Download Lack of Efficacy of the Statins Atorvastatin and Lovastatin in Rodent

Cancer Prevention Research
Lack of Efficacy of the Statins Atorvastatin and Lovastatin in Rodent
Mammary Carcinogenesis
Ronald A. Lubet,1 Daniel Boring,1 Vernon E. Steele,1 J. Michael Ruppert,2
M. Margaret Juliana2 and Clinton J. Grubbs2
Abstract
The statins are highly effective in lowering cholesterol by inhibiting 3-hydroxy-3-methylglutaryl CoA reductase. Recently, there has been conflicting epidemiologic data indicating
that statins decrease the incidence of certain types of cancer, including breast cancer. Atorvastatin and lovastatin, statins with different lipophicilities, were administered in diet either
as single agents or in combination with suboptimal doses of tamoxifen or the retinoid X receptor agonist bexarotene were evaluated for prevention of estrogen receptor–positive
mammary cancers induced in the rat with methylnitrosourea. Atorvastatin (125 or 500 mg/
kg diet) alone did not significantly alter cancer incidence or multiplicity. Suboptimal doses of
tamoxifen (0.4 mg/kg diet) or bexarotene (80 mg/kg diet) reduced cancer multiplicity from 3.8
(control) to 2.9 and 0.9, respectively. Combining atorvastatin (500 mg/kg diet) with either of
these effective agents minimally altered their efficacy. Although this dose of atorvastatin did
not decrease serum triglyceride levels in control rats, it significantly decreased triglyceride
levels that had been increased in bexarotene-treated rats. Experiments done with a second
statin, lovastatin (100 and 400 mg/kg diet), yielded similar results: (a) limited activity when
administered alone, (b) no obvious synergy with bexarotene, and (c) an ability to decrease
bexarotene-induced increases in serum triglycerides. Thus, the statins had minimal activity
in this model of mammary cancer in which approximately half of the cancers are mutated in
the Ha Ras oncogene. Similarly, atorvastatin failed to alter the development of estrogen receptor–negative mammary carcinomas in a new animal model using bitransgenic mice
(MMTV-Neu+/−/p53KO+/−), whereas bexarotene (250 mg/kg diet) was effective.
T he statins reduce serum cholesterol by inhibiting the
upstream enzyme 3-hydroxy-3-methylglutaryl CoA (HMGCoA) reductase. Recently, there have been epidemiologic
reports that this class of agents may reduce the incidence of
cancer (1–3). Although perhaps the most convincing epidemiologic data have been associated with colon cancer (2) and
prostate cancer (3), there are data implying that statins may
also alter the incidence of breast cancer (4–6). One early hypothesis was that by inhibiting cholesterol synthesis, the statins would decrease the production of farnesyl PPi and
geranyl PPi (7). These metabolites are used in the prenylation
and activation of a wide variety of proteins. Included among
the prenylated proteins are the known oncogenes Ha Ras and
Ki Ras, and proteins such as RHO A,B,C and CEBPE/F. Many
of these are either known to be or are hypothesized to contribute to the oncogenic process (8). In addition, the statins have
been shown, at least in vitro, to increase cellular levels of the
two cyclin-dependent kinase inhibitors p21 and p27, which diminish cell proliferation (7).
The methylnitrosourea (MNU)–treated Sprague-Dawley
rats develop multiple estrogen receptor–positive (ER+) cancers that have a histopathology (9) and gene expression profile
(10) similar to highly differentiated ER + breast cancer in
women. Tumors in this model have proven to be highly susceptible to a wide variety of antihormonal agents (e.g., tamoxifen, aromatase inhibitors) that are effective against human
ER+ breast cancer. In addition, the cancers are susceptible
to other agents such as retinoid X receptor (RXR) agonists
(11–13) and epidermal growth factor receptor inhibitors (14)
that may not work directly by inhibiting the hormonal axis.
Unlike human breast cancer where Ras mutations are rarely
observed, approximately half of the MNU-xinduced cancers
have Ha Ras mutations (15). Tumors with Ha Ras mutations
are particularly sensitive to the preventive and therapeutic effects of the farnesyl transferase inhibitor tipifarnib (16). Because statins should decrease prenylation, the presence of
Ha Ras in 50% of MNU-induced mammary tumors (15)
should have made this model particularly sensitive to prevention by statins.
