Abstract

Various estrogen preparations can be used in hormone replacement therapy for peri- and postmenopausal women. Different progestagens can be combined with these estrogens and different methods of administration, different dosages and administration schemes can be used. The pharmacokinetics can vary greatly in the same treatment according to the individual, and there are also significant individual differences in sensitivity to the hormonal effects. The doctor's task to adapt a replacement therapy to the needs of his patients is therefore not simple. This necessitates a pragmatic policy, supported by some knowledge of several principles of the pharmacology of hormone replacement. This article will briefly review the Pharmacokinetics of estrogens with special attention to the various methods of administration.

by J.M. Kaufman.

Introduction

When discussing the pharmacokinetics of estrogens for different modes of administration and their potential clinical implications in hormone replacement therapy, it is useful to keep in mind some of the characteristics of the physiological patterns of circulating estrogens.

Estrogen production during the ovarial cycle in the premenopausal woman is marked by great variation. The plasma levels of 17b-estradiol, the most active estrogen secreted by the ovarial follicles, can multiply more than ten-fold between the early follicular and the pre-ovulatory phase of the cycle, from 40 pg/ml or lower to 400 pg/ml or higher.1-2 In addition to these cyclic changes in estrogen levels, there are important variations in the short term (hours) in which variations in both production and in clearance of the hormone may play a role.3 The levels of estrone also fluctuates cyclically. Estrone to an important extent originates from the metabolic change of 17b-estradiol and as a rule the circulating concentrations remain lower than those of 17b-estradiol.1,2

17b-estradiol production is drastically reduced after the menopause with serum levels usually lower than 30 pg/ml, whilst the estrone level remains at a relatively higher level, generally above 40 pg/ml. The circulating estrogens after the menopause originate mainly from aromatisation of adrenal gland androgens, mainly androstenedione to estrone, which can itself be further converted to 17b-estradiol. This aromatization occurs principally in the fat tissue so that the circulating estrogen concentrations are relatively higher with obese postmenopausal women.2,4-6

There is also a large variation in the obtained estrogen serum levels with women treated with estrogen preparations. This is a variation from person to person, and also in the same person as a function of time. This applies to a certain extent to all modes of administration.7,8

Variations in estrogen levels nevertheless do not immediately have to translate into tangible clinical phenomena. Although most pharmacological effects of estrogens are, within certain limits, plasma concentration-dependent, it is usually not possible to determine precise and universally applicable "effective therapeutic levels". It is pertinent in this context to remember that the estrogen dose needed to effectively prevent or treat the consequences of postmenopausal estrogen deficiency may vary according to the considered aspect of the "postmenopausal syndrome". As an example, in many women vasomotor symptoms are treated satisfactorily which a lower estrogen dose than the average dose believed to be required for effective prevention of postmenopausal bone loss.9-11 Along the same lines, also the chosen treatment regimen may have different implications according to the considered postmenopausal symptom, with a cyclic treatment (estrogen administration 3 weeks out of 4) being adequate for prevention of postmenopausal osteoporosis,10 while unacceptable recurrence of vasomotor symptoms may occur in the estrogen-free intervals. Moreover, it should be pointed out that for potentially important areas of clinical applications of hormone replacement therapy, such as in the prevention of cardiovascular diseases, little or no information is presently available on the required estrogen dose or duration of treatment.12,13

Determinants of Estrogen Effects

Specific estrogen effects are exercised on a variety of target cells, carriers of specific (intra-cellular) estrogen receptors. The liver occupies a particular place with, on the one hand, a major impact on the pharmacokinetics of estrogens, through synthesis of circulating estrogen-binding proteins and a predominant role in estrogen bio-transformation,7,14 while it is on the other hand also a target tissue with an important role in estrogen induced metabolic changes with potential side effects.15-17 The estrogen effects on the target cells and the hepatocytes depend on the nature (intrinsic activity) of the estrogens taken up by these cells and on the attained intra-cellular concentrations.

