Pharmacology of progesterone

Mechanism of action

Progesterone is a progestogen, or an agonist of the nuclear progesterone receptors (PRs), the PR-A, PR-B, and PR-C.^[13] In one study, progesterone showed EC₅₀ values of 7.7 nM for the human PR-A and 8.0 nM for the human PR-B.^[17] In addition to the PRs, progesterone is an agonist of the membrane progesterone receptors (mPRs), including the mPRα, mPRβ, mPRγ, mPRδ, and mPRϵ.^[18][19] It is also a potent antimineralocorticoid (antagonist of the mineralocorticoid receptor (MR)),^[20][21] as well as a very weak glucocorticoid (agonist of the glucocorticoid receptor).^[22][23] Progesterone does not bind to the androgen receptor (AR) or to the estrogen receptor (ER).^[13] In addition to its activity as a steroid hormone, progesterone is a neurosteroid.^[24] Specifically, it is an antagonist of the sigma σ₁ receptor,^[25][26] a negative allosteric modulator of nicotinic acetylcholine receptors,^[24] and, via its active metabolites allopregnanolone and pregnanolone, a potent positive allosteric modulator of the GABA_A receptor, the major signaling receptor of the inhibitory neurotransmitter γ-aminobutyric acid (GABA).^[27]

Effects in the body and brain

The PRs are expressed widely throughout the body, including in the uterus, cervix, vagina, fallopian tubes, breasts, fat, skin, pituitary gland, hypothalamus, and elsewhere throughout the brain.^[13][33] Through activation of the PRs (as well as the mPRs), progesterone has many effects, including the following:^[13][33]

• Induces endometrial secretory transformation in preparation for pregnancy
• Prevents estrogen-induced endometrial hyperplasia and increased endometrial cancer risk
• Maintains pregnancy via effects in endometrium (with withdrawal resulting in miscarriage)
• Reduces amount and fibrosity of cervical mucus and causes cervix to become firmer and more tightly closed^[34]
• Controls motility and composition of fluid in the fallopian tubes
• Reduced cornification and maturation of the vaginal lining^[35]
• Causes water retention in the breasts resulting in temporary enlargement during the menstrual cycle^[36][37]
• Mediates lobuloalveolar development of the breasts necessary for lactation
• Suppresses lactation initiation and triggers lactation upon withdrawal (as with parturition)
• Maintains skin health, integrity, appearance, and hydration and slows the rate of aging of the skin^[38][39]
• Modulates brain function, with effects on mood, emotionality, and sexuality, as well as cognition and memory
• Exerts negative feedback on the hypothalamic–pituitary–gonadal axis (HPG axis) by suppressing the secretion of the gonadotropins FSH and LH from the pituitary gland (including the mid-cycle gonadotropin surge), thereby inhibiting gonadal sex hormone production as well as ovulation and fertility
• Increases basal body temperature (by 0.3–0.6 °C (0.5–1.0 °F) relative to preovulation) via the hypothalamus^[40]
• Reduces hot flashes via the hypothalamus^[41][42]
• Stimulates respiration via the hypothalamus and/or respiratory center^[43][44]
• Influences the risk and/or progression of hormone-sensitive cancers including breast cancer and endometrial cancer

It should be noted that many of the effects of progesterone require estrogen, as estrogens prime tissues for progesterone by inducing expression of the PRs.^[13][33] The PRs are induced in the breasts by estrogens, and for this reason, it is assumed that progestogens cannot mediate breast changes in the absence of estrogens.^[45]

Progesterone also lowers blood pressure and reduces water and salt retention among other effects via its antimineralocorticoid activity.^[13][46]

Progesterone can produce sedative, hypnotic, anxiolytic, euphoric, cognitive-, memory-, and motor-impairing, anticonvulsant, and even anesthetic effects via formation of sufficiently high concentrations of its neurosteroid metabolites and consequent GABA_A receptor potentiation in the brain.^{[47][48][49][50]}

Antiestrogenic effects

Progesterone, like all progestogens, has antiestrogenic effects in certain tissues such as the uterus, cervix, and vagina and possibly also the breasts and brain.^[13][51][52] These effects are mediated by activation of the PR in these tissues.^[13] Progesterone does not have antiestrogenic effects in the more conventional sense of binding to and antagonizing the ER or binding to and inhibiting enzymes involved in estrogen biosynthesis.^[13] Instead, for instance in the endometrium, progesterone causes downregulation of the ER and upregulation of the estrogen-inactivating enzymes 17β-hydroxysteroid dehydrogenase 2 (converts estradiol into estrone) and estrone sulfotransferase (converts estrone into estrone sulfate).^[13] In the breasts, progesterone similarly downregulates the ER as well as the estrogen-activating enzymes steroid sulfatase (converts estrone sulfate into estrone) and 17β-hydroxysteroid-dehydrogenase 1 (converts estrone into estradiol) and upregulates estrone sulfotransferase.^[51][52] The antiestrogenic effects of progesterone and other progestogens form the basis for their only approved indication in menopausal hormone therapy: prevention of long-term unopposed estrogen-induced endometrial hyperplasia and increased endometrial cancer risk in women with intact uteruses.^[13]

It has been hypothesized that progestogens may counteract various effects of estrogens in the brain such as stimulatory and excitatory effects on neuronal activity.^[13] Progesterone moreover has a special position among progestogens concerning such actions due to its inhibitory neurosteroid metabolites and their central depressant effects.^[13] It has been suggested that these actions of progestogens may explain the unfavorable effects on mood that have been observed with these medications in some women.^[13] However, the mutual interactions of estrogens and progestogens in the brain in general are controversial and require more research.^[13]

Progesterone can also have body-wide antiestrogenic effects at very high doses in both women and men via its antigonadotropic effects and consequent suppression of gonadal estrogen production (see below).^[13][53] These antigonadotropic effects are mediated by hyperactivation of the PR.^[13][53]

