Pharmacokinetics of estradiol

Absorption and bioavailability

The oral bioavailability of estradiol is very low.^[2][35][36] This is due to the fact that estradiol is poorly soluble in water, which limits its dissolution and absorption, and is additionally subject to extensive metabolism during the first pass through the intestines and liver.^[3][37] Estradiol is micronized and/or conjugated with an ester, as in estradiol valerate or estradiol acetate, to improve its oral bioavailability and potency.^[2][35][36] Micronization decreases the particle size of estradiol crystals and hence increases the surface area for absorption, thereby improving the rate and extent of absorption.^[38][3][39] In addition, there is an improvement in metabolic stability.^[3][39] Oral micronized estradiol consists of more than 80% of estradiol particles micronized to a size smaller than 20 μm in diameter, or to about 1 to 3 μm on average.^[40][41][39] All oral formulations of estradiol available today are micronized,^[35] and oral estradiol valerate tablets also seem to be micronized.^[42]

Oral non-micronized estradiol and oral micronized estradiol do not appear to have ever been directly compared in a study.^{[43][41][44][45]} Both have been assessed independently however, and have been found to produce significant estrogenic effects.^{[43][41][44][46]} Micronization of other poorly water-soluble steroids such as spironolactone and norethisterone acetate has been found to increase their potency by several-fold.^{[47][48][49][50][51]} In accordance, studies of the amount of oral estradiol necessary for endometrial proliferation in women have reported a total dose of 60 mg for micronized estradiol^[52] relative to 120 to 300 mg or more for non-micronized estradiol.^[53][54][55] As such, micronization has been said to substantially improve the potency of oral estradiol.^[39]

Estradiol levels with a single 2 mg dose of oral estradiol micronized to different particle sizes in postmenopausal women.^[37]

A study compared different particle sizes of oral micronized estradiol.^[37][56][57] A preparation with the smallest particles (mainly <0.6 μm) was found to have the most rapid absorption and the highest bioavailability.^[37][57] However, a sharp peak in estradiol levels, without an accompanying rise in estrone levels, was observed during the first 2 hours with this particle size.^[37][57] It was suggested that the smallest estradiol particles may have been absorbed by the lymphatic system, partially bypassing first-pass metabolism and resulting in very high initial estradiol levels.^[37] The preparations with the larger particle sizes (mainly <3.5 μm and <20 μm) were found to be absorbed more slowly, without a pronounced initial peak in estradiol levels.^[37][57] Levels of estradiol were more even and similar to physiological levels with these particle sizes.^[37][57] Differences in area-under-the-curve estradiol levels with the different particle sizes were relatively small.^[37] As such, micronization may improve absorption but does not necessarily improve therapeutic effect.^[57]

Micronized estradiol is rapidly and completely absorbed with oral administration.^[3][12] This is true for oral doses of 2 mg and 4 mg, but absorption was found to be incomplete for an oral dose of 8 mg.^[3][58] This dose showed 76% of the expected bioavailability based on dose proportionality and area-under-the-curve levels, indicating a small deviation from linearity.^[3][58] The absolute bioavailability of oral micronized estradiol is approximately 5%, with a possible range of 0.1% to 12%.^[1][2][57] As such, the bioavailability of oral estradiol is very low even with micronization.^[57] There is high interindividual variability in the levels of estradiol achieved with oral estradiol, which is likely related to the high first-pass effect.^[12] This variation has been reported to be in the range of 28 to 127%, or about 4.6-fold maximal difference in levels between individuals, in terms of mean area-under-the-curve levels of estradiol.^[12]

In postmenopausal women, a dosage of 1 mg/day oral micronized estradiol has been found to produce circulating concentrations of 30 to 50 pg/mL estradiol and 150 to 300 pg/mL estrone, while a dosage of 2 mg/day has been found to result in circulating levels of 50 to 180 pg/mL estradiol and 300 to 850 pg/mL estrone.^[19] A study of oral micronized estradiol in transgender women found that mean estradiol levels across a dosage range of 1 to 8 mg/day were about 50 pg/mL at 1 mg/day, 100 pg/mL at 4 mg/day, and 150 pg/mL at 8 mg/day, with a wide degree of variation.^[59] In another study, mean estradiol levels at steady state with 4 mg/day and 6 mg/day oral micronized estradiol were approximately 180 pg/mL and 265 pg/mL, respectively.^[60] A study that used high to very high-dose oral micronized estradiol in postmenopausal women found that steady-state estradiol levels with 6 mg/day were about 300 pg/mL and with 30 mg/day were about 2,400 pg/mL.^[61]

