Lecture 1

Second Semester

Dr.Narmin Hama Amin

Steroid Hormones

Steroid Hormones

Steroid hormones in mammals are biosynthesized from

cholesterol

Which in turn is made in vivo from acetyl-coenzyme,A

(acetyl-CoA) via the mevalonate pathwayand differ only in

the ring structure and side chains attached to it

Steroids are lipid soluble

They can pass through the cell membrane as they are fat-

soluble,and then bind to steroid hormone receptors,

cholesterol

Types of steroid hormones

Steroid hormones can be grouped into two classes:

1- Sex Hormones(typically made in the gonads or placenta).

a) Estrogens, including estradiol and estrone

b) Progestogens such as progesterone

c) Androgens such as testosterone

2- corticosteroids (typically made in the adrenal cortex

a) Glucocorticoids such as cortisol

b) Mineralocorticoids such as aldosterone

Steroid Biosynthesis

Conversion of cholesterol to pregnenolone is the ratelimiting step in steroid hormone biosynthesis.An enzyme-denoted cytochrome P450scc (SCC stands for side-chain cleavage) mediates the cleavage of the C17 side chain on the D ring of the sterol to provide pregnenolone, the C21 precursor of the steroids.

This enzyme mediates a three-step process involved in the oxidative metabolism of the side chain. Successive hydroxylations at C20 and C22 are followed by oxidative cleavage of the C20–C22 bond, providing pregnenolone.

Pregnenolone can be either directly converted into progesterone or modified for synthesis of GCs, estrogens, and androgens.

Introduction of unsaturation into the A ring leads to the formation of progesterone. Specifically, oxidation of the alcohol at C3 to the ketone provides a substrate in which isomerization of the 5,6-double bond to the 4,5-double bond is facilitated. This transformation is mediated by a bifunctional enzyme, 3β-hydroxysteroid dehydrogenase/5-4 isomerase (3β-HSD). This enzyme can act on several 3-ol-5ene steroids in addition to pregnenolone

Hydroxylation at C17 provides the precursor for both sex steroid hormones and GCs. Cytochrome P450c17 hydroxylates pregnenolone and progesterone to provide the corresponding 17-hydroxylated compounds. 17-Hydroxypregnenolone can be converted to 17-hydroxyprogesterone by 3β-HSD.

If 17α-hydroxyprogesterone is the substrate for the lyase, androstenedione results. The conversion of 17hydroxyprogesterone to androstenedione is limited in humans, although in other species this is an important pathway. DHEA is converted to androstenedione by the action of 3β-HSD.

Androstenedione can either be converted to testosterone by the action of 17β-hydroxysteroid dehydrogenase (17β-HSD) or be transformed into estrone by aromatase, a unique cytochrome P450 that aromatizes the A ring of certain steroid precursors.

Testosterone is aromatized to 17β-estradiol by the same enzyme. 17β-HSD acts on estrone to form 17-estradiol. If testosterone is acted on by 5α-reductase, 5α-dihydrotestosterone (DHT), an androgen important in the prostate, is produced

Steroid Biosynthesis Progestrone , Aldosterone and hydrocorticone

Steroid Biosynthesis Estrogen and testosterone

Gonadotropin-Releasing

Hormone And Gonadotropins

The gonadotropins are peptides that have a close functional

relationship to estrogen, progesterone, and testosterone.

They are called gonadotropins because of their actions on the gonads.

They control ovulation, spermatogenesis, and development of sex organs, and they maintain pregnancy

Included in this group are the following: GnRH

Luteinizing hormone (LH)

Follicle-stimulating hormone (FSH)

Chorionic gonadotropin (CG; hCG is human chorionic gonadotropin)

Gonadotropin-Releasing Hormone

The hypothalamus releases GnRH, a peptide that stimulates the anterior

pituitary to secrete LH and FSH in males and females. This peptide controls and

regulates both male and female reproduction

In females, LH and FSH regulate the menstrual cycle

At the start of the cycle, plasma concentrations of estradiol and other estrogens and progesterone are low.

