Structure –Function Relationships, Pharmacokinetics, and Potency of Orally and Parenterally Administered Progestogens

~HAPTER 5 z

Structure- Functio n Relationships, Pharmacokinetics, and Potency of Orally and Parenterally Administered Progestogens FRANK Z . STANCZYK

Departmentsof Obstetrics & Gynecology, and Preventive Medicine, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033

I. I N T R O D U C T I O N

progestin (1). In recent years, progestin has been often used when referring to synthetic progestogens such as norethindrone and levonorgestrel (1). This name excludes the "natural" progestogen, which is progesterone. The North American Menopause Society (NAMS) has been using this nomenclature (2). The use of different names to denote a compound with progestational activity can be confusing. It has been proposed that an international consensus be established to determine a single name for all progestational compounds. This would avoid ambiguity and would be in the best interest of communication and information retrieval. An obvious choice is the name progestogen, which would be consistent with the

A variety of progestogens are available for treatment of postmenopausal women, primarily for protecting the endometrium against hyperplasia during estrogen therapy. Progestogens are compounds that exhibit progestational activity. The most widely recognized progestational activity is transformation of proliferative to secretory endometrium in estrogenprimed uteri. Progestogens act primarily by activating the progesterone receptor. The term progestogen has been used interchangeably with other terms that include progestagen, gestogen, gestagen, and T R E A T M E N T OF T H E POSTMENOPAUSAL W O M A N

779

Copyright 9 2007 by Elsevier,Inc. All rights of reproduction in any form reserved.

780

FRANK Z. STANCZYK

name for a compound with estrogenic or androgenic activity (e.g., estrogen or androgen, respectively). In the present chapter, the NAMS nomenclature will be used when referring to progestational compounds. There is a concern that some progestogens cause adverse metabolic effects, including alterations in lipoprotein fractions and carbohydrate intolerance. The choice of progestogen, the dose, and the number of days of administration appear to be important. It is, therefore, helpful to understand the chemical structures of progestogens, what happens to them after they are administered, and their biologic activity. The objective of the present chapter is to discuss progestogens with respect to their classification, structure-function relationships, pharmacokinetics, and potency. Classification of progestogens separates these compounds on the basis of their chemical structures and enables us to have a clearer picture of which types are available for therapeutic use. Knowing the chemical structure of a progestogen allows us to understand better how modification of its structure affects its function. Knowledge about the pharmacokinetics of progestogens informs us about which of them are pro-drugs and provides us with information about two clinically relevant pharmacokinetics parameters, namely, their bioavailability and half-life. Finally, progestogen potency is poorly understood, and some of the misconceptions related to this topic will be addressed.

TABLE 54.1

Classification of Progestogens

Natural: progesterone Synthetic Structurally related to progesterone Pregnane derivatives Acetylated: medroxyprogesterone acetate, megestrol acetate, cyproterone acetate, chlormadinone acetate, medrogestone Nonacetylated: dydrogesterone, medrogestone 19-Norpregnane derivatives Acetylated: nomegestrol acetate, nesterone Nonacetylated: demegestone, promegestone, trimegestone Structurally related to testosterone Ethinylated Estranes: norethindrone, norethindrone acetate, ethynodiol diacetate, tibolone, norethynodrel, lynestrenol, 13-Ethylgonanes: levonorgestrel, desogestrel, norgestimate, gestodene Non-ethinylated: dienogest, drospirenone

II. CLASSIFICATION OF P R O G E S T O G E N S Table 54.1 shows a classification scheme for progestogens. Progestogens can be dMded into two types: natural and synthetic (3-6). There is only one natural progestogen, and that is progesterone; its chemical structure is shown in Fig. 54.1. The term natural, as used here, denotes that the compound is found in a living organism. It should be realized that progesterone is found in humans and certain animals, but not in plants. However, progesterone can be synthesized from plantderived sterols, such as diosgenin, by multiple chemical reactions (Fig. 54.2). A syntbetic progestogen (progestin) is defined

FIGURE 54.2

FIGURE54.1 The natural progestogen,progesterone. here as a progestational compound that is not found in living organisms. Diosgenin is also an important starting compound for the chemical synthesis of a variety of progestins. Because a substantial number of progestins are available for therapeutic use, it is sometimes convenient to

Conversionof diosgenin to progesterone.

CHAPTER 54 Structure-Function Relationships, Pharmacokinetics, and Potency of Progestogens classify them on the basis of their chemical structures into two groups: (a) those related to progesterone and (b) those related to testosterone (3-6). This classification scheme, however, does not necessarily imply the source of the steroid precursor used to synthesize the progestin. For example, the progestin levonorgestrel is synthesized chemically from estrone by multiple chemical reactions. On the other hand, norethindrone is synthesized from diosgenin. Progestins structurally related to progesterone can be subdivided into those with and without a methyl group at carbon 10 (i.e., pregnane and 19-norpregnane derivatives). These derivatives can be subdivided further into those that are acetylated and those that are not. In contrast to the progestins chemically related to progesterone, those structurally related to testosterone can be subdivided into those that contain an ethinyl group at carbon 17 versus those that do not (non-ethinylated). The ethinylated derivatives can be subdivided further into those that have an estrane structure and those with an 18-ethylgonane structure.

781

structure from MPA only in that it has a double bond between carbons 6 and 7. Chlormadinone acetate and cyproterone acetate also have a double bond between carbons 6 and 7, and in addition they both have a chloral group substituted for the methyl group at carbon 6. Cyproterone acetate differs from chlormadinone acetate in that it has a methylene group attached to carbons 1 and 2. A unique progestin that has a methyl group at carbon 10 but is not acetylated is dydrogesterone. This progestin belongs to a group of compounds called retroprogesterones. Their unique feature is that the methyl group at carbon 10 is present in the oL-orientation instead of the [3-orientation, as in progesterone and the pregnane subclass of progestins. Dydrogesterone also has a double bond between carbons 6 and 7. Compared with progesterone, this progestin apparently does not inhibit ovulation when given throughout the menstrual cycle and does not alter the basal body temperature. Thus, the change in spatial orientation of the methyl group at carbon 10 may result in altered peripheral and central effects. Another compound in the non-acetylated group of progestins that contain a methyl group at carbon 10 is medrogestone.

III. STRUCTURE-FUNCTION RELATIONSHIPS OF PROGESTOGENS It is often useful to consider how a change in the chemical structure of a progestogen relates to a change in its biologic activity. This is especially important when considering potency of progestogens, which will be discussed later in this chapter. Figs. 54.3 to 54.7 show the chemical structures of the progestins classified in Table 54.1.

A. Progestins Structurally Related to Progesterone l. PREGNANEDERIVATIVES

As stated earlier, the two reference compounds in the classification of synthetic progestins are progesterone and testosterone. If we begin with the progesterone molecule and add a hydroxyl group at carbon 17, the altered progesterone molecule loses its progestational activity. However, if we acetylate the hydroxyl group, the new molecule acquires some progestational activity and some oral activity. If we go one step further and add a methyl group at carbon 6, the altered molecule, medroxyprogesterone acetate (MPA), acquires relatively high progestational activity and oral activity. Chemical manipulation of the MPA molecule by addition of a double bond between carbons 6 and 7 and/or substitution of the methyl group at carbon 6 with a chloro substituent gives rise to three highly potent progestins that are more potent than MPA: megestrol acetate, chlormadinone acetate, and cyproterone acetate. Megestrol acetate differs in chemical

FIGURE 54.3 Progestinsstructurally related to progesterone: pregnane derivatives-- acetylated and non-acetylated.

782

FRANK Z. STANCZYK

FIGURE 54.4 Progestins structurally related to progesterone: 19-norpregnane derivatives uacetylated and non-acetylated.

FIGURE 54.5

Progestins structurally related to testosterone: ethinylated derivatives - - estranes.

CHAPTER 54 Structure-Function Relationships, Pharmacoldnetics, and Potency of Progestogens

FIGURE 54.6

783

Progestins structurally related to testosterone: ethinylated derivatives m 13-ethylgonanes.

This progestin differs from progesterone in that it contains a methyl group at carbons 6 and 17 and a double bond between carbons 6 and 7.

B. Progestins Structurally Related to T e s t o s t e r o n e 1. ETHINYLATED DERIVATIVES

2. 19-NORPREGNANE DERIVATIVES

The norpregnane derivatives include nomogestrol acetate, demegestone, promegestone, trimegestone, and nesterone. All lack a methyl group at carbon 10. Except for the absence of this group, the chemical structure of nomogestrel acetate is identical to that of megestrol acetate. Nesterone differs from nomogestrol acetate in that it has a methylene group at carbon 16 and lacks the methyl group at carbon 6 and the double bond between carbons 6 and 7. In contrast, the structures of demegestone, promegestone, and trimegestone all have a double bond between carbons 9 and 10 and a methyl group substituted for the acetate group at carbon 17. Promegestone and trimegestone have an additional methyl group on the two-carbon side chain at carbon 17; the penultimate carbon is hydroxylated in the structure of trimegestone.

FIGURE 54.7 Progestins structurally related to testosterone: nonethinylated derivatives.

Using the testosterone molecule as the starting point, we can show how manipulation of its chemical structure alters its biologic activity dramatically. Addition of an ethinyl group to the molecule causes the steroid to lose androgenicity substantially and to acquire both progestational properties and oral activity. The new compound is 17~-ethinyltestosterone, which was given the common name etbisterone. Removal of the methyl group at carbon 10 of ethisterone further increases the progestogenic and oral activities of the molecule and virtually eliminates its androgenicity. The resulting product is norethindrone (the U.S. name), which is also called norethisterone (the name used in Europe and other countries). Substitution of an ethyl group for the methyl group at carbon 13 of norethindrone yields norgestrel, which consists of a racemic mixture of D-(-)-norgestrel (levonorgestrel) and L-(+)-norgestrel (dextronorgestrel) when norgestrel is synthesized (7). Levonorgestrel is the biologically active form of norgestrel and is one of the most potent orally active progestins. Progestins structurally related to norethindrone and levonorgestrel have been synthesized. The two groups are sometimes referred to as the norethindrone and levonorgestrelfamilies. The norethindrone family of progestins is often referred to as estranes because they all have the same 18-carbon steroid nucleus as the parent steroid, estrane. On the other hand, the levonorgestrel family of progestins is sometimes referred to as gonanes. The latter name, however, is not appropriate because all steroids by definition are gonanes since they contain the 4-ring carbon nucleus (gonane). A more appropriate name for these progestins is 13-ethylgonanes (8).