Estradiol valerate is rapidly hydrolyzed into estradiol in the intestines.^[10][62][63] For this reason, oral estradiol and oral estradiol valerate have very similar pharmacokinetics.^[10][62][63] Due to the presence of its valeric acid ester and differences in molecular weight, estradiol valerate contains about 76% of the same amount of estradiol by weight.^{[64][65][66][67]} As a result, 2 mg oral estradiol valerate produces equivalent estradiol levels to about 1.5 mg oral estradiol.^{[64][65][66][67]}

Metabolism and elimination

When taken orally, about 95% of a dose of estradiol is metabolized in the intestines and liver into estrone and estrogen conjugates such as estrone sulfate, estrone glucuronide, and estradiol sulfate, among others, prior to entering the circulation.^{[3][86][87][88]} As a result, circulating estrone and estrogen conjugate levels are markedly elevated, in a highly unphysiological manner, with oral estradiol.^[86][89] Whereas the ratio of circulating estradiol to estrone is about 1:1 in premenopausal women and with transdermal estradiol, oral estradiol produces a ratio of about 1:5 on average and as high as 1:20 in some women.^{[1][10][90][58]} In addition, whereas levels of estradiol with menopausal replacement dosages of oral estradiol are in the range of the follicular phase of the normal menstrual cycle, levels of estrone resemble those during the first trimester of pregnancy.^[91][92] Moreover, whereas normal physiological estrone sulfate levels are 10 to 25 times higher than those of estradiol and estrone in premenopausal women,^[93] levels of estrone sulfate with oral estradiol are an additional 8 to 20 times higher than normal premenopausal or postmenopausal estrone sulfate levels.^[89][94][95] One study found that estrone sulfate levels were 200-fold higher than estradiol levels with 2 mg/day oral micronized estradiol or oral estradiol valerate, and estrone sulfate levels can reach up to nearly 1,000-fold higher concentrations than estradiol in some cases.^[10][12] In contrast to oral estradiol, due to the lack of the first pass, an excess in estrone and estrogen conjugate levels does not occur with transdermal estradiol or other parenteral estradiol routes.^[86][89]

The transformation of estradiol into estrone and estrogen conjugates is reversible.^[10] As such, these metabolites can be converted back into estradiol.^[10] About 15% of orally administered estradiol is transformed into estrone and 65% into estrone sulfate.^[12] About 5% of estrone and 1.4% of estrone sulfate is converted back into estradiol.^[12] An additional 21% of estrone sulfate is converted into estrone, whereas transformation of estrone into estrone sulfate is approximately 54%.^[12] The interconversion between estradiol and estrone is mediated by 17β-hydroxysteroid dehydrogenases (17β-HSDs),^[12] whereas the conversion of estrone into estrone sulfate is mediated by estrogen sulfotransferases (ESTs) and the transformation of estrone sulfate into estrone by steroid sulfatase (STS).^[96][97] The metabolic clearance rates and hence blood half-lives of estrogen conjugates like estrone sulfate are much longer than those of estradiol and estrone.^[10][12][89] Estrogen conjugates, primarily estrone sulfate, serve as a large circulating reservoir for estradiol, and because of this, they function to greatly extend the biological half-life of oral estradiol.^[10][12] As such, the biological half-life of oral estradiol is a composite parameter that is dependent on interconversion between estradiol and estrogen conjugates, as well as on enterohepatic recirculation.^[12] Whereas the biological half-life of estradiol given by intravenous injection is about 0.5 to 2 hours, the biological half-life of oral estradiol has a range of 13 to 20 hours due to the large and long-lasting pool of estrogen conjugates that is formed during first-pass metabolism and that serves to continuously replenish circulating estradiol levels.^[12][10][9]

First-pass effect and differences from other routes

The first-pass effect that occurs with oral estradiol results in unusually high levels of estrone and estrogen conjugates in the circulation as well as of estradiol in the liver.^[10] These unique properties of oral estradiol result in a number of pharmacological differences relative to the other routes of administration of estradiol.^[10]

