Female and Male Reproductive Parts Medicinal Chemistry

Estrogens — steroid classification

C-18 steroids; characterized by an aromatic A ring with a 3-phenolic hydroxyl group.

Progestins — steroid classification

C-21 steroids; characterized by an unsaturated 3-keto-4-ene structure in the A ring and a ketone at C-20.

Androgens — steroid classification

C-19 steroids; have ketone and hydroxyl groups at the C-3 and C-17 positions.

Sex hormone biosynthesis — starting material

All androgens, estrogens, and progestins are synthesized from cholesterol via pregnenolone, stimulated by LH.

Aromatase enzyme — role in sex hormone biosynthesis

Converts androstenedione → estrone and testosterone → 17β-estradiol.

Placenta — role during pregnancy

Main source of estrogen and progesterone during pregnancy.

17β-Estradiol metabolism — oxidation product

Oxidized to estrone (less potent) by estradiol dehydrogenase.

17β-Estradiol metabolism — 16α-hydroxylation product

Forms estriol (weak estrogen); major metabolite excreted in urine.

17β-Estradiol metabolism — CYP3A4 products

Oxidation at positions 2 or 4 produces catechol estrogens (2- and 4-hydroxyestrogens).

17β-Estradiol — excretion forms

Excreted as glucuronide, sulfate, and glutathione conjugates in urine.

Progesterone metabolism — major urinary metabolite

5β-pregnane-3α,20-diol glucuronide (inactive).

Progesterone — oral activity

Ineffective orally; completely metabolized in one hepatic passage; predominantly given parenterally.

Estrogen SAR — C-3 phenol group role

Provides hydrogen bond donor (HBD) interactions with Glu353 and Arg394 of the estrogen receptor.

Estrogen SAR — optimal distance between hydroxyl groups

10.3 to 12.1 Å between the C-3 and C-17 hydroxyl groups for maximal ER binding.

Estrogen SAR — C-17 hydroxyl group orientation

Must be in the β-orientation; provides hydrogen bond acceptor (HBA) with His524.

Estrogen SAR — hydrophobic spacer role

Flat hydrophobic bulk (planar-rigid system) binds ER through hydrophobic interactions.

trans-Diethylstilbestrol (DES) vs. cis-DES — potency difference

Trans-isomer is 10× more potent than the cis-isomer because it more closely resembles 17β-estradiol.

17β-Estradiol — oral bioavailability limitation

Limited oral bioavailability; degraded by GI tract microorganisms and promptly metabolized by the liver.

17β-Estradiol — clinical uses

Prevention of postmenopausal osteoporosis; treatment of menopausal symptoms (e.g., hot flashes).

Ethinyl estradiol (EE) — structural modification and effect

17α-ethinyl group added to 17β-estradiol; blocks C-17 oxidation during first-pass metabolism → orally active; most used estrogen in oral contraceptives.

Mestranol — classification and mechanism

3-methoxy ether of ethinyl estradiol; orally active prodrug (demethylated in vivo to EE).

17β-Estradiol esters (cypionate, valerate) — route and advantage

Given IM; slowly hydrolyzed in vivo to release free estradiol; prolonged estrogenic action up to 4 weeks.

Estradiol acetate (Femring) — route and use

Cured silicone elastomer vaginal ring; treats moderate-to-severe vasomotor symptoms or vaginal atrophy due to menopause.

Premarin — composition

Mixture of conjugated estrogens from pregnant mare urine; ~50-65% sodium estrone sulfate, ~20-35% sodium equilin sulfate, plus four others.

Conjugated estrogens (Premarin, Menest, Estropipate) — mechanism of oral activity

Sulfate ester prodrugs hydrolyzed in the intestine to active phenols.

Estropipate — chemical description

Piperazine salt of estrone sulfate; orally active prodrug.

Progesterone SAR — key structural features for progestational activity

Pregnane nucleus (21-C), 3-keto-4-ene A ring, and 17β-ketone are all required.

17α-Acetoxyprogesterone — advantage over progesterone

Orally active; the 17α-acyl group slows metabolism of the C-20 ketone, increasing duration of action.