Authors' Affiliations: 1Chemopreventive Agent Development Research Group,
Division of Cancer Prevention, National Cancer Institute, Bethesda, Maryland
and 2Departments of Surgery, Medicine, and Genetics, University of Alabama at
Birmingham, Birmingham, Alabama
Received 07/07/2008; revised 09/23/2008; accepted 11/20/2008.
Grant support: National Cancer Institute contract HHSN261200433001C
and grant R01 CA127405.
Requests for reprints: Ronald A. Lubet, National Cancer Institute, Executive
Plaza North, Suite 2110, 6130 Executive Boulevard, Bethesda, MD 20852.
Phone: 301-594-0457; Fax: 301-402-0553; E-mail: [email protected].
©2009 American Association for Cancer Research.
doi:10.1158/1940-6207.CAPR-08-0134
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Cancer Prevention Research
time period) at 63, 94, and 126 d after the administration of the carcinogen to determine serum triglyceride levels. For the study that
evaluated lovastatin, blood was collected only at termination of the
study. In both studies, blood was collected from rats not receiving
the carcinogen. After centrifugation of the blood, the serum was frozen at −85°C until it was analyzed for triglycerides (22). The Infinity
triglyceride assay kit was purchased from Thermo DMA.
As indicated earlier, chemically induced cancers in rats are
primarily ER+ (17), whereas many of the mammary cancer
models in transgenic mice yield ER− tumors. The mouse mammary tumor virus (MMTV)-Neu transgenic model uses overexpression of NEU under the control of a MMTV promoter to
induce the development of multiple ER− mammary carcinomas. However, the majority of the resulting tumors have a
mutation in the transmembrane domain in Neu unlike most
human Neu-expressing tumors (18). By making a bitransgenic
animal that expresses the MMTV-Neu transgene and that also
has an alteration in the p53 tumor suppressor gene, the resulting tumors overexpress Neu and have an altered p53 (19).
However, the resulting tumors do not have a mutation in
the transmembrane domain. This lack of mutation in Neu
and an alteration in p53 are characteristic of human ER− tumors that overexpress Neu.
In the present experiments, the ability of two structurally
distinct statins (lovastatin and atorvastatin) to prevent the development of ER+ cancers in rats was evaluated. The statins
were tested either alone or in combination with suboptimal
doses of tamoxifen or the RXR agonist bexarotene. Both of
these agents have proven to be highly effective in inhibiting
tumor development (multiplicity and incidence) in this model
when used at higher doses. In addition, we examined the ability of the statins to modulate serum triglyceride levels in normal rats as well as in rats treated with bexarotene. Finally, the
efficacies of atorvastatin and bexarotene to prevent ER− mammary cancers in heterozygous MMTV-Neu+/−/p53KO+/− bitransgenic mice were also determined.
MMTV-Neu+/−/p53KO+/− mammary cancer model
Female MMTV-Neu+/−/p53KO+/− mice were placed on a diet supplemented with atorvastatin (200 mg/kg diet) or bexarotene (250 mg/
kg diet) beginning at 60 d of age. The numbers of mice of group were
as follows: controls, 31; atorvastatin, 27; and bexarotene, 28. The animals remained on the diet until the termination of the study (at 310 d
of age). The mice were weighed weekly, palpated for mammary tumors twice per week, and checked daily for signs of toxicity. At necropsy, all mammary lesions were removed and histologically
evaluated (20).
Statistical analysis
Chemopreventive effects of the statins on cancer incidence and latency were determined using the log-rank analysis, and differences in
cancer multiplicity were determined using the Armitage test (23, 24).
The Student's t test was used to compare differences in animal body
weights and serum triglyceride levels.
Results
In an initial study (data not shown), the efficacy of lovastatin and atorvastatin against MNU-induced mammary cancers
were evaluated at relatively low doses: 100 and 125 mg/kg
diet, respectively. Because these dose levels did not alter tumor multiplicity or incidence, additional studies using higher
doses of the agents were done. Statin, tamoxifen, or the combination of these agents did not alter the body weight gain of
the animals in any of the studies >7% from the respective controls. Atorvastatin was given at 500 mg/kg diet beginning
5 days after MNU and continuing for the duration of the studies
(Table 1; Fig. 1). The number of mammary cancers at the end
of the study in the MNU-treated-only group was 3.8 per rat.
