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Endocrinology Vol. 149, No. 12 6084-6091
Copyright © 2008 by The Endocrine Society

The Pairing of a Selective Estrogen Receptor Modulator, Bazedoxifene, with Conjugated Estrogens as a New Paradigm for the Treatment of Menopausal Symptoms and Osteoporosis Prevention

Yogendra Kharode, Peter V. N. Bodine, Christopher P. Miller, C. Richard Lyttle and Barry S. Komm

Wyeth Research, Women’s Health and Musculoskeletal Biology, Collegeville, Pennsylvania 19426

Address all correspondence and requests for reprints to: Barry S. Komm, Ph.D., Senior Director, Women’s Health and Musculoskeletal Biology–Nuclear, Wyeth Research, Collegeville, Pennsylvania 19426. E-mail: KOMMB{at}wyeth.com.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The menopausal transition is associated with decreased ovarian function and concomitant decline in estrogen production, which may result in physiological effects such as hot flashes, reduced bone mass, and altered lipid profile. It is well established that these unfavorable changes are effectively offset with estrogen therapy (ET) or, in women with a uterus, estrogens in combination with a progestin (hormone therapy). Selective estrogen receptor (ER) modulators (SERMs), which exhibit both ER agonist and antagonist activities depending on the target tissue, have been regarded as offering the potential to provide the benefits of ET and hormone therapy with an improved safety and tolerability profile. To date, no SERM alone has demonstrated an ideal benefit-risk profile for menopausal therapy. The tissue-selective estrogen complex, or the pairing of a SERM with estrogens, may provide an optimal blend of ER agonist and antagonist activities. We evaluated the physiological profile of this novel therapeutic paradigm by using various in vivo models to assess uterine, vasomotor, lipid, and skeletal responses to a tissue-selective estrogen complex partnering bazedoxifene with conjugated estrogens (CE). Bazedoxifene at 3.0 mg/kg effectively antagonized CE-induced uterine stimulation without reversing the positive effects of CE on vasomotor instability. When paired with CE, bazedoxifene at 3.0 mg/kg reduced total cholesterol levels by up to 20% compared with CE alone and significantly increased total bone density relative to control. These preclinical findings showed that the appropriate dose combination of bazedoxifene/CE exhibits positive vasomotor, lipid, and skeletal responses with minimal uterine stimulation.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
ESTROGENS HAVE LONG been known to mediate a broad range of physiological processes, the most notable being uterine and mammary gland development/differentiation and associated reproductive functions (1, 2), maintenance of skeletal bone mass (3, 4), and regulation of lipid metabolism (5) and vasomotor response (6). As women age, decreased ovarian function and concomitant decline in levels of circulating estrogens may result in a number of physiological changes (7). Among the most common effects associated with perimenopause and menopause are vasomotor instability (or an increase in hot flashes) and symptoms related to vulvar-vaginal atrophy (VVA) (8, 9, 10, 11). Loss of bone mass (4, 12, 13, 14) and an altered lipid profile (5, 15) may also occur. Estrogen therapy (ET) has been demonstrated to effectively offset these changes (4, 5, 8, 12, 13, 16); however, in women with a uterus, exogenously administered estrogens are associated with endometrial stimulation and resulting hyperplasia (17, 18, 19, 20), which necessitates coadministration with a progestin [hormone therapy (HT)] to inhibit the abnormal proliferative response (21).

Evidence indicates that ET and HT are highly effective for the relief of bothersome menopausal symptoms (e.g. hot flashes and VVA) (8), preservation of bone mass (i.e. prevention of osteoporosis) (4, 12, 13), and normalization of lipid profiles [i.e. lowering low-density lipoprotein (LDL) and raising high-density lipoprotein (HDL) cholesterol] (5, 16). Continued efforts to provide women with efficacious menopausal therapies with improved safety and tolerability have generated interest in the development of selective estrogen receptor (ER) modulators (SERMs) (22). Similar to estrogens, SERMs have been shown to bind to ERs with high affinity and to regulate transcriptional events (23, 24) in a variety of target tissues, including the uterus, mammary gland, cardiovascular system, skeleton, central nervous system, and liver, among others (2, 25, 26, 27). Whereas estrogens typically exhibit ER agonist effects in all tissues, SERMs demonstrate mixed functional activity (ER agonist/antagonist) depending on the target tissue (28).

