PCOL 826B Steroid Lecture 2 (2022) PDF

Medicinal Chemistry of Steroids Focus on Estrogens Steroid lecture 2 PCOL 826B [pages 1386- 1433 in Foye’s MedChem] [email protected] 1 Drugs targeting the estrogen receptor 2 oral contraception breast cancer chemoprevention and chemotherapy infertility hormone replacement therapy [HRT] o...

Medicinal Chemistry of Steroids Focus on Estrogens Steroid lecture 2 PCOL 826B [pages 1386- 1433 in Foye’s MedChem] [email protected] 1 Drugs targeting the estrogen receptor 2 oral contraception breast cancer chemoprevention and chemotherapy infertility hormone replacement therapy [HRT] osteoporosis Drugs targeting the Estrogen Receptor SERM [selective ER modulators] ER agonists OH natural synthetic H H H HO OH O H HO H H O3S O O H ER antagonists H O3S O 3 Estrogen Receptor alpha: an estrogen-activated transcription factor dimerized ERa DNA binding domains [DBD] with DNA: gray spheres are Zn atoms contained in DNA binding Zn fingers. 4 dimerized ERa ligandbinding domains [LBD] ERa & b fulvestrant mammary gland bone, vascular endothelial cells etc. 5 Steroid hormone receptors: the classic model The estrogen receptor (ER) is a ligand-activated transcription factor 6 estrogen receptor signaling 7 Exam question Select the correct statement that refers to estrogen receptors as drug targets: A. Activation of ERa occurs by proteolytic cleavage. B. Drug binding to ERa occurs at the Zinc finger domain. C. ERa is the natural antagonist of ERb. D. ERa is a ligand activated nuclear protease. E. ERa is a ligand-activated transcription factor. 8 Estrogen Receptor Signaling ERE: estrogen response element in promotor region of target genes ERa ó ERb Nuclear ó membrane ER; direct transcriptional activator ócoactivator 9 Ligand-induced ER-conformational changes A B Fully transcriptionally active partially transcriptionally active OH HO Crystallographic structures of diethylstilbestrol-bound (A), and raloxifen-bound (B) LBD of ERa. 10 Targeting ERa: Estradiol versus Tamoxifen transcriptionally active transcriptionally inactive Anti-oestrogens such as tamoxifen and fulvestrant (ICI-182780) act by binding to ER-alpha and causing a conformational change that prevents the recruitment of co-activators to ER-alpha. 11 ERa ligand binding I: Interactions with E2 and 4-OH-tamoxifen estradiol ER-inactive 4-OH-tamoxifen ER-ligand activated by hepatic O-demethylation Mestranol: 3-O-methyl-EE, a prodrug 12 17a-ethinylestradiol [EE]: metabolically stable estradiol ERa ligand binding II: Interactions with E2 and 4-OH-tamoxifen weak ER ligand potent ER-ligand CYP2D6 HO tamoxifen, the inactive prodrug 4-OH-tamoxifen, the active metabolite 13 Estrogen receptor ligands: estradiol versus raloxifene Arg394 Glu353 His524 Asp 351 14 Drugs targeting the estrogen receptor oral contraception breast cancer chemoprevention and chemotherapy infertility hormone replacement therapy [HRT] osteoporosis 15 Drugs targeting the Estrogen Receptor SERM [selective ER modulators] ER agonists OH natural synthetic H H H HO OH O H HO H H O3S O O H O3S O 16 H ER antagonists Endogenous Estrogens: relative potency and metaboIic interconversion E2 E1 E3 conjugation [glucuronidation/sulfation] excretion (bile /urine) E2: low oral bioavailability due to rapid hepatic metabolism into E1 and E3 ‘E2>E1>E3’ 17 Enterohepatic estrogen recycling Enterohepatic recycling through intestinal microbial deconjugation and reuptake of free estrogens enhances estrogen action. 18 SAR: estrogens - aromatic A ring; C3 hydroxyl group [=phenol group] - 17b-hydroxyl group, - distance between C3 and C17: 10.3 to 12.1 A), - planar hydrophobic scaffolding, - unsaturation in B ring enhances estrogenic potency, - 17a-ethinyl and 17a-vinyl substituent: oral availability through inhibition of D-ring directed metabolic inactivation. 