Bazedoxifene acetate: a selective estrogen receptor modulator with improved selectivity

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.

Selective estrogen receptor modulators: tissue specificity and clinical utility

Selective estrogen receptor modulators (SERMs) are a diverse group of nonsteroidal compounds that function as agonists or antagonists for estrogen receptors (ERs) in a target gene-specific and tissue-specific fashion. SERM specificity involves tissue-specific expression of ER subtypes, differential expression of co-regulatory proteins in various tissues, and varying ER conformational changes induced by ligand binding. To date, the major clinical applications of SERMs are their use in the prevention and treatment of breast cancer, the prevention of osteoporosis, and the maintenance of beneficial serum lipid profiles in postmenopausal women. However, SERMs have also been found to promote adverse effects, including thromboembolic events and, in some cases, carcinogenesis, that have proven to be obstacles in their clinical utility. In this review, we discuss the mechanisms of SERM tissue specificity and highlight the therapeutic application of well-known and emergent SERMs.

Novel ligands balance estrogen receptor β and α agonism for safe and effective suppression of the vasomotor response in the ovariectomized female rat model of menopause

Vasomotor thermo-dysregulation (hot flashes) are an often debilitating symptom of menopause. Effective treatment is achieved primarily through activation of the estrogen receptor (ER)α with estrogens but is also associated with increased risk for breast and uterine cancer. In this study, we have tested novel compounds lacking the B ring of 17-hydroxy-β-estradiol (E2) (A-CD compounds) with differing ratios of ERα:ERβ binding affinities for the ability to reduce diurnal/nocturnal tail-skin temperatures (TSTs) in the ovariectomized female rat menopausal hot flash model. Normal mammary tissue expresses the predominantly antiproliferative ERβ. Therefore, we hypothesized that a preferential ERβ agonist with fractional ERα activity would safely reduce TSTs. The A-CD compound, L17, is a preferential ERβ agonist that has a ratio of ERβ:ERα binding affinity relative to E2 of 9.3 (where ERβ:ERα for E2, 1.0). In the ovariectomized rat, daily administration of low doses (1 mg/kg) of the A-CD compound TD81 (ERα:ERβ relative affinity, 15.2) was ineffective in temperature regulation, whereas L17 showed a trend toward TST reduction. Both E2 and the A-CD compound, TD3 (ERβ:ERα relative affinity, 5.0), also reduced TSTs but had marked proliferative effects on mammary and uterine tissues. At 2 mg/kg, L17 strongly reduced TSTs even more effectively than E2 but, importantly, had only minimal effect on uterine weight and mammary tissues. Both E2- and L17-treated rats showed similar weight reduction over the treatment period. E2 is rapidly metabolized to highly reactive quinones, and we show that L17 has 2-fold greater metabolic stability than E2. Finally, L17 and E2 similarly mediated induction of c-fos expression in neurons within the rat thermoregulatory hypothalamic median preoptic nucleus. Thus, the A-CD compound, L17, may represent a safe and effective approach to the treatment of menopausal hot flashes.

Use of SERMs for treatment in postmenopausal women

Selective estrogen receptor modulators (SERMs) are synthetic non-steroidal agents which have varying estrogen agonist and antagonist activities in different tissues, most likely due to the receptor conformation changes associated with that SERM's binding and the subsequent effect on transcription. Clinical trials aim to differentiate amongst SERMs on selected target tissues for use in postmenopausal women including effects on breast, bone, cardiovascular venous thrombosis risk, endometrium, vagina, vasomotor symptoms, and brain. This paper describes differences in clinical effects on selected target tissues of SERMs that are approved, discontinued or in development. FDA approved SERMs include tamoxifen and toremifene used for prevention and treatment of breast cancer, raloxifene approved for prevention and treatment of osteoporosis and prevention of invasive breast cancer, and ospemifene approved for treatment of dyspareunia from menopausal vaginal atrophy. The FDA approved first tissue selective estrogen complex (TSEC) a pairing of conjugated equine estrogens with the SERM, bazedoxifene. This pairing reduces the risk of endometrial hyperplasia that can occur with the estrogenic component of the TSEC without the need for a progestogen in women with a uterus. It also allows for the estrogenic benefits on relief of hot flashes and prevention of bone loss without stimulating the breast or the endometrium. In clinical practice, the tissue-selective actions of SERMs, alone or paired with estrogens, allow for individualization in meeting the treatment needs of postmenopausal women by providing targeted tissue effects. This article is part of a Special Issue entitled 'Menopause'.

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.