Antigonadotropic effects

Progestogens have antigonadotropic effects at sufficiently high dosages via activation of the PR and consequent negative feedback on and hence suppression of the hypothalamic–pituitary–gonadal axis (HPG axis).^[53] This results in suppression of gonadotropin secretion and by extension interference with fertility and gonadal sex hormone production.^[53] The minimum ovulation-inhibiting (i.e., contraceptive) dosage of oral progesterone in women, this effect being the result of suppression of the mid-cycle surge in gonadotropin secretion, is 300 mg/day.^[10][54][13] However, this figure is based on limited clinical data.^[10] Conversely, treatment with a high dosage of oral progesterone of 100 mg four times per day (or 400 mg/day total) in men for 10 days that resulted in approximately mid-luteal phase levels of progesterone (7.9–9.4 ng/mL; measured by RIA) did not cause any change in testosterone levels, suggesting that progesterone has little or no antigonadotropic effect in males at typical clinical dosages.^[47][55] In addition, a study found that administration of 1,000 mg/day oral progesterone for 3 months had no significant effect on urinary gonadotropin excretion.^[10] On the other hand, a single 50 mg intramuscular injection of progesterone, which is associated with high progesterone levels of approximately 50 ng/mL (or early- to mid-pregnancy levels),^[56][57][58] resulted in substantial (50–60%) suppression of luteinizing hormone, follicle-stimulating hormone, and testosterone levels in men.^[59] Similarly, continuous or intermittent intravenous injections of 100 to 400 mg/day progesterone for 10 days significantly decreased urinary gonadotropin excretion.^[10] Progestogens in general are able to suppress gonadal testosterone production in men by a maximum of about 70 to 80% or to just above castrate levels when used at sufficiently high dosages.^[60][61]

Antimineralocorticoid activity

Progesterone is a potent antimineralocorticoid.^[20][21] It has similar or higher affinity for the MR than aldosterone, the major endogenous agonist of the receptor.^[13][20] Progesterone produces antimineralocorticoid effects such as natriuresis (excretion of sodium in the urine) at normal physiological concentrations.^[21] A 200 mg dose of oral progesterone is considered to be approximately equivalent in antimineralocorticoid effect to a 25 to 50 mg dose of the potent antimineralocorticoid spironolactone, which itself is a derivative of progesterone.^[62] The antimineralocorticoid effects of progesterone underlie its ability to lower blood pressure and reduce water and salt retention and its potential application in the treatment of hypertension.^[47][13][46] An active metabolite of progesterone, 11-deoxycorticosterone (21-hydroxyprogesterone), is a precursor of aldosterone and has strong mineralocorticoid activity (i.e., is a strong agonist of the MR).^[62] However, it is formed in relatively small amounts, and any such effects produced by it are usually outweighed by the antimineralocorticoid activity of progesterone.^[62]

Glucocorticoid activity

Progesterone is a partial agonist of the glucocorticoid receptor.^{[13][22][23][63][64]} It has about 10 or 11% of the affinity of the corticosteroid dexamethasone for this receptor.^[13][64] However, progesterone appears to show weak or no glucocorticoid activity and no antiglucocorticoid activity in vitro and in animals.^[64]

Androgenic and antiandrogenic activities

Progesterone does not bind importantly to the AR, the biological target of androgens like testosterone and dihydrotestosterone (DHT), and hence does not possess any direct androgenic or antiandrogenic activity.^[13][65][66] This is in contrast to many progestins, such as 19-nortestosterone derivatives (e.g., norethisterone, levonorgestrel, dienogest) and 17α-hydroxyprogesterone derivatives (e.g., cyproterone acetate, medroxyprogesterone acetate), which do bind to the AR and have been associated with androgenic or antiandrogenic effects depending on the progestin in question.^[13][66]

Steroidogenesis, showing progesterone as an intermediate in the biosynthesis of the androgens.^[67]

Although progesterone does not bind directly to the AR, it is a precursor and intermediate, albeit distant, in the biosynthesis of androgens from cholesterol.^[67][68] For this reason, there has been some speculation that exogenous progesterone could be transformed into androgens by certain tissues that express the requisite enzymes.^[68][69] Progesterone is converted by 17α-hydroxylase into 17α-hydroxyprogesterone; 17α-hydroxyprogesterone is converted by 17,20-lyase into androstenedione; and androstenedione is converted by 17β-hydroxysteroid dehydrogenases into testosterone.^[67] CYP17A1, the cytochrome P450 gene that encodes 17α-hydroxylase and 17,20-lyase, is expressed mainly in the gonads (ovaries and testes) and the adrenal glands.^[70] However, while it is theoretically possible that progesterone could be transformed in the body into androgens, clinical studies in which women were treated with 100 to 300 mg/day oral progesterone have found no or only a small increase in levels of 17α-hydroxyprogesterone and no change in androgen levels, including of dehydroepiandrosterone, androstenedione, and testosterone.^[71][72][73] In these studies, levels of estradiol and cortisol, which progesterone is also a precursor of, did not change either, although levels of 11-deoxycorticosterone did increase significantly.^[72][73] In accordance with the lack of changes in androgen levels, progesterone, unlike various progestins,^[13] has not been associated with androgenic effects in clinical studies, including changes in the blood lipid profile or sex hormone-binding globulin levels,^[74][71] acne, skin oiliness, hirsutism, or voice deepening, or induction of teratogenicity (i.e., virilization of female fetuses).^[73][75][76]

