Hormone Replacement Therapy in Menopausal Women: Past Problems and Future Possibilities

By Schmidt, John W; Wollner, Debra; Curcio, Jessica; Riedlinger, June; Kim, Linda S

Abstract

Oral administration of conjugated equine estrogens (CEE) with and without the synthetic progestin medroxyprogesterone acetate (MPA) in postmenopausal women is associated with side-effects that include increased risk of stroke and breast cancer. The current evidence that transdermal administration of estradiol may provide a safer alternative to orally administered CEE is reviewed. Transdermally administered estradiol has been shown to be an efficacious treatment for hot flushes possibly without the increase in blood clotting that is associated with administration of oral CEE. Further, natural progesterone may have a more beneficial spectrum of physiological effects than synthetic progestins. The substantial differences between CEE compared with estradiol and estriol, as well as the differences between synthetic MPA and natural progesterone, are detailed. Estriol is an increasingly popular alternative hormone therapy used for menopausal symptoms. There is evidence that estriol, by binding preferentially to estrogen receptor-β, may inhibit some of the unwanted effects of estradiol. New clinical trials are needed to evaluate the safety and efficacy of topically or transdermally administered combinations of estradiol, estriol and progesterone. Future studies should focus on relatively young women who begin estrogen supplement use near the start of menopause.

Keywords: Hormone replacement therapy, bioidentical hormone replacement therapy, menopause, review

Introduction

For decades, hormone replacement therapy (HRT) has been widely used to treat the debilitating and unpleasant symptoms of menopause in healthy women. Due to efficacy, cost and convenience, the primary treatment of choice in the USA has until recently been oral conjugated equine estrogens (CEE; sold as Premarin(TM)) with or without a progestin (usually medroxyprogesterone acetate, MPA; sold as Provera(TM)). However, since the beginning of the current century, several large-scale studies have been published that demonstrate a significant possibility of serious-side effects, particularly in older women many years past menopause, using these treatments. This has led many women and their physicians to question whether the benefits of oral HRT outweigh the risks. There is a need for safer alternatives; however, caution must be exercised to demonstrate safety and efficacy of alternative treatments before another generation of women begins treatment with inadequately tested therapies that also may eventually prove unsafe. Here we review recent studies of HRT, and suggest possible alternative therapies that must undergo rigorous testing to provide evidence of safety before they are prescribed for popular use.

HRT is often prescribed to treat symptoms associated with menopause, including hot flushes, night sweats and sleep disturbances [I]. This paper is designed to address concerns of healthy women seeking relief of moderate to severe menopausal symptoms (e.g. hot flushes). With the correct type of estrogen for treatment of vasomotor symptoms, there may also be health benefits that target bones and other tissues, including the cardiovascular system, in relatively young menopausal women.

This review specifically compares HRT using the different types of hormones clinically available, and notes some significant differences among the results obtained when studying chemically distinct forms of the same parent hormone. CEE are not equivalent to the human hormones estradiol and estriol, and the effects of synthetic progestins, e.g. MPA, are different from those of the major human progestogen, progesterone. Other progestins besides MPA are often used, particularly outside the USA, but this review focuses on differences between MPA and progesterone. Also, the various methods of administration of the hormone provide different results, as topical application is not equivalent to oral. This review addresses these differences in order to suggest safer HRT protocols for symptomatic menopausal women.

Placebo effect in hormone replacement therapy studies

Early clinical trials testing estrogen therapy for the treatment of vasomotor symptoms demonstrated that, for at least some populations of menopausal women, placebo control groups achieve large decreases in hot flush frequency [2]. According to a meta- analysis of 21 clinical trials with a total of 2511 participants, the average reported decline in hot flush frequency for placebo groups during estrogen supplementation studies was about 50% [3]. The central nervous system of many women naturally adapts to the lower level of estrogen after menopause, either without a period of hot flushes or with declining numbers of hot flushes over some years. These data from placebo-controlled studies suggest that many women seeking treatments for hot flushes may receive only a small benefit from estrogen supplementation over and above the natural decrease in symptoms they probably would have experienced without treatment; therefore it is important to have placebo-controlled clinical trials testing the efficacy of estrogen supplements. Crossover studies may optimally measure the natural decline in symptoms that occurs without drug intervention and the rebound of symptoms that can occur when estrogen supplements are withdrawn (discussed below, ‘Rebound hot flushes and hormone dependency’).

Current guidelines regarding hormone replacement therapy

Given the potential side-effects of estrogen supplements [4], and the fact that many menopausal women will see a natural reduction in incidence of hot flushes without treatment, the current view is that estrogen supplement use should be minimized (position statement of the North American Menopause Society [5]). Current HRT strategies involve use of the lowest effective dose for management of severe hot flushes and restrict hormone supplement use to the shortest possible duration. Unfortunately, very little is known about how to successfully remove postmenopausal women from estrogen supplements without triggering a return of vasomotor symptoms [6]. However, if managed properly in terms of dose and length of treatment, route of administration, correct combination of hormones and integration with appropriate behavior modification, the benefits of sex steroid hormone supplement use can be greater than the risks. Below, we review clinical trial results which suggest that there need be no urgency in ending the standard use of estrogen supplementation in women undergoing menopause and we discuss alternative forms of HRT that may have advantages over the type that was used in the Women’s Health Initiative (WHI) studies.

