75-1 and MCF cells, has been observed in preclinical studies.^153-155

In summary, both arzoxifene and acolbifene are promising new SERMS for prevention in that they both have some activity in tamoxifen-resistant metastatic breast cancer. Moreover, preclinical studies predict greater preservation of bone mineral density but fewer unfavorable effects on the uterus than with tamoxifen or raloxifene in tamoxifen-naïve women.

Circumventing Resistance to SERMs With Combination Therapy
The partial estrogen agonist profile of tamoxifen and other SERMs may promote the health of other several estrogen responsive organs, but it also may enable the development of resistance via upregulation of the MAP kinase and AKT pathways. Upregulation of these pathways in turn results in an increase in transcriptional activity of coactivators and ER.^{23,136,156-159} Lack of ER expression, as well as reduction in the level of corepressors, are other potential mechanisms of tamoxifen resistance.³⁵ Combination therapy employing a SERM plus another agent which might reduce MAP kinase and PI-3 kinase signaling would be predicted to reduce the development of tamoxifen-resistant breast cancer.^160,161

Approximately 30% of breast cancers are ER-negative, although 40% of ER-negative tumors are ERß-positive.⁹ ER expression may be lost through hypermethylation of the ER promoter, which could theoretically be overcome by combining a histone deacetylase inhibitor with a demethylating agent.^162-164 Preclinical work has indicated that ER can be re-expressed,^165,166 but currently available demethylating agents or histone deacetylase inhibitors may have too many side effects for use in the prevention setting.^167-169 Phytoestrogens such as soy may have histone deacetylase inhibitory properties.¹⁷⁰ Furthermore, soy products contain genestein, which has been observed to inhibit HER-2/neu activation and promote apoptosis in breast cancer cell lines.¹⁷¹ An in vitro study by Tanos et al¹⁷² indicated synergistic effects of genestein and tamoxifen on human dysplastic and malignant epithelial cells. On the other hand, genestein blocked the inhibitory effects of tamoxifen on growth in an ovarectomized xenograft model.¹⁷³ The different results of studies involving genestein and other phytoestrogens may in some part be due to differences in dose as well as hormonal environment and timing of exposure.^174-176 The estrogen antagonist effects of phytoestrogens may be mediated via their preferential binding of ERß, which blunts the full agonist effects of ER.^177-179 The lignan component of flaxseed, another phytoestrogen, may inhibit the PI-3 kinase pathway via inhibition of activation of vascular endothelial growth factor receptor and IGFR.^180-182 Reduced proliferation has been observed with flaxseed in an MDA-MB-435 mouse mammary tumor model.¹⁸⁰ Clinical prevention trials with various phytoestrogens are underway. Pilot trials of combination therapy with phytoestrogens and SERMs should be considered in the future.

Gefitinib is an EGFR tyrosine kinase inhibitor that exhibited only minimal activity in treatment of refractory metastatic breast cancer as a single agent.¹⁸³ However, it has shown activity in the ER-negative HER-2/neu transgenic mouse model¹⁸⁴ and was able to restore tamoxifen sensitivity in the MCF-7/HER-2 overexpressing mouse xenograft model.¹⁸⁵ In a placebo-controlled trial using the presurgical model, 14 days of gefitinib significantly reduced proliferation in progesterone receptor–positive DCIS, as well as normal adjacent breast tissue.¹⁸⁶ Although gefitinib will undoubtedly be tested in combination with SERMs and other endocrine agents in women with established breast cancer, concerns regarding the side effect profile (rash, diarrhea, nausea, and lung toxicity) may limit enthusiasm for evaluation in the prevention setting.¹⁸⁷