784 In addition to norethindrone, the norethindrone family of progestins also includes norethindrone acetate, ethynodiol diacetate, norethynodrel, and lynestrenol. Norethindrone acetate and ethynodiol diacetate differ from the parent compound by having an acetate group at carbon 3, and at carbons 3 and 17, respectively. Except for a shift in the double bond between carbons 4 and 5 of norethindrone to a double bond between carbons 5 and 10 in the norethynodrel molecule, both of these progestins have identical structures. Lynestrenol differs from norethindrone in structure by the absence of an oxygenated functional group at carbon 3. Another progestin that can be classified in the estrane group is tibolone. Tibolone differs structurally from norethindrone only in that it has a double bond between carbons 5 and 10, instead of 4 and 5, and a methyl group at carbon 7. Although tibolone is not classified in the norethindrone family of progestins, its chemical structure can be viewed as a derivative of norethindrone. In addition to levonorgestrel, other progestins in the levonorgestrel family (13-ethylgonanes) include desogestrel, norgestimate, and gestodene. These compounds are often referred to as the "new" progestins because they were marketed more recently compared with levonorgestrel, norethindrone, and progestins structurally related to norethindrone. In comparison to the chemical structure of levonorgestrel, desogestrel contains no oxygenated functional group at carbon 3 and has a methylene group at carbon 11, whereas norgestimate has an oxime group at carbon 3 and an acetate group at carbon 17. In contrast, gestodene differs from levonorgestrel only in that it has a double bond between carbons 15 and 16.

2. NoN-ETHINYLATEB DERIVATIVES

Two compounds can be included in the non-ethinylated subgroup of progestins; namely, dienogest and drospirenone. When compared with the chemical structure of norethindrone, dienogest contains a cyanomethyl group instead of an ethinyl group at carbon 17, and a double bond between carbons 10 and 11. Drospirenone has the basic chemical structure of the parent compound, androstane (19 carbons) and is an analog of spironolactone. It contains one methylene group attached to carbons 6 and 7 and another attached to carbons 15 and 16. In addition, a carbolactone group is present at carbon 17.

IV. PHARMACOKINETICS OF ORALLY ADMINISTERED PROGESToGENS Considering the wide use of progestogens for postmenopausal hormone therapy and steroidal contraception, surprisingly, there is generally little information about the pharmacokinetics of most orally administered progesto-

FRANKZ. STANCZYK

gens. A variety of pharmacokinetic parameters of a progestogen can be calculated from its serum or plasma levels obtained at frequent intervals after oral administration of the progestogen. The present discussion will focus on the pharmacokinetics of orally administered progestogens and on the following pharmacokinetic parameters: maximum concentration (Cmax), the time to reach Cmax (Tmax), bioavailability, and half-life. These parameters are determined by quantifying the progestogens in serum or plasma obtained at frequent intervals over a 24-hour period after dosing. Cmax and Tmax are obtained from the highest concentration of the drug. Bioavailability and half-life are two clinically relevant pharmacokinetic parameters. Bioavailability is defined as the extent to which an administered drug reaches the systemic circulation after undergoing hepatic first-pass metabolism. It is usually estimated by comparing the areas under the serum or plasma drug concentration-time curves (AUCs) obtained following oral and intravenous administration of a given dose of the drug. Half-life is the time required for a drug's blood level to fall to 50% of its maximal value. Table 54.2 contains a summary of the bioavailabilities and half-lives of different progestogens. There is large intrasubject variability and, to a lesser extent, intrasub]ect variability in circulating levels and pharmacokinetic parameters of all the progestogens that are administered orally. A fivefold difference between two women in serum levels of a progestogen, following oral administration of the same type and dose of progestogen, is not uncommon. It is also important to realize that the pharmacokinetics of progestogens in elderly women (>65 years) is altered compared with adult women and younger postmenopausal women. Elderly women have increased oral bioavailability of drugs due to decreased hepatic cytochrome P450 content; this results in decreased hepatic first-pass metabolism. They also have a more extensive volume of distribution of lipidsoluble drugs and a less extensive volume of distribution of water-soluble drugs. Perhaps the most important cause for altered pharmacokinetics of drugs in the elderly is the decline in renal clearance of a drug.

A. Progesterone Crystalline progesterone is poorly absorbed. However, when it is broken down to minute particles by the process of micronization, its absorption is improved substantially. The micronization process gives rise to a greater surface area of the compound, allowing it to be dissolved in the aqueous medium of the intestine. Increased surface area of a water-insoluble compound such as progesterone can also be achieved by dispersing the compound in watersoluble carriers such as sterols. For example, oral administration of a mixture of progesterone with cholesterol

CHAPTER

785

54 Structure-Function Relationships, Pharmacoldnetics, and Potency of Progestogens TABLE 54.2

Average Bioavailabilities and Half-Lives of Progestogens

Progestogen

Dose ( m g )

Progesterone Medroxyprogesterone acetate Megestrol acetate Cyproterone acetate Chlormadinone acetate Medrogestone Dydrogesterone Trimegestone Norethindrone Levonorgestrel Desogestrel Gestodene Dienogest Drospirenone Nomegestrol acetate

100, 200, 300 10 160 2 2 5 10 0.5 1 0.15-0.25 0.15 0.075 4 3 5

Bioavailability (%)

Half-Life (Hrs)

--* m

16.2-18.3 24 22.3 54.0-78.6 80.1 34.9 14-17 15 8 9.9, 13.2 11.9,23.8 12-14 10.8, 11.6 31.1-32.5 40

~ 100 m ~-100 64 89, 99 62, 76 87, 99 96.2 66 63

*Dashed line indicates that no data were available. pivolate on a lactose carrier increases plasma progesterone levels. Although the processes just described allow progesterone to be readily absorbed, its bioavailability is also limited by the extensive hepatic first-pass metabolism that it undergoes (9). This is due to the presence of a double bond and two ketone groups on the progesterone molecule, making it highly vulnerable to enzymatic reduction by reductases and hydroxysteroid dehydrogenases. These transformations occur in the small intestine by enzymes in bacterial flora and intestinal mucosa, and to a greater extent by hepatic enzymes. The bioavailability of orally administered micronized progesterone is generally considered to be <10%. The extensive intestinal and hepatic metabolism of progesterone requires administration of large doses of this compound compared with other progestins when used therapeutically. Dose proportionality of micronized progesterone has been evaluated in a three-way crossover design study in which doses of 100, 200, or 300 mg of micronized progesterone were ingested daily for 5 days by healthy, fasting, postmenopausal women (Fig. 54.8) (10). The Cmax and AUg0_24h values increased proportionately with increasing dose. Mean Cmaxvalues were 6.5, 13.8, and 32.3 ng/mL, and Tmax values ranged between 2.0 and 2.7 hours. These values meet or exceed the mean levels found in the midluteal phase of a spontaneous menstrual cycle, indicating that the doses administered in that study would be expected to achieve therapeutic circulating progesterone levels. However, after peaking, serum progesterone levels fell precipitously during the first 6 hours. After 6 hours post-treatment, mean progesterone levels were approximately 2, 4.5, and 8 ng/mL, and after 10 hours they were only 1.5, 2.5 and 3.5 ng/mL, for the 100, 200, and 300 mg doses, respectively. At 24 hours, the levels ranged from <1 to <2 ng/mL. The half-lives of elimination were estimated and shown to be 18.3, 16.8, and 16.2 hours, respectively. In the same study, it was shown that on the basis of AUG0_24h the absorption of

progesterone was enhanced twofold and Cmax approximately fivefold, with essentially no change in Tmax, when micronized progesterone was administered with food. Because oral micronized progesterone is metabolized so extensively, it was initially suggested that twice-daily doses were necessary to stabilize serum progesterone levels for endometrium protection (11). However, more recently a dose of 200 to 300 mg has been recommended when administered sequentially 10 to 14 days per month during estrogen therapy (12).

B. Medroxyprogesterone Acetate Considering that it has been used widely throughout the world by many women for many years, it is surprising that so little is known about the pharmacokinetics of MPA.

"~E 100 1 Ig

~

iJJ z 0a: LU

t 10

I-. UJ 0 :E

:::) i,tJ

K/)

o.1 0

4

o

12

16

20

24

TIME (hrs)

54.8 Serum progesterone concentrations (mean + standard error) prior to and followingoral administrationof micronizedprogesterone to postmenopausalwomen,usingthree different doses: 100 mg, 200 mg, and 300 mg. (From ref. 6, with permission.) FIGURE

786

FRANK Z. STANCZYK

Doses of 2.5 or 5 mg of MPA are used continuously combined with conjugated equine estrogens (CEEs), and 2.5, 5, and 10 mg doses are used sequentially with different estrogens. W h e n a single 10-mg tablet containing MPA was administered orally to each of three premenopausal women, mean peak levels ranged from 3 to 5 ng/mL and were attained between 1 and 4 hours (Fig. 54.9) (13). The levels then fell precipitously until 6 hours following dosing and then declined gradually until 24 hours. At this time the levels were in the range of 0.3 to 0.6 ng/mL. MPA levels were also measured in a triple-crossover design study in which four different drugs were administered orally to three

., :t "

C. Norethindrone

h

,i"

,,,

m

IItL1 qt

li' m

k,\

I-i t 1

o

to six postmenopausal women on three different occasions (14). Similar findings to those just described with the 10-mg dose of M P A were obtained after a tablet containing the same dose of M P A combined with 2 mg of estradiol valerate was administered. Cmax and 24-hour medroxyprogesterone values were a little lower when a tablet containing 5 mg of medroxyprogesterone combined with 2 mg of estradiol valerate was ingested. In the circulation, MPA is bound nonspecifically to albumin and undergoes extensive metabolism by A-ring reduction, hydroxylation (primarily at carbons 6 and 20), and conjugation (primarily glucuronidation) (15). The half-life of MPA following a 10-mg oral dose has been reported to be 24 hours (16).