Current challenges in the treatment of osteoporosis: an opportunity for bazedoxifene

BACKGROUND

Osteoporosis affects millions of postmenopausal women worldwide. Osteoporosis-related fractures can lead to chronic pain, disability, systemic complications, and increased risk of recurrent fractures, resulting in increased healthcare costs and mortality. Because currently available therapies have unique benefit/risk profiles, challenges remain in selecting the most appropriate treatment for each osteoporotic woman. RESEARCH AND RESULTS: Bazedoxifene (BZA), a new selective estrogen receptor modulator, is being developed for postmenopausal osteoporosis. In a 3 year, global, phase 3 study, BZA significantly reduced the risk of new vertebral fractures and nonvertebral fractures in women with higher baseline fracture risk compared with placebo. In two extensions of this study, the efficacy of BZA in reducing vertebral fracture risk was sustained over 7 years. BZA improved lumbar spine and total hip bone mineral density compared with placebo at 3 and 5 years, and demonstrated a favorable safety/tolerability profile, with no endometrial or breast stimulation. BZA was cost-effective compared with raloxifene in a 3 year, head-to-head comparative trial. Indirect comparisons further suggest that BZA may be as effective as bisphosphonates in reducing risk of nonvertebral fractures in women at high risk of fracture. BZA demonstrated efficacy and safety for treating postmenopausal osteoporosis over 7 years, particularly in women at a higher fracture risk.

CONCLUSION

Because of its specific pharmacologic profile, BZA may be appropriate for postmenopausal women seeking a tolerable, safe, effective, and cost-effective long-term osteoporosis treatment.

Conjugated estrogens and bazedoxifene

Each month, subscribers to The Formulary Monograph Service receive 5 to 6 well-documented monographs on drugs that are newly released or are in late phase 3 trials. The monographs are targeted to Pharmacy & Therapeutics Committees. Subscribers also receive monthly 1-page summary monographs on agents that are useful for agendas and pharmacy/nursing in-services. A comprehensive target drug utilization evaluation/medication use evaluation (DUE/MUE) is also provided each month. With a subscription, the monographs are sent in print and are also available on-line. Monographs can be customized to meet the needs of a facility. A drug class review is now published monthly with The Formulary Monograph Service. Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. For more information about The Formulary Monograph Service, call The Formulary at 800-322-4349. The March 2014 monograph topics are umeclidinium and vilanterol inhalation powder, sucroferric oxyhydroxide, luliconazole, edoxaban, and secukinumab. The DUE/MUE is on fentanyl transdermal system.

Menopausal hormone therapy and menopausal symptoms

A majority of women will experience bothersome symptoms related to declining and/or fluctuating levels of estrogen during their menopausal transition. Vasomotor symptoms, vaginal dryness, poor sleep, and depressed mood have all been found to worsen during the menopausal transition. While vasomotor symptoms gradually improve after menopause, the time course can be many years. Vaginal dryness does not improve without treatment, while the long-term course of sleep and mood deterioration is not clearly defined at this time. A small minority of women have vasomotor symptoms that persist throughout the remainder of their lives. These common menopausal symptoms all improve with estrogen treatment. Over the last 10 years, we have witnessed a dramatic reduction in enthusiasm for menopausal hormone therapy, despite its high efficacy relative to other treatments. We have also seen the emergence of sound, evidence-based clinical trials of non-hormonal alternatives that can control the common menopausal symptoms. Understanding the natural history of menopausal symptoms, and the risks and benefits of both hormonal and non-hormonal alternatives, helps the clinician individualize management plans to improve quality of life.

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.

Gq-mER signaling has opposite effects on hypothalamic orexigenic and anorexigenic neurons

Two populations of cells within the hypothalamus exert opposite actions on food intake: proopiomelanocortin (POMC) neurons decrease it, while neuropeptide Y (NPY)/agouti-related peptide (AgRP) neurons increase it. 17β-Estradiol (E2) is a potent anorexigenic hormone that exerts both genomic and non-genomic, rapid actions on these metabolic neurons. This review focuses on the rapid membrane effects of E2 in both POMC and NPY/AgRP neurons and how these combined effects mediate the anorexigenic effects of this steroid.

A double-blind, randomized, ascending, multiple-dose study of bazedoxifene in healthy postmenopausal women

Bazedoxifene is a novel selective estrogen receptor modulator in clinical development for the prevention and treatment of postmenopausal osteoporosis. This phase 1, double-blind, randomized, placebo-controlled study (N = 107) of healthy postmenopausal women examined the pharmacokinetics and safety/tolerability profile of multiple doses of bazedoxifene (1, 2.5, 5, 10, 20, 40, and 80 mg) administered orally once daily for 30 days. Bazedoxifene demonstrated a half-life of 25 to 30 hours, reached steady state within 7 days, and exhibited linear pharmacokinetics over a dose range of 5-80 mg. Fibrinogen levels decreased with bazedoxifene doses of 5 mg and greater; these changes were significant for bazedoxifene 20, 40, and 80 mg (P ≤ .05 vs placebo), but were not dose dependent. Bazedoxifene was associated with increased levels of sex hormone-binding globulin, thyroxine-binding globulin, and cortisol-binding globulin (CBG); only increases in the levels of CBG appeared to be dose related. Bazedoxifene was safe and well tolerated within the tested dose range. Bazedoxifene showed no differences from placebo in adverse event reports, vital sign measurements, or electrocardiogram findings.