To date, no SERM alone has been able to achieve an ideal balance of ER agonist and antagonist activity for an optimal menopausal therapy. However, it may be possible to achieve optimal results based on the blended tissue-selective activities of a SERM and estrogens in a novel approach termed the tissue-selective estrogen complex (TSEC). The SERM chosen to test our hypothesis was bazedoxifene, which has demonstrated favorable effects on bone and lipid profile without endometrial or breast stimulation in both preclinical and clinical studies (29, 30). To determine whether the pairing of bazedoxifene with conjugated estrogens (CE) would result in a physiological profile distinct from that of bazedoxifene or CE alone, we evaluated uterine, lipid, central nervous system/vasomotor, and skeletal responses using various in vivo models.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Detailed methodology for the studies included in this article was previously reported (30) and is briefly outlined here. Chemicals and reagents (of reagent grade or better) were purchased from various vendors. The Wyeth Research compound library (Princeton, NJ) supplied 17{alpha}-ethinyl estradiol (EE), CE (a mixture of the 10 most abundant CE in Premarin), and bazedoxifene, which were either purchased or generated by Wyeth Medicinal Chemistry (Collegeville, PA) and underwent analytical confirmation before use.

Animals
Sprague Dawley rats (aged 10–12 wk, 225–250 g at the time of ovariectomy) were used for all in vivo experiments and were obtained from Taconic Farms (Germantown, NY). Animals were appropriately housed and cared for as previously described (30). All procedures were approved by the Institutional Animal Care and Use Committee and performed in accordance with all federal, state, and local guidelines.

Uterine evaluation
A mature ovariectomized (OVX) rat model of osteopenia was used to evaluate uterine response over a range of CE doses and to determine the dose of bazedoxifene required to antagonize CE-induced uterine stimulation. Uterine wet weight in response to treatment with vehicle control (saline, 2% Tween 80, 0.5% methylcellulose), EE 0.3 mg/kg, CE 0.5, 1.0, 2.5, or 5.0 mg/kg alone, bazedoxifene 3.0 mg/kg alone, or bazedoxifene 3.0 mg/kg in combination with CE was assessed as previously described (30). Compounds were administered orally in saline vehicle to rats once daily for 6 wk. Animals were euthanized approximately 24 h after administration of the last dose. Uteri were then excised and weighed after removal of associated fat and luminal fluids.

Vasomotor instability
Assessment of vasomotor instability using a rodent hot flush model has been described previously (31). Ovariectomies were performed on 60-d-old female rats at least 7 d before initiation of treatment. Briefly, OVX rats were treated for 3 d with vehicle control, EE 0.3 mg/kg, CE 10 mg/kg alone (determined to be the minimally efficacious dose in this model), or CE 10 mg/kg plus bazedoxifene 0.3, 3.0, or 10.0 mg/kg. On the third day of treatment, each animal was administered a morphine pellet sc. An additional two pellets were administered on the fifth day after treatment initiation. To measure tail skin temperature (a surrogate for hot flush response), a thermistor was adhered to the animal’s tail on the eighth day after treatment initiation. Baseline temperature was obtained during the first 15 min, after which naloxone 1 mg/kg was injected sc. Tail skin temperature readings were obtained for 1 h after injection.

Total serum cholesterol measurement
One week after assessment of bone mineral density (BMD) in OVX rats, total serum cholesterol levels of each animal were measured using a Hitachi 911 autoanalyzer (Roche Diagnostics, Indianapolis, IN).