The high levels of estrone and estrogen conjugates that occur with oral estradiol raise the question of the pharmacodynamic significance of these metabolites.^[10] In contrast to estradiol however, estrone has very low activity as an estrogen.^[10][98][99] The affinities of estrone for the human ERs and its estrogenic activity have been reported to be approximately 3 to 4% of those of estradiol.^[10] In addition, unlike estradiol and estriol, estrone is not accumulated in target tissues.^[10] Because estrone can be transformed into estradiol, most of its activity in vivo is actually due to conversion into estradiol.^[10] In accordance, doses of oral and transdermal estradiol that achieve similar levels of estradiol have been found, in spite of markedly elevated levels of estrone with oral estradiol but not with transdermal estradiol, to possess equivalent and non-significantly different potency in terms of clinical measures including suppression of LH and FSH levels, inhibition of bone resorption, and relief of menopausal symptoms such as hot flashes.^{[10][86][100][101][94][102]} In addition, estradiol levels were found to correlate with these effects, while estrone levels did not.^[86][100] These findings suggest that estrone contributes very little or not at all to the estrogenic potency of estradiol, while also not antagonizing the estrogenic activity of estradiol.^{[10][86][100][101]} This contradicts some cell-free in-vitro research suggesting that high concentrations of estrone might be able to partially antagonize the actions of estradiol.^{[103][104][105]}

Distribution of 17β-HSD activities for formation of estradiol (E2) versus formation of estrone (E1) in human tissues.^[106][11]

Distribution of STS and EST activities for interconversion of estrone (E1) and estrone sulfate (E1S) in adult human tissues.^[107]

Relative activation and inactivation rates of estradiol to and from estrone and estrone sulfate in tissues throughout the body.

On the other hand, it has been suggested by some authors that the high levels of estrone and/or estrone conjugates with oral estradiol may result in excessive estradiol levels in certain tissues such as the breasts and endometrium, due to high expression in these tissues of the requisite enzymes (17β-HSDs and STS) necessary to transform these metabolites back into estradiol.^{[92][89][108][109]} In accordance, circulating levels of estrone sulfate have been found to be positively associated with breast density in postmenopausal women treated with oral estradiol, with 1.3% higher breast density observed for every 1 ng/mL greater level of estrone sulfate.^[110][111] Similarly, levels of estradiol, estrone, and estrone sulfate are all strongly associated with the risk of breast cancer in women.^[110] Preclinical studies have shown that estrone sulfate, via local transformation into estradiol, stimulates the growth of mammary cancer cells.^[112][113]

Due to the first pass through the liver, disproportionate and supraphysiological levels of estrogens occur locally in the liver with oral estradiol.^[114][12] These levels are approximately 4- to 5-fold higher than in the circulation, based on differences in hepatic estrogenic potency for oral estradiol relative to transdermal estradiol.^[114][10] As a result, there is abnormally high estrogenic signaling in the liver with oral estradiol, and a variety of unphysiological effects on liver protein synthesis result.^[10][12] Through modulation of liver protein synthesis, conjugated oral estrogen increases the risk of blood clots,^[115] increases circulating levels of a variety of binding proteins including thyroid binding globulin (TBG), cortisol binding globulin (CBG), sex hormone binding globulin (SHBG), growth hormone binding protein (GHBP),^[116] insulin-like growth factor-binding proteins (IGFBPs),^[117] and copper binding protein (CBP),^[87][118] suppresses growth hormone (GH)-mediated insulin-like growth factor 1 (IGF-1) production,^[119][120] and produces positive blood lipid changes, among a variety of other effects.^{[88][121][12]} In contrast to oral estradiol, transdermal estradiol has relatively minimal impact on liver protein synthesis.^[10] As an example, a study found that 1 mg/day oral estradiol significantly increased SHBG levels by 45%, while 50 µg/day transdermal estradiol increased SHBG levels non-significantly by only 12%.^{[122][123][124]}