Endometrial profile of bazedoxifene acetate alone and in combination with conjugated equine estrogens in a primate model

OBJECTIVE

Concerns of breast cancer risk in postmenopausal women taking combined estrogen + progestin therapy have generated interest in the use of selective estrogen receptor modulators (SERMs) as potential progestin alternatives. Endometrial proliferation and cancer risk are major concerns, however, for estrogens and certain types of SERMs when given alone. The primary aim of this study was to evaluate the endometrial profile of bazedoxifene acetate (BZA), a third-generation SERM, alone and in combination with conjugated equine estrogens (CEE) in a postmenopausal primate model.

METHODS

Ninety-eight ovariectomized cynomolgus monkeys (Macaca fascicularis) were randomized to receive no hormone treatment (controls), BZA 20 mg, CEE 0.45 mg, or the combination of BZA 20 mg + CEE 0.45 mg once daily for 20 months in a parallel-arm study design. The primary outcome measure was endometrial epithelial proliferation.

RESULTS

BZA + CEE and BZA treatment resulted in significantly less endometrial epithelial area and Ki67 expression compared with CEE (P < 0.001 for all). The prevalence of endometrial hyperplasia and other estrogen-induced morphologic changes in the BZA + CEE and BZA groups was not significantly different from controls. The addition of BZA to CEE completely inhibited the expression of estrogen receptor-α-regulated genes (TFF1 and PGR), whereas BZA alone had no effect. BZA + CEE and BZA treatment also resulted in lower estrogen receptor-α protein expression in the endometrium compared with the control and CEE groups (P < 0.05 for all).

CONCLUSIONS

BZA given at a clinically relevant dose inhibits estrogen effects on the endometrium and lacks uterotropic effects when given alone.

Tissue-selective estrogen complexes with bazedoxifene prevent metabolic dysfunction in female mice

Pairing the selective estrogen receptor modulator bazedoxifene (BZA) with estrogen as a tissue-selective estrogen complex (TSEC) is a novel menopausal therapy. We investigated estrogen, BZA and TSEC effects in preventing diabetisity in ovariectomized mice during high-fat feeding. Estrogen, BZA or TSEC prevented fat accumulation in adipose tissue, liver and skeletal muscle, and improved insulin resistance and glucose intolerance without stimulating uterine growth. Estrogen, BZA and TSEC improved energy homeostasis by increasing lipid oxidation and energy expenditure, and promoted insulin action by enhancing insulin-stimulated glucose disposal and suppressing hepatic glucose production. While estrogen improved metabolic homeostasis, at least partially, by increasing hepatic production of FGF21, BZA increased hepatic expression of Sirtuin1, PPARα and AMPK activity. The metabolic benefits of BZA were lost in estrogen receptor-α deficient mice. Thus, BZA alone or in TSEC produces metabolic signals of fasting and caloric restriction and improves energy and glucose homeostasis in female mice.

Vaginal effects of ospemifene in the ovariectomized rat preclinical model of menopause

Ospemifene is a unique tissue-selective estrogen agonist/antagonist (also known as a selective estrogen receptor modulator [SERM]) with demonstrated efficacy in Phase 3 studies of postmenopausal women with vulvar and vaginal atrophy (VVA). This report describes preclinical studies on the effects of ospemifene in the ovariectomized (OVX) rat model of menopause. Ospemifene (10mg/kg/day) and the SERM comparator, raloxifene (10mg/kg/day) were administered for 2 weeks and both increased vaginal weight; ospemifene was more effective than raloxifene. In addition, ospemifene had a greater effect on increasing vaginal epithelial height compared with raloxifene. The effect on uterine weight was less pronounced for both ospemifene and raloxifene. The ED50 of ospemifene on vaginal epithelial height was 0.39mg/kg/day and the magnitude was nearly the same as was seen with the positive control, 17α-ethinyl estradiol (EE2). In a histological analysis of ospemifene-treated rat vaginas, basal cells were overlaid by 2 to 3 cell layers of thickened goblet-like mucified cells apically; however, the cornification observed with EE2 was absent. Estrogenic activity of ospemifene was confirmed by upregulation of progesterone receptors in vaginal epithelium and stroma. Ospemifene showed similar affinity for estrogen receptor (ER)-α and ER-β, but an overall lower affinity than estradiol. Ospemifene antagonized estrogen response element (ERE)-mediated transactivation on MCF-7 cells, confirming its anti-estrogenic activity in breast cancer cells. The dose response for ospemifene in the rat is consistent with that observed in clinical studies of ospemifene 30 and 60mg, showing that the OVX rat is a highly predictive model of SERM activity in postmenopausal VVA.

Profile of ospemifene in the breast

Vulvar and vaginal atrophy (VVA) is a chronic, progressive medical condition prevalent among postmenopausal women, which produces symptoms such as dyspareunia, vaginal dryness, and vaginal irritation. Currently, the only prescription options are systemic and vaginal estrogen therapies that may be limited by concerns about long-term safety and breast cancer risk. Ospemifene is a tissue-selective estrogen agonist/antagonist (a selective estrogen receptor modulator) recently approved by the US Food and Drug Administration for treatment of dyspareunia, a symptom of VVA, due to menopause. Ospemifene, the first nonestrogen oral treatment for this indication, may provide an alternative to treatment with estrogen. Animal models with ospemifene suggest an inhibitory effect on growth of malignant breast tissue, but animal data cannot necessarily be extrapolated to humans. Clinical trials, including 3 long-term studies assessing the overall safety of ospemifene, support that ospemifene is generally well tolerated, with beneficial effects on the vagina, neutral effects on the breast, and minimal effects on the endometrium.