Given the variability of vasomotor symptoms between women [7] and variability in estrogen metabolism (discussed below), treatment should be individualized for menopausal women seeking relief from hot flushes, night sweats and associated sleep disruption. Non-drug alternatives such as behavior modification may successfully mitigate some vasomotor symptoms and correct sleep patterns for some women [8]. The data indicate that not all postmenopausal women with vasomotor symptoms benefit from estrogen supplements, so non- hormonal therapies should be properly considered in such women.

Rebound hot flushes and hormone dependency

While it is recommended that treatment of symptoms of menopause using HRT last for only a short period of time, the rebound hot flushes experienced by many women on cessation of hormone therapy [2] has sometimes led to the continuous use of HRT for many years. Women may become trapped into extended use because HRT may interfere with the body’s natural ability to adjust to the lower estrogen levels that occur in postmenopausal women. This ‘trapping’ effect, or dependency, of estrogen supplementation can be considered a side- effect of estrogen therapy and is an example of a general phenomenon observed when neuroactive substances are administered and then withdrawn [9]. In order to begin to address this issue, clinical studies of estrogen supplements for treating vasomotor symptoms in postmenopausal women should include a follow-up phase in which hot flush frequency is measured in women who stop using HRT. The collection of such data will begin to provide a basis for making better decisions about how to successfully wean women off estrogen supplements when the risks of long-term continuation exceed the benefits.

Risks of oral hormone replacement therapy

Women in the large, randomized, placebo-controlled WHI clinical trial had an average age at study screening of 63 years and were not selected specifically for active vasomotor symptoms [1O]. However, data specific for the youngest women in the study have been reported and analyzed separately, providing a comprehensive dataset for a wide range of health effects of HRT. Evidence of potentially serious risks with CEE and synthetic progestins, particularly in older women, has been accumulating. These risks include stroke, pulmonary embolism, other forms of cardiovascular disease (CVD), certain cancers and possibly dementia. Comparison of results from the WHPs ‘estrogen plus progestin’ (16608 women treated with Prempro(TM) (CEE plus MPA)) and ‘estrogen alone’ (10739 women treated with Premarin(TM)) arms allows distinctions to be made between the side- effects of Prempro\(TM) and Premarin(TM) (Table I) [10-16]. The major sideeffects associated with CEE plus MPA were blood clots, breast cancer and dementia, and with CEE alone the major side- effect was the formation of blood clots. Theses results indicate that increased blood clotting is a major risk with oral HRT in postmenopausal women.

Oral administration of estrogen and risk of blood clots

Since 1968, the ability of oral estrogens to stimulate blood clotting has been recognized as a side-effect [17], and noted in early clinical trials for postmenopausal women [2]. The widespread use of estrogen supplements for treating postmenopausal women was advocated within a context of such known risks [18]. Given the dangers with HRT, individualization, low doses and short-term use were all advised [19]. However, certain populations of women, such as those at risk for osteoporosis, were soon identified as candidates for long-term HRT [2O].

Table I. Side-effects observed with different hormone replacement therapy regimens in the Women’s Health Initiative [10,12-16].*

Enthusiasm that estrogen use would have great benefits for bone mineralization and the cardiovascular system [21] led to the abandonment of caution regarding the risk of increased blood clotting. Beneficial effects of HRT on osteoporosis have been confirmed by the WHI studies, but for older postmenopausal women without symptoms of estrogen deficiency, risks such as stroke with long-term use outweigh the potential benefits [22].

The WHI CEE plus MPA arm [10] found a 1.41fold increased risk of stroke (95% confidence interval (CI) 1.07-1.85). In the CEE alone arm [11], a similar 1.39-fold increased risk of stroke was reported (95% CI 1.10-1.77). Thus both the Heart and Estrogen/progestin Replacement Study [23] and the WHI failed to demonstrate cardiovascular benefits of using HRT for older postmenopausal women (Table I, total CVD). The WHI Observational Study of 89 914 women also showed that hysterectomy was associated with a 1.26-fold (p

Impact of age on side-effects of hormone replacement therapy

The results from the WHI estrogen alone arm are generally better for younger women than for older women [12]. For coronary heart disease, women who started CEE alone while in their fifties had a trend towards a 0.56-fold reduced risk (95% CI 0.30-1.03) compared with women in the placebo control group [12]. A trend towards decreased risk of coronary heart disease with estrogen use was not seen for women in the age group 70-79 years, where the hazard ratio (HR) was 1.04 (95% CI 0.75-1.44).

For women of all ages, there was a statistically significant 1.47- fold increased risk (95% CI 1.042.08) of deep vein thrombosis (Table II). However, for women who started the study in their fifties, there was a trend towards a smaller 1.22-fold increased risk (95% CI 0.62-2.42) that did not achieve statistical significance. Women who started CEE alone while in their fifties had an HR for stroke of 1.08 (95% CI 0.57-2.04), not statistically different from placebo [12]. These results suggest that if the estrogen alone study had been continued for its full duration, oral estrogen use by young menopausal women (starting estrogen use in their fifties) might have shown statistically significant benefits for coronary heart disease without as large a risk of blood clotting as was seen for older women (Table II). The results of the estrogen alone arm (Table II) do not authoritatively support the avoidance of estrogen supplement use by young menopausal women suffering from vasomotor symptoms.

Hormone replacement therapy and risk of endometrial cancer

Historically, there was a suspected association between estrogen supplementation and endometrial cancer [25], and the incidence of endometrial cancer cases increased dramatically following the introduction of widespread CEE use [26]. It was known that progestogens can have an inhibitory effect on the growth of endometrial tumors [27]. Controlled studies quickly showed that synthetic progestins can block the induction of endometrial cancer associated with estrogen supplementation [28]. Co-administration of a progestogen is still the standard method used to prevent estrogen- induced endometrial hyperplasia [29], but co-administration of a synthetic progestin has its own side-effects [10,11].

Table II. Disease incidence in the estrogen alone (CEE, Premarin(TM)) arm of the Women’s Health Initiative [12].*

Hormone replacement therapy and risk of breast cancer

An association has been demonstrated between combined synthetic progestin and estrogen use and elevated breast cancer risk in postmenopausal women [3O]. In the WHI study, use of CEE plus MPA (Table I) was associated with a 1.26-fold increased risk of breast cancer (95% CI 1.00-1.59), while CEE alone was associated with a 0.77-fold reduced risk (95% CI 0.59-1.01, p > 0.05). These results suggest that the association between HRT and increased risk of breast cancer that has been discussed for many years [31-36] is due to combined estrogen and progestin (MPA) use, not supplementation with estrogen alone, as had been suggested by earlier research [37]. The data for the WHI study indicated increasing breast cancer risk in the estrogen plus progestin group with increasing duration of HRT use. For just the 74% of women enrolled with no previous use of HRT, there was an observed 1.06-fold increased breast cancer risk (95% CI 0.81-1.38) [1O].

Hormone replacement therapy and risk of ovarian cancer

In the WHI CEE plus MPA arm, there was a trend towards increased ovarian cancer in the estrogentreated group [13]. The HR for estrogen plus progestin use compared with placebo was 1.58 (95% CI 0.77-3.24, p

An association between estrogen supplement use and increased risk of ovarian cancer has been discussed for several decades [39,4O]. For postmenopausal women, estrogen use for more than 10 years was found to have a statistically significant increased risk of death due to ovarian cancer (rate ratio =1.71) compared with non-users (95% CI 1.06-2.77) [41]. Other studies have also reported ovarian cancer increases in estrogen users compared with non-users [42].

Hormone replacement therapy and cognitive functions

Progestins such as MPA may counter some of the cardioprotective effects of estrogen [43] and may interfere with estrogen-induced improvements in cognitive function [44]. Observational studies suggest a reduction in the incidence of dementia for postmenopausal women using HRT [45] and even after the results of the WHI estrogen plus progestin arm were known, enthusiasm for the potential benefits of estrogen supplements on cognition has continued [46]. However, in the placebo-controlled WHI study, CEE plus MPA use was associated with a 2.05-fold increased risk of dementia (95% CI 1.21-3.48) and use of CEE alone with a 1.49-fold increased risk (95% CI 0.83-2.66, p > 0.05) (Table I). The results for dementia and cognitive function [14,15] indicate that exposure to MPA has deleterious effects on the brains of older postmenopausal women. It is not clear if the increased incidence of blood clots in women using oral HRT is related to declines in cognitive abilities.

The results for the incidence of dementia from the CEE alone arm may not be relevant to young postmenopausal women. The WHI dementia and cognition studies did not involve women under the age of 65 years. The cognitive benefits from estrogen were originally suggested by non-randomized studies of HRT users, many of whom were young and seeking treatment for vasomotor symptoms [47]. Possible cognitive benefits of estrogen may be seen most robustly in women who begin estrogen use near the time of menopause, perhaps before the structural brain changes associated with dementia become significant [47]. Such issues require further exploration.

Other doses or forms of estrogen may produce different results than those observed for orally administered CEE. Results from experimental animals that indicate positive effects of estrogen on cognition generally do not involve oral CEE [49]. A small placebo- controlled study with transdermal estradiol suggested the possibility of improvement for women with existing cognitive decline [49]. Asthana and colleagues reported an estrogen supplement- induced enhancement in verbal memory that correlated with the circulating levels of estradiol (r= 0.96, p

The results reviewed above indicate that MPA used with CEE is associated with a worsening of the major side-effects, as well as cognitive decline and elevated risk of breast cancer. There may be other progestogens that can be used, such as progesterone rather than MPA [52]. The partial estrogen agonist estriol may also provide benefits against cancer (see below) and cognitive decline [53].

