Effects of various selective estrogen receptor modulators with or without conjugated estrogens on mouse mammary gland

Palazestrant (OP-1250), A Complete Estrogen Receptor Antagonist, Inhibits Wild-type and Mutant ER-positive Breast Cancer Models as Monotherapy and in Combination

The estrogen receptor (ER) is a well-established target for the treatment of breast cancer, with the majority of patients presenting as ER-positive (ER+). Endocrine therapy is a mainstay of breast cancer treatment but the development of resistance mutations in response to aromatase inhibitors, poor pharmacokinetic properties of fulvestrant, agonist activity of tamoxifen, and limited benefit for elacestrant leave unmet needs for patients with or without resistance mutations in ESR1, the gene that encodes the ER protein. Here we describe palazestrant (OP-1250), a novel, orally bioavailable complete ER antagonist and selective ER degrader. OP-1250, like fulvestrant, has no agonist activity on the ER and completely blocks estrogen-induced transcriptional activity. In addition, OP-1250 demonstrates favorable biochemical binding affinity, ER degradation, and antiproliferative activity in ER+ breast cancer models that is comparable or superior to other agents of interest. OP-1250 has superior pharmacokinetic properties relative to fulvestrant, including oral bioavailability and brain penetrance, as well as superior performance in wild-type and ESR1-mutant breast cancer xenograft studies. OP-1250 combines well with cyclin-dependent kinase 4 and 6 inhibitors in xenograft studies of ER+ breast cancer models and effectively shrinks intracranially implanted tumors, resulting in prolonged animal survival. With demonstrated preclinical efficacy exceeding fulvestrant in wild-type models, elacestrant in ESR1-mutant models, and tamoxifen in intracranial xenografts, OP-1250 has the potential to benefit patients with ER+ breast cancer.

Protective effects of combining SERMs with estrogen on metabolic parameters in postmenopausal diabetic cardiovascular dysfunction: The role of cytokines and angiotensin II

INTRODUCTION

The beneficial effects of the administration of selective estrogen receptor modulators (SERMs) and estrogen (E2), alone or in combination with each other, have been reported in postmenopausal diabetic cardiovascular dysfunction. In the present study, we determined the mechanism of action of SERMs and E2 on inflammatory balance, angiotensin II (Ang II) serum levels, and glycemic profile in a postmenopausal diabetic rat model.

METHODS

Ovariectomized rats with type 2 diabetes received daily SERMs (tamoxifen and raloxifene) and E2 for one month. After treatment, cardiovascular risk indices, glycemic profile, and serum Ang II, TNF-α and IL-10 levels were measured.

RESULTS

Type 2 diabetes caused an abnormal glycemic profile, which was exacerbated by ovariectomy. All treatments inhibited the effects of diabetes and ovariectomy on the glycemic profile, with combined treatments (SERMs + E2) showing stronger effects. Cardiovascular risk indices that became abnormal by diabetes and worsened by ovariectomy were improved in all treatment modalities. Also, combined treatment reduced serum Ang II, TNF-α, and the ratio of TNF-α to IL-10, indicating an improvement in inflammatory balance.

CONCLUSION

Our study showed the administration of SERMs and E2, alone or in combination, could be an effective alternative in the treatment of menopausal diabetes, and generally, the beneficial effects of combined treatments were more effective than the effects of E2 or SERMs alone. It appears that E2 or SERMs benefit the cardiovascular system by improving inflammatory balance and reducing Ang II levels.

The Use of ERE-Luc Reporter Mice to Monitor Estrogen Receptor Transcriptional Activity in a Spatio-Temporal Dimension

In spite of the fact that women spend 1/3 of their lives in postmenopause, the search for appropriate therapies able to counteract the derangements associated with the menopause still represents a sort of sought after the "Holy Grail."Nowadays, the combination of estrogens and selective estrogen receptor modulators (SERMs), a class of compounds with a mixed agonist/antagonistic activity on the estrogen receptor (ER) in various tissues, represents the most promising approach to improve postmenopausal women's health, by preserving the benefits while avoiding the side effects of estrogen-based therapy.Given their complex mechanisms of action, the evaluation of SERM activity in combination with conjugated estrogens (CE) requires a multifactorial analysis that takes into account the multifaceted and dynamic effects of these compounds in target tissues, even in relation to the physiological/pathological status.To accomplish such a goal, we took advantage of the ERE-Luc model, a reporter mouse that allows the monitoring of ER transcriptional activity in a spatio-temporal dimension. Cluster analyses performed on in vivo/ex vivo bioluminescence (BLI) data and ex vivo luciferase activity enabled to sustain the combination of CE plus bazedoxifene (TSEC, tissue-selective estrogen complex) as a valuable option for the pharmacological treatment of the postmenopause.

Tissue-Selective Estrogen Complex and Breast

Although estrogen-progestin therapy has traditionally been standard care for postmenopausal women with an intact uterus experiencing bothersome menopausal symptoms, concerns about side effects related to menopausal hormone therapy (MHT) have led to a dramatic decrease in MHT use over recent decades. As many MHT side effects are now believed to be associated with the progestin component of MHT, efforts have been made to develop a progestin-free alternative to conventional MHT. Recently, a tissue-selective estrogen complex (TSEC), a combination of conjugated estrogen and bazedoxifene, was developed as a progestin-free MHT and is now approved and used worldwide for the relief of vasomotor symptoms and the prevention of bone loss in postmenopausal women. Replacement of synthetic progestin with bazedoxifene could allow more favorable safety profiles, such as those for pain or tenderness, mammographic density, and cancer incidence, for the breast. This review examined the effects of the TSEC on breasts and demonstrated evidence from preclinical and clinical studies supporting TSEC use in clinical practice.

Tissue selective estrogen complex (TSEC): a review

Objective:

This review describes historical development of selective estrogen receptor modulators (SERMs) and their combination with estrogens, termed a tissue selective estrogen complex (TSEC), and considers the potential for future TSEC development.

Methods:

This narrative review is based on literature identified on PubMed and the TSEC research and development experience of the authors.

