Introduction to Drugs Acting on the Endocrine System (Steroids)
This unit dives deep into the chemistry of steroids — one of the most important molecular families in pharmacology. Starting with the ABCD ring system nomenclature and stereochemistry, it covers the sex hormones (Androgens, Estrogens, Progestins), drugs for erectile dysfunction, oral contraceptives, anti-inflammatory Corticosteroids, and Thyroid/Antithyroid drugs. Understanding how minute structural modifications to the steroid nucleus completely alter biological activity is the key to mastering this unit.
Syllabus & Topics
- 1Steroid Nomenclature: The steroid skeleton consists of the cyclopentanoperhydrophenanthrene (ABCD) ring system – three six-membered rings (A, B, C) and one five-membered ring (D). 17 carbon atoms in the basic nucleus. Naming: Estrane (C18), Androstane (C19), Pregnane (C21).
- 2Steroid Stereochemistry: The ABCD rings can fuse in cis or trans configurations. All natural active steroids have trans B/C and trans C/D ring junctions. Angular methyl groups at C-10 and C-13 are β-oriented (above the plane). The 5α (trans A/B) and 5β (cis A/B) configurations dramatically alter biological activity.
- 3Steroid Metabolism: Steroids are metabolized primarily in the liver by reduction of the Δ4-3-ketone group (Ring A), hydroxylation, and conjugation (glucuronidation/sulfation) for renal excretion.
- 4Androgens – Testosterone: The primary male sex hormone. Synthesized in Leydig cells of testes. Responsible for development of male secondary sexual characteristics, anabolic effects on muscle/bone. Poor oral bioavailability (extensive first-pass metabolism).
- 5Androgens – Nandrolone: A 19-nortestosterone derivative (lacks the C-19 methyl group). Exhibits higher anabolic:androgenic ratio than testosterone. Used clinically for anemia and wasting diseases.
- 6Estrogens – Estradiol, Estrone, Estriol: Estradiol (E2) is the most potent natural estrogen. Aromatized Ring A (phenolic A ring) is the hallmark structural feature. Estrone (E1) is the primary post-menopausal estrogen. Estriol (E3) is the weakest but predominant during pregnancy.
- 7Synthetic Estrogen – Diethylstilbestrol (DES): Non-steroidal synthetic estrogen. Despite lacking the steroid ring, the distance between the two hydroxyl groups mimics the spatial geometry of estradiol → fits the estrogen receptor. Historically infamous for causing clear cell vaginal adenocarcinoma in daughters of women who took DES during pregnancy.
- 8Progestins – Progesterone: The key C21 steroid hormone from the corpus luteum. Prepares the endometrium for implantation. Poor oral bioavailability. Synthetic progestins (Norgestrel, Levonorgestrel) have greatly improved oral activity.
- 9Drugs for Erectile Dysfunction: Sildenafil (Viagra) and Tadalafil (Cialis) – selectively inhibit Phosphodiesterase type 5 (PDE5) → prevent breakdown of cGMP in corpus cavernosum → sustained smooth muscle relaxation → increased blood flow → erection. Tadalafil has exceptionally long half-life (~17.5 hours).
- 10Oral Contraceptives: Mifepristone (RU-486 – progesterone receptor antagonist, medical abortion), Norgestrel and Levonorgestrel (potent synthetic progestins used in combined oral contraceptive pills and emergency contraception).
- 11Corticosteroids – Glucocorticoids: Cortisone (inactive prodrug → Hydrocortisone), Hydrocortisone (cortisol – prototype, both anti-inflammatory and mineralocorticoid activity), Prednisolone (Δ1 double bond → 4x more potent, less mineralocorticoid), Betamethasone and Dexamethasone (9α-fluoro + 16-methyl → 25-30x potent, negligible mineralocorticoid activity).
- 12SAR of Corticosteroids: Δ4-3-keto group essential for activity. 11β-OH essential for glucocorticoid activity. 17α-OH increases glucocorticoid potency. 9α-F increases anti-inflammatory potency enormously. 16α/β methyl eliminates salt-retaining (mineralocorticoid) property.
- 13Thyroid Hormones: L-Thyroxine (T4, prohormone – 4 iodines) and L-Thyronine (T3, active form – 3 iodines). Synthesized in thyroid gland from Thyroglobulin + Iodine. Regulate basal metabolic rate, growth, and development.
- 14Antithyroid Drugs: Propylthiouracil (PