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.     

Is natural progesterone the missing link in osteoporosis prevention and treatment?

Conventional treatment with vitamin D, calcium, and estrogen will delay but not reverse osteoporosis. The addition of fluoride may increase bone mass but fails to increase bone strength; fracture incidence is actually increased in non-vertebral bone by fluoride. Clearly, successful treatment of osteoporosis remains an unsolved problem. In women, osteoporosis coincides with menopause. The hypothesis that progesterone and not estrogen is the missing factor was tested in a clinical setting and was found to be extraordinarily effective in reversing osteoporosis.     

Use of FRAX®-based fracture risk assessments to identify patients who will benefit from osteoporosis therapy

Several pharmacological interventions, including selective estrogen receptor modulators (SERMs), bisphosphonates, denosumab, and strontium ranelate have demonstrated efficacy in reducing the incidence of osteoporotic fractures, the most severe consequence of postmenopausal osteoporosis. Until recently, bone mineral density (BMD) was the primary factor used to determine which postmenopausal women may require osteoporosis treatment. However, clinical guidelines now recommend the use of the Fracture Risk Assessment Tool (FRAX^®), a computer-based algorithm introduced by the World Health Organization, to help primary care physicians identify postmenopausal women who may be candidates for pharmacological osteoporosis therapy based on the level of fracture risk. Beyond its utility as a resource for determining whether or not to initiate osteoporosis treatment, clinical studies have begun to evaluate the correlation between FRAX^®-based 10-year fracture probability and efficacy of different osteoporosis treatments. Bazedoxifene, clodronate, and denosumab have shown greater fracture risk reduction at higher FRAX^®-based 10-year fracture probabilities, but the efficacy of raloxifene, alendronate, and strontium ranelate were relatively stable regardless of fracture probability. In summary, these data suggest that the relationship between FRAX^®-based fracture probability and efficacy of different osteoporosis treatments varies depending upon the agent in question.     

Treatment of osteoporosis with denosumab

Osteoporosis is a common skeletal disease associated with an imbalance in bone remodeling resulting in a reduction in bone strength and increased fracture risk. The principal regulator of osteoclastic bone resorption is receptor activator of nuclear factor-κB ligand (RANKL), a cytokine member of the tumor necrosis factor family. The binding of RANKL to its receptor on the cell surface of osteoclasts and pre-osteoclasts increases the formation, activity, and survival of osteoclasts. Denosumab is an investigational fully human monoclonal antibody to RANKL. By binding to RANKL, denosumab prevents RANKL from binding to its receptor, resulting in a decrease in bone resorption due to reduction in the formation, activity, and survival of osteoclasts. In postmenopausal women with osteoporosis, denosumab 60 mg by subcutaneous injection every 6 months increased bone mineral density (BMD), reduced bone turnover markers, and reduced the risk of vertebral, hip, and non-vertebral fractures. In postmenopausal women with low BMD, denosumab increased BMD and reduced bone turnover markers. It was well tolerated with a safety profile generally similar to placebo. Denosumab is a promising emerging drug for the prevention and treatment of postmenopausal osteoporosis. It may be particularly useful in clinical practice for the treatment of patients with gastrointestinal contraindications or side effects with oral bisphosphonates and for patients with malabsorption.     

Rationale for using raloxifene to prevent both osteoporosis and breast cancer in postmenopausal women