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.

A selective membrane estrogen receptor agonist maintains autonomic functions in hypoestrogenic states

It is well known that many of the actions of estrogens in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. But there is also a compelling evidence for the involvement of membrane estrogen receptors in hypothalamic and other CNS functions. However, it is not well understood how estrogens signal via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for sometime that estrogens can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, estrogens can affect second messenger systems including calcium mobilization and a plethora of kinases within neurons to alter cellular functions. Therefore, this brief review will summarize our current understanding of rapid membrane-initiated and intracellular signaling by estrogens in the hypothalamus, the nature of receptors involved and how these receptors contribute to maintenance of homeostatic functions, many of which go awry in menopausal states. This article is part of a Special Issue entitled Hormone Therapy.

The discovery and development of selective estrogen receptor modulators (SERMs) for clinical practice

Selective estrogen receptor modulators (SERMs) are structurally different compounds that interact with intracellular estrogen receptors in target organs as estrogen receptor agonists or antagonists. These drugs have been intensively studied over the past decade and have proven to be a highly versatile group for the treatment of different conditions associated with postmenopausal women's health, including hormone responsive cancer and osteoporosis. Tamoxifen, a failed contraceptive is currently used to treat all stages of breast cancer, chemoprevention in women at high risk for breast cancer and also has beneficial effects on bone mineral density and serum lipids in postmenopausal women. Raloxifene, a failed breast cancer drug, is the only SERM approved internationally for the prevention and treatment of postmenopausal osteoporosis and vertebral fractures. However, although these SERMs have many benefits, they also have some potentially serious adverse effects, such as thromboembolic disorders and, in the case of tamoxifen, uterine cancer. These adverse effects represent a major concern given that long-term therapy is required to prevent osteoporosis or prevent and treat breast cancer. The search for the 'ideal' SERM, which would have estrogenic effects on bone and serum lipids, neutral effects on the uterus, and antiestrogenic effects on breast tissue, but none of the adverse effects associated with current therapies, is currently under way. Ospemifene, lasofoxifene, bazedoxifene and arzoxifene, which are new SERM molecules with potentially greater efficacy and potency than previous SERMs, have been investigated for use in the treatment and prevention of osteoporosis. These drugs have been shown to be comparably effective to conventional hormone replacement therapy in animal models, with potential indications for an improved safety profile. Clinical efficacy data from ongoing phase III trials are available or are awaited for each SERM so that a true understanding of the therapeutic potential of these compounds can be obtained. In this article, we describe the discovery and development of the group of medicines called SERMs. The newer SERMs in late development: ospemifene, lasofoxifene, bazedoxifene, are arzoxifene are described in detail.

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.

Bazedoxifene exhibits antiestrogenic activity in animal models of tamoxifen-resistant breast cancer: implications for treatment of advanced disease

PURPOSE

There is compelling evidence to suggest that drugs that function as pure estrogen receptor (ER-α) antagonists, or that downregulate the expression of ER-α, would have clinical use in the treatment of advanced tamoxifen- and aromatase-resistant breast cancer. Although such compounds are currently in development, we reasoned, based on our understanding of ER-α pharmacology, that there may already exist among the most recently developed selective estrogen receptor modulators (SERM) compounds that would have usage as breast cancer therapeutics. Thus, our objective was to identify among available SERMs those with unique pharmacologic activities and to evaluate their potential clinical use with predictive models of advanced breast cancer.

EXPERIMENTAL DESIGN

A validated molecular profiling technology was used to classify clinically relevant SERMs based on their impact on ER-α conformation. The functional consequences of these observed mechanistic differences on (i) gene expression, (ii) receptor stability, and (iii) activity in cellular and animal models of advanced endocrine-resistant breast cancer were assessed.

RESULTS

The high-affinity SERM bazedoxifene was shown to function as a pure ER-α antagonist in cellular models of breast cancer and effectively inhibited the growth of both tamoxifen-sensitive and -resistant breast tumor xenografts. Interestingly, bazedoxifene induced a unique conformational change in ER-α that resulted in its proteasomal degradation, although the latter activity was dispensable for its antagonist efficacy.

CONCLUSION

Bazedoxifene was recently approved for use in the European Union for the treatment of osteoporosis and thus may represent a near-term therapeutic option for patients with advanced breast cancer.