17α-Hydroxyprogesterone caproate (Makena) — administration

Given IM or SC injection; slow hydrolysis prolongs action.

Medroxyprogesterone acetate (MPA/Depo-Provera) — structural modification

6α-methyl group added to 17α-acetoxyprogesterone; hinders metabolism and increases lipid solubility, enhancing oral contraceptive action.

Megestrol acetate (Megace) — structural feature enhancing activity

6,7 double bond on a 6-substituted 17α-acetoxyprogesterone; enhances progestational activity.

Ethisterone — classification and origin

17α-ethynyl testosterone; synthesized from androstane male sex hormones; orally active progestin with weak androgen.

19-Norprogesterone vs. progesterone — activity comparison

19-Norprogesterone (C-19 methyl replaced by H) is 8× more active parenterally; absence of 19β-methyl increases progestational activity.

Norethindrone and Norethynodrel — classification

17α-ethynyl 19-norsteroids; first-generation progestins; orally active; used in oral contraceptives with estrogens (e.g., mestranol).

17α-ethynyl group on 19-norsteroids — effects on activity

Blocks metabolism to 17-ketone, increases progestational activity, and decreases androgenic activity.

Norethindrone acetate — classification and metabolism

Prodrug; rapidly deacetylated to norethindrone after oral administration.

Levonorgestrel — structural origin and activity

18β-ethyl analog of norethindrone (second generation); active levo-stereoisomer of norgestrel; 18β-ethyl decreases androgenic and increases progestational activity.

Norgestimate — metabolism to active form

17β-ester cleaved to active 17β-hydroxyl in intestine/liver; oxime group can bind PR directly or is metabolized to 3-keto in the liver.

Norelgestromin (Ortho Evra patch) — metabolic relationship

Third-generation progestin; metabolized to levonorgestrel (back to 3-keto).

Desogestrel and Etonogestrel — structural feature

Third-generation progestins; contain a methylidene group at the 11-position; desogestrel is the prodrug of etonogestrel.

Drospirenone (Yasmin) — structural features and activity

Fourth-generation progestin; two cyclopropane rings at positions 6,7 and 15,16; γ-lactone at position 17; does not possess androgenic activity.

Dienogest — structural features

Fourth-generation progestin; double bond between C-9 and C-10; 17α-propenonitrile group.

Mifepristone (RU-486) — mechanism and structural feature

Competitive PR antagonist; 11β-(4-dimethylaminophenyl) side chain destabilizes PR agonist conformation; used with misoprostol as abortifacient.

Ulipristal acetate (Ella) — comparison to mifepristone

More lipophilic due to methyl ketone and acetoxy groups at C-17; longer half-life; used as emergency contraceptive.

Ulipristal acetate — metabolism

Metabolized by CYP3A4 to mono-demethyl-ulipristal acetate (active) and di-demethyl-ulipristal acetate (inactive).

SERM pharmacophore — three required elements

(1) Three aromatic groups held by a nonsteroidal linker; (2) trans-aryl groups bearing a phenol or bioisostere; (3) cis-aryl group with a phenoxy-alkyl-basic tertiary amine chain.

Tamoxifen — prodrug activation

Activated by CYP2D6 to 4-hydroxytamoxifen (4-OH TAM); both tamoxifen and 4-OH TAM metabolized by CYP3A4 to the major active metabolite endoxifen (4-OH-NDM TAM). Active metabolites bind ER with up to 30× greater affinity.

Toremifene vs. tamoxifen — structural difference

Toremifene has a chlorine atom added to the ethyl side chain; metabolized by CYP3A4.

Ospemifene — origin and use

Dealkylation metabolite of toremifene; FDA-approved SERM for dyspareunia; agonist in vaginal epithelia, antagonist in breast and uterus.

Raloxifene — chemical class and indication

Benzothiophene SERM; FDA-approved chemopreventive to reduce risk of invasive breast cancer in postmenopausal women with osteoporosis or at high risk.