Low-dose strategy hormone replacement therapy to minimize side- effects

When estrogen levels were monitored during estrogen therapy and compared with subjective well-being [54], higher plasma levels of estrogen were associated with unpleasant side-effects such as nausea and breast tenderness. It is important to prescribe the lowest dose that provides relief on an individualized basis. Some women find relief from hot flushes with doses that are lower than those used in the WHI studies [55], e.g. a\bout half of the dose (CEE 0.3 mg/ day) is adequate for some women [56]. Given the known variability in metabolism of estrogen, individualization of the estrogen dose continues to be recommended, as is common practice for any drug with a low therapeutic index [57]. For women who have relief of symptoms with lower levels of estrogen, co-administered progestogens can also be in lower doses. It was found that endometrial hyperplasia was adequately blocked in women given CEE 0.3 mg/day and MPA 1.5 mg/day [58].

Conventional versus bioidentical hormones

The combination of CEE 0.625 mg/day with MPA 2.5 mg/day (Prempro(TM)) has been the conventional form of HRT in the USA since the 1990s for women having undergone natural menopause with intact uteri. Premarin(TM) and Prempro(TM) contain a complex mixture of hormone metabolites, some of which can be metabolically converted in women to the potent human estrogen hormone estradiol. However, some of the minor components of CEE preparations make major contributions to the estrogenic activity of CEE and have poorly characterized or unrecognized actions inside the human body [59]. For example, conjugated equilenin and equilin account for more than 10% of CEE and some of their metabolites in the human body may have antiestrogenic or other effects [59].

The term ‘bioidentical’ has been adopted to refer to the forms of hormones found endogenously in women: estradiol, estriol and progesterone [60]. The most commonly used sources for these compounds are soy and wild yam, which contain molecular components that can be chemically modified to match the structures of the endogenous hormones. Supplemental administration of the endogenous human sex steroid hormones has a long history [61] and is referred to as bioidentical hormone therapy (BHT) [62]. The sketchy nature of the published literature regarding use of BHT was recently reviewed [63]. Due to paucity of efficacy and safety data, there is a need for clinical trials testing combinations of bioidentical hormones in the treatment of vasomotor symptoms.

Endogenous human hormones

In the non-pregnant human female, some estradiol is ultimately metabolized to estriol [64]. Estradiol has generally been found to be present at levels higher than that of estriol except during pregnancy (Table III) [66-72]. Before menopause, ovarian follicles are the source of greater than 90% of estrogen [73] and estradiol is the major biologically active estrogen made by non-pregnant human females during their reproductively active years. Relief of hot flushes in postmenopausal women can usually be achieved by maintaining serum estradiol levels at 40-50 pg/ml [74], the lowest level of estradiol that is expected to be seen in a typical menstrual cycle. Conversion of androstendione to estrone by non- ovarian tissues is the main source of estrogens after menopause. Using radioactive estradiol and estrone, Longcope demonstrated that there is very little (

Table III. Sex steroid hormone levels in menstruating, pregnant and postmenopausal women [65-72].

Two major isoforms of human estrogen receptor have been characterized, ERa and ERβ. The dissociation constant (k^sub d^) of estriol for binding to the ligand-binding domain of human ERβ is 5.6 nM, while for the ERα ligand-binding domain k^sub d^ is 18 nM [75]. In contrast, estradiol has usually been reported to have preferential binding to ERα. The k^sub d^ for estradiol binding to human ERα was reported to be as low as 0.06 nM [76] while estradiol has been reported to bind to two ERβ receptor forms with affinities in the range of 1-8 nM [77]. Thus, estriol and estradiol bind to ERβ with similar affinities while estradiol has higher affinity for ERα. A comparison of the relative affinities of estradiol and estriol for just the human ERα ligand-binding domain indicated a 15-fold higher affinity for estradiol than for estriol [75].

During pregnancy when estradiol levels are highest in women, estriol levels are even higher (Table III). Estriol is produced in large amounts by a metabolic pathway that involves the fetus [78]. The addition of estriol to estradiol to treat symptoms of menopause may prove beneficial and there may be physiological functions of ERβ in target tissues such as the brain [53,79] that can be facilitated by administration of an ERβ-preferring estrogen such as estriol. Since estradiol and estriol cause different kinds of conformational changes in estrogen receptors [80], an estrogen supplement containing only estradiol may not be able to accomplish all of the physiological functions of a combination of estradiol and estriol.