Both osteoporosis with fracture and breast cancer are important health issues for postmenopausal women. It is well known that estrogen and estrogen receptors (ERs) play an important role in the pathogenesis of both diseases. In past decades, hormone therapy (HT), mainly estrogen plus progestin (EPT), has been frequently used for the purpose of preventing and treating postmenopausal osteoporosis because of its efficacy, but it also contributes to a significant increase in breast cancer. Currently, there is a dilemma regarding the use of estrogen for postmenopausal women. Fortunately, an increasing understanding of the action of estrogen has led ultimately to the design of new drugs that work by virtue of their interaction with the ER; these drugs have come to be known as selective estrogen receptor modulators (SERMs), and are not only effective in preventing osteoporosis and managing those with osteoporosis, but also in decreasing the incidence of breast cancer. Among these SERMs, raloxifene may be the most attractive agent based on the evidence from five recent large trials (Multiple Outcomes of Raloxifene Evaluation [MORE], Continuing Outcomes Relevant to Evista [CORE], Raloxifene Use for the Heart [RUTH], Study of Tamoxifen and Raloxifene [STAR], and Evista Versus Alendronate [EVA]). The former three trials showed that raloxifene not only decreases the incidence of osteoporosis-associated fractures, but also has efficacy in breast cancer prevention. The head-to-head comparison with the anti-fracture agent alendronate (EVA trial) and the chemoprevention agent tamoxifen (STAR trial) further confirmed that raloxifene is a better choice. We concluded that since there is an absence of a therapeutic effect on relieving climacteric symptoms and there is the presence of a potential risk of thromboembolism in the use of raloxifene, this drug can be prescribed for clear indications, such as the management of osteoporosis, the prevention of fracture, and decreasing the incidence of invasive breast cancer, with careful monitoring for thromboembolism. It is reasonable to use raloxifene as an appropriate medicine that targets climacteric symptom-free postmenopausal women because of its overall favorable risk-benefit safety profile using the global index proposed by the Women's Health Initiation (WHI).     

Sex steroids and bone

Sex steroids are essential for skeletal development and the maintenance of bone health throughout adult life, and estrogen deficiency at menopause is a major pathogenetic factor in the development of osteoporosis in postmenopausal women. The mechanisms by which the skeletal effects of sex steroids are mediated remain incompletely understood, but in recent years there have been considerable advances in our knowledge of how estrogens and, to a lesser extent androgens, influence bone modeling and remodeling in health and disease. New insights into estrogen receptor structure and function, recent discoveries about the development and activity of osteoclasts, and lessons learned from human and animal genetic mutations have all contributed to increased understanding of the skeletal effects of estrogen, both in males and females. Studies of untreated and treated osteoporosis in postmenopausal women have also contributed to this knowledge and have provided unequivocal evidence for the potential of high-dose estrogen therapy to have anabolic skeletal effects. The development of selective estrogen receptor modulators has provided a new approach to the prevention of osteoporosis and other major diseases of menopause and has implications for the therapeutic use of other steroid hormones, including androgens. Further elucidation of the mechanisms by which sex steroids affect bone thus has the potential to improve the clinical management not only of osteoporosis, both in men and women, but also of a number of other diseases related to sex hormone status.     

Modulators of androgen and estrogen receptor activity

This review focuses on significant recent findings regarding modulators of androgen and estrogen receptor activity. Selective androgen receptor modulators (SARMs) interact with androgen receptors (ARs), and selective estrogen receptor modulators (SERMs) interact with estrogen receptors (ERs), with variable tissue selectivity. SERMs, which interact with both ERб and ERв in a tissue-specific manner to produce diverse outcomes in multiple tissues, continue to generate significant interest for clinical application. Development of SARMs for clinical application has been slower to date because of potential adverse effects, but these diverse compounds continue to be investigated for use in disorders in which modulation of the AR is important. SARMs have been investigated mostly at the basic and preclinical level to date, with few human clinical trials published. These compounds have been evaluated mostly for application in different stages of prostate cancer to date, but they hold promise for multiple other applications. Publication of the large STAR and RUTH clinical trials demonstrated that the SERMs tamoxifen and raloxifene have interesting similarities and differences in tissues that contain ERs. Lasofoxifene, bazedoxifene, and arzoxifene are newer SERMs that have been demonstrated in clinical trials to more potently increase bone mineral density and lower serum cholesterol values than tamoxifen or raloxifene. Both SARMs and SERMs hold great promise for therapeutic use in multiple disorders in which tissue-specific effects are mediated by their respective receptors.     

A pharmacological review of selective oestrogen receptor modulators

Selective oestrogen receptor modulators (SERMs) are structurally diverse non-steroidal compounds that bind to oestrogen receptors and produce oestrogen agonist effects in some tissues and oestrogen antagonist effects in others. SERMs are being evaluated for a number of oestrogen-related diseases, including post-menopausal osteoporosis, hormone-dependent cancers, and cardiovascular disease. Several compounds that exhibit a SERM profile are currently available for clinical use, including clomiphene, tamoxifen, and toremifene (which are triphenylethylenes) and raloxifene (a benzothiophene). Clomiphene is used for the induction of ovulation in sub-fertile women attempting pregnancy. Tamoxifen and toremifene are both used to treat breast cancer. Tamoxifen may have beneficial effects on bone mineral density and serum lipids. The effects of toremifene on serum lipids are similar to that of tamoxifen. Both compounds have stimulatory effects on the endometrium. Raloxifene, indicated for the treatment and prevention of post-menopausal osteoporosis, has beneficial effects on bone mineral density and serum lipids, but does not increase the risk of endometrial hyperplasia or endometrial cancer. Recently, raloxifene was shown to reduce the incidence of vertebral fractures in otherwise healthy women with osteoporosis; in the same study, a reduced incidence of breast cancer was also observed. Similar to oestrogens, SERMs increase the incidence of venous thromboembolism. Several newer compounds that exhibit a SERM profile are also in clinical development, including other triphenylethylenes (droloxifene, idoxifene) and benzothiophenes (LY353381.HCl), benzopyrans (EM-800), and naphthalenes (CP-336,156).     

Assessment of hormonally active agents in the reproductive tract of female nonhuman primates

Using the ovariectomized macaque model of postmenopausal women's health, we investigated the effects of long-term treatments (5 weeks-3 years) with estradiol, conjugated equine estrogens (CEE), esterified estrogens, progestins such as medroxyprogesterone acetate (MPA) and nomegestrol acetate, CEE + MPA, tamoxifen, soybean phytoestrogens (SPEs), a variety of putative selective estrogen receptor modulators (SERMs), and androgens. Agents tested were selected on the basis of beneficial effects on arteries and/or bone. Doses were scaled on a caloric or serum-concentration basis to approximate human clinical doses. We evaluated endometrial and mammary gland histopathology and morphometry and used immunohistochemistry to evaluate cell proliferation and expression of estrogen receptor alpha and progesterone receptor (PR). Both estradiol and CEE induced endometrial hyperplasia. MPA antagonized epithelial proliferation induced by CEE in endometrium and induced pseudodecidual stromal hyperplasia in some animals. Tamoxifen induced endometrial polyps, cystic hyperplasia, stromal fibrosis, and PR expression but not Ki-67 expression. SPEs were not estrogenic at dietary doses and antagonized estrogen-induced proliferation in the endometrium and breast. Nandrolone induced mucometra and an adenomyosis-like change. The potential SERM 17 alpha dihydroequilenin did not have uterotrophic or mammotrophic effects. In general, experimental findings in macaques have been predictive of outcomes in human clinical trials of the same agents.     

Sex steroids and bone: current perspectives

Although the process of bone remodelling or its control has not yet been fully elucidated there is, at present, sufficient information available to conclude that ovarian steroids (estrogens, androgens, progesterone) play an essential role in skeletal homeostasis. The mechanism of action of sex steroids on the skeleton is still not entirely clear, but it has traditionally included indirect effects on systemic hormones that regulate calcium balance and a direct receptor-mediated action. More recently, changes in cytokine production within the bone marrow, as well as pro-apoptotic and anti-apoptotic effects in the osteoblastic cells, have been proposed as new perspectives on the cellular and molecular mechanisms by which sex steroids influence adult bone homeostasis. Mechanical loading, when combined with estrogens or androgens, results in a greater osteogenic response than either condition separately. Women are especially at risk for osteoporosis if they have had a premature or surgical menopause and have not received hormone replacement therapy (HRT). Other reproductive factors that can help to identify women with osteopenia and emphasize the role of sex steroids in preserving bone mass in premenopausal women include: age at menarche, menstrual history and irregularities (including those associated with excessive exercise), age at menopause, previous hysterectomy, hyperprolactinaemia, anorexia nervosa, scoliosis, ovarian dysgenesis, pregnancy and lactation, and pharmacological ovarian suppression. The prevention of osteoporosis starts with the onset of the menarche. A combination of exercise, appropriate nutrition and a healthy lifestyle all maximize bone mineral accrual and result in optimal peak bone mass; normal ovarian function is essential to this process. Unfortunately, many women actually become aware of the need for osteoporosis prevention much later in life, usually after they have already become menopausal. HRT, however, has important limitations for prevention of fractures in post-menopausal women. Future perspectives for treatment of osteoporosis include androgen therapy and anabolic agents. Specifically, synthetic ligands of the estrogen receptor that can evoke the non-genotrophic but not the genotrophic signal of the receptor may be bone anabolic agents, as opposed to natural estrogens or selective estrogen receptor modulators that are anti-resorptive agents. The same ligands may circumvent the side effects associated with conventional HRT.     

Evidence of a linkage disequilibrium between polymorphisms in the human estrogen receptor alpha gene and their relationship to bone mass variation in postmenopausal Italian women

Bone mineral density (BMD), the major determinant of osteoporotic fracture risk, has a strong genetic component. The discovery that inactivation of estrogen receptor alpha (ERalpha) gene is associated with low BMD indicated ERalpha as a candidate gene for osteoporosis. We have investigated the role of three ERalpha gene polymorphisms [intron 1 PVU:II and XBA:I RFLPs and TA dinucleotide repeat polymorphism 5' upstream of exon 1] in 610 postmenopausal women. There was a strong linkage disequilibrium between intron 1 polymorphic sites and also between these sites and the microsatellite (TA)(n) dinucleotide polymorphism, with a high degree of coincidence of the short TA alleles and the presence of PVU:II and XBA:I restriction sites. No significant relationship between intron 1 RFLPs and BMD was observed. A statistically significant correlation between (TA)(n) repeat allelic variants and lumbar BMD was observed (P = 0.04, ANCOVA), with subjects with a low number of repeats (TA < 15) showing the lowest BMD values. We observed a statistically significant difference in the mean +/- SD number of TA repeats between analyzed women with a vertebral fracture (n = 73) and the non-fracture group, equivalent to 2.9 (95% CI 1.56-5.72) increased fracture risk in women with a low number of repeats (TA < 15). We conclude that in this large population sample the (TA)(n) dinucleotide repeat polymorphism at the 5' end of the ERalpha gene accounts for part of the heritable component of BMD and might prove useful in the prediction of vertebral fracture risk in postmenopausal osteoporosis.     

选择性雌激素受体调节剂

雌激素替代疗法(estrogen replacement therapy,ERT)是治疗绝经后综合征的首选治疗方案,但是长期应用导致子宫内膜增生、乳腺癌等.选择性雌激素受体调节剂主要通过ER 亚型、共调节子、靶启动子、雌激素受体相关受体等机制实现其组织选择性,在发挥骨骼、心血管保护作用的同时,减少了对乳腺及生殖系统的副作用.目前,选择性雌激素受体调节剂的种类、作用的组织特异性及其临床应用在医学界引起广泛关注,具有广阔的发展前景.     

Mechanisms of sex steroids. Future developments

The discussion on the risks of hormone therapy supports the search for alternative drugs such as selective estrogen receptor modulators (SERMs). These compounds are suitable for special preventive goals, but cannot be expected to replace the use of estrogens in patients with estrogen deficiency. The development of selective progesterone receptor modulators (SPRMs) which has to resolve various problems, might be a promising approach. Hormone replacement therapy (HRT) with natural estrogens remains the measure of choice for treatment of symptoms caused by estrogen deficiency. Recent findings suggest that the additional progestogen which is used for the protection of the endometrium, plays a crucial role with regard to the risk of breast cancer and cardiovascular disease. As surrogate parameters cannot predict the extent of risks, suitable tools for the selection of progestogens with the least potential for causing adverse effects, are urgently needed. Experimental, clinical and epidemiological data suggest that the elevation in breast cancer risk is due to the proliferative effect of estrogens on breast tissue which is largely enhanced by progestogens. A short-term in vivo-test might be helpful for the evaluation of proliferative effects of estrogen–progestogen preparations. Similarly, a strictly standardized in vivo-test for the assessment of the atherogenic potential of estrogen–progestogen preparations might help to select the preparations with the lowest risk for ischemic diseases. The available data suggest that it is probably not the androgenic but the glucocorticoid activity of a progestogen which plays a role in the development of cardiovascular disease. Progestogens with glucocorticoid effects may up-regulate the thrombin receptor in the vessel wall which is involved in the development of atherosclerosis and stimulation of extrinsic coagulation.     

Phytoestrogens and post reproductive health

The use of phytoestrogens for various perceived health benefits is widespread. Despite 20 years of research the evidence for any significant health benefits remains inconclusive. Pre clinical trials have demonstrated both non-genomic and genomic actions of constituents of phytoestrogens including selective, but weak, binding to estrogen receptors, with a preference for ER B over ER A. Evidence of clinically relevant biological effects from observational studies and randomized trials has, in general, been lacking. Despite many trials there remains little evidence that phytoestrogens, whether dietary or supplemented, significantly relieve menopausal vasomotor symptoms or cognition. Several potential mechanisms for a positive effect on bone and cardiovascular health have been demonstrated however no fracture prevention data or cardiovascular end point benefit has yet been demonstrated. In vitro effects of phytoestrogens on breast cells have been both stimulatory and inhibitory however net effects appear neutral with observational studies finding no change in breast cancer risk. No effect has been seen on endometrial or other cancers and side effect profiles have, in general, been mild.     

Osteoporotic fractures: background and prevention strategies

Objectives: To review current knowledge of the epidemiology, pathogenesis, prevention and treatment of osteoporosis, with particular reference to issues related to the menopause. Methods: Peer-reviewed publications were assessed. Results: Much international variation exists in the prevalence of osteoporosis and the incidence of fracture. Risk fractures for osteoporosis are numerous. The menopause and other causes of hypogonadism in both women and men strongly predispose to osteoporosis. Various endocrinopathies, especially glucocorticoid excess, also are important. The contribution of family history may be explained by one or more genetic markers. Poor vitamin D and calcium nutrition, smoking, high alcohol consumption and inactivity increase risk. Reduced bone mass is a major risk factor for fracture, although the magnitude of that risk may vary between populations. In addition, bone fragility, length of the femoral neck (for hip fracture), history of prior fracture (for vertebral fracture) and falls affect fracture risk. Useful methods for measuring bone density are available for both epidemiologic surveillance and for clinical practice. Dual energy x-ray absorptiometry is the most desirable method in clinical care settings. Some risk factors can be modified for prevention of osteoporosis. Postmenopausal bone loss can be inhibited with estrogen or estrogen plus progestin therapy. Bone loss in the elderly may be moderated with calcium and vitamin D supplementation. Maintenance of muscle tone and strength through exercise may reduce falls. Conclusions: Osteoporosis is a large and growing health problem in many countries. Prevention of osteoporosis is a high priority, especially because treatment of the established disease remains sub-optimal. Prevention requires immediate, intermediate-term and long-term strategies. First line therapy for established osteoporosis in women in many countries is estrogen or estrogen plus progestin, calcium and vitamin D. Prospects for improved prevention of osteoporotic fractures are encouraging.     

Prevention of postmenopausal osteoporosis with oestrogen replacement therapy and associated compounds: update on clinical trials since 1995

Hormonal replacement therapy (HRT) is generally regarded as first choice for pharmacological prevention of osteoporosis in women. We reviewed recent studies of HRT regimens and selective oestrogen receptor modulators (SERMs), including controlled trials of at least one-year duration published since 1995 until February 2000 providing data on bone mineral density (BMD) or fractures. Natural and synthetic oestrogens exert a continuum of positive effects on BMD in a dose-dependent, though non-proportional, fashion independent of age and mode of administration. Bone loss may be largely prevented by 25 microg transdermal patch oestradiol, 0.3 mg conjugated equine or 0.3 mg esterified oestrogens. Progestogens neither attenuate nor augment the effect of oestrogens; sole use of tibolone prevents bone loss. Both the SERMs, tamoxifen and raloxifene, slightly increase BMD. There are no adequately powered fracture trials for any HRT regimen. Raloxifene 60 mg daily decreases the relative risk of vertebral fractures by at least 30%, as demonstrated by one 3-year fracture study of osteoporotic women. In conclusion, the recommendation to use oestrogen for postmenopausal osteoporosis, given both the lack of fracture trials and the rare trials on long-term use of HRT in (late) postmenopausal women, is not well supported. Fracture trials could overcome shortcomings of the current level of evidence.     

