U.S. Pharmacist Continuing Education
ACPE Program No. 430-000-99-004-H01
This program provides 2.0 hours of credit (0.2 CEU).
Lesson Expires: end of April 2001

This program is made possible by an unrestricted educational grant from
Wyeth-Ayerst Laboratories..

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Evolving knowledge about the mechanism of estrogen action represents one of the most significant scientific advances today. In the early 1970s, estrogen’s action was believed to be restricted to reproductive tissues and to be mediated through a single estrogen receptor (ER). More recently, the effect of estrogens in nonreproductive tissues has been discovered. Studies involving long-term replacement of estrogens have identified benefits that include maintaining skeletal integrity, improving cardiovascular profile and cognitive function, preventing colon cancer, and protecting against tooth loss and macular degeneration. The concept that estrogen could have effects on numerous target organs was difficult to explain until the theory of hormone selectivity evolved and an additional ER was identified.^1-4

The discovery of a second ER may be useful for future development of menopausal therapies, and begins to explain the paradoxical behavior of estrogen in different tissue types. For example, both estrogen receptors are not expressed to the same level in all cells or tissues, which may explain why some tissues are more responsive to estrogen than others. When different estrogens bind to the receptor they may differentially alter the properties of that receptor, ultimately influencing its action. In addition, once estrogen is bound to the receptor, different estrogen/receptor complexes are not recognized the same way in all cells. Adding to the complexity of estrogen action are the facts that ERs are expressed at different levels in various target tissues, and that some tissues can convert estrogens to more, or less, active forms. These findings have led to the understanding of how different estrogens acting through the same ER can induce different biological actions. Thus, all estrogens do not act in a similar fashion and are not interchangeable.

Estrogen Pharmacology
Human estrogens are formed from androgenic precursors through an enzymatic process known as aromatization. 17beta-estradiol (E2), the predominant and most potent estrogen in premenopausal women, is synthesized by developing ovarian follicles. Estradiol is secreted into the bloodstream, bound partially by circulating sex hormone binding globulin, and then transported to cells throughout the body. The principal path of estradiol metabolism is reversible oxidation to estrone (E1), a weaker estrogen, and then to estriol (E3). Estrone can also be produced in peripheral tissues through aromatization of androstenedione, an androgen precursor produced by both the ovaries and the adrenal glands. All of these compounds are metabolized into sulfate and glucuronide forms for excretion.⁵

Peak estradiol (200–400 pg/mL) and estrone (170–200 pg/mL) concentrations are achieved during the late follicular phase, thereafter decreasing to their lowest concentration (estradiol, 40–60 pg/mL; estrone, 40–60 pg/mL) during the early follicular phase. The estradiol/estrone ratio before menopause is generally greater than one.⁶ After menopause, estrone derived from the conversion of adrenal androstenedione becomes the predominant estrogen. Since aromatization readily occurs in adipose tissue, body composition affects the production of estrone (i.e., more adipose tissue, more estrogen). Both estradiol and estrone can be hepatically metabolized to the very weak estrogenic compound estriol.⁷ Average estradiol and estrone concentrations are 5–20 pg/mL and 30–70 pg/mL, respectively, with an estradiol/estrone ratio of less than one.

After menopause, when ovarian function declines, women experience a state of estrogen deficiency. Hormone replacement therapy is recommended for some women to alleviate postmenopausal symptoms, manage and prevent osteoporosis, and modify the increased risk for cardiovascular disease and other age-related diseases. The main goal of estrogen replacement therapy after menopause is to provide levels of estrogen that alleviate symptoms of estrogen deficiency and that minimize the potential long-term health consequences of estrogen deficiency. It has been shown that clinically effective doses of conjugated equine estrogens (CEE) [Premarin] produce estradiol and estrone levels comparable to those of the menstrual cycle.⁸ Transdermal or percutaneous estradiol administration results in higher estradiol concentrations than oral estrogen treatment.⁶ Although the predominant forms of systemic estrogen are estradiol and estrone, other estrogen metabolites have estrogenic activity. Thus, the combination of estrogens and their metabolites determines the overall estrogenic effect in women.

