Endocrine Reviews 20 (3): 253-278
Copyright © 1999 by The Endocrine Society
Tamoxifen, Raloxifene, and the Prevention of Breast Cancer1
V. Craig Jordan and
Monica Morrow
Departments of Molecular Pharmacology, Biological Chemistry
(V.C.J.), and Surgery (M.M.), Robert H. Lurie Comprehensive Cancer
Center, Northwestern University Medical School, Chicago, Illinois
60611
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Abstract
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- I. Introduction
- II. Lacassagnes Prevention Principle: A Target and an Estrogen Antagonist
- III. Tamoxifen as an Antitumor Agent
- A. ER status and the duration of tamoxifen
- B. Contralateral breast cancer
- C. Endometrial cancer
- D. Conclusions
- IV. Selective Estrogen Receptor Modulation
- A. Antiestrogen activity at the ER
- B. Coactivators for ER
- C. Alternate response elements on DNA
- D. An alternate ER-ERß
- V. Biological Basis for Tamoxifen as a Breast Cancer Preventive
- A. Animal models
- B. Bones
- C. Lipids
- D. Uterus
- VI. Risk Factors for Breast Cancer
- A. Interactions among risk factors
- B. Identification of candidates for chemoprevention
- VII. Prevention of Breast Cancer with Tamoxifen
- A. Royal Marsden Pilot Study
- B. NSABP/NCI Study
- C. Italian study
- D. Conclusions
- VIII. Biological Basis for Raloxifene as a Breast Cancer Preventive
- A. Antitumor actions
- B. Bones
- C. Lipids
- D. Uterus
- IX. Study of Tamoxifen And Raloxifene (STAR)
- X. The Future of Prevention
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I. Introduction
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IN 1896, George Beatson demonstrated
that the removal of the ovaries from premenopausal women with
metastatic breast cancer could, in some cases, cause regression of the
disease and improve the prognosis of the patient (1). However, by 1900
Stanley Boyd had established that only one in three patients would
obtain improvement for about 1 yr (2). Despite this disappointment, a
link was established between an ovarian factor and the growth of some
breast cancers. This observation was to become the foundation of modern
clinical practice and the rationale for the use of antiestrogens to
treat breast cancer (3, 4). At the turn of the century, studies were
being conducted in the laboratory to complement the clinical effort.
Inbred strains of mice were being established for medical research, and
it was found that certain strains of mice developed a high incidence of
mammary tumors. Lathrop and Loeb (5) reported that an early ovariectomy
could prevent the spontaneous development of tumors but it was not
until Allen and Doisey (6) identified "estrus stimulating
principle" that ovarian hormones could be linked to the development
of breast cancer. By 1936, Professor Antoine Lacassagne, again working
with high-incidence strains of mice, suggested that if breast cancer
was caused by a special hereditary sensitivity to estrogen, then the
disease could be prevented by developing a therapeutic antagonist to
estrogen action in the breast (7). However, there were no therapeutic
antagonists of estrogen at that time, nor was there a target to design
drug molecules. Nevertheless, exciting developments in the discovery of
nonsteroidal estrogens would establish the structural basis of carrier
molecules, which resulted in the design of the two drugs, tamoxifen and
raloxifene (Fig. 1
), both originally
described as antiestrogens and used today in a clinical trial to
prevent breast cancer in high-risk women (see Section IX).

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Figure 1. The discovery of MER-25, and the knowledge that a
strategically placed alkylaminoethoxy side chain confers antiestrogenic
properties, was important to develop the antiestrogens tamoxifen and
raloxifene from the known nonsteroidal estrogens, triphenylethylene and
diethylstilbestrol. Tamoxifen is used for the endocrine treatment of
all stages of breast cancer and is available for the reduction of
breast cancer incidence in high-risk women. Raloxifene is used for the
prevention of osteoporosis in postmenopausal women, but because a
preliminary evaluation shows a reduction in the risk of breast cancer,
raloxifene is to be evaluated for the prevention of breast cancer in
high-risk postmenopausal women. The clinical trial STAR started to
recruit the 22,000 volunteers in 1999.
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Estrogen action in the 1930s was assayed using ovariectomized mice as
originally described by Allen and Doisy (6). Using this technique,
parallel research ventures resulted in the discovery of the
triphenylethylene-based estrogens (8, 9, 10) and the stilbene-based
estrogens (Fig. 1
). The triphenylethylenes are long acting and are
stored in body fat (11, 12, 13, 14), whereas the hydroxystilbene derivatives
are short acting (9, 15, 16), primarily because of rapid-phase II
metabolism after absorption. However, Dodds and associates (17, 18)
described an extremely potent compound, diethylstilbestrol (Fig. 1
),
that was widely used in gynecology and also subsequently used, at high
doses, as a treatment for advanced breast cancer in postmenopausal
women (19, 20).
In the 1950s and 1960s it became clear that adrenalectomy, with
glucocorticoid support, could also improve the prognosis of some
postmenopausal women with advanced breast cancer (21). In fact, about
one third of the women responded, i.e., about the same
proportion as premenopausal women after oophorectomy. The reason for
the apparently arbitrary responses, however, would not become clear
until the discovery of the estrogen receptor (ER) by Jensen and
Jacobson (22) and the subsequent application of this knowledge to
predict the hormone responsiveness of a patients tumor to endocrine
ablation (23). This was an extremely important finding because it
prevented those patients who had an ER-negative tumor from having
additional major surgery with little hope of a response. Only patients
whose tumors had high levels of ER were likely to respond to endocrine
ablative surgery (24). It is important, however, to stress that in the
early 1970s, there was no significant clinical experience available
with the class of drugs called antiestrogens, and no large clinical
studies had linked the efficacy of antiestrogens with the
presence or absence of the ER. Several antiestrogens had been tested in
small clinical studies, but tamoxifen, the first clinically useful
antiestrogen for the treatment of advanced breast cancer in
postmenopausal women, was not approved by the Food and Drug
Administration in the United States until 1977 (25).
Although antiestrogens are important therapeutic agents today, 40 yr
ago there was very little interest in treating breast cancer
with new hormonal drugs, and most of the research in endocrinology was
focused on an understanding of reproduction. The discovery of the
nonsteroidal antiestrogens was serendipitous and resulted from an
interest in contraception in the 1950s. The first nonsteroidal
antiestrogen to be reported in the literature, MER25 (Fig. 1
), was
described by Lerner and co-workers in 1958 (26) as an agent that had no
other hormonal or antihormonal properties in any species tested. In
fact, it was a blocking drug for estrogen action with almost no
estrogenic properties. The drug failed in clinical trial because large
doses were required (MER25 has low potency), which caused serious
central nervous system side effects (27). On one hand this was
disappointing but, it must be stressed, that a pure antiestrogen such
as MER25 would ultimately have been catastrophic as an agent to prevent
breast cancer. Drug discovery switched to the triphenylethylene-based
compounds that resulted first in clomiphene and then tamoxifen (25).
Subsequently, drug discovery concentrated on compounds with a high
affinity for ER (25). Only a research focus on cancer in the 1970s
facilitated tamoxifens development as a breast cancer therapy for all
stages of the disease (25).
The critical property of the so called "antiestrogens," which
permitted their subsequent development as long-term preventives for
breast cancer, was that they are antiestrogens at some sites, like the
breast, but had estrogen-like properties at other sites to maintain
bone density and lower circulating cholesterol (28, 29, 30, 31, 32, 33, 34, 35). The unusual
target site-specific action as an estrogen or as an antiestrogen was
true for both tamoxifen and raloxifene (29, 30). Lacassagnes
prediction (7) of developing an antagonist to estrogen action to
prevent breast cancer in healthy women would not have occurred if the
available drugs had increased the risk of osteoporosis and coronary
heart disease.
The goal of this review is to provide an up-to-date analysis of the
current status of efforts to prevent breast cancer in women by the
strategic use of antiestrogens. One aim of our review is to identify
the principles, established in the laboratory, that have, through the
clinical trial process, proven to be valid in patients with breast
cancer or women at risk for breast cancer. The review will also provide
the scientific basis for the ongoing trial called Study of Tamoxifen
And Raloxifene (STAR). This trial is examining the worth of raloxifene,
a drug approved for the prevention of osteoporosis, to prevent breast
cancer in postmenopausal women with elevated risk factors. The
biological basis for consideration of the antiestrogen raloxifene to be
used as a breast cancer preventive is described in Section
VIII. In the interests of space it is not possible to review the
antiestrogen literature exhaustively, but we intend to provide
sufficient background to link laboratory research with clinical
results.
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II. Lacassagnes Prevention Principle: A Target and an Estrogen
Antagonist
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In 1962, Jensen and Jacobson (22) demonstrated that
[3H]estradiol bound to and was retained by estrogen
target tissue, e.g., uterus, vagina, and pituitary gland, in
the immature female rat. By contrast, tissues that did not respond to
estrogen did not retain [3H]estradiol. Jensen proposed
that an ER must be present in estrogen target tissues to capture
circulating steroids and initiate the cascade of biochemical events
associated with estrogen action in that particular tissue. Gorski and
co-workers (36, 37) first identified the ER as an extractable protein
from rat uterus. Subsequently, the groups of Gorski et al.
(38) and Jensen et al. (39) independently proposed
subcellular models of estrogen action in target tissues. However,
Jensen and associates (23) took the process one step further by
proposing the clinical ER assay to predict hormone-responsive breast
cancer. Thus, a mechanistic link between estrogen action and the growth
of breast cancer was established.
The MCF-7 breast cancer cell line is ER positive (40, 41), and the
cells have found ubiquitous applications in cancer research
laboratories throughout the world (42). Most importantly, access to
these cells has resulted in a fundamental change in the understanding
of hormone action that has resulted in the discovery of the steroid
receptor superfamily of receptors (43, 44). Jensen and co-workers (45, 46) first developed monoclonal antibodies to ER derived from MCF-7
cells. The antibodies were used to establish that the ER was a nuclear
protein (47), and the technology of immunocytochemistry is now standard
for the determination of receptor status in breast biopsies (48, 49).
However, the application of monoclonal antibodies as probes to clone
and sequence the ER gene (50, 51, 52) is of fundamental significance for
the understanding of the ER as a nuclear transcription factor.
The ER is a nuclear protein (47, 53, 54), which, for convenience, is
subdivided into six functional domains (Fig. 2
) (55, 56). The E regions make up the
steroid-binding domain that undergoes a conformation change to lock
estradiol into its hydrophobic pocket (see Section IV.A).
Changes in conformation of the ER permits the binding of coactivators
to the activating function 1 and 2 regions (AF-1 and 2) (57, 58, 59) and
facilitates the interaction of the ER with DNA through the DNA-binding
domain (C region) (60, 61). The transcription unit is selectively
located as a homodimer in the promoter region of estrogen-responsive
genes to initiate the events associated with estrogen-stimulated cell
replication. Unfortunately, it is not possible to provide much more
than the basic concepts in hormone action because our article is
focused on progress in breast cancer chemoprevention. The topic of gene
regulation is a rapidly evolving story, so we strongly recommend that
interested readers consult recently published reviews that provide
further information (62, 63, 64).