[Third generation selective estrogen receptor modulators: benefits beyond bone. II, endometrial action]

The selective estrogen receptor modulators (SERMs) are substances with estrogenic/anti-estrogen effect that act differently depending on the tissue and composition. Since the discovery that tamoxifen and raloxifene (RLX) had a breast cancer preventive effect, the search for the perfect SERM has been the goal. The evidence that tamoxifen significantly increased the risk of endometrial cancer as compared to placebo made this tissue the center of interest in developing new SERMs. Thus, ospemifen, arzoxifene, lasofoxifene (LFX) and bazedoxifene (BZA) appeared as third-generation SERMs but only BZA reached the stage of clinical use. Both experimental and clinical data available on the effects of RLX or third-generation SERMs reaching clinical stage (LFX and BZA) show either neutrality or anti-estrogenic effects at endometrial level. BZA has shown to be equivalent to vehicle in several experimental conditions and acts as anti-estrogen in models were estrogens (conjugated equine estrogens [CEE] or E2) were co-administered. In a 7 years pivotal study the incidence of endometrial adenocarcinoma has been significantly lower in the BZA than in the placebo group. Moreover, in a clinical trial to evaluate the ability of a combination of BZA and CEE to prevent hot flushes in symptomatic postmenopausal women, doses of 20mg or higher of BZA have significantly decreased the risk of presenting endometrial hyperplasia when co-administered with either 0.650 or 0.450mg of CEE.

Estrogen-mediated mechanisms to control the growth and apoptosis of breast cancer cells: a translational research success story

The treatment and prevention of solid tumors have proved to be a major challenge for medical science. The paradigms for success in the treatment of childhood leukemia, Hodgkin's disease, Burkett's lymphoma, and testicular carcinoma with cytotoxic chemotherapy did not translate to success in solid tumors--the majority of cancers that kill. In contrast, significant success has accrued for patients with breast cancer with antihormone treatments (tamoxifen or aromatase inhibitors) that are proved to enhance survivorship, and remarkably, there are now two approved prevention strategies using either tamoxifen or raloxifene. This was considered impossible 40 years ago. We describe the major clinical advances with nonsteroidal antiestrogens that evolved into selective estrogen receptor modulators (SERMs) which successfully exploited the ER target selectively inside a woman's body. The standard paradigm that estrogen stimulates breast cancer growth has been successfully exploited for over 4 decades with therapeutic strategies that block (tamoxifen, raloxifene) or reduce (aromatase inhibitors) circulating estrogens in patients to stop breast tumor growth. But this did not explain why high-dose estrogen treatment that was the standard of care to treat postmenopausal breast cancer for 3 decades before tamoxifen caused tumor regression. This paradox was resolved with the discovery that breast cancer resistance to long-term estrogen deprivation causes tumor regression with physiologic estrogen through apoptosis. The new biology of estrogen action has been utilized to explain the findings in the Women's Health Initiative that conjugated equine estrogen alone given to postmenopausal women, average age 68, will produce a reduction of breast cancer incidence and mortality compared to no treatment. Estrogen is killing nascent breast cancer cells in the ducts of healthy postmenopausal women. The modulation of the ER using multifunctional medicines called SERMs has provided not only significant improvements in women's health and survivorship not anticipated 40 years ago but also has been the catalyst to enhance our knowledge of estrogen's apoptotic action that can be further exploited in the future.

[Third generation selective estrogen receptor modulators benefits beyond bone: effects on breast]

The selective estrogen receptor modulators (SERMs) are substances with estrogenic/anti-estrogen effect that act differently depending on the tissue and composition. Since the discovery that tamoxifen and raloxifene (RLX) had a breast cancer preventive effect, the search for the perfect SERM has been the goal. Thus, ospemifen, arzoxifene, lasofoxifene and bazedoxifene (BZA) appeared as third-generation SERMs. Among all them, only BZA reached the stage of clinical use. BZA has been shown to have an anti-estrogen effect in experimental studies, but not a protective effect on clinical breast cancer in pivotal clinical trials (301 and extensions). However, in these studies comparing BZA versus RLX and placebo, RLX has not shown the expected preventive effect on breast cancer. This lack of effect can be the consequence of the size and characteristics of BZA's studies in a population with low incidence of breast cancer.

Tissue selective estrogen complexes (TSECs) differentially modulate markers of proliferation and differentiation in endometrial cells

Selective estrogen receptor modulators (SERMs) have tissue-specific estrogen receptor (ER) modulating properties. Combining an SERM with one or more estrogens to form a tissue selective estrogen complex (TSEC) can provide an improved blend of tissue-specific ER agonist and antagonist effects. While both estrogens and SERMs affect the uterine endometrium, not all TSECs reverse the endometrial effects of estrogens preventing endometrial proliferation and hyperplasia. Their action in uterine cells is not completely understood. HOXA 10, leukemia inhibitory factor (LIF), progesterone receptor (PR), and EMX2 are genes known to regulate endometrial proliferation and differentiation. The expression of these genes was used to assess endometrial effects of SERMs and TSECs. We evaluated the effects of raloxifene (RAL), tamoxifen (TAM), lasofoxifene (LAS), bazedoxifene acetate (BZA), and progesterone (P) alone and in combination with estradiol (E2) in Ishikawa cells. Increased HOXA10, LIF, PR, and EMX2 messenger RNA (mRNA) expression was noted in E2-treated cells compared with vehicle-treated controls. All TSECs maintained E2-induced PR expression and all except TAM prevented estrogen-induced LIF expression. The TSEC containing BZA uniquely decreased HOXA10 expression and increased EMX2 expression. The TSECs alter endometrial cell proliferation by selective modulation of estrogen responsive genes, maintaining the antiproliferative effects mediated by PR and inhibiting LIF. The differential effect of TSECs on endometrial gene expression suggests a mechanism by which they manifest differential effects on endometrial safety against the risk of estrogen-induced endometrial hyperplasia.