17b-estradiol estrone 17a-ethinylestradiol CYP2D6 Diethylstilbestrol [DES] tamoxifen HO 4-OH-tamoxifen 19 Estradiol ester prodrugs [=cypionate] for i.m. administration; duration of action 14-28 d. 20 17a-Ethinylestradiol [EE]: Metabolically stable, oral estradiol 3-O-glucoronide first path metabolism 3-O-sulfate 2-OH-EE enterohepatic recycling 2-OCH3-EE 17a-ethinyl substituent: Ethinyl goup does not reduce estrogen receptor affinity. Serves as a metabolic shield that impedes D-ring hydroxylation. 3-O-glucoronide and 3-O-sulfate metabolites undergo enterohepatic recycling. Antibacterial agents potentially interfere with enterohepatic cycling of EE used as a major oral contraceptive (reduced efficacy). 21 Components of representative oral contraceptives EE 22 Synthetic steroidal estrogens with prolonged activity: EE-ether derivatives (mestranol, quinestrol) H quinestrol ethinyl estradiol 3-cyclopentyl ether) R = H: 17a-ethinylestradiol (chemically inert ethinyl-C17 substituent blocks oxidative metabolism at D-ring) R = methyl: mestranol prodrugs activated by hepatic O-dealkylation R = cyclopentyl : quinestrol Lipophilic EE-ether derivatives display prolonged estrogenic activity after oral dosing attributed to deposition in adipose tissue from which they are released slowly and converted to EE. Metabolic O-demethylation essential for mestranol /quinestrol estrogenicity. 23 Conjugated Equine Estrogens: CEE The equine estrogens contain one or two additional double bonds in the steroidal B ring leading to potentiation of estrogenic activity. ‘estropipate’ Conjugated equine estrogen CEE: ring B saturated (45% estrone sulfate), ring B unsaturated components (25% equilin sulfate; 15% 17a-dihydroequilin). O ‘estronesulfate, E1-S’ H O3S O 24 H enterohepatic recycling of E1-S Exam question O OH 2 H 1 H H H H H O3S O O O 4 3 H 5 H O3S O Five estrogenic agents that target the estrogen receptor are shown above. Identify those that can undergo enterohepatic recycling: A. 1, 2, 3 B. 2, 3, 4 C. 1, 3, 5 D. 1, 2, 3, 4 E. 1, 2, 3, 4, 5 25 Hormonal Replacement Therapy for the Treatment of Menopausal Symptoms Route of administration: oral, transdermal, intravaginal estrogen only [‘unopposed estrogen’]: e.g. estradiol acetate, conjugated equine estrogens [CEE], micronized 17b-estradiol, estropipate estrogen/progestin combinations: e.g. CEE + medroxyprogesterone acetate (MPA), ethynylestradiol [EE] + norethindrone 26 estrogen transdermal patches Estradiol transdermal system, is designed to release estradiol through a rate-limiting membrane continuously upon application to intact skin. Estradiol 0.05 mg/day, 0.1 mg/day; transdermal patch. 27 Transdermal patch for systemic delivery cardiovascular agents glyceryl trinitrate analgesics fentanyl (opioid) hormones estrogen androgen contraceptives modulators of neutransmission scopolamine (anti-nausea) nicotine (withdrawal symptoms) The first commercially available prescription patch was approved by the U.S. Food and Drug Administration in December 1979, which administered scopolamine for motion sickness. 28 Estrogen Interconversion and Oxidative Metabolism D-ring hydroxylation D A A-ring hydroxylation O-methylation interconversion Catecholic extradiol metabolites methoxyestradiols only weakly estrogenic 29 estrogen-metabolites as genotoxic intermediates 30 Long term administration of steroidal estrogens including estradiol and estrone has been associated with a slight increase in the incidence of reproductive tract cancer. It is generally believed that metabolic activation of estradiol leading to the formation of catechol estrogens is a prerequisite for its genotoxic activity. Formation of electrophilic intermediates during estrogen metabolism induces DNA base adduction by quinone-estrogens. Estrogenic Compounds 2 3 endogenous / synthetic prodrug conjugated estrogen 4 OH H SERM enterohepatic recycling H H 1 HO O O H H H H O3S O3S 7 O 5 O H 6 31 Drugs targeting the Estrogen Receptor SERM [selective ER modulators] ER agonists OH natural synthetic H H H HO OH O H HO H H O3S O O H O3S O 32 H ER antagonists Targeting ER transcriptional activity agonist [SERM] [antagonist] Hsp90 Mechanism of action of estradiol, tamoxifen and fulvestrant at the level of transcriptional regulation. Estradiol binds to the ER, leading to dimerization, conformational change and binding to estrogen response elements [EREs]. Tamoxifen competes with estradiol for ER binding whereas fulvestrant binds, blocks and degrades the ER, leading to complete inhibition of estrogen signaling. AF1, AF2: ER activation domains 1 and 2 (‘activating function 1 and 2’, respectively). 