The complexity of estrogen action is illustrated further by an alternative path of estrogen metabolism via 2-hydroxylation. This results in the formation of catecholestrogens. This process is much more important in the brain than in the peripheral tissues. Thus, estrogen may exert its effects by altering catecholamine metabolism.⁵ This is important because catecholamines can interact with dopamine, alpha1-adrenergic, and serotonin receptors. In addition, hydroxy derivatives have different estrogenic activity. While 4-hydroxy estrone has estrogenic activity, the 2-hydroxy estrone does not. However, the 2-hydroxy derivatives of estradiol have estrogenic as well as catecholaminergic activity, indicating some crossover between systems.⁵ This pathway may explain in part the mechanism by which estrogens can exert effects on the central nervous system.

Estrogen Receptors
Until recently, it was generally accepted that simply one ER (now termed ER-alpha) mediated all the actions of estrogen. Whether or not a tissue or cell responded to estrogen was dependent on the presence of the ER in that particular tissue or cell. In this model, a tissue responded to estrogen when the ER was activated by binding to estrogen. Once activated, the receptor interacted with the control region of a target gene, whose expression was then regulated by the activated receptor. Thus, it was thought that the binding affinity to an estrogen determined the biological activity of estrogen in the target tissue. Estrogen action through its receptor is now understood to be a more complex phenomenon. This is evident when considering some of the individual estrogenic components in CEE. Although these components bind to the ER with different affinities,⁹ their binding affinities do not necessarily reflect their individual biological potency or activity.¹⁰ For example, delta8,9-dehydroestrone ranks ninth in receptor binding affinity, but it is the second most potent estrogen component in terms of its biological activity (TABLE 1).

Table 1: Relationship Between Estrogen Receptor Binding Affinity and Biological Activity
Rank	Human ER Binding	Biological Potency†
1	17beta-Estradiol	17beta-Estradiol
2	17beta-Dihydroequilin	Delta8,9-Dehydroestrone
3	17beta-Dihydroequilenin	Estrone
4	17alphaDihydroequilin	17beta-Dihydroequilenin
5	17alphaEstradiol	Equilenin
6	Estrone	17beta-Dihydroequilin
7	Equilin	Equilin
8	17alpha-Dihydroequilenin	17alphaDihydroequilin
9	Delta8,9-Dehydroestrone	17alpha-Dihydroequilenin
10	Equilenin	17alphaEstradiol
†Measured by C3 gene activation Source: reference 10

The complexity of estrogen action through its receptor is further supported by the identification of a second ER subtype, termed ER-beta.³ The human and rat ER-beta subtypes were first discovered in the human testes and rat prostate and ovary.^1,2 Since this initial discovery, the mouse and human counterparts of the rat ER-beta have shown to be similar to ER-alpha.¹¹ When considering the possible biological roles for ER-beta in estrogen action, it is important to consider that ER-alpha and ER-beta have virtually identical (97% amino acid similarity) areas, or domains, where they bind to DNA (DNA-binding domain).¹² This suggests that ER-alpha and ER-beta interact with and activate the same genes (FIGURE 1).

Figure 1: Amino Acid Compositions of ER-alpha and ER-beta

* numbers in boxes indicate number of amino acids. Estrogen acts biologically through at least two receptors. ER-alpha and ER-beta are 97% similar in the amino acids of their DNA-binding domain. In contrast, there is only approximately 60% similarity between their ligand-binding domains. This suggests that the two receptors interact with and activate the same genes, and that different estrogens or compounds may differentially bind to the two estrogen receptors. Source: references 10,12

However, there is only approximately 60% similarity between areas on the receptor where the ligand binds (ligand-binding domains).¹² Thus, various estrogens may differentially bind to the two estrogen receptors.^11,12 The discovery of this second ER may help to explain the tissue effects of the blend of the multiple components of CEE, since the individual steroidal components of CEE may bind to the two receptors in a discriminating fashion.