Breast cancer development is often associated with loss of expression of the tumor suppressor gene retinoic acid receptor beta 2 (RARß2).¹⁸⁸ Retinoic acid derivatives can induce reexpression of RARß and inhibit signaling mediated through the MAP kinase pathway.¹⁸⁸ A variety of retinoic acid analogues have been found to prevent both ER-positive and ER-negative breast cancer.^189,190 The retinoic acid analogue fenretinide appeared to have modest activity in preventing contralateral breast cancer in premenopausal women in an Italian adjuvant trial.¹⁹¹ A variety of preclinical studies have suggested enhancement of apoptosis when SERMs are combined with various retinoic acid derivatives.^192-197 Decensi et al¹⁹⁸ recently reported that fenretinide did not appear to improve risk biomarker modulation over that observed with low-dose tamoxifen alone. The type of SERM and retinoic acid analog may be critical to the success of combination treatment. Suh et al¹⁹² have found that the combination of arzoxifene and a retinoic acid X receptor agonist LG100268 (a targretin analog) was the most powerful inducer of apoptosis of all SERMs and retinoids tested by that laboratory to date. The mechanism appeared to be a profound induction of TGFß-3.¹⁹⁴ Synergism may also allow lower doses of retinoic acid analogs to be used, which would reduce side effects.^160,199 Low doses of these two agents were also effective in preventing breast cancer in both ER-positive and ER-negative preclinical models.^192,194 The increase in apoptosis suggests the combination might be effective in restoring tissue homeostasis following short-term use rather than requiring long-term chronic exposure over years.¹⁶¹

The Future
Another direction in SERM prevention research includes development of highly specific ERß agonists in the hope of avoiding estrogen agonist effects on breast and uterus mediated via ER, but providing estrogen agonist effects on other organs. Unfortunately, one of the early compounds, ERß-041, was not effective in preventing bone loss or vasomotor instability in ovarectomized animal models, although it did not manifest any estrogenic response in the breast or uterus.²⁰⁰ Development of compounds that do not cross the blood brain barrier might, to some degree, avoid the hot flashes encountered to date with all SERMs. Development of transdermal formulations would avoid a first-pass effect through the liver and theoretically might reduce procoagulant effects observed with SERMs. The development of predictive biomarkers such as those for tamoxifen in the adjuvant setting²⁰¹ might be possible from benign precancerous tissue in the prevention setting as well.

With the continuing development of a strong biologic rationale for selective modulation of specific estrogen receptors, it would appear that there is much promise in the use of SERMs for primary prevention of breast cancer and for women's health.

	Authors' Disclosures of Potential Conflicts of Interest

TOP INTRODUCTION BIOLOGY BEHIND THE MULTIFACETED... TAMOXIFEN Authors' Disclosures of... REFERENCES

 
The following authors or their immediate family members have indicated a financial interest. No conflict exists for drugs or devices used in a study if they are not being evaluated as part of the investigation. Consultant: Carol J. Fabian, Pfizer, Amgen, AstraZeneca, Sankyo. Honoraria: Carol J. Fabian, Pfizer, Amgen, AstraZeneca, Sankyo. Research Funding: Carol J. Fabian, Pfizer, Novartis; Bruce F. Kimler, Pfizer, Novartis. For a detailed description of these categories, or for more information about ASCO's conflict of interest policy, please refer to the Author Disclosure Declaration and the Disclosures of Potential Conflicts of Interest section of Information for Contributors found in the front of every issue.

	NOTES

 
Authors' disclosures of potential conflicts of interest are found at the end of this article.

	REFERENCES

TOP INTRODUCTION BIOLOGY BEHIND THE MULTIFACETED... TAMOXIFEN Authors' Disclosures of... REFERENCES

 
1. Goldstein SR: Selective estrogen receptor modulators: A new category of therapeutic agents for extending the health of postmenopausal women. Am J Obstet Gynecol 179:1479-1484, 1998[CrossRef][Medline]

2. Levin ER: Cell localization, physiology, and nongenomic actions of estrogen receptors. J Appl Physiol 91:1860-1867, 2001[Abstract/Free Full Text]

3. Frasor J, Stossi F, Danes JM, et al: Selective estrogen receptor modulators: Discrimination of agonistic versus antagonistic activities by gene expression profiling in breast cancer cells. Cancer Res 64:1522-1533, 2004[Abstract/Free Full Text]

4. Horwitz KB, Jackson TA, Bain DL, et al: Nuclear receptor coactivators and corepressors. Mol Endocrinol 10:1167-1177, 1996[Abstract]