Peripheral quantitative computed tomography (pQCT)
Mature OVX rats (2.5–3 months old, 250 g) were used for BMD assessment. Animals were treated with vehicle control, EE 0.3 mg/kg, ascending doses of bazedoxifene alone, CE alone, or bazedoxifene in combination with CE. Treatments were initiated 3 d after ovariectomy and continued for 6 wk. Volumetric total bone densities (e.g. cortical plus trabecular bone densities) were evaluated in anesthetized OVX rats using an XCT-960M pQCT instrument (Stratec Medizintechnik, Pforzheim, Germany). Details of pQCT methods and analyses (proximal tibia) have been described previously (30). Bone density was reported in milligrams per cubic centimeter.

Tibial histology
Bone histology was assessed in mature OVX rats treated with vehicle control, bazedoxifene 3.0 mg/kg alone, CE 2.5 mg/kg alone, or bazedoxifene plus CE at respective doses. Preparation and histological evaluation of proximal tibiae have been described previously (30). Proximal tibia cancellous bone volume was calculated as bone volume per tissue volume (BV/TV, %).

Statistical analyses
Between-group treatment comparisons were done by one-way ANOVA with the Dunnett test. Analyses were performed using SAS software version 6.1.2 (SAS Institute, Cary, NC). Data are expressed as mean ± SEM.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Uterine response
In the OVX rat model, 6 wk treatment with a range of CE doses (0.5, 1.0, 2.5, and 5.0 mg/kg) demonstrated a dose-dependent increase in uterine wet weight, a surrogate measure for an estrogenic stimulatory response. Uterine wet weight was significantly increased with CE 1.0, 2.5, and 5.0 mg/kg compared with vehicle control (P < 0.01; Fig. 1Go); associated morphological changes were also observed (data not shown). Specifically, increasing doses of CE resulted in an increase in luminal epithelial hypertrophy compared with vehicle control. Maximal uterine stimulation was observed with CE 2.5 mg/kg (Fig. 1Go), which was similar to that seen with EE 0.3 mg/kg and normal levels of circulating estrogens/progestins in sham-operated rats. When coadministered with bazedoxifene 3.0 mg/kg, uterine stimulation induced by CE 2.5 mg/kg was reduced to vehicle control levels. Uterine wet weights of rats treated with bazedoxifene 3.0 mg/kg alone and in combination with the CE 0.5- and 1.0-mg/kg doses were also similar to those with vehicle control, whereas uterine wet weight was slightly, but significantly, increased with bazedoxifene 3.0 mg/kg in combination with CE 5.0 mg/kg compared with vehicle control (P < 0.01; Fig. 1Go).


Figure 1
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  		FIG. 1. Uterine response of bazedoxifene/CE in mature OVX rats. After 6 wk treatment, uterine wet weights (milligrams) in response to EE, CE alone, bazedoxifene alone, and bazedoxifene 3.0 mg/kg paired with ascending doses of CE were determined (n = 8 animals per treatment group). A dose-dependent increase in uterine wet weight, similar to that of EE, was observed with CE. When paired with CE, bazedoxifene 3.0 mg/kg antagonized the CE-induced increase in uterine wet weight to a level similar to that of vehicle control.

 
Vasomotor instability
In the rat tail skin temperature model, CE 10 mg/kg significantly reduced vasomotor instability compared with vehicle control (P < 0.05) and exhibited a temperature reduction similar to that with EE 0.3 mg/kg (Fig. 2Go). Specifically, the approximate 5 C increase in tail skin temperature observed in vehicle control-treated animals after naloxone injection was attenuated by approximately 60% (or 3 C) with both CE 10 mg/kg and EE 0.3 mg/kg (Fig. 2Go). The effects of bazedoxifene alone, which has previously been shown to have no effect on tail skin temperature reduction (29), were not evaluated in these experiments. Here, the significant reduction in tail skin temperature relative to vehicle control was maintained when CE was paired with bazedoxifene 0.3, 3.0, or 10.0 mg/kg (P < 0.05). There were no significant between-group differences in temperature reduction for any bazedoxifene/CE dose combination or CE 10.0 mg/kg alone.


Figure 2
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  		FIG. 2. Vasomotor response of bazedoxifene/conjugated estrogens (CE) in mature OVX rats. CE 10 mg/kg effectively reduced tail skin temperature compared with vehicle control-treated animals (n = 8 per group). This positive effect on vasomotor response was not antagonized when CE was paired with bazedoxifene.