FSH and LH stimulate several ovarian follicles to enlarge and begin developing more rapidly than others. After a few days, only one follicle continues developing to the release of a mature ovum. The granulosa cells of the maturing follicles begin secreting estrogens, which then cause the uterine endometrium to thicken.

Vaginal and cervical secretions increase. Gonadotropins and estrogen reach their maximum plasma concentrations at about day 14 of the cycle. The release in LH causes the follicle to break open, releasing a mature ovum. Under the stimulation of LH, the follicle changes into the corpus luteum, which begins secreting progesterone as well as estrogen.

The increased concentrations of estrogens and progesterone regulate the hypothalamus and the anterior pituitary by a feedback inhibition process that decreases GnRH, LH, and FSH production. The result is that further ovulation is inhibited. As described later in this chapter, this is the primary mechanism by which steroid birth control products inhibit ovulation.

If fertilization does not occur by about day 25, the corpus luteum

begins to degenerate, slowing down its production of hormones.

The concentrations of estrogens and progesterone become too low

to maintain the vascularization of the endometrium, and

menstruation results.

The pharmacological actions of hCG are essentially the same as

those of LH. In females during pregnancy, the hCG secreted by the

placenta maintains the corpus luteum to continue secretion of

estrogen and progesterone, thus inhibiting ovulation and

menstruation. In males , LH stimulates testosterone synthesis by the

testes, and together, testosterone and LH promote spermatogenesis

(sperm production) and development of the testes.

Testosterone is also essential for the development of secondary sex

characteristics in males. FSH stimulates production of proteins and

nutrients required for sperm maturation.

Estrogens

Endogenous Estrogens:

The active endogenous estrogens are estradiol, estrone, and

estriol.

Estradiol provides the greatest estrogenic activity, with less

activity for estrone, and the least activity with estriol.

Biosynthesis of estrogen

The estrogens are synthesized by the action of the enzyme aromatase on androstenedione or testosterone .

They are normally produced in relatively large quantities in the ovaries and the placenta, in lower amounts in the adrenal glands, and in trace quantities in the testes.

In postmenopausal women, most estrogens are synthesized in adipose tissue and other nonovarian sites.

During the menstrual cycle about 50 to 300 microg/d of estradiol are produced by the ovaries (especially the corpus luteum)

During pregnancy, the estrogen blood levels are up to 1,000 times higher than during the menstrual cycle.

Metabolism of estrogens

SARs of Estrogen

The most potent naturally occurring estrogen in humans for both

the Estrogen Receptor alpha and beta mediated actions is 17beta-

estradiol, followed by estrone and estriol.

Each estrogen contains a phenolic A ring with a hydroxyl group at

carbon 3 and a beta-OH or ketone in position 17 of ring D.

The phenolic A ring is the principal structural feature responsible

for selective, high-affinity binding to both receptors.

Ring A and 3-OH, 17β-OH groups is necessary for activity.

SARs of Estrogen

Activity depends on mode of action

Subcutaneous= Estradiol > Estrone > Estriol

Oral = Estriol > Estradiol > Estrone

Ethinyl estradiol having greatest activity, because of resistant

to metabolism in liver and degradation by enzymes in GIT.

Removal of 3-OH group, decrease the activity.

Compound without steroid nucleus also shows activity.

Estrogen products

Estradiol, United States Pharmacopeia (USP).

• Estradiol, estra-1,3,5(10)-triene-3,17β-diol, is the most active of the natural steroid

estrogens. Although its 17β-OH group is vulnerable to bacterial and enzymatic oxidation to estrone, it can be temporarily protected as an ester at C3 or C17, or permanently protected by adding a 17α-alkyl group (e.g., 17-ethinyl estradiol, the most commonly used estrogen in oral contraceptives).

• The increased oil solubility of the 17β-esters (relative to estradiol) permits the esters to remain in oil at the IM injection site for extended periods.

Ethinyl Estradiol, USP.

• 17α-Ethinyl estradiol has the greatest advantage over other estradiol products of being orally active. It is equal to estradiol in potency by injection but is 15 to 20 times more orally active.

• The primary metabolic path for ethinyl estradiol is 2-hydroxylation by cytochrome P450 isozyme 3A4 (CYP3A4), followed by conversion to the 2- and 3-methyl ethers by catechol-Omethyltransferase.