Alternatives to conventional estrogen therapy

The hydrophobic hormones estradiol and estriol are usually bound to carrier proteins present in the fluid compartments of the body and are rapidly metabolized to more hydrophilic conjugates such as sulfates and glucuronides [81]. Estriol and estradiol can be well absorbed when administered orally. However, when these estrogens are absorbed from the gastrointestinal tract they are subject to efficient first-pass metabolism in the liver [82]. Topical and transdermal formulations permit application without first-pass metabolism, and may reduce the stimulation of liver production of clotting proteins (discussed further below).

Use of bioidentical hormones

A recent meta-analysis of placebo-controlled studies compared the efficacy of topically administered estradiol, oral estradiol and oral CEE for the treatment of hot flushes in postmenopausal women [83]. Nelson’s analysis of data from 14 clinical trials indicated that the number of hot flushes per week decreased in women using each of these forms of estrogens compared with placebo control groups. For the one clinical trial with oral CEE that met the criteria for the meta-analysis [2], there was a mean change in the reported number of hot flushes per week of -19.1 (95% CI -33.0 to – 5.1). For oral estradiol the results from five trials demonstrated a mean difference of -16.8 (95% CI -23.4 to -10.2). For transdermal estradiol the results from six trials indicated a mean difference of -22.4 (95% CI -35.9 to -10.4). These results indicate that estradiol is as efficacious as CEE for treating hot flushes and that transdermal estradiol can be as efficacious as oral administration [83].

Oral administration of combinations of steroid hormones

Use of the bioidentical sex steroid hormones estradiol, estriol and progesterone (as opposed to CEE and MPA) for HRT introduces pharmacokinetic challenges due to their hydrophobicity and metabolism [66]. Absorption into the body can be enhanced by making sure that the hormones are finely ground [84]. However, even if absorption from the gastrointestinal tract is efficient, the bioavailability of sex steroid hormones after oral administration is generally low and highly variable. For example, a study of one preparation of estradiol found oral bioavailability to vary from 1 to 12% [78]. Some of the variability seen with oral administration is due to differences in estrogen metabolism between individuals [86].

Pharmacokinetic studies of sex steroid hormones demonstrate rapid clearance. The half-life of estradiol in postmenopausal women is typically 2-3 h [87]. Time to peak serum concentration after oral administration of sex steroid hormones is about 1-3 h. Serum estradiol levels typically return to near baseline within 24 h after a single oral dose [66]. These pharmacokinetics of bioidentical steroid hormone supplements prompted the use of synthetic estrogens and progestins that have more convenient pharmacokinetics [88]. However, multiple oral doses of sex steroid hormones can be used during a 24-h period in order to limit the large swings in serum concentration that occur with a single daily oral dose, and can achieve a therapeutic benefit.

A daily oral dose of 200 mg micronized progesterone is effective at preventing endometrial hyperplasia in women receiving estrogen [89]. An oral dose of 200 mg progesterone produces peak serum levels similar to progesterone levels of premenopausal women [9O]. Absorption and clearance of oral progesterone is rapid [9O]. As much as 90% of an oral dose of progesterone is metabolized by the liver in a first-pass effect [91]. Some metabolites of progesterone have actions in the brain [92] and some women experience serious neurological sideeffects such as dizziness and drowsiness following oral administration of progesterone [91].

In summary, while bioidentical hormones can be administered orally, they have poor oral bioavailability. The rapid metabolism by the liver and the hydrophobic nature of sex steroid hormones suggests that topical administration should be carefully considered as a viable route of administration [93].

Topical administration of steroid hormones

While there are bioavailability issues with oral bioidentical hormones, transdermal or topical bioidentical hormones can provide additional alternatives [94]. The two main advantages of transdermal administration are the opportunity for sustained release of steroids from a juxtacutaneous depot source and the lack of the large first- pass metabolism by the liver [95].

Transdermal administration of estradiol

Transdermal administration of estradiol can maintain serum levels within a relatively narrow range for an extended period of time [96], with relatively low variation in bioavailability between individuals [87], due to lack of the large first-pass conversion of estradiol to inactive metabolites and estrone [82]. Compliance with transdermal estradiol is good when a convenient means of hormone \application is provided that is not irritating to the skin [97]. Based on comparative bioavailability, some investigators began to advocate topical estradiol over oral estrogen in the early 1980s [98].

Women who applied two new transdermal patches (0.2 mg estradiol in 24 h; Estraderm(TM)) each day for 15 days attained about twice the serum concentration of estradiol as for oral 1.0 mg estradiol every 12 h [99]. Various estradiol-containing creams and gels have been approved for topical administration of estradiol [10O]. Estradiol cream applied topically in different amounts can allow patients to control and individualize their serum hormone level [101]. Transdermal steroid delivery patches can help avoid transfer of topical steroids to family members through physical contact.