Results:

SERMs have estrogenic and antiestrogenic effects in various tissues; however, no single agent has achieved an optimal balance of agonist and antagonist effects for the treatment of menopausal symptoms. Clinically, a number of SERMs protect against osteoporosis and breast cancer but can exacerbate vasomotor symptoms. Estrogens alleviate menopausal hot flushes and genitourinary symptoms as well as reduce bone loss, but the addition of a progestogen to menopausal hormone therapy to protect against endometrial cancer increases vaginal bleeding risk, breast tenderness, and potentially breast cancer. The search for an effective menopausal therapy with better tolerability led to the investigation of TSECs. Clinical development of a TSEC consisting of conjugated estrogens/bazedoxifene increased understanding of the importance of a careful consideration of the combination's components and their respective doses to balance safety and efficacy. Bazedoxifene is an estrogen receptor agonist in bone but an antagonist/degrader in the endometrium, which has contributed to its success as a TSEC component. Other oral TSEC combinations studied thus far have not demonstrated similar endometrial safety.

Conclusions:

Choice of SERM, selection of doses, and clinical trial data evaluating safety and efficacy are key to ensuring safety and adequate therapeutic effect of TSECs for addressing menopausal symptoms.

Initial investigation into the optimal dose ratio of conjugated estrogens and bazedoxifene: a double-blind, randomized, placebo-controlled phase 2 dose-finding study

OBJECTIVE

The aim of the study was to explore dose-related endometrial effects of conjugated estrogens/bazedoxifene (CE/BZA).

METHODS

In this randomized, double-blind, phase 2 study, 408 nonhysterectomized, symptomatic (with hot flushes [HFs]) postmenopausal women received ≥1 dose of CE 0.3 or 0.625 mg alone or with BZA 5, 10, or 20 mg/d; placebo; BZA 5 mg/d alone; or CE 0.625 mg with medroxyprogesterone acetate 2.5 mg/d for 84 days. The primary outcome was endometrial thickness on transvaginal ultrasound. HF frequency and severity based on diaries were key secondary outcomes.

RESULTS

CE 0.625 mg alone increased endometrial thickness compared with placebo (mean 5.5 vs 2.95 mm, P < 0.001); BZA countered this in a dose-related manner such that average thickness with the addition of BZA 5, 10, and 20 mg was 5.99, 4.33, and 3.54 mm, respectively. On average, endometrium was significantly less thick with CE 0.625 mg/BZA 20 mg than CE 0.625 mg (P < 0.001) and CE 0.3 mg/BZA 20 mg versus CE 0.3 mg (2.94 vs 3.92 mm, P < 0.05); endometrial thickness was similar to placebo with CE 0.625 mg/BZA 20 mg. Lower BZA doses failed to reduce endometrial thickness relative to the same dose of CE alone. Regimens containing CE 0.625 mg reduced HF frequency and severity versus placebo; CE 0.3 mg with BZA 10 or 20 mg was ineffective.

CONCLUSIONS

BZA ≥20 mg is needed to counter endometrial growth resulting from treatment with CE 0.3 or 0.625 mg. CE 0.3 mg inadequately controls HFs if given with BZA 20 mg.

Bazedoxifene for the treatment of osteoporosis

Introduction: Bazedoxifene (BZD) is a third-generation selective estrogen receptor modulator approved for the treatment of postmenopausal osteoporosis with additional favorable effects in lipids, uterine and breast tissue. Areas covered: In this review, the authors outline clinical data regarding the efficacy, safety, and tolerability of continuous BZD administration up to seven years in randomized, placebo-controlled, phase III clinical trials. Long-term treatment with BZD for postmenopausal osteoporosis is generally safe and well tolerated. BZD achieves small but significant increases in the bone mineral density of the lumbar spine but not the total hip. In addition, BZD reduces significantly the risk of vertebral fractures but not of non-vertebral and hip fractures, with the exception of high fracture risk postmenopausal women in whom BZD significantly reduces non-vertebral fractures. Expert opinion: BZD does not seem to offer significant advantages over the other available antiresorptive agents. However, considering the need for long-term management of osteoporosis, BZD may have a place in the long-term therapeutic planning of the disease.

[Towards an optimization of the modulation of the estrogen receptor during menopausal hormonal therapy]

Women now live more than a third of their lives after the onset of menopause. The decline in endogenous estrogen production during this period is accompanied by functional disorders that affect quality of life. These symptoms may be relieved by menopausal hormone therapy (MHT) initially based on the administration of equine conjugated estrogens (mainly in the United States, oral route) or the natural estrogen, 17β-estradiol (in Europe, transdermal route). Estrogen receptor α (ERα), but not ERβ, mediates most of the physiological effects of estrogens. ERα belongs to the superfamily of nuclear receptors and regulates the transcription of genes via its activation functions AF1 and AF2. In addition to these classical genomic actions, estrogens can activate a subpopulation of ERα present at the cell membrane and thereby induce rapid signals. In this review, we will summarize the evolution of MHTs in last decades, as well as treatments that use various selective estrogen receptor modulators (SERMs). Next, we will describe recent advances in the understanding of the mechanisms of estrogen action, in particular the respective roles of nuclear and membrane ERα as well as the potential implications for future therapies.

What do TSECs provide in the menopausal hormone therapy?

Tissue-selective estrogen complex (TSEC) is projected as a progestogen-free option for the treatment of estrogen deficiency symptoms in postmenopausal, non-hysterectomized women. TSEC combines the benefits of estrogen with a selective estrogen receptor modulator (SERM), in this case bazedoxifene acetate (BZA), which has an antagonistic effect on the endometrium, thus avoiding the use of progestins. The authorized TSEC combination (conjugated estrogens [CE] 0.45 mg/BZA 20 mg) for the alleviation of vasomotor symptoms has been demonstrated in randomized clinical trials compared with placebo or menopausal hormone therapy (MHT). In addition, TSEC has shown improvements in quality of life and vaginal atrophy. In respect to MHT using progestins, the benefits of TSEC are found mainly in the bleeding pattern, amenorrhea rate, and reduction in mammary repercussion (i.e., breast tenderness and radiological density). The objective of this guide will be to analyze the efficacy and safety of TSEC consisting of CE/BZA in postmenopausal women.