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Figure 2. A subcellular model of estradiol (E2)
action in a target tissue. Estradiol diffuses into all cells but binds
to the ER specifically located in estrogen target tissues
(e.g., uterus, vagina, some breast cancers, etc.). The
steroid receptor complex undergoes a conformational change and
dimerizes before binding to an estrogen response element (ERE) in the
promotor region of an estrogen-responsive gene. A transcription unit is
formed by interaction with coactivator (CoA) molecules to initiate RNA
synthesis and ultimately the estrogen-stimulated cellular response.
Corepressor (CoR) modules are believed to prevent transcription by
exclusively interacting with antiestrogen ER complexes. The ER is
divided into six regions (AF). The DNA-binding domain (C) is
essential for the interaction of the ER with the ERE. The
ligand-binding domain (E) is the site of E2 binding and the
site of competitive binding by antiestrogens. The AA351 is identified
in the E region because it is the known site of an interaction with the
alkylaminoethoxy side chain of raloxifene. The activating function
(AF-1 and -2) regions are the areas of the ER that interact with
coactivators to form the transcription unit at an estrogen-responsive
gene.
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The second facet of Lacassagnes hypothesis is the requirement for an
antiestrogen to block estrogen action. Tamoxifen blocks the binding of
[3H]estradiol to the ER derived from rat uterus (65, 66, 67, 68)
or human tumor (69, 70). However, initial clinical studies with
tamoxifen were conducted exclusively on unselected populations of
postmenopausal women with advanced breast cancer (3, 4), and not until
1977 was it noted that tamoxifen was more likely to be effective in
ER-positive breast cancer (71). Tamoxifen is currently used as a
palliative therapy in the treatment of pre- and postmenopausal patients
with ER-positive advanced (Stage IV) breast cancer. By contrast, the
application of the concept of adjuvant therapy has revolutionized the
treatment of breast cancer. Systemic adjuvant therapy is used after
breast surgery to destroy undetected micrometastases around a womans
body.
Adjuvant studies with tamoxifen have proved to be successful in
increasing survival (72, 73, 74) but, perhaps most importantly, the
interaction between laboratory and clinical research endeavors has
ultimately elucidated both the principal mechanism of action of
tamoxifen as an antitumor agent in women and identified those women
most likely to benefit from adjuvant tamoxifen treatment. During the
past 12 yr there has been some confusion in the literature about
whether tamoxifen was active in ER-positive breast cancer exclusively
or whether ER-negative breast cancer could respond (75, 76, 77).
Additionally, there was controversy as to whether tamoxifen was
significantly active as an adjuvant in premenopausal women (78). The
reader is referred to recent reviews on the clinical investigation and
development of tamoxifen (79, 80, 81, 82, 83) for further information, but we will
summarize the latest findings of the world-wide randomized clinical
trials (84). It is important to appreciate that the general principles
derived from the use of tamoxifen as a therapy for breast cancer can be
used as a basis for consideration of tamoxifen as a estrogen antagonist
for the prevention of breast cancer.
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III. Tamoxifen as an Antitumor Agent
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The 1998 Oxford Overview Analysis (84) involved any randomized
trial that began before 1990. The analysis included 55 trials of
adjuvant tamoxifen vs. no tamoxifen before recurrence. The
study population comprised 37,000 women with node-positive and
node-negative breast cancer, thus comprising 87% of world evidence of
known randomized clinical trials. Of these women, fewer than 8,000 had
a very low or zero level of ER and 18,000 were classified as ER
positive. The ER status of the remaining nearly 12,000 women was
unknown, but based on the normal distribution of ER in random
populations, the authors estimated that two-thirds would be ER
positive.
This clinical trial data base (84) can now be used to answer the
questions raised over the past two decades by laboratory results and
hypotheses. In the 1970s three laboratory observations emerged that
merited evaluation in clinical trial: 1) tamoxifen blocks estrogen
binding to the ER, making patients with ER-positive disease more likely
to respond than those with ER-negative disease (85), 2) tamoxifen
prevents mammary cancer in rats (86, 87) so the drug could reduce the
incidence of primary breast cancer, and 3) long-term treatment was
better than short-term treatment to prevent rat mammary carcinogenesis;
therefore, longer adjuvant therapy with tamoxifen should be superior to
short-term adjuvant therapy (88, 89, 90), i.e., 5 yr of
tamoxifen should be superior to 1 yr of tamoxifen. By the late 1980s,
tamoxifen had been shown in the laboratory to block estrogen-stimulated
breast tumor growth but to encourage the growth of human endometrial
cancer implanted in the same athymic mouse (91, 92). The clinical
question therefore became "are patients, who are receiving long-term
adjuvant tamoxifen therapy, at risk for an increased incidence of
endometrial cancer?" (92).
The process of evaluating the impact of translational research is
important to establish what works and achieves clinical progress and
what does not. A clinical trial should not begin without a strong
hypothesis and the incorporation of relevant scientific results. For
convenience, the discussion in this section will be subdivided, but the
end points of duration of tamoxifen usage, menopausal status, and ER
status interact, making the size of a pharmacological effect subject to
change.
A. ER status and the duration of tamoxifen
The ER status of the patient is highly predictive of a treatment
response to long-term tamoxifen therapy. The treatment effect, based on
receptor status, is summarized in Table 1
. The recurrence reductions produced by
tamoxifen in ER-positive patients are all highly significant
(2P < 0.00001), and the trend between the different
durations of tamoxifen is also highly significant
(
2 = 45.5, 2P < 0.00001). By contrast,
the therapeutic effect of tamoxifen on ER-negative patients is minimal.
Additionally, the questions could be asked, "Does more ER give a
better response to tamoxifen?" and "Does an additional progesterone
receptor (PgR) assay help to improve the results with tamoxifen?" In
the trials of about 5 yr of tamoxifen treatment, the proportional
reductions of recurrence were 43 ± 5% and 60 ± 6% for
patients with below or above 100 fmol/mg cytosol ER protein. This
translated to a reduction in mortality of 23 ± 6% and 36 ±
7%, respectively. Clearly, one can conclude the ER is a powerful
predictor of tamoxifen response, a conclusion consistent with
tamoxifens proven mechanism of action as an estrogen antagonist in
breast cancer (82). Although PgR-positive status might be thought to be
of benefit, these data show that there was little additional value if
the tumor was already ER positive. A comparison of interactions is
shown in Table 2
. A comparison of the
2,000 women who had ER-positive and PgR-negative tumors and the 7,000
women who had ER-positive and PgR-positive tumors shows there was no
apparent difference in the effect of tamoxifen on either the recurrence
rates or mortality rates. Additionally, the numbers were too few (602
women) in the Overview Analysis (84) to allow a meaningful prediction
of the benefits of tamoxifen in patients who had an ER-negative but
PgR-positive tumor.
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Table 2. A comparison of the proportional risk reduction of
adjuvant tamoxifen therapy based on PgR status in populations of
ER-positive patients
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The Overview Analysis also provides unequivocal proof of the laboratory
principle (88, 89, 90) that longer adjuvant tamoxifen therapy was predicted
to provide more benefit. The duration of therapy is extremely important
for the ER-positive premenopausal woman with large amounts of
circulating estrogen that can rapidly reverse the effect of short-term
tamoxifen treatment. The effect of the duration of tamoxifen treatment
on the reduction of recurrence rates and the reduction of death rates
is shown in Fig. 3
. The duration of
tamoxifen therapy is critical for the premenopausal patient: the effect
of 1 yr of treatment is virtually nonexistent compared with the benefit
of 5 yr of treatment. It is also important to point out that the
reduction of death rates in women under 50 yr of age and over 60 yr of
age treated with 5 yr of tamoxifen is identical, at around 33% (Table 3
). By contrast, the effect of tamoxifen
duration on women over the age of 60 is less dramatic because 1 yr of
tamoxifen is much more effective in postmenopausal women. These data
are illustrated in Table 3
, which shows a 2- to 3-fold increase in the
effectiveness of tamoxifen when the duration is increased from 1 to 5
yr, whereas there is a 20-fold increase in tamoxifens effectiveness
for premenopausal women with an increased duration of 15 yr (Fig. 3
).

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Figure 3. The relationship between the duration of adjuvant
tamoxifen therapy in ER-positive premenopausal patients and the
reduction in recurrence and death rate. A longer duration of treatment
has a dramatic effective on patient survival. [Derived from Ref.
84.]
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Table 3. Proportional risk reductions in 60- to 69-yr-old
breast cancer patients when the known ER-poor patients are excluded
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B. Contralateral breast cancer
Tamoxifen consistently reduces the risk of contralateral breast
cancer (i.e., a second primary breast cancer in the other
breast) independent of age (84). Women have a proportional risk
reduction that is 27 ± 11% or 31 ± 7% if they are below
or above the age of 50, respectively. The principle "longer is
better" is also true for the reduction of risk for contralateral
breast cancer with adjuvant tamoxifen therapy. Five years is better
then 2 yr or 1 yr of adjuvant therapy with tamoxifen (Fig. 4
). In fact, 1 yr of adjuvant tamoxifen
does not significantly reduce the incidence of contralateral breast
cancer compared with control because the SD is so large
(13 ± 13% reduction compared with control).

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Figure 4. The relationship between the duration of adjuvant
tamoxifen and the reduction in contralateral breast cancer. A longer
duration is clearly superior, and the 5-yr result that produces a 47%
reduction in contralateral breast cancer is equivalent to the result
observed in the tamoxifen prevention trial presented in Fig. 9 .
[Derived from Ref. 84.]
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It is interesting to note that a quarter of the women allocated to the
known adjuvant trials in the Overview Analysis (84) were Japanese, who
have an annual incidence of contralateral breast cancer in patients not
receiving tamoxifen of 2 per 1,000 compared with 6 per 1,000 elsewhere
in the world. Therefore, if 5 yr of tamoxifen therapy can halve
contralateral breast cancer, then the absolute benefit for Japanese
women would be 1 per 1,000 and 3 per 1,000 elsewhere for both young and
old women. Finally, the proportional reduction in contralateral breast
cancer appears to be similar in women whose initial tumor being treated
with tamoxifen was ER-poor (29 ± 15%) compared with the rest of
the study population (30 ± 6%). This is an important result for
the potential application of tamoxifen for the reduction of
contralateral breast cancer in the woman with a primary breast cancer
that is unequivocally ER negative.
C. Endometrial cancer
The overall increase in the incidence of endometrial cancer in the
Overview Analysis was 2- to 3-fold (84). There was no association with
dose, i.e., 20 mg and 3040 mg daily produced relative risk
(RR) ratios of 2.7 and 2.4, respectively. However, there was a
suggestion that 1 and 2 yr of tamoxifen doubled the incidence of
endometrial cancer and 5 yr quadrupled the incidence. However, the side
effect is so rare (i.e., the numbers are too small) that the
risk ratios are not significantly different from one another for each
duration of tamoxifen. It is important, however, to state that the
absolute increase in endometrial cancer was only half as big as the
absolute decrease in contralateral breast cancer.
The Overview Analysis was able to identify 3,673 women who took 5 yr of
adjuvant tamoxifen. With 26,400 woman years of follow-up before breast
cancer recurrence in this group, there were seven endometrial cancer
deaths. It is estimated that during the whole first decade, the
cumulative risk was two deaths per 1,000 women. It is important to
state that the current knowledge about the association of tamoxifen
with endometrial cancer will improve these statistics. In general, the
reported trials were conducted without awareness of the endometrial
side effects of tamoxifen. This is no longer the situation, and early
detection will improve mortality figures associated with tamoxifen.