X

",,i

o,

....... T

~4 ....

\

!

,Xi

.

!

\i N

~, .............. r

Plasma norethindrone levels versus time profiles for doses of 1000, 500, and 300 pg of norethindrone administered orally to normal cycling women are shown in Fig. 54.10 (17,18). The highest dose of norethindrone was administered in combination with 120 pg of ethinylestradiol, whereas the other two doses were given without estrogen. A dose response was obtained with all three formulations. Mean peak norethindrone levels of approximately 16, 6, and 4 ng/mL were attained with the 1000-, 500-, and 300-pg doses of norethindrone, respectively, within I to 2 hours following the dosing. Thereafter, all three levels fell precipitously at first and then declined gradually until 24 hours. At that time, the mean levels were approximately 0.5 ng/mL or less. The AUCs calculated for the three different norethindrone doses were shown to be proportional to the administered doses.

#,

....... m

FIO.I

14"

r

:]

|

'

i;

4

?

,-

rA

1"4

? .............

0

,

"

~,

24

".,j "

--

411

I2-

, \ | \

I

',

'X

:i

'"

! " ,i,, !

iX,

0 a

Norelhindrone dose:

,

9 1.0 mo (n-24) 0 05 rng (n,,5) " 0.3 mo (n-5)

10-

! ,,

'!" ,

.,!,

"f "'

8"

^,

)

4,

.................

I

72

2-

O" "O. " ~ ~ ' ~ " - ' - - - - ~ _ . _ . .

FIO.I

0

FIGURE 54.9 Serum concentrations of medroxyprogesterone acetate (MPA) prior to and following oral administration of a 10-mg dose of MPA to 3 premenopausalwomen. Frequentblood sampfingwas obtained on day 1 of treatment, but on subsequent days,blood sampleswere obtained at 2 and 24 hours after dosing. (From ref. 6.)

~ 9 . 3' . 9 ~ 9 9

r9 "--

12' '

" 1'5"

" 18"-" ' 21"

" 24

Hours after administration

FIGURE 54.10 Plasmanorethindrone level-time curves for three different doses of norethindrone administered orally to premenopausal women. (From ref. 6.)

787

CHAPTER 54 Structure-Function Relationships, Pharmacokinetics, and Potency of Progestogens Most of the information about the pharmacokinetics of norethindrone is based on a study in which I mg of norethindrone acetate in combination with 50 lag of ethinylestradiol was administered as a single dose orally and intravenously to a group of 6 women (19). The results show that the absolute bioavailability ranged from 47% to 73%, with a mean standard deviation of 64 +_ 10%. In the same study it was also shown that after the intravenous dose the values (mean ___ standard deviation) for the clearance, apparent volume of distribution, and half-life of elimination were, respectively, 355 _+ 68 mL/ hr/kg (range, 260 to 422 mL/hr/kg), 3.6 _+ 2.0 L/kg (range, 2.09 to 6.90 L/kg), and 8.0 _+ 3.3 hours (range, 5.2 to 12.8 hours). The values for these parameters were not significantly different from those obtained after the oral dosing. It has been shown that following the administration of norethindrone to 10 women, more than 95% of circulating norethindrone is bound to sex hormone-binding globulin (SHBG) and albumin (20). The following distribution of norethindrone in serum was found: SHBG-bound, 35.5 _+ 13.6%; albumin-bound, 60.8 ___ 12.9%; unbound, 3.7 ___ 0.9% (mean _+ standard deviation). Norethindrone undergoes extensive ring A reduction, forming dihydro- and tetrahydro-norethindrone metabolites, which undergo conjugation (7). It also appears to be aromatizable. Low serum ethinylestradiol levels have been measured in postmenopausal women, following oral administration of relatively large doses of norethindrone acetate or norethindrone (21). On the basis of AUCs determined for ethinylestradiol and norethindrone, it was shown that the mean conversion ratio of norethindrone to ethinylestradiol was 0.7% and 1.0% at doses of 5 and 10 rag, respectively. The authors calculated that this corresponds to an oral dose equivalent of about 6 txg of ethinylestradiol per milligram of norethindrone acetate. Similarly, the authors showed that a dose of 5 mg of orally administered norethindrone was equivalent to about 4 Ixg of ethinylestradiol per milligram of norethindrone. On the basis of these calculations, it was estimated that lower doses of norethindrone or its acetate (e.g., 0.5 to 1.0 mg) used in combination with ethinylestradiol would add between 2 and 10 Ixg of ethinylestradiol to the existing ethinylestradiol. It should be realized that the estimations for these lower doses were extrapolated from high doses of norethindrone and its acetate, which were not combined with ethinylestradiol. Nevertheless, the data show that significant amounts of ethinylestradiol were formed from norethindrone, and the amounts formed appear to be highly variable. Conversion of norethindrone acetate to ethinylestradiol was confirmed very recently in premenopausal women who were treated with 10, 20, or 40 mg of the progestin for 7 days in the early follicular phase of the menstrual cycle (22). On the basis of mean serum norethindrone levels of 5.5 ng/mL at 24 hours following oral administration of 10 mg of norethindrone acetate, the approximate conversion of this progestin to ethinylestradiol was calculated to be 0.2%.

D. P r o g e s t i n s S t r u c t u r a l l y R e l a t e d to N o r e t h i n d r o n e It is generally considered that the progestins structurally related to norethindrone are pro-drugs and that their progestational activity is due to norethindrone. After oral administration, norethindrone acetate and ethynodiol diacetare are rapidly converted to the parent compound by esterases during hepatic first-pass metabolism. Mthough less is known about the transformation of lynestrenol and norethynodrel (7), it appears that lynestrenol first undergoes hydroxylation at carbon 3 and then oxidation of the hydroxyl group, forming norethindrone. The possibility that some norethynodrel is metabolized by pathways not involving norethindrone as an intermediate has not been excluded, but there is no evidence in its favor. Thus, it generally appears that the pharmacokinetics of progestins structurally related to norethindrone is determined by the pharmacokinetics of norethindrone.

E. L e v o n o r g e s t r e l As depicted in Fig. 54.11, dose-response curves are obtained when doses of 250, 150, and 75 lag oflevonorgestrel are administered orally to normal cycling women (23-25). The 250-lag dose was administered in the form of the racemic mixture of norgestrel, that is, DL-(+)-norgestrel (500 lag). Also, all three levonorgestrel doses were given in combination with 30 to 50 lag of ethinyl-estradiol. Mean peak levels of approximately 6.0, 3.5, and 2.5 ng/mL were attained at 1 to 3 hours with the 250-, 150-, and 75-lag doses, respectively. At 24 hours, the mean levonorgestrel level was 1 to 2 ng/mL

Levonorgestreldose: 9 0.25 rng:(n,,~) 0 0.15 rng(n.24) 9 0 075 mo (n--,?4)

,,~ = 7 ~

i

-i %,--I._

o, \

O-o..~ "m-l----u_ 6

12

~q~-....,._ .

c:)

18 24 30 HoursafterMministration

-'m~~ll 38

42

48

FIGURE 54.11 Plasmalevonorgestrellevel-time curves for three differ-

ent doses of levonorgestrel administered orally to premenopausalwomen. (From ref. 6.)

788

FRANK Z. STANCZYK

with the highest dose and less than 0.5 ng/mL with the other two doses. Pharmacokinetic data have been obtained for the 150and 250-lag doses oflevonorgestrel (26,27) but are sparse for the 75-lag dose. The mean absolute bioavailability of levonorgestrel has been generally accepted to be virtually 100%. However, it should be realized that this conclusion is based on only two studies. In one of the studies (26), absolute bioavailabilities were determined for the 250- and 150-lag doses of levonorgestrel, each of which was administered in combination with ethinylestradiol to only five women. The absolute bioavailability for the 250-lag dose of levonorgestrel ranged from 72% to 125% (mean + standard deviation, 99 + 20%), and for the 150-lag dose the range was 65% to 108% (mean _+ standard deviation, 89 + 13%). Three of the five subjects who received the 250-lag dose of levonorgestrel had absolute bioavailabilities substantially greater than 100%. It is not clear why the AUCs in those subjects were greater by the oral route compared with the parenteral route, unless there were methodologic problems in the study. Nevertheless, it is incorrect to determine a mean absolute bioavailability using those values. In a second study (27), the absolute bioavailability of levonorgestrel was investigated using a 30-lag dose of the drug in three women; values of 80%, 83%, and 97% were obtained. On the basis of the very limited data on the bioavailability of levonorgestrel, it appears that, in general, this drug is not subject to an appreciable first-pass effect. However, a significant number of women receiving the 150- or 250-lag dose of levonorgestrel may undergo a substantial first-pass effect (absolute bioavailability <75%). Other pharmacokinetic parameters oflevonorgestrel have been calculated following either intravenous or oral dosing. In the same study in which the absolute bioavailabilities of levonorgestrel were determined for the 150- and 250-lag doses (26), the clearance, apparent volume of distribution, and half-life of elimination were found to be 105 + 36 and 113 + 31 mL/hr/kg, 1.9 _+ 0.7 and 1.6 + 0.7 L/kg, and 13.2 + 6.0 and 9.9 + 0.7 hours (mean + standard deviation), for the two doses, respectively, when administered intravenously. The half-life of elimination following oral dosing was similar to the values obtained after intravenous dosing. The following distribution of levonorgestrel in serum has been reported: SHBG-bound, 47.5 _+ 11.7%; albumin-bound, 50.0 ___ 11.0%; unbound, 2.5 +_ 0.7% (mean _+ standard deviation) (20).

F. N o r g e s t i m a t e Very little is known about the pharmacokinetics of orally administered norgestimate. However, it is known that norgestimate is a relatively complex pro-drug. After its oral

administration, the acetate group at carbon 17 is rapidly removed during hepatic first-pass metabolism. The product formed, levonorgestrel-3-oxime, has progestational activity. It has also been referred to as deacetylated norgestimate, and more recently it has been assigned the common name nordgestromin by the pharmaceutical company that markets this progestin. Rapid formation of norelgestromin from norgestimate was demonstrated in a study in which serum levels of norelgestromin were measured after single and multiple oral dose administration of 360 Ixg of norgestimate combined with 70 lxg ethinylestradiol to 10 women (28). Mean peak serum levels of norelgestromin were approximately 4 ng/mL and were attained after about 1 hour; the levels remained elevated as long as 36 hours after treatment. In contrast, peak levels of norgestimate were only about 0.1 ng/mL. It has also been shown that norgestimate is converted to levonorgestrel. In a randomized, comparative pharmacokinetic study, 12 women received single oral doses of 250 Ixg of norgestimate combined with 35 Ixg ethinylestradiol and 250 Ixg of levonorgestrel combined with 50 txg of ethinylestradiol (29). The levonorgestrel AUC ratios were determined after administration of both formulations and were used to calculate the bioavailability of norgestimate-derived levonorgestrel. The results showed that, on the average, about 22% of the dose of administered norgestimate became systemically available as levonorgestrel. More recently, it was demonstrated that substantial amounts (~ 2.5 ng/mL) of serum levonorgestrel levels are found in premenopausal women, following administration of 250 lag of norgestimate combined with ethinylestradiol (35 lag) orally or 150 tag/day of norelgestromin combined with ethinylestradiol (20 lag/day) transdermally (30). In addition to norelgestromin and levonorgestrel, a third progestationally active metabolite of orally administered norgestimate, namely levonorgestrel-17-acetate, is formed. However, it is barely detectable in serum (Wyeth-Ayerst Laboratories, personal communication).

G. D e s o g e s t r e l It is well established that desogestrel is a pro-drug and that its progestational action is mediated through one of its metabolites, namely 3-ketodesogestrel. This metabolite has recently been named etonogestrel by the pharmaceutical company that markets this progestin. Initial evidence for this finding came from a study in which a single 2.5-mg dose of desogestrel was administered to one woman and a peak circulating etonogestrel level of 12.7 ng/mL was attained within 1.5 hours after treatment (31). In contrast, the peak level of desogestrel was only about 0.7 ng/mL. This finding was supported by data showing that 3H-etonogestrel was a major metabolite resulting from incubation of human liver homogenates with 3H-desogestrel (32). Further evidence that

CHAPTER 54 Structure-Function Relationships, Pharmacokinetics, and Potency of Progestogens

desogestrel acts via etonogestrel came from a study in which 10 women received 150 lag of desogestrel in combination with 30 lag of ethinylestradiol and another 10 women received 150 lag of etonogestrel combined with 30 lag of ethinylestradiol (33,34). Each combination was ingested as a single dose, and serum samples were obtained at frequent intervals over a 24-hour period. The results show that desogestrel was undetectable following treatment. In contrast, etonogestrel levels rose and fell, and its mean levels were virtually superimposeable between the two groups of women, even though the intersubject variability in the levels was very high. The bioavailability of desogestrel was determined in a crossover study in which nine women received an oral dose of 150 lag of desogestrel in combination with 30 lag of ethinylestradiol and an intravenous dose of 150 lag etonogestrel combined with 30 lag of ethinylestradiol (35). The absolute bioavailability of etonogestrel was 76 __ 22% (mean _+ standard deviation). In a subsequent study (36), the absolute bioavailability of the same progestin was reported to be 62 _+ 7% (mean __ standard deviation). In the former study (35), the clearance of etonogestrel following its administration intravenously and desogestrel orally was 8.7 _+ 2.9 and 12.1 _+ 4.7 L/hr, respectively. Although the apparent volume of distribution of etonogestrel was not reported in that study, the data were utilized by others to calculate this parameter; a value of 3.0 __ 1.3 L/kg has been reported (37). In another study, the pharmacokinetics of 150 lag of desogestrel administered orally in combination with 30 lag of ethinylestradiol was investigated in 25 women (38). A mean Cm~x of 3.69 __ 0.97 ng/mL and Tm~x of 1.6 _+ 0.97 hours (mean _+ standard deviation) were reported for etonogestrel. In the same study, a reliable estimate of the elimination half-life of etonogestrel was obtained by following its serum levels for up to 72 hours; the mean elimination halflife was 23.8 _+ 5.3 hours. This value is approximately twice as high as that reported previously in a study in which blood samples were collected only up to 24 hours after oral or intravenous dosing (35). In most pharmacokinetic studies, blood sampling is carried out only for 24 hours, which does not provide an accurate half-life value. Serum levels of etonogestrel have also been quantified following multiple dosing with desogestrel (39). A dose of 150 lag of desogestrel in combination with 30 lag of ethinylestradiol was administered to 11 women during 12 continuous treatment cycles. Blood sampling was carried out at frequent intervals on days 1, 10, and 21 in cycles 1, 3, 6, and 12. This experimental design was part of a study (40) designed to compare circulating levels of ethinylestradiol following administration of ethinylestradiol with either desogestrel or gestodene. Comparison of the mean serum etonogestrel levels measured in the samples on days 1, 10, and 21 showed that the levels were relatively low on day 1 of treatment but rose progressively and were higher on day 21 of treatment in all study cycles, except cycle 12.

789

Increases in etonogestrel levels have been attributed to elevated serum levels of SHBG induced by the estrogenic component of the pill. In the study described in the previous paragraph (39), mean SHBG levels rose dramatically (131%) between days I and 10 of the first cycle but increased by only 10% between days 10 and 21 of the same cycle. On day 1 of subsequent cycles, mean SHBG levels were 84% to 115% higher than the SHBG level on day 1 of the first cycle. Also, in the subsequent cycles, the rise in mean SHBG levels between days 1 and 10 was considerably lower (25% to 54%) than the rise observed in the first cycle, and the rise in mean SHBG levels between days 10 and 21 did not exceed 20%. Mean SHBG levels on day 21 of subsequent cycles were 2.5- to 3.3-fold greater than the level on day 1 of the first cycle. The following distribution of etonogestrel in serum has been reported: SHBG-bound, 31.6 + 12.0%; albumin-bound, 65.9 + 11.9%, unbound, 2.5 _+ 0.2% (mean _+ standard deviation) (41).

H. Gestodene It has been shown that after oral administration of 14C-labeled gestodene to 3 women, the compound was converted to reduced and hydroxylated metabolites, but a substantial number of the metabolites were not identified (42). Gestodene was not excreted in urine to any significant extent in an unchanged form, and it was not converted to levonorgestrel. It is generally accepted that gestodene is not a pro-drug. Single-dose pharmacokinetics of gestodene after intravenous and oral administration was investigated in 6 women who received four different treatments: 75 lag of gestodene given intravenously and 50, 75, or 125 lag of gestodene administered orally, each in combination with 30 lag of ethinylestradiol (37). After oral administration of the 50-, 75-, and 125-lag doses, maximum plasma levels of 1.0, 3.6, and 7.0 ng/mL, respectively, were attained between 1.4 and 1.9 hours post-treatment. After the maximum levels were reached, the subsequent levels of gestodene showed two disposition phases with half-lives of approximately 1 hour and 12 to 14 hours for each of the three doses. The absolute bioavailability was 99 + 11% (mean ___ standard deviation) (range 86% to 111%) for the 75-lag oral dose of gestodene. A subsequent study (36) showed that the absolute bioavailability of gestodene was 87 + 19% (mean _+ standard deviation) (range 64% to 126%) when 75 lag of the progestin in combination with 30 lag of ethinylestradiol was administered orally and intravenously to a group of 10 women. The data from these studies show that gestodene is highly bioavailable. Both the clearance and volume of distribution of gestodene were calculated from data obtained in the latter two

790 studies (36,37). Values (mean _+ standard deviation) of 3.4 + 1.5 L/hr and 0.80 + 0.53 mL/min/kg were reported for the clearance and 47.3 _+ 24 L and 0.66 _+ 0.43 L/kg for the volume of distribution, respectively. The values for the clearance and volume of distribution obtained from the first study (36) can also be expressed on the basis of an estimated average weight of 60 kg per subject; they are 0.94 + 0.42 mL/min/kg and 0.79 ___0.40 L/kg, respectively. As mentioned earlier, a multiple-dosing study measured levels of gestodene in serum. The gestodene levels were quantified in samples obtained from 11 women at frequent intervals on days 1, 10, and 21 of several cycles during 12 continuous cycles of treatment with 75 lag of gestodene in combination with 30 lag of ethinylestradiol (43). The results showed a dramatic rise in the mean gestodene levels between day 1 and day 10 and a further rise between day 10 and day 21 in all study cycles. These findings are similar to those obtained when multiple dosing was performed with desogestrel. The multiple-dosing study with gestodene/ethinylestradiol (43) also showed that the increases in mean SHBG levels during the first cycle and in subsequent cycles of treatment were very similar to the increases observed during long-term treatment with desogestrel/ethinylestradiol. There was a 2.7- to 3.0-fold increase in mean SHBG levels on day 21 of each cycle relative to the mean SHBG level on day I of the first cycle. In a recent review, it was pointed out that after administration of 75 lag of gestodene in combination with 30 lag of ethinylestradiol to women, circulating levels of gestodene were high relative to levels of other progestins measured after treatment with combined oral contraceptives (44). These elevated levels occurred after both single and multiple doses of gestodene/ethinylestradiol. The finding is surprising because the 75-lag dose of gestodene is the lowest of any progestin in a combination pill. Two factors may contribute to high circulating levels of gestodene: elevated circulating SHBG levels and a high affinity of SHBG for gestodene. Elevated SHBG levels result from the estrogenic component of combination pills, which has been shown to increase SHBG as much as threefold from pretreatment levels (43). However, a similar increase in SHBG levels was found after treatment with desogestrel/ ethinylestradiol, although serum etonogestrel levels remain relatively low (39). It has been reported that gestodene is distributed in serum as follows: SHBG-bound, 75.3 ___9.1%; albumin-bound, 24.1 _+ 9.1%; unbound, 0.6 _+ 0.1% (mean ___standard deviation) (41). Thus, approximately 75% of total circulating gestodene is bound to SHBG, in contrast to 32% for etonogestrel, 35% for norethindrone, and 47% for levonorgestrel (37). As a consequence, gestodene has a lower metabolic clearance rate and a greater concentration in the circulation. It is probably the affinity of SHBG for gestodene rather than an increase in circulating SHBG levels that is responsible for elevating the serum levels of this progestin.