The antagonist properties of Bazedoxifene after acute treatment are shifted to stimulatory action after chronic exposure in the liver but not in the uterus

A promising alternative to conventional hormone therapy for postmenopausal symptoms is treatment combining Bazedoxifene (BZA), a third-generation selective estrogen receptor modulator (SERM), and conjugated equine estrogen (CE). This combination is also known as a tissue-selective estrogen complex (TSEC). Understanding the tissue-specific actions of SERMs and the TSEC remains a major challenge to try to predict their clinical effects. The aim of this study was to compare acute versus chronic treatment with BZA, CE or CE + BZA in two major targets of estrogens, the uterus and the liver. In these two tissues, acute treatment with CE, but not with BZA, induced similar gene expression change than the most important endogenous estrogen, 17-β estradiol (E2). Acute induction of gene expression by E2 or by CE was antagonized by the addition of BZA. Concomitantly, BZA alone or in combination with E2 or CE induced a partial degradation of ERα protein after acute exposure. In uterus, chronic treatment of BZA alone had no impact on tissue weight gain or on epithelial cell proliferation, and also antagonized CE-effect in uterus, thereby mimicking the acute effect. By contrast, in the liver, chronic BZA and CE + BZA elicited agonistic transcriptional effects similar to those of CE alone. In addition, at variance to BZA acute effect, no change in ERα protein abundance was observed after chronic treatment in this tissue. These experimental in vivo data highlight a new aspect of the time-dependent tissue-specific action of BZA or TSEC, i.e. they can act acutely as antagonists but become agonists after chronic treatment. This shift was observed in liver tissue, but not in proliferative sex target such as the uterus.

Effect of a tissue selective estrogen complex on breast cancer: Role of unique properties of conjugated equine estrogen

The Women's Health Initiative studies reported that the menopausal hormone therapy (MHT) regimen containing conjugated equine estrogen (CEE) and medroxyprogesterone acetate increased, whereas CEE alone reduced breast cancer incidence. These observations suggest the possibility that CEE might exert unique actions on breast and also suggest the need to eliminate the progestogen from MHT regimens. A MHT regimen called a tissue selective estrogen complex (TSEC), containing CEE plus bazedoxifene (BZA), to avoid the need for a progestogen, was developed and FDA approved. Our study addressed two questions regarding this TSEC: (i) whether CEE exert effects on breast cancer which differ from those of estradiol (E2 ) and (ii) whether BZA antagonize the effects of E2 and CEE on breast cancer? Two rodent models (NMU and ACI) were used to compare the effect of CEE with E2 on mammary tumor formation, proliferation and apoptosis. In both the NMU and ACI models, E2 significantly increased tumor incidence and multiplicity whereas in striking contrast CEE did not, even though the estrogenic effects of CEE and E2 on uterine weight were identical. Mechanistically E2 blocked whereas CEE stimulated apoptosis (cleaved caspase-3) in ACI animals and only E2 stimulated proliferation (Ki67). BZA exerted highly potent anti-estrogenic effects on tumors by completely blocking palpable tumor formation. These data suggest that the CEE/BZA TSEC may be a safer, breast-antagonistic, MHT agent for women and might have potential to prevent breast cancer while relieving menopausal symptoms.

Towards optimization of estrogen receptor modulation in medicine

Women now spend more than one-third of their lives in the postmenopausal years, and the decline of endogenous estrogen production during menopause is accompanied by a series of functional disorders that affect the quality of life. These symptoms could be alleviated or even totally suppressed by menopausal hormone therapy (MHT), initially based on natural estrogens extracted from the urine of pregnant mares (mainly in the USA, using the oral route) and later from the synthesis of the natural estrogen, 17β-estradiol (mainly in Europe, in particular using the transdermal route). Estrogen receptor (ER) α is the main receptor mediating the physiological effects of estrogens. ERα belongs to the nuclear receptor superfamily and activates gene transcription in a time and tissue-specific manner through two distinct activation functions (AF), AF1 and AF2. In addition to these classical genomic actions, ERα also mediates membrane initiated signaling enabling rapid actions of estrogen, potentially along or in interaction with other receptors. Here, we provide a brief historical overview of MHT, and we then highlight recent advances in the characterization of new treatments based on the association of estrogens with selective estrogen receptor modulators (SERMs) or on the modulation of nuclear or membrane ERα.

Membrane and Nuclear Estrogen Receptor Alpha Actions: From Tissue Specificity to Medical Implications

Estrogen receptor alpha (ERα) has been recognized now for several decades as playing a key role in reproduction and exerting functions in numerous nonreproductive tissues. In this review, we attempt to summarize the in vitro studies that are the basis of our current understanding of the mechanisms of action of ERα as a nuclear receptor and the key roles played by its two activation functions (AFs) in its transcriptional activities. We then depict the consequences of the selective inactivation of these AFs in mouse models, focusing on the prominent roles played by ERα in the reproductive tract and in the vascular system. Evidence has accumulated over the two last decades that ERα is also associated with the plasma membrane and activates non-nuclear signaling from this site. These rapid/nongenomic/membrane-initiated steroid signals (MISS) have been characterized in a variety of cell lines, and in particular in endothelial cells. The development of selective pharmacological tools that specifically activate MISS and the generation of mice expressing an ERα protein impeded for membrane localization have begun to unravel the physiological role of MISS in vivo. Finally, we discuss novel perspectives for the design of tissue-selective ER modulators based on the integration of the physiological and pathophysiological roles of MISS actions of estrogens.

Glyceollins trigger anti-proliferative effects through estradiol-dependent and independent pathways in breast cancer cells

Background

Estrogen receptors (ER) α and β are found in both women and men in many tissues, where they have different functions, including having roles in cell proliferation and differentiation of the reproductive tract. In addition to estradiol (E2), a natural hormone, numerous compounds are able to bind ERs and modulate their activities. Among these compounds, phytoestrogens such as isoflavones, which are found in plants, are promising therapeutics for several pathologies. Glyceollins are second metabolites of isoflavones that are mainly produced in soybean in response to an elicitor. They have potentially therapeutic actions in breast cancer by reducing the proliferation of cancer cells. However, the molecular mechanisms driving these effects remain elusive.

Methods

First, to determine the proliferative or anti-proliferative effects of glyceollins, in vivo and in vitro approaches were used. The length of epithelial duct in mammary gland as well as uterotrophy after treatment by E2 and glyceollins and their effect on proliferation of different breast cell line were assessed. Secondly, the ability of glyceollin to activate ER was assessed by luciferase assay. Finally, to unravel molecular mechanisms involved by glyceollins, transcriptomic analysis was performed on MCF-7 breast cancer cells.

Results

In this study, we show that synthetic versions of glyceollin I and II exert anti-proliferative effects in vivo in mouse mammary glands and in vitro in different ER-positive and ER-negative breast cell lines. Using transcriptomic analysis, we produce for the first time an integrated view of gene regulation in response to glyceollins and reveal that these phytochemicals act through at least two major pathways. One pathway involving FOXM1 and ERα is directly linked to proliferation. The other involves the HIF family and reveals that stress is a potential factor in the anti-proliferative effects of glyceollins due to its role in increasing the expression of REDD1, an mTORC1 inhibitor.