D. Conclusions
Tamoxifen has been extensively tested in clinical trials of
adjuvant therapy for 20 yr. The Overview shows that the proportional
mortality reductions were similar for women with node-positive or
node-negative disease (84). However, the absolute reductions in
mortality were much greater in node-positive than node-negative
disease. Additionally, patients with ER-positive disease have an
increased reduction in death rate with longer duration of tamoxifen
treatment, whereas patients who are ER-negative do not benefit from
tamoxifen, regardless of the duration of therapy. The value of a long
duration of treatment is most important for the premenopausal patient
(Fig. 3
). This latter finding is new, as the results for premenopausal
women could not be ascertained with certainty in earlier overviews
(74). The Oxford Overview Analysis has established the veracity of the
laboratory concepts that tamoxifen would be most effective in
ER-positive disease, longer duration would be more beneficial, and
tamoxifen would prevent primary breast cancer, in this case
contralateral disease (85, 86, 87, 88, 89, 90).
Overall, the absolute improvement in recurrence was greater during the
first 5 yr after surgery, but improvement in survival increased
steadily throughout the first 10 yr. This is an important finding
because the patient is clearly benefiting from tamoxifen even after
therapy has been discontinued. There is an accumulation of the
tumoristatic/tumoricidal actions of tamoxifen for at least the first 5
yr of treatment, but the benefit continues after therapy stops. This is
also true for the reduction in contralateral breast cancer; the breast
seems to be protected so the value remains after therapy stops. This
observation is extremely important for the application of tamoxifen as
a preventive because a 5-yr course of tamoxifen would be expected to
protect a woman from breast cancer for many years afterward.
Finally, the risk/benefit ratio of tamoxifen therapy can be stated to
be strongly in the benefit category. The risk of endometrial cancer, a
concept derived from laboratory studies (92), is of concern, but the
benefits clearly outweigh the risks. In contrast, early concerns about
the carcinogenic effects of tamoxifen in the rat liver (see
Section VII) do not translate to the clinic as there is no
evidence from the Overview Analysis of an increase in either liver or
colorectal cancer in patients who take tamoxifen (84)
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IV. Selective ER Modulation
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Nonsteroidal antiestrogens were originally defined as compounds
that would inhibit estradiol-stimulated rat uterine weight. The
compounds tamoxifen (ICI 46,474) (93), nafoxidine (U, 11,100A) (94),
nitromifene (CI628) (95), and clomiphene (MRL 41)(96) are all partial
estrogen agonists in the uterus that also inhibit
dimethylbenzanthracene (DMBA)-induced rat mammary tumor growth (67, 97, 98) and the growth of ER-positive MCF-7 breast cancer cell growth
in vitro (99). Thus, in the 1960s and 1970s,
antiestrogenicity was correlated with antitumor activity. However, the
finding that the compounds expressed increased estrogenic properties,
i.e., vaginal cornification and increased uterine weight in
the mouse (93, 100), raised questions about the reasons for the species
specificity. One obvious possibility was species-specific metabolism,
i.e., the mouse converts antiestrogens to estrogens via
novel metabolic pathways. However, no species-specific metabolic routes
to known estrogens (101, 102) have been identified, but knowledge of
the mouse model created a new dimension for study that ultimately led
to the recognition of the target site-specific actions of
antiestrogens. This concept was subsequently referred to as selective
ER modulation (SERM) to describe the target site-specific effects of
raloxifene, an antiestrogen originally targeted for an application in
breast cancer but now used, paradioxically, as a preventive for
osteoporosis (see Section VII). Now the whole class of drugs
are known as SERMs.
The ER-positive breast cancer cell line MCF-7 (for a review see Ref.
42) can be heterotransplanted into immune-deficient athymic mice but
the cells will only grow into tumors with estrogen support.
Paradoxically, tamoxifen, an estrogen in the mouse, does not support
tumor growth (103) but stimulates mouse uterine growth with the same
spectrum of tamoxifen metabolites present in both the uterus and the
human tumor (28). To explain the selective actions of tamoxifen in
different targets of the same host, it was suggested that the ER
complex could be interpreted as a stimulatory or inhibitory signal at
different sites (28). The concept was consolidated with experimental
evidence from two further models. First, tamoxifen and raloxifene
maintain bone density in the ovariectomized rat, but both compounds
inhibit estradiol-stimulated uterine weight (29) and prevent
carcinogen-induced mammary tumorigenesis (30). Second, the finding that
tamoxifen would partially stimulate the growth of a human endometrial
carcinoma transplanted into athymic mice (91) allowed the investigation
of two human tumors bitransplanted in the same mouse to determine
whether tamoxifen could inhibit estrogen-stimulated growth of two
tumors in the same host equally (92). Tamoxifen demonstrated target
site specificity: breast tumor growth was controlled but endometrial
tumors continued to grow. Again the range of tamoxifen metabolites were
consistent in all target tissues despite the contrasting biological
responses, so it was concluded that the ER complexes must be
interpreted differently in different target tissues.
During the past decade an intense effort has been made to
discover the reason for the target site-specific effects of
antiestrogens. Not only will this knowledge permit a rational
application of tamoxifen and raloxifene in patients, but also the
discovery of new mechanisms for drug selectively will open the door for
new innovations in drug discovery. For the sake of completeness, we
will briefly consider some of the current hypotheses that could explain
the molecular mechanisms of antiestrogen action in different tissue
sites.
A. Antiestrogenic activity at the ER
The crystallization of the ligand-binding domain of the ER with
estradiol and raloxifene has provided an important insight into the
conformational changes that occur in the receptor (104) liganded with
an estrogen or an antiestrogen, respectively. Estradiol causes helix 12
to seal the ligand inside the hydrophobic pocket of the ligand-binding
domain (Fig. 5A
). This causes receptor
activation through the binding of coactivators on the surface of helix
12 (see Section IV.B). By contrast, the binding of
raloxifene prevents helix 12 from sealing the hydrophobic pocket (Fig. 5B
), and gene transcription cannot occur because coactivators cannot
bind. Unfortunately, the final shape of the ER and anti-ER
complexes do not tell us how the tertiary changes in protein
structure occur. However, the crystal structure provides proof of the
critical importance of AA351 (aspartate) for raloxifene action. The
alkylaminoethoxy side chain is the essential structural feature of
nonsteroidal antiestrogens (for a review see Ref. 105). The distance
between the nitrogen and the oxygen must be optimal (106), the
conformations available to the side chain must not be restricted (107),
and the basicity of the nitrogen must be correct (108). Removal of the
side chain results in loss of all activity or an increase in estrogenic
properties (109). The side chain was originally predicted (110, 111) to
bind to an "antiestrogenic region" in the ligand-binding domain of
the ER to neutralize the estrogenic properties of the receptor. Simply
stated, the antiestrogen was perceived to act like a stick to prevent
the jaws of the ER from closing around the ligand. Looked at another
way, an estrogenic complex would only be created by the protein
enveloping the ligand. For the nonsteroidal antiestrogens, the
"antiestrogenic region" is now known to be AA351 on helix 3. The
discovery of an ER mutant in a tamoxifen-stimulated MCF-7 breast tumor
(112) and the finding that it can increase the estrogenic properties of
4-hydroxytamoxifen (113, 114), the active metabolite of tamoxifen
(115), and convert raloxifene from an antiestrogen to an estrogen (116, 117) is valuable biological proof that AA 351 is important for the
antiestrogenic activity of these specific compounds. This interaction,
at the critical contact point of helix 3 and helix 12, prevents helix
12 from sealing the ligand into the binding pocket (Fig. 5B
).
Interestingly, a recent report of the crystal structure of
4-hydroxytamoxifen and the ligand-binding domain of ER (118) shows a
complex interaction of the side chain with several amino acids
including AA 351. The distance between AA 351 and the nitrogen of
4-hydroxytamoxifen is further than the comparable interaction in
raloxifene. This difference between the crystal structure of the
raloxifene ER complex (104) and the 4-hydroxytamoxifen ER complex
(118) may explain the promiscuous nature of the 4-hydroxytamoxifen ER
complex. However, it must be stressed the AA 351 has no role for the
antiestrogenic action of the pure antiestrogen ICI 182,780 (117) and
other amino acids may be found to be involved in the antiestrogenic
mechanisms of novel compounds in the future.

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Figure 5. Comparison of the binding of estradiol (diagram A)
and raloxifene (diagram B) in the ligand-binding domain of the human
ER. The key event is the repositioning of helix 12 to seal the steroid
in the hydrophobic pocket. This event allows the ER complex to recruit
coactivators for the transcription complex. The side chain of
raloxifene prevents recruitment of coactivators by first masking AA 351
in helix 3, which is critical for the relocation of helix 12.
Coactivators cannot bind to the AF2 region of helix 12 because it
cannot seal the ligand-binding domain. [Derived from Ref. 104.]
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Nevertheless, this two-dimensional model, which describes antiestrogen
and estrogen action, is too simple to encompass all the complexities of
the target site-specific actions of the antiestrogens. Furthermore, the
crystallization data do not include the conformational information
about the other half (i.e., the A, B, C, or F domains
illustrated in Fig. 2
) of the ER that control interaction with
transcription factors and binding to DNA. Subtle changes in the whole
protein shape could be responsible for changes in the intrinsic
efficacy of the receptor complex at different target sites.
B. Coactivators for ER
A host of coactivator and corepressor proteins has been implicated
in the construction of a transcription complex in target cells (63, 64, 119, 120, 121, 122, 123). The finding that an antiestrogen ER complex could become
increasingly estrogenic in different cell contexts (124, 125) raised
the possibility that the differential distribution of coactivators or
corepressors could be responsible for changes in estrogenicity between
the breast and, for example, bones (126, 127). The activating function
(AF-2) region in the ligand-binding domain (Fig. 2
) is known to be
repressed by tamoxifen and raloxifene, but the AF-1 region is
unaffected by tamoxifen binding (125). Clearly, the shape of a
particular complex of ligand and ER will be different for different
drugs (125). Thus coactivators could modulate estrogenicity
differentially in different target sites. The candidate proteins could
therefore amplify the anti-ER complex into an estrogenic complex.
Alternatively, the anti-ER complex might recruit completely new
proteins at a specific target site to induce or to suppress gene
transcription.
C. Alternate response elements on DNA
Anti-ER complexes can bind to an estrogen response element but
cannot recruit coactivators to initiate transcription of
estrogen-responsive genes (114). However, it is possible that the
anti-ER complex can bind to alternate sites in the promoter region to
initiate transcription. Alu DNA repeats were originally thought to be
functionally inert, but these elements may be able to activate gene
transcription with antiestrogens through an ER-related mechanism (128, 129). Additionally, a specific region of the transforming growth
factor-ß promoter is believed to be activated by raloxifene (130). A
raloxifene response element has been identified (131), but the authors
are now convinced that a simple protein DNA interaction does not occur
(132).
D. An alternate ER-ERß
The discovery of a second ER, named ERß (133), has introduced a
new dimension into the possible mechanisms of tamoxifen or raloxifene
action. Although ERß has similar functional homology in the
DNA-binding domain, there is only 55% homology between ERß and ER
in the ligand-binding domain. Clearly, one possibility to explain the
target site specificity and altered estrogenicity of antiestrogens is a
differential distribution of ER
and -ß to different tissues (134).