33 Tissue-associated estrogenic activities of various estrogen receptor ligands a pure ER antagonist competitive antagonist interferes with ER a dimerization causes ER a proteasomal degradation [ICI 182780, 2002 FDA-approved] Fulvestrant (ICI-182780, FDA approved 2002 is a pure estrogen antagonist without agonist properties. Fulvestrant binds to oestrogen receptors (ERs) competing with estradiol and also induces estrogen receptor degradation; used to treat estrogen receptor-positive metastatic breast cancer in postmenopausal women when other hormonal therapies, such as tamoxifen, have failed. 34 Exam question Fulvestrant is a potent anti-estrogen. Identify the correct statement: A. Fulvestrant is an estradiol-derivative substituted at C19. B. Fulvestrant is a non-aromatic estradiol-derivative. C. Fulvestrant is a nonsteroidal anti-estrogen. D. Fulvestrant bears a penta-fluorinated alkyl-substituent E. Fulvestrant is more water soluble than estradiol. 35 Tissue-associated estrogenic activities of various estrogen receptor ligands 36 Selective Estrogen Receptor Modulators [SERMS] 37 Estrogen receptor ligands: estradiol versus raloxifene 38 Selective estrogen receptor modulators [SERMS]: percent response Targeting osteoporosis without unwanted estrogenic effects 39 Tamoxifen and beyond. The clinical impact of tamoxifen and an understanding of its pharmacology has facilitated the clinical development of selective oestrogen-receptor modulators (SERMs) and the introduction of third-generation aromatase inhibitors (anastrozole, letrozole and exemestane) and a new pure anti-oestrogen (fulvestrant). Most importantly, the SERM principle—exemplified by the clinical usefulness of raloxifene to prevent osteoporosis, but with breast and endometrial safety—is being applied to other members of the steroid hormone superfamily of receptors. 40 Tamoxifen: A chemopreventive/chemotherapeutic anti-cancer drug Tamoxifen is currently used for the treatment of both early and advanced estrogen receptor positive breast cancer in pre- and post-menopausal women. It is also approved by the FDA for the prevention of breast cancer in women at high risk of developing the disease. Tamoxifen is a prodrug with little affinity for the estrogen receptor metabolized in the liver by the cytochrome P450 isoform CYP2D6 into the active metabolite 4-hydroxytamoxifen. In breast tissue, 4-hydroxytamoxifen acts as an antagonist so that transcription of estrogen-responsive genes is inhibited. Patients with variant forms of the gene CYP2D6 (also called simply 2D6) may not receive full benefit from tamoxifen because of too slow metabolism of the tamoxifen prodrug into its active metabolite 4-hydroxytamoxifen. Even though it is an antagonist in breast tissue it acts as partial agonist on the endometrium and has been linked to endometrial cancer in some women. Moreover, its anti-osteoporotic effects resulting from estrogenic activity on bone tissue have been recognized. 41 Tamoxifen Metabolism Tamoxifen (TAM) has been shown to be metabolized by CYP2D6 to the more potent metabolites 4-OH-TAM and endoxifen. Poor metabolizers of tamoxifen have lower levels of endoxifen and poorer clinical outcomes as compared to extensive metabolizers. 42 Raloxifene: SERM targeting osteoporosis and breast carcinogenesis Raloxifene, a selective estrogen receptor modulator [SERM] is approved for treatment and prevention of osteoporosis in postmenopausal women. In September 2007 the FDA approved the use of raloxifene as chemopreventive medication against invasive breast cancer in postmenopausal women. 43