In the circulation, approximately 38% of estradiol is reversibly bound to SHBG and 60% is reversibly bound to albumin in women under normal physiological circumstances, with 2 to 3% of total estradiol circulating free or unbound at any given time.^[3][2][1] Only estradiol that is free or unbound is able to be enter target cells and hence is biologically active.^{[1][12]: 249 [17]} The increase in SHBG levels with oral estradiol (e.g., +50%) can result in a clinically meaningful increase in the fractions of sex hormones like estradiol and testosterone that are bound to SHBG, whereas this is not the case with typical clinical dosages of transdermal estradiol.^[125][17] The increase in the fraction of estradiol bound to SHBG results in a significant decrease in the percentage of free or unbound and hence bioactive estradiol.^[1][17] As a result, the bioavailability and potency of oral estradiol may be diminished relative to parenteral estradiol routes by some amount.^[17][1] However, a study found that the free fraction of estradiol was similar with doses of oral and topical estradiol that resulted in equivalent total estradiol levels.^[126]

Experimental oral formulations

Estradiol decanoate, estradiol cyclooctyl acetate, estradiol 3-saccharinylmethyl ether, and EC508 (estradiol 17β-(1-(4-(aminosulfonyl)benzoyl)-L-proline)) are estradiol esters and novel oral forms of estradiol that have been developed with improved properties, such as greater bioavailability and reduced first-pass effect.^{[127][128][129][130][131][132][133][134][135][136][137][138]} Estradiol decanoate and estradiol cyclooctyl acetate were studied for potential use in menopausal hormone therapy and birth control pills but were never marketed.^{[127][128][129][130][131][132][133][134]} EC508 is currently under active development for use in menopausal hormone therapy.^[137][138]

Graphs

Clinical effects

The total endometrial proliferation dose of sublingual estradiol in women is 60 to 140 mg per cycle or 14 days and of sublingual estradiol benzoate in women is 60 to 180 mg per cycle or 14 days.^[73]: 310 Both sublingual estradiol and sublingual estradiol benzoate have a persistence of estrogenic effect after a dose of only one day.^[73]: 310 The effects of sublingual estradiol on gonadotropin levels have also been studied in postmenopausal women.^{[149][171][150][172]} After a dose of sublingual estradiol, levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) decrease precipitously within 4 hours.^[149] Following this, LH and FSH levels gradually increase, and return to near-baseline levels by 24 hours.^[149] One study found no difference between oral and sublingual estradiol in suppression of LH levels.^[149] However, FSH levels were suppressed to a greater extent with sublingual estradiol than with oral estradiol in the study.^[149]

It is notable that the magnitude of the genomic effects of estradiol (i.e., signaling through the nuclear ERs) may, at least in some cases, be dependent on the total estrogenic exposure as opposed to the duration of exposure.^[10] For instance, in normal human epithelial breast cells and ER-positive breast cancer cells, the rate of breast cell proliferation has been found not to differ with estradiol incubation of 1 nM for 24 hours and incubation of 24 nM for 1 hour.^[10] In other words, short-term high concentrations and long-term low concentrations of estradiol appear to have the same degree of effect in terms of genomic estrogenic signaling, at least in terms of breast cell proliferation over a 24-hour period.^[10] On the other hand, non-genomic actions of estradiol, such as signaling through membrane estrogen receptors like the GPER, may be reduced with short-term high concentrations of estradiol relative to more sustained levels.^[10] For instance, although daily intranasal administration of estradiol is associated with comparable clinical effectiveness (e.g., for hot flashes) relative to longer acting routes of estradiol administration in postmenopausal women, it is also associated with significantly lower rates of breast tension (tenderness and enlargement) relative to longer acting estradiol routes, and this is thought to reflect comparatively diminished non-genomic signaling.^[10]

Graphs

Transdermal patches

Vivelle-Dot, an estradiol patch.

Estradiol patches have an extended duration and are available for twice-weekly (3–4-day) and once-weekly (7-day) application, while gels, emulsions, and sprays are administered daily.^{[182][19][10][206]} There are two types of estradiol patches: reservoir patches, which have been described as first-generation patches, and matrix patches, which are considered to be improved second-generation patches.^{[10][12][182]} Reservoir patches were designed for twice-weekly application, while matrix patches have been produced for both twice-weekly and once-weekly application.^[12] Reservoir patches of estradiol (e.g., Estraderm) are mostly no longer used, with most estradiol patches available today being matrix patches (e.g., Alora, Climara, Esclim, Estradot, FemPatch, Menostar, Oesclim, Vivelle, and Vivelle-Dot).^[182]