The mechanism of action of the differential pharmacology between ER
and -ß may also involve different methods of gene activation. A novel
signal transduction pathway has been identified as a protein-protein
interaction between ERß anti-ER complexes and AP-1 (fos and jun)
(135) that is capable of activating a reporter gene. Estradiol,
however, does not activate the reporter. Therefore, the pathway would
be of pharmacological rather than physiological significance.
Interestingly, an ER
tamoxifen complex will activate AP-1 reporter
systems in the context of an endometrial cancer cell (136). This has
led to speculation that the target site specificity of antiestrogens
could be both receptor and context selective.
 |
V. Biological Basis for Tamoxifen as a Breast Cancer Preventive
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With the molecular mechanisms of antiestrogens as a background, it
is now appropriate to consider the scientific rationale for selecting
tamoxifen to be tested as a breast cancer preventive, based on its
pharmacological properties. Knowledge converged over the past 25 yr to
make the choice of testing tamoxifen in well women a logical extension
of clinical experience. Tamoxifen was selected for testing as a
preventive based on 1) animal studies that demonstrated it could
prevent carcinogenesis, 2) an extensive clinical experience that showed
few serious side effects, 3) a beneficial profile of estrogen-like
action in maintaining bone density, and 4) tamoxifen reduces
circulating cholesterol. The fact that tamoxifen was already known to
reduce the incidence of contralateral breast cancer made the drug the
primary agent to test in high-risk women. The pharmacological
properties of tamoxifen have been recently reviewed extensively (82);
therefore, the purpose of this section is to act as a framework for a
comparison with raloxifene in Section VIII and as a prelude
to considering the current STAR trial (Section IX).
A. Animal models
Tamoxifen prevents rat mammary carcinogenesis induced by
dimethylbenzanthracene, N-nitrosomethylurea, and
ionizing radiation (86, 87, 88, 89, 30, 137), and long-term treatment prevents
spontaneous carcinogenesis in C3H/OUJ mice infected with mouse mammary
tumor virus (138). The latter result is of interest because tamoxifen
is classified as an estrogen in the uterus and vagina of the mouse (93, 100). This, again, illustrates the target site specificity of tamoxifen
in the mouse model, as mammary cancer is prevented almost completely.
Although the athymic mouse model heterotransplanted with breast cancer
cell lines has been extremely instructive for the use of therapeutic
tamoxifen (103), and valuable as a model for understanding drug
resistance (112), there are few parallels to chemoprevention. The MCF-7
cell line is derived from a pleural effusion, and the cells are,
therefore, metastatic breast cancer (40). As such, the cell line does
not replicate carcinogenesis in the breast or mimic primary breast
cancer cells that have not developed the metastatic phenotype.
B. Bones
Tamoxifen maintains bone density in the ovariectomized rat (29, 31), and these observations have been translated to clinical trials.
Sporadic reports (32, 139) and placebo-controlled randomized trials
(33, 140) demonstrate that tamoxifen can increase bone density in the
lumbar spine, forearm, and neck of the femur by 12%. Although the
increases are modest compared with the results obtained with estrogen
use or bisphosphonates (
5% increase in bone density), tamoxifen
produced a significant decrease in hip and wrist fractures as a
secondary end point in the breast cancer prevention trial (141). There
is, however, a report that tamoxifen can reduce bone density in
premenopausal women by 12% (142), but the decrease appears to be
without clinical significance as there is no increase in the fracture
rate. The reason for this may be the fact that a small decrease in bone
density in a premenopausal woman is still well above the range of bone
densities observed for women in their late 60s and 70s at risk for
fractures.
C. Lipids
Tamoxifen reduces circulating cholesterol (34, 35). Low-density
lipoprotein cholesterol is reduced by about 15%, but high-density
lipoprotein cholesterol is maintained. It is hypothesized that this
magnitude of fall in circulating cholesterol is a good surrogate marker
for protection from coronary heart disease and atherosclerosis. In this
regard there is evidence that woman who have been treated with 5 yr of
adjuvant tamoxifen for breast cancer have a reduced incidence of fatal
myocardial infarction (143, 144). Additionally, longer treatment (5 yr)
appears to be superior to shorter treatment (2 yr) in reducing the
number of hospital admissions for any cardiac condition (145).
Conversely, a large study in the United States of five or more years of
tamoxifen for the adjuvant treatment of breast cancer found no
statistically strong evidence for the protection of women from coronary
heart disease (146). Nevertheless, the incidence of coronary heart
disease doubled once tamoxifen treatment was stopped, and, most
importantly, there was no evidence for a detrimental effect of
tamoxifen, i.e., tamoxifen did not increase the rate of
coronary heart disease in pre- or postmenopausal women. The reasons for
the disparate results probably reflect the populations studied. Breast
cancer clinical trials usually require a good general health status
before enrollment. Obviously, women at high risk for a second disease,
coronary heart disease, would not be enrolled into a trial evaluating
the efficacy of an antiestrogenic therapy in, for example,
node-negative breast cancer where the prognosis, for the majority of
women, would be expected to be good. Only a prospective randomized
trial in a high-risk population would provide accurate data to support
a claim for cardio-protection. At this point in time, there is no
evidence that tamoxifen is detrimental based on current clinical
evaluations, but there is no prospective clinical evidence that
tamoxifen will reduce the risk of coronary heart disease.
D. Uterus
It is well known that tamoxifen produces a partial agonist action
in the rat uterus (93), but the histology is different than the
epithelial hyperplasia noted with estradiol (147). Until the late
1980s, there was very little information about the actions of tamoxifen
in the normal human uterus. However, it is now clear that a variety of
endometrial changes occur in unselected populations of women (148). The
most significant finding is an increase in the stromal component rather
than endometrial hyperplasia (149, 150). Despite the fact that
tamoxifen has been used to treat endometrial cancer, the laboratory
data suggesting that tamoxifen has the potential to encourage the
growth of preexisting disease harbored in the uterus (91, 92) provoked
an intense investigation of the rates of detection of endometrial
cancer in women using adjuvant tamoxifen treatment for breast cancer.
These data have been reviewed (151), and it is clear from the recent
results of the tamoxifen prevention trial (141) that tamoxifen does not
cause an excess of endometrial cancer in premenopausal women but does
increase risk by 3- to 4-fold in postmenopausal women. This is
consistent with the fact that women harbor 45 times the level of
endometrial cancer than is detected clinically (152). In other words,
the increase in the detection of endometrial cancer from 1 per 1,000
women per year to 3 per 1,000 women per year is consistent with the
known rate of occult disease. Most importantly, the stage and grade of
endometrial cancer observed in women taking tamoxifen is the same
as those in the general population (141, 153).
 |
VI. Risk Factors for Breast Cancer
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If tamoxifen is an appropriate agent to test as a chemopreventive
primarily because of its extensive clinical experience for the
treatment of breast cancer, then the issue becomes identification of
women at risk to select as a target population for recruitment to
definitive clinical trials. Family history is probably the most well
recognized risk factor for breast cancer, and it is now known that two
forms of risk are associated with a family history of the disease. An
inherited gene mutation predisposing to breast cancer is believed to
account for only 510% of breast cancer cases (154, 155). Although
infrequent, these mutations are significant since they are associated
with a lifetime risk of breast cancer development of 5080% (156, 157). At present, two predisposition genes, BRCA1, located on
chromosome 17q21 (158), and BRCA2, located on chromosome 13q1213
(159), have been identified. Both genes are inherited in an autosomal
dominant fashion and are characterized by an extremely high risk of
breast cancer development, which begins at a young age. Both genes also
confer an increased risk of ovarian cancer development, which in BRCA1
carriers is estimated to be 10% by age 60 (160), and is lower in BRCA2
carriers. In addition, germ line mutations of the tumor suppressor gene
p53, as seen in patients with the Li-Fraumeni syndrome, may account for
about 1% of breast cancer cases occurring in women age 40 and younger
(161, 162).
Most women with a family history of breast cancer do not have the
genetically transmitted form of the disease, and therefore their
increase in risk is much less than that seen in women who have
inherited a predisposition gene. The cumulative probability that a
30-yr-old woman with a mother and sister with breast cancer will
develop breast cancer by the age of 70 is reported to be between 7%
and 18% (163, 164). While this risk increases as the number of
relatives with breast cancer increases, the probability of cancer
development if both a mother and sister have bilateral breast cancer
has been reported to be only 25% (162, 164). The cumulative risk of
breast cancer development in women with a family history of breast
cancer rarely exceeds 30%, making it critically important to
distinguish those women with hereditary breast cancer from those with a
family history of the disease. Factors that should increase the
clinicians index of suspicion that a woman is at risk for genetically
transmitted breast cancer include multiple relatives (maternal or
paternal) with the disease, a family history of ovarian cancer in
association with breast cancer, and a family history of bilateral
and/or early onset of breast cancer. Although not all women with these
factors will have genetically transmitted breast cancer, a referral for
genetic counseling will allow the construction of a detailed pedigree
to estimate both breast cancer risk and the competing causes of death
due to an increased risk of the development of other types of cancer.
Breast cancer is clearly related to endogenous hormones, and numerous
studies have linked breast cancer risk to the age of menarche,
menopause, and first pregnancy. Although the absolute age-specific
incidence of breast cancer is higher in postmenopausal than
premenopausal women (165), the absolute rate of rise of the curve is
greatest up to the time of menopause, and then slows to one-sixth of
that seen in the premenopausal period. Further support for the
promotional role of estrogen in breast cancer development comes from
the observations that early menarche (166), late menopause (167),
nulliparity, and late age at first birth (168) all increase the risk of
breast cancer development. An increased number of ovulatory cycles is
suggested to be the common mechanism of increased risk.
Other hormonal risk factors have been suggested but are not as well
established. Abortion, whether spontaneous or induced, has been
reported by some authors to increase risk (169, 170), while other
studies have found no relationship between abortion and breast cancer
risk (171, 172). Studies of the effect of lactation on breast cancer
risk have also been inconclusive (173, 174), but recent studies have
suggested that a long duration of lactation reduces breast cancer risk
in premenopausal women (175). Physical activity in adolescence is
reported to decrease risk, perhaps due to a higher rate of anovulatory
cycles (176, 177), but an increased level of physical activity later in
life has not been shown to reduce breast cancer risk (178).
Postmenopausal obesity has also been shown to increase risk (179),
perhaps due to increased peripheral estrogen production, but this
relationship between weight and risk is not observed in premenopausal
women. In fact, some studies have reported an inverse relationship
between weight and risk at a younger age (180).
The effects of exogenous hormones in the form of oral contraceptives
and hormone replacement therapy on breast cancer risk have been studied
extensively, but few firm conclusions may be drawn. Overall, there is
no convincing evidence of an increase in breast cancer risk in women
who have ever used oral contraceptives (181). However, some studies
have suggested that the long-term use of oral contraceptives in young
women before first birth may increase breast cancer risk (182, 183).
Two recent meta-analyses of the effect of estrogen replacement therapy
demonstrate small but statistically significant increases in risk for
users (184, 185). However, Steinberg et al. (184) noted no
increase in risk until after at least 5 yr of estrogen use, after which
a proportional increase in risk for each year of estrogen use was
observed, while Sillero-Arenas et al. (185) did not observe
a significant association between duration of hormone replacement
therapy and breast cancer risk. In summary, although hormonal risk
factors are clearly implicated in the pathogenesis of breast cancer,
most of them are associated with a RR of 3 or less of breast cancer
development (Table 4
), and the presence
of a single hormonal risk factor is insufficient to classify a woman as
high risk.