Conclusion

Overall, our study clearly shows that glyceollins exert anti-proliferative effects by reducing the expression of genes encoding cell cycle and mitosis-associated factors and biomarkers overexpressed in cancers and by increasing the expression of growth arrest-related genes. These results reinforce the therapeutic potential of glyceollins for breast cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-017-0182-1) contains supplementary material, which is available to authorized users.

An Overview of the Clinical Efficacy and Safety of Tissue Selective Estrogen Complex: From the Selective Estrogens, Menopause, and Response to Therapy (SMART) Trials

Hormone therapy (HT) is the most effective treatment for menopausal symptoms, and reduces both spinal and non-spinal postmenopausal osteoporotic fractures. However, a Women's Health Initiative (WHI) trial revealed that progestin-containing HT is associated with higher incidences of breast cancer and coronary heart disease than those associated with placebo. Tissue selective estrogen complex (TSEC) is a novel progestin-free HT option composed of conjugated estrogens (CE) and a selective estrogen receptor modulator. CE at a dose of 0.45 mg combined with 20 mg of bazedoxifene was the first TSEC medication approved in the United States and Korea for women with moderate to severe menopause-related vasomotor symptoms (VMS) and for preventing postmenopausal osteoporosis. This review summarizes the clinical efficacy, safety, and tolerability of TSEC as obtained from the five SMART clinical trials.

Preclinical breast effects of a tissue selective estrogen complex (TSEC) including conjugated estrogen with bazedoxifene

The first tissue-selective estrogen complex (TSEC), consisting of a combination of a conjugated equine estrogen (CEE) and bazedoxifene (BZA), has been approved for treatment of the menopause in the USA and European Union. We have postulated that this TSEC might block the estrogenic effects of CEE on breast tissue and thereby prevent breast cancer growth. This manuscript, representing a presentation at a Festschrift honoring Evan Simpson, reviews our published data BZA blocked the in vitro effects of both estradiol and CEE on cell growth and gene expression in MCF-7 cells. BZA completely blocked CEE- or E₂-stimulated ductal and terminal end bud growth of immature murine mammary glands and the growth of experimental breast cancers. These findings provide a rationale for future clinical studies to determine whether this TSEC prevents the growth of occult breast cancer in women.

Genistein modulation of seizure: involvement of estrogen and serotonin receptors

Genistein, a major source of phytoestrogen exposure for humans and animals, has been shown to mediate neuroprotection in Alzheimer's disease and status epilepticus. In the present study, we investigated the effect of genistein on pentylenetetrazole-induced seizures in ovariectomized mice and the possible involvement of estrogenic and serotonergic pathways in the probable effects of genistein. Intraperitoneal (i.p.) administration of genistein (10 mg/kg) significantly increased the seizure threshold 30 min prior to induction of seizures 14 days after ovariectomy surgery. Administration of fulvestrant (1 mg/kg, i.p.), an estrogen receptor antagonist, completely reversed the anticonvulsant effect of genistein (10 mg/kg) in ovariectomized mice. Administration of the antagonist of serotonin receptor (5-HT3), tropisetron (10 mg/kg, i.p.), eliminated the anticonvulsant effect of genistein, whereas co-administration of m-chlorophenylbiguanide (5-HT3 receptor agonist; 1 mg/kg) and a non-effective dose of genistein (5 mg/kg) increased the seizure threshold. To conclude, it seems that estrogenic/serotonergic systems might be involved in the anticonvulsant properties of genistein.

The evolving role of oral hormonal therapies and review of conjugated estrogens/bazedoxifene for the management of menopausal symptoms

This review describes the evolving role of oral hormone therapy (HT) for treating menopausal symptoms and preventing osteoporosis, focusing on conjugated estrogens/bazedoxifene (CE/BZA). Estrogens alleviate hot flushes and prevent bone loss associated with menopause. In nonhysterectomized women, a progestin should be added to estrogens to reduce the risk of endometrial cancer. Use of HT declined since the Women's Health Initiative (WHI) studies showed that HT does not prevent coronary heart disease (CHD) and that conjugated estrogens/medroxyprogesterone acetate increased the risk of invasive breast cancer after nearly 5 years of use. However, re-analyses of the WHI data suggest that some risks (eg, CHD, all-cause mortality) may be reduced when HT is initiated in women <60 years of age and <10 years since menopause, compared with later. CE/BZA is the first menopausal HT without a progestogen for nonhysterectomized women. Instead, BZA, a selective estrogen receptor modulator, in combination with CE, protects against estrogenic effects on uterine and breast tissue. Data from 5 large, randomized clinical trials show that CE/BZA reduces hot flush frequency/severity, prevents bone loss, reduces bone turnover, improves the vaginal maturation index and ease of lubrication, and improves some measures of sleep and menopause-specific quality of life. In studies of up to 2 years, there was no increase in endometrial hyperplasia, vaginal bleeding, breast density, or breast pain/tenderness compared with placebo. Venous thromboembolism and stroke are risks of all estrogen-based therapies. The choice of HT should be individualized, with consideration of the risk/benefit profile and tolerability of therapy, as well as patient preferences.

The Tissue-Selective Estrogen Complex (Bazedoxifene/Conjugated Estrogens) for the Treatment of Menopause

The tissue-selective estrogen complex (TSEC) pairs conjugated estrogens (CE) with a selective estrogen receptor modulator (SERM), bazedoxifene acetate (BZA). A 2-year treatment with the TSEC improved vasomotor symptoms, quality of life, and vaginal atrophy in healthy postmenopausal women. In addition, the TSEC prevented vertebral and hip bone loss without increasing mammographic density, breast tenderness, the risk of myocardial infarction, stroke, or venous thromboembolism. Finally, the BZA 20 mg/CE 0.45 mg dose did not increase the risk of endometrial hyperplasia. Based on these findings, the TSEC can be considered as a first-line treatment for symptomatic postmenopausal women.