5. MacGregor JI, Jordan VC: Basic guide to the mechanisms of antiestrogen action. Pharmacol Rev 50:151-196, 1998[Abstract/Free Full Text]

6. Fuqua SA, Russo J, Shackney SE, et al: Estrogen, estrogen receptors and selective estrogen receptor modulators in human breast cancer. J Womens Cancer 2:21-32, 2000

7. Brzozowski AM, Pike AC, Dauter Z, et al: Molecular basis of agonism and antagonism in the oestrogen receptor. Nature 389:753-758, 1997[CrossRef][Medline]

8. Jensen EV, Jordan VC: The estrogen receptor: A model for molecular medicine. Clin Cancer Res 9:1980-1989, 2003[Abstract/Free Full Text]

9. Howell A, Howell SJ, Clarke R, et al: Where do selective estrogen receptor modulators (SERMs) and aromatase inhibitors (AIs) now fit into breast cancer treatment algorithms? J Steroid Biochem Mol Biol 79:227-237, 2001[CrossRef][Medline]

10. Bentrem D, Fox JE, Pearce ST, et al: Distinct molecular conformations of the estrogen receptor alpha complex exploited by environmental estrogens. Cancer Res 63:7490-7496, 2003[Abstract/Free Full Text]

11. Katzenellenbogen JA, O'Malley BW, Katzenellenbogen BS: Tripartite steroid hormone receptor pharmacology: Interaction with multiple effector sites as a basis for the cell- and promoter-specific action of these hormones. Mol Endocrinol 10:119-131, 1996[CrossRef][Medline]

12. Bord S, Horner A, Beavan S, et al: Estrogen receptors alpha and beta are differentially expressed in developing human bone. J Clin Endocrinol Metab 86:2309-2314, 2001[Abstract/Free Full Text]

13. Kuiper GG, Carlsson B, Grandien K, et al: Comparison of the ligand binding specificity and transcript tissue distribution of estrogen receptors alpha and beta. Endocrinology 138:863-870, 1997[Abstract/Free Full Text]

14. Gustafsson JA: Estrogen receptor beta–A new dimension in estrogen mechanism of action. J Endocrinol 163:379-383, 1999[CrossRef][Medline]

15. Paech K, Webb P, Kuiper GG, et al: Differential ligand activation of estrogen receptors ERalpha and ERbeta at AP1 sites. Science 277:1508-1510, 1997[Abstract/Free Full Text]

16. Pettersson K, Delaunay F, Gustafsson JA: Estrogen receptor beta acts as a dominant regulator of estrogen signaling. Oncogene 19:4970-4978, 2000[CrossRef][Medline]

17. Hall JM, McDonnell DP: The estrogen receptor beta-isoform (ERbeta) of the human estrogen receptor modulates ERalpha transcriptional activity and is a key regulator of the cellular response to estrogens and antiestrogens. Endocrinology 140:5566-5578, 1999[Abstract/Free Full Text]

18. Shiau AK, Barstad D, Loria PM, et al: The structural basis of estrogen receptor/coactivator recognition and the antagonism of this interaction by tamoxifen. Cell 95:927-937, 1998[CrossRef][Medline]

19. Montano MM, Muller V, Trobaugh A, et al: The carboxy-terminal F domain of the human estrogen receptor: Role in the transcriptional activity of the receptor and the effectiveness of antiestrogens as estrogen antagonists. Mol Endocrinol 9:814-825, 1995[Abstract]

20. Allred DC, Mohsin SK, Fuqua SA: Histological and biological evolution of human premalignant breast disease. Endocr Relat Cancer 8:47-61, 2001[Abstract]

21. Shaaban AM, O'Neill PA, Davies MPA, et al: Declining estrogen receptor-ß expression defines malignant progression of human breast neoplasia. Am J Surg Pathol 27:1502-1512, 2003[Medline]

22. Speirs V, Malone C, Walton DS, et al: Increased expression of estrogen receptor beta mRNA in tamoxifen-resistant breast cancer patients. Cancer Res 59:5421-5424, 1999[Abstract/Free Full Text]

23. Clarke R, Liu MC, Bouker KB, et al: Antiestrogen resistance in breast cancer and the role of estrogen receptor signaling. Oncogene 22:7316-7339, 2003[CrossRef][Medline]