Differences in activity of oral and transdermal/ topical hormones may reflect different levels of stimulation of liver actions. In one study, the incidence of venous thromboembolism was found to be higher in users of oral estrogen than for transdermal administration. The odds ratio for venous thromboembolism in current users of oral and transdermal estrogens compared with non-users was 3.5 (95% CI 1.86.8) and 0.9 (95% CI 0.5-1.6), respectively [102]. Markers of blood clotting risk such as activated protein C resistance that are associated with oral estrogen use are not seen with transdermal administration [103].

Transdermal absorption of estriol

Estriol administered by the vaginal route has been used to treat menopause-related symptoms [104], but no studies have been published for human topical delivery of estriol into the bloodstream. A study using mouse skin organ culture indicated that estradiol is absorbed through mouse skin about eight times more efficiently than the more hydrophilic estriol [105]. Human studies of estriol bioavailability after transdermal administration are needed.

Topical administration of progesterone

It is expected that topical administration of progesterone would also have benefits over oral administration [91]. Progesterone production in women is thought to be about 25 mg/day during the mid- luteal phase [91]. The large first-pass metabolism prevents most orally administered progesterone from reaching the blood plasma and generates metabolites with undesirable side-effects [91]. In order to protect the endometrium from the effects of estrogen, a large amount (often 200 mg) of progesterone is administered by the oral route.

Both nasally [106] and vaginally [107] administered progesterone have been shown to result in enough bioavailability to have effects on the endometrium. Good vaginal absorption of progesterone has been widely reported [108] and there is better vaginal bioavailability of progesterone than oral bioavailability [109]. As is the case for estrogens, progesterone has a much greater bioavailability (about 40- fold) after vaginal administration than oral administration [109]. However, it has been suggested that the topical route of administration does not allow for adequate absorption [91].

Given the large person-to-person variability reported for progesterone bioavailability [110], suitably large populations are needed to obtain valid averaged results. Monitoring of progesterone levels in individuals may be required to make sure that desired circulating levels are maintained.

These results for progesterone bioavailability indicate that a definitive study is needed comparing the bioavailability of progesterone after oral and topical administration. A small study with 12 postmenopausal women who used either 200 mg oral progesterone once daily or progesterone cream 40 mg twice daily found similar dose-adjusted bioavailability for the two routes of administration [111], suggesting that topically applied progesterone creams are effective for delivery of progesterone. Previously published bioavailability data for both oral and topical administration vary considerably from study to study. However, topical administration of progesterone might provide enough progesterone to the blood plasma to protect the endometrium from the effects of estrogen supplementation, particularly if low doses of estrogen are used. In one study, twice daily use of 1.5% progesterone cream blocked endometrial cell proliferation induced by CEE 0.625 mg/day [112].

Leonetti and associates reported that topical progesterone 20 mg/ day applied in a cream can block estrogen-stimulated endometrial proliferation [113]. However, others have reported that topically applied progesterone up to 64 mg/day had no detectable effect on the endometrium [114]. Additional studies are needed to confirm if topically administered progesterone can prevent estrogen supplement- induced uterine hyperplasia.

Progesterone versus medroxyprogesterone acetate

After the results of the WHI estrogen plus progestin arm were published, the US Food and Dug Administration (FDA) asked marketers of estrogen supplements to emphasize the risks associated with HRT use. Now that the results of the WHI estrogen alone arm are also available, new FDA guidelines for warnings about the health problems (blood clotting, breast cancer, dementia) caused by synthetic progestins (MPA) are needed. As discussed above, orally administered progesterone (and possibly topically applied progesterone) has the ability to block estrogen-induced endometrial hyperplasia. Below, we review the existing evidence suggesting that progesterone may be safer than MPA, with a different side-effect profile.

Human tissues contain at least two major forms of nuclear progesterone receptors, PRA and PRB. PRA lacks 164 amino acids that are present at the NH2-terminal end of PRB. For many genes that are regulated by progestogens, PRB is the more effective transcription activator while PRA often has a dominant negative effect on PRB [115]. A screen for progesterone-regulated genes found 65 that were regulated by PRB, four by PRA, and 25 by both [116].

The normal physiological roles of the two progesterone receptor forms in the uterus are still under investigation. Based on results with cultured endometrial cells and synthetic progesterone receptor gene transfection experiments, it has been suggested that antiproliferative actions of progesterone on uterine cells may depend only on PRB expression [117]. However, uterine responses to progesterone were reported to be normal in mice that lack expression of PRB [118].

Differences between the roles of PRA and PRB in different tissues may explain how progestins can prevent uterine cancer and promote breast cancer. In contrast to the results for the uterus discussed above, breast development and response to progesterone are not normal in the absence of PRB [118]. It has been reported that the two progesterone receptor types seem to be coordinately regulated in normal breast epithelial cells, but this coordination is frequently lost in breast cancer cells [119]. It has been suggested that selective modulation of PRA activity by progestins may be particularly important for the regulation of mammary gland hyperplasias [118]. Synthetic PR subtype and cell type-selective progesterone receptor modulators are now being investigated [120] with the hope that some of the side-effects observed with existing progestins might be avoided if more selective progestins are used. Use of the bioidentical progesterone supplement in combination with estrogens may avoid some of the deleterious effects associated with synthetic progestin use.

