Effects of the antiestrogens tamoxifen, toremifene, and ICI 182,780 on endometrial cancer growth

Emerging principles for the development of resistance to antihormonal therapy: implications for the clinical utility of fulvestrant

We seek to evaluate the clinical consequences of resistance to antihormonal therapy by studying analogous animal xenograft models. Two approaches were taken: (1) MCF-7 tumors were serially transplanted into selective estrogen receptor modulator (SERM)-treated immunocompromised mice to mimic 5 years of SERM treatment. The studies in vivo were designed to replicate the development of acquired resistance to SERMs over years of clinical exposure. (2) MCF-7 cells were cultured long-term under SERM-treated or estrogen withdrawn conditions (to mimic aromatase inhibitors), and then injected into mice to generate endocrine-resistant xenografts. These tumor models have allowed us to define Phase I and Phase II antihormonal resistance according to their responses to E(2) and fulvestrant. Phase I SERM-resistant tumors were growth stimulated in response to estradiol (E(2)), but paradoxically, Phase II SERM and estrogen withdrawn-resistant tumors were growth inhibited by E(2). Fulvestrant did not support growth of Phases I and II SERM-resistant tumors, but did allow growth of Phase II estrogen withdrawn-resistant tumors. Importantly, fulvestrant plus E(2) in Phase II antihormone-resistant tumors reversed the E(2)-induced inhibition and instead resulted in growth stimulation. These data have important clinical implications. Based on these and prior laboratory findings, we propose a clinical strategy for optimal third-line therapy: patients who have responded to and then failed at least two antihormonal treatments may respond favorably to short-term low-dose estrogen due to E(2)-induced apoptosis, followed by treatment with fulvestrant plus an aromatase inhibitor to maintain low tumor burden and avoid a negative interaction between physiologic E(2) and fulvestrant.

Development and therapeutic options for the treatment of raloxifene-stimulated breast cancer in athymic mice

PURPOSE

Selective estrogen receptor modulators (SERM) are used for the treatment and prevention of breast cancer (tamoxifen) and osteoporosis (raloxifene). Mechanisms of tamoxifen-resistance in breast cancer are incompletely understood but current research is focused on crosstalk between growth factor receptors and the estrogen receptor alpha (ERalpha) pathway. There is increasing clinical use of raloxifene for the treatment of osteoporosis, but the widespread use of this SERM will have consequences for the treatment of breast cancer in raloxifene-exposed women.

EXPERIMENTAL DESIGN

We took the strategic step of developing a raloxifene-resistant tumor (MCF-7RALT) model in vivo and investigating the mechanisms responsible for resistance.

RESULTS

MCF-7RALT tumors exhibited phase I SERM resistance, growing in response to SERMs and 17beta-estradiol. Epidermal growth factor receptor/HER1 and HER2/neu mRNAs were increased in MCF-7RALT tumors. The HER2/neu blocker, trastuzumab, but not the epidermal growth factor receptor blocker, gefitinib, decreased the growth of MCF-7RALT tumors in vivo. Consequently, trastuzumab decreased prosurvival/proliferative proteins: phospho-HER2/neu, total HER2/neu, phospho-Akt (protein kinase B), glycogen synthetase kinase-3, cyclin D1, and the antiapoptotic protein X chromosome-linked inhibitor of apoptosis, whereas increasing the proapoptotic protein, caspase-7, in raloxifene-treated MCF-7RALT tumors. Interestingly, ERalpha protein was overexpressed in untreated MCF-7RALT tumors and hyperactivated in cells derived from these tumors. Only fulvestrant completely inhibited the growth and ERalpha activity of MCF-7RALT tumors. The coactivator of ERalpha, amplified in breast cancer-1 protein was modestly increased in the raloxifene-treated MCF-7RALT tumors and increased both basal and estradiol-induced activity of ERalpha in cells derived from the MCF-7RALT tumors.

CONCLUSIONS

These results suggest that overexpression and increased activity of HER2/neu might be responsible for the development of raloxifene-resistant breast cancer. The results also suggest that increased expression of basal activity of ERalpha could contribute to the hypersensitivity of MCF-7RALT tumors in response to estradiol because only fulvestrant blocked growth and ERalpha activity.

Decreased responsiveness of naturally occurring mutants of human estrogen receptor alpha to estrogens and antiestrogens

Estrogen receptor alpha (ERalpha) is a ligand-inducible transcription factor that mediates the biological effects of estrogens and antiestrogens. Many point mutations in the human ERalpha gene have been reported to be associated with breast cancer, endometrial cancer, and psychiatric diseases. However, functional analyses for most mutants with amino acid changes are still lacking. In the present study, to investigate the effects of point mutations on the function, gel-shift assays and luciferase assays were performed for eight kinds of mutated ERalpha proteins, including a single nucleotide change of C207G (N69K), G478T (G160C), T887C (L296P), A908G (K303R), C926T (S309F), A1058T (E353V), A1186G (M396V), and G1231deletion (411fsX7). The mutated ERalpha expression plasmids were constructed by site-directed mutagenesis. With gel-shift assays using in vitro translated ERalpha proteins, binding to the consensus estrogen response element (ERE) was observed for the mutated ERalpha proteins except ERalpha (G160C) and ERalpha (411fsX7), the binding of which was comparable with that of the wild type. Western blot analyses showed that ERalpha (G160C) could not be efficiently translated with the in vitro transcription/translation system and that ERalpha (411fsX7) produced a truncated protein. To investigate the transactivation potency, wild-type or mutated ERalpha expression plasmids were co-transfected with pGL3-3EREc38 reporter plasmid into human breast adenocarcinoma MDA-MB-435 cells. The concentration-response curves (10pM-100nM E2) of the mutant ERalpha proteins except ERalpha (E353V) and ERalpha (411fsX7) were similar to that of wild-type ERalpha. However, at a low level of E2 (100pM), the mutants ERalpha (N69K), ERalpha (L296P), ERalpha (S309F), and ERalpha (M396V) showed a significant decrease of transactivation compared with that of the wild-type ERalpha. The mutants ERalpha (E353V) and ERalpha (411fsX7) did not show responsiveness to E2 and antiestrogens, 4-hydroxytamoxifen (4OHT) and ICI 182,780. The mutant ERalpha (S309F) showed decreased responsiveness for the antiestrogenicity of 4OHT. In conclusion, we found that some of the naturally occurring human ERalpha mutants with amino acid changes may have an altered responsiveness to estrogen and antiestrogens.

David Kupfer, Ph.D. A mentor and a scientist

I worked with the late Dr. David Kupfer for nearly nine years at the Worcester Foundation/University of Massachusetts Medical School, Worcester, MA. I was involved in the metabolism of methoxychlor and tamoxifen, the areas of research close to David's heart. We demonstrated the metabolic pathways of these compounds in rats and humans, and the covalent binding to microsomal proteins that could result in long-term toxic manifestations. I learned a lot from David, who was a mentor and friend/colleague. His death has left a void in my heart and he will be sorely missed.