5α-Reductase inhibition

Progesterone is a substrate for 5α-reductase, and has been found to act as a competitive inhibitor of this enzyme in vitro in a variety of studies.^[13] In one study, it showed IC₅₀ values of 1,375 nM and 88 nM (in the presence of 50 nM androstenedione as the substrate) for 5α-reductase types 1 and 2, respectively.^[77] 5α-Reductase is highly expressed in skin, hair follicles, and prostate gland, and is responsible for the transformation of testosterone into the several-fold more potent androgen DHT in such tissues.^[78][79] As such, it has been suggested that progesterone might possess some antiandrogenic effects via acting as a 5α-reductase inhibitor.^[13] However, inhibition of 5α-reductase by progesterone is described as a weak effect that has only been demonstrated in vitro and at supraphysiological concentrations.^[80][81] In accordance, physiological levels of circulating progesterone have not been found to importantly influence circulating DHT concentrations.^[82][83]

Congenital 5α-reductase 2 deficiency is a rare intersex condition which is associated with ambiguous genitalia in male fetuses due to a deficiency in DHT production during genital differentiation.^[79] Experimental prenatal exposure to established 5α-reductase inhibitors like finasteride has been found to produce similar feminized genital defects in male animals including rodents and monkeys.^[84] In contrast, exogenous administration of progesterone to pregnant rodents has been found not to produce fetal defects in either male or female pups.^[85][86][10] In addition, endogenous progesterone levels naturally increase to very high concentrations during pregnancy (e.g., 100-fold by term), yet genital defects do not occur.^[87] In accordance, while total concentrations of progesterone in pregnant women at term are around 150 ng/mL (~500 nM), free or unbound and hence bioactive concentrations of progesterone are only about 3 ng/mL (~10 nM) due to the high plasma protein binding of progesterone, and these concentrations are still well below the aforementioned IC₅₀ values for inhibition of 5α-reductase types 1 and 2.^[88][89] As with endogenous progesterone during pregnancy, exogenous administration of additional progesterone during pregnancy has been found not to increase the risk of hypospadias in male infants.^[90]

Although systemic progesterone does not appear to be an effective 5α-reductase inhibitor, topical progesterone may produce potent inhibition of 5α-reductase in the skin due to the very high local concentrations produced.^[91][92] A study found that topical progesterone inhibited 5α-reductase in male pubic skin by more than 70% in vivo.^[92]

Neurosteroid effects and fluctuations

Levels of progesterone (P4), allopregnanolone, and pregnanolone in premenopausal women following a single dose of 200 mg oral progesterone or 400 mg vaginal progesterone (as a suppository).^[93] Allopregnanolone and pregnanolone levels were determined by GC-MS, while progesterone levels were determined by RIA with chromatographic separation via LC–MS.^[93]

Progesterone, through the actions of neurosteroid active metabolites such as allopregnanolone and pregnanolone, is a potent positive allosteric modulator of the GABA_A receptor, the major signaling receptor of the inhibitory neurotransmitter γ-aminobutyric acid (GABA).^[27] It can produce sedative, hypnotic, anxiolytic, euphoric, cognitive-, memory-, and motor-impairing, anticonvulsant, and even anesthetic effects with formation of sufficiently high concentrations of its neurosteroid metabolites and consequent GABA_A receptor potentiation in the brain.^{[47][48][49][50]} These actions and effects are characteristically similar to those of other GABA_A receptor positive allosteric modulators like alcohol, barbiturates, and benzodiazepines.^[50]

Progesterone is extensively metabolized into allopregnanolone and pregnanolone upon oral ingestion due to a large first-pass effect (80–90% or greater metabolism).^[94][95][96] In accordance, with oral administration, allopregnanolone and pregnanolone circulate at higher concentrations than does progesterone.^[13][93] These neurosteroid metabolites of progesterone have relatively short biological half-lives in the circulation.^[97][98] Because of this, there are dramatic and highly supraphysiological spikes in allopregnanolone and pregnanolone concentrations followed by steep declines with each oral intake of progesterone.^[94][95][93] As such, neurosteroid levels fluctuate substantially (e.g., 15-fold in the case of allopregnanolone) and in an unphysiological manner with oral progesterone therapy.^[94][96] In addition, consumption of food with oral progesterone increases its absorption by two-fold, and this may also further amplify fluctuations in neurosteroid levels, particularly if food intake with progesterone is not consistent from dose to dose.^[8]

Similarly to other GABA_A receptor positive allosteric modulators like alcohol, barbiturates, and benzodiazepines, tolerance has been found to develop with exposure to increased levels of allopregnanolone and related inhibitory neurosteroids.^[97][99] This includes downregulation and desensitization of the GABA_A receptor, reduced effects of allopregnanolone and other GABA_A receptor activators (e.g., GABA and benzodiazepines), and rebound or withdrawal effects upon falls in allopregnanolone levels.^[97][99] In addition, changes in allopregnanolone levels have been implicated in adverse neuropsychiatric effects associated with the menstrual cycle (e.g., dysphoria, depression, anxiety, irritability) and postpartum period (e.g., postpartum depression), as well as in catamenial epilepsy (seizures).^[100][101] Low and high levels of allopregnanolone seem to have a neutral effect on mood, whereas moderate levels have a negative effect, which may underlie the symptoms of premenstrual syndrome and premenstrual dysphoric disorder that are observed in 30 to 40% of premenopausal women.^{[100][101][102]} This U-shaped effect on mood appears to be a common property of GABA_A receptor positive allosteric modulators.^[100][101]

In contrast to oral administration, parenteral progesterone, such as with vaginal administration, avoids the first-pass effect, and is not associated with supraphysiological concentrations of neurosteroid metabolites, nor with spikes or marked fluctuations in neurosteroid levels.^[94] Parenteral routes can be used instead of oral administration to avoid adverse effects related to neurosteroid fluctuations if they prove to be problematic.^[96][13] Lower doses of oral progesterone (e.g., 100 mg/day) are also associated with relatively reduced formation of neurosteroid metabolites, and may similarly help to alleviate such side effects.^[13] In addition, the 5α-reductase inhibitor dutasteride, which blocks the production of allopregnanolone (though not of pregnanolone) from progesterone, has been found to diminish symptoms of premenstrual syndrome.^[103]