Recent findings have revealed a far more complex regulation of the expression of estrogen receptor-mediated effects than previously appreciated. Indeed, the estrogen receptor, in combination with different estrogenic compounds, regulates more than one DNA response element, which allows for the possibility that estrogenic compounds can have differential effects in different estrogen-responsive tissues, through activation of specific pathways.18

Passage through the cell membrane depends on a whole series of factors, including the physico-chemical properties of the molecule (rapid passage of the generally strong lipophilic estrogens, limited passage of the more water-soluble estrogen conjugates), the permeability of the cell membrane (much larger for the hepatocytes than for the "classical" target cells), the concentration gradients (with influences from protein compounds in the plasma and intra-cellular), the contact area and the duration of the contact between the estrogen and the cell membrane (importance of protein-binding and micro-circulation).7,14,19

The Bio-Transformation of Oestrogens

A series of metabolites can be detected in the general circulation following estrogen administration in addition to the parent compound. Part of these metabolites is already formed before the original compound reaches the circulation, others originate from bio-transformation of the parent molecule and metabolites already present in the circulation. There are generally significant differences in biological activity between the mother molecule and metabolites. 17b-estradiol is clearly a stronger estrogen than estrone, the most important metabolite, whilst conjugation to estrone sulphate results in an inactive estrogen. The estrogen metabolism consists however partly of equilibrium reactions which are reversible as a function of concentration gradients. The metabolites estrone and estrone sulphate are therefore also precursors of 17b-estradiol; high circulating concentrations of estrone and estrone sulphate therefore also act as a reservoir for the much more active 17b-estradiol. The estrogen action obtained with the administration of an estrogen is therefore a reflection of the complex activity of the mother molecule and a series of metabolites.7,11,14,19,20

Central Role of the Liver

There is an important "first passage phenomenon" with extensive metabolic change in the intestinal wall and the liver with oral administration of estrogens before the molecule reaches the general circulation; in some cases virtually no unaltered substance will reach the circulation.21,22 The estrogen concentrations attained in the liver sinusoids during the first passage are many multiples of the maximum serum concentrations.22,23 This explains why oral administration can be associated with significant estrogen effects in the liver, with for example stimulation of the synthesis of a series of proteins, such as carrier proteins for hormones - including the sex hormone binding globulin (SHBG), proteins involved in the regulation of the haemostasis and the renin substrate, another example of hepatic effects being alterations of the lipid metabolism.15-17

In addition to the first passage phenomenon, which is important for the bio-transformation and the metabolic effects of orally administered estrogens, the liver is also involved in the bio-transformation of estrogens after they have already reached the circulation. The liver is in this way responsible for the formation of conjugated metabolites, that are found in the circulation in high concentrations and are eliminated in the urine and also secreted in the gall.7,11,14,19,24 Circulating estrogen conjugates can be re-hydrolysed and thus function as a reservoir for biologically active estrogens.25 Hydrolysation of the conjugates also occurs in the intestine after excretion in the gall, after which the estrogens can be taken up again in an entero-hepatic (re)-circulation.7,26,27 The capacity of the liver to bio-transform estrogens can vary strongly from individual to individual and is influenced by hormonal factors and by pharmaceuticals, including progestogens.28

The Protein-Binding of Oestrogens

The estrogens in the circulation are freely available for exchange with the tissues if they are not protein-bound or if they are weakly bound to albumin. If estrogens are bound with high affinity to SHBG, a dynamic equilibrium develops between the protein-bound estrogens and a non-bound, immediately available, free fraction. The role played by protein binding in uptake of estrogens from the circulation in the tissues is complex and not yet fully understood. The extent of protein binding differs according to the estrogen: 17b-estradiol thus binds with high affinity to SHBG, whilst binding of estrone is rather weak; there is no significant binding of estriol or ethinyl estradiol to SHBG.7 Progestogens can affect hepatic SHBG synthesis, and thus indirectly protein-binding of estrogens.29

Natural and Synthetic Oestrogens

Various estrogen preparations are available for therapy. These can be classified into "natural estrogens" and synthetic estrogens. Among the natural estrogens are 17b-estradiol, estrone, estriol and estrogen conjugates such as estrone sulphate; also available is a series of esterified forms of estradiol such as estradiol valerate, estradiol cypionate, estradiol phenyl propionate and estradiol benzoate.7 A special position is occupied by a mixture of conjugated estrogens, excreted in the urine of pregnant mares. This mixture has a complex composition with at least nine different estrogens, including estrone sulphate and a series of estrogens which is admittedly natural but which does not occur in humans, e.g. equiline sulphate and 17a-dihydroequiline sulphate.26 Widely used synthetic estrogens include ethinylestradiol and to a much lesser extent mestranol, which does not bind to the estrogen receptor but is rapidly converted in vivo to ethinylestradiol following oral administration.7