24. Smith CL, Nawaz Z, O'Malley BW: Coactivator and corepressor regulation of the agonist/antagonist activity of the mixed antiestrogen, 4-hydroxytamoxifen. Mol Endocrinol 11:657-666, 1997[Abstract/Free Full Text]

25. Migliaccio A, Pagano M, Auricchio F: Immediate and transient stimulation of protein tyrosine phosphorylation by estradiol in MCF-7 cells. Oncogene 8:2183-2191, 1993[Medline]

26. Migliaccio A, Di Domenico M, Castoria G, et al: Tyrosine kinase/p21ras/MAP-kinase pathway activation by estradiol-receptor complex in MCF-7 cells. EMBO J 15:1292-1300, 1996[Medline]

27. Lee SK, Choi HS, Song MR, et al: Estrogen receptor, a common interaction partner for a subset of nuclear receptors. Mol Endocrinol 12:1184-1192, 1998[Abstract/Free Full Text]

28. Endoh H, Sasaki H, Maruyama K, et al: Rapid activation of MAP kinase by estrogen in the bone cell line. Biochem Biophys Res Commun 235:99-102, 1997[CrossRef][Medline]

29. Razandi M, Pedram A, Park ST, et al: Proximal events in signaling by plasma membrane estrogen receptors. J Biol Chem 278:2701-2712, 2003[Abstract/Free Full Text]

30. Goldstein SR, Siddhanti S, Ciaccia AV, et al: A pharmacological review of selective oestrogen receptor modulators. Hum Reprod Update 6:212-224, 2000[Abstract/Free Full Text]

31. Wijayaratne AL, McDonnell DP: The human estrogen receptor-alpha is a ubiquitinated protein whose stability is affected differentially by agonists, antagonists, and selective estrogen receptor modulators. J Biol Chem 276:35684-35692, 2001[Abstract/Free Full Text]

32. Osborne CK, Zhao H, Fuqua SA: Selective estrogen receptor modulators: Structure, function, and clinical use. J Clin Oncol 18:3172-3186, 2000[Abstract/Free Full Text]

33. Hong SH, Privalsky ML: The SMRT corepressor is regulated by a MEK-1 kinase pathway: Inhibition of corepressor function is associated with SMRT phosphorylation and nuclear export. Mol Cell Biol 20:6612-6625, 2000[Abstract/Free Full Text]

34. Osborne CK, Bardou V, Hopp TA, et al: Role of the estrogen receptor coactivator AIB1 (SRC-3) and HER-2/neu in tamoxifen resistance in breast cancer. J Natl Cancer Inst 95:353-361, 2003[Abstract/Free Full Text]

35. Lavinsky RM, Jepsen K, Heinzel T, et al: Diverse signaling pathways modulate nuclear receptor recruitment of N-CoR and SMRT complexes. Proc Natl Acad Sci USA 95:2920-2925, 1998[Abstract/Free Full Text]

36. Carroll JS, Lynch DK, Swarbrick A, et al: p27(Kip1) induces quiescence and growth factor insensitivity in tamoxifen-treated breast cancer cells. Cancer Res 63:4322-4326, 2003[Abstract/Free Full Text]

37. Powles TJ, Hickish T, Kanis JA, et al: Effect of tamoxifen on bone mineral density measured by dual energy x-ray absorptionmetry in health premenopausal and postmenopausal women. J Clin Oncol 14:78-84, 1996[Abstract]

38. Jordan VC: Estrogen, selective estrogen receptor modulation, and coronary heart disease: Something or nothing. J Natl Cancer Inst 93:2-4, 2001[Free Full Text]

39. Grey AB, Stapleton JP, Evans MC, et al: The effect of the anti-estrogen tamoxifen on cardiovascular risk factors in normal postmenopausal women. J Clin Endocrinol Metab 80:3191-3195, 1995[Abstract]

40. Love RR, Surawicz TS, Williams EC: Antithrombin III level, fibrinogen level and platelet count change with adjuvant tamoxifen therapy. Arch Intern Med 152:317-320, 1992[Abstract]