The existence of the two ER subtypes can provide, in part, an explanation for the selective actions of estrogens in different target tissues. The selective effects of estrogens and their analogs may be due to the differential distribution of each ER subtype in various tissues.³ The tissue distribution and/or the relative levels of ER-alpha and ER-beta expression in rats is quite different. There is a moderate to high expression of ER-alpha in the uterus, testes, pituitary, ovary, kidney, epididymis, and adrenal gland, and a relatively high expression of ER-beta in the prostate, ovary, lung, bladder, brain, uterus, and testes. The differential expression of ER-alpha and ER-beta within individual tissues may contribute to the selective effects of estrogen within individual tissues. For example, in the rat and human prostate, ER-alpha and ER-beta are differentially expressed in the secretory epithelia and stromal tissues.¹¹ It is also clear that estrogens exert organizational effects on the rat and mouse prostate, since neonatal exposure to 17beta-estradiol or diethylstilbestrol causes permanent changes not only in the size of the prostate, but in the expression level of certain genes. Additionally, in the rat ovary, the levels of ER-beta expression vary within compartments of ovarian tissue, while ER-alpha is expressed at a low level throughout the rat ovary with no particular cellular localization. Finally, in primary osteoblastic cells isolated from the bone of neonatal rats, both ER-alpha and ER-beta were detected, although the level of ER-beta was much higher.¹¹ The fact that both estrogen receptors are expressed in bone helps to explain the beneficial effects of estrogens on bone mineral density.

Based on the relatively abundant distribution of ER-beta in the rat brain, it is likely that the actions mediated by this ER regulate many important neuronal functions. Some of these functions were previously known to be influenced by estrogen, although little or no ER-alpha was present in these regions. The role of estrogens in the brain is supported by observations that estrogen replacement therapy improves cognitive function in postmenopausal women.¹³ Until the discovery of ER-beta and its localization in regions associated with learning and memory (neocortex, hippocampus and nuclei of the basal forebrain), these data were difficult to explain because ER-alpha is sparse or absent in these areas.³ There is also evidence that the differential distribution of the estrogen receptors in the brain may provide a mechanism for the selective modulation of the central regulation of reproduction as well as the regulation of nonreproductive events, such as learning and memory. Findings in ER-alphadepleted mice, which appear to have normal levels of ER-beta, indicate that these animals have deficiencies in reproductive behavior, but appear to have normal cognitive function.³ Collectively, the described differential expression between the ER subtypes in various tissues could contribute to the selective action of estrogen agonists and antagonists in these tissues.³

Estrogen Mechanism of Hormone Selectivity
Although estrogen was once thought to exert its activity primarily on reproductive tissues, such as the uterus and mammary glands, evidence now demonstrates that estrogens have effects on multiple target organs. In addition to the identification of a second ER, the concept that estrogens could have effects on several target organs is supported by theories of hormone selectivity.^3,4 The selectivity of hormones for nuclear receptors displayed at different sites may be mediated by three distinct mechanisms: 1) ligand-based selectivity; 2) receptor-based selectivity; and 3) tissue-specific or effector site-based selectivity.⁴

Ligand-based Selectivity: A ligand is defined as any molecule that binds to a receptor (e.g., a protein, steroid, or compound).¹⁴ Selectivity at the tissue or cell level may be achieved by differences in ligand pharmacokinetics or metabolism. Although the same hormone can be presented to different target tissues through the circulation, its relative amounts within a cell can be altered by differential uptake or metabolism of the ligand at the level of the target cell.⁴

Receptor-based Selectivity: This selectivity occurs when the same hormone elicits different responses in various tissues because the tissues have a different composition of receptors. This difference could include variations in concentrations or ratios of receptor subtypes, the molecular form of the receptor, or biochemical modification of the receptor protein.⁴

Effector Site-based Selectivity: This selectivity is depicted by the different biological activities of estrogens or their analogs in different target tissues.⁴ Once an estrogen enters the cell, it binds to a receptor and becomes a ligand/receptor complex. The complex undergoes dimerization (becomes two parts), which results in a change in configuration that is unique for each ligand. The complex then binds to an adaptor, or promotor, protein which allows the binding of the complex to a gene and ultimately influences the expression of that gene.¹⁵ Not only are there different receptors and ligands, but there are also different adaptor proteins that may be exclusively expressed in one type of tissue, for example, bone or uterus (FIGURE 2).¹⁶