Membrane-initiated actions of estradiol that regulate reproduction, energy balance and body temperature

It is well known that many of the actions of estrogens in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there now exists compelling evidence for membrane estrogen receptors in hypothalamic and other brain neurons. But, it is not well understood how estrogens signal via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for sometime that estrogens can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, estrogens can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by estrogens in the hypothalamus, the nature of receptors involved and how they contribute to homeostatic functions.

The tissue selective estrogen complex: a promising new menopausal therapy

Menopause is associated with health concerns including vasomotor symptoms, vulvar/vaginal atrophy (VVA), and osteoporosis. Estrogen therapy or combined estrogen-progestin therapy (EPT) are primary treatment options for menopausal symptom relief and osteoporosis prevention. Because EPT has been associated with some safety/tolerability concerns relating to undesirable effects of estrogen and progestin, alternative options are needed. The tissue selective estrogen complex (TSEC) is a novel class of agents pairing a selective estrogen receptor modulator (SERM) with 1 or more estrogens. The TSEC combines the established efficacy of estrogens on menopausal symptoms and bone with the protective effects of a SERM on the reproductive tract. The pairing of bazedoxifene (BZA) with conjugated estrogens (CE) has been evaluated in a series of phase 3 clinical trials. BZA 20 mg/CE 0.45 mg and BZA 20 mg/CE 0.625 mg have shown efficacy in reducing the frequency and severity of hot flushes, relieving VVA symptoms, and maintaining bone mass while protecting the endometrium and breast. These BZA/CE doses have been associated with a favorable safety/tolerability profile, with higher rates of cumulative amenorrhea and lower incidences of breast pain than those reported for EPT. Thus, BZA/CE may be a promising alternative to conventional EPT for treating non-hysterectomized, postmenopausal women.

Bazedoxifene: the evolving role of third-generation selective estrogen-receptor modulators in the management of postmenopausal osteoporosis

Osteoporosis is a significant public health concern, particularly for postmenopausal women. Current treatment options may not be appropriate for all women. Selective estrogen-receptor modulators (SERMs) are a class of molecules with tissue-selective activity. Bazedoxifene is currently in clinical development for the prevention and treatment of postmenopausal osteoporosis. In a 2-year, phase III, osteoporosis prevention study (N = 1583), bazedoxifene 10, 20, and 40 mg was shown to preserve bone mineral density and decrease biochemical markers of bone turnover compared with placebo in postmenopausal women at risk for osteoporosis. In a pivotal 3-year, phase III, osteoporosis treatment study (N = 7492), bazedoxifene 20 and 40 mg significantly reduced the incidence of new vertebral fractures compared with placebo (p < 0.05 for both) in postmenopausal women with osteoporosis. In a post hoc subgroup analysis of women at higher risk for fracture (n = 1772), bazedoxifene 20 mg significantly reduced the risk of nonvertebral fractures versus placebo (p = 0.02) and raloxifene 60 mg (p = 0.05). Bazedoxifene 20 mg has demonstrated sustained efficacy in reducing the risk of vertebral fractures over 5 and 7 years. Overall, bazedoxifene was generally safe and well tolerated, with favorable endometrial and breast safety profiles. As with other SERMs, the rate of deep vein thrombosis was higher in the bazedoxifene groups compared with placebo at 3 and 5 years. Considering its demonstrated efficacy and safety, bazedoxifene may be an appropriate osteoporosis therapy for women who cannot take or are unwilling to take bisphosphonates because of safety or tolerability issues. Bazedoxifene may also be appropriate for younger women at increased fracture risk who are concerned about the effects of long-term bisphosphonate therapy. This article reviews the results of key clinical trials of bazedoxifene for the prevention and treatment of postmenopausal osteoporosis and describes its role in clinical practice.

The evolution of selective estrogen receptor modulators in osteoporosis therapy

Selective estrogen receptor modulators (SERMs), which exhibit estrogen receptor agonist or antagonist activity based on the target tissue, have evolved through multiple generations for the prevention and/or treatment of postmenopausal osteoporosis. An ideal SERM would protect bone without stimulating the breast or endometrium. Raloxifene, lasofoxifene, and bazedoxifene have demonstrated unique preclinical profiles. Raloxifene, lasofoxifene, and bazedoxifene have shown significant reduction in the risk of vertebral fracture and improvement in bone mineral density versus placebo in postmenopausal women with osteoporosis. Raloxifene has been shown to reduce the risk of non-vertebral fractures in women with severe prevalent fractures at baseline. Lasofoxifene 0.5 mg, but not lasofoxifene 0.25 mg, has shown reduction in the incidence of non-vertebral fractures. Bazedoxifene 20 mg has been associated with a significant reduction in the risk of non-vertebral fracture versus placebo and raloxifene 60 mg in women at higher baseline fracture risk. Neither raloxifene, lasofoxifene, nor bazedoxifene has shown an increase in the incidence of endometrial hyperplasia or carcinoma. All SERMs have been associated with increased venous thromboembolic events and hot flushes. SERMs are effective alternatives for women who cannot tolerate or are unwilling to take bisphosphonates and may be appropriate for women at higher risk of fracture, particularly younger women who expect to remain on therapy for many years and are concerned about the long-term safety of bisphosphonates.