Absence of DNA Adduct in the Leukocytes from Breast Cancer Patients Treated with Toremifene

Tamoxifen (TAM) causes cancer in rat liver and human endometrium, whereas the carcinogenicity of its chlorinated analogue toremifene (TOR) has not been observed. To elucidate the genotoxicity of TOR, the capability of forming DNA adducts by TOR was examined in the leukocytes of patients treated with TOR. Leukocytes were collected from 27 breast cancer patients (57.7 +/- 11.4 years old) taking TOR (40 mg/day for 25 patients, 80 mg/day for one patient, and 120 mg/day for one patient; average duration, approximately 12 months) and 20 untreated breast cancer patients (58.2 +/- 12.3 years old). The DNA extracted was analyzed by (32)P-postlabeling/high-performance liquid chromatography. No DNA adducts were detected in the leukocytes of either TOR-treated or nontreated patients. Our results contrast to the previous observation detecting TAM-DNA adducts in several patients treated with TAM, indicating that TOR is less genotoxic to humans.

Durable response of metastatic endometrial carcinoma to treatment with fulvestrant (Faslodex®) after prior progestin and anastrozole therapy

BACKGROUND

Patients with metastatic endometrial carcinoma may respond to hormonal therapy with progestins. There is a need for new therapies for hormone-responsive disease.

CASE

We report a patient with metastatic endometrial carcinoma to the lungs, who after progressing on progestin therapy, had a lengthy remission with anastrozole; upon further progression, fulvestrant (Faslodex) was instituted, with a resultant partial remission, which has been sustained for almost 3 years.

CONCLUSION

In this case, fulvestrant therapy was successful even in the face of prior anastrozole and megestrol. The activity of fulvestrant in patients with metastatic endometrial carcinoma should be further explored, especially in situations in which the tumor is well differentiated and/or expresses hormone receptors.

Intrinsic mechanism of estradiol-induced apoptosis in breast cancer cells resistant to estrogen deprivation

BACKGROUND

We previously developed an estrogen receptor (ER)-positive breast cancer cell line (MCF-7:5C) that is resistant to long-term estrogen deprivation and undergoes rapid and complete apoptosis in the presence of physiologic concentrations of 17beta-estradiol. Here, we investigated the role of the mitochondrial apoptotic pathway in this process.

METHODS

Apoptosis in MCF-7:5C cells treated with estradiol, fulvestrant, or vehicle (control) was investigated by annexin V-propidium iodide double staining and 4',6-diamidino-2-phenylindole (DAPI) staining. Apoptosis was also analyzed in MCF-7:5C cells transiently transfected with small interfering RNAs (siRNAs) to apoptotic pathway components. Expression of apoptotic pathway intermediates was measured by western blot analysis. Mitochondrial transmembrane potential (psim) was determined by rhodamine-123 retention assay. Mitochondrial pathway activity was determined by cytochrome c release and cleavage of poly(ADP-ribose) polymerase (PARP) protein. Tumorigenesis was studied in ovariectomized athymic mice that were injected with MCF-7:5C cells. Differences between the treatment groups and control group were determined by two-sample t test or one-factor analysis of variance. All statistical tests were two-sided.

RESULTS

MCF-7:5C cells treated with estradiol underwent apoptosis and showed increased expression of proapoptotic proteins, decreased psim, enhanced cytochrome c release, and PARP cleavage compared with cells treated with fulvestrant or vehicle. Blockade of Bax, Bim, and p53 mRNA expression by siRNA reduced estradiol-induced apoptosis relative to control by 76% [95% confidence interval (CI) = 73% to 79%, P < .001], 85% [95% CI = 90% to 80%, P < .001], and 40% [95% CI = 45% to 35%, P < .001], respectively, whereas blockade of FasL by siRNA had no effect. Estradiol caused complete regression of MCF-7:5C tumors in vivo.

CONCLUSION

The mitochondrial pathway of apoptosis plays a critical role in estradiol-induced apoptosis in long-term estrogen-deprived breast cancer cells. Physiologic concentrations of estradiol could potentially be used to induce apoptosis and tumor regression in tumors that have developed resistance to aromatase inhibitors.

Trastuzumab therapy for tamoxifen-stimulated endometrial cancer

A novel in vivo model of tamoxifen-stimulated endometrial cancer was developed and the role of HER-2/neu investigated by using trastuzumab. Tamoxifen-stimulated tumors (ECC-1TAM) were growth stimulated by 17beta-estradiol (E2), tamoxifen, or raloxifene. Trastuzumab inhibited growth of E2-stimulated ECC-1E2 tumors by 50% and tamoxifen-stimulated ECC-1TAM tumors by 100%. ECC-1 tumors expressed functional estrogen receptor alpha (ER alpha) as measured by induction of pS2 and c-myc mRNAs. E2 induced pS2 and c-myc mRNAs up to 40-fold in ECC-1E2 and ECC-1TAM. Tamoxifen induced pS2 and c-myc mRNAs up to 5-fold in ECC-1E2 tumors and up to 10-fold in ECC-TAM tumors. Trastuzumab blocked E2-induced pS2 mRNA (P < 0.01) in ECC-1E2 by 50% and tamoxifen-induced c-myc mRNA (P < 0.1) in ECC-1TAM tumors by 70%. Trastuzumab decreased phosphorylated and total HER-2/neu protein in ECC-1E2 and ECC-1TAM tumors. However, only phospho-ERK-1/2 and not phospho-Akt protein was decreased by trastuzumab in tamoxifen-treated ECC-1TAM tumors. The insulin-like growth factor (IGF-I) signaling pathway also activates extracellular signal-related kinase (ERK)-1/2 and could block the efficacy of trastuzumab in ECC-1E2 tumors. The results showed that IGF-I, IGF-IR mRNAs, and phospho-insulin receptor substrate-1 (IRS-1) protein were decreased in ECC-1TAM compared with ECC-1E2 tumors. The results show that trastuzumab is an effective therapy for both E2-stimulated and tamoxifen-stimulated endometrial cancer. The data suggest estrogenic activities of E2 and tamoxifen at ER alpha-regulated pS2 and c-myc genes are in part mediated by HER-2/neu. However, trastuzumab is a better growth inhibitor of ECC-1TAM tumors where there is diminished IGF-I signaling allowing for complete blockade of the downstream phospho-ERK-1/2 signal.