The relationship of benign breast disease to breast carcinoma was a
subject of confusion for many years. The use of a standard
classification of benign breast diseases as nonproliferative,
proliferative, or proliferative with atypia has resolved much of the
controversy. The histological diagnoses comprising these categories are
shown in Table 5
. Nonproliferative
disease is associated with no increase in breast cancer risk, while
proliferative disease increases risk by a factor of 1.52.0, and
atypical hyperplasia by a factor of 45. Approximately 70% of
palpable breast masses contain nonproliferative disease (186), and only
3.6% are atypical hyperplasia. The incidence of atypia is somewhat
higher in biopsies performed for mammographic lesions, ranging from
710% (187, 188). However, the risk of breast cancer development 15
yr after a diagnosis of atypical hyperplasia is only 8% in the absence
of a family history of breast cancer. Proliferative breast disease is
also noted more frequently in women with a significant family history
of breast cancer than in controls, further supporting its role as a
risk factor (189).
Another benign breast lesion that is clearly associated with an
increased risk of breast cancer development is lobular carcinoma
in situ (LCIS). In the past, LCIS was thought to be a
malignant lesion, albeit one with a favorable prognosis. However, the
finding that LCIS is associated with a risk of breast cancer
development of approximately 1% per yr, the observation that the risk
of carcinoma is equal in both breasts, and the finding that neither the
extent of LCIS in the breast nor its presence at a margin of resection
influence the risk of subsequent cancer have led LCIS to be regarded as
a risk factor for breast cancer development rather than the actual
precursor of carcinoma (190).
A number of environmental factors have also been linked to breast
cancer risk. Exposure to ionizing radiation, whether secondary to
nuclear explosion or medical procedures, has been clearly demonstrated
to increase breast cancer risk (191, 192, 193). The level of risk varies
with the age of exposure, with a minimal increase in risk observed for
exposure in women older than 40 yr. A larger amount of attention has
been directed toward the role of diet in the etiology of breast cancer.
This link has been suggested by the large international variation in
breast cancer incidence rates and the observation that national per
capita fat consumption correlates with breast cancer incidence and
mortality (194). However, prospective studies of diet and breast cancer
risk have failed to identify a relationship between dietary fat intake
and breast cancer incidence for up to 10 yr of follow-up (195). The
lack of a relationship between dietary fat intake and cancer risk
within the context of a Western diet is confirmed by a pooled analysis
of seven cohort studies involving a total of 337,816 women, which
demonstrated no difference in risk for women with the lowest and
highest quintile of fat intake (196). However, all of these studies
have addressed fat intake during adult life, and they do not exclude
the possibility that fat intake during childhood and adolescence may
influence subsequent breast cancer risk.
Stronger evidence exists to support an association between alcohol and
breast cancer. A meta-analysis of 12 case control studies demonstrated
a RR of 1.4 for each 24 g of alcohol consumed daily (197).
Defining a relationship between age of alcohol consumption and breast
cancer risk is more difficult, with conflicting data on the importance
of drinking early in life (198, 199). A summary of the magnitude of
increase in risk associated with the factors discussed is provided in
Table 4
.
A. Interactions among risk factors
A major problem in the clinical identification of the "high risk
woman" is the lack of knowledge of the interactions among the various
factors known to alter breast cancer risk, since the majority of
studies have focused on defining individual risk factors. Most women
have a combination of factors that both increase and decrease risk,
complicating the assessment of an individuals level of risk. In
addition, it is unclear whether the risk conferred by multiple risk
factors is additive, multiplicative, or varies with the risk factor
under study.
The interactions between a family history of breast cancer and other
risk factors have been examined, often with conflicting results. Dupont
and Page (186) observed that the combination of atypical hyperplasia
and a family history of a first-degree relative with breast cancer
increased the RR of breast cancer to 11 times that of an index
population, compared with a RR of 4.4 for atypia alone. However, Rosen
et al. (200) found that the presence of a family history of
breast carcinoma did not alter the level of risk after a diagnosis of
LCIS, a lesion often considered part of a continuum with atypical
hyperplasia. An analysis of data from the Nurses Health Study (201)
found that in women with a mother or sister with breast cancer, known
risk factors of age at menarche or menopause, parity, age at first
birth, alcohol use, and the presence of benign breast disease did not
further alter risk. In contrast, Anderson and Badzioch (202) and
Brinton et al. (203) have reported that hormonal factors
further modulate risk in women with a family history of breast cancer,
although the effect varies with the factor under study.
Studies of the interaction between estrogen replacement therapy and
other known breast cancer risk factors also have variable results,
depending on the risk factor under study. In a meta-analysis of 16
published studies, Steinberg et al. (184) found that the
effect of estrogen replacement did not differ among parous and
nulliparous women and those with or without benign breast disease.
However, an enhanced risk was observed in women with a family history
of breast cancer. The analysis of the interaction among risk factors is
further complicated by the fact that some factors may be important for
the risk of premenopausal, but not postmenopausal, cancer and
vice versa, and these effects may not be constant over time.
A model to predict the risk of breast cancer development in women at a
given age over a defined time interval was developed by Gail et
al. (204) using data from 4,496 matched pairs of cases and
controls in the Breast Cancer Diagnosis and Demonstration Project. The
model incorporates the risk factors of age at menarche, age at first
live birth, number of first-degree relatives with breast cancer, and
number of previous breast biopsies, and has been shown to predict risk
accurately in two validation studies of women undergoing annual
mammographic screening (205, 206). However, the model overpredicts
breast cancer risk by 33% among women age 60 and younger who do not
undergo annual screening. There are several other limitations of the
model. Because only first-degree relatives are considered, it is not an
appropriate model for women with extensive family histories of breast
cancer, where risk may be underestimated. In women with risk due to
LCIS or atypical hyperplasia, the model underestimates risk, since the
highest RR for breast biopsy is 2.0. Similarly, for the woman with
nonproliferative disease, the model may overestimate risk. In spite of
these limitations, the model is a clinically important tool for
identifying a womans level of risk over a clinically relevant time
period, after correction for competing causes of mortality. The Risk
Disk, which is available from the National Cancer Institute, uses the
Gail model to provide a numeric estimate of a womans 5 yr and
lifetime risk of developing breast cancer compared with an "average
risk" woman of the same age. Examples are given in Figs. 6
and 7
.

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Figure 6. Risk assessment for breast cancer in a woman with
two risk factors. The woman illustrated here has a mother with breast
cancer and has never had any children. The combination of these factors
means that her 5-yr risk of breast cancer development is 1.6%,
compared with 0.5% in a woman with no risk factors. If she lives to
the age of 70, her risk will be 18% compared with 6.4% for the woman
with no risk factors. This level of risk would not have qualified the
woman to participate in the recently completed Breast Cancer Prevention
Trial with tamoxifen.
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Figure 7. Risk assessment for breast cancer in a woman with
multiple risk factors. The woman illustrated here has early menarche,
late age at first birth, and has a mother and sister with breast
cancer. In addition, she has had a breast biopsy showing atypical
hyperplasia. This combination of risk factors makes her 5-yr risk of
breast cancer development 7.5% and her lifetime risk 53.4%. She is an
ideal candidate to consider tamoxifen for risk reduction.
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B. Identification of candidates for chemoprevention
Women at increased risk for breast cancer would seem to be ideal
candidates for chemoprevention initiatives. However, from the preceding
discussion it is apparent that the problem of identification of the
high-risk woman is far from solved. There is no consensus regarding
what level of increase in risk is clinically relevant. The interactions
among risk factors and their variability over time are poorly
understood, and most of the data on risk come from studies of white
women, so little is known about the impact of ethnic diversity on risk.
Finally, with the exception of women with mutations of breast cancer
predisposition genes, the majority of women with risk factors will not
develop breast carcinoma. In addition, a recent study of the fraction
of breast cancer cases in the United States due to attributable risk
factors (207) found that fewer that 50% of women who develop the
disease have any identifiable risk factors. A family history of breast
cancer accounted for only 9.1% of cases, while relatively minor risk
factors such as later age at first birth and nulliparity contributed
29.5% of cases. In a similar study, Seidman et al. (208)
noted that only 21% of breast cancer cases in women age 3054 and
29% of cases in women age 5584 occurred in women with 1 of 10 common
breast cancer risk factors. The majority of women in the studies
described had minor risk factors, which increased the RR of breast
cancer only 2-fold, and most had only a single risk factor. This level
of "increased risk" would not meet the entry criteria for the
trials of breast cancer prevention in high-risk women discussed below.
These data suggest that even if women with a very small increase in
breast cancer risk were targeted for prevention initiatives, a large
number of cases would continue to be missed.
 |
VII. Prevention of Breast Cancer with Tamoxifen
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This section will explore progress that has been achieved in the
last decade to answer the question, "Does tamoxifen have worth in the
prevention of breast cancer in high risk women?". Two studies have
claimed to address this questionThe Royal Marsden Pilot Study (209)
and the National Surgical Adjuvant Breast and Bowel Project
(NSABP) Protocol P-1 (141). However, the Marsden study was not
designed to answer the question about the prevention of breast cancer.
It was a pilot toxicology study (142) that was subsequently part of a
nationwide clinical trial in Britain that planned to recruit a total of
20,000 high-risk women. The main British study is ongoing.
Additionally, an Italian report of the efficacy of tamoxifen in a small
number of low-risk women (
5,000) has been published (210), but again
this is a small component of a 20,000-volunteer trail that has now been
stopped.
A. Royal Marsden Pilot Study
Powles and co-workers (211) recruited high-risk women aged 3070
to a placebo-controlled trial using 20 mg of tamoxifen daily for up to
8 yr. Women were eligible if their risk of breast cancer was increased
due to family history. Each participant had at least one first-degree
relative with breast cancer under age 50, or a first-degree relative
affected at any age plus an additional affected first- or second-degree
relative, or a first-degree relative with bilateral breast cancer.
Women with a history of benign breast biopsy and an affected
first-degree relative of any age were also eligible. Women with a
history of venous thrombosis, any previous malignancy, or an estimated
life expectancy of fewer than 10 yr were excluded (209, 212). A total
of 2,494 women consented to participate in the study, and 23 were
excluded from final analysis due to the presence of preexisting ductal
carcinoma in situ (DCIS) or invasive breast carcinoma (209)
The trial was originally undertaken to evaluate the problems of
accrual, acute symptomatic toxicity, compliance, and safety as a basis
for subsequent large national, multicenter trials designed to test
whether tamoxifen can prevent breast cancer (211). However, the trial
has also been analyzed for breast cancer incidence (209). The stated
goal of this pilot study was to act as a vangard for a 20,000 strong
volunteer trial throughout the United Kingdom and Australia. The
national study is ongoing, but the recruitment goal has been cut to
12,000.
Acute symptomatic toxicity was low for participants on tamoxifen or
placebo in the pilot study, and compliance remained correspondingly
high: 77% of women on tamoxifen and 82% of women on placebo remained
on medication at 5 yr (212). There was a significant increase in hot
flashes (34% vs. 20%), mostly in premenopausal women
(P < 0.005); vaginal discharge (16% vs.