41. Chang J, Powles TJ, Ashley SE, et al: The effect of tamoxifen and hormone replacement in serum cholesterol, bone mineral density and coagulation factors in healthy postmenopausal women participating in a randomised controlled tamoxifen prevention study. Ann Oncol 7:671-675, 1996[Abstract/Free Full Text]

42. Shewmon DA, Stock JL, Abusamra LC, et al: Tamoxifen decreases lipoprotein (a) in patients with breast cancer. Metabolism 43:531-532, 1994[CrossRef][Medline]

43. Anker G, Lonning PE, Ueland PM, et al: Plasma levels of the atherogenic amino acid homocysteine in post-menopausal women with breast cancer treated with tamoxifen. Int J Cancer 60:365-368, 1995[Medline]

44. Bergman L, Beelen ML, Gallee MP, et al: Risk and prognosis of endometrial cancer after tamoxifen for breast cancer. Comprehensive Cancer Centres' ALERT Group. Assessment of liver and endometrial cancer risk following tamoxifen. Lancet 356:881-887, 2000[CrossRef][Medline]

45. Ellmen J, Hakulinen P, Partanen A, et al: Estrogenic effects of toremifene and tamoxifen in postmenopausal breast cancer patients. Breast Cancer Res Treat 82:103-111, 2003[CrossRef][Medline]

46. Chlebowski RT, Collyar DE, Somerfield MR, et al: American Society of Clinical Oncology technology assessment of breast cancer risk reduction strategies: Tamoxifen and raloxifene. J Clin Oncol 17:1939-1955, 1999[Abstract/Free Full Text]

47. Fisher B, Costantino JP, Wickerham DL, et al: Tamoxifen for prevention of breast cancer: Report of the National Surgical Adjuvant Breast and Bowel Project P-1 Study. J Natl Cancer Inst 90:1371-1388, 1998[Abstract/Free Full Text]

48. Cuzick J, Forbes J, Edwards R, et al: First results from the International Breast Cancer Intervention Study (IBIS-I): A randomised prevention trial. Lancet 360:817-824, 2002[CrossRef][Medline]

49. Powles T, Eeles R, Ashley S, et al: Interim analysis of the incidence of breast cancer in the Royal Marsden Hospital tamoxifen randomised chemoprevention trial. Lancet 352:98-101, 1998[Medline]

50. Veronesi U, Maisonneuve P, Costa A, et al: Prevention of breast cancer with tamoxifen: Preliminary findings from the Italian randomised trial among hysterectomised women. Lancet 352:93-97, 1998[Medline]

51. Cuzick J, Powles T, Veronesi U, et al: Overview of the main outcomes in breast-cancer prevention trials. Lancet 361:296-300, 2003[CrossRef][Medline]

52. Klotz DM, Hewitt SC, Korach KS, et al: Activation of a uterine insulin-like growth factor I signaling pathway by clinical and environmental estrogens: requirement of estrogen receptor-alpha. Endocrinology 141:3430-3439, 2000[Abstract/Free Full Text]

53. Braithwaite RS, Chlebowsk RT, Lau J, et al: Tamoxifen and breast cancer meta-analysis of vascular and neoplastic events associated with tamoxifen. J Gen Intern Med 18:937-947, 2003[CrossRef][Medline]

54. Port ER, Montgomery LL, Heerdt AS, et al: Patient reluctance toward tamoxifen use for breast cancer primary prevention. Ann Surg Oncol 8:580-585, 2001[Abstract/Free Full Text]

55. Morrow M, Hou N, Tchou J, et al: Acceptance of tamoxifen (TAM) chemoprevention varies with the cause of breast cancer risk. Proc Am Soc Clin Oncol 22:538, 2003 (abstr 2165)

56. Vogel VG, Costantino JP, Wickerham DL, et al: Re: tamoxifen for prevention of breast cancer: report of the National Surgical Adjuvant Breast and Bowel Project P-1 Study. J Natl Cancer Inst 94:1504, 2002[Free Full Text]