Formation of tamoxifen-DNA adducts in human endometrial explants exposed to alpha-hydroxytamoxifen

An increased risk of developing endometrial cancer has been observed in women receiving tamoxifen (TAM) endocrine therapy and chemoprevention. The genotoxic damage induced by TAM metabolites may be involved in the development of endometrial cancer. To investigate the capability of endometrial tissues to form TAM-DNA adducts, primary cultured human endometrial explants were exposed to alpha-hydroxytamoxifen (alpha-OHTAM) and used for quantitative analysis of TAM-DNA adducts, using (32)P-postlabeling/HPLC analysis. A trans isoform of alpha-(N(2)-deoxyguanosinyl)tamoxifen (dG-N(2)-TAM) was detected as the major adduct in eight of nine endometrial explants exposed to 100 microM alpha-OHTAM at levels of 7.7 +/- 5.3 (mean +/- SD) adducts/10(7) nucleotides. Approximately 25- and 37-fold lower amounts of the cis form of dG-N(2)-TAM and another trans isoform were also detected. The dG-N(2)-TAM adduct (3.3 adducts/10(7) nucleotides) was detected in one of three endometrial explants exposed to 25 microM alpha-OHTAM. No TAM-DNA adducts were detected in any unexposed tissues. These results indicate that TAM-DNA adducts are capable of forming through O-sulfonation and/or O-acetylation of alpha-OHTAM in the endometrium. The endometrial explant culture can be used as a model system to explore the genotoxic mechanism of antiestrogens for humans.

K-ras mutation in the endometrium of tamoxifen-treated breast cancer patients, with a comparison of tamoxifen and toremifene

The putative presence of a mutation in codon 12 of the K-ras gene was investigated in the endometrium of tamoxifen (TAM) and toremifene (TOR)-treated breast cancer patients. DNA was extracted from fresh cytologic samples of the endometrium in 86 TAM and 21 TOR-treated breast cancer patients. Mutations were detected by enriched PCR and an enzyme-linked mini-sequence assay (ELMA). K-ras mutation was found in 35 TAM-treated endometrial samples, and in only one TOR-treated endometrium (P<0.003). In 24 premenopausal patients, K-ras mutation was found in seven (43.8%) of 16 patients with less than 47 months of TAM treatment, while none was found in eight patients with more than 48 months of TAM treatment (P<0.03). In 62 postmenopausal-amenorrheic patients, K-ras mutation was found in three (15.8%) of 19 patients with less than 23 months of TAM treatment, while it was found in 16 (61.5%) of 26 patients with 24–47 months of TAM treatment and nine (52.9%) of 17 patients with more than 48 months of TAM treatment (P=0.002). The presence of K-ras mutation is significantly influenced by the duration of TAM treatment and menstrual status of the patients. TOR may have a lower potential genotoxicity than TAM.

Endocrinology and hormone therapy in breast cancer: selective oestrogen receptor modulators and downregulators for breast cancer - have they lost their way?

Although tamoxifen has been an effective treatment for breast cancer, several novel anti-oestrogen compounds have been developed with a reduced agonist profile on breast and gynaecological tissues. These include selective oestrogen receptor modulators (SERMs; both 'tamoxifen-like' and 'fixed-ring' SERMs) and selective oestrogen receptor downregulators (SERDs), although none has been proved superior in efficacy to tamoxifen in various advanced breast cancer trials. Thus, many have questioned whether a need for SERMs in breast cancer still exists, although chemoprevention remains a possible niche setting. In contrast, SERDs may have useful efficacy following aromatase inhibitors because of their unique mechanism of action, and clinical trials to determine their optimal use or sequence are ongoing.

Reversal of tamoxifen resistant breast cancer by low dose estrogen therapy

Currently, the standard of care for estrogen receptor (ER)-positive breast cancer is 5 years of tamoxifen (TAM) or an aromatase inhibitor (AI) such as anastrozole. New studies indicate that extending antiestrogen therapy beyond 5 years with sequential regimens will improve disease-free survival. Based on the emerging concept that longer therapies are better, we have developed sequential models of tamoxifen-resistant breast cancer in vivo to mimic the clinical scenario of long-term antiestrogen therapy. The goal of the current study was to investigate the consequences of long-term treatment with tamoxifen on the growth of breast tumors in athymic mice. The results demonstrate that there are distinct phases of resistance to tamoxifen that correlate with time of treatment and expression of HER2/neu mRNA. In the treatment phase, 17beta-estradiol (E2) stimulated growth, while TAM inhibited growth of MCF-7 tumors (MCF-7E2). The withdrawal of treatment, mimicking the use of an AI, completely prevented growth. In Phase I resistance, the tumors (MCF-7TAMST) were growth-stimulated by either E2 or TAM, but inhibited by no treatment, fulvestrant, or E2 + fulvestrant. Phase II-resistant tumors (MCF-7TAMLT) were treated for more than 5 years and growth-stimulated by TAM. However, no treatment, fulvestrant, or E2 completely inhibited growth. Interestingly, the few tumors (MCF-7TAMLT) that survived in response to E2 were robustly re-stimulated by E2 after transplantation into new generations of athymic mice. These E2-stimulated tumors (MCF-7TAME) were inhibited by TAM in a dose-dependent similar to their parental tumors (MCF-7E2). In addition, the MCF-7TAME tumors were inhibited by either no treatment or fulvestrant. HER2/neu and HER3 mRNAs were over-expressed in TAM-stimulated MCF-7TAMLT tumors and remained high in E2-stimulated MCF-7TAME tumors. The data indicate that complete reversal of resistance to TAM can be achieved with the use of low dose E2 therapy. Also, these data suggest that over-expression of HER2/neu alone is insufficient to predict resistance to TAM. Based on the results, we suggest using an alternating treatment regimen, cycling antiestrogen with estrogen therapy to avoid drug-resistance.

Estrogen-induced apoptosis in a breast cancer model resistant to long-term estrogen withdrawal

Estrogen suppression through the use of an aromatase inhibitor is an effective endocrine treatment option for postmenopausal breast cancer patients with estrogen receptor (ER)-positive disease, however, there are concerns that long-term estrogen deprivation will inevitably lead to resistance. To address the issue of acquired resistance to long-term estrogen deprivation our laboratory has developed an ER+/PR- hormone-independent breast cancer cell line, MCF-7:5C which is a variant clone of wild-type MCF-7 cells. Originally, these cells were cultured in estrogen-free MEM containing 5% charcoal-stripped calf serum and were found to be resistant to both estradiol (E(2)) and antiestrogens. Interestingly, a completely different phenomenon was observed when MCF-7:5C cells were cultured in phenol red-free RPMI 1640 medium containing 10% charcoal-stripped fetal bovine serum (SFS). Using DNA quantitation assays, we examined the effect of E(2) on the growth of MCF-7:5C cells under different media conditions. Our results showed that 10(-9)M E(2) caused a dramatic 90% reduction in the growth of MCF-7:5C cells cultured in RPMI medium containing 10% SFS but did not have any significant inhibitory effects on cells cultured in MEM media. Additional experiments were performed to determine whether the medium or the serum facilitated the inhibitory effects of E(2) and the results indicated that it was the serum. Annexin V and DAPI staining confirmed that the E(2)-induced growth inhibition of MCF-7:5C cells was due to apoptosis. We also examined the tumorigenic potential of MCF-7:5C cells by injecting 1x10(7)cells/site into ovariectomized athymic mice and found that these cells, previously cultured in RPMI media, spontaneously grew into tumors in the absence of E(2). Overall, these results show that low concentrations (>10(-11)M) of E(2) are capable of inducing apoptosis in an aromatase resistant breast cancer cell model and that this effect is highly influenced by the medium in which the cells are grown.