Pregnenolone, an over-the-counter supplement and close analogue of progesterone, is extensively converted into neurosteroids such as allopregnanolone and pregnanolone with oral administration similarly to progesterone.^{[104][105][106][107]} Conversely, this was not seen with transdermal administration of pregnenolone.^[107]

Oral administration

Progesterone levels measured by RIA and LC–MS following a single dose of 100 mg oral micronized progesterone or vaginal micronized progesterone in postmenopausal women.^[1][117] The progesterone levels with oral progesterone measured by RIA are falsely high due to cross-reactivity with progesterone's metabolites.^[1][117] Conversely, LC–MS is an exact method, and is reliable.^[1][117]

Knowledge about the pharmacokinetics of oral progesterone has been complicated by the use of flawed analytical techniques.^{[121][117][118]} When progesterone is taken orally, due to first-pass metabolism, very high levels of its metabolites occur.^{[121][117][118]} Most previous studies have used a method known as radioimmunoassay (RIA) to measure progesterone levels.^{[121][117][118]} However, RIA has high cross-reactivity and is unable to differentiate between progesterone and metabolites such as allopregnanolone and pregnanolone.^{[121][117][118]} As a result, studies that have assessed the pharmacokinetics of oral progesterone using RIA have reported falsely high progesterone levels and inaccurate dependent pharmacokinetic parameters.^{[121][117][118]} Comparative studies using reliable and exact methods such as liquid chromatography–mass spectrometry (LC–MS) and RIA in conjunction with adequate chromatographic separation (CS) have shown that RIA overestimates levels of progesterone by 5- to 8-fold.^{[121][117][118]} For this reason, the use of reliable assays is mandatory when studying the pharmacokinetics of oral progesterone, and an awareness of these methodological issues is likewise essential in understanding the pharmacokinetics of oral progesterone.^{[121][117][118]} Conversely, the same issues are not applicable to parenteral routes of progesterone such as vaginal administration and intramuscular injection, because these routes are not subject to a first pass and relatively low levels of progesterone metabolites are formed.^{[121][117][118]} Aside from the relevant methodological issues, it is also important to note that when the term "oral progesterone" is used, what is used clinically and what is almost always being referred to, unless noted otherwise, is micronized progesterone suspended in oil.^{[122][13][121]}

The oral bioavailability of progesterone is very low.^[122] Studies using RIA have generally measured the bioavailability of oral progesterone as less than 10%,^[122] with one study reporting values of 6.2 to 8.6%.^[113][8] However, these values are overestimations; a study using LC–MS found that the bioavailability of oral progesterone was only 2.4% relative to vaginal progesterone gel^[1] (and notably not relative to the standard of progesterone by intramuscular injection, which has much higher bioavailability than vaginal progesterone).^[123] The very low bioavailability of oral progesterone is due to the fact that it is poorly absorbed from the gastrointestinal tract and undergoes massive metabolism, resulting in almost complete inactivation during the first pass through the liver.^[122][124] Because of its poor oral bioavailability, very high doses of progesterone must be used by the oral route to achieve significant circulating progesterone levels.^[122] In addition, oral progesterone is always micronized and suspended in oil.^{[122][75][123][125]} This improves the bioavailability of oral progesterone significantly compared to plain milled progesterone, and allows it to be used at practical doses.^[122]

Micronization is the process of reducing the average diameter of the particles of a solid material.^[125] By micronizing progesterone, its particles are made smaller (mainly <10 μM) and its surface area is increased, thereby enhancing absorption from the intestines.^[122][125] Suspension and partial solubilization^[126] of progesterone in oil containing medium- to long-chain fatty acids likewise improves the bioavailability of oral progesterone.^{[13][127][128]} Progesterone is a lipophilic compound and it has been theorized that suspension of progesterone in oil may improve its absorption by the lymphatic system, thereby allowing a portion of oral progesterone to bypass the first pass through the liver and hence enhancing its bioavailability.^{[122][129][130][131]} Compared to plain milled progesterone, peak levels of progesterone following a single 200 mg oral dose were increased 1.4-fold by micronization, 1.2-fold by suspension in oil, and 3.2-fold by the combination of micronization and suspension in oil.^[131] Oral micronized progesterone suspended in oil is rapidly and almost completely absorbed from the intestines.^[10] There is wide interindividual variability in the bioavailability of oral progesterone.^[13][8] As progesterone was not used orally for many decades due to its poor bioavailability (until the introduction of oral micronized progesterone in oil-filled gelatin capsules in 1980),^[123] oral progestins (synthetic progestogens) with improved metabolic stability and high oral bioavailability were developed and have been used clinically instead.^[132]

When oral progesterone is used at typical clinical dosages, only very low levels of progesterone are measured using reliable methods.^{[121][117][118]} Following single doses of oral progesterone, peak levels of progesterone of 1.5 to 2.4 ng/mL with 100 mg and 2.8 to 4.7 ng/mL with 200 mg have been measured using LC–MS, liquid chromatography–tandem mass spectrometry (LC–MS/MS), and RIA with adequate CS.^{[121][133][1]} In one such study, although peak levels of progesterone were 2.2 ng/mL after a single 100 mg dose of oral progesterone, levels of progesterone remained significantly elevated for less than about 4 hours, and the average progesterone levels over a period of 24 hours were only 0.14 ng/mL.^[118][1] For comparison, normal progesterone levels during the luteal phase of the menstrual cycle with LC–MS/MS are 6.7 to 22.2 ng/mL.^[134] When the unreliable method of standalone RIA has been used to measure progesterone levels with oral progesterone, far higher peak levels of 6.5 to 10.2 ng/mL, 13.8 to 19.9 ng/mL, and 32.3 to 49.8 ng/mL have been erroneously observed after single 100, 200, and 300 mg doses, respectively.^[113][8] In addition, progesterone levels have been inaccurately measured by RIA as remaining elevated for 12 to 24 hours.^[1][75] Regardless of assay method, peak levels of progesterone following a dose of oral progesterone occur after about 1 to 3 hours.^[117] The elimination half-life of progesterone in the circulation is very short at a range of about 3 to 90 minutes.^[10] Previous studies using RIA have reported an overestimated half-life of oral progesterone of about 16 to 18 hours.^[75] A subsequent, reliable study using high-performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) reported a half-life of oral progesterone of about 4.6 to 5.2 hours when it was taken with food.^[4] Due to the short half-life and duration of action of oral progesterone, it is often taken in divided doses of two or three times per day.^[75][135]