4%; P < 0.005); and menstrual irregularities (14%
vs. 9%; P < 0.005), respectively. At the
most recent follow-up, 320 women had discontinued tamoxifen and 176 had
discontinued placebo before the studys completion (P
< 0.005) (209).
Until their report in 1994, the Marsden group (212) observed no
thromboembolic episodes; a detailed analysis of other coagulation
parameters in a sequential subset of women also found no significant
changes in Protein S, Protein C, or cross-linked fibrinogen degradation
products. At 70 months, no significant difference in the incidence of
deep vein thrombosis or pulmonary embolism was observed between groups.
A significant fall in total plasma cholesterol occurred within 3 months
and was sustained over 5 yr of treatment (142, 213, 214). The decrease
affected low-density lipoproteins with no change in apolipoproteins A
and B or high-density lipoprotein cholesterol.
In contrast, tamoxifen exerted antiestrogenic or estrogenic effects on
bone density, depending on menopausal status. In premenopausal women,
early findings demonstrated a small but significant (P
< 0.05) loss of bone in both the lumbar spine and hip at 3 yr (142).
It will be most important to evaluate the results at 5 and 8 yr of
therapy, as current indications suggest bone stabilization rather than
continued loss. In contrast, postmenopausal women had increased bone
mineral density in the spine (P < 0.005) and hip
(P < 0.001) compared with untreated women (142).
Finally, the Marsden group has made an extensive study of gynecological
complications associated with tamoxifen treatment in healthy women.
Since ovarian and uterine assessment by transvaginal ultrasound became
available some time after the trials start, many subjects did not
have a baseline evaluation. Ovarian screening demonstrated a
significantly increased risk (P < 0.005) of detecting
benign ovarian cysts in premenopausal women who had received tamoxifen
for more than 3 months compared with controls. There were no changes in
ovarian appearance in postmenopausal women (212). A careful examination
of the uterus with transvaginal ultrasonography using color Doppler
imaging in women taking tamoxifen showed that the organ was usually
larger; moreover, women with sonographic abnormalities had
significantly thicker endometria (215). Of particular interest in this
regard was the recent observation (150) that 20 mg of tamoxifen daily
caused a time-dependent proliferation of the endometrium in
premenopausal and early postmenopausal women. This effect appeared to
be mediated by the stromal component, since no cases of cancer or even
epithelial hyperplasia were observed among the tamoxifen-treated group
in this Italian study with 33 women (150).
Although the Marsden pilot study has provided invaluable information
about the biological effects of tamoxifen in healthy women, the trial
was not designed to answer the question of whether tamoxifen prevents
breast cancer. In spite of this, an analysis of breast cancer incidence
was reported at a median follow-up of 70 months, when 42% of the
participants had completed therapy or withdrawn (209). During the
study, 336 women receiving tamoxifen and 305 on placebo received
hormone-replacement therapy. No difference in the incidence of breast
cancer was observed between the groups. There were 34 carcinomas in the
tamoxifen group and 36 in the placebo groupa RR of 0.98. Of the 70
cancers, only 8 were ductal carcinoma in situ. An analysis
of the subset of women on hormone-replacement therapy did not
demonstrate an interaction with tamoxifen treatment. At present there
is no satisfactory answer to the question of why the Marsden Pilot
study shows no decrease in breast cancer incidence in the tamoxifen
arm. The authors suggest (209) that perhaps they have a high population
of BRCA-1 and -2 carriers that are hormone unresponsive but this is
unproven. The fact that women on the Marsden trial were allowed to take
hormone replacement therapy, but the women on the NSABP P-1 trial were
not is unlikely to be responsible for the Marsden result. We believe
that the study, by chance, is underpowered to show a difference. If
this is true, then it appears that a precise instrument to evaluate
risk may be a key factor in the success of the NSABP study as well as
the large volunteer population with adequate events to answer the
question posed.
B. NSABP/NCI Study
This prospective clinical trial opened in the United States and
Canada in May of 1992 with an accrual goal of 16,000 women to be
recruited at 100 North American sites. The specific aim was to test the
worth of tamoxifen as a preventive for breast cancer. It closed after
accruing 13,388 in 1997 because of the exceptionally high-risk status
of the participants. This means that the events would be adequate to
establish statistical significance. The study design is illustrated in
Fig. 8
. Those eligible for entry included
any woman over the age of 60, or women between the ages of 35 and 59
whose 5-yr risk of developing breast cancer, as predicted by the Gail
model (204), was equal to that of a 60-yr-old woman. Additionally, any
woman over age 35 with a diagnosis of LCIS treated by biopsy alone was
eligible for entry to the study. In the absence of LCIS, the risk
factors necessary to enter the study varied with age, such that a
35-yr-old woman must have had a RR of 5.07, whereas the required RR for
a 45-yr-old woman was 1.79. Routine endometrial biopsies to evaluate
the incidence of endometrial carcinoma in both arms of the study were
also performed.

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Figure 8. The eligibility and design of the NSABP tamoxifen
prevention trial. Originally the recruitment goal was 16,000
volunteers, but the actual calculated risk for the recruited group was
higher than anticipated and resulted in a change in recruitment goals.
A total of 13,388 women were recruited by Summer 1997, and the
preliminary results were reported in April 1998. A full report was
presented in September 1998 (141 ).
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The breast cancer risk of women enrolled in the study was extremely
high, with no age group having an RR of less than 4including the
over-60 group. Recruitment was also balanced, with about one-third
younger than 50 yr, one-third between 5060 yr, and one-third older
than 60 yr. Secondary end points of the study included the effect of
tamoxifen on the incidence of fractures and cardiovascular deaths. Most
importantly, the study expects to provide the first prospective
information about the role of genetic markers in the etiology of breast
cancer. It will also establish whether tamoxifen has a role to
play in the treatment of women who are found to carry somatic mutations
in the BRCA-1 gene. (Laboratory results are not yet available.)
The first results of the NSABP study were reported in September 1998,
after a mean follow-up of 47.7 months (141). There were a total of
363 invasive and noninvasive breast cancers in the participants:
124 in the tamoxifen group and 239 in the placebo group. A 49%
reduction in the risk of invasive breast cancer was seen in the
tamoxifen group, and a 50% reduction in the risk of noninvasive breast
cancer was observed. A subset analysis of women at risk due to a
diagnosis of LCIS demonstrated a 56% reduction in this group. The most
dramatic reduction was seen in women at risk due to atypical
hyperplasia, where risk was reduced by 86%.
The benefits of tamoxifen were observed in all age groups, with a RR of
breast cancer ranging from 0.45 in women aged 60 and older to 0.49 for
those in the 5059 yr age group, and 0.56 for women aged 49 and
younger (Fig. 9
). A benefit for tamoxifen
was also observed for women with all levels of breast cancer risk
within the study, indicating that the benefits of tamoxifen are not
confined to a particular lower-risk or higher-risk subset. Benefits
were observed in women at risk on the basis of family history and those
whose risk was due to other factors.

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Figure 9. The overall reduction in invasive breast cancer
observed in the NSABP tamoxifen prevention trial P-1 in women at high
risk for the disease, recruited to receive either tamoxifen (20 mg
daily) or placebo (see Fig. 8 ). The women were also subdivided into age
groups, and the same reduction in the incidence of breast cancer was
observed. The numbers of breast cancers are shown on the top of
each histogram for each treatment arm. [Derived from Ref.
141.]
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As expected, the effect of tamoxifen was seen on the incidence of
ER-positive tumors, which was reduced by 69% per year. The rate of
ER-negative tumors in the tamoxifen group (1.46 per 1,000 women) did
not significantly differ from that of the placebo group (1.20 per 1000
women) (Fig. 10
). Tamoxifen reduced the
rate of invasive cancers of all sizes, but the greatest difference
between the groups was in the incidence of tumors 2.0 cm or less in
size. Tamoxifen also reduced the incidence of both node-positive and
node-negative breast cancer. The beneficial effects of tamoxifen were
observed for each year of follow-up in the study. After year 1, the
risk was reduced by 33%, and in year 5, by 69%.

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Figure 10. The incidence of ER-positive and ER-negative
breast cancer in the placebo and tamoxifen-treated arms of the NSABP
tamoxifen prevention trial P-1 (see Fig. 8 ). The antiestrogen reduces
the risk of developing ER-positive breast cancer, but there is no
change in the incidence of ER-negative breast cancer. The number of
breast cancers are shown on the top of each histogram
for each treatment arm. [Adapted from B. Fisher et al.:
J Natl Cancer Inst 90:13711388, 1998 (141 ). ©
Oxford University Press.]
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Tamoxifen also reduced the overall incidence of osteoporotic fractures
of the hip, spine, and radius by 19% (Fig. 11
) (141). However, the overall
difference approached, but did not reach, statistical significance.
This reduction was greatest in women who were 50 and older at study
entry. No difference in the risk of myocardial infarction, angina,
coronary artery bypass grafting, or angioplasty was noted between
groups (141).

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Figure 11. The incidence of osteoporotic fractures of the
hip, wrist, and spine observed in the placebo and tamoxifen-treated
arms of the NSABP tamoxifen prevention trial P-1. The numbers of
fractures are shown on the top of each histogram for
each treatment arm. [Derived from Ref. 141.]
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This study confirmed the association between tamoxifen and endometrial
carcinoma (151, 153). The RR of endometrial cancer in the tamoxifen
group was 2.5. The increased risk was seen in women aged 50 and older,
whose RR was 4.01. All endometrial cancers in the tamoxifen group were
grade 1, and none of the women receiving tamoxifen died of endometrial
cancer. There was one endometrial cancer death in the placebo group.
Although there is no doubt that tamoxifen increases the risk of
endometrial cancer, it is important to recognize that this increase
translates to an incidence of 2.3 women per 1,000 per year who develop
endometrial carcinoma.
More women in the tamoxifen group developed deep vein thrombosis
than in the placebo group (141). Again, this excess risk was confined
to women aged 50 and older. The RR of deep vein thrombosis in the older
age group was 1.71 (95% confidence interval 0.85 to 3.58). An
increase in pulmonary emboli was also seen in the older women taking
tamoxifen, with a RR of approximately 3. Three deaths from pulmonary
emboli occurred in the tamoxifen arm, but all were in women with
significant comorbidities. An increased incidence of stroke (RR 1.75)
was also seen in the tamoxifen group, but this did not reach
statistical significance
An assessment of the incidence of cataract formation was made using
patient self-report. A small increase in cataracts was noted in the
tamoxifen groupa rate of 24.8 women per 1,000 compared with 21.7 per
1,000 in the placebo group. There was also an increased risk of
cataract surgery in the women on tamoxifen. These differences were
marginally statistically significant and were observed in the older
patients in the study. These findings emphasize the need to assess the
patients overall health status before making a decision to use
tamoxifen for reduction of breast cancer risk.
An assessment of quality of life showed no difference in depression
scores between groups. Hot flashes were noted in 81% of the women on
tamoxifen compared with 69% of the placebo group, and the
tamoxifen-associated hot flashes appeared to be of no greater severity
than those in the placebo group. Moderately bothersome or severe
vaginal discharge was reported by 29% of the women in the tamoxifen
group and 13% in the placebo group. No differences in the occurrence
of irregular menses, nausea, fluid retention, skin changes, or weight
gain or loss were reported.