INTERACTIONS OF THE STEREOISOMERS OF α-HYDROXYTAMOXIFEN WITH HUMAN HYDROXYSTEROID SULFOTRANSFERASE SULT2A1 AND RAT HYDROXYSTEROID SULFOTRANSFERASE STA

Tamoxifen (TAM) is a nonsteroidal antiestrogenic drug that is widely used for the treatment of estrogen receptor-dependent breast cancer. An increased risk of endometrial cancer in some patients treated with TAM has been linked to the metabolic formation of alpha-hydroxytamoxifen (alpha-OHTAM) and its subsequent sulfation. Alpha-OHTAM has been found to be a substrate for rat and human hydroxysteroid sulfotransferases (STa and SULT2A1, respectively). Since stereochemistry plays an important role in the interactions of hydroxysteroid sulfotransferases with their substrates, we have now investigated the interactions of each of the stereoisomers of alpha-OHTAM with highly purified recombinant STa and SULT2A1. Methods for the preparation of the enantiomers of E- and Z-alpha-OHTAM were developed. When each of the four enantiomers was examined with rat STa, E-(+)-alpha-OHTAM was the only substrate for the enzyme, whereas E-(-)-alpha-OHTAM, Z-(+)-alpha-OHTAM, and Z-(-)-alpha-OHTAM were inhibitors of the sulfation of E-(+)-alpha-OHTAM catalyzed by STa. The dissociation constants for the alpha-OHTAM enantiomers indicated that they bound to STa with similar affinity, but only the E-(+)-enantiomer was a substrate. In contrast to the results obtained with rat hydroxysteroid sulfotransferase STa, all enantiomers of alpha-OHTAM were substrates for the human SULT2A1. Moreover, kcat/Km values with SULT2A1 were higher with the Z enantiomers than with the E enantiomers. As a result of the potential for interconversion of the E and Z geometric isomers upon metabolism, the sulfation of the Z isomers may be of greater concern in human tissues than has been previously assumed.

Genotoxic Mechanism of Tamoxifen in Developing Endometrial Cancer

Increased risk of developing endometrial cancers has been observed in women treated with tamoxifen (TAM), a widely used drug for breast cancer therapy and chemoprevention. The carcinogenic effect may be due to genotoxic DNA damage induced by TAM. In fact, TAM-DNA adducts were detected in the endometrium of women treated with this drug. TAM is alpha-hydroxylated by cytochrome P450 3A4 followed by O-sulfonation by hydroxysteroid sulfotransferase, and reacts with guanine residues in DNA, resulting in the formation of alpha-(N2-deoxyguanosinyl)tamoxifen adducts. During this metabolic process, short-lived carbocations are produced at the ethyl moiety of TAM as reactive intermediates. TAM-DNA adducts promote primarily G -->T transversions in mammalian cells. The same mutations have been frequently detected at codon 12 of the K-ras gene in the endometrial tissue of women treated with this drug. TAM-DNA adducts, if not readily repaired, may act as initiators, leading to development of endometrial cancers. The reactivity of TAM metabolites with DNA is inhibited in toremifene, where the hydrogen atom has been replaced by a chlorine atom at the ethyl moiety. Therefore, toremifene may be a safer alternative to TAM. This article describes an overview of the mechanism of TAM-DNA adduct formation, mutagenic events of this adduct, and detection of TAM-DNA adducts in the endometrium of women treated with TAM.

The use of selective estrogen receptor modulators and selective estrogen receptor down-regulators in breast cancer

Tamoxifen is one of the most effective treatments for breast cancer through its ability to antagonize estrogen-dependent growth by binding estrogen receptors (ERs) and inhibiting proliferation of breast epithelial cells. However, tamoxifen has estrogenic agonist effects in other tissues such as bone and endometrium due to liganded ER activating target genes in these different types of cell. Several novel anti-estrogen compounds have been developed which have a reduced agonist profile on breast and gynaecological tissues. These compounds offer the potential for enhanced efficacy and reduced toxicity compared with tamoxifen. In advanced breast cancer clinical data exist for two groups of agents: the selective estrogen receptor modulators (SERMs), further divided into "tamoxifen-like" (e.g. toremifene, droloxifene and idoxifene) and "fixed ring" compounds (e.g. raloxifene, arzoxifene and EM-800), and the selective estrogen receptor down-regulators (SERDs; e.g. fulvestrant (ICI 182780), SR 16234 and ZK 191703) also termed "pure anti-estrogens". In phase II trials in tamoxifen-resistant metastatic breast cancer the SERMs show low objective response rates (range 0-15%), suggesting cross resistance with tamoxifen. Randomized phase III trials for toremifene and idoxifene in over 1500 patients showed no significant difference compared with tamoxifen. Fewer clinical data exist for the "fixed ring" SERMs and it remains unclear whether any clinical advantage exists for the "fixed ring" SERMs over tamoxifen as first-line therapy. The main advantage for SERMs such as tamoxifen and raloxifene probably remains in early-stage disease (adjuvant therapy or prevention). Fulvestrant and the other SERDs have a high affinity for the estrogen receptor (ER) compared to tamoxifen, but none of its agonist activities. Of the SERDs, only fulvestrant has entered the clinic and this new agent is showing promising clinical activity in the treatment of advanced breast cancer. Recently published phase III studies have shown fulvestrant to be at least as effective as the third-generation aromatase inhibitor anastrozole in patients whose disease has relapsed or progressed on prior endocrine therapy. Surprisingly, however, in a phase III trial versus tamoxifen for the first-line therapy of advanced breast cancer fulvestrant did not attain the requirements for equivalence to tamoxifen, and in terms of time-to-treatment failure was inferior (5.9 versus 7.8 months for fulvestrant and tamoxifen, respectively; P=0.029). Future clinical studies will evaluate fulvestrant in the neoadjuvant setting together with its optimal sequencing in relation to tamoxifen and other endocrine therapies in advanced disease.

Novel agents for the prevention of breast cancer: targeting transcription factors and signal transduction pathways

Transformation of breast cells occurs through loss or mutation of tumor suppressor genes, or activation or amplification of oncogenes, leading to deregulation of signal transduction pathways, abnormal amplification of growth signals, and aberrant expression of genes that ultimately transform the cells into invasive cancer. The goal of cancer preventive therapy, or "chemoprevention," is to eliminate premalignant cells or to block the progression of normal cells into cancer. Multiple alterations in signal pathways and transcription factors are observed in mammary gland tumorigenesis. In particular, estrogen receptor (ER) deregulation plays a critical role in breast cancer development and progress, and targeting ER with selective ER modulators (SERMs) has achieved significant reduction of breast cancer incidence in women at high risk for breast cancer. However, not all breast cancer is prevented by SERMs, because 30-40% of the tumors are ER-negative. Other receptors for retinoids, vitamin D analogs and peroxisome proliferator-activiator, along with transcription factors such as AP-1, NF-kappaB, and STATs (signal transducers and activators of transcription) affect breast tumorigenesis. This is also true for the signal transduction pathways, for example cyclooxygenase 2 (Cox-2), HER2/neu, mitogen-activated protein kinase (MAPK), and PI3K/Akt. Therefore, proteins in pathways that are altered during the process of mammary tumorigenesis may be promising targets of future chemopreventive drugs. Many newly-developed synthetic or natural compounds/agents are now under testing in preclinical studies and clinical trials. Receptor selective retinoids, receptor tyrosine kinase inhibitors (TKIs), SERMs, Cox-2 inhibitors, and others are some of the promising novel agents for the prevention of breast cancer. The chemopreventive activity of these agents and other novel signal transduction inhibitors are discussed in this chapter.