Because of studies that used RIA, it was incorrectly believed for many years that oral progesterone could easily achieve luteal phase levels of progesterone or beyond and could produce considerable progestogenic effects.^{[121][117][118]} In actuality, the very low levels of progesterone with oral administration, as measured by reliable methods like LC–MS, appear to be insufficient for adequate and full progestogenic effects.^[118][1] This is evidenced by the fact that, in contrast to almost all progestins, an increased risk of endometrial cancer has been observed when oral progesterone is combined with an estrogen in menopausal hormone therapy.^[118][1] This finding indicates that typical clinical dosages of oral progesterone are insufficient for full endometrial protection.^[118][1] However, in spite of the very low levels of progesterone achieved, typical clinical dosages of oral progesterone do seem to be able to adequately prevent estrogen-induced endometrial hyperplasia.^[121][117] On the other hand, oral progesterone fails to produce full endometrial secretory transformation, and is considered to be inappropriate for use in assisted reproduction, whereas vaginal and intramuscular progesterone are effective.^[136][137] Even very high doses of 600 mg/day oral progesterone fail to produce full luteal-phase endometrial changes,^[96] although doses of 300 to 600 mg/day oral progesterone have reportedly been used for luteal support in assisted reproduction.^[137]

The very low levels of progesterone achieved with oral progesterone may also explain the absence of an increase in risk of breast cancer and venous thromboembolism when oral progesterone is added to estrogen therapy in postmenopausal women.^[118] Such risks are increased by progestins, which are PR agonists similarly to progesterone, but have not been found to be increased by oral progesterone.^[117][118] Since typical clinical dosages of oral progesterone achieve only very low levels of progesterone, and progesterone therapy resulting in adequate progesterone levels has never been properly evaluated in sufficiently large clinical studies, it has been said that notion that progesterone somehow differs from progestins and does not increase the risk of breast cancer or venous thromboembolism are unsubstantiated.^{[117][118][133]} Moreover, it has been said that in the absence of adequate data to the contrary, it would be reasonable to consider progesterone at least equivalent to progestins as a potential risk factor for such complications.^{[117][118][133]} Indeed, the French E3N study did observe a significant increase in risk of breast cancer with estrogen and progesterone therapy in postmenopausal women after long-term (>5-year) administration, which is consistent with a weak proliferative effect of oral progesterone on the breasts such that a longer duration of exposure is necessary for a significant increase in breast cancer incidence to manifest.^[117][118]

Progesterone levels with HPLC–MS/MS after a single dose of 200 mg oral progesterone with food in postmenopausal women.^[4] Although not depicted, C_max levels were actually 89.2 ng/mL.^[4]

Progesterone levels with RIA after a single dose of 200 mg oral progesterone with or without food in postmenopausal women.^[8]

When oral progesterone is taken with food instead of taken in a fasting state, it appears that maximal levels of progesterone and its overall bioavailability may be greatly improved.^[8][4] Measured with the unreliable method of RIA, peak levels of progesterone were increased by 5-fold and area-under-the-curve levels of progesterone by 2-fold when oral progesterone was taken with food.^[8] In this RIA study, a single dose of 200 mg oral progesterone in fasting and fed conditions resulted in peak levels of progesterone of 13.4 ng/mL and 69.5 ng/mL, respectively, and AUC_0–24 levels of progesterone of 91.5 ng/mL and 182.5 ng/mL, respectively.^[8] Likewise however, a study using the reliable method of HPLC–MS/MS found that peak levels of progesterone were 69.7 to 89.2 ng/mL and AUC_0–24 levels of progesterone were 111.6 to 125.9 ng/mL after a single dose of 200 mg oral progesterone with a high-fat, high-calorie meal.^[4] These data, which are fairly similar, indicate that the previous findings of the RIA study may not have been fully erroneous and may in considerable part have indeed represented actual levels of progesterone rather than those of its metabolites.^[8][4] On the basis of the HPLC–MS/MS study findings and previous studies using reliable methods, peak levels of progesterone when oral progesterone is taken with food may be on the order of 15- to 20-fold greater than when it is taken in a fasting state.^[4][1][133] The apparent substantial improvement in progesterone levels and bioavailability when oral progesterone is taken with food may be due to enhanced lymphatic absorption, allowing oral progesterone to bypass some first-pass metabolism.^{[8][122][129][130]} In accordance, a related steroid medication, oral testosterone undecanoate in oil-filled capsules, is absorbed by the lymphatic system but must be taken with meals that contain at least a moderate or "normal" amount of fat for adequate bioavailability.^[138][139] Taking oral testosterone undecanoate with food has been found to increase peak testosterone levels by 16-fold and the overall bioavailability by 11-fold relative to when the medication is taken in a fasting state.^[140][141] Although the bioavailability of oral progesterone is greatly increased if it is taken with food, its overall bioavailability is still relatively low, even if measured using RIA.^[14]