There are studies that indicate progesterone may have fewer side- effects than synthetic progestins. Postmenopausal women using CEE 0.625 mg/day plus MPA 5 mg/day have more vaginal bleeding and breast tenderness than those using CEE plus progesterone (200 mg/day, oral) [121]. In the Postmenopausal Estrogen/Progestin Interventions clinical trial, the use of progesterone 200 mg/day resulted in 0.62- to 0.68-fold fewer musculoskeletal sideeffects than MPA 2.5 mg/day [122]. Synthetic progestins may have detrimental side-effects on the cardiovascular system [123] and the brain [44] that are not observed with progesterone. This might account for the higher incidences of CVD and dementia in users of combined CEE plus MPA compared with CEE alone (Table I). Results from clinical trials and basic research on breast cancer, dementia and CVD suggest that synthetic progestins may be more deleterious than progesterone when used as part of HRT for postmenopausal women [52]. Long-term clinical trials testing progesterone are required to clarify the differences in side-effect profile of progesterone and synthetic progestins such as MPA. Some recent reports suggest that it might be possible to use progesterone rather than synthetic progestins and avoid progestin-induced increases in breast cancer risk [124].

Efficacy and bioavailability of estriol

Although basic research on the physiological effects of estriol has been limited [125] compared with the amount of research on estradiol, there is interest in estriol as a drug with a different spectrum of sideeffects than estradiol [126]. Much of the research on estriol as a treatment for menopause symptoms has focused on results showing that estriol can alleviate symptoms such as hot flushes and night sweats while not providing as much stimulation of endometrial cell growth as is seen with the more potent estrogen agonist estradiol [127]. However, estriol is not entirely without growth stimulatory effect on the uterus, so ‘estriol alone’ does not seem to be a viable therapy for women with a uterus [29]. A weak association between vaginal estriol administration and endometrial cancer was detected in a case-control study [128].

Following oral estriol, estriol metabolites are present in the circulation at higher levels than estriol itself while only 1-2% of the administered estriol reaches the circulation in unconjugated form [88]. Estriol sulfate levels increase dramatically in the blood after or\al estriol [129]. The poor bioavailability of oral estriol is strong motivation for the use of topically applied estriol, but most of the existing efficacy data for estriol are for the vaginal and oral routes of administration with very little published information concerning topical delivery of estriol. Once estriol reaches the blood plasma, it is rapidly removed from circulation [65].

It was reported that estriol levels can remain elevated for many hours after a single oral dose, possibly because of enterohepatic recycling [13O]. Peak plasma levels of estriol following 1 mg oral estriol averaged about 50 pmol/ml and were still above baseline after 24 h [131]. Plasma levels near 100 pmol/1 were reported 24 h after a 3 mg oral estriol dose, with peak levels of about 300 pmol/ 1 several hours after estriol ingestion [13O]. When a 12 mg oral dose of estriol was used the average peak plasma level of estriol was reported to be about 500 pmol/1 and a level of about 300 pmol/1 was still detected 24 h later [132]. Similar but generally lower levels of estriol were observed following oral estriol administration under fasting conditions [133].

Several studies have reported that menopausal symptoms can be relieved with estriol [134,135]. However, it is difficult to compare such studies with studies of other estrogens and there have been few studies directly comparing oral estriol’s effectiveness to estradiol or CEE in treating vasomotor symptoms. In one study, estriol 2-4 mg/ day was found to be less effective than CEE 0.625 mg/day [136].

There are conflicting results from several clinical trials that involved treating vasomotor symptoms with various oral doses of estriol. There have also been attempts to examine follicle- stimulating hormone (FSH) as a biomarker for biological activity of estriol supplements. A daily oral dose of 2 mg estriol was found to be effective in only those women in whom FSH levels showed a relatively large decrease [134]. A daily oral dose of 2 mg estriol was found to decrease the FSH levels of most postmenopausal women only slightly [137]. Other studies with oral estriol 2 mg/day reported no change in the average FSH level [127,138]. Schiff and co- workers also found that a 4 mg oral dose of estriol has little effect on FSH levels [129]. When estriol was provided at the level of three daily 2 mg oral doses there was a significant decrease in FSH levels [139]. One study that tested oral estriol for relief of severe vasomotor symptoms at a range of doses (2, 4, 6 and 8 mg/ day) reported a direct relationship between dose and effectiveness as measured by the Kupperman index [14O]. In this study FSH levels were decreased following estriol use, most dramatically by the 8 mg/ day dose (reduced to about one-third of the baseline value). Another study found 4 mg/day generally ineffective while 12 mg/day showed some effectiveness in some patients [141]. Estriol appears to be efficacious for the treatment of vasomotor symptoms in some women, at variable doses. However, since estriol binds poorly to steroid hormone-binding proteins in the blood [142] and is cleared from the blood with a half-time of a few minutes [65], circulating levels of estriol might not be particularly useful for monitoring estriol bioavailability to target tissues.