A mechanism of drug resistance to tamoxifen in breast cancer

Drug resistance to tamoxifen (Tam) is a significant clinical problem but the mechanism through which this occurs remains elusive. We have developed a number of xenograft models of Tam-stimulated growth that model breast cancer progression using estrogen receptor positive MCF-7 or T47D breast cancer cells. When estrogen-stimulated T47D:E2 tumors are treated long term with Tam, Tam-stimulated tumors develop (T47D:Tam) that are stimulated by both estrogen and Tam. When HER-2/neu status is determined, it is clear that the T47D:Tam tumors express significantly higher levels of HER-2/neu protein by immunohistochemistry and mRNA as measured by real-time RT-PCR. The T47D:Tam tumors also express higher levels of estrogen receptor and progesterone receptor protein than their estrogen-stimulated T47D:E2 counterparts. We compared out results to the MCF-7 model of Tam-stimulated growth. The MCF-7:Tam ST (estrogen- and Tam-stimulated) and MCF-7:Tam LT (estrogen-inhibited, Tam-stimulated) were bilaterally transplanted to account for any mouse to mouse variation and characteristic growth patterns were observed. TUNEL staining was performed on MCF-7:Tam LT treated with either estrogen or Tam and it was concluded that estrogen-inhibited tumor growth was a result of increased apoptosis. Three phases of tumor progression are described that involve increases in HER-2/neu expression, de-regulation of estrogen receptor expression and increases in apoptosis which in concert determine the phenotype of drug resistance to Tam.

Regulation of estrogen target genes and growth by selective estrogen-receptor modulators in endometrial cancer cells

OBJECTIVE

Tamoxifen has mixed agonist/antagonist activities, leading to tissue-specific estrogen-like actions and endometrial cancer. The purpose of this study was to evaluate the effects of antiestrogens on the growth of estrogen receptor (ER)-positive ECC-1 endometrial cancer cells in vitro and in vivo.

METHODS

We performed growth studies and luciferase assays using ERE-tK and AP-1 reporters. ERalpha protein expression was measured by Western blot after antiestrogen treatments. We investigated the actions of antiestrogens on the transcription of the pS2 gene in situ measured by Northern blot and the actions of antiestrogens on the VEGF protein secreted by ELISA. ERalpha, ERbeta, EGFR, and HER2/neu mRNAs were determined by RT-PCR. Last, ECC-1 tumors were developed by inoculation of cells into ovariectomized athymic mice and treated with estradiol (E2), tamoxifen, raloxifene, and a combination.

RESULTS

E2 induced cell proliferation while antiestrogens did not. E2 and raloxifene down regulated ERalpha protein; in contrast, 4OHT did not. ICI182,780 completely degraded the receptor. ECC-1 cells express ERbeta at insignificant levels. Luciferase assays did not show any induction in ERE- nor AP-1-mediated transcription by antiestrogens. E2 caused a concentration-dependent increase in pS2 mRNA but antiestrogens did not. E2 increased VEGF expression in a dose-dependent manner and antiestrogens blocked E2 action. E2 down regulated HER2/neu while 4OHT and raloxifene did not change HER2/neu levels compared to control. In addition, EGFR mRNA was down regulated by E2 but raloxifene did not change it. Tamoxifen and raloxifene did not promote tumor growth in vivo. However, raloxifene (1.5 mg daily) only partially blocked E2-stimulated growth.

CONCLUSION

Tamoxifen and raloxifene are antiproliferative agents and antiestrogens in ECC-1 endometrial cells in vitro and in vivo. The observation that selective estrogen-receptor modulators do not down regulate EGFR and HER2/neu mRNA may provide a potential role for these oncogenes in the development of raloxifene- or tamoxifen-stimulated endometrial cancer. The ECC-1 cell line could provide important new clues about the evolution of drug resistance to tamoxifen and raloxifene.

The search for the ideal SERM

Selective oestrogen receptor modulators (SERMs) are compounds that interact with the oestrogen receptor and have tissue-specific effects distinct from those of oestradiol, acting as an oestrogen agonist in some tissues and as an antagonist in others. The development of SERMs that selectively interact with specific receptors, coactivators and corepressors in different organ systems offers the possibility of improving the risk:benefit profile relative to hormone replacement therapy. Tamoxifen is a SERM that acts as an oestrogen antagonist in breast tissue and is currently being used for the treatment and prevention of breast cancer. Tamoxifen also exhibits oestrogen-agonistic properties in the endometrium and increases the risk of endometrial cancer. Oestrogen and another SERM, raloxifene, have been shown to prevent osteoporosis. The effects of oestrogens on cognitive functions are currently being investigated. Recent data reveal the lack of secondary prevention of coronary heart disease with oestrogen. Oestrogen has been used to treat menopausal symptoms, whereas the SERMs have been shown to induce hot flushes. Current research is focused on producing the ideal SERM, which would have benefits over existing SERMs in terms of preventing cancer, cardiovascular disease, osteoporosis and menopausal symptoms, improving cognitive functions, and have a significantly better toxicity profile in terms of endometrial cancer and thromboembolic events.

Comparison of the effects of EM-652 (SCH57068), tamoxifen, toremifene, droloxifene, idoxifene, GW-5638 and raloxifene on the growth of human ZR-75-1 breast tumors in nude mice

EM-652 exerts pure antiestrogenic activity in the mammary gland and endometrium, while tamoxifen, the antiestrogen most widely used for the treatment of breast cancer, exerts mixed antiestrogenic-estrogenic activity in these tissues. Our objective was to compare the agonistic and antagonistic effects of EM-652 with tamoxifen and 5 other antiestrogens on the growth of ZR-75-1 human breast xenografts in ovariectomized nude mice. During the 23 weeks of treatment at a daily oral dose of 50 microg, EM-652 was the only compound that decreased tumor size relative to pretreatment values, whereas the 6 other antiestrogens only decreased to various extents the progression rate stimulated by estrone. Under estrone stimulation, all groups of animals had more than 60% of their tumors in the progression category except for the EM-652-treated group, where only 7% of the tumors progressed. In the absence of estrone stimulation, progression was seen in 60%, 33%, 21% and 12% of tumors in the tamoxifen-, idoxifene-, toremifene- and raloxifene-treated groups, respectively, while only 4% of tumors progressed in the EM-652-treated group. The agonistic and antagonistic actions of each antiestrogen were also measured on endometrial epithelial cell thickness. Our present findings indicate that EM-652, in addition to being the most potent antiestrogen on human breast tumor growth, has no agonistic effect in breast and endometrial tissues. Since previous data have shown benefits of EM-652 on bone density and lipid profile, this compound could be an ideal candidate for chemoprevention of breast and uterine cancers, while protecting against osteoporosis and cardiovascular disease.