Bazedoxifene — structural relationship

Indole derivative of raloxifene (benzothiophene); binds ERα similarly.

Prasterone (DHEA/Intrarosa) — use and metabolism

Approved for dyspareunia due to vulvar/vaginal atrophy from menopause; metabolized to testosterone and then 17β-estradiol.

Aromatase inhibitors — mechanism of action (overview)

Bind to the Fe atom of the heme group in aromatase, preventing androgen binding and inhibiting enzyme turnover; used to treat breast cancers.

Aromatase catalytic mechanism — number of steps

Three successive hydroxylations using 3 moles each of NADPH and O₂; first two hydroxylate C-19 methyl to gem-diol; final step eliminates C-19 as formate to complete aromatization.

Letrozole — class and mechanism

Nonsteroidal reversible aromatase inhibitor; triazole ring binds via N-4 lone pair to the Fe atom of aromatase heme, blocking O₂ binding.

Anastrozole — class and structural feature

Nonsteroidal reversible aromatase inhibitor (benzyltriazole); two dimethylacetonitrile groups at both meta positions of benzene ring.

Exemestane (Aromasin) — class and mechanism

Steroidal irreversible 'suicide' inhibitor; resembles androstenedione; forms gem-diol intermediate then irreversibly captures a nucleophile from the enzyme. Differs from androstenedione by C1-C2 double bond and C-6 methylidene group.

Exemestane — pharmacokinetic note

Absorption is improved when taken following a high-fat meal.

Testosterone — oral activity

Ineffective orally due to extensive presystemic first-pass metabolism in GI mucosa and liver.

Testosterone IM esters (cypionate, undecanoate, propionate) — mechanism of prolonged action

Depot formulations; slowly hydrolyzed in vivo at injection site to release free testosterone over an extended period.

Testosterone metabolism — 5α-reductase product

Converts testosterone to 5α-dihydrotestosterone (DHT) in the prostate; DHT is the active metabolite.

Testosterone metabolism — aromatase product

Aromatase converts testosterone to 17β-estradiol.

Testosterone metabolism — 17β-HSD product

17β-Hydroxysteroid dehydrogenase converts testosterone to androstenedione (weaker androgen).

Testosterone metabolism — major urinary excretion product

Etiocholanolone (inactive, major), plus androsterone and epiandrosterone (inactive, minor); excreted as glucuronide/sulfate conjugates.

17α-Methyltestosterone — structural modification and effect

17α-methyl group reduces hepatic oxidative metabolism; oral bioavailability ~70%.

Fluoxymesterone — structural modification and potency

9α-fluoro group added to 17α-methyltestosterone analog; 20× anabolic and 10× androgenic activity vs. 17α-methyltestosterone; associated with sodium/water retention.

Androgen SAR — 17β-OH group role

Important for receptor binding via hydrogen bonding.

Androgen SAR — 17α-alkyl groups role

Small 17α-alkyl groups block C-17 metabolism, conferring oral activity with increased bioavailability and duration.

Androgen SAR — 3-ketone or 3α-OH role

Presence of either enhances androgenic activity.

Androgen SAR — 5β-reduction effect

Reduction to 5β-derivatives (etiocholane series) eliminates androgenic and anabolic activity; 5α-derivatives retain activity.

Androgen SAR — ring expansion/contraction effect

Significantly reduces or destroys androgenic and anabolic activities.

Androgen SAR — A-ring modifications for anabolic activity

Double bond at C-1, oxygen at position 2, and vinyl alcohol at C-2 all enhance anabolic activity.

19-Norandrogens — effect of removing C-19 methyl group

Reduces androgenic properties while retaining anabolic (tissue-building) properties.

Methandrostenolone — structural origin and properties

C-1,2 double bond added to 17α-methyltestosterone; severalfold increased anabolic activity; low androgenic activity; can produce mammogenic effects in men via estrogenic metabolites.

Oxandrolone — structural feature and activity

2-Oxasteroid analog of 17α-methyltestosterone containing a lactone in the A ring; slight androgenic activity.

Oxymetholone — structural feature

Vinyl alcohol group at position 2 of the A ring.