Patient considerations in the management of menopausal symptoms: role of conjugated estrogens with bazedoxifene

Menopausal symptoms (eg, hot flushes and vaginal symptoms) are common, often bothersome, and can adversely impact women's sexual functioning, relationships, and quality of life. Estrogen-progestin therapy was previously considered the standard care for hormone therapy (HT) for managing these symptoms in nonhysterectomized women, but has a number of safety and tolerability concerns (eg, breast cancer, stroke, pulmonary embolism, breast pain/tenderness, and vaginal bleeding) and its use has declined dramatically in the past decade since the release of the Women's Health Initiative trial results. Conjugated estrogens paired with bazedoxifene (CE/BZA) represent a newer progestin-free alternative to traditional HT for nonhysterectomized women. CE/BZA has demonstrated efficacy in reducing the frequency and severity of vasomotor symptoms and preventing loss of bone mineral density in postmenopausal women. CE/BZA provides an acceptable level of protection against endometrial hyperplasia and does not increase mammographic breast density. Compared with traditional estrogen-progestin therapy, it is associated with lower rates of breast pain/tenderness and vaginal bleeding. Patient-reported outcomes indicate that CE/BZA improves menopause-specific quality of life, sleep, some measures of sexual function (especially ease of lubrication), and treatment satisfaction. This review looks at the rationale for selection and combination of CE with BZA at the dose ratio in the approved product and provides a detailed look at the efficacy, safety, tolerability, and patient-reported outcomes from the five Phase III trials. Patient considerations in the choice between CE/BZA and traditional HT (eg, tolerability, individual symptoms, and preferences for route of administration) are also considered.

Selective Estrogen Receptor Modulators and the Tissue-Selective Estrogen Complex: Analysis of Cell Type-Specific Effects Using In Vivo Imaging of a Reporter Mouse Model

Selective estrogen receptor modulators (SERMs) are a class of compounds that act differentially on the estrogen receptor (ER) in various tissues with a mixed agonist/antagonistic activity (agonistic in some tissues while antagonist in others). This peculiarity represents a challenge for developing new hormone replacement therapies (HRTs) and highlights the need of new tools to evaluate the specific effects of a given SERM in different organs/tissues of an entire organism and with time. Reporter mice represent invaluable tools in pharmacology to analyze specific signaling in physiological conditions and monitor the effects of drugs acting on these signals in a spatio-temporal dimension. Here, we describe an in vivo protocol to examine the effects of different SERMs on estrogen receptor activity by using the ERE-Luc reporter model, a mouse that reports ER transcriptional activity.

Selective estrogen receptor modulators and the combination therapy conjugated estrogens/bazedoxifene: A review of effects on the breast

Traditional menopausal hormone therapy containing estrogens/progestin has been associated with an increased risk of breast cancer, and estrogen exposure is known to promote growth and proliferation of a majority of breast cancers. Therefore, it is important for clinicians to consider the breast safety profile of any hormone-based therapy used in postmenopausal women. This review provides an overview of the breast safety and tolerability profiles of currently marketed selective estrogen receptor modulators, antiestrogens, and the first tissue selective estrogen complex combining conjugated estrogens with the selective estrogen receptor modulator bazedoxifene in postmenopausal women. Selective estrogen receptor modulators and antiestrogens act as estrogen receptor antagonists in the breast. Tamoxifen, toremifene, and the selective estrogen receptor degrader fulvestrant are used to treat breast cancer, and tamoxifen and raloxifene protect against breast cancer in high-risk women. Postmenopausal women using selective estrogen receptor modulators for prevention or treatment of osteoporosis (raloxifene, bazedoxifene) can be reassured that these hormonal treatments do not adversely affect their risk of breast cancer and may, in the case of raloxifene, even be protective. There are limited data on breast cancer in women who use ospemifene for dyspareunia. Conjugated estrogens/bazedoxifene use for up to two years did not increase mammographic breast density or breast pain/tenderness, and there was no evidence of an increased risk of breast cancer, suggesting that conjugated estrogens/bazedoxifene has an improved breast safety profile compared with traditional menopausal hormone therapies. Future research will continue to focus on development of selective estrogen receptor modulators and selective estrogen receptor modulator combinations capable of achieving the ideal balance of estrogen receptor agonist and antagonist effects.

New antiresorptive therapies for postmenopausal osteoporosis

Osteoporosis is a systemic skeletal disease whose risk increases with age and it is common among postmenopausal women. Currently, almost all pharmacological agents for osteoporosis target the bone resorption component of bone remodeling activity. Current antiresorptive agents are effective, but the effectiveness of some agents is limited by real or perceived intolerance, longterm adverse events (AEs), coexisting comorbidities, and inadequate long-term adherence. New antiresorptive therapies that may expand options for the prevention and treatment of osteoporosis include denosumab, combination of conjugated estrogen/bazedoxifene and cathepsin K inhibitors. However, the long-term efficacy and AEs of these antiresorptive therapies need to be confirmed in studies with a longer follow-up period.

Incorporating bazedoxifene into the treatment paradigm for postmenopausal osteoporosis in Japan

The incidence of osteoporosis-related fractures in Asian countries is steadily increasing. Optimizing osteoporosis treatment is especially important in Japan, where the rate of aging is increasing rapidlyelderly population is increasing rapidly and life expectancy is among the longest in the world. There are several therapies currently available in Japan for the treatment of postmenopausal osteoporosis, each with a unique risk/benefit profile. A novel selective estrogen receptor modulator, bazedoxifene (BZA), was recently approved for the treatment of postmenopausal osteoporosis in Japan. Results from a 2-year, phase 2 trial in postmenopausal Japanese women showed that BZA significantly improved lumbar spine and total hip bone mineral density compared with placebo, while maintaining endometrial and breast safety, consistent with results from 2 global, phase 3 trials including a 2-year osteoporosis prevention study and a 3-year osteoporosis treatment study. In the pivotal 3-year treatment study, BZA significantly reduced the incidence of new vertebral fractures compared with placebo; in a post hoc analysis of a subgroup of women at higher risk of fractures, BZA significantly reduced the risk of nonvertebral fractures compared with placebo and raloxifene. A 2-year extension of the 3-year treatment study demonstrated the sustained efficacy of BZA over 5 years of treatment. BZA was generally safe and well tolerated in these studies. In a "super-aging" society such as Japan, long-term treatment for postmenopausal osteoporosis is a considerable need. BZA may be considered as a first choice for younger women anticipating long-term treatment, and also an appropriate option for older women who are unable or unwilling to take bisphosphonates.