Progesterone is metabolized into allopregnanolone and pregnanolone, which are neurosteroids and potent potentiators of the GABA_A receptor.^[48][49] The conversion of progesterone into these metabolites is catalayzed by the enzymes 5α- and 5β-reductase and 3α-hydroxysteroid dehydrogenase, and occurs primarily in the liver, but also in reproductive endocrine tissues, the skin, the brain, and other tissues.^[142] Due to the first pass with oral progesterone, about 90% of progesterone is rapidly transformed into these metabolites, and massive quantities of these neurosteroids are consequently formed and circulate.^[137] It is for this reason that commonly reported side effects of progesterone include dizziness, drowsiness, sedation, somnolence, and fatigue.^[48][49] Both oral and sufficiently high doses of intramuscular progesterone can produce these sedative effects, indicating that first-pass metabolism in the liver is not necessarily essential for the conversion to take place.^{[143][144][145]} However, the levels of these neurosteroids have been found to be comparatively very low with parenteral routes like vaginal and intramuscular progesterone relative to oral progesterone.^[93][144] As with the bioavailability of oral progesterone, there is high interindividual variability in the formation and levels of allopregnanolone and pregnanolone with oral progesterone.^[13] As a result, some individuals may experience considerable central depressant effects with oral progesterone, whereas others may experience few such effects.^[13]

Buccal administration

Progesterone has been studied for use by buccal administration.^[13][146]

Sublingual administration

Levels of progesterone following a single sublingual dose of 50 or 100 mg progesterone in a suspension in women.^[147][148]

A micronized tablet formulation of progesterone marketed under the brand name Luteina is indicated for use by sublingual administration in addition to the vaginal route and is approved for use in Poland and Ukraine.^[149] It is used by the sublingual route at dosages of 50 to 150 mg three to four times per day.^[149][5] A single 100 mg sublingual dose of Luteina has been found to reach mean peak progesterone levels of 13.5 ng/mL after 1 to 4 hours, with an elimination half-life of about 6 to 7 hours.^[149][5]

A number of other studies have also investigated the use of progesterone by sublingual administration.^{[150][151][147][148]} Older studies have also explored sublingual progesterone.^{[152][153][154]} A study of sublingual progesterone for luteal support in patients undergoing embryo transfer found that after sublingual administration of 50 or 100 mg progesterone dissolved in a 1 mL suspension, peak levels of progesterone were reached in 30 to 60 minutes and were on average 17.61 ± 3.78 ng/mL with the 100-mg dose.^[150][151] However, the duration was short, with levels of less than 5 ng/mL at 6 hours, and re-administration had to be done two or three times per day for adequate circulating levels of progesterone to be maintained throughout the day.^[150][151] Another study found that sublingual progesterone had to be administered at a dose of 400 mg every 8 hours to achieve circulating levels similar to those produced by 100 mg/day intramuscular progesterone.^[150] One study administered 400 mg sublingual progesterone three times per day and achieved mean progesterone levels of 57.8 ± 37.4 ng/mL, which were similar to those produced by 50 mg/day intramuscular progesterone.^[151]

Intranasal administration

Progesterone has been evaluated by the intranasal route, in the form of a nasal spray, in one study.^{[150][13][155]}

Transdermal administration

Progesterone levels measured via LC–MS/MS during treatment with 200 mg/day oral progesterone or over-the-counter 80 mg/day topical progesterone cream (Pro-Gest) in postmenopausal women.^[133]

Progesterone for transdermal or topical administration is not approved by the FDA in the United States but is available from custom compounding pharmacies and is also notably available over-the-counter without a prescription in this country.^{[156][157][133]} It is available from these sources in the form of creams and gels.^[156][157] Topical progesterone has been used as a component of menopausal hormone therapy to treat thousands of women in the United States and Europe.^[156] However, these products are unregulated and have not been clinically tested, often with little being known about their pharmacokinetics.^[156] Moreover, the effectiveness of topical progesterone for systemic therapy, as in menopausal hormone therapy, is controversial.^[156][157] Clinical studies have found only very low levels (<3.5 ng/mL) of progesterone in circulation with the use of topical progesterone, and these levels are thought to be insufficient to confer endometrial protection from unopposed estrogen.^[156][157] The range of circulating levels of progesterone that has been observed in clinical studies with various preparations and doses of transdermal progesterone is 0.38 to 3.5 ng/mL.^[123][156]

Although very low levels of progesterone have been observed in venous blood, very high and in fact greatly supraphysiological levels of progesterone have been found in saliva and capillary blood with topical progesterone.^{[156][157][158]} In one study, the levels of progesterone in saliva and capillary blood were 10- and 100-fold greater than levels in venous blood, respectively.^{[156][157][158]} Levels of salivary progesterone that have been observed in clinical studies with transdermal progesterone have ranged from 2.9 to 2,840 ng/mL.^[123] Findings of high salivary and capillary blood levels of progesterone suggest that in spite of the fact that progesterone levels in circulation remain low, and for reasons that are poorly understood, systemic distribution with topically administered progesterone may be occurring somehow and there may be substantial exposure of some tissues, such as the endometrium, to the hormone.^{[156][157][158]} However, the few clinical studies that have assessed the effects of topical progesterone on the endometrium have had mixed findings, and further research is needed to determine whether topical progesterone can confer adequate endometrial protection in menopausal hormone therapy.^[156][157]

Topical progesterone is usually supplied in the form of creams and water-based gels, and the studies in which very low levels of progesterone in circulation were observed with topical progesterone used these formulations.^[156][157] However, a study of topical progesterone in the form of an alcohol-based gel found relatively high concentrations of progesterone in circulation that corresponded to luteal phase levels, and were theoretically sufficient to confer endometrial protection.^[156][157]

A study that investigated the pharmacokinetics of topical progesterone using a hydrophilic (gel), lipophilic, or emulsion-type base found that in all three cases the time to peak concentrations was around 4 hours and the elimination half-life was in the range of 30 to 40 hours.^[7] The venous blood levels observed were very low.^[7]