 
Total cholesterol
Blood samples collected from animals that were treated similarly to those assessed for uterine response were analyzed for total cholesterol content. An expected increase in total cholesterol after ovariectomy was observed in this study; specifically, vehicle control-treated OVX animals demonstrated a 134 mg/100 ml total cholesterol level, which was significantly greater compared with the 98.8 mg/100 ml level observed in sham-operated animals (P < 0.05; Fig. 3Go). Although reductions in total cholesterol levels were not as profound as those associated with EE 0.3 mg/kg (approximately a 30% reduction vs. vehicle control, P < 0.05), significant dose-dependent decreases in total cholesterol levels were observed with all CE doses (0.5, 1.0, 2.5, and 5.0 mg/kg) compared with vehicle control (P < 0.01; Fig. 3Go). When CE was coadministered with bazedoxifene 3.0 mg/kg (uterine-protective dose), a 10–20% greater reduction in total cholesterol levels was seen. Total cholesterol levels with all bazedoxifene/CE dose combinations and bazedoxifene 3.0 mg/kg alone were significantly reduced compared with vehicle control (P < 0.01; Fig. 3Go). Bazedoxifene 3.0 mg/kg alone was the most effective treatment in this study and further reduced total cholesterol levels by approximately 10–20% relative to bazedoxifene 3.0 mg/kg in combination with CE.


Figure 3
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  		FIG. 3. Effects of bazedoxifene/CE on total cholesterol levels in mature OVX rats. Blood samples from OVX rats (n = 8 per group) were evaluated after 6 wk treatment with vehicle control, EE, bazedoxifene alone, CE alone, or bazedoxifene paired with CE. All treatments significantly reduced total serum cholesterol levels relative to vehicle control. When CE was paired with bazedoxifene 3.0 mg/kg (the uterine-protective dose), total cholesterol was reduced by up to 20% relative to CE alone.

 
Skeletal response
A dose-ranging study of CE in a rat osteopenia model demonstrated that CE 2.5 and 5.0 mg/kg significantly increased total bone density compared with vehicle control (P < 0.01; Fig. 4Go). Previous dose-response evaluations have consistently shown that CE 2.5 mg/kg is the most efficacious bone-sparing dose of CE in this rat model (data not shown). Bazedoxifene 3.0 mg/kg alone demonstrated a similar skeletal response relative to CE 2.5 mg/kg and significantly increased total bone density compared with vehicle control (P < 0.01; Fig. 4Go). Coadministration of bazedoxifene 3.0 mg/kg with ascending doses of CE (0.5–5.0 mg/kg) was associated with a significant increase in total bone density compared with vehicle control for all dose combinations (P < 0.01; Fig. 4Go). There were no significant between-group differences in total bone density for any bazedoxifene/CE dose combination and bazedoxifene 3.0 mg/kg alone, CE 2.5 or 5.0 mg/kg alone, EE 0.3 mg/kg alone, and sham control.


Figure 4
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  		FIG. 4. Skeletal effects of CE and bazedoxifene with ascending doses of CE in a rat osteopenia model. Bazedoxifene 3.0 mg/kg alone or in combination with ascending doses of CE was found to significantly increase total bone density (proximal tibia) compared with vehicle control-treated animals (n = 8 per group).

 
Similar results were obtained when ascending doses of bazedoxifene (0.1–3.0 mg/kg) were coadministered with a fixed dose of CE 2.5 mg/kg (Fig. 5Go). Significant increases in total bone density were found for all dose combinations of bazedoxifene/CE compared with vehicle control (P < 0.01; Fig. 5Go). These increases were not significantly different between groups and were similar to that with bazedoxifene 0.1, 0.3, 1.0, and 3.0 mg/kg alone, EE 0.3 mg/kg alone, and sham control.


Figure 5
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  		FIG. 5. Skeletal effects of bazedoxifene with or without CE in a rat osteopenia model. When a fixed dose of CE 2.5 mg/kg was paired with ascending doses of bazedoxifene, total bone density (proximal tibia) was significantly increased relative to vehicle control-treated animals (n = 8 per group).