Effects of raloxifene after tamoxifen on breast and endometrial tumor growth in athymic mice

BACKGROUND

In patients with early-stage breast cancer, 5 years of treatment with the selective estrogen receptor modulator (SERM) tamoxifen reduces breast cancer recurrence and mortality, whereas more than 5 years of tamoxifen does not further reduce breast cancer recurrence and doubles the risk of endometrial cancer. We evaluated the effects on tumor growth of raloxifene, another SERM, after tamoxifen treatment in mouse models of breast and endometrial cancers.

METHODS

Athymic, ovariectomized mice were bitransplanted with tumors derived from human breast cancer and endometrial cancer cells that either were tamoxifen-naive or had been exposed to tamoxifen for short (6 months) or long (>5 years) terms. The effects of raloxifene (two dose levels) and tamoxifen on tumor growth in the presence and absence of low-dose estrogen were evaluated. All statistical tests were two-sided.

RESULTS

Raloxifene was less effective than tamoxifen in blocking the stimulatory effects of low-dose estrogen on the growth of tamoxifen-naive breast (P<.001) and endometrial (P =.001) tumors. Raloxifene and tamoxifen had similar inhibitory effects on the growth of short-term tamoxifen-exposed breast tumors. Raloxifene and tamoxifen had similar stimulatory effects on the growth of breast and endometrial tumors that had been exposed to at least 5 years of tamoxifen. However, neither drug blocked the stimulatory effects of estrogen on the growth of these tumors. Raloxifene was less effective than tamoxifen (P<.001) in blocking the stimulatory effects of estrogen on endometrial tumors that had been exposed to tamoxifen in the past.

CONCLUSIONS

Raloxifene and tamoxifen had similar effects on these mouse models of tamoxifen-naive and tamoxifen-resistant breast and endometrial cancer. Treatment with raloxifene following 5 years of adjuvant tamoxifen may not further decrease breast cancer recurrence and may increase endometrial cancer incidence.

Role of antiestrogens and aromatase inhibitors in breast cancer treatment

This review explores the recent experience with, and the basis for, the use of selective estrogen receptor modulators to treat and prevent breast cancer. As new agents are unveiled, they will continue to be tested against tamoxifen. A number of new selective estrogen receptor modulators are in clinical development in an attempt to decrease the unwanted effects of tamoxifen. Raloxifene holds the promise of treating osteoporosis with the beneficial side effect of breast cancer prevention. Additionally, two different classes of hormonal agents, the aromatase inhibitors and estrogen receptor down-regulators, which have no estrogen-like properties at any site, appear to be promising new treatments for advanced breast cancer.

Use of SERMs for the adjuvant therapy of early-stage breast cancer

Tamoxifen was the first in a class of drugs now commonly referred to as selective estrogen receptor modulators or SERMs. SERMs exhibit tissue-specific estrogenic agonist/antagonist activity through their ability to bind to the estrogen receptor alpha (ER) protein and interact with coregulatory proteins, thereby modulating transcription of estrogen target genes. Since its first approval by the United States Food and Drug Administration (FDA) in 1977, tamoxifen has been found to (a) lower the risk of recurrence and death for women with early-stage hormone receptor-positive breast cancer, irrespective of menopausal and node status or use of adjuvant chemotherapy; (b) reduce the risk of invasive breast cancer following breast conservation in women with ductal carcinoma in situ (DCIS); and (c) reduce the risk of breast cancer in high-risk women. Toremifene is the only other SERM approved by the FDA for breast cancer treatment. However, it offers no clear clinical advantage over tamoxifen in the adjuvant or metastatic settings. Several other SERMs are in various phases of clinical development. In addition, strategies to combine SERMs with other endocrine therapy like ovarian suppression or aromatase inhibitors are active areas of investigations. At present, SERMs are recognized as the first targeted and relatively nontoxic medical therapy for women with high-risk or steroid hormone receptor-positive breast cancer.

Synthesis and reactivity of potential toxic metabolites of tamoxifen analogues: droloxifene and toremifene o-quinones

Tamoxifen remains the endocrine therapy of choice in the treatment of all stages of hormone-dependent breast cancer. However, tamoxifen has been shown to increase the risk of endometrial cancer which has stimulated research for new effective antiestrogens, such as droloxifene and toremifene. In this study, the potential for these compounds to cause cytotoxic effects was investigated. One potential cytotoxic mechanism could involve metabolism of droloxifene and toremifene to catechols, followed by oxidation to reactive o-quinones. Another cytotoxic pathway could involve the oxidation of 4-hydroxytoremifene to an electrophilic quinone methide. Comparison of the amounts of GSH conjugates formed from 4-hydroxytamoxifen, droloxifene, and 4-hydroxytoremifene suggested that 4-hydroxytoremifene is more effective at formation of a quinone methide. However, all three substrates formed similar amounts of o-quinones. Both the tamoxifen-o-quinone and toremifene-o-quinone reacted with deoxynucleosides to give corresponding adducts. However, the toremifene-o-quinone was shown to be considerably more reactive than the tamoxifen-o-quinone in terms of both kinetic data as well as the yield and type of deoxynucleoside adducts formed. Since thymidine formed the most abundant adducts with the toremifene-o-quinone, sufficient material was obtained for characterization by (1)H NMR, COSY-NMR, DEPT-NMR, and tandem mass spectrometry. Cytotoxicity studies with tamoxifen, droloxifene, 4-hydroxytamoxifen, 4-hydroxytoremifene, and their catechol metabolites were carried out in the human breast cancer cell lines S30 and MDA-MB-231. All of the metabolites tested showed cytotoxic effects that were similar to the parent antiestrogens which suggests that o-quinone formation from tamoxifen, droloxifene, and 4-hydroxytoremifene is unlikely to contribute to their cytotoxicity. However, the fact that the o-quinones formed adducts with deoxynucleosides in vitro implies that the o-quinone pathway might contribute to the genotoxicity of the antiestrogens in vivo.

What would be the properties of an ideal SERM?

Selective estrogen receptor modulators (SERMs) are drugs that bind to the estrogen receptor (ER); in some tissues they act like estrogen (agonists), while in other tissues they oppose the action of estrogen (antagonists). The SERM tamoxifen acts as an estrogen antagonist in the breast in that it prevents and treats breast cancer, but it acts as an estrogen agonist in the endometrium, where it can induce cancer. Estrogen, and to a lesser extent SERMs, are effective in preventing and treating osteoporosis. Contrary to the prevalent hypothesis that estrogen provides benefit to women with regard to secondary prevention of coronary heart disease (CHD), randomized clinical trials have demonstrated that estrogen is associated with an increased risk of CHD in this population of women. Conflicting results have been reported on the effect of estrogens on cognitive function. The latest and largest randomized clinical trials have demonstrated a beneficial role in short-term memory in nondemented women, in contrast to the absence of such benefit in improving symptoms in women with Alzheimer's disease. Although estrogens have been used successfully to treat some menopausal symptoms such as hot flashes, the SERMs tamoxifen and raloxifene actually induce or increase hot flashes. Data on the beneficial and adverse effects of estrogen and SERMs are reported along with an elaboration of the constellation of properties that would characterize an ideal SERM working through the ER.