C. Italian study
The third tamoxifen prevention study, performed in Italy, began in
October 1992, and randomized 5,408 women aged 35 to 70 to 20 mg of
tamoxifen daily for 5 yr (210). Originally, 20,000 volunteers without
risk factors were to be recruited, but the study was stopped
prematurely because of poor recruitment and compliance. Women were
required to have had a hysterectomy for a nonneoplastic condition to
obviate concerns about an increased risk of endometrial carcinoma.
There was no requirement that participants be at risk for breast cancer
development, and, in fact, those who underwent premenopausal
oophorectomy with hysterectomy (47%) actually had a reduced risk of
breast cancer development. Women with endometriosis, cardiac disease,
and deep venous thrombosis were excluded from the study. Although 5,408
women (mean age 51 yr old) were randomized into this study, 1,422
withdrew and only 149 completed 5 yr of treatment. A valid breast
cancer prevention study would only be possible if more than 10,000
normal-risk women had completed 5 yr of tamoxifen vs. 10,000
on placebo.
The incidence of breast cancer did not differ between groups, with 19
cases in the tamoxifen group and 22 in the placebo group. Tumor
characteristics, including size, grade, lymph node status, and receptor
status, also did not differ between groups.
The incidence of thrombophlebitis was increased in the tamoxifen group.
Fifty-six women experienced a total of 64 events: 38 women in the
tamoxifen group and 18 women in the placebo group (P =
0.0053). However, 42 of these cases were superficial phlebitis. No
differences in the incidence of cerebrovascular ischemic events were
observed.
D. Conclusions
Based on a single trial with a positive result and two with
negative results, it may seem, at first glance, that the role of
tamoxifen in breast cancer prevention remains unresolved. However,
critical differences exist between these three studies (see
characteristics in Table 6
).
The negative finding in the Italian study (210) is readily explained by
the relatively low risk of breast cancer development in the study
population, the high dropout rate, the fact that the volunteers were
young women, and the small number of participants who completed 5 yr of
treatment. Despite these problems, the Italian study shows a trend
toward statistical significance among women who took tamoxifen for more
than 1 yr, suggesting that with further follow-up, the results of this
study may become positive with benefit for the women who took
tamoxifen. At present, the only conclusion that can be drawn from this
study is that the possible benefits of tamoxifen are likely to be small
in women with an average or decreased risk of breast cancer.
The Royal Marsden study was initially described as a pilot study to
examine toxicity and compliance (211, 212, 214), which would serve as a
feasibility assessment for a larger trial to determine whether
tamoxifen prevents breast cancer. In spite of being designed as a pilot
study, the trial is now said to have a 90% power to detect a 50%
reduction in breast cancer incidence, yet shows no effect (209). The
authors suggest that the positive results of the NSABP trial at 3.5 yr
of follow-up are most likely due to the treatment of clinically occult
carcinoma, rather than the prevention of new breast cancers. However,
of the 363 total cancers in the NSABP study (141), 99 (28%)
were DCIS, compared with 11% of the 70 cancers in the Royal Marsden
study. The higher percentage of DCIS in the NSABP trial indicates that
the detection of subclinical cancers occurred, and that any treated
occult cancer was not truly amenable to detection by currently
available means. Whether occult carcinoma was treated or true
prevention occurred, a significantly greater number of women were
spared surgery, irradiation, and chemotherapy. The data from the
Overview Analysis (84) do not support the contention that these cancers
will become clinically evident when tamoxifen is stopped, since the
reduction in contralateral breast cancer persists through 10 yr even
though tamoxifen treatment was stopped at 5 yr.
Overall, the results of the NSABP trial (141), with its large study
population, clearly support the benefit of tamoxifen for breast cancer
prevention in high-risk women. These findings are consistent with
laboratory observations and with the contralateral breast cancer risk
reduction seen with tamoxifen therapy (Fig. 4
).
Tamoxifen was approved in 1998 for the reduction of risk in pre- and
postmenopausal women with a high risk of breast cancer. The results of
the NSABP prevention trial have established tamoxifen as the current
standard of care and also opened the door for the evaluation of other
agents, which might have an improved safety or efficacy profile in
clinical trial. Tamoxifen causes rat liver carcinogenesis (216, 217),
and there is an association with an increased incidence of endometrial
cancer (151, 153). Although it is believed that metabolic activation of
tamoxifen to a DNA adduct (218, 219) is unique to the rat liver
(220, 221, 222, 223, 224, 225, 226), an agent that did not produce liver tumors in rats and was
less estrogenic in the rodent uterus might have value for application
as a breast cancer preventive. Raloxifene is being tested in the STAR
trial (Section IX) against tamoxifen primarily because there
is no evidence of rat liver carcinogenesis and there is less
estrogen-like actions in the rodent uterus. It is possible that these
laboratory qualities could translate into fewer endometrial cancers in
women during raloxifene therapy, but this can only be established in
large prospective clinical trials.
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VIII. Biological Basis for Raloxifene as a Breast Cancer Preventive
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Raloxifene [originally named keoxifene or LY 156758 (227)] was
discovered as part of the breast cancer program at the laboratories of
Eli Lilly & Co. in Indianapolis, IN. The drug has a high
binding affinity for ER (228, 229) primarily because it has
strategically located phenolic groups (see Fig. 1
). However, raloxifene
and an analog LY 117018 (90, 230, 231, 232) are short acting compounds
because of poor bioavailability due to rapid phase II metabolism.
Indeed, a concern in the early clinical trials for the treatment of
breast cancer was an inability to monitor blood levels. Although
numerous assays are available to monitor tamoxifen and its metabolites
(105), the structure of raloxifene does not permit the use of similar
chemical methods of detection. Tamoxifen is easily converted from a
triphenylethylene to a phenanthrene by UV light so that fluorescence
detection has been used for two decades to measure drug levels as small
as 1 ng/ml serum. The analytical technique currently used to
monitor raloxifene has not been published. Nevertheless, raloxifene has
limited clinical experience for the treatment of breast cancer. The
initial study conducted at the MD Anderson Hospital in Houston showed
no responses in heavily pretreated patients with stage IV disease
(233). A second small study of 18 ER-positive patients with previously
untreated metastatic disease showed modest response rates of 30%, with
a dose of 300 mg daily (234). The key issue, which has not yet been
addressed, is cross-resistance between raloxifene and tamoxifen. The
use of raloxifene for the prevention of osteoporosis after 5 yr of
adjuvant tamoxifen cannot be assumed to be safe for the patient with
breast cancer until this issue is resolved. A clinical trial comparing
adjuvant treatment with tamoxifen for 5 yr to treatment with tamoxifen
followed by raloxifene is needed to determine whether
raloxifene-stimulated tumor growth is a clinical reality.
The rationale for the use of raloxifene as a breast cancer preventive
is based solely on an hypothesis formulated when SERM was first
recognized (25, 29).
We have obtained valuable clinical information about this group
of drugs that can be applied in other disease states. Research does not
travel in straight lines, and observations in one field of science
often become major discoveries in another. Important clues have been
garnered about the effects of tamoxifen on bone and lipids, so it is
possible that derivatives could find targeted applications to retard
osteoporosis or atherosclerosis. The ubiquitous application of novel
compounds to prevent diseases associated with the progressive changes
after menopause may, as a side effect, significantly retard the
development of breast cancer. The target population would be
postmenopausal women in general, thereby avoiding the requirement
to select a high-risk group to prevent breast cancer (25).
This new strategy to prevent breast cancer was described in 1990 (25).
The evidence for the application of raloxifene in this new paradigm
will be presented.
A. Antitumor actions
Raloxifene inhibits the growth of dimethylbenzanthracene-induced
rat mammary carcinomata (235) but, dose for dose, tamoxifen is more
effective. More importantly for the proposed evaluation as a
preventative, raloxifene reduces the incidence of
N-nitrosomethylurea-induced tumors (30, 236) if given after
the carcinogen but before the appearance of palpable tumors (Fig. 12
). However, as would be anticipated
with a drug that has a short biological half-life, raloxifene is not
superior to tamoxifen at equivalent doses (30). There is no doubt that
raloxifene and its analogs are effective and potent inhibitors of the
growth of breast cancer cells in culture (237, 238), but the
complication of first-pass metabolism in vivo reduces
potency. For this reason, doses above 60 mg raloxifene daily have been
tested in clinical trial to prevent osteoporosis. As stated previously,
raloxifene has only modest antitumor activity for the treatment of
advanced disease (234). Only high-dose therapy (up to 300 mg daily) has
been tested.

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Figure 12. A comparison of the effects of raloxifene (Ra1)
on femur ash density in ovariectomized (OVX) rats (29 ) and the
incidence of rat mammary tumors after the administration of NMU. Rats
were treated with 100 µg and 500 µg raloxifene daily to prevent
mammary carcinogenesis (30 ). These data illustrate the target site
specificity of a drug predicted (25 ) to have the potential to prevent
osteoporosis and breast cancer simultaneously. E2B,
Estradiol benzoate, 50 µg daily orally.
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Based on the hypothesis that raloxifene could reduce the incidence of
breast cancer as a beneficial side effect of the prevention of
osteoporosis (25), the placebo-controlled trials with raloxifene have
been monitored. There are two separate data bases to test the
hypothesis. First, an ongoing single trial entitled Multiple Outcomes
of Raloxifene Evaluation (MORE) has randomized 7,704 postmenopausal
women (mean age 66.5 yr) who had osteoporosis (hip or spine bone
density at least 2.5 SD below normal mean or had vertebrate
fractures) and no history of breast or endometrial cancer, to placebo,
or 60 or 120 mg raloxifene daily. Results at 2 yr, with a total of 32
cases of breast cancer confirmed, indicate a 70% reduction in the risk
of breast cancer (239). The second data base pools all
placebo-controlled trials and includes 10,553 women monitored for, on
average, 3 yr. In this group a 54% reduction in the incidence
of breast cancer in the raloxifene-treated patients is observed (240, 241). As was noted in the tamoxifen study, raloxifene reduces the
incidence of ER-positive breast cancer and has no effect on the
incidence of ER-negative breast cancer. A comparison of the overall
results with raloxifene, the NSABP prevention trial, and the small
chemoprevention studies with tamoxifen is shown in Fig. 13
. It should be pointed out that the
data from the raloxifene study really represent three groups: one
placebo control and two doses of raloxifene, 60 and 120 mg daily. Since
the raloxifene data are pooled and represented in the abstracts as a
percent of control, the events that can be calculated are artificially
high. A more accurate representation of total events would be 30
(control), 15 (60 mg raloxifene daily), and 15 (120 mg raloxifene
daily) for all reported invasive and noninvasive cancers at 3 yr.
Clearly, these are small numbers compared with the NSABP study.
However, the result with raloxifene is strong preliminary data as a
basis for the STAR trial, which compares tamoxifen, the standard of
care, with the test drug raloxifene in women with a high-risk for
breast cancer (see Section IX).

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Figure 13. A comparison of the able-to-be-evaluated events
observed in the studies to reduce the incidence of breast cancer. The
NSABP P-1 trial is the only prospective clinical trial designed to test
the worth of an antiestrogen to prevent breast cancer in 13,388
high-risk women. The figure illustrates the effect of tamoxifen on both
invasive and noninvasive (ductal carcinoma in situ DCIS)
breast cancer. By contrast, the Royal Marsden Study is a pilot project
(209 ) originally designed to be a toxicity evaluation (211 ) in 2,471
high-risk women, and the Italian study reports (210 ) at least one
years data from an original population of 5,408 young women of normal
risk. Finally, the raloxifene data that can only be estimated from
published abstracts (240 241 ), constitute a secondary end point from
10,553 postmenopausal women in osteoporosis trials. The reported cases
are both invasive and noninvasive breast cancers.