Figure 2: Tissue-Specific Actions of Estrogen
ER/ligand	ER/ligand complex	Ligand/ receptor dimer	Gene promoter complex (adaptor)
A				Uterus
B				Brain
C				Bone
ER = estrogen receptor Each target tissue responds uniquely to different estrogens. The ligand/receptor dimer binds to a protein known as an adaptor. In addition to different receptors and ligands, the presence of different adaptors in various tissues allows for the tissue-specific actions of estrogen. Source: References 10, 16

This is exemplified by tamoxifen’s estrogen-like actions (i.e., as an agonist) on bone mineral density, lipoproteins, and the uterus, whereas tamoxifen acts as an antagonist in the breast, where it is shown to reduce the recurrence of breast cancer.⁴

Estrogen Physiology
The principal biological activity of estrogen is to influence the growth, differentiation and function of many reproductive tissues, including the mammary gland, uterus and ovary.¹⁷ Estrogens stimulate growth of the endometrium, myometrium, and the vaginal and urethral epithelium. Estrogens also enhance vascular flow in the genital tract, increase cervical gland secretions, and induce expression of progesterone and luteinizing hormone receptors. In addition to developing the female reproductive organs and secondary sex characteristics at puberty, estrogens are responsible for skeletal growth and development, and fat distribution in women. Estrogen receptors have been identified in many nonreproductive tissues, and in women, endogenous estrogens are known to beneficially affect lipid metabolism, skin and collagen tissue, neuronal function, and the cardiovascular system. Estrogen deficiency leads to vasomotor instability (hot flashes and night sweats), while long-term deprivation leads to urogenital atrophy, osteoporosis and tooth loss, atherosclerosis and coronary heart disease, and potentially, increases the risk of dementias.^18,19 In postmenopausal women, exogenous estrogens relieve the vasomotor and urogenital symptoms associated with estrogen deficiency, prevent and manage osteoporosis, and reduce risk factors associated with cardiovascular disease.¹⁸

All biologically active estrogens do not exert similar effects. The estrogenic potency of estradiol is 12 times that of estrone and 80 times that of estriol.¹⁸ Because the various target tissues exhibit different sensitivities to exogenous estrogens, potency of exogenous estrogens is difficult to compare. However, data from preclinical studies can illustrate this concept. When different estrogens or estrogenic components of CEE were investigated in rat cortical neurons,^20,21 their potencies were quite different (TABLE 2).¹⁰

Table 2: Comparison of Neurotrophic and Neuroprotective Effects of Different Estrogenic Compounds on Rat Cortical Neurons
Estrogen	Neuronal Outgrowth	Neuronal Survival	Protection Against Oxidative Damage
Conjugated equine	+++	+++	+++
17beta-estradiol	++	+++	+
Equilin	++	?	?
delta8,9-dehydroestrone	++++	?	+
Source: reference 10

Estrogen Products
Estrogens are either obtained from natural sources or chemically synthesized. Natural estrogens are found in animals and plants, and may or may not be chemically modified. Conjugated equine estrogens is a natural estrogen product collected from the urine of pregnant horses and marketed without chemical modifications. Natural estrogens that are chemically modified include 17beta-estradiol, estrone, estrogen sulfate, and those derived from soy and Mexican yams. The Mexican yam contains steroidal precursors used to synthesize esterified estrogen (e.g., Estratab, Menest), a mixture of sodium salts of estrogenic substances, primarily estrone (75%–85%) and equilin (6%–15%).²² Plants such as soybeans, dates, and pomegranates contain phytoestrogens, which are protective nonsteroidal plant chemicals, some of which structurally resemble endogenous estrogens from humans and animals. Natural estrogens derived from plants may act as estrogen agonists and/or antagonists in humans, and more research is required to determine their role in HRT.²³ While synthetic estrogens are widely used for oral contraception, these estrogens are rarely used for estrogen replacement therapy in postmenopausal women. The composition of commercially available estrogen products in the U.S. recommended for the treatment of postmenopausal symptoms is considerably different. Some available estrogen products are Premarin (CEE), Estrace (17beta-estradiol; micronized oral), Estraderm (17beta-estradiol; transdermal), Estinyl or Feminone (ethinyl estradiol), Ogen (estrone), and Estratab or Menest (esterified estrogens).