Atorvastatin caused a 30% increase in tumor number, whereas
bexarotene (80 mg/kg diet) and tamoxifen (0.4 mg/kg diet)
caused 76% and 24% decreases, respectively. The combinations of atorvastatin with bexarotene or tamoxifen did not
alter the preventive efficacy from that observed when bexarotene or tamoxifen were given alone. The effects of the agents
on serum triglyceride levels were determined at three time intervals during the study (at 63, 94, and 126 days after initiating
treatment in rats not receiving the carcinogen). As indicated in
Table 2, bexarotene greatly increased triglycerides when given
alone. Atorvastatin treatment did not reduce triglyceride levels in rats when given alone, but diminished the increase in
triglycerides by bexarotene when the two agents were given in
combination. Tamoxifen, as expected, did not modify triglycerides either when given alone or in combination with atorvastatin.
In the second experiment, lovastatin (400 mg/kg diet) and
bexarotene (80 mg/kg diet) were given alone or in combination to determine their effects on mammary cancer prevention.
In this study, the control group developed 3.6 cancers per rat
with a mammary cancer incidence of 93% (Table 3; Fig. 2).
Lovastatin treatment caused a slight increase (19%) in cancer
multiplicity. As seen with atorvastatin, the combination of lovastatin with bexarotene was no more or less effective than the
Materials and Methods
Experimental animals and chemicals
Female Sprague-Dawley rats were obtained from Harlan SpragueDawley, Inc., at 4 wk of age. The bitransgenic (MMTV-Neu+/−/p53
KO+/−) female mice were generated in the Chemoprevention Center
at the University of Alabama at Birmingham. The p53-deficient line
p53N5-T was purchased in a C57BL background (Taconic) and backcrossed at least five times onto a FBV/N background (The Jackson
Laboratory). MMTV-Neu transgenic mice [strain FVB/N-Tg
(MMTVneu) 202Mul/J] were purchased from The Jackson Laboratory.
For generation of MMTV-Neu+/−/p53KO+/− females, p53−/− males
were crossed to MMTV-Neu+/+ females (20, 21). All animals were
housed in groups of five per cage in a room maintained at 22 ± 2°C
and artificially lighted 12 h/d. Atorvastatin, lovastatin, and bexarotene were obtained from the National Cancer Institute Prevention Repository. Tamoxifen was purchased from Sigma Chemical Company,
and MNU was from the National Cancer Institute Chemical Carcinogen Repository. All agents evaluated for prevention activity were incorporated into the diet by mixing with mash feed using a liquid-solid
blender with intensifier bar (Patterson-Kelly).
MNU mammary cancer model
When rats were 50 d of age, they were injected i.v. (via the jugular
vein) with MNU (75 mg/kg body weight) as previously described
(13). Rats (15 per group) were given the statins, bexarotene, and/or
tamoxifen (either as a single agent or in combination) beginning 5 d
after MNU. Rats were weighed weekly, palpated for mammary tumors twice per week, and checked daily for signs of toxicity. The studies were terminated 126 d after MNU. Mammary tumors were
removed at necropsy of the rats and examined histopathologically
as previously described (9).
During the study that evaluated atorvastatin, blood (0.5 mL) was
collected from the jugular vein of anesthetized rats (5 per group per
Cancer Prev Res 2009;2(2) February 2009
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Statins and Mammary Cancer
Table 1. Effects of atorvastatin, bexarotene, and tamoxifen (alone and in combination) in the prevention of
MNU-induced mammary cancers
Group
Carcinogen*
1
2
3
4
5
6
MNU
MNU
MNU
MNU
MNU
MNU
Treatment†
Mammary adenocarcinomas‡
None
Atorvastatin, 500 mg/kg diet
Bexarotene, 80 mg/kg diet
Bexarotene, 80 mg/kg diet plus atorvastatin, 500 mg/kg diet
Tamoxifen, 0.4 mg/kg diet
Tamoxifen, 0.4 mg/kg diet plus atorvastatin, 500 mg/kg diet
Incidence (%)
Multiplicity
93
93
60
47¶
73
73
3.8 ± 0.7
4.9 ± 0.7 (29%↑)§,∥
0.9 ± 0.2 (76%↓)¶,**
0.9 ± 0.3 (76%↓)¶
2.9 ± 0.8 (24%↓)∥,††
2.2 ± 0.6 (42%↓)∥
*Female Sprague-Dawley rats received MNU at 50 d of age.
†
Chemopreventive agents administered when the rats were 55 d of age.
‡
Study terminated 126 d after MNU.
§
Values are mean ± SE (n = 15 rats per group). The number in parenthesis is the percent difference from control group.