Saliva has been suggested as a suitable biological fluid for measuring estriol levels [143]. It is not clear that measures of estriol in the blood are the best indicators of an effective dose of estriol, particularly when topically administered. An estriol metabolite such as estriol sulfate, estriol levels in saliva or levels of some estriol metabolite in urine are better indicators of estriol bioavailability than is the circulating level of estriol in the blood. Alternatives to measuring blood levels of estriol should also be further investigated for their utility in monitoring individual variations in estriol bioavailability.

Estriol may have beneficial effects by acting as an agonist at ERβ or through its ability to inhibit activation of ERa by estradiol. There have been suggestions of important physiological roles for ERβ in both the brain and the cardiovascular system. For example, ERβ knockout mice have brain changes that resemble those associated with Alzheimer’s disease [53] and vasoconstriction in response to estrogen and hypertension [144]. Estriol has also been suggested to have a physiological function in preventing cancer, discussed briefly in the next section. Even if estriol alone is not efficacious for treatment of vasomotor systems, estriol may be useful as an adjunct in estrogen combination therapies, e.g. estradiol and estriol combination (Biest(TM)), discussed below.

Can estriol inhibit carcinogenesis by acting on estrogen receptor β?

Results showing that estriol actions through ERβ can modulate or complement the effects of estradiol binding to ERa [145,146] have stimulated interest in the development of synthetic ERβ-selective estrogens [147]. Estrogen receptor subtype- selective ligands should help to reveal which effects of estrogen on target tissues are mediated by ERa and which by ERβ. One of the most studied effects of estrogens is stimulation of cell proliferation and its association with carcinogenesis. Most evidence is consistent with a key role for ERa in estrogen-induced cell proliferation. Several lines of investigation have suggested that estriol might be able to act in a complementary way with ERα- preferring estrogens to limit estrogen-induced cell proliferation and, possibly, carcinogenesis. Investigations have begun into the existence of mutations in the human ERβ gene that might contribute to breast cancer risk [ 148]. Estriol stimulates the development of uterine cancer less than estradiol [127].

It was originally suggested in the 1960s that there might be an association between low estriol levels and increased risk of breast cancer [149]. Experiments with laboratory rodents also suggested that estriol has a protective effect against the induction of breast cancer [15O]. Exposure of rats to estriol prior to exposure to carcinogens such as 7,12-dimethylbenz(a) anthracene resulted in the prevention of 80-90% of the tumors that could be induced in the absence of exposure to estriol [151]. Similar studies of inhibition of carcinogenesis in rodents following activation of sex steroid hormone receptors have continued [152].

Recent results have led to the suggestion that ERβ- preferring estrogens may be useful for combating cancer [153] and that ERβ can function as a tumor suppressor [154,155]. These results suggest reasons for investigating if combinations of sex steroid hormones that include an ERβ-preferring estrogen such as estriol are safer than estradiol alone.

Conclusions

Long-term, placebo-controlled safety and efficacy clinical trials should be conducted for topically applied combinations of estradiol, estriol and progesterone. Existing evidence suggests that transdermal administration of combinations of these bioidentical hormones may have advantages over conventional oral HRT. In particular, the risk of blood clotting may be minimized for some combinations of hormones consisting of topical estrogen administration with progesterone rather than synthetic progestin. Clinical trials testing low-dose, topically applied estrogens such as estradiol with estriol without synthetic progestin for young postmenopausal women may be able to demonstrate cardiovascular benefits of these combination estrogen therapies.

Strategies to avoid the rebound of vasomotor symptoms upon estrogen supplement withdrawal are needed when minimizing length of therapy with hormones. Possibly certain forms of estrogen supplementation in effective doses will make tapering estrogen doses easier.

Exploration of alternative estrogen supplements such as Biest(TM) (combined estradiol and estriol) plus progesterone needs to take place within a research environment where individualization of treatment is emphasized and a comprehensive approach to the treatment of menopause symptoms is used, one that integrates the therapy with lifestyle changes (e.g. smoking cessation, weight loss, diet modifications, exercise). The development of simple tests and monitoring devices that will allow women to easily monitor their estrogen levels to control their vasomotor symptoms more precisely will be helpful. Long-term monitoring and record-keeping of patient histories, treatments and therapy outcomes, which will serve to aid in making individually appropriate decisions about estrogen supplement use by postmenopausal women, is absolutely necessary to develop beneficial treatment for menopausal symptoms that has minimal risk.

Acknowledgements

We thank Dr John Dye, Dr Jennifer Orlowski, Dr Jennifer Nevels and Dr Debi Smolinski for stimulating discussions and all their other contributions during the preparation of this review. The review was prepared with funding support from Southwest College of Naturopathic Medicine. There were no conflicts of interest in the preparation of this manuscript.

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