Because progesterone is a lipophilic and hydrophobic compound, it is not absorbed efficiently by the skin.^[14][150] Moreover, because the body produces about 25 mg progesterone per day on average during the luteal phase, around 50% of the body would need to be used as a surface of absorption to deliver a similar quantity of progesterone.^[14][150] As such, the transdermal route cannot easily achieve adequate circulating progesterone levels, and this makes transdermal progesterone impractical for systemic therapy.^[14][150]

Topical application of progesterone with the intention of systemic therapy should not be equated with local treatment.^[123] Despite the fact that it is not approved for use in menopausal hormone therapy, topical progesterone is registered in some countries under the brand name Progestogel as a 1% gel for direct local application to the breasts to treat premenstrual mastodynia (breast pain).^[159][123] It has been found in clinical studies to inhibit estrogen-induced proliferation of breast epithelial cells and to abolish breast pain and tenderness in women with the condition.^[123] The effectiveness of topical progesterone for this indication may be related in part to the fact that the site of application of topical progesterone has been found to significantly influence its absorption.^[157] A study observed a significant increase in serum levels of progesterone when it was applied as a topical ointment to the breasts but not when it was applied to other areas like the thigh or abdomen.^[157]

Some unregulated topical progesterone products contain "wild yam extract" derived from Dioscorea villosa, but there is no evidence that the human body can convert its active ingredient (diosgenin, the plant steroid that is chemically converted to produce progesterone industrially)^[160] into progesterone.^[161][162]

Vaginal administration

Progesterone for vaginal administration is available in the form of a gel or insert (suppository). With vaginal administration, a 100 mg dose of progesterone results in peak levels at 4 hours and 8 hours after dosing, respectively, with the levels achieved being in the luteal phase range.^[110] Following peak levels, there is a gradual decline in circulating concentrations, and after 24 hours, levels typical of the follicular phase are reached.^[110] The bioavailability of vaginal progesterone is about 40-fold greater than that of oral progesterone.^[163][1] There is a uterine first-pass effect with vaginal progesterone, such that progesterone levels are far greater in the uterus than in the circulation.^[123]

Rectal administration

Progesterone levels after a single vaginal or rectal 100 mg progesterone suppository in women.^[147]

Progesterone can be taken by rectal administration.^{[47][14][110]} A suppository sold under the brand name Cyclogest is indicated for rectal use in addition to the vaginal route.^{[125][164][165]} Daily rectal administration of progesterone is inconvenient and poorly accepted for long-term therapy.^[113][47] Nonetheless, rectal progesterone can be a useful alternative to the vaginal route in the context of vaginal infection, cystitis, recent childbirth, or when barrier contraception methods are used.^[47]

A number of studies have assessed progesterone by the rectal route.^{[166][167][168][169][170][155]} Levels of progesterone following rectal administration have been found to be 6.4 ng/mL after a single 25 mg suppository, 22.5 ng/mL after a single 100 mg suppository, and 19.4 to 20.3 ng/mL after a single 200 mg suppository.^[47] The absorption of the rectal route is variable, with a wide range of maximal concentrations of 15 to 52 ng/mL progesterone after a single rectal dose of 100 mg progesterone.^[14][168] Levels of progesterone peak after 6 to 8 hours and then gradually decrease.^[14][47] Progesterone levels have been found to be similar and non-significantly different after administration of rectal and vaginal suppositories in several studies.^[47]

Progesterone is delivered directly into the circulation when it is absorbed by the lower portion of the rectum and transported by the inferior and middle rectal veins.^[14] Conversely, if it is absorbed by the upper portion of the rectum, progesterone is subject to hepatic first-pass metabolism due to entry into the hepatic portal system via the superior rectal vein.^[14] As such, although rectal administration is a parenteral route, it can still be subject to some first-pass metabolism similarly to oral progesterone.^[14]

Intramuscular injection

Progesterone (P4) levels after a single intramuscular injection of different doses of progesterone in oil in postmenopausal women.^[144]

When used by intramuscular injection, progesterone bypasses first-pass metabolism in the intestines and liver and achieves very high circulating progesterone levels.^[13][123] Levels of progesterone with 100 mg/day intramuscular progesterone were substantially higher than with 800 mg/day vaginal progesterone (about 70 ng/mL and 12 ng/mL, respectively), although local progesterone levels in the uterus were 10 times higher with the vaginal route due to a uterine first-pass effect (around 1.5 ng/mL and almost 12 ng/mL, respectively).^[123] The duration of progesterone is extended by the intramuscular route due to a depot effect in which it is stored locally in adipose tissue, and can be administered once every 1 to 3 days.^[14] The half-life of intramuscular progesterone is significantly longer when it is injected into the gluteal muscles of the buttocks rather than the deltoid muscle of the upper arm.^[14] Intramuscular progesterone has traditionally been the most popular form of progesterone used for luteal support in assisted reproduction in the United States, although vaginal progesterone is also used and effective.^[123][14]

With intramuscular injection of 10 mg progesterone in vegetable oil, maximum plasma concentrations (C_max) are reached at approximately 8 hours after administration, and serum levels remain above baseline for about 24 hours.^[56] Doses of 10, 25, and 50 mg via intramuscular injection have been found to result in average maximal concentrations of 7, 28, and 50 ng/mL, respectively.^[56] With intramuscular injection, a dose of 25 mg results in normal luteal phase serum levels of progesterone within 8 hours, and a 100 mg dose produces mid-pregnancy levels of 40 to 80 ng/mL at peak.^[110] At these doses, levels of progesterone remain elevated above baseline for at least 48 hours (6 ng/mL at this point for 100 mg),^[110] with an elimination half-life of about 22 hours.^[9]