Research resource: Transcriptional profiling in a cellular model of breast cancer reveals functional and mechanistic differences between clinically relevant SERM and between SERM/estrogen complexes

Exploitation of the relationship between estrogen receptor (ER) structure and activity has led to the development of 1) selective ER modulators (SERM), compounds whose relative agonist/antagonist activities differ between target tissues; 2) selective ER degraders (SERD), compounds that induce a conformational change in the receptor that targets it for proteasomal degradation; and 3) tissue-selective estrogen complexes (TSEC), drugs in which a SERM and an ER agonist are combined to yield a blended activity that results in distinct clinical profiles. In this study, we have performed a comprehensive head-to-head analysis of the transcriptional activity of these different classes of ERM in a cellular model of breast cancer. Not surprisingly, these studies highlighted important functional differences and similarities among the existing SERM, selective ER degraders, and TSEC. Of particular importance was the identification of genes that were regulated by various TSEC combinations but not by an estrogen or SERM alone. Cumulatively, the findings of this analysis are informative with respect to the mechanisms by which ER is engaged by different enhancers/promoters and highlights how promoter context influences the pharmacological activity of ER ligands.

In vitro bioactivation of bazedoxifene and 2-(4-hydroxyphenyl)-3-methyl-1H-indol-5-ol in human liver microsomes

Bazedoxifene is a selective estrogen receptor modulator (SERM) that has been developed for use in post-menopausal osteoporosis. However, it contains a potentially toxic 5-hydroxy-3-methylindole moiety. Previous studies on the 5-hydroxyindole and the 3-alkylindole-containing drugs indometacine, zafirlukast and MK-0524 structural analogs have shown that they are bioactivated by cytochrome P450s through a dehydrogenation process to form quinoneimine or 3-methyleneindolenine electrophilic species. In the present study, bazedoxifene was synthesized and then evaluated, together with raloxifene and 2-(4-hydroxyphenyl)-3-methyl-1H-indol-5-ol (13), a 3-methyl-5-hydroxyindole-based structural fragment of bazedoxifene, for its ability to form reactive electrophilic species when incubated with human liver microsomes (HLMs) or recombinant CYP isozymes. We showed that bazedoxifene was bioactivated only in trace amounts with recombinant CYP isozymes. In contrast, the N-dealkylated fragment of bazedoxifene (2-(4-hydroxyphenyl)-3-methyl-1H-indol-5-ol) was bioactivated in considerable amounts to an electrophilic intermediate, which was trapped with glutathione and identified by LC-MS/MS. This suggests that bazedoxifene would require initial N-dealkylation, which could subsequently lead to the formation of the reactive intermediate. However, such an N-dealkylated metabolite of bazedoxifene was not detected after the incubation of bazedoxifene in HLM or recombinant CYP isozymes.

Incorporating bazedoxifene/conjugated estrogens into the current paradigm of menopausal therapy

Many women experience bothersome vasomotor and vaginal symptoms during the menopausal transition. Decreasing levels of estrogens during menopause are also associated with reduced bone density and an increased risk of osteoporosis. Combined estrogen/progestin therapy (hormone therapy) effectively treats menopausal symptoms and prevents bone loss, but has been associated with some safety and tolerability concerns. A novel menopausal therapy is the tissue selective estrogen complex, which pairs a selective estrogen receptor modulator with one or more estrogens. In preclinical studies, the tissue selective estrogen complex partnering bazedoxifene (BZA) with conjugated estrogens (CE) antagonized stimulation of breast and endometrial tissue, reduced vasomotor instability, and preserved bone mass in rat and mouse models. The specific attributes seen with BZA/CE were different from those observed with other selective estrogen receptor modulator/estrogen pairings. BZA/CE has undergone clinical evaluation in the Phase III Selective estrogens, Menopause, And Response to Therapy (SMART) trials in postmenopausal women with an intact uterus. Of the various doses of BZA/CE evaluated, BZA 20 mg/CE 0.45 mg and 0.625 mg were associated with a low incidence of endometrial hyperplasia (<1%) similar to placebo, and showed significant improvements in hot flushes and vulvar/vaginal symptoms and increases in bone mineral density. BZA 20 mg/CE 0.45 mg and 0.625 mg were associated with a low incidence of breast-related adverse events and demonstrated no difference from placebo in age-related changes in mammographic breast density. Both BZA/ CE doses showed a favorable tolerability profile, with no increases in uterine bleeding or breast tenderness, and had positive effects on metabolic parameters and quality of life. BZA/CE may be a promising alternative to hormone therapy for the treatment of menopausal symptoms and prevention of osteoporosis in nonhysterectomized postmenopausal women.