∥
No statistical difference from group 1; P > 0.05.
¶
Statistically different from group 1; P < 0.05.
**
No statistical difference from group 4; P > 0.05.
††
No statistical difference from group 6; P > 0.05.
the statins is the relatively recent and somewhat conflicting
epidemiology reports regarding the efficacy of this class of
agents in preventing breast cancer (4–7). In the present experiments, the statins were first examined as potential preventive agents in a chemically induced model of mammary
carcinogenesis in rats. Each of the statins was initially tested
at lower doses (atorvastatin, 125 mg/kg diet, and lovastatin,
administration of the RXR agonist alone. Lovastatin also prevented the large increase in serum triglycerides caused by bexarotene but did not alter these levels from controls when given
alone (Table 3).
Finally, atorvastatin (and a positive control bexarotene),
when given as single agents to MMTV-Neu+/−/p53KO+/− bitransgenic mice, was evaluated for its preventive efficacy
against ER− mammary cancers. Other lesions (e.g., skin cancers, lymphomas) were less than three per group and were
not related to treatment. As seen in Fig. 3, atorvastatin had
minimal effects on the development of mammary cancers
when compared with control bitransgenic mice, whereas bexarotene was effective in preventing the appearance of tumors
(P < 0.05).
Discussion
A carcinogen-induced model of mammary cancer in rats
was developed five decades ago by Huggins and coworkers
(17). The resulting mammary cancers appear histologically
similar to invasive ductal adenocarcinoma in women. Recent
gene array analyses of these tumors have shown them to be
similar to highly differentiated human ER+ tumors (10). As expected, these tumors are responsive to a wide variety of agents
that are effective in prevention and/or therapy of human cancers, including selective ER modulators (SERM), aromatase inhibitors, and pregnancy (25, 26). Because of its relative ease of
performance, this model has been used to screen for potential
chemopreventive efficacy using the widest range of agents.
Various nonhormonal agents (e.g., RXR agonists and, more recently, farnesyltransferase inhibitors and epidermal growth
factor receptor inhibitors) have all been shown to be highly
active (11–14).
Lovastatin and atorvastatin are two small-molecule inhibitors of HMG-CoA reductase that alter cholesterol metabolism (1). Both have KIs in the low nanomolar range when
used against purified HMG-CoA. The impetus for testing
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Fig. 1. Effects of atorvastatin, bexarotene, and tamoxifen (alone and in
combination) on the time of appearance of mammary cancers induced with
MNU. The groups were as follows: atorvastatin (500 mg/kg diet), ; bexarotene
(80 mg/kg diet),
; tamoxifen (0.4 mg/kg diet), □; atorvastatin (500 mg/kg diet)
plus bexarotene (80 mg/kg diet); ; atorvastatin (500 mg/kg diet) plus
tamoxifen (0.4 mg/kg diet), ; none, . The latency of the cancers developing
in the bexarotene and the atorvastatin plus bexarotene groups was
statistically different from the controls (P < 0.05).
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Table 2. Effects of atorvastatin, bexarotene, and tamoxifen on serum triglyceride levels in female SpragueDawley rats
Group
1
2
3
4
5
6
Serum triglycerides (mg/dL)*
Treatment
None
Atorvastatin, 500 mg/kg diet,
Bexarotene, 80 mg/kg diet
Bexarotene, 80 mg/kg diet plus atorvastatin, 500 mg/kg diet
Tamoxifen, 0.4 mg/kg diet
Tamoxifen, 0.4 mg/kg diet plus atorvastatin 500 mg/kg
63 d
94 d
126 d
40 ± 5†
41 ± 9
211 ± 36‡
89 ± 17§
39 ± 4
24 ± 4
27 ± 3
25 ± 6
150 ± 24‡
69 ± 11§
47 ± 5
20 ± 3
65 ± 6
53 ± 12
264 ± 34‡
141 ± 23§
80 ± 9
49 ± 3
*Serum triglycerides were determined at the indicated time periods after initiating treatment with the chemopreventive agents.
n = 5. Values are mean ± SE.
‡
Statistically different from group 1; P < 0.05.
§
Statistically different from group 3; P < 0.05.
†
However, the evaluation of how lipophilicity might influence statin chemoprevention is complex. Lipophilicity ranking of compounds is routinely expressed by the use of
either experimentally determined or computed log P values
(log of the n-octanol/water partition coefficient). Although
an approximate log P value is readily generated, extrapolation of the measured log P in a highly complex organism is
fraught with questions. The log P value is premised on the
basis of a neutral molecule partitioning between two separate but chemically homogenous static solvent systems.