 
Relative to sham-operated rats, a substantial reduction in proximal tibia bone volume (BV/TV) and connectivity was observed in OVX vehicle control rats (Fig. 6Go and Table 1Go). Histological assessment revealed a predominance of marrow space and adipocytes in the vehicle control animals (Fig. 6BGo), whereas normal histological bone quality was maintained in the active treatment groups (Fig. 6Go, C–E). Treatment with CE 2.5 mg/kg or bazedoxifene 3.0 mg/kg alone significantly increased BV/TV relative to vehicle control (P < 0.01); a similar increase was found with bazedoxifene 3.0 mg/kg in combination with CE 2.5 mg/kg (P < 0.01 vs. vehicle control; Table 1Go).


Figure 6
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  		FIG. 6. Effects of bazedoxifene/CE on bone histology in a rat osteopenia model. Representative cross-sectional areas of the growth plate/primary spongiosa region of the proximal tibia of sham-operated rats (A) and OVX vehicle control-treated rats (B) are shown (n = 8 per group). In this region, the connectivity and thickness of trabecular bone (blue-green) were reduced, whereas marrow space (purple) and presence of adipocytes was increased in the vehicle control animals relative to sham controls. Total proximal tibia bone volume (BV/TV; indicated in parentheses) was increased and normal bone histology maintained when OVX rats were treated with CE 2.5 mg/kg alone (C), bazedoxifene 3.0 mg/kg alone (D), or bazedoxifene in combination with CE at respective doses (E) relative to vehicle control (n = 8 per group).

 

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  		TABLE 1. Proximal tibia bone volume

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The purpose of this series of in vivo evaluations was to establish proof-of-concept for a novel treatment paradigm, the TSEC, for the treatment of menopausal symptoms and osteoporosis prevention. This concept was based on the hypothesis that the pairing of CE with the SERM bazedoxifene would result in a physiological profile distinct from the SERM or CE component alone. In particular, the goal was to demonstrate that bazedoxifene/CE has the potential to provide positive tissue-selective responses (e.g. protection of uterine tissue, reduction of vasomotor instability, regulation of lipid profile, and preservation of bone mass) without the negative effects of either component; in other words, an improved pharmacological profile compared with what has been observed with ET, HT, or SERMs alone.

A key physiological response that requires close monitoring in evaluations of estrogens and/or SERMs is uterine stimulation, because these molecules function directly through the ERs, which are abundant in the uterus. In preclinical rodent models, ER-antagonistic activity in the uterus has been found to be a relevant predictor for human endometrial response to SERMs, such as bazedoxifene (29, 32). Such in vivo models are also expected to be useful in predicting the level of inhibition imparted by SERMs when coadministered with CE in the clinical setting. The optimal response would be minimal or no detectable uterine stimulation associated with the chosen dose combination. In the current study, maximal uterine stimulation with CE alone in the OVX rat was observed at CE 2.5 mg/kg. In contrast, bazedoxifene 3.0 mg/kg alone did not stimulate a significant increase in rat uterine wet weight compared with vehicle control. Of note, it was previously demonstrated that a 5-mg/kg dose of bazedoxifene alone did not significantly increase uterine wet weight, or associated morphological changes, compared with vehicle control in a 3-day immature rodent model (30). When bazedoxifene 3.0 mg/kg was coadministered with CE, uterine wet weight and morphological changes to the endometrium were reduced to vehicle control levels. Because the bazedoxifene 3.0-mg/kg dose was approximately 10-fold higher than the established bone-protective dose of bazedoxifene in OVX rats (30), an obvious question that arose was whether this relatively high dose of bazedoxifene would unfavorably antagonize the positive effects of CE on vasomotor, lipid, and skeletal responses.