Anti-oestrogenic drugs and endometrial cancers

Tamoxifen is one of the most effective drugs to be used in the treatment of women with breast cancer and as a chemopreventive agent in women 'at risk' from this disease. Tamoxifen can be regarded as a paradigm for a new range of selective oestrogen receptor modulators that include toremifene, used in the treatment of metastatic breast cancer and raloxifene, presently approved for use in postmenopausal women for the treatment of osteoporosis. Tamoxifen treatment of women leads to a small increase in the incidence of endometrial cancers. It is important to understand the mechanism for this side effect in order to predict the likely human risk for other drugs of this class. Two such mechanisms have been proposed: (1) conversion of the drug to electrophilic metabolites that damage cellular DNA; and (2) an oestrogen agonist action on the uterus, promoting endogenous lesions. In rats, long-term tamoxifen treatment results in liver cancer via a genotoxic mechanism. However, it seems most likely that, in women treated with tamoxifen, endometrial cancer is related to an oestrogen agonist effect of this drug, promoting uterine cell proliferation.

Estrogens as endogenous genotoxic agents--DNA adducts and mutations

Estrogens induce tumors in laboratory animals and have been associated with breast and uterine cancers in humans. In relation to the role of estrogens in the induction of cancer, we examine formation of DNA adducts by reactive electrophilic estrogen metabolites, formation of reactive oxygen species by estrogens and the resulting indirect DNA damage by these oxidants, and, finally, genomic and gene mutations induced by estrogens. Quinone intermediates derived by oxidation of the catechol estrogens 4-hydroxyestradiol or 4-hydroxyestrone may react with purine bases of DNA to form depurinating adducts that generate highly mutagenic apurinic sites. In contrast, quinones of 2-hydroxylated estrogens produce less harmful, stable DNA adducts. The catechol estrogen metabolites may also generate potentially mutagenic oxygen radicals by metabolic redox cycling or other mechanisms. Several types of indirect DNA damage are caused by estrogen-induced oxidants, such as oxidized DNA bases, DNA strand breakage, and adduct formation by reactive aldehydes derived from lipid hydroperoxides. Estradiol and the synthetic estrogen diethylstilbestrol also induce numerical and structural chromosomal aberrations and several types of gene mutations in cells in culture and in vivo. In conclusion, estrogens, including the natural hormones estradiol and estrone, must be considered genotoxic carcinogens on the basis of the evidence outlined in this chapter.

Molecular mechanism of action at estrogen receptor alpha of a new clinically relevant antiestrogen (GW7604) related to tamoxifen

Tamoxifen is the endocrine treatment of choice for all stages of estrogen receptor (ER)-positive breast cancer, and it is the first drug approved to reduce the incidence of breast cancer in high-risk women. Unfortunately, tamoxifen also possesses some estrogen-like effects in the uterus that cause a modest increase in the risk of endometrial cancer. GW5638 is a tamoxifen derivative with a novel carboxylic acid side chain with no uterotropic activity in the rat (Willson et al., J Med Chem, 1994, 37:1550-1552). We have compared and contrasted the actions of 4-hydroxytamoxifen (4-OHT, the active metabolite of tamoxifen) with GW7604 [the presumed metabolite of GW5638 in breast (MCF-7) and endometrial (ECC-1) cell lines in vitro]. GW7604 did not cause the growth of ECC-1 cells at any concentration (10(-11)-10(-6) M), but 4-OHT was weakly estrogen-like at low concentrations (10(-11)-10(-10) M). Compounds (10(-7) M) blocked the growth promoting action of estradiol (10(-10) M) in both ECC-1 and MCF-7 cells. Western blotting was used to show that GW7604 and raloxifene did not affect ER levels significantly, compared with controls, in MCF-7 cells; whereas the pure antiestrogen ICI182,780 decreased ER levels (P < 0.05). An assay system was used that can classify compounds into tamoxifen-like, raloxifene-like, or pure antiestrogens. The assay depends on the activation of the transforming growth factor alpha (TGFalpha) gene in situ by wild-type or D351Y mutant ER stably transfected into MDA-MB-231 cells (MacGregor-Schafer et al., Cancer Res, 1999, 59:4308-4313). GW7604 inhibited both estradiol (10(-9) M) and 4-OHT (10(-8), 10(-7) M) induction of TGFalpha in a concentration related manner (10(-9)-10(-6) M). GW7604 and raloxifene stimulated TGFalpha with the D351Y ER. In contrast, ICI 182,780 (10(-6) M) did not initiate TGFalpha and blocked the induction of TGFalpha with GW7604, raloxifene, and 4-OHT in D351Y-transfected cells. Using computer-assisted molecular models of ER complexes, we found that the antiestrogenic side chain of 4-OHT weakly interacted with the surface amino acid 351 (aspartate), but the carboxylic acid of GW7604 caused a strong repulsion of aspartate 351. We propose that GW7604 is less estrogen-like than 4-OHT, because it disrupts the surface charge around aa351 required for coactivator docking in the 4-OHT:ER complex. This charge is restored in the D351Y ER, thus converting GW7604 from an antiestrogen to an estrogen-like molecule.

Selective estrogen receptor modulators: structure, function, and clinical use

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.

Effects of angiogenesis inhibitor TNP-470 on tamoxifen-stimulated MCF-7 breast tumors in nude mice

The antitumor effect of TNP-470, antiangiogenic drug, was analyzed in the tamoxifen-stimulated MT-2 tumors inoculated in the athymic nude mice. TNP-470 was injected subcutaneously at a dose of 30 mg/kg body weight twice a week to mice which were randomized into three treatments: control (sham treatment), tamoxifen alone, and tamoxifen plus TNP-470. TNP-470 inhibited the growth of the tamoxifen-stimulated MT2 tumors without any major side effects or significant weight loss compared with tamoxifen-treated mice alone. The mean tumor area of the mice treated with tamoxifen plus TNP-470 was reduced 50% to those treated with tamoxifen alone. TNP-470 was shown to inhibit tumor neovascularization and to increase incidence of apoptosis in tumor cells. TNP-470 did not affect tamoxifen metabolism of the mice. In conclusion, TNP-470 could be evaluated clinically in patients with tamoxifen failure.