When ionizable groups are present in a molecule, the protonation state will change with pH. Therefore, the log P value must be adjusted to reflect the pH of the medium and
100 mg/kg diet) and found to have minimal effects on tumor incidence and multiplicity. They were subsequently
tested at higher, albeit nontoxic, doses (atorvastatin, 500
mg/kg diet, and lovastatin, 400 mg/kg diet), which similarly failed to significantly decrease cancer multiplicity (Tables
1 and 3). In fact, increases in tumor multiplicity of 20% to
30% were observed; however, these increases were not statistically significant. The results were somewhat unexpected
because of one unusual characteristic of the model: ∼50% of
the MNU-induced cancers have mutations in the Ha Ras
oncogene, specifically at codon 12 (15). We have previously
shown that the Ha Ras–mutated tumors are highly sensitive
to the preventive and therapeutic activity of the farnesyltransferase inhibitor tipifarnib (R115777; ref. 16). This is presumably due to blocking of farnesylation of Ha Ras, which
is necessary for activation of Ras proteins. Because statins
would be expected to block the prenylation process upstream of an FTI inhibitor, one might expect these cancers
to be sensitive to prevention by statins. Finally, atorvastatin
and bexarotene were tested as single agents in an ER− model of mammary cancers in bitransgenic mice. The MMTV/
Neu model was developed almost 15 years ago by Mueller
and colleagues (21). The specific bitransgenic mice used in
this study were heterozygous both for the MMTV-Neu
transgene and KO of p53. The resulting mice developed
mammary carcinomas that overexpress Neu and have an alteration in p53 (19), which is similar to human ER− cancers
expressing Neu. As observed in the rat, no effect of atorvastatin on cancer formation was found but bexarotene (at the
dose used) reduced tumor multiplicity by ∼60%. The efficacy of bexarotene is even more striking given that only 23%
of the bexarotene-treated bitransgenic mice had developed a
tumor at 310 days, whereas 50% of control mice had developed tumors at 274 days of age.
We chose the two specific statins (lovastatin and atorvastatin) because they both have been commonly used in humans
and because atorvastatin is considered relatively lipophilic
and lovastatin less lipophilic. Some investigators had proposed that relatively lipophilic statins (e.g., atorvastatin)
might exhibit significantly greater chemopreventive activity
compared with less lipophilic statins (7).
Cancer Prev Res 2009;2(2) February 2009
Fig. 2. Effects of lovastatin and bexarotene (alone and in combination) on the
time of appearance of mammary cancers induced with MNU. The groups were
as follows: lovastatin (400 mg/kg diet), ; bexarotene (80 mg/kg diet),
;
lovastatin (400 mg/kg diet) plus bexarotene (80 mg/kg diet), □; none, . The
latency of the cancers developing in the bexarotene and the lovastatin plus
bexarotene groups was statistically different from the controls (P < 0.05).
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Statins and Mammary Cancer
Table 3. Efficacy of lovastatin and bexarotene (alone or in combination) in the prevention of MNU-induced
mammary cancers
Group
1
2
3
4
Carcinogen*
MNU
MNU
MNU
MNU
Treatment†
None
Lovastatin, 400 mg/kg diet
Bexarotene, 80 mg/kg diet
Bexarotene, 80 mg/kg diet ±
Lovastatin 400 mg/kg diet
Serum triglyceride levels (mg/dL)§
Mammary
adenocarcinomas‡
Incidence (%)
Multiplicity
93
93
67
67
3.6 ± 0.5
4.3 ± 0.9 (19%↑)∥
1.3 ± 0.3 (64%↓)¶
1.2 ± 0.3 (67%↓)¶,**
23 ±
24 ±
225 ±
97 ±
3
4
18¶
9 ¶,††
*Female Sprague-Dawley rats received MNU at 50 d of age. n = 15 rats per group.
†
Chemopreventive agents administered when the rats were 55 d of age.
‡
Study terminated 126 d after MNU.
§
Serum triglycerides determined at the end of study (n = 5). Values are mean ± SE.
∥
The number in parenthesis is the percent difference from control group.
¶
Statistically different from group 1; P < 0.05.
**
No statistical difference from group 3; P > 0.05.