Diethylstilbestrol.

The synthetic nonsteroidal molecules such as diethylstilbestrol (DES) could

have the same activity as estradiol or other estrogens. DES can be viewed,

however, as a form of estradiol with rings B and C open and a six-carbon ring

D. The activity of DES analogs was explained in 1946.

It was proposed that the distance between the two DES phenol OH groups was

the same as the 3-OH to 17-OH distance of estradiol; therefore, they could

both fit the same receptor.

Although DES was used for many years, it was discovered that the daughters of

women who had taken DES during pregnancy (DES babies) had a high risk of

vaginal, cervical, and uterine abnormalities, along with a low risk of vaginal

clear cell adenocarcinoma. Because of the safety concerns associated with DES,

it was completely removed from the U.S. market in the late 1990s.

DES is still available as an estrogen for use in veterinary medicine.

Selective estrogen receptor modulators

and antiestrogen Tamoxifen Citrate, USP.

Tamoxifen is a triphenylethylene SERM used to treat early and advanced breast

carcinoma in postmenopausal women.

Tamoxifen is used as adjuvant treatment for breast cancer in women following

mastectomy and breast irradiation. It reduces the occurrence of contralateral breast

cancer in patients receiving adjuvant tamoxifen therapy. It is also effective in the

treatment of metastatic breast cancer in both women and men. In premenopausal

women with metastatic breast cancer, tamoxifen is an alternative to oophorectomy or

ovarian irradiation.

The major metabolite of tamoxifen is N-desmethyltamoxifen, which reaches steady-

state levels higher than tamoxifen itself. It is believed that N-desmethyltamoxifen

contributes significantly to the overall antiestrogenic effect. Another metabolite, 4-

hydroxytamoxifen, is a more potent antiestrogen than tamoxifen.

Clomiphene Citrate, USP.

Clomiphene citrate, (Clomid), is used as an ovulation stimulant in women desiring

pregnancy

Raloxifene, USP.

Raloxifene is a benzothiophene derivative that differs slightly from the triphenylethylene SERMs. A key structural difference is the carbonyl ―hinge‖ that connects the modified phenolic side chain to the benzothiophene ring system.

This hinge is the key structural element that leads to the differing actions at the ERs.

Raloxifene, unlike tamoxifen and toremifene, has antagonist properties on the endometrium and breast tissue and agonist properties on bone and the cardiovascular system.

The lack of agonist action on endometrial tissue has been suggested as a reason for the lack of endometrial cancer associated with raloxifene use. Raloxifene is approved for the prevention and treatment of osteoporosis in postmenopausal women.

Bazedoxifene, USP.

• Bazedoxifene, was declared to be ―approvable‖ by the FDA in 2007 for the prevention of postmenopausal osteoporosis. As of mid-2009, final approval in the United States is still pending. Bazedoxifene was approved in Europe in 2009 for the treatment of post–menopausal osteoporosis.

Selective estrogen receptor modulators

and antiestrogen

Selective estrogen receptor modulators

and antiestrogen

Anastrozole, USP.

Anastrozole, was the first specific aromatase inhibitor approved in the United States. It is

indicated for first-line treatment of postmenopausal women with advanced or metastatic

breast cancer, for second-line treatment of postmenopausal patients with advanced breast

cancer who have had disease progression following tamoxifen therapy, and for adjuvant

treatment.

Anastrozole reduces serum estradiol approximately 80% after 14 days of daily dosing.

Because of an elimination half-life of 50 hours, anastrozole is effective with once-daily

dosing (1 mg). Metabolism of anastrozole includes hydroxylation and glucuronidation, as

well as N-dealkylation to produce triazole. The metabolites of anastrozole are inactive.

Letrozole, USP.

Letrozole, (Femara), is used for most of the same indications as anastrozole.

It reduces concentrations of estrogens by 75% to 95%, with maximal suppression

achieved within 2 to 3 days. CYPs 3A4 and 2A6 are involved in the metabolism of

letrozole to the major carbinol metabolite, which is inactive.