57. Gail MH, Costantino JP, Bryant J, et al: Weighing the risks and benefits of tamoxifen treatment for preventing breast cancer. J Natl Cancer Inst 91:1829-1846, 1999[Abstract/Free Full Text]

58. Freedman AN, Graubard BI, Rao SR, et al: Estimates of the number of US women who could benefit from tamoxifen for breast cancer chemoprevention. J Natl Cancer Inst 95:526-532, 2003[Abstract/Free Full Text]

59. Page DL, Dupont WD: Anatomic markers of human premalignancy and risk of breast cancer. Cancer 66:1326-1335, 1990[CrossRef][Medline]

60. Cancer risks in BRCA2 mutation carriers. The Breast Cancer Linkage Consortium. J Natl Cancer Inst 91:1310-1316, 1999[Abstract/Free Full Text]

61. Brose MS, Rebbeck TR, Calzone KA, et al: Cancer risk estimates for BRCA1 mutation carriers identified in a risk evaluation program. J Natl Cancer Inst 94:1365-1372, 2002[Abstract/Free Full Text]

62. Fisher B, Dignam J, Wolmark N, et al: Tamoxifen in treatment of intraductal breast cancer: National Surgical Adjuvant Breast and Bowel Project B-24 randomised controlled trial. Lancet 353:1993-2000, 1999[CrossRef][Medline]

63. Narod SA, Brunet JS, Ghadirian P, et al: Tamoxifen and risk of contralateral breast cancer in BRCA1 and BRCA2 mutation carriers: A case-control study. Hereditary Breast Cancer Clinical Study Group. Lancet 356:1876-1881, 2000[CrossRef][Medline]

64. Foulkes WD, Goffin J, Brunet JS, et al: Tamoxifen may be an effective adjuvant treatment for BRCA1-related breast cancer irrespective of estrogen receptor status. J Natl Cancer Inst 94:1504-1506, 2002

65. Cappelletti V, Veneroni S, Coradini D, et al: Re: Tamoxifen may be an effective treatment for BRCA1-related breast cancer irrespective of estrogen receptor status. J Natl Cancer Inst 95:629-630, 2003[Free Full Text]

66. King MC, Wieand S, Hale K, et al: Tamoxifen and breast cancer incidence among women with inherited mutations in BRCA1 and BRCA2: National Surgical Adjuvant Breast and Bowel Project (NSABP-P1) Breast Cancer Prevention Trial. JAMA 286:2251-2256, 2001[Abstract/Free Full Text]

67. Early Breast Cancer Trialists' Collaborative Group: Tamoxifen for early breast cancer: An overview of the randomized trials. Lancet 351:1451-1467, 1998[CrossRef][Medline]

68. Bardou VJ, Arpino G, Elledge RM, et al: Progesterone receptor status significantly improves outcome prediction over estrogen receptor status alone for adjuvant endocrine therapy in two large breast cancer databases. J Clin Oncol 21:1973-1979, 2003[Abstract/Free Full Text]

69. Allred D, Bryant J, Land S, et al: Estrogen receptor expression as a predictive marker of the effectiveness of tamoxifen in the treatment of DCIS: Findings from NSABP rotocol B-24. Breast Cancer Res Treat 76:S36, 2002 (abstr 30; suppl 1)

70. Hershman D, Sundararajan V, Jacobson JS, et al: Outcomes of tamoxifen chemoprevention for breast cancer in very high-risk women: A cost-effectiveness analysis. J Clin Oncol 20:9-16, 2002[Abstract/Free Full Text]

71. Hoogerbrugge N, Bult P, deWidt-Levert LM, et al: High prevalence of premalignant lesions in prophylactically removed breasts from women at hereditary risk for breast cancer. J Clin Oncol 21:41-45, 2003[Abstract/Free Full Text]

72. Fabian CJ, Kimler BF, Zalles CM, et al: Short-term breast cancer prediction by random periareolar fine-needle aspiration cytology and the Gail risk model. J Natl Cancer Inst 92:1217-1227, 2000[Abstract/Free Full Text]

73. Dooley WC, Ljung BM, Veronesi U, et al: Ductal lavage for detection of cellular atypia in women at high risk for breast cancer. J Natl Cancer Inst 93:1624-1632, 2001[Abstract/Free Full Text]