††
Statistically different from group 3; P < 0.05.
the ionization constant of the acid. Thus, a standard log P
value does not capture true lipophilicity of ionizable compounds such as the statins. The log D value (log D = log
P− [1 + 10 (pH-pKa)] for an acid) is a more accurate measure
of true lipophilicity. However, even with the addition of pH,
factors such as multiple phases, temperature fluxes, specific
drug receptors, transport mechanisms, multiple ionized species, and drug metabolism are not modeled by the log P or
log D equation.
Another factor that complicates interpreting the true influence of lipophilicity on the activity of the statins in a biological
matrix is the ability of statins to equilibrate between a cyclic
lactone (non-ionized) and free acid (ionized) structure. The lac-
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tone and open acid can be interconverted spontaneously both
through pH-dependent chemical hydrolysis and enzymatic
actions. Each individual form undergoes separate binding
and partitioning with specific lipophilicity influences operating on each form. These many issues are illustrated in the
Scheme and highlight the complexity involved in attempting
to correlate lipophilicity with tissue concentrations and relative
efficacy.
Although atorvastatin and lovastatin given alone were ineffective, we evaluated the combination of statins with other
agents with known preventive efficacy. This was to test for
possible enhanced or decreased efficacy of the combinations
of agents, as well as to determine whether the expected
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1 and 3, no increase in efficacy was observed when combining a suboptimal dose of bexarotene with either of the statins. Bexarotene and various RXR agonists strongly increase
triglyceride levels (29). We, therefore, determined whether
the combination of bexarotene and either lovastatin or atorvastatin would diminish the increase in triglyceride levels
induced by bexarotene. As shown in Tables 2 and 3, both
lovastatin and atorvastatin significantly decreased the bexarotene-induced increase in serum triglycerides. These studies
also showed that the statins do not interfere with the efficacy of this RXR agonist. This is important because the present clinical use of bexarotene in cutaneous T-cell lymphoma
has used this agent in combination with a statin.
In summary, the statins were not effective by themselves in
this model of breast cancer despite the fact that the doses were
relatively high. In contrast, these doses were effective in decreasing azoxymethane-induced colon cancer in rats (30).
Our data demonstrating lack of efficacy of the statins in two
mammary models is in contrast to a prior study examining
statins in transgenic mice (31), which showed significant, albeit limited, activity of lovastatin. The dose used was lower than
our dose on a mg/kg body weight basis. However, the methods of administration (diet in the present study versus i.p.)
make it difficult to directly compare because i.p. administration circumvents typical concerns of absorption and hepatic
metabolism. Similarly, two articles examining the effects of
statins on growth of grafted tumor cells (32, 33) also used i.
p. administration of the statins and showed limited efficacy.
This may contribute to the different results obtained in the
various studies. The present experiments, however, clearly
show the lack of efficacy of these statins following dietary dosing in two in vivo models of mammary cancer used routinely
in screening for preventive agents.
Fig. 3. Effects of atorvastatin and bexarotene on the time of appearance of
mammary cancers occurring spontaneously in MMTV-Neu+/−/p53KO+/−. The
groups were as follows: atorvastatin (200 mg/kg diet), □; bexarotene (80 mg/kg
diet), ; none, . Statistical analysis of tumor latency indicated no difference
between atorvastatin and controls but a statistical difference between
bexarotene and controls (P < 0.05). The mean times of tumor appearance for
each of the groups were as follows: controls, 217 d; atorvastatin, 232 d; and
bexarotene, 250 d.
○
•
physiologic effect of statins (decreased serum triglycerides)
was achieved. In these studies, the statins were combined
with suboptimal doses of two agents, the SERM tamoxifen
and the RXR agonist bexarotone. Tamoxifen is a SERM that
functions as an ERα antagonist in the breast (27). Tamoxifen
is highly effective in prevention of ER+ breast cancer, both
in this animal model and in human cancer. As seen (Table
1), there was no improvement in efficacy by combining
atorvastatin with a low suboptimal dose of tamoxifen. The
second agent used was the RXR agonist bexarotene. Bexarotene and related RXR agonists have proven to be highly
effective in preventing ER+ tumors as well as ER− mammary tumors in rodent models (11–13, 28). As shown in Tables
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Acknowledgments
We thank Jeanne Hale and Mary Jo Cagle for editorial services; Tom
Morgan, Bonnie Mould, and Caroline Kirkner for performing all aspects of the
animal studies; and Julie Gray for analytic determinations on diets and serum.
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