Conjugated Equine Estrogens: Oral CEE has been the most widely used form of estrogen replacement therapy in the United States.²⁴ The clinical efficacy of CEE has been documented since it was approved by the FDA in 1942. At the time CEE was first marketed, only two estrogenic compounds, the conjugates of estrone and equilin, were identified as the major constituents in CEE, although additional estrogens were also present.²⁵ In 1970, CEE was first officially defined in the United States Pharmacopeia (USP) monograph as a mixture of sodium estrone sulfate and sodium equilin sulfate. Currently, although at least 10 estrogenic components have been identified and shown to have biological activity,⁹ the USP monograph for CEE includes only five steroids: estrone sulfate, equilin sulfate, 17alpha-dihydroequilin sulfate, 17beta-dihydroequilin sulfate, and 17alpha-estradiol sulfate. In 1994, a citizen petition was filed requesting that estrogen delta8,9-dehydroestrone sulfate be reclassified as a component in conjugated estrogens,²⁶ an issue that is still unresolved. More than 200 individual steroidal components have been identified in CEE,²⁷ with all components not yet fully characterized in terms of both their chemical and biological activity.⁹ Because all of the estrogens present in CEE have estrogenic activity, the clinical efficacy of CEE may be the result of the combined effects of all of the individual components.⁹ Thus, preparations lacking some of these important components may not offer the same degree of benefit reported with CEE.⁹ Generic conjugated estrogens were marketed at a time when the understanding of the pharmacologic actions of estrogens was not as well defined. In 1991, however, the FDA withdrew approval for all abbreviated new drug applications (ANDAs) for conjugated estrogen tablets because the documented lack of bioequivalence led the agency to conclude that these products could not be considered safe or effective for their intended uses.²⁸

In May 1997, emerging scientific evidence indicating that all estrogens do not exert uniform effects on different target tissues, together with the absence of full characterization of all of the components in CEE, prompted the CDER of the FDA to conclude that any ANDA submitted for a synthetic version of conjugated estrogens could not be approved since the generic conjugated estrogens may not work in the same way as CEE in treating postmenopausal symptoms and preventing osteoporosis.²⁹ In order for the FDA to approve a generic drug product, it must have the same active ingredient and it must be bioequivalent to the brand name drug. Because not all of the ingredients of CEE are fully characterized, there are no generic equivalents available. Therefore, any new synthetically manufactured product that contains only some components of CEE cannot be considered therapeutically equivalent to CEE, and may not potentially offer the same degree of benefit as CEE.⁹

Estradiol: The use of micronized estradiol preparations has resulted in reliable absorption of estradiol and is clinically effective. Previously, oral estradiol tablets were absorbed poorly and the estradiol was rapidly metabolized and inactivated. Micronized estradiol is subject to gastrointestinal metabolism, with significant conversion to estrone and other estrogens before entry into the general circulation.⁸

Estradiol is also available as a transdermal preparation. Absorption of estradiol through the skin occurs rapidly with the application of the first patch. There appears to be little intermediate metabolism through the epidermis. Transdermal delivery may reduce some of the adverse side effects associated with oral preparations.¹⁸ Estradiol may also be administered intravaginally as a cream or ring.⁶

Estrone: Oral administration of estrone has been proposed as a safe form of estrogen therapy because it was believed to induce less stimulation of the endometrium. However, it seems unlikely that estrone, at clinically useful doses, would spare the endometrium. Estrone at appropriate doses provides clinical relief of postmenopausal symptoms.^22,30