Natural estrogens are usually recommended for hormone replacement therapy, whereas ethinylestradiol is the classical estrogen component of oral contraceptives. Ethinylestradiol (or mestranol) is no longer used for hormone replacement therapy because it has more pronounced effects on hepatic metabolism, which is explained by the pharmacokinetics of this compound. The presence of an ethinyl group at the 17a position makes it's metabolism to less active metabolites in hepatocytes slower, high liver concentrations of ethinylestradiol being attained in the first passage following oral administration.7 Moreover, bio-transformation of ethinylestradiol produces highly reactive metabolites that may inactivate certain liver enzymes and bind irreversibly to microsomal proteins.7,30

Therapy With Estradiol and Estradiol Esters

Oral administration

Using "micronised" preparations, estradiol is readily taken up with maximum concentrations usually attained between two to six hours.7,8,31 The estradiol undergoes an important bio-transformation in the intestinal mucosa and in the liver on the first passage, with most of it (up to 80 to 90% of the dose) being metabolized to estrone and conjugates; estrone and estrone sulphate are the most important metabolites found in the circulation.7,11,19,32 The estrone concentrations attained after oral administration are therefore also three to six times higher than the estradiol levels. The circulating concentrations of the estrone sulphate, which has a longer plasma half-life, are again appreciably higher than those of estrone.7,32 The development of a reversible equilibrium between estradiol, estrone and estrone sulphate5,24,25 causes the estrone sulphate reserve to play an important role in the maintenance of the estrone and estradiol levels for several hours after administration of estradiol.7,24 During repeated daily administration, the estradiol levels are between 80 and 150 pg/ml for several hours after taking in 2 mg micronised estradiol and usually above 40 tot 50 pg/ml after 24 hours. The estrone/estradiol ratio increases with postmenopausal women from about 2:1 before treatment to about 4:1 with oral estradiol.7,8,31

Plasma elimination half-life of estradiol is approximately 1h (independent of the route of administration), with a plasma clearance rate between 650 and 900 L/day/m2.11 Estrone undergoes irreversible bio-transformation to catecholoestrogens or to estriol. A major pathway for bio-transformation is the formation of a series of conjugates, i.e. sulphates and glucoronides; the sulphates are present in high concentrations in the circulation, while the glucoronides are excreted in the bile and in urine. Less than 1 percent of the dose is recovered in the urine as unchanged estradiol; 50 to 80 percent is excreted in the urine as conjugates. Estrogens excreted in the bile can recirculate after hydrolysis of conjugates in the intestines.7,11,19,27,32

Estradiol valerate is rapidly and completely hydrolysed to estradiol after oral administration so that the pharmacokinetics and the effects of estradiol and estradiol valerate are fully comparable with the same dosage.11,34

Transdermal administration

In practice two forms of parenteral administration of estradiol through the intact skin are used, i.e. with an estradiol-containing hydro-alcoholic gel35,36 or by using a transdermal therapeutic system with controlled release of estradiol, the patch or plaster.31,37 In a first generation of patches, an alcoholic estradiol solution is held in a reservoir and is separated from the skin by a rate controlling membrane.31,37,38 In a more recently developed type of patches, estradiol is directly dissolved or dispersed into the adhesive matrix of the patch, the rate of release being controlled by the specific formulation of the matrix.39-41

The estrogen penetrates the stratum corneum within several minutes of the estrogen-containing gel being applied to the skin, where most of it is stored, and then diffuses out during the subsequent hours to the capillary plexus in the dermis so as to reach the general circulation. About 10% of the applied dose reaches the circulation in an equilibrium situation. Daily application of 1.5 to 3 mg estradiol (0.6 mg estradiol/gram gel) causes the estradiol serum levels to rise during the first 3 to 5 days, after which a plateau is reached with average concentrations of about 70-80 pg/ml and 120-150 pg/ml respectively, with however considerable interindividual variations (with a variation cefficient of 40-45%); the observed variations in achieved serum estradiol levels within a single individual are also rather large, although less pronounced than between individuals. First-pass hepatic metabolism being avoided using the transdermal route, the estrone/estradiol ratio in the circulation is about 1 or lower, which is somewhat lower than in untreated postmenopausal women.8,35,36,42 The biological availability of estradiol improves and the variability of the serum levels decrease with the extent to which a dose is smeared out on a larger skin surface; a reduction of the surface of drug application from 750 to 400 cm2 may result in a 50% reduction of the achieved steady state levels. The choice of cutaneous area for drug application is not of critical importance.35,42,43