Tamoxifen reduces the levels of letrozole significantly if they are used together, so

combination treatment with these agents is not recommended.

Progestins:

Progestin's include the naturally occurring hormone progesterone

Progesterone is secreted mainly by the corpus luteum of the ovary.

The corpus luteum secretes the most progesterone, 20 to 30 mg/d during

the last or ―luteal‖ stage of the menstrual cycle.

Normal men secrete about 1 to 5 mg of progesterone daily

Function: Progesterone is critical for the maintenance of pregnancy by suppressing

menstruation and decreasing uterine contractility.

Progesterone has important actions in the breasts during pregnancy, acting

in conjunction with estrogens to prepare for lactation.

Progesterone has a half-life of only about 5 minutes when taken

orally, because of rapid metabolism.

Biosynthesis

Progestin Products

Progesterone, USP.

Progesterone, pregn-4-en-3,20dione, is so rapidly metabolized that it is not particularly effective orally, being only one twelfth as active as intramuscularly.

An oral formulation of micronized progesterone (Prometrium) is available. Progesterone given intramuscularly can be very irritating. A vaginal gel containing 4% or 8% progesterone offers an alternative dosage form. Progesterone was originally obtained from animal ovaries but is now prepared synthetically from plant sterol precursors.

Progesterone (and all other steroid 4-ene-3-ones) is light sensitive and should be protected from light.

Medroxyprogesterone Acetate, USP.

MPA, 17-acetyloxy-6α-methylpregn-4-ene-3,20-dione (Provera), adds a 6-methyl group to the basic 17-hydroxyprogesterone structure to greatly decrease the rate of reduction of the 4ene-3-one system.

The 17α-acetate group also decreases reduction of the 20-one, similar to the 17α-caproate. MPA is very active orally and has such a long duration of action intramuscularly that it cannot be routinely used intramuscularly for treating many menstrual disorders.

Structural classes—progestins

Hydroxyprogesterone caproate, is much more active and longer acting thanprogesterone

probably because the 17 ester hinders reduction to the 20-ol.

In contrast, hydroxyprogesterone itself lacks progestational activity. The caproate ester is given

only intramuscularly. The ester greatly increases oil solubility, allowing it to be slowly released

from depot preparations

Norethindrone is about 10 times more active than norethynodrel indicating that

isomerization is not as facile in vivo as one might predict. Although they are less active

than progesterone when given subcutaneously, they have the important advantage of being

orally active.

Norgestrel is a racemic mixture, while levonorgestrel is the single active levorotatory

enantiomer. Norgestrel is used only in oral contraceptives. Levonorgestrel is used in both

oral combination birth control products and polymeric implants that provide

contraception for up to 5 years

yasamin

Drospirenone, USP. Drospirenone differs structurally from all the other commercially available progestins. Its

structure is similar to that of spironolactone, an MR antagonist, and it does have

antimineralocorticoid activity as well as progestational activity.

It is also reported to have some antiandrogenic effects

The spirolactone at C17 and the two cyclopropyl groups at C6 C7 and C15-C16 contribute to

these unique actions.

Progesterone Receptor Antagonists and Selective

Progesterone Receptor Modulators

Progesterone receptor antagonists and SPRM.

Mifepristone (Mifeprex), a PR antagonist, is

approved by the FDA to induce abortion in the

first 49 days of pregnancy .

Mifepristone acts directly by antagonizing the

effects of progesterone at PRs, as well as

indirectly by causing a decrease in progesterone

secretion from the corpus luteum. These

combined effects lead to an increase in the level

of prostaglandins, which stimulates uterine

contractions.

Mifepristone treatment is followed by the use of

misoprostol, a prostaglandin E2 analog, to

ensure a complete abortion. Mifepristone also

has antagonist action at GR

Asoprisnil has antagonist properties in breasts

and partial agonist actions in the uterus and acts

an SPRM.

Contraceptive agents

Ovulation Inhibitors and Related

Hormonal Contraceptives

(Minipill).

Combined Estrogen/Progestin

Hormone Replacement Therapy Osteoporosis prevention and treatment

Hormone replacement therapy, heart disease, and cancer—women’s health

initiative trials.