Miscellaneous Estrogens: Synthetic estrogens (e.g., ethinyl estradiol, quinestrol, diethylstilbestrol) have the most potent hepatic effects, which may lead to undesirable effects on blood pressure and clotting factors. They are most commonly used for oral contraception. Because of their pronounced effect on liver proteins, these are not traditionally considered an initial choice for postmenopausal estrogen therapy. Esterified estrogen is a mixture of sodium salts of estrogenic substances consisting primary of estrone (75%–85%) and equilin (6%–15%).²² Diethylstilbestrol is a potent nonsteroidal estrogenic compound available for palliative treatment of prostate cancer. Quinestrol appears to be stored in adipose tissue after oral administration, and is then gradually released back into the circulation. Because it may be difficult or impossible to stop therapy quickly, this product has gained little popularity.^8,18,22

Differential Effects of Estrogens in Various Systems
While most estrogen replacement products relieve symptoms of estrogen deficiency in postmenopausal women, head-to-head comparisons among different estrogen products are not abundant. Because CEE has been used for over 56 years and is the most widely used estrogen replacement therapy,²⁴

Women will spend about one-third of their lives postmenopause; to serve and educate them, pharmacists must be informed of new advances in estrogen research.

the clinical efficacy of postmenopausal estrogen, especially its long-term benefits, are derived principally from studies using CEE. However, some studies have indicated that there are distinct differences in estrogen action between various estrogen products, as well as between the individual estrogen components in CEE. These studies are summarized below based on their physiological effect.

Gonadotropin Secretion: A biochemical technique used to quantify estrogen action is the measurement of gonadotropin secretion; this marker is suppressed after estrogen administration. Whereas CEE, piperazine estrone sulfate, and micronized estradiol all decrease gonadotropins equivalently,³¹ differences between the individual components of CEE have been reported. Equilin sulfate has been shown to be 4–8 times more potent than estrone sulfate in suppressing uterine gonadotropins.³² Equilin sulfate also suppresses gonadotropin secretion to a greater extent than CEE.³² This latter study further supports the previous suggestion that the overall effect of CEE results from the combined effects of all of its components.

Uterine Growth: A classic bioassay used to measure estrogen action is the quantification of uterine growth in rats. Dorfman and Dorfman found that the growth effects on the uterus with 17beta-estradiol and 17beta-dihydroequilin are 2.6- and 8-fold more potent, respectively, than estrone sulfate.³³ Most estrogen products elicit hyperplastic activity on the endometrium unless a progestin is included in the hormone therapy.^34-37 CEE has been shown to increase thickness and induce morphometric changes in the endometria of postmenopausal women in a dose-dependent manner.³⁸ Recent investigations with 17alpha-dihydroequilenin sulfate, one component of CEE, reported no uterotrophic effects in female rhesus monkeys.³⁹ Even though all oral estrogen treatments may stimulate endometrial growth, low doses of local estradiol treatment can relieve urogenital symptoms, without having hyperplastic effects on the endometrium.⁴⁰

Vasomotor and Urogenital Symptoms: Most estrogen replacement products relieve symptoms of estrogen deficiency in postmenopausal women in a similar fashion. In studies of the individual components of CEE, estrone sulfate, equilin sulfate, and 17beta-dihydroequilin sulfate were all equally beneficial on climacteric and urogenital symptoms when compared with CEE.^9,26,41 When CEE and transdermal estradiol products were compared, they equally reduced the incidence and severity of vasovagal and urogenital symptoms.^42,43

Cardiovascular Disease: The beneficial effects of different estrogens on serum lipoproteins vary; this may modify the efficacy of estrogens in preventing cardiovascular disease. For example, estrone sulfate has equally beneficial effects on lipid profiles, compared with CEE.²⁶ However, equilin sulfate has been shown to exert a more potent effect on lipoprotein profiles than CEE.^31,44 When CEE, piperazine estrone sulfate, and micronized estradiol were compared, CEE was the most potent in stimulating hepatic function,³¹ resulting in beneficial increases in HDL cholesterol. Both oral and transdermal products decrease total and LDL cholesterol; however, only oral administration increases HDL.^45,46 CEE has been shown to decrease LDL cholesterol to a greater degree than estradiol valerate or an estradiol implant.⁴⁷ An increase in HDL cholesterol and triglycerides was also reported with CEE, whereas an estradiol implant decreased triglycerides.⁴⁷