Due to the high concentrations achieved, progesterone by intramuscular injection at the usual clinical dose range is able to suppress gonadotropin secretion from the pituitary gland, demonstrating antigonadotropic efficacy (and therefore suppression of gonadal sex steroid production).^[56]

Intramuscular progesterone often causes pain when injected.^[14] It irritates tissues and is associated with injection site reactions such as changes in skin color, pain, redness, transient indurations (due to inflammation), ecchymosis (bruising/discoloration), and others.^[171][14] Rarely, sterile abscesses can occur.^[14]

An intramuscular suspension formulation of progesterone contained in microspheres is marketed under the brand name ProSphere in Mexico.^{[171][172][173]} It is far longer-lasting than regular intramuscular progesterone and is administered once weekly or once monthly, depending on the indication.^[171]

Subcutaneous injection

Progesterone levels following a single intramuscular injection of 25, 50, or 100 mg progesterone complexed with β-cyclodextrin in an aqueous solution (Prolutex) in postmenopausal women.^[9]

Progesterone levels following a single intramuscular or subcutaneous injection of 100 mg progesterone in an aqueous solution (Prolutex) or oil solution (Prontogest) in postmenopausal women.^[9]

Progesterone can also be administered alternatively via subcutaneous injection, with the aqueous formulation Prolutex in Europe being intended specifically for once-daily administration by this route.^{[9][174][175]} This formulation is rapidly absorbed and has been found to result in higher serum peak progesterone levels relative to intramuscular oil formulations.^[175] In addition, subcutaneous injection of progesterone is considered to be easier, safer (less risk of injection site reactions), and less painful relative to intramuscular injection.^[175] The elimination half-life of this formulation is 13 to 18 hours,^[9] compared to 20 to 28 hours for intramuscular injection in oil.^[8][6][9]

Intrauterine administration

A one-year progesterone intrauterine device (IUD) for hormonal birth control was previously available in the United States and a few other countries under the brand name Progestasert.^[176][177] It was marketed between 1976 and 2001.^[176] The IUD was never widely used due to a relatively high contraceptive failure rate of 2.9% and the requirement of annual replacement.^[176] It contained 38 mg progesterone and released 65 μg progesterone into the uterus per day (adding up to about 35 mg after one year).^[176][177] For comparison, a woman's body produces on average about 25 mg progesterone per day during the luteal phase.^[14][150] While effective as a form of contraception and for decreasing menstrual bleeding and discomfort, long-lived IUDs can fundamentally only deliver small amounts of progesterone per day, and hence intrauterine progesterone cannot achieve adequate circulating progesterone levels and is unsuitable as a form of systemic therapy.^[150]

General (ADME)

Metabolism

See also: Progesterone § Metabolism

With oral administration, progesterone is rapidly metabolized in the gastrointestinal tract and liver.^[159] As many as 30 different metabolites have been found to be formed from progesterone with oral ingestion.^[159] Regardless of the route of administration, 5α-reductase is the major enzyme involved in the metabolism of progesterone and is responsible for approximately 60 to 65% of its metabolism.^[94] 5β-Reductase is also a major enzyme in the metabolism of progesterone.^[94] 5α-Reduction of progesterone occurs predominantly in the intestines (specifically the duodenum), whereas 5β-reduction occurs almost exclusively in the liver.^[94] The metabolites of progesterone produced by 5α-reductase and 5β-reductase (after further transformation by 3α-hydroxysteroid dehydrogenase) are allopregnanolone and pregnanolone, respectively.^[159] With oral administration of progesterone, they occur in circulation at very high and in fact supraphysiological concentrations that are well in excess of those of progesterone itself (peak concentrations of 30 ng/mL for allopregnanolone and 60 ng/mL for pregnanolone versus 12 ng/mL for progesterone at 4 hours after a single 200-mg oral dose of progesterone).^[159]

The percentage constitutions of progesterone and its metabolites as reflected in serum levels have been determined for a single 100 mg dose of oral or vaginal progesterone.^[96] With oral administration, progesterone accounts for less than 20% of the dose in circulation while 5α- and 5β-reduced products like allopregnanolone and pregnanolone account for around 80%.^[96] With vaginal administration, progesterone accounts for around 50% of the dose and 5α- and 5β-reduced metabolites for around 40%.^[96]

A small amount of progesterone is converted by 21-hydroxylase into 11-deoxycorticosterone.^[62][94] Increases in levels of 11-deoxycorticosterone are markedly higher when progesterone is given orally as opposed to via parenteral routes like vaginal or intramuscular injection.^[94] The conversion of progesterone into 11-deoxycorticosterone occurs in the intestines (specifically the duodenum) and in the kidneys.^[62][94] 21-Hydroxylase appears to be absent in the liver, so conversion of progesterone into 11-deoxycorticosterone is thought not to occur in this part of the body.^[94]

The biological half-life of progesterone in the circulation is very short; with intravenous injection, its half-life has ranged widely from 3 to 90 minutes in various studies.^[10] The metabolic clearance rate of progesterone ranges between 2,100 and 2,800 L/day, and is constant across the menstrual cycle.^[10][179]

Metabolism of progesterone in humans[10] Progesterone 17α-Hydroxyprogesterone 11-Deoxycorticosterone Pregnanetriols (8 isomers) 5α-Dihydroprogesterone 5β-Dihydroprogesterone Allopregnanolone Isopregnanolone Pregnanolone Epipregnanolone Pregnanediols (8 isomers) 17α-Hydroxylase 21-Hydroxylase Multiple 5α-Reductase 5β-Reductase 3α-HSD 3β-HSD 3α-HSD 3β-HSD 20α-HSD 20β-HSD This diagram illustrates the metabolic pathways involved in the metabolism of progesterone in humans. In addition to the transformations shown in the diagram, conjugation (sulfation and glucuronidation) occurs with progesterone metabolites that have one or more available hydroxyl (-OH) groups.