Bazedoxifene: a review of its use in the treatment of postmenopausal osteoporosis

Bazedoxifene (Conbriza®, Viviant®) is the first third-generation selective estrogen receptor modulator (SERM) and it is approved for the treatment of postmenopausal osteoporosis in the EU and Japan. Bazedoxifene contains an indole-based core binding domain that binds with high affinity to estrogen receptors and exhibits favourable effects on bone and lipid profiles, with no clinically relevant endometrial or breast stimulation. Oral bazedoxifene once daily reduced the incidence of new vertebral fractures in patients with postmenopausal osteoporosis in a large, well designed trial of 3 years' duration; both bazedoxifene and raloxifene were significantly more effective than placebo. Neither bazedoxifene nor raloxifene reduced the incidence of nonvertebral fractures in the overall study population; however, bazedoxifene, but not raloxifene, reduced the rate of nonvertebral fractures in high-risk patients. Moreover, data from patients who continued to receive the drug during a 2-year extension phase of this trial indicate that bazedoxifene continues to provide protection against new vertebral fractures for up to 5 years. Bazedoxifene also increases bone mineral density and reduces the levels of bone turnover markers. Bazedoxifene was generally well tolerated and did not detrimentally affect the reproductive tract or breast tissue in clinical trials, thereby demonstrating a favourable risk-benefit profile. A pharmacoeconomic analysis conducted from an EU perspective predicted bazedoxifene to be cost effective in some EU countries. Therefore, bazedoxifene presents another useful option for the treatment of postmenopausal osteoporosis, especially in those at high risk for osteoporotic fracture.

Sustained efficacy and safety of bazedoxifene in preventing fractures in postmenopausal women with osteoporosis: results of a 5-year, randomized, placebo-controlled study

In this 2-year extension of a 3-year study, bazedoxifene showed sustained efficacy in preventing new vertebral fractures in postmenopausal women with osteoporosis and in preventing non-vertebral fractures in higher-risk women. Bazedoxifene significantly increased bone mineral density and reduced bone turnover versus placebo and was generally safe and well tolerated.

INTRODUCTION

This study evaluated the efficacy and safety of bazedoxifene for the treatment of postmenopausal osteoporosis over 5 years.

METHODS

A total of 4,216 postmenopausal women with osteoporosis were enrolled in this 2-year extension of a 3-year, randomized, double-blind, placebo-controlled, phase 3 trial. In the core study (N = 7,492), subjects received bazedoxifene 20 or 40 mg/day, raloxifene 60 mg/day, or placebo. The raloxifene arm was discontinued after 3 years; subjects receiving bazedoxifene 40 mg were transitioned to bazedoxifene 20 mg after 4 years. Five-year findings are reported for bazedoxifene 20 and 40/20 mg and placebo. Endpoints included incidence of new vertebral fractures (primary) and non-vertebral fractures, and changes in bone mineral density (BMD) and bone turnover markers.

RESULTS

At 5 years, the incidence of new vertebral fractures in the intent-to-treat population was significantly lower with bazedoxifene 20 mg (4.5%) and 40/20 mg (3.9%) versus placebo (6.8%; P < 0.05), with relative risk reductions of 35% and 40%, respectively. Non-vertebral fracture incidence was similar among groups. In a subgroup of higher-risk women (n = 1,324; femoral neck T-score ≤-3.0 and/or ≥ 1 moderate or severe or ≥ 2 mild vertebral fracture[s]), bazedoxifene 20 mg reduced non-vertebral fracture risk versus placebo (37%; P = 0.06); combined data for bazedoxifene 20 and 40/20 mg reached statistical significance (34% reduction; P < 0.05). Bazedoxifene significantly increased BMD and reduced bone turnover versus placebo (P < 0.05) and was generally safe and well tolerated.

CONCLUSIONS

The findings support a sustained anti-fracture effect of bazedoxifene on new vertebral fractures in postmenopausal osteoporotic women and on non-vertebral fractures in the higher-risk subgroup of women.

Safety, Efficacy and Patient Acceptability of Bazedoxifene Acetate in the Management of Postmenopausal Osteoporosis

Many pharmacological agents are available for treatment of postmenopausal osteoporosis, including estrogen and the selective modulators of estrogen receptor (SERMS). Bazedoxifene is a third-generation SERM, which acts as estrogen agonist in bone and lipid metabolism and as an antagonist in the breast and endometrium. Studies demonstrated that bazedoxifene reduced significantly the risk of vertebral fractures. In a subgroup of patients at high risk (post-hoc analysis), a reduction of nonvertebral fractures risk was reported. Moreover, the combination of conjugated estrogens with bazedoxifene seems to offer an alternative to classical hormone therapy, improving the vasomotor symptoms and vaginal atrophy, without the use of a progestin. Bazedoxifene is a promising drug for the treatment and prevention of osteoporosis in postmenopausal women; however a safety concern regarding venous thromboembolic events is needed before starting treatment.