74. Wrensch MR, Petrakis NL, Miike R, et al: Breast cancer risk in women with abnormal cytology in nipple aspirates of breast fluid. J Natl Cancer Inst 93:1791-1798, 2001[Abstract/Free Full Text]

75. Endogenous Hormones and Breast Cancer Collaborative Group: Endogenous sex hormones and breast cancer in postmenopausal women: reanalysis of nine prospective studies. J Natl Cancer Inst 94:606-616, 2002[Abstract/Free Full Text]

76. Hankinson SE, Willett WC, Colditz GA, et al: Circulating concentrations of insulin-like growth factor-I and risk of breast cancer. Lancet 351:1393-1396, 1998[CrossRef][Medline]

77. Boyd NF, Lockwood GA, Bying JW, et al: Mammographic densities and breast cancer risk. Cancer Epidemiol Biomarkers Prev 7:1133-1144, 1998[Abstract/Free Full Text]

78. Bonanni B, Johansson H, Gandini S, et al: Effect of low dose tamoxifen on the insulin-like growth factor system in healthy women. Breast Cancer Res Treat 69:21-27, 2001[CrossRef][Medline]

79. Cuzick J, Warwick J, Pinney E, et al: Tamoxifen and breast density in women at increased risk of breast cancer. J Natl Cancer Inst 96:621-628, 2004[Abstract/Free Full Text]

80. Tan-Chiu E, Wang J, Costantino JP, et al: Effects of tamoxifen on benign breast disease in women at high risk for breast cancer. J Natl Cancer Inst 95:302-307, 2003[Abstract/Free Full Text]

81. Fabian CJ, Kimler BF, Elledge RM, et al: Models for early chemoprevention trials in breast cancer. Hematol Oncol Clin North Am 12:993-1017, 1998[CrossRef][Medline]

82. Fabian CJ, Kimler BF, Mayo MS, et al: Clinical approaches to the discovery and testing of new breast cancer prevention drugs, in Kelloff GJ, Hawk ET, Sigman CC (eds): Cancer Chemoprevention Volume 2: Strategies for chemoprevention. Humana Press, Totowa, NJ, pp 13-238, 2004

83. Swain SM, Wilson JW, Mamounas EP, et al: Estrogen receptor status of primary breast cancer is predictive of estrogen receptor status of contralateral breast cancer. J Natl Cancer Inst 96:516-523, 2004[Abstract/Free Full Text]

84. Chang J, Powles TJ, Allred DC, et al: Prediction of clinical outcome from primary tamoxifen by expression of biologic markers in breast cancer patients. Clin Cancer Res 6:616-621, 2000[Abstract/Free Full Text]

85. Decensi A, Robertson C, Viale G, et al: A randomized trial of low-dose tamoxifen on breast cancer proliferation and blood estrogenic biomarkers. J Natl Cancer Inst 95:779-790, 2003[Abstract/Free Full Text]

86. Kisanga ER, Gjerde J, Guerrieri-Gonzaga A, et al: Tamoxifen and metabolite concentrations in serum and breast cancer tissue during three dose regimens in a randomized preoperative trial. Clin Cancer Res 10:2336-2343, 2004[Abstract/Free Full Text]

87. de Lima GR, Facina G, Shida JY, et al: Effects of low dose tamoxifen on normal breast tissue from premenopausal women. Eur J Cancer 39:891-898, 2003

88. Grese TA, Sluka JP, Bryant HU, et al: Molecular determinants of tissue selectivity in estrogen receptor modulators. Proc Natl Acad Sci U S A 94:14105-14110, 1997[Abstract/Free Full Text]

89. Black LJ, Sato M, Rowley ER, et al: Raloxifene (LY139481 HCI) prevents bone loss and reduces serum cholesterol without causing uterine hypertrophy in ovariectomized rats. J Clin Invest 93:63-69, 1994

90. Lee ES, Schafer JM, Yao K, et al: Cross-resistance of triphenylethylene-type antiestrogens but not ICI 182,780 in tamoxifen-stimulated breast tumors grown in athymic mice. Clin Cancer Res 6:4893-4899, 2000[Abstract/Free Full Text]