A dosage of 1 mg/day oral estradiol is considered to be roughly equivalent to 25 or 50 µg/day transdermal estradiol and a dosage of 2 mg/day oral estradiol is considered to be equivalent to 50 or 100 µg/day transdermal estradiol depending on the source.^{[209][95][12][10]} Estradiol patches delivering a daily dosage of 0.05 mg (50 µg) achieve mean estradiol and estrone levels of 30 to 65 pg/mL and 40 to 45 pg/mL, respectively, while a daily dosage of 0.1 mg (100 µg) attains respective mean levels of 50 to 90 pg/mL and 30 to 65 pg/mL of estradiol and estrone.^[19] In general, Climara-type estradiol transdermal patches have an approximate 1:1 ratio of estradiol delivered in μg/day relative to circulating estradiol concentration in pg/mL.^[201] In other words, a 100 μg/day Climara estradiol patch may be expected to produce circulating estradiol levels of around 100 pg/mL.^[201] Transdermal estradiol patches produce an estradiol to estrone ratio of about 1:1.^[10][12] Following removal of an estradiol patch, circulating estradiol levels decrease to baseline within 24 hours.^[10]

Typical dosages of estradiol patches are intended to provide the minimum amount of estrogen replacement necessary for the effective alleviation of menopausal symptoms, and for this reason, they achieve relatively low levels of estradiol.^[10] A dosage of two to six 100 µg/day transdermal estradiol patches can achieve mean levels of estradiol in the area of 200 to 400 pg/mL and can be used as a form of high-dose estrogen therapy, for instance to suppress testosterone levels in the treatment of prostate cancer in men and in feminizing hormone therapy for transgender women.^{[14][210][211]} High-dose transdermal estradiol patches have also been studied in the treatment of postpartum depression and postpartum psychosis; in one such study, 200, 400, and 800 μg/day estradiol in the form of transdermal patches resulted in estradiol levels of 286 pg/mL, 675 pg/mL, and 1032 pg/mL, respectively.^[212] In another study, estradiol levels with 800 μg/day estradiol in the form of transdermal patches (Estraderm TTS) resulted in estradiol levels of 690 to 815 pg/mL.^[213] However, there is erratic absorption and considerable variation in estradiol levels using high-dose estradiol patches both between and within individuals, with one study finding that estradiol levels ranged from 70 pg/mL to 1,045 pg/mL (mean 460.7 pg/mL) with six 100 μg/day estradiol patches.^[214][215]

The Prostate Adenocarcinoma: TransCutaneous Hormones (PATCH) study is a randomized controlled trial of high-dose transdermal estradiol patches versus gonadotropin-releasing hormone agonist monotherapy in the treatment of prostate cancer in approximately 2,200 men.^{[216][217][218]} It is specifically comparing three to four 100 μg/day estradiol patches (FemSeven) against goserelin implants.^[216] The study was started in March 2006 and is estimated for completion in August 2021.^[216] Its objectives include comparison of survival, cardiovascular mortality and morbidity, pharmacological activity (e.g., suppression of testosterone levels), other side effects and toxicities, and quality of life.^[216] In addition to the PATCH trial, the Systemic Therapy in Advancing or Metastatic Prostate Cancer: Evaluation of Drug Efficacy (STAMPEDE) study added a high-dose estradiol patches arm (~2,000 men) in July 2017.^{[219][217][218]}

Estradiol patches are associated with local skin reactions and such as irritation in 14.2% of individuals (with reservoir patches), mild-to-moderate erythema (redness) in 50 to 60% of individuals, and allergic reactions due to cutaneous sensitization.^[10][12] Up to 5% of people using reservoir patches may discontinue therapy due to skin reactions.^[12] Visible adhesive residues are also often left by estradiol patches following their removal.^[10] Transdermal estradiol gel can serve as an alternative to transdermal estradiol patches for individuals who experience intolerable skin reactions with them.^[220] Estradiol patches should not be applied to the breast as this may result in high local levels of estradiol in the breasts and hence an increased likelihood of breast tenderness.^[221]

Transdermal gel

Estradiol is available as a transdermal gel in the form of gel dispensers and gel packets. Major estradiol gel dispenser products include EstroGel and Elestrin while major estradiol gel packet products include DiviGel and Sandrena. Estradiol gels are administered daily.^{[182][19][10][208]} When estradiol is administered as a hydroalcoholic gel, it dries within 2 to 5 minutes following application to the skin.^[190] A single application of a transdermal estradiol gel results in a sustained increase in estradiol levels for at least 24 hours.^[17][190] The apparent elimination half-life of estradiol with transdermal estradiol gel is 36 hours.^[190]