Estrogens also have antioxidant properties that may help prevent cardiovascular disease.^48,49 Of estrone sulfate, equilin sulfate and 17alpha-dihydroequilin sulfate (the three most abundant components in CEE), estrone sulfate was the most potent LDL antioxidant, while 17alpha-dihydroequilin sulfate affected LDL oxidation the least.⁵⁰ However, 17alpha-dihydroequilenin sulfate was reported to prevent endothelium-dependent vasoconstriction in ovariectomized female rhesus monkeys.³⁹ Both equilin and equilenin have equivalent effects on fatty acid oxidation, although these estrogens are more potent antioxidants than estradiol and estrone.⁵¹ A group of researchers found that CEE, but not the transdermal estrogen studied, had significant antioxidant activity.⁵²

Osteoporosis: The beneficial effects of estrogens on bone mineral density and fracture risk are well known. Differences among estrogen products are difficult to evaluate; few studies have reported direct comparisons. With oral and transdermal estrogens, there is a similar increase in bone density,^52,53 which is due in part to a reduction in bone loss.^53,54 CEE and estriol were shown to have comparable bone-preserving effects,⁵⁵ and CEE and percutaneous estradiol equally prevented bone loss and increased bone mineral density.⁵⁶

Several studies have evaluated the effects of the separate components in CEE on bone.^44,57-59 These studies indicate that CEE have a more potent effect on preserving bone than does equilin sulfate alone.^44,57,58 In a recent investigation, delta8,9-dehydroestrone decreased urinary N-telopeptide excretion, a surrogate marker for bone resorption.⁵⁹ These latter studies demonstrate that the combination of the multiple CEE components may help explain its clinical efficacy in maintaining bone mineral density, potentially reducing fracture risk in postmenopausal women.

Cognitive Function: Comparative studies of different estrogens on cognitive function are scarce. A reduced risk of Alzheimer’s disease and related dementia with both oral and nonoral estrogens was reported, with longer duration of use of CEE enhancing this risk reduction for Alzheimer’s disease.⁶⁰ Equilin, a component of CEE, showed the greatest effect on nerve cell growth in vitro when compared to estradiol, estrone and estriol (TABLE 2).²¹ These findings may help to explain the potential beneficial effect of CEE in preventing Alzheimer’s disease.⁶¹

Compliance
Compliance of HRT use may also be affected by estrogen formulation. More women switch from the patch to oral treatment than from oral treatment to the patch.⁶² Additionally, transdermal estrogen users have a greater relative risk of discontinuing hormone therapy than women who take oral CEE.⁶²

Future Directions
Information about the mechanism of estrogen action has demonstrated that the action of estrogen is very intricate. Estrogen’s action involves both tissue- and ligand-specific aspects of ER biology. The discovery of ER-beta, and its differential expression relative to ER-alpha, may permit the targeting of one ER independent of the other. Thus, the development of drugs with more specific and selective effects is ongoing. Compounds known as selective ER modulators (e.g., tamoxifen, raloxifene, droloxifene) are being studied for their selective ability to act like estrogen in bone and cardiovascular tissue, while blocking estrogenic effects in the breast and uterus. However, their long-term effects or clinical efficacy in managing postmenopausal health are unclear.⁶³ Opportunities to develop new drugs having greater tissue specificity will help in treating osteoporosis, cardiovascular disease, and Alzheimer’s disease.⁶⁴

Conclusion
Until recently it was believed that all the actions of estrogen were mediated by a single, high-affinity ER. This concept drastically changed with the understanding of the complex action of the ER and the discovery of ER-beta. With this new information, the basic pharmacology, pharmacodynamics and physiologic actions of all estrogens are being reevaluated.

New ER data reinforce the fact that estrogens are not interchangeable. Thus, one should select a particular form of estrogen on an individual basis, with consideration of safety, effectiveness, and patient convenience. When a patient obtains a desired therapeutic response from an estrogen product, substitution of another “equivalent product,?is not advisable. Women spend about one-third of their lives postmenopause. To continue to serve and educate their female patients, pharmacists must be informed of new advances in estrogen research.

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