Chemoprevention of breast cancer

Despite a recent trend toward improvement in the U.S. breast cancer mortality rate, breast cancer incidence (182,800 new cases anticipated in 2000) and mortality figures (over 40,800 anticipated deaths) remain the highest and second highest, respectively, of all cancers in U.S. women. In 1998, the selective-estrogen-receptor-modulator (SERM) tamoxifen achieved positive results in the Breast Cancer Prevention Trial (BCPT), leading to the Food and Drug Administration (FDA) approval of tamoxifen for risk reduction in women at high risk of breast cancer (the historic first FDA approval of a cancer preventive agent). This brought about a paradigm shift in new approaches for controlling breast cancer toward pharmacologic preventive regimens, called chemoprevention. This paper presents a comprehensive clinical review of breast cancer prevention study, highlighting issues of the extensive study of tamoxifen. These issues include the record of primary tamoxifen results in several breast-cancer risk-reduction settings (primary, adjuvant, and ductal carcinoma in situ [DCIS]); critical secondary BCPT risk-benefit findings (including quality of life issues) and their effects on counseling patients on use of tamoxifen for prevention; ethic minorities; optimal tamoxifen dose/duration; and potential impact on mortality and other issues involved with potential net benefit to society. Other breast-cancer chemoprevention issues reviewed here include women at high genetic risk (especially BRCA1 mutation carriers); raloxifene in breast cancer prevention; other SERMs; SERM resistance; and new agents and combinations currently in development. Very recent developments involving PPAR-gamma ligands, COX-2 inhibitors, and RXR-ligands are discussed in the section on new drug development.

Antiestrogens--tamoxifen, SERMs and beyond

Estrogens play a central role in reproductive physiology. The cellular effects of estrogens are mediated by binding to nuclear receptors (ER) which activate transcription of genes involved in cellular growth control. At least two such receptors, designated ERalpha and ERbeta, mediate these effects in conjunction with a number of coactivators. These receptors can directly interact with other members of the steroid receptor superfamily. A complex cross-talk exists between the estrogen-signaling pathways and the downstream signaling events initiated by growth factors, such as epidermal growth factor and insulin-like growth factors. Estrogens are also a causative factor in the pathogenesis of a variety of neoplastic and non-neoplastic diseases, including breast cancer, endometrial cancer, endometriosis, and uterine fibroids, among others. Antiestrogens, such as tamoxifen, are widely used for the treatment of breast cancer. Tamoxifen produces objective tumor shrinkage in advanced breast cancer, reduces the risk of relapse in women treated for invasive breast cancer, and prevents breast cancer in high-risk women. Although, initially developed as an antiestrogen, tamoxifen can also prevent postmenopausal osteoporosis as well as reduce cholesterol, due to its estrogen-agonist effects. Its estrogen-agonist activity, however, can lead to significant side-effects such as endometrial cancer and thromboembolic phenomena. This has led to the concept of "ideal" selective estrogen receptor modulators (SERMs), drugs that would have the desired, tissue selective, estrogen-agonist or -antagonist effects. Raloxifene is a SERM which has the desirable mixed agonist/antagonist effects of tamoxifen but does not cause uterine stimulation. "Pure" antiestrogens may provide very potent estrogen-antagonist drugs, but are likely to be devoid of beneficial effects on bone and lipids. Future drug development efforts should focus on developing superior SERMs that have a greater efficacy against ER-positive tumors and do not cause hot flashes or thromboembolism, and explore combination strategies to simultaneously target hormone-dependent as well as hormone-independent breast cancer.

The evolving role of specific estrogen receptor modulators (SERMs)

Estrogens are the most effective therapy for women with postmenopausal problems. However, relatively few women use estrogen and then often for a limited time because of the fear of its carcinogenic effects on the uterus and breast; in addition, estrogen is not advised for women who have had breast cancer. Selective estrogen receptor modulators (SERMs) are agents with antagonist action on the uterus and breast and agonist action on the bones, cardiovascular system, and brain. Unlike estrogens, however, existing SERMs do not help alleviate the vasomotor and urogenital problems associated with menopause. A comprehensive review of the literature published from January 1995 to June 1999 was conducted. Reports were identified using Medline and Cancer Lit. The effect of menopausal problems on the health of women and the socioeconomic effects of menopause are discussed. All currently available and investigational SERMs are reviewed and discussed, including their mechanism of action, metabolism, dose scheduling, antitumor activity, and potential role in maintaining the health of menopausal women and in preventing breast cancer.

Uterotrophic effects of tamoxifen, toremifene, and raloxifene do not predict endometrial cell proliferation in the ovariectomized CD1 mouse

The uterotrophic responses of ovariectomized CD1 mice to tamoxifen, toremifene, and raloxifene have been compared to 17beta-estradiol after a treatment period of 72 h. Uterine and vaginal weight, luminal epithelial thickening, and 5-bromodeoxyuridine (BrdU) labeling index in the endometrial stroma were examined. All three pharmaceuticals, as well as 17beta-estradiol, produced increases in the classic estrogen-dependent variables of uterine and vaginal weights after the 3-day treatment period. Tamoxifen, toremifene, raloxifene, and estradiol all increased luminal epithelial thickness, and increased the BrdU labeling index in the endometrial stroma of the uterus. Although the dose response for the uterotrophic effect and the vaginal weight increases for toremifene differed from tamoxifen and raloxifene, in that there was no dose at which these effects were maximal, the stimulation of BrdU labeling index in the endometrial stroma was dose dependent and very similar for all three, at the clinically relevant doses. Treatment-related hypertrophic effects were estimated by examination of the nuclear profile density in the endometrial stroma. Estradiol and tamoxifen caused a greater hypertrophic effect than toremifene and raloxifene, indicating that factors other than an increase in cell number contribute to the overall uterotrophic effect. This demonstrates that the use of uterine weight to estimate the relative estrogenicity of drugs could give a misleading impression of the response of the uterus to estrogen agonists. Variables, such as increased DNA replication, which may be more important to a subsequent potential carcinogenic process in the uterus, for a particular drug, requires separate evaluation.

New Hormonal Therapies for Breast Cancer

BACKGROUND: There has been an explosion in the development of hormonal therapies for the treatment of breast cancer. Several new agents have been approved for the treatment of breast cancer in the metastatic setting, and trials are ongoing in the adjuvant and prevention setting to improve hormonal therapy for the prevention and treatment of breast cancer. METHODS: The literature on new hormonal therapies for the treatment of breast cancer is reviewed, with an emphasis on newer agents. RESULTS: Two antiestrogens are now approved in the United States for the treatment of metastatic breast cancer. Other antiestrogens have activity in metastatic breast cancer as well as in osteoporosis. Newer pure antiestrogens may overcome resistance to tamoxifen. Several aromatase inhibitors are available for the treatment of metastatic breast cancer. CONCLUSIONS: Many hormonal agents are now available for both adjuvant and advanced disease settings. Developments will depend on clarifying mechanisms of resistance to antiestrogens and identifying new classes of agents that lack cross-resistance to standard therapy.

Hormonal therapy for breast cancer. An update

The use of endocrine manipulation for the treatment of breast cancer has been available for 100 years, but in recent years the number of therapeutic options available to patients has increased dramatically. This article considers new developments in the use of hormonal agents for the treatment and prevention of breast cancer.