Once daily application of 1.25 g topical gel containing 0.75 mg estradiol (brand name EstroGel) for 2 weeks was found to produce mean peak estradiol and estrone levels of 46.4 pg/mL and 64.2 pg/mL, respectively.^[190] The time-averaged levels of circulating estradiol and estrone with this formulation over the 24-hour dose interval were 28.3 pg/mL and 48.6 pg/mL, respectively.^[190] Levels of estradiol and estrone are stable and change relatively little over the course of the 24 hours following an application, indicating a long duration of action of this route.^[190] Steady-state levels of estradiol are achieved after 3 days of application.^[190] A higher dosage of estradiol gel containing 1.5 mg estradiol per daily application has been found to produce mean estradiol levels of 40 to 100 pg/mL and estrone levels of 90 pg/mL, while 3 mg per day has been found to result in respective mean estradiol and estrone levels of 60 to 140 pg/mL and 45 to 155 pg/mL.^[19] Topical estradiol gel at a dosage of 3 mg/day has been reported to be equipotent with 2 mg oral estradiol in terms of therapeutic effects and FSH suppression, as well as to produce similar estradiol levels.^[126] Transdermal estradiol gel produces an estradiol to estrone ratio of about 1:1.^[10][12]

Transdermal estradiol gel can be used as a form of high-dose estrogen in transgender women.^[220] However, the doses needed require application to a large surface of skin that amounts to the combined area of both legs for proper absorption.^[220] As a result, high-dose transdermal estradiol gel is not a primary choice of estrogen therapy for most transgender individuals.^[220] Similarly to transdermal estradiol patches, high-dose transdermal estradiol gel has been studied in the treatment of prostate cancer as well.^{[231][232][233][234][235][236][237]} In these studies, levels of estradiol with estradiol gel or ointment were 84 pg/mL with 3 mg/day, 185 pg/mL with 6 mg/day, 107 pg/mL with 10 mg/day, and 473 pg/mL with 20 mg/day.^{[232][233][234][235][236][237]}

Studies have found that topical application of estradiol to the breasts increases local levels of estradiol in breast tissue.^{[238][239][240][241]}

The total endometrial proliferation dose of transdermal estradiol gel in women has been reported to be 150 mg per cycle or 14 days.^{[242][73]: 310} However, it has also been found that 6 mg/day estradiol gel is effective for endometrial proliferation in women.^[243]

Other transdermal formulations

Estradiol levels with different doses of an estradiol transdermal spray (brand name Lenzetto) in postmenopausal women.^[246]

Estradiol levels with different doses of an estradiol transdermal emulsion (brand name Estrasorb) in postmenopausal women.^[247][248]

Estradiol is available in the form of transdermal emulsions (e.g., Estrasorb) and sprays (e.g., Lenzetto, Evamist).^[183] Estradiol emulsions and sprays are administered daily.^{[182][19][10][208]} The pharmacokinetics of these preparations have been studied.^{[246][247][248]}

Variability in pharmacokinetics

Transdermal estradiol patches are described as delivering a fixed amount of estradiol such as 50 µg/day or 100 µg/day.^[10] However, there is large interindividual variability and intraindividual variability in the pharmacokinetic parameters of transdermal estradiol, and fluctuations in circulating estradiol levels with estradiol patches are almost as great as with oral estradiol.^{[10][94][12][17]} As such, the actual delivery rate of estradiol and mean levels of estradiol achieved with transdermal estradiol patches may be different from what is described and from the mean levels observed in clinical studies, respectively.^[10]

A wide range of estradiol levels are measured in women using the same estradiol patch or gel and dosage, with an up to about 10-fold difference in levels.^[10][94][17] In a study of estradiol gel and patches, the maximal difference in peak levels between individuals was 11-fold for the gel and 7-fold for the patch, and the maximal difference in area-under-the-curve levels (total exposure) was 6-fold for the gel and 8-fold for the patch.^[94] It has likewise been reported that the interindividual variability in bioavailability with Estraderm reservoir patches ranges from 25 to 225%.^[17] In as many as 30% of women treated with a 50 µg/day estradiol patch, estradiol levels are low.^[10] There are also significant short-term intraindividual differences in estradiol levels with estradiol patches; estradiol levels can fluctuate considerably from hour to hour.^[10][184] In addition, estradiol levels with estradiol patches are higher in the evening than in the morning, which may be due to circadian variations in skin blood flow that may influence absorption.^[10] In terms of area-under-the-curve levels of estradiol, the interindividual variability of transdermal estradiol has been found to be 20 to 44% using different transdermal formulations, and the intraindividual variability with transdermal estradiol has been found to be 20%.^[12]