FRANK Z. STANCZYK

I. Dienogest Studies on the pharmacokinetics of dienogest have been carried out by Oettel and colleagues (45) following oral and intravenous administration of different doses of the drug. A dose response was observed in serum dienogest levels after oral administration of four single doses of dienogest (1, 2, 4, and 8 mg) in randomized order during 4 consecutive menstrual cycles in 12 women. Following administration of the 1 mg dose, the Cmax levels were 23.4 + 5.9 ng/mL and Tmax was 2.2 ___ 1.1 hours (mean _+ standard deviation); the half-life of elimination was 6.5 hours. The absolute bioavailability of dienogest was determined in 17 women who ingested a single dose of 2 tablets, each containing 2 mg of dienogest and 30 Ixg of ethinylestradiol; the average value reported for this parameter was 96.2%. In the same study the average terminal half-life was reported to be 10.8 and 11.6 hours after the oral and intravenous doses, respectively. In the studies by Oettel and colleagues (45), it became obvious that circulating levels of dienogest are relatively high compared with those found with similar doses of other progestins. In blood, dienogest is mostly weakly bound to albumin (45), yet its clearance appears to be lower than that of other progestins (4). No significant accumulation was observed in serum levels of dienogest during its daily intake (45).

J. Drospirenone Pharmacokinetic characteristics of drospirenone (3 mg) combined with ethinylestradiol (30 Ixg) were assessed in 13 women during 13 continuous cycles, each of which consisted of 21 continuous days of treatment, followed by a 7-day pill-free interval (46). Frequent blood sampling was carried out on day 21 of treatment cycles 1, 6, 9, and 13. After administration of the first tablet, the Cmax of drospirenone was 36.9 _+ 4.8 ng/mL (mean _+ standard deviation). This value rose to 87.5 _+ 51.4 ng/mL on day 21 of the first cycle and ranged from 78.7 to 84.2 ng/mL on day 21 of the next three sampling cycles. The corresponding Tma~ values ranged from 1.6 to 1.8 hours, and the half-life of elimination values was 31.1 to 32.5 hours. Other pharmacokinetic characteristics of drospirenone, based on data obtained by the manufacturer of the oral contraceptive containing 3 mg of drospirenone combined with 30 Ixg of ethinylestradiol (Yasmin, Berlex Laboratories, Wayne, NJ), were provided in a review article (47). It was reported that a steady state in circulating drospirenone levels was achieved after 1 week of treatment. Also, a dose response in circulating drospirenone levels was obtained, following oral administration of doses ranging from 1 to 10 ng/mL. In addition, the absolute bioavailability was reported to be 66% on average.

CHAPTER 54 Structure-Function Relationships, Pharmacoldnetics, and Potency of Progestogens

V. PHARMACOKINETICS OF PROGESTOGENS ADMINISTERED PARENTERALLY Although the oral route of progestogen administration is most common, there is increasing interest in obtaining an effective parenteral route for progestogens, primarily to avoid the hepatic first-pass effect. Progestogens can be administered by a variety of parenteral routes, which include intramuscular, vaginal, percutaneous, intranasal, sublingual, and rectal. There is, however, a paucity of data on the pharmacokinetics of progestogens administered parenterally. Three of the more commonly used parenteral routes are discussed here.

A. Intramuscular Route Dose proportionality of progesterone was evaluated in a study of six premenopausal women who received intramuscular administration of 10, 25, 50, or 100 mg of progesterone in oil (48). The results showed that peak concentrations of plasma progesterone for the different doses increased proportionally with increasing dose and were attained within 8 hours of dosing; the peak levels were 7, 28, 50, and 68 ng/mL for each of the doses, respectively. Elevated progesterone levels persisted for 24 to 48 hours, which is consistent with a depot effect of progesterone after intramuscular administration. The data indicate that a single intramuscular injection of 25 mg of progesterone in oil can give rise to circulating progesterone levels comparable with those found during the luteal phase of a spontaneously occurring menstrual cycle. In another study, a comparison was made between intramuscular and oral administration of progesterone (49). Three premenopausal women received a single oral 200-mg dose of micronized progesterone during days 2 to 5 of the menstrual cycle. Two days later, the same women received an intramuscular injection of 25 mg of progesterone in sesame oil. Blood and urine were collected at baseline and at 1, 4, 8, 12, and 24 hours post-treatment. Progesterone was measured in both serum and urine, and pregnanediol glucuronide, the predominant metabolite of progesterone, was measured in urine. Following the oral dose, serum progesterone levels rose rapidly, and peak levels ranging from 8.6 to 11.7 ng/mL were attained at 3 to 4 hours after dosing. By 24 hours, the peak progesterone concentrations fell to approximately 10% of the peak values. In contrast, urinary progesterone and pregnanediol glucuronide levels peaked later (i.e., about 4 to 8 hours after dosing), and at 24 hours the levels of both compounds decreased to 5% to 8% of the peak values. In comparison to the oral dosing, the results of the intramuscular administration showed that serum pro-

791

gesterone levels rose rapidly and were elevated at a level of about 10 ng/mL after 12 hours in all three subjects. Urinary progesterone and pregnanediol glucuronide levels peaked later and were still elevated even 24 hours after dosing. Both the oral and intramuscular routes of administration showed that urinary pregnanediol glucuronide concentrations were much higher and much more variable than those of urinary progesterone. The clinical significance of this study is that luteal phase serum progesterone levels can be achieved by both routes of administration; however, the intramuscular route provides a prolonged effect on these levels.

B. Intravaginal Route Vaginal administration of micronized progesterone has been shown to be an effective, acceptable, and convenient alternative to intramuscular injections (50). In one study, 200 mg of micronized progesterone was administered intravaginally every 6 hours to one group of 15 women and 50 mg of progesterone in oil was injected intramuscularly twice into a second group of women during a 24-hour study period. The results show that after intramuscular administration a rapid rise in serum progesterone levels occurred, with a plateau in levels of about 16 ng/mL after 4 hours of treatment. In contrast, after vaginal administration of progesterone a slow rise in serum progesterone levels occurred, which reached a peak of about 7 ng/mL after 4 hours. In the same study, endometrial concentrations of progesterone were also measured after intravaginal and intramuscular progesterone treatments. The results showed that endometrial progesterone concentrations following intravaginal treatment were considerably higher than after intramuscular treatment, even though serum progesterone levels were higher after intramuscular injection. The high endometrial progesterone concentrations obtained by the intravaginal route indicate the potential importance of this route.

C. Percutaneous Route Progesterone can be administered percutaneously in the form of a topical cream or gel. A major concern about progesterone creams is that serum progesterone levels achieved with the creams are too low to have a secretory effect on the endometrium (51). However, antiproliferative effects on the endometrium have been demonstrated with progesterone creams when circulating levels of progesterone are low (<4 ng/mL). Thus, effects of topical progesterone creams on the endometrium should not be based on serum progesterone levels but rather on histologic examination of the endometrium. Despite the low serum progesterone levels achieved with the creams, salivary progesterone levels are very high, indicating that

792 progesterone levels in serum do not necessarily reflect those in tissues. The mechanism by which the serum progesterone levels remain low is not known. However, one explanation is that after absorption through the skin, the lipophilic ingredients of creams, including progesterone, may have a preference for saturating the fatty layer below the dermis. Because there appears to be rapid uptake and release of steroids by red blood cells passing through capillaries, these cells may play an important role in transporting progesterone to salivary glands and other tissues. In contrast to progesterone creams, progesterone gels are water soluble and appear to enter the microcirculation rapidly, thus giving rise to elevated serum progesterone levels with progesterone doses comparable to those used in creams. Preliminary data were obtained in one study in which one of two different doses (30 or 100 mg) of progesterone gel was applied on the inner portion of the arms of postmenopausal women daily for 1 month (52). Blood samples were obtained at frequent intervals on the first day of treatment. The results showed that following administration of the 30-mg dose of progesterone gel, serum progesterone levels increased by 50% to 100%, but remained in the follicular phase range (<0.5 ng/mL). With the 100-mg progesterone dose, peak progesterone levels of 5.8 to 8.0 ng/mL were formed at 2 to 3 hours after gel application. Similar levels of progesterone were found at 1, 2, and 4 weeks of treatment. Progesterone cream products are readily available over the counter. However, some of these products do not contain progesterone but contain wild yam extract instead. This extract contains diosgenin, which can be converted to progesterone by a series of chemical reactions (see Fig. 54.2) that do not occur in the body. It is important to inform patients using progesterone creams that creams containing wild yam extract without progesterone will not provide a progestational effect.

FRANK Z. STANCZYK

excretion. All these factors play an important role in determining the biologic response of a progestogen. The potency of a drug can be defined as an estimate of a specific biologic effect of the drug. From the technical point of view, a drug's potency is always related to that of a standard drug and is quantified by measuring the difference of the parallel dose-response curves produced by the reference drug and the test drug. A variety of qualitative and quantitative tests using either human or animal species to establish potencies of progestogens have been performed. The tests can be divided into the following three types: (1) in vitro receptor binding assays, (2) bioassays, and (3) clinical tests. Problems encountered in estimating potencies of progestogens using those tests are well recognized (53-56). A major source of the difficulties can be found in the variables associated with each test. In general, the variables include (1) type of animal species; (2) type of tissue or target organ; (3) the specific response of the tissue or target organ; (4) temporal considerations (e.g., time of test following dosing); (5) the route of, and vehicle for, drug administration; (6) the type of reference drug; and (7) assay conditions (e.g., incubation time and temperature). Interlaboratory assay variations preclude precise comparisons of relative potencies. Another important limitation of receptor binding tests or bioassays is that no estrogen is usually added to the progestogen being tested. Addition of estrogen to a progestogen may increase the potency of the progestogen substantially. Difficulties also arise from the fact that potency estimates from animal tests cannot be extrapolated to humans. Although these difficulties exist, there is a substantial amount of information pertaining to potencies of progestogens, and some generalizations can be made. An overview of some of the more relevant data pertaining to progestogen potency follows.