In general, SERMs do not exhibit a positive vasomotor response as is observed with estrogens in rodent models of vasomotor instability. For instance, it was previously shown that bazedoxifene and raloxifene alone did not reduce vasomotor instability compared with vehicle control in an OVX rat tail skin temperature model (30). In the current study, treatment with CE significantly reduced vasomotor instability compared with vehicle control in the same model, as was expected. When coadministered with CE 10.0 mg/kg, doses of bazedoxifene up to 10 mg/kg had minimal effects on this positive CE-induced reduction in tail skin temperature, suggesting a neutral to minimal antagonist activity of bazedoxifene. Thus, when paired with CE doses lower than 10 mg/kg, the uterine-protective bazedoxifene 3.0 mg/kg dose would be expected to maintain the positive CE-induced vasomotor and skeletal responses. Additionally, although the rat hot flush model is not designed to evaluate uterine wet weight, it was shown that bazedoxifene can efficaciously antagonize the stimulatory effects of CE on the uterus, even at high doses of bazedoxifene/CE. These data suggest that favorable responses can be achieved for several physiological endpoints, including vasomotor instability, with the appropriate dose combination of bazedoxifene and CE.

Although the rat model does not directly mimic human lipid metabolism with regard to LDL and HDL cholesterol concentrations/ratios (33), a reduction in total cholesterol serves as an acceptable surrogate for a reduction in LDL cholesterol, as has been observed in humans after treatment with estrogens and SERMs. Evaluation of total cholesterol data in the rat osteopenia model suggested that bazedoxifene 3.0 mg/kg may be more efficacious in improving lipid profile compared with the maximal effective dose of CE 2.5 mg/kg alone. In contrast to bone density data, total cholesterol levels appeared to improve (e.g. further reductions compared with CE alone) when bazedoxifene 3.0 mg/kg was paired with CE. It is clear that bazedoxifene/CE imparts a positive effect on lipid profile, specifically a decrease in total cholesterol that is representative of the reduced LDL cholesterol levels observed with CE alone in humans.

Data on skeletal effects of bazedoxifene paired with CE were not as easily defined as that of uterine and vasomotor responses. In the rat osteopenia model, this may be attributable to the relatively high doses of CE that were required for optimal bone-preserving efficacy compared with EE. Importantly, the data demonstrated that bazedoxifene 3.0 mg/kg, when paired with CE 2.5 mg/kg, protected the skeleton (i.e. preserved bone mass, increased bone volume, and maintained normal histological bone quality) over 6 wk treatment without any apparent antagonism of the positive CE skeletal response. In this model, the effects of bazedoxifene and CE alone on BMD, as well as proximal tibia bone histology and volume, were not different compared with that of bazedoxifene plus CE at any dose combination. Thus, the data indicate that, at a minimum, the skeletal protection provided by the combination (as measured by maintenance of BMD) would be no less than that with either component alone.

Other SERM and estrogen combinations have been evaluated. Consistent with our preclinical findings for bazedoxifene paired with CE, raloxifene 60 mg/d coadministered with 17β-estradiol 1 mg/d reduced vasomotor instability compared with raloxifene alone in a recent clinical trial (34). However, raloxifene was not able to antagonize estrogen-induced uterine stimulation, and the level of resultant endometrial hyperplasia was unacceptable (34). Lasofoxifene has been coadministered at a dose of 100 µg/kg · d with EE 30 µg/kg · d in OVX rats (35). Uterine wet weights with this combination were significantly reduced compared with EE alone and were similar to that with lasofoxifene alone (35); however, treatment of OVX rats with lasofoxifene alone significantly increased uterine wet weight compared with control (35, 36). To date, no other tissue-specific responses to lasofoxifene in combination with estrogens have been evaluated. Collectively, available data on SERM/estrogen combinations reinforce the requirement for appropriate dosing and the importance of choosing suitably matched SERM and estrogen components to obtain an optimal physiological profile.

A wide range of chemically diverse molecules, including the classic steroidal estrogens (e.g. estrone, 17β-estradiol, EE, and estriol) and SERMs, interact with and affect the functional activity of the ERs to elicit either agonist or antagonist activities in the target tissues. The manner in which a target cell simultaneously responds to a mixture of ER ligands (e.g. both the SERM and estrogen component of a TSEC) is not entirely clear. Hypothetically, the different molecules would compete for the available pool of ERs in the target cell, and simple receptor kinetics (i.e. the bioavailability, concentration, and ER affinity of each molecule) would dictate the dominant effector molecule. It is known, for instance, that bazedoxifene and each of the CE components have different binding affinities, which may directly affect competition for the ERs (37, 38).