Factors which may contribute to inter- and intraindividual variability with transdermal estradiol include skin location and thickness; hair follicle density; solvent (alcohol) evaporation; skin dehydration, ambient temperature, and humidity; and reservoir size.^[17]

Aqueous solutions

Aqueous solutions are solutions of a compound with water. In contrast to other formulations, such as oil solutions, aqueous suspensions, and emulsions, aqueous solutions of estradiol and estradiol esters by intramuscular injection are not depot injections. Instead, they are rapidly absorbed and eliminated, analogously to intravenous injections of estradiol and estradiol esters. The durations of aqueous solutions of estradiol and estradiol esters by intramuscular injection are measured in hours.

Oil solutions

Oil solutions are solutions of a compound with oil, for instance sesame oil or castor oil. When free steroids like estradiol are administered in oil solution by intramuscular injection, they are rapidly absorbed and the duration is relatively short.^[267][272] A single 1 to 2 mg dose of estradiol in oil solution by intramuscular injection has a duration of about 1 or 2 days.^{[262][273][274]} Little prolongation of duration is achieved with the use of larger doses.^{[267][275][276]} Nonetheless, the duration of estradiol in oil solution by intramuscular injection is significantly longer than an intravenous injection of estradiol or estradiol valerate, which show a duration of only a few hours.^{[10][12][4][58][277][278]}

Conversely, intramuscular injections of estradiol esters in oil solution have durations of days to months, depending on the ester administered.^[262] Following a single 4 or 5 mg intramuscular injection in oil solution, peak estradiol levels are about 950 pg/mL with estradiol benzoate after 2 days, 400 to 650 pg/mL with estradiol valerate after 2 days, and 250 to 350 pg/mL with estradiol cypionate after 4 days.^{[271][16][270]} The durations with a 5 mg dose are 4 or 5 days with estradiol benzoate, 7 or 8 days with estradiol valerate, and 11 to 14 days with estradiol cypionate.^{[271][16][270]} The differences in estradiol levels and the different durations with estradiol levels are due to their different rates of release from the oily depot at the injection site.^[271] The longer and hence more lipophilic the fatty acid ester, the slower the release from the depot, the lower the peak estradiol levels, and the longer the duration.^{[271][10][262]}

The duration of estradiol esters in oil solution by intramuscular injection is dose-dependent.^[279] With estradiol valerate, it is reported that a dose of 5 mg has a duration of 7 to 8 days,^[271] 10 mg a duration of 10 to 14 days,^[262][279] 40 mg a duration of 2 to 3 weeks, and 100 mg a duration of 3 to 4 weeks.^[279] High doses of estradiol valerate, such as 40 mg per week, can achieve pregnancy levels of estradiol.^[280] A study of pseudopregnancy with intramuscular injections of 40 mg/week estradiol valerate and 250 mg/week hydroxyprogesterone caproate observed estradiol levels of about 2,500 to 3,000 pg/mL.^[280]

Aqueous suspensions

Aqueous suspensions are suspensions of crystal particles of a compound in water. Estradiol in microcrystalline aqueous suspension for use by intramuscular injection was previously marketed in the 1950s under brand names such as Aquadiol, Diogyn, Progynon Aqueous Suspension, and Progynon Micropellets.^{[281][282][283][284][285][286][287][288]} It was used at a dose of 0.5 to 1.5 mg 2 or 3 times per week.^[287] Newman (1950) found that 0.5 to 2 mg once per week was satisfactory.^[289] As such, the preparation presumably had a duration in the range of 2 to 7 days.^[287][289]