When using a reservoir-type patch,37 a small quantity of estrogen passes through a "controlling" membrane at a relatively constant rate and is delivered to a rather small area of the skin, corresponding to the area of the patch itself. A continual, although not really uniform, delivery is ensured for 3.5 days so that 2 applications per week will suffice. The total delivery of estradiol depends on the area of the patch, the delivery aimed for being about 25,50 or 100 ug estradiol per day with the patches available. Peak concentrations of estradiol will be attained within 2 to 8 hours, after which the levels tend to decrease progressively over the remaining period of application. Average estradiol concentrations in serum of about 40 pg/ml are attained with continual application and a change of plaster twice a week with 50 ug/day plasters; the estrone/estradiol ratios are similar to those observed with the use of the gel.8,31,38 The variation in achieved estradiol levels between individuals is very large in this case as well (variation coefficient about 50%)42; the site of application is not of critical importance in this regard.44

The plasma concentrations decrease rapidly if the plaster is removed (or becomes loose). The concentrations decrease more progressively if an estrogen gel treatment is terminated because of the presence of a significant estrogen reserve in the corneous layer. The latter store of estradiol in the skin may also explain observations of a somewhat smaller intra-individual variation of serum levels when using the gel as compared to the reservoir-type patch.42

With the more recently introduced matrix-type patches, the estradiol contained in the adhesive matrix is delivered progressively to the underlying skin, the surface covered by the patch being in this case also proportional to the aimed delivery of approximately 25, 50 or 100 ug/day.39-41 Serum estradiol levels rise rapidly following application of the patch, with values around 90% of the maximum levels usually attained within 12 hours; serum levels are maintained around maximum values for 1 to 2 days, decline thereafter slowly over the remainder of the 7-day patch wear period and return within 12 hours to baseline values after removal of the patch.39,41 Peak estradiol concentrations attained with use of a 50 ug/day patch are around 50 pg/ml, with mean steady state concentrations around 30 to 40 pg/ml, small differences being observed according to the manufacturer; estrone/estradiol levels are, as expected, around 1.39,41 During continuous treatment, consecutive application of the patches on either alternate sites or a same site result in similar estradiol pharmacokinetics.45 The observed variations of estradiol serum levels within a same individual over a prolonged period of time are somewhat smaller with use of the once-a-week matrix-type patch than with the twice-a-week reservoir-type patch.39,41

Other methods of administration

Fairly constant serum levels are attained over several months with subcutaneous implantation of a rod or pellet of crystalline estradiol.7,11,20 Variability of serum levels over prolonged periods of time may be smaller than with the transdermal delivery systems.46 There are nevertheless significant variations in the total time during which adequate levels are maintained. This method of administration leaves little room for individual dose adaptations. Accumulation can occur with a rise in the serum levels if a new tablet is implanted before the estrogen concentrations have returned to their initial values (with a return of menopausal symptoms as well). It can be very difficult to remove the pellet if an interruption of treatment is desired.7,11,20 Injections of estrogen esters in oil is a now little used mode of administration. The concentrations attained as a function of time are very variable with the same preparation, with high peak estradiol concentrations during the first days following injection.7,20

Estradiol is absorbed readily across any epithelium, in particular the vaginal mucosa. Vaginal application of micronised estradiol rapidly gives high serum concentrations with little metabolism to estrone, but these also decrease rapidly. Vaginal rings with estradiol in a polymer, with continual release of the estrogen, allow relatively constant serum levels to be attained over a period of weeks or months although serum levels may be initially high.7,11,20 Estradiol is also readily absorbed across the nasal and sublingual mucosae.11

Therapy With Other Oestrogens

Estriol

Estriol is a weak estrogen which cannot be converted to estradiol. It is almost completely conjugated in the intestine to glucoronides and sulphates after oral intake; only 1-2% estriol reaches the circulation. Enterohepatic circulation can contribute significantly to the estriol levels, with food ingestion resulting in a seconddary rise of estriol levels.27 It may therefore be advisable that estriol is taken in the evening in order to avoid endometrial proliferation during unopposed treatment.7 It is noteworthy that unexpectedly high systematic concentrations can be attained with vaginal applications (creams, ovules). The metabolism of estriol is indeed much more limited after resorption through the vaginal mucosa; comparable estriol serum concentrations are thus obtained after vaginal application of 0.5 mg estriol and oral administration of 8 mg estriol.7,47,48