Nandrolone decanoate — classification and administration

Ester of 19-nortestosterone; given IM; slow hydrolysis releases 19-nortestosterone over a prolonged period; anabolic with longer duration of action.

PDE5 inhibitors — mechanism of action

Inhibit phosphodiesterase 5 (PDE5), which normally converts cGMP (active) → GMP (inactive). Inhibition sustains cGMP levels in corpus cavernosum → smooth muscle relaxation → vasodilation → erection.

Sildenafil and vardenafil — structural mimicry of cGMP

Modified purine ring system mimics the guanine ring of cGMP; other substituents mimic the ribose and phosphate.

Vardenafil vs. sildenafil — potency comparison

Vardenafil is ~20× more potent than sildenafil as a PDE5 inhibitor due to its imidazotriazinone heterocyclic ring system.

Tadalafil — duration of action and key structural feature

Duration up to 48 hours (vs. ~4 hr for sildenafil/vardenafil); 3,4-methylenedioxy substitution on the phenyl ring increases potency and duration; minimal presystemic metabolism.

Avanafil — onset of action and structural feature

Fastest onset (~15 min); pyrimidine derivative; (S)-enantiomer of 2-hydroxymethyl group on pyrrolidine is 7× more potent than (R)-enantiomer; extensively metabolized by CYP3A4.

PDE5 inhibitors — shared metabolic enzyme

All are metabolized by CYP3A4 (primary); sildenafil also involves CYP2C9, CYP2C19, CYP2D6.

Sildenafil — primary active metabolite

N-desmethylsildenafil (produced by CYP3A4); piperazine ring opening is a minor/inactive pathway.

Tadalafil — metabolites

Both CYP3A4 metabolites (catechol products) are inactive.

α1-Adrenergic antagonists for BPH — mechanism

Block α1A-adrenoceptors in prostate and urethral smooth muscle → muscle relaxation → relief of lower urinary tract symptoms (LUTS).

Quinazoline α1-blockers (alfuzosin, terazosin, doxazosin) — shared pharmacophore

All share a 4-amino-6,7-dimethoxyquinazoline ring system; differ at position 2 (piperazine vs. open chain) and in acyl substituents.

Tamsulosin — structural class and selectivity

Catecholamine-sulfonamide; selective α1A-adrenoceptor antagonist (uroselectivity); first-line BPH treatment; avoid in severe sulfa allergy.

Silodosin — selectivity

Most uroselective α1-adrenoceptor antagonist; indole carboxamide class.

5α-Reductase inhibitors (5ARIs) — mechanism

Suppress intraprostatic DHT production by inhibiting 5α-reductase (5AR), which converts testosterone → DHT; reduces prostate size.

Finasteride — selectivity and structural class

Selective inhibitor of 5AR2; 4-azasteroid-17-amide; mimics pathway of testosterone → DHT reduction.

Dutasteride — selectivity

Nonselective inhibitor of both 5AR1 and 5AR2; suppresses DHT more completely than finasteride.

Finasteride — mechanism of irreversible-like inhibition

Accepts hydride from NADPH at 1α-position; 4-aza group prevents tautomerization to keto; forms dihydrofinasteride-NADP adduct with t½ of 1 month.

Finasteride — major CYP3A4 metabolites

Monohydroxy finasteride (t-butyl side chain), then further oxidized via aldehyde intermediate to finasteride carboxylic acid.

Dutasteride — CYP3A4 metabolites

6′-hydroxydutasteride (comparable potency to parent), 4′-hydroxydutasteride and 1,2-dihydrodutasteride (less potent).

Finasteride and dutasteride — pregnancy handling warning

Can be absorbed through skin; capsules must not be handled by pregnant women or those who could become pregnant due to risk of fetal exposure.

5ARIs — onset of clinical effect

Slow onset; can take 6–12 months to exert maximum clinical effect.

BPH combination therapy indication

α1-adrenergic antagonist + 5ARI used when prostate volume is large (>40 g); α1-antagonists do not reduce prostate volume alone.