Controversy about uterine effects and safety of SERMs: the saga continues

From the perspective of endometrial safety, there has been great controversy about what special management, if any, tamoxifen-treated patients should undergo. Periodic blind endometrial sampling or transvaginal ultrasound has been advocated by some. Because of the problems associated with either of these techniques alone, we recommended an approach that used transvaginal ultrasound and then proceeded to sonohysterography when the endometrial echo on transvaginal ultrasound was not reliably thin and distinct. The American College of Obstetricians and Gynecologists (ACOG), in its committee opinion, stated that patients receiving tamoxifen therapy should only have an annual pelvic exam with pap smear if they remain asymptomatic. Newer data suggest, however, that there are high- and low-risk groups that can be identified by pretreatment screening. Before tamoxifen therapy, 17% of patients have polyps. These patients have 17 times the incidence of atypical hyperplasia than those whose uterus was negative before tamoxifen therapy. Such findings call into question the validity of the only study of raloxifene where uterine safety was the primary endpoint. In that study, any woman with baseline endometrial findings other than pristinely negative (i.e., low risk) was excluded. However, other raloxifene studies without pretreatment screening show relative risk (RR) = 0.8 (95% CI = 0.2, 2.7) for endometrial carcinoma. This compares with the women over 50 years of age in the Breast Cancer Prevention Trial (National Surgical Adjuvant Breast and Bowel Project P-1) with tamoxifen when the RR = 4.01 (95% CI= 1.70, 10.90). The existence of potentially high- and low-risk groups should be taken into account in any future clinical trials looking at the endometrial safety of selective estrogen receptor modulators (SERMs).     

Emerging therapies for postmenopausal vaginal atrophy

Symptoms related to vaginal atrophy are a significant problem for postmenopausal women and estrogen has been the gold standard for its treatment. A number of recent reviews of vaginal estrogen products are available. This review will, therefore, focus on other products and potential products for this indication, including the tissue selective estrogen complex and selective estrogen receptor modulators. Additionally, lesser-studied approaches will be discussed.     

SERMs: Progress and future perspectives

Selective estrogen receptor modulators (SERMs) interact with estrogen receptors as agonists or antagonists depending on the target tissue. Currently available SERMs are used to treat and prevent breast cancer and osteoporosis, to treat ovulatory dysfunction in women, and for contraception. Because current therapies do not adequately treat menopausal symptoms, the search continues for the optimal SERM for postmenopausal women, which would relieve hot flushes, treat vaginal atrophy, and prevent fractures, while protecting the endometrium, breast, and cardiovascular system. Future use of SERMs may also include their use in a tissue selective estrogen complex (TSEC), a therapy that combines a SERM with estrogen(s), designed to deliver the efficacy of each component with improved overall tolerability for the treatment of postmenopausal women. The future of SERMs may also include their use in men for the treatment of osteoporosis and various syndromes associated with secondary hypogonadism and possibly prostate cancer. Continued research should allow the full potential of SERMs to be uncovered.     

Low-dose estrogen therapy for prevention of osteoporosis: working our way back to monotherapy

The risks of low bone mineral density, osteoporosis and fractures, are major concerns in postmenopausal women. Although postmenopausal hormone therapy is effective for reducing these risks, safety issues have been raised by the results of studies such as the Women's Health Initiative. Although there are scientifically valid reasons to be wary of the general applicability of the Women's Health Initiative findings, the study has underscored the continuing need for research into new forms of menopausal hormone therapy. Low-dose transdermal estrogen monotherapy can preserve bone density while relieving vasomotor symptoms. Transdermal administration may offer advantages, including lack of first-pass liver metabolism, which permits the use of lower doses and avoids a negative impact on the lipid profile. Moreover, a recently published 2-year study of ultra-low-dose transdermal estrogen monotherapy in an older population similar to that of the WHI reported significant increases in bone mineral density, accompanied by significant reductions in markers of bone turnover, with no increased risk of endometrial hyperplasia or other side effects. Additional studies are warranted to shed further light on the possible benefits of low-dose estrogen monotherapy for the prevention of bone loss in postmenopausal women.