It is expected that each molecule would uniquely regulate transcription of target genes and subsequent protein expression and that the final physiological response would be attributed to a blend of all of the molecules’ activities. The binding of various ER ligands has been eloquently shown to elicit slightly different receptor conformations, which directly impacts coactivator/corepressor interactions, biochemical modifications (e.g. phosphorylation of key amino acids), and subsequent transcriptional efficacy of the ligand-receptor complex (2, 24, 39, 40, 41). Data have shown both qualitative and quantitative differences in gene expression profiles between SERMs and estrogens in specific cell types (42). It is expected that a similarly unique gene expression profile and subsequent physiological response would be observed with a TSEC when compared with the SERM or estrogen component alone.

In the current studies, evaluated physiological responses appeared to be influenced predominantly by one of the TSEC components, with the overall physiological profile reflecting a blend of both bazedoxifene and CE activities. The dominant component (either bazedoxifene or CE) as reflected in the final physiological response varied between target tissues. In the uterus, bazedoxifene was shown to effectively abrogate the stimulatory effects of CE on the endometrium. Both CE and bazedoxifene demonstrated positive agonistic effects on the lipid profile, although bazedoxifene alone was the most efficacious treatment in reducing total cholesterol levels. Vasomotor data indicated that the positive temperature reduction observed with CE alone was maintained with all dose combinations of bazedoxifene/CE. Thus, the antagonistic effects of bazedoxifene were low enough to achieve optimal vasomotor response with the appropriate bazedoxifene/CE combination.

Both bazedoxifene and CE alone significantly increased total bone density in the rat osteopenia model relative to vehicle control; however, in contrast to other evaluated endpoints, skeletal responses did not reveal a dominant effector molecule or suggest a mechanism of additive agonist activity. One potential explanation is that positive skeletal effects may be maximal with the evaluated doses of bazedoxifene and CE alone; thus, no synergistic or additive effects could be observed with the dose combinations evaluated in this model. Importantly, all doses of bazedoxifene/CE maintained total bone density and proximal tibia bone volume at a level similar to that of both bazedoxifene and CE alone.

The novel TSEC that pairs bazedoxifene with CE appears to provide, for the first time, an optimal physiological profile aligned with the goals of treating menopausal symptoms and preventing osteoporosis, along with endometrial safety, an important consideration with new menopausal therapies. These data show that bazedoxifene/CE elicits favorable vasomotor, lipid, and skeletal responses, with minimal uterine stimulation at the appropriate dose combinations. Thus, bazedoxifene/CE represents a promising therapeutic option and warrants further clinical evaluation. Other TSECs are likely to be evaluated, and as we learn more about the complex interactions of these ER ligands at the transcriptional level, we may better predict combinations that will achieve optimal responses in target tissues with improved safety and tolerability profiles.


   Footnotes
 
This work was funded by Wyeth Research, Collegeville, PA.

Current address for C.P.M. and C.R.L.: Radius Health, Cambridge, Massachusetts.

Disclosure Statement: Y.K., P.B., and B.K. are employed by Wyeth. C.M. and C.R.L. were previously employed by Wyeth.

First Published Online August 14, 2008

Abbreviations: BMD, Bone mineral density; BV/TV, bone volume per tissue volume; CE, conjugated estrogens; EE, 17{alpha}-ethinyl estradiol; ER, estrogen receptor; ET, estrogen therapy; HDL, high-density lipoprotein; LDL, low-density lipoprotein; OVX, ovariectomized; pQCT, peripheral quantitative computed tomography; SERM, selective ER modulator; TSEC, tissue-selective estrogen complex; VVA, vulvar-vaginal atrophy.

Received June 2, 2008.

Accepted for publication August 7, 2008.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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