A. Progestational Activity VI. POTENCY OF P R O G E S T O G E N S The efficacy of a progestogen depends on a number of factors. Basic prerequisites for progestational activity of a steroid include adequate affinity for the progesterone receptor, induction of conformational change of the steroidreceptor complex, and duration of binding to DNA. The number of receptors occupied by the steroid is a function of its concentration in the target cell. Intracellular steroid concentration depends on how much steroid enters the cell and is metabolized and stored. The extent to which the steroid enters the cell is, in turn, dependent on its circulating level in a bioavailable (non-SHBG-bound) or free form. The blood level of the steroid depends on its pharmacokinetics; in other words, its absorption and metabolism during the hepatic first pass, rates of distribution and elimination, and

1. RECEPTOR BINDING TESTS

Uteri from various animal species, including humans, in various conditions of age and pretreatment have been used as a source of progesterone receptors for binding studies. In practice, the binding affinity of a test steroid for the progesterone receptor is determined by the concentration of the steroid that corresponds to 50% inhibition of the total binding (IC50) of a radiolabeled progestational marker (e.g., tritiated progesterone) to the receptor. Because progesterone is the natural progestational agent of all mammals, it is an obvious choice to be considered the prototype for comparison. Progesterone initially served as the reference steroid and was used in conjunction with 3Hprogesterone in competitive binding studies with progesterone receptors. However, because most progestins have

CHAPTER 54 Structure-Function Relationships, Pharmacokinetics, and Potency of Progestogens considerably greater progestational activity than progesterone, a highly potent synthetic progestin, referred to as R5020 (17,21-dimethyl- 19-nor-pregna-4,9-diene-3,20-dione), has replaced progesterone as the reference compound. Comparison of the binding affinity of a test steroid for a specific receptor relative to that of a steroid standard is expressed by the relative binding affinity (RBA). Thus, the RBA of a test steroid for uterine progesterone receptors can be calculated from the IC50 of a reference steroid divided by the corresponding IC50 of the test steroid. The ratio is multiplied by 100 and expressed as a percentage. Table 54.3 shows that when R5020 was employed as the reference steroid, etonogestrel, levonorgestrel, MPA, cyproterone acetate, and gestodene were all bound with high affinity to the human uterine progesterone receptors (57). The RBA values of these compounds ranged from 85% to 130% and were 2.1- to 3.2-fold higher than the RBA of progesterone (40%). No measurable binding affinity to the progesterone receptors was demonstrated for desogestrel or norgestimate. The lack of significant binding of norgestimate and desogestrel to the progesterone receptors supports the view that these compounds are pro-drugs and must be transformed to a biologically active form for progestogenic action. A different pattern in hierarchy of RBA values was obtained with the test substances when etonogestrel, gestodene, or levonorgestrel were used as reference steroids (57). On the basis of the data shown in Table 54.3, it appears that gestodene binds to the progesterone receptors with higher affinity than either etonogestrel or levonorgestrel and that etonogestrel has a slightly higher affinity than levonorgestrel. Profound differences in progestational activity are often observed between human and animal tissues used in progestogen potency tests. This is clearly evident in the results of two studies in which RBAs for uterine progesterone receptors were determined (58,59). In one of the studies (58), rabbit

TABLE 54.3

uterine tissue was used, and it was shown that norgestimate was bound to the progesterone receptor with high affinity (RBA, 124%). Also, norelgestromin and levonorgestrel had RBAs of 94% and 541%, respectively. In contrast, data from the other study (59), in which human uterine tissue was used, showed there was no significant binding of norgestimate to the progesterone receptor (RBA, 0.8%) and the binding of norelgestromin was low (RBA, 8%); the RBA of levonorgestrel was 250%. The discrepancy in the potency data of norgestimate and its principal active metabolites between human and rabbit tissues emphasizes the potential problems that may arise when extrapolating animal data to the human.

2. BIOASSAYS

The most common bioassays used to evaluate progestational potency determine the effect of the test compound on uterine glandular proliferation, pregnancy maintenance, delay of parturition, or inhibition of ovulation in rabbits or rats. The most widely used bioassay for progestational agents has been the Clauberg test. This test is based on initial observations made by Clauberg in the 1920s. In 1934 McPhail organized these observations into specific protocols (60). In general, the test procedure involves priming immature female rabbits with estrogen, followed by either oral or parenteral treatment with the test substance. Progestogens induce the development of complicated glandular structures in the estrogenic endometrium with simple glands. A standardized scale for grading glandular proliferation of the rabbit endometrium was provided by McPhail. This scale ranges from 0, which corresponds to no glandular development, to a value of +4, corresponding to maximal glandular development. Active progestational compounds are compared at a dose level that produces a +2 on the McPhail scale. Although the Clauberg test is the most widely used bioassay for progestational agents, it is subject to considerable

Relative Binding Affinities (%) of Different Progestogens for Human Uterine Progesterone Receptor

Compound

R5020

Progesterone Gestodene Levonorgestre1120 R5020 Org2058 Desogestrel Etonogestrel Norgestimate Cyproterone acetate Medroxyprogesterone acetate

40 85 90 100 350 1 130 < 0.1 90 115

ND, not determined. (From ref. 9, with permission.)

793

Etonogestrel

Gestodene

15 110 75 20 ND 0.3 100 < 0.1 ND ND

2 100 100 20 35 <0.1 55 < 0.1 ND ND

Levonorgestrel 18 150 22 ND 0.3 110 < 0.1 ND ND

794 divergence in estimates of potency (61). Interpretation of the test is complicated by the fact that the dose-response curves for commonly employed test substances are not parallel and the maximum response varies. The other commonly used bioassays also have limitations (61). Estrogens are ineffective in the pregnancy maintenance bioassays and will inhibit the active progestogens when administered at sufficient dose levels. The delay of parturition bioassay fails to distinguish among the various progestogens. Finally, the ovulation inhibition bioassay gives different progestogen potencies compared with those obtained in women. 3. CLINICALTESTS

A variety of clinical tests have been used to assess the relative potency of progestogens in women. These include tests based on induction of secretory changes in the endometrium, inhibition of ovulation, and changes in vaginal cytology and cervical mucus. Because endometrial effects of progestogens are relatively simple to assess clinically, the earliest efforts to compare the potency of progestogens relied on the delay-of-menses test, which was first described by Greenblatt et al. (62). The test is based on the fact that uterine bleeding is induced by withdrawal of hormonal support of the endometrium at the end of the menstrual cycle. In general, the assay protocol involves administration of the test substance beginning on the sixth or seventh day after ovulation and continuation of treatment for 3 weeks or more. An effective progestogen will delay menstrual bleeding until 2 to 3 days after treatment is discontinued. The delay-of-menses test was subsequently standardized by Swyer and used for comparative potency evaluations of progestogens; this test is sometimes referred to as the Swyer-Greenblatt test (63). In 1985 the potencies of progestogens were compared on the basis of available human data in the literature obtained from studies in which the effect of progestogens on the delay of menses, subnuclear vacuolization, and glycogen deposition, as well as on lipids and lipoproteins, was assessed (64). In the review of those data, the objective was to examine the scientific evidence, which generally supported the view that the potency of the progestogens used in oral contraceptive formulations marketed in the United States was similar. The conclusion of the review was that norethindrone, norethindrone acetate, and ethynodiol diacetate are approximately equivalent in potency, whereas norgestrel and its biologically active enantiomer, levonorgestrel, are about 5 to 10 and 10 to 20 times as potent as norethindrone, respectively. Another approach to determining progestogen potency from clinical data emerged from a series of studies by King and coworkers (65-71), in which progestogen effects were assessed by analyzing biochemical and morphologic features of endometria from estrogen-primed postmenopausal women. The postmenopausal women were treated daily

FRANKZ. STANCZYK

with either 0.625 or 1.25 mg of CEEs. Effects of at least three different doses of each of five orally administered progestogensmspecifically, norethindrone, levonorgestrel, MPA, dydrogesterone and progesteronemwere tested after 6 days of sequential progestogen treatment during the last 6 to 12 days of the month. The biochemical parameters analyzed included the nuclear estradiol receptor, DNA synthesis, and isocitric and estradiol dehydrogenases. Using data from these studies, King and Whitehead (72) recalculated the results to allow comparisons with corresponding premenopausal, secretory phase values. Relative to a value of 1 for norethindrone, it was shown that levonorgestrel had a potency that was eightfold greater, whereas the potencies ofMPA, dydrogesterone, and progesterone were 10, 50, and 500 times lower, respectively. The progestogen potencies just shown, based on studies by King and Whitehead (72), are consistent with oral progestogen doses that will provide endometrial protection (Table 54.4). In general, the doses are 1 mg, 0.15 mg, 2.5-10 mg, 20 mg, and 100-300 mg for norethindrone (or its acetate), levonorgestrel, MPA, dydrogesterone, and progesterone, respectively. Although these doses are commonly administered therapeutically, the specific dose used depends on whether the progestogen is given sequentially for 10 to 14 days per month or continuously combined, as well as on the type of estrogen administered. In addition to assessment of progestogen potency based on endometrial effects, ovulation inhibition has also been used to compare potency of progestogens. Progestogen doses required for ovulation inhibition have been determined for various combination oral contraceptives containing 30 to 35 p~g ethinylestradiol (73). The data show that high doses are required for norethindrone (400 pLg) and norgestimate (200 p.g) and relatively low doses for gestodene (30 ~g), levonorgestrel (60 ~zg), and desogestrel (60 ~g). On the basis of the clinical evaluations of progestogen potency that have been discussed here, certain generalizations can be made. Progestins structurally related to testosterone are considerably more potent than progesterone and TABLE 54.4

Comparison of Different Progestogen Potencies Determined Experimentally with Corresponding Therapeutic Oral Doses

Progestin Levonorgestrel Norethindrone Medroxyprogesterone acetate Dydrogesterone Progesterone

Experimental

Potency* Basedon dose

8 1

6.7 (0.15 mg) 1 (1 mg)

0.1 0.02 0.002

0.1-0.4 (2.5-10 mg) 0.05 (20 mg) 0.01-0.003 (100- 300 rag)

*Relativeto a valueof 1 for norethindrone.

CHAPTER 54 Structure-Function Relationships, Pharmacokinetics, and Potency of Progestogens

progestins structurally related to progesterone. Among the latter group of progestogens, progesterone is less potent than dydrogesterone, which in turn is less potent than MPA. In the 17ot-ethinylated 19-nortestosterone group, gestodene, levonorgestrel, and desogestrel are more potent than norgestimate, and norethindrone and its pro-drugs (e.g., norethindrone acetate). As pointed out earlier, desogestrel and norgestimate are pro-drugs and their progestational potency is exhibited through their active metabolites.