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B. Bones
Raloxifene can maintain bone density in ovariectomized rats (Fig. 12
) (28, 242, 243, 244, 245, 246, 247, 248). Urinary pyridinoline and serum osteocalcin are
elevated after ovariectomy, but the elevations are reduced by
raloxifene, thus indicating that raloxifene inhibits bone loss by
reducing bone resorption (243). In general, raloxifene does not
maintain bone density in the ovariectomized rat to the level observed
in an intact animal (242), but efficacy appears equivalent to estrogen
treatment although without increases in uterine weight (242).
Preliminary studies in 251 normal postmenopausal women randomized to
placebo, raloxifene (200 mg daily), raloxifene (600 mg daily), or
Premarin (Wyeth-Ayerst Laboratories, Inc., Philadelphia, PA; 0.625 mg daily) show decreases in serum
alkaline phosphatase, serum osteocalcin, urinary pyridinoline, and
urinary calcium excretion with raloxifene that was no different than
estrogen (249). However, the doses of raloxifene are far higher
than the 60 mg/day currently recommended for the prevention of
osteoporosis. Evaluation of raloxifene, 60 mg/day, on bone remodeling
in early postmenopausal woman, using calcium tracer kinetic methods,
found that although remodeling suppression was greater for estrogen,
the remodeling balance was the same for the two agents (250). The
authors concluded that raloxifene acted on bone as an estrogen agonist.
Indeed, these results are consistent with the finding that raloxifene
increases bone density by 2.4 ± 0.4% in the lumber spine and
2.4 ± 0.4% for the total hip (251). Although the percent
increases in bone density are not as high as would be anticipated with
estrogen or bisphosphonates, it is now clear that raloxifene produces a
40% decrease in spine fractures. There are, however, no reports of a
significant decrease in hip fractures with raloxifene. This contrasts
with the 50% decrease noted with tamoxifen in the prevention study
(141) and adds further support for the need to compare and contrast the
clinical endocrinology of tamoxifen and raloxifene in the STAR trial
(see Section IX).
C. Lipids
Raloxifene produces a significant decrease in low-density
lipoprotein cholesterol, but high-density lipoprotein cholesterol
remains the same (249, 251, 252). Additionally, triglycerides do not
rise during raloxifene treatment. Laboratory data from the rabbit (253)
strongly support the value of raloxifene to prevent atherosclerosis.
However, data from primates fed high-cholesterol diets do not show a
benefit for raloxifene (254). These results have proved to be
controversial (255), as both hormone replacement therapy and tamoxifen
show positive results in the primate model. To address the issue
directly, a prospective randomized clinical trial is in place to
address the question of whether raloxifene has merit for the reduction
of risk for coronary heart disease in postmenopausal women with
elevated risk factors. The study, referred to as Raloxifene Use for The
Heart (RUTH), will randomize 10,000 high-risk women to placebo or
raloxifene (60 mg daily) treatment for 5 yr. Results should be
available by 2005.
D. Uterus
Raloxifene and its analogs have low estrogen-like actions in the
rat uterus (227, 229, 230, 231). Indeed, the raloxifene analog LY117018 is
able to block (at high doses) the estrogen-like effect of tamoxifen on
the rat uterus (231). However, raloxifene and its analogs cannot be
classified as pure antiestrogens in these tests. There is not a
complete lack of uterotropic properties (256, 257), and
estrogen-regulated genes, such as the PgR, are partially activated
(232). Also, mechanistically raloxifene cannot be described as a pure
antiestrogen (117).
Raloxifene is receiving a rigorous evaluation in the human uterus. This
is important because the drug is used to prevent osteoporosis. A
current evaluation in women screened to ensure the absence of
preexisting endometrial abnormalities shows that raloxifene, unlike
estrogen, does not increase endometrial thickness (258). Although there
is not enough information to prove that raloxifene reduces the
incidence of endometrial cancer in treated women, there is no evidence
to suggest that raloxifene increases the risk for endometrial cancer.
Raloxifene does have less estrogenicity in the uterus than tamoxifen,
and it only increases the growth of human endometrial carcinomas under
laboratory conditions by about 50% of that noted with tamoxifen (259).
This, coupled with the preliminary data with raloxifene as a potential
preventive for breast cancer in elderly woman (239, 240, 241), is sufficient
to propose testing against the current standard of care, tamoxifen.
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IX. Study of Tamoxifen And Raloxifene (STAR)
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The STAR trial is a phase III, double-blind trial that will assign
eligible postmenopausal women to either daily tamoxifen (20 mg orally)
or raloxifene (60 mg orally) therapy for 5 yr. Trial participants will
also complete a minimum of two additional years of follow-up after
therapy is stopped.
The STAR trials primary aim is to determine whether long-term
raloxifene therapy is effective in preventing the occurrence of
invasive breast cancer in postmenopausal women who are identified as
being at high risk for the disease. The comparison is to be made to the
established drug, tamoxifen. Its secondary aim is to establish the net
effect of raloxifene therapy, by a comparison of cardiovascular data,
fracture data, and general toxicities with tamoxifen. It is clear that
the activation or suppression of various target sites around a womans
body is similar for tamoxifen and raloxifene, but an evaluation of the
overall comparative benefits of the agents will be an important new
clinical data base for raloxifene in postmenopausal women.
Premenopausal women at risk for breast cancer are, currently, not
eligible for the STAR trial. Although there is extensive information
about the efficacy of tamoxifen in premenopausal breast cancer patients
(84) and women at risk for breast cancer (141), clinical experience
with raloxifene is confined to monitoring the action of the drug in
postmenopausal women. Raloxifene is classified as an antiestrogen with
less estrogen-like actions than tamoxifen (229, 230, 231). However,
tamoxifen has been shown to produce a small decrease in bone density in
premenopausal women (142), and there is concern that raloxifene might
produce greater decreases in bone density. The National Cancer
Institute is currently conducting a randomized study of raloxifene (60
mg daily and 300 mg daily) in high-risk premenopausal women to address
the issue of raloxifene and bone density. Additionally, short-term
raloxifene treatment (5 days or 28 days) causes elevations in
circulating estradiol but does not prevent ovulation (260), consistent
with the known elevation of steroid hormones produced by tamoxifen in
premenopausal breast cancer patients (261). The changes in endocrine
function produced by raloxifene will also be assessed as a prelude to
the recruitment of premenopausal high-risk women to the STAR trial.
The results from the STAR are anticipated by 2006. Clearly it will be
invaluable to establish the overall benefits of the drugs with regard
to breast cancer incidence, coronary heart disease, and osteoporosis.
The comparisons of endometrial cancer will be most instructive because
the standard of care, i.e., self-reporting, will be employed
in the STAR trial rather than routine screening with annual biopsies.
 |
X. The Future of Prevention
|
|---|
The concept of the chemoprevention of cancer was proposed in the
mid-1970s by Sporn and colleagues (262, 263). Essentially, effective
implementation of the strategy requires a target to prevent either the
initiation or the promotion of the cancer cell (or both). The key to
success is, therefore, a well defined target so that a selective action
can be applied without general toxicity. The idea that breast cancer
could be prevented was first proposed by Lacassagne (7) in 1936. He
suggested that an antagonist to estrogen action could prevent the
disease. The target became the ER (22), but serendipitously the
antiestrogens tamoxifen and raloxifene, which block breast cancer cell
growth, were also found to modulate the physiological requirements for
estrogen action selectively at other target sites (28, 29, 30).
The major clinical question for the current application of tamoxifen as
a chemopreventive is when should the 5-yr course be taken and how long
will the effects last to protect a woman at elevated risk for breast
cancer? The simple answer to the first part of the question is that a
woman who is found to fit the elevated risk criteria for breast cancer
will receive benefit through a 55% risk reduction whenever she takes
tamoxifen. However, since there are no rules that can define when a
woman will develop breast cancer if she is found to be at risk, then
earlier rather than later would seem to be the appropriate strategy.
The answer to the duration of benefit part of the question is less
clear, but there are clues that indicate that 5 yr of tamoxifen therapy
results in protection, i.e., risk reduction, for at least 5
yr after the drug is stopped, based on the data about contralateral
breast cancer from the Overview Analysis (84). At this time, further
follow-up is not available. Clearly, it will be important to discover
the mechanism for the long-term beneficial effects of tamoxifen as a
chemopreventive, as this could be exploited further. Similar questions
about the optimal duration of raloxifene therapy for prevention will
need to be addressed in the future. In the case of raloxifene, this is
not as important an issue because long-term therapy for the prevention
of osteoporosis is necessary.
Finally, there is the issue of whether 5 yr of tamoxifen will be
sufficient for prolonged protection from breast cancer, or whether
longer durations of initial treatment will provide longer periods of
protection. For example, after 10 or 15 yr of treatment can a woman get
20 yr of risk reduction? However, there is a reluctance to consider
this type of clinical experiment at present because of a concern about
the development of tamoxifen-stimulated primary breast cancer. The
concern comes from the literature about breast cancer treatment.
Although 10 yr of adjuvant tamoxifen appears to be less effective than
5 yr for the treatment of breast cancer (264), this is not useful
evidence to support the restricted application of tamoxifen as a
chemopreventive. Drug resistance by metastatic breast cancer cells can
probably develop much more rapidly than can occur during the process of
carcinogenesis for primary breast cancer. As a result, it may be
important to evaluate longer durations of tamoxifen as a
chemopreventive agent in clinical trial. Be that as it may, the
challenge for the present is first to establish the efficacy of
raloxifene compared with tamoxifen in the STAR trial, and then to
determine the optimal application of SERMs as a new drug group for the
benefit of womens health.
Finally, a new generation of agents that are specifically designed to
modulate ER
and -ß selectively will become available for clinical
testing within the next decade. A number of postmenopausal diseases
will be targeted, such as osteoporosis and coronary heart disease, but
the beneficial side effects should include a reduction in uterine and
breast cancer in the general population. The present reduction of
breast cancer in high-risk women by 50% is an important first step
that has resulted from the rational application of translational
research. The challenge for the future is to apply developing
laboratory knowledge about the mechanisms of carcinogenesis to prevent
breast cancer completely.
 |
Dedication and Acknowledgment
|
|---|
Our article is dedicated to Professor Elwood V. Jensen of the
Karolinska Research Institute, Stockholm, Sweden, who identified the
estrogen receptor as a target that made the prevention of breast cancer
a practical possibility. We thank Gloria Duncan for typing the
manuscript and Alexander de los Reyes for preparing the figures.
 |
Footnotes
|
|---|
Address reprint requests to: V. Craig Jordan, Ph.D., D.Sc., Director, Lynn Sage Breast Cancer Research Program, Robert H. Lurie Comprehensive Cancer Center, Olson Pavilion 8258, 303 E. Chicago Avenue, Chicago, Illinois 60611 USA.
1 Supported by the generosity of the Lynn Sage Breast Cancer Research
Foundation of Northwestern Memorial Hospital. 
 |
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E. Leygue, H. Dotzlaw, P. H. Watson, and L. C. Murphy
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V C. Jordan and M Morrow
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J. M. Hall, J. F. Couse, and K. S. Korach
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