Microcrystalline aqueous suspensions of estradiol esters, for instance of estradiol benzoate (brand names Agofollin Depot alone and Follivirin in combination with testosterone isobutyrate),^[290][291] have been found to have longer duration of actions than oil solutions of the same esters when administered via intramuscular injection.^{[292][293][268][294][295][47][296]: 310} Whereas the duration of a single intramuscular injection of amorphous estradiol benzoate in oil solution is 6 days, the duration of a single intramuscular injection of microcrystalline estradiol benzoate in aqueous suspension is 16 to 21 days.^{[73][293][297][298]}

The duration of crystalline aqueous suspensions is highly dependent on crystal size.^{[299][300][295][301][302]} Steroids and steroid fatty acid esters are lipophilic and have very low water solubility.^[303] When they are suspended in the form of crystals in water, these crystals dissolve slowly, releasing steroid from their surfaces in the process.^[303][304] The larger the particle sizes of the crystals, the slower the dissolution rate.^[303] When a crystalline aqueous suspension of steroid is administered via intramuscular injection, a crystalline depot suspended in fluid is formed locally within the muscle.^[303][304] These crystals slowly dissolve and the steroid is gradually absorbed into the body, resulting in the long durations of such preparations.^[303][304] Particle sizes of 10 μm or less have no apparent depot effect.^[305]

A larger needle size is needed for aqueous suspensions of steroids to allow the steroid crystals to pass through the needle lumen.^[306][307] Aqueous suspensions pose a risk of injection site reactions such as local irritation, swelling, and redness, with often severe pain.^[300][307] The reactions are worse with larger crystal sizes.^[300][308] Particle sizes of more than 300 μm in the case of estradiol benzoate have been found to be too painful for use.^[308] The local injection site reactions, which do not occur with oil solutions, have limited the clinical use of aqueous suspensions of estradiol and its esters as well as other steroids.^{[309][310][311]}

Emulsions

Emulsions are mixtures of immiscible liquids. Water-in-oil emulsions of estradiol benzoate were evaluated as long-acting preparations for use by intramuscular injection in the 1940s and 1950s.^[295][268] Formulations of estradiol benzoate alone under the brand name Menformon-Emulsion and with progesterone under the brand name Di-Pro-Emulsion were previously marketed.^[312][313] A 10 mg dose of estradiol benzoate in emulsion by intramuscular injection is said to have a duration of about 2 to 3 weeks.^[312] This is similar to the duration of an aqueous suspension of 10 mg estradiol benzoate or an oil solution of 10 mg estradiol valerate.^[312] Emulsions of steroids by intramuscular injection have similar properties (e.g., duration) relative to aqueous suspensions.^[295][268] Painful injection site reactions have been reported with emulsions similarly to suspensions.^[314]

Polymers

Polymers are large molecules of repeating subunits. Polyestradiol phosphate (brand name Estradurin) is a water-soluble estradiol ester in the form of a polymer and a very slowly hydrolyzed prodrug of estradiol.^[315][316] It is formulated as an aqueous solution and is given by intramuscular injection.^[315][316] The medication has an exceptionally long duration of action, with an elimination half-life of about 70 days or 10 weeks following a single injection.^[317] Estradiol levels during polyestradiol phosphate therapy are very constant and uniform.^[317] Levels of estradiol after 6 months of treatment with polyestradiol phosphate were about 350, 450, and 650 pg/mL with doses of 160, 240, and 320 mg once per month, respectively.^[13] Polyestradiol phosphate has mostly been discontinued and remains available only in a few countries.^[316][318]

Microspheres

Microspheres are microscopic spherical particles which can be used to encapsulate compounds. Estradiol is available in the form of an aqueous suspension of 1.0 mg estradiol in microspheres for use by intramuscular injection once a month under the brand name Juvenum E in Mexico.^[319][320] It achieves circulating estradiol levels of 163 pg/mL to 219 pg/mL in the first 3 to 12 hours following injection, which decrease to 42 to 66 pg/mL during the first 4 days post-injection and to 20 to 35 pg/mL after 8 days, with levels remaining in this range thereafter over 30 days.^[319] These estradiol levels are similar to the normal levels that occur during the early follicular phase of the menstrual cycle in premenopausal women (24 to 75 pg/mL).^[319] The elimination of the formulation follows three phases: a rapid phase in the first 2 days, a second phase during days 2 to 12 days with a biological half-life of 7 to 10 days, and a third phase in which estradiol levels remain elevated above baseline for up to 30 days.^[319]

Graphs