B. Other Biologic Activities of Progestogens In addition to the characteristic progestational activity of progestogens, they may also possess anti-estrogenic, androgenic, anti-androgenic, and/or anti-mineralocorticoid activities. The most controversial and confusing of these activities is the androgenicity of certain progestins. 1. ANDROGENICAND ANTI-ANDROGENIC ACTIVITY The two most notable androgenic progestins are levonorgestrel and norethindrone. Primary evidence for their androgenicity includes significant binding affinity for the rat prostatic androgen receptor, stimulation of ventral prostate growth in immature castrated rats, and suppression of serum SHBG and lipoprotein levels. However, there are deficiencies in studies of progestin androgenicity. First, data from studies using the rat ventral prostate may not be relevant to a woman's tissues. Also, the doses used in such studies are considerably higher than those required for ovulation inhibition in women, and the estrogenic component is usually not considered. It is well recognized that some progestins have a suppressive effect on hepatic proteins such as SHBG and high-density lipoprotein (HDL), and for this reason they are considered to be androgenic. However, proper interpretation of data from studies investigating androgenic effects of progestins using SHBG as the end point is often lacking. When two progestins are compared in such studies, the data may show that one has a lower percentage of free or bioavailable (non-SHBG-bound) testosterone than the other based on a higher SHBG level, suggesting that the former progestin is less androgenic. However, this suggestion is not valid unless the effect on the total testosterone concentration is taken into account. This has been shown very clearly in a study by Thorneycroft et al. (74)in which the effect of 100 Ixg oflevonorgestrel was compared with 1000 txg of norethindrone acetate, each combined with 20 Ixg of ethinylestradiol, on SHBG and bioavailable testosterone concentrations. The results showed that bioavailable testosterone was suppressed significantly and to the same extent from baseline with each formulation, but by different mechanisms. With the norethindrone acetate formulation, bioavailable testosterone concentrations were suppressed due to decreased SHBG levels. However, with the levonorgestrel formulation,

795

there was a lesser increase in SHBG levels, which was compensated for by a greater suppression of total testosterone levels compared with the norethindrone acetate formulation. It is important to realize that androgenic progestins such as levonorgestrel and norethindrone also have anti-androgenic effects. They suppress serum levels of androgens produced by the adrenal, ovarian, and peripheral compartments. In the study just described (74), the adrenal androgen marker, DHEAS1 and the markers of peripheral androgen production, dihydrotestosterone and 3ot-androstanediol glucuronide, were each similarly and significantly reduced from baseline in the two treatment groups. These data demonstrate that both levonorgestrel and norethindrone, in combination with ethinylestradiol, have profound anti-androgenic effects with respect to androgen production when administered orally. In addition, an important anti-androgenic effect of combined oral contraceptives, including those containing levonorgestrel and norethindrone, is observed clinically in women who show improvement in hirsutism and acne. 2. ESTROGENICACTIVITY

The binding affinity of a variety of progestogens for the uterine estrogen receptors has been investigated using incubation procedures similar to those described for the progesterone receptor and androgen receptor assays. Weak displacement oftritiated estradiol from rabbit uterine cytosol receptors has been demonstrated by norethindrone, norethynodrel, and ethynodiol diacetate but not by levonorgestrel, MPA, and megestrol acetate (56). In a similar study, relative binding affinities of a number of different progestogens were determined for binding to human estrogen receptors. The results showed no affinity of the estrogen receptor for gestodene, levonorgestrel, MPA, or cyproterone acetate (RBAs < 0.1%). The estrogenicity of norethindrone, norethynodrel, and ethynodiol diacetate has also been demonstrated in bioassays (56). Estrogenic effects of both norethynodrel and ethynodiol diacetate have been shown in short-term assays for vaginal cornification in rats (Allen-Doisy vaginal smear test). In contrast, norethindrone causes vaginal changes in the rat only when long-term protocols involving the oral route of administration are used (75). It is not effective by parenteral routes of administration. Clinical evaluation of estrogenicity, as measured by actions on the cervix and changes in vaginal cytology, shows that the estrogenic effects of norethynodrel and ethynodiol diacetate in humans parallel those in the rat (56). However, there is no clear-cut evidence that norethindrone exhibits an estrogenic effect clinically.

3. ANTI-MINERALOCORTICOIDACTIVITY

It is well recognized that progesterone has substantial anti-mineralocorticoid activity. It binds with high affinity to the mineralocorticoid receptor, acting as an antagonist, with

796

FRANK Z. STANCZYK

obvious significance for electrolyte homeostasis, and a variety of mineralocorticoid-related functions in the circulation and central nervous system. Drospirenone is the first progestin with anti-mineralocorticoid activity. This activity is well established at doses that exert progestational and antiovulatory effects in rats and women, respectively (76). T h e anti-aldosterone potency of drospirenone is in the range of 549% to 1095% of that of spironolactone (76).

VII. CONCLUSIONS In the pharmacokinetic studies discussed earlier, large intersubject variability was found in serum or plasma levels and pharmacokinetic parameters, including bioavailability and half-life, of all the progestogens. Higher bioavailability of a progestogen may result in a lower required dosage and less intersubject and intrasubject variability. A long serum half-life may be associated with a greater protective effect in the event of missed pills and may provide more consistent cycle control. Assessment of progestogen potency is problematic due to the large number of variables and assumptions associated with quantitative and qualitative tests of potency. Difficulties also arise when potency estimates from animal tests are extrapolated to humans. Nevertheless, certain clinical tests in which potency of orally administered progestogens is assessed in w o m e n can provide useful information from which optimal therapeutic progestogen doses can be determined. Biologic activities of progestogens, other than progestational effects, have been poorly studied. There is a misconception about progestin androgenicity. This is due primarily to extrapolation of data from rat studies to the human and misinterpretation of data that show effects of progestins on S H B G and free testosterone. A better understanding of the pharmacokinetics and potency of progestins will help clinicians to choose the optimal type and dose of progestogen for individualized treatment of patients.

References 1. Stanczyk FZ, Henzl MR. Use of the name "progestin". Contraception 2001;64:1-2. 2. Position Statement. Role of progestogen in hormone therapy for postmenopausal women: position statement of the North American Menopause Society.Menopause 2003;10:113-132. 3. Henzl MR. Contraceptive hormones and their clinical use. In: Yen SSC, Jaffe RE, eds. Reproductive endocrinology,physiology, pathophysiology, and clinical management. Philadelphia: W.B. Saunders, 1986: 643 -682. 4. StanczykFZ. Introduction: structure-function relationships,metabofism, pharmacokinetics and potency of progestins. Drugs Today 1996;32:1-14. 5. Stanczyk FZ. Pharmacokinetics and potency of progestins used for hormone replacement therapy and contraception. Rev Endocr Metabol Disord 2002;3:211-224.

6. Stanczyk FZ. All progestins are not created equal. Steroids 2003;68: 879-890. 7. Stanczyk FZ, Roy S. Metabolism of levonorgestrel, norethindrone, and structurally related contraceptive steroids. Contraception 1990;42:67-96. 8. Edgren RA, Stanczyk FZ. Nomenclature of the gonane progestins. Contraception 1999;60:313. 9. StanczykFZ. Pharmacokineticsof progesterone administered orally and parenterally. In: Sitruk-Ware R, Mishell DR Jr, eds. Progestins and antiprogestins in dinicalpractice. New York: Marcel-Dekker, 2000:393-400. 10. Simon JA, Robinson DE, Andrews MC, et al. The absorption of oral micronized progesterone: the effect of food, dose proportionality, and comparison with intramuscular progesterone. Fertil Steri11993;60: 26-33. 11. Padwick ML, Endacott J, Matson C, Whitehead MI. Absorption and metabolism of oral progesterone when administered twice daily. Fertil Steri11986;46:402-407. 12. Sitruk-Ware R. Progestins in hormonal replacement therapy and prevention of endometrial disease. In: Sitruk-Ware R, Mishell DR Jr, eds. Progestins and anti-progestins in clinical practice. New York: Marcel Dekker, 2000:269-287. 13. Hiroi M, Stanczyk FZ, Goebelsmann U, et al. Radioimmunoassay of serum medroxyprogesteroneacetate (Provera) in women following oral and intravaginal administration. Steroids 1975;26:373-386. 14. Svensson L-O, Johnson SH, Olsson S-E. Plasma concentrations of medroxyprogesterone acetate, estradiol and estrone following oral administration of Klimaxil,Trisequence/Provera and Divina. A randomized, single-blind, triple cross-over bioavailability study in menopausal women. Maturitas 1994;18:229-238. 15. Mathrubutham M, Fotherby K. Medroxyprogesterone acetate in human serum.J Steroid Biochem 1981;14:783-786. 16. VictorA, Johansson ED. Pharmacokineticobservations on medroxyprogesterone acetate administered orally and intravaginally. Contraception 1976;14:319-329. 17. Stanczyk FZ, Mroszczak EJ, Ling T, et al. Plasma levels and pharmacokinetics of norethindrone and ethinylestradiol administered in solution and as tablets to women. Contraception 1983;28:241-251. 18. Odlind V, Weiner E, Victor A, Johansson EDB. Plasma levels of norethindrone after single oral dose administration of norethindrone and lynestrenol. Clin Endocrino11979;10:29- 38. 19. Back DJ, Brackenridge AM, Crawford FE, et al. Kinetics of norethindrone in women. II. Single-dose kinetics. Clin Pharmacol Ther 1978;24: 448-453. 20. Hammond GL, Lahteenmaki PL, Luukkainen T, et al. Distribution and percentages of non-protein bound contraceptive steroids in human serum.J Steroid Biochem 1982;17:375-380. 21. Kuhnz W, Huener A, Humpel M, Seifert W, Michaelis K. In vivo conversion of norethisterone and norethisterone acetate to ethinyl estradiol in postmenopausal women. Contraception 1997;56:379- 385. 22. Chu MC, Nakhuda GS, Zhang X, Stanczyk FZ, Lobo RA. Formation of ethinyl estradiol in women during treatment with norethindrone acetate.J Clin Endocrinol Metab, in press. 23. Humpel M, Wendr H, Dogs G, et al. Intraindividual comparison of pharmacokinetic parameters of d-norgestrel, lynestrenol and cyproterone acetate in 6 women. Contraception 1977;16:199-215. 24. Goebelsmann U, Hoffman D, Chiang S. Woutersz T. The relative bioavailability of levonorgestrel and ethinyl estradiol administered as a low-dose combination oral contraceptive. Contraception 1986;34: 341-351. 25. Stanczyk FZ, Lobo RA, Chiang ST, Woutersz TB. Pharmacokinetic comparison of two triphasic oral contraceptive formulations containing levonorgestrel and ethinylestradiol. Contraception 1990;41:39-53. 26. Back DJ, Bates M, Breckenridge AM, Hall JM, Maclver M. Levonorgestrel and ethinylestradiol in women m studies with Ovran and Ovranette. Contraception 1981;23:229- 239.