Effects of genistein in combination with conjugated estrogens on endometrial hyperplasia and metabolic dysfunction in ovariectomized mice

Tissue-selective estrogen complex (TSEC), which combines a selective estrogen receptor modulator (SERM) with one or more estrogens, is a novel approach to menopausal therapy. It has been demonstrated that the phytoestrogen genistein (GEN) exhibits mixed estrogen receptor agonist and antagonist activity, suggesting that GEN may have potential for use as a natural SERM. We evaluated, for the first time, the effects of GEN, conjugated estrogens (CE), and their pairing effects as a TSEC treatment on estrogen-induced endometrial hyperplasia and metabolic dysfunction in ovariectomized (OVX) mice fed a high-fat diet. CE replacement prevented fat accumulation in the adipose tissue and liver, improved glucose homeostasis, and induced endometrial hyperplasia in OVX mice. GEN at 100 mg/kg showed CE mimetic effects in preventing ovariectomy-induced metabolic dysfunctions without endometrial stimulation. Combination treatments with CE and GEN prevented metabolic dysfunctions more strongly than CE alone, but at both low and high doses, GEN did not reverse CE-induced endometrial hyperplasia. In addition, we found that in a TSEC regimen, a typical SERM raloxifene maintains the metabolic benefits of CE while simultaneously protecting the endometrium in OVX mice. These findings indicate that GEN acts as an estrogen agonist in metabolic regulation, but has no SERM function in the uteri of OVX mice.

Development of conjugated estrogens/bazedoxifene, the first tissue selective estrogen complex (TSEC) for management of menopausal hot flashes and postmenopausal bone loss

Conjugated estrogens (CE) combined with the selective estrogen receptor modulator (SERM) bazedoxifene (BZA) is a new option for alleviating menopausal symptoms and preventing postmenopausal bone loss. The rationale for developing the tissue selective estrogen complex (TSEC) CE/BZA was to combine CE's benefits with the SERM's tissue-specific properties to offset estrogenic stimulation of endometrial and breast tissue. TSECs provide a progestin-free alternative to traditional estrogen-progestin therapy (EPT) in women with a uterus. Preclinical studies supported bazedoxifene as the SERM of choice and demonstrated that CE/BZA provided an optimal balance of estrogen receptor agonist/antagonist activity compared with other potential TSEC pairings. Initial clinical development of CE/BZA focused on determining the appropriate dose ratio that would demonstrate efficacy with minimal to no stimulation of the breast or endometrium. Clinical studies confirmed the efficacy of the selected doses for maintaining bone mass; relieving vasomotor symptoms, vulvar-vaginal atrophy, and dyspareunia; and improving sexual function in postmenopausal women. Reduction of hot flashes also translated into improved menopause-specific quality of life and sleep. Unlike EPT, the FDA-approved dose of CE 0.45 mg/BZA 20mg does not cause a change in breast density or the endometrium, or increase breast pain compared with placebo. In clinical trials up to 2 years, CE 0.45 mg/BZA 20 mg has a favorable tolerability profile and rates of coronary heart disease, venous thromboembolism, and amenorrhea similar to placebo. Therefore, CE 0.45 mg/BZA 20 mg is an effective, well-tolerated alternative to EPT for menopausal symptom relief and osteoporosis prevention for postmenopausal women with a uterus.

An overview of current and emerging SERMs

Selective estrogen receptor modulators (SERMs) are compounds that exhibit tissue-specific estrogen receptor (ER) agonist or antagonist activity, and are used for various indications, including treatment of breast cancer, osteoporosis, and menopausal symptoms. Endometrial safety has been a key differentiator between SERMs in clinical practice. For example, tamoxifen exhibits ER agonist activity in the uterus, resulting in an increased risk of endometrial hyperplasia and malignancy, whereas raloxifene and bazedoxifene have neutral effects on the uterus. Based on their efficacy and long-term safety, SERMs are increasingly being prescribed for women who cannot tolerate other treatment options and for younger women at an increased risk of fracture who may remain on therapy for long periods of time. Continuing advances in the understanding of SERM mechanisms of action and structural interactions with the ER may lead to the development of new agents and combinations of agents to provide optimal treatments to meet the varying needs of postmenopausal women. One such example is the tissue selective estrogen complex, which partners a SERM with 1 or more estrogens, with the aim of blending the desired estrogen-receptor agonist activities of estrogens on vasomotor symptoms, vulvar-vaginal atrophy, and loss of bone mass with the tissue selectivity of a SERM.

Breast-related effects of selective estrogen receptor modulators and tissue-selective estrogen complexes

A number of available treatments provide relief of menopausal symptoms and prevention of postmenopausal osteoporosis. However, as breast safety is a major concern, new options are needed, particularly agents with an improved mammary safety profile. Results from several large randomized and observational studies have shown an association between hormone therapy, particularly combined estrogen-progestin therapy, and a small increased risk of breast cancer and breast pain or tenderness. In addition, progestin-containing hormone therapy has been shown to increase mammographic breast density, which is an important risk factor for breast cancer. Selective estrogen receptor modulators (SERMs) provide bone protection, are generally well tolerated, and have demonstrated reductions in breast cancer risk, but do not relieve menopausal symptoms (that is, vasomotor symptoms). Tissue-selective estrogen complexes (TSECs) pair a SERM with one or more estrogens and aim to blend the positive effects of the components to provide relief of menopausal symptoms and prevention of postmenopausal osteoporosis without stimulating the breast or endometrium. One TSEC combination pairing conjugated estrogens (CEs) with the SERM bazedoxifene (BZA) has completed clinical development and is now available as an alternative option for menopausal therapy. Preclinical evidence suggests that CE/BZA induces inhibitory effects on breast tissue, and phase 3 clinical studies suggest breast neutrality, with no increases seen in breast tenderness, breast density, or cancer. In non-hysterectomized postmenopausal women, CE/BZA was associated with increased bone mineral density and relief of menopausal symptoms, along with endometrial safety. Taken together, these results support the potential of CE/BZA for the relief of menopausal symptoms and prevention of postmenopausal osteoporosis combined with breast and endometrial safety.