Equine estrogen conjugates

The complex composition of the clinically used mixture of sulphated estrogens separated from horse urine26 makes it extremely difficult to study the kinetics and there is an almost complete absence of data for various components of this estrogen mixture. Following oral administration, part of the conjugates are absorbed as unchanged sulphates, the remainder is hydrolised in the gut and resulphated following absorption. Sulphated estrogens and the corresponding unconjugated form can be interconverted in the liver and target tissues, so that the high concentrations of albumine bound conjugates in the circulation represent a reservoir for the respective unconjugated estrogens.7,26,49

The main components of the conjugated estrogen mixture separated from horse urine are estrone sulphate (50 to 60%), equiline sulphate (22.5 to 30%) and dehydro-equilinesulphate (15%). Following oral administration, there is an extensive bio-transformation; oral administration is followed by a rise of the serum concentrations of conjugates and of unconjugated estrone, estradiol and specific equine estrogens. Due to their long half-life, conjugated equine estrogens can be demonstrated in the circulation up to several weeks after termination of the treatment.7,26,31,50,51 After oral administration, hepatic metabolic effects are pronounced due to the permeability of hepatocytes membranes for estrogen sulphates and the rather pronounced intrinsic hepatic effects of the equine estrogensulphates with unsaturated B ring.7,23,26 Conjugated estrogens are also readily absorbed trough the vaginal mucosa, with more limited intermediate metabolism.51

Differences Between Oral and Transdermal Administration

The differences between oral and transdermal administration were given ample attention in recent years. The essential difference between the two methods of administration is that with transdermal administration, as is the case with other non-oral routes of administration (e.g. subdermal or vaginal) extensive metabolism during first passage through intestine and liver is avoided. This means that the hepatic metabolic effects are much more limited after transdermal administration.7,17,19,20 Differences in the relative concentrations of metabolites and parent steroid in the circulation is another aspect influenced by the route of administration, with for example much higher plasma concentrations of estrone following oral administration of estradiol, than is the case for the transdermal route.8,11

There may be a misunderstanding about the clinical relevance of some differences between the two administration methods. A reference is thus often made to the more "physiologic" relationship between the serum levels of estradiol and estrone after transdermal administration, which mimics more closely the situation in the ovarial cycle. We have however no indication that higher serum levels of estrone, a much weaker estrogen than estradiol, should in themselves be detrimental; as discussed earlier, high concentrations of estrone and the biologically inactive estrone sulphate help, through equilibrium reactions, to maintain adequate estradiol levels.7

The limitation of the metabolic effects after transdermal administration (e.g. less pronounced effects on the regulation of the haemostasis) undoubtedly offers theoretical advantages at least to some patients. Nevertheless, it has been argued, but not confirmed, that the favourable effects of estrogen therapy on lipid metabolism (lowering of the total cholesterol and increasing the HDL cholesterol) can be more limited with transdermal administration than with oral administration. There have been no current studies which allow the proposal that either of the administration methods offers clinically tangible clinical advantages. It must be remembered here that the choice of the progestagen which is associated with the estrogen treatment can also have important metabolic consequences.

Conclusions

The pharmacokinetics of estrogens is marked by a large interindividual variability. The determination of the plasma concentrations is, however, of little use for monitoring of the treatment in individuals. Indeed, for a same treatment plasma estrogen levels may vary greatly in function of time in a single individual. Moreover, extensive metabolic change causes a complex situation where the active metabolites and not just the parent steroid(s) also contribute to the longer term therapeutic effect. In this context, it should also be pointed out that determination of estrogen plasma levels in subjects under estrogen replacement therapy poses specific technical problems related to the specificity of each radio-immuno assay, with some active metabolites not being and/or (active or inactive) metabolites causing aspecific interferences. Finally, it should be pointed out that for most clinical aspects of hormone replacement therapy there is no well documented, simple relationship between estrogen plasma levels and effects.

As to the route of administration, important differences in the pharmacokinetics do not necessarily translate into tangible differences in the therapeutic results. Claims concerning the superiority of a particular mode of administration or treatment regimen should therefore always be supported by the findings of randomised clinical trials with relevant clinical endpoints.

For the choice of treatment in individuals, this general context forces a pragmatic policy in which a scrupulous clinical evaluation remains of crucial importance. The prescribing doctor should allow himself to acquire sufficient clinical experience with a limited number of treatment regimens, but the clinical findings, the wishes of the patient and her own evaluation of the clinical results should always remain decisive elements when choosing or adjusting a treatment.