CHAPTER 54 Structure-Function Relationships, Pharmacokinetics, and Potency of Progestogens 27. Humpel M, Wendt H, Pommerenke G, Weib CHR, Speck U. Investigations of pharmacokinetics of levonorgestrel to specific consideration of a possible first pass effect in women. Contraception 1978;17:207-220. 28. McGuire JL, Phillips A, Hahn DW, et al. Pharmacologic and pharmacokinetic characteristics of norgestimate. Am J Obstet Gynecol 1990; 163:2127-2131. 29. Kuhnz W, Blode H, Mahler M. Systemic availability of levonorgestrel after single oral administration of a norgestimate-containing oral contraceptive to 12 women. Contraception 1993b;47:283-294. 30. White T, Ozel B, Jain JK, Stanczyk FZ. Effects of transdermal and oral steroidal contraceptives on estrogen-sensitive hepatic proteins, levonorgestrel, serum-binding globulins, and C-reactive protein. Contraception 2006;74:293-296. 31. Viinikka L. Radioimmunoassay of a new progestogen, ORG 2969, and its metabolite.J Steroid Biochem 1978;10:353-357. 32. Viinikka L. Metabolism of a new synthetic progestogen, ORG 2969, by human liver in vitro. J Steroid Biochem 1979;10:353-357. 33. Hasenack HG, Bosch AMG, Kaar K. Serum levels of etonogestrel after oral administration of desogestrel and etonogestrel. Contraception 1986;33:591-596. 34. Hammerstein J. Prodrugs: advantage or disadvantage? Am J Obstet Gyneco11990;163:2198 - 2203. 35. Back DJ, Grimmer SFM, Shenoy N, Orme MLE. Plasma concentrations of etonogestrel after oral administration of desogestrel and intravenous administration of etonogestrel. Contraception 1987;35:619- 626. 36. Orme M, Back DJ, Ward S, Green S. The pharmacokinetics of ethinyl estradiol in the presence and absence of gestodene and desogestrel. Contraception 1991;43:305 - 316. 37. Tauber U, Tack JW, Matthes H. Single dose pharmacokinetics of gestodene in women after intravenous and oral administration. Contraception 1989;40:461-479. 38. Bergink W, Assendorp R, Koosterboer L, et al. Serum pharmacokinetics of orally administered desogestrel and binding of contraceptive progestogens to sex hormone-binding globulin. Am J Obstet Gynecol 1990;163:2132-2137. 39. Kuhl H, Jung-Hoffman C, Heidt E Serum levels of etonogestrel and SHBG during 12 cycles of treatment with 30 pg desogestrel. Contraception 1988a;38:381 - 390. 40. Jung-Hoffman C, Kuhl H. Interaction with the pharmacokinetics of ethinyl estradiol and progestogens contained in oral contraceptives. Contraception 1989;40:299- 312. 41. Kuhnz W, Pfeffer M, Al-Yacoub G. Protein binding of the contraceptive steroids gestodene, 3-keto-desogestrel and ethinylestradiol in human serum.J Steroid Biochem 1990;35:313-318. 42. Dusterberg B, Tack J-W, Krause W, Humpel M. Pharmacokinetics and biotransformation of gestodene in man. In: Elstein M, ed. Gestodene." development of a new gestodene-containing lozo-dose oral contraceptive. London: Parthenon, 1987:35-44. 43. Kuhl H, Jung-Hoffman C, Heidt E Alterations in the serum levels of gestodene and SHBG during 12 cycles of treatment with 30 mg ethinyl estradiol and 75 mg gestodene. Contraception 1988b;38:477-486. 44. Fotherby K. Potency and pharmacokinetics of gestagens. Contraception 1990;41:533-550. 45. Oettel M, Bervoas-Mart/n S, Elger W, et al. A 19-norprogestin without a 17a-ethinyl group. I: Dienogest from a pharmacokinetic point of view. DrugsToday 1995;31:499-516. 46. Blode H, Wuttke W, Loock W, R611G, Heithecker R. A 1-year pharmacokinetic investigation of a novel oral contraceptive containing drospirenone in healthy female volunteers. Eur J Contracept Reprod Health Care 2000;5:256-264. 47. Krattenmacher R. Drospirenone: pharmacology and pharmacokinetics of a unique progestogen. Contraception 2000;62:29-38. 48. Nillius SJ, Johansson EDB. Plasma levels of progesterone after vaginal, rectal, or intramuscular administration of progesterone. Am J Obstet Gyneco11971;110:470-477.

797

49. Stanczyk FZ, Gentzschein E, Ary BA, et al. Urinary progesterone and pregnanediol. Use for monitoring progesterone treatment. J Reprod Med 1997;42:216-222. 50. Miles RA, Paulson RJ, Lobo RA. Pharmacokinetics and endometrial tissue levels of progesterone after administration by intramuscular and vaginal routes: a comparative study. Fertil Steri11994;62:485-490. 51. Stanczyk FZ, Paulson R, Roy S. Percutaneous administration of progesterone: blood levels and endometrial protection. Menopause 2005; 12:232-237. 52. Bello SM, Merzow G, Shoupe D, Stanczyk FZ. Administration of progesterone by use of percutaneous gel in postmenopausal women. Proceedings of the Annual Meeting of the Pacific Coast Fertility Society, Indian Wells, CA, April 9-13, 1997. 53. Edgren RA, Sturtevant FM. Potencies of oral contraceptives. Am J Obstet Gyneco11976;125:1029-1038. 54. Edgren RA. Progestational potency of oral contraceptives: a polemic. IntJ Ferti11978;23:162-169. 55. Edgren RA. Relative potencies of oral contraceptives. In: Moghissi KS, ed. Controversiesto contraception. Baltimore: Williams &Wilkins, 1979: 1-19. 56. Edgren RA. Progestins. In: Givens JR, ed. Clinical use of sex steroids. Chicago: Year Book Medical, 1980:1-29. 57. Juchem M, Pollow K. Binding of oral contraceptive progestogens to serum proteins and cytoplasmic receptor. AmJ Obstet Gyneco11990;163: 2171-2183. 58. Phillips A, Demarest DW, Hahn F, Wong F, McGuire JL. Progestational and androgenic receptor binding affinities and in vivo activities of norgestimate and other progestins. Contraception 1990;41: 399-410. 59. Juchem M, PoUow E, Elger W, Hoffman G, Mobus V. Receptor binding of norgestimatema new orally active synthetic progestational compound. Contraception 1993;47:283-294. 60. McPhail MK. The assay of progestin. J Physio11934;83:145-156. 61. Edgren RA. Issues in animal pharmacology. In: Goldzieher JW, Fotherby K, eds. Pharmacology of the contraceptive steroids. New York: Raven, 1994;81-97. 62. Greenblatt RB, Jungck EC, Barfield WE. A new test of efficacy of progestational compounds. Ann NYAcad Sci 1958;71:717. 63. Swyer GIM, Little V. Clinical assessment of relative potency of progestogens. J Reprod Fertil Supp11968;5:63-68. 64. Dorflinger LJ. Relative potency of progestins used in oral contraceptives. Contraception 1985;31:557-570. 65. Whitehead MI, Townsend PT, Pryse-Davies J, Ryder TA, King RJB. Effects of estrogens and progestins on the biochemistry and morphology of the postmenopausal endometrium. N EnglJ ivied 1981; 305:1599. 66. Whitehead MI, Townsend PT, Pryse-Davies J, et al. Effects of various types and dosages of progestins on the postmenopausal endometrium. J Reprod Med 1982;27:539. 67. Lane G, Siddle NC, Ryder TA, et al. Dose dependent effects of oral progesterone on the oestrogenised postmenopausal endometrium. Br MedJ 1983;287:1241. 68. Whitehead M, Lane G, Siddle N, Townsend P, King R. Avoidance of endometrial hyperstimulation in estrogen-treated postmenopausal women. Semin Reprod Endocrino11983a;1:41. 69. Whitehead MI, Siddle NC, Townsend PT, Lane G, King RJB. The use of progestins and progesterone in the treatment of climacteric and postmenopausal symptoms. In: Bardin CW, Milgrom E, Mauvais Jarvis P, eds. Progesteroneandprogestins. New York: Raven, 1983b:277. 70. Lane G, Siddle NC, Ryder TA, et al. Effects of dydrogesterone on the oestrogenized postmenopausal endometrium. Br J Obstet Gynaecol 1986a;93:55.

798 71. Lane G, Siddle NC, Ryder TA, et al. Is Provera the ideal progestin for addition to postmenopausal estrogen therapy? Fertil Steril 1986b; 45:345. 72. King RJB, Whitehead MI. Assessment of the potency of orally administered progestins in women. Fertility and Sterility 1986;46: 1062-1066. 73. Teichman AT. Levonorgestrel. Stuart-George Thieme Verlag. 1990;43.

FRANK Z. STANCZYK 74. Thorneycroft IH, Stanczyk FZ, Bradshaw KD, et al. Effect of lowdose contraceptives on androgenic markers and acne. Contraception 1999;60:255-262. 75. Jones RC, Edgren RA. The effects of various steroids on the vaginal histology in the rat. Fertil Steri11973;24:284. 76. Elger W, Beier S, Pollow K, et al. Conception and pharmacodynamic profile of drospirenone. Steroids 2003;68:891-905.