91. O'Regan RM, Gajdos C, Dardes RC, et al: Effects of raloxifene after tamoxifen on breast and endometrial tumor growth in athymic mice. J Natl Cancer Inst 94:274-283, 2002[Abstract/Free Full Text]

92. Buzdar AU, Marcus C, Holmes F, et al: Phase II evaluation of LY156758 in metastatic breast cancer. Oncology 45:344-345, 1988[Medline]

93. Gradishar W, Glusman J, Lu Y, et al: Effects of high dose raloxifene in selected patients with advanced breast carcinoma. Cancer 88:2047-2053, 2000[CrossRef][Medline]

94. Ettinger B, Black DM, Mitlak BH, et al: Reduction of vertebral fracture risk in postmenopausal women with osteoporosis treated with raloxifene: Results from a 3-year randomized clinical trial. Multiple Outcomes of Raloxifene Evaluation (MORE) Investigators. JAMA 282:637-645, 1999[Abstract/Free Full Text]

95. Kanis JA, Johnell O, Black DM, et al: Effect of raloxifene on the risk of new vertebral fracture in postmenopausal women with osteopenia or osteoporosis: A reanalysis of the Multiple Outcomes of Raloxifene Evaluation trial. Bone 33:293-300, 2003[Medline]

96. Cummings SR, Eckert S, Krueger KA, et al: The effect of raloxifene on risk of breast cancer in postmenopausal women: Results from the MORE Randomized Trial. Multiple outcomes of raloxifene evaluation. JAMA 281:2189-2197, 1999[Abstract/Free Full Text]

97. Cauley JA, Norton L, Lippman ME, et al: Continued breast cancer risk reduction in postmenopausal women treated with raloxifene: 4-year results from the MORE trial. Multiple outcomes of raloxifene evaluation. Breast Cancer Res Treat 65:125-134, 2001[CrossRef][Medline]

98. Walsh BW, Kuller LH, Wild RA, et al: Effects of raloxifene on serum lipids and coagulation factors in healthy postmenopausal women. JAMA 279:1445-1451, 1998[Abstract/Free Full Text]

99. Jordan VC, Fritz NF, Tormey DC: Endocrine effects of adjuvant chemotherapy and long-term tamoxifen administration on node-positive patients with breast cancer. Cancer Res 47:624-630, 1987[Abstract/Free Full Text]

100. Baker VL, Draper M, Paul S, et al: Reproductive endocrine and endometrial effects of raloxifene hydrochloride, a selective estrogen receptor modulator, in women with regular menstrual cycles. J Clin Endocrinol Metab 83:6-13, 1998[Abstract/Free Full Text]

101. Cheng WC, Yen ML, Hsu SH, et al: Effects of raloxifene, one of the selective estrogen receptor modulators, on pituitary-ovary axis and prolactin in postmenopausal women. Endocrine 23:215-218, 2004[CrossRef][Medline]

102. Draper MW, Flowers DE, Huster WJ, et al: A controlled trial of raloxifene Hcl: Impact on bone turnover and serum lipid profile in healthy postmenopausal women. J Bone Miner Res 11:835-842, 1996[Medline]

103. Davies GC, Huster WJ, Lu Y, et al: Adverse events reported by postmenopausal women in controlled trials with raloxifene. Obstet Gynecol 93:558-565, 1999[Abstract/Free Full Text]

104. Cohen FJ, Lu Y: Characterization of hot flashes reported by healthy postmenopausal women receiving raloxifene or placebo during osteoporosis prevention trials. Maturitas 34:65-73, 2000[CrossRef][Medline]

105. Ganz PA, Castellon SA, Silverman DH: Estrogen, tamoxifen, and the brain. J Natl Cancer Inst 94:547-549, 2002[Free Full Text]

106. Day R, Ganz PA, Costantino JP: Tamoxifen and depression: More evidence from the National Surgical Adjuvant Breast and Bowel Project's Breast Cancer Prevention (P-1) Randomized Study. J Natl Cancer Inst 93:1615-1623, 2001[Abstract/Free Full Text]