Endometriosis impairs bone marrow-derived stem cell recruitment to the uterus whereas bazedoxifene treatment leads to endometriosis regression and improved uterine stem cell engraftment

Endometriosis is a disease defined by the ectopic growth of uterine endometrium. Stem cells contribute to the generation of endometriosis as well as to repair and regeneration of normal endometrium. Here we demonstrate that the selective estrogen receptor modulator bazedoxifene (BZA), administered with conjugated estrogens (CEs), leads to regression of endometriosis lesions as well as reduction in stem cell recruitment to the lesions. Female mice underwent transplantation of male bone marrow. Endometrium was transplanted in the peritoneal cavity of half to create experimental endometriosis. Mice with or without experimental endometriosis were randomized to BZA/CE or vehicle treatment. Endometriosis lesions, bone marrow-derived mesenchymal stem cell engraftment of the lesions, and eutopic endometrium as well as ovarian stimulation were assessed. BZA treatment significantly reduced lesion size, gland number, and expression of proliferation marker proliferating cell nuclear antigen. Ovarian weight was not affected. Stem cells were recruited to the endometriosis lesions, and this recruitment was dramatically reduced by BZA/CE treatment. Stem cell engraftment was reduced in the uterus of animals with endometriosis; however the number of stem cells engrafting the uterus was completely restored by treatment with BZA/CE. Competition between endometriosis and the eutopic endometrium for a limited supply of stem cells and depletion of normal stem cells flux to the uterus is a novel mechanism by which endometriosis interferes with endometrial function and fertility. BZA/CE not only treats lesions of endometriosis, it also dramatically reduces stem cell recruitment to the lesions and restores stem cell engraftment of the uterine endometrium.

Conjugated estrogens combined with bazedoxifene: the first approved tissue selective estrogen complex therapy

Menopausal therapy with a tissue selective estrogen complex combines estrogens with a selective estrogen receptor modulator, with the goal of blending the desirable effects of estrogens on menopausal symptoms and bone with the tissue selective properties of a selective estrogen receptor modulator. The first tissue selective estrogen complex to receive regulatory approval is a combination of conjugated estrogens (CE) with bazedoxifene (BZA). Clinical trials with CE/BZA in postmenopausal women have shown improvement in vasomotor symptoms, vulvo-vaginal atrophy, and bone mineral density, without stimulation of the endometrium or breast tissue, with a generally favorable safety and tolerability profile. CE/BZA represents a new approach to the management of menopausal symptoms in women with a uterus.

Current and evolving approaches to individualizing estrogen receptor-based therapy for menopausal women

CONTEXT

Adding progestogens to estrogens changes the risk profile of hormonal therapy for menopausal women, and recent data support the need for progestogen-free options. Several current and evolving approaches to managing estrogen deficiency allow for progestogen omission. We review the mechanisms of estrogen activity and provide an overview of emerging and available estrogen receptor (ER)-based therapies.

EVIDENCE ACQUISITION

PubMed was searched for relevant English-language articles using keywords pertaining to estrogen deficiency, menopause, hormone therapy, and estrogen-only therapy. Pivotal or recent randomized controlled trials, large observational studies, comprehensive meta-analyses, and established therapeutic guidelines were compiled.

EVIDENCE SYNTHESIS

Advances in our understanding of ER pharmacology have led to therapies designed to optimize ER activity, including selective ER modulators (SERMs) and tissue-selective estrogen complexes (TSECs). Each estrogen, SERM, and TSEC exhibits a unique profile of tissue-specific activity, spanning the spectrum from ER agonism to antagonism. Systemic estrogens unopposed by progestogens effectively manage menopausal symptoms in hysterectomized postmenopausal women but require progestogen use in postmenopausal women with a uterus. SERMs are effective for managing certain aspects of estrogen deficiency in postmenopausal women, but data suggest that pairing a SERM with estrogens to form a TSEC provides a more optimal therapeutic profile for women with a uterus.

CONCLUSIONS

Treating signs and symptoms of estrogen deficiency requires an individualized approach based on a woman's goals and the purported risks of different therapies. New and emerging agents have demonstrated efficacy in postmenopausal women with a uterus, while allowing these women to avoid progestogens and their possible adverse effects.

Rapid oestrogenic regulation of social and nonsocial learning

Much research on oestrogens has focused on their long-term action, exerting behavioural effects within hours to days through gene transcription. Oestrogens also affect behaviour on a much shorter time scale. These rapid effects are assumed to occur through cell signalling and can elicit a behavioural effect as early as 15 min after treatment. These effects on behaviour have primarily been explored through the action of oestradiol at three well-known oestrogen receptors (ERs): ERα, ERβ and the more recently described G protein-coupled ER1 (GPER1). The rapid effects of oestradiol and ER agonists have been tested on both social and nonsocial learning paradigms. Social learning refers to a paradigm in which an animal acquires information and modifies its behaviour based on observation of another animal, commonly studied using the social transmission of food preferences paradigm. When administered shortly before testing, oestradiol rapidly improves social learning on this task, although no ER agonist has definitive, comparable improving effects. Some evidence points to GPER1, whereas ERα impairs, and ERβ activation has no effect on social learning. Conversely, ERα and GPER1 play a larger role than ERβ in the rapid improving effect of oestrogens on nonsocial learning, including social and object recognition. In addition, when administered immediately post-acquisition, oestrogens also rapidly improve memory consolidation in a variety of learning paradigms: object recognition, object placement, inhibitory avoidance and the Morris water maze, indicating that oestradiol affects the consolidation of multiple types of memory. Evidence suggests that these improvements are the result of oestrogens acting in the dorsal hippocampus where selective activation of all three ERs shows rapid improving effects on spatial learning comparable to oestradiol. However, the hippocampus is not necessary for rapid oestradiol improvements on social recognition. Although acute treatment with oestradiol enhances learning and memory on various social and nonsocial learning paradigms, the specific ERs play different roles in each type of learning. Future research should aim to further determine the roles of ERs with respect to the enhancing effects of oestradiol on learning and memory, and also determine where in the brain oestradiol acts to affect social and nonsocial learning.

Profile of bazedoxifene/conjugated estrogens for the treatment of estrogen deficiency symptoms and osteoporosis in women at risk of fracture

Decreasing levels of estrogens during menopause are associated with reduced bone density and an increased risk of osteoporosis. Many women also experience bothersome vasomotor and vaginal symptoms during the menopausal transition. Results of systematic reviews and meta-analyses of randomized controlled trials have shown that both systemic estrogen therapy or hormone therapy (estrogen combined with a progestin) are useful to prevent bone loss, and they are the most effective treatment for such climacteric symptoms as hot flushes, sweating, vaginal dryness, and dyspareunia. Unfortunately, estrogen therapy and hormone therapy increase the risk of endometrial and breast cancer, respectively. The selective estrogen receptor modulators (SERMs) result in positive estrogenic effects on bone, with no negative effects on the endometrium and breast but do not provide relief from postmenopausal symptoms. The combination of a SERM with estrogen as a tissue selective estrogen complex (TSEC) is a new strategy for the prevention of bone loss and the treatment of climacteric symptoms. This combination is particularly interesting from a clinical point of view, taking into account that estrogen alone did not increase breast cancer risk by the Women’s Health Initiative. TSEC is hypothesized to provide the benefits of estrogen-alone therapy, with an improved tolerability profile because the SERM component can make possible the elimination of progestin. The objective of this review was to critically evaluate the evidence from the reports published to date on the use of bazedoxifene (a third-generation SERM) in combination with conjugated estrogens in postmenopausal women. The conclusion is that effectively, the combination of bazedoxifene and conjugated estrogens may be a promising alternative to hormone therapy for the prevention of osteoporosis and the treatment of postmenopausal symptoms in non-hysterectomized postmenopausal women.

Tissue-selective estrogen complexes for postmenopausal women

Although hormone therapy using estrogens plus progestogens (EPT) is effective for the management of menopausal symptoms (e.g., vasomotor symptoms and vulvar/vaginal atrophy) and prevention/treatment of postmenopausal osteoporosis, EPT is associated with safety and tolerability concerns. A new alternative to EPT is the tissue selective estrogen complex (TSEC), which partners a selective estrogen receptor modulator (SERM) with one or more estrogens and is designed to treat menopausal symptoms and prevent postmenopausal osteoporosis without the tolerability concerns associated with EPT. The first TSEC to reach advanced clinical development is a combination of the SERM bazedoxifene (BZA) with conjugated estrogens (CE). BZA has been shown to inhibit the stimulatory activity of CE on uterine tissue and breast in vitro and in vivo. In clinical studies, BZA/CE treatment has been associated with significant improvements in menopausal symptoms including hot flushes and vulvar/vaginal atrophy and significant increases in bone mineral density, coupled with reductions in bone turnover marker levels and improvements in sleep and health-related quality of life. Additionally, BZA/CE has been shown to have a neutral effect on endometrial and breast tissue because BZA inhibits the stimulatory effects of estrogens in tissue-selective fashion in these 2 organs. Taken together, results of these preclinical and clinical studies indicate that the benefits of estrogens for treating menopausal symptoms are maintained with BZA/CE without endometrial or breast stimulation, resulting in a safe and effective treatment for symptomatic postmenopausal women.

Carcinogenicity and hormone studies with the tissue-selective estrogen receptor modulator bazadoxifene

Bazedoxifene Acetate (BZA) is a selective estrogen receptor modulator (SERM) that is approved for the prevention and/or treatment of osteoporosis in postmenopausal women. To assess for carcinogenic potential, BZA was administered ad libitum in the diet to rats for 2 years. BZA caused an increase in benign ovarian tumors in female rats and decreased incidences of mammary tumors (females) and pituitary tumors (males and females). In addition, BZA provided a significant survival benefit at all dosages tested, which correlated with a significant reduction in pituitary and mammary gland tumors and decreased body weight gain (both genders). Additional studies were subsequently conducted in rats and monkeys to further explore the mechanisms likely responsible for the observed effects. Results from studies in hypophysectomized and chemically castrated female rats indicated that BZA did not directly stimulate formation of ovarian cysts, but an intact pituitary was required for cyst formation. Further, BZA increased estradiol concentrations in rats and monkeys. In monkeys, BZA increased concentrations of luteinizing hormone (LH) after onset of treatment and prohibited the preovulatory surge of LH until after cessation of treatment. These hormonal changes suggest that BZA inhibited both the positive and negative feedback effects of estrogen on gonadotropins and the resulting increase in LH caused formation and persistence of ovarian cysts, which eventually transformed into benign ovarian granulosa cell tumors in the rat carcinogenicity study. These results also suggest that the reductions in pituitary and mammary gland tumors were attributed to BZA-related antagonism of endogenous estrogens at the estrogen receptors.

Effects of bazedoxifene acetate with and without conjugated equine estrogens on the breast of postmenopausal monkeys

OBJECTIVE

Concerns about increased breast cancer risk with estrogen and progestin therapy have led to an increased interest in progestin alternatives. The main objective of this study was to determine if bazedoxifene acetate (BZA), a new selective estrogen receptor modulator, will antagonize the proliferative and transcriptional effects of conjugated equine estrogens (CEE) in the breast.

METHODS

As part of a 20-month preclinical trial, 95 ovariectomized cynomolgus macaques (Macaca fascicularis) were randomized to receive no treatment or treatment with BZA (20 mg/d), CEE (0.45 mg/d), or BZA and CEE in combination (women's daily equivalent doses). The data presented here include breast effects after 6 months of treatment. Endpoints included histomorphometry, histopathological evaluations, gene microarray assays, polymerase chain reaction quantification of specific estrogen receptor α (ER-α) activity markers, and immunohistochemical detection of sex steroid receptors, and the proliferation marker Ki67.

RESULTS

BZA + CEE and BZA resulted in significantly less total epithelial density, lobular enlargement, and Ki67 immunolabeling in the terminal ducts compared with CEE alone (P < 0.05 for all). The addition of BZA to CEE antagonized the expression of ER-α-regulated genes such as GREB1 and TFF1 (P < 0.01 for both), whereas BZA alone had minimal effects on ER-α-mediated transcriptional activity. BZA and BZA + CEE did not significantly up-regulate genes related to cell cycle progression and proliferation. BZA with and without CEE also resulted in less lobular and terminal duct ER-α immunolabeling compared with control and CEE (P < 0.0001 for all).

CONCLUSIONS

These findings demonstrate that BZA given at a clinically relevant dose is an estrogen antagonist in the breast, supporting the idea that CEE + BZA may provide a lower breast cancer risk profile compared with traditional estrogen + progestin therapies.

Nuclear receptors and their selective pharmacologic modulators

Nuclear receptors are ligand-activated transcription factors and include the receptors for steroid hormones, lipophilic vitamins, sterols, and bile acids. These receptors serve as targets for development of myriad drugs that target a range of disorders. Classically defined ligands that bind to the ligand-binding domain of nuclear receptors, whether they are endogenous or synthetic, either activate receptor activity (agonists) or block activation (antagonists) and due to the ability to alter activity of the receptors are often termed receptor "modulators." The complex pharmacology of nuclear receptors has provided a class of ligands distinct from these simple modulators where ligands display agonist/partial agonist/antagonist function in a tissue or gene selective manner. This class of ligands is defined as selective modulators. Here, we review the development and pharmacology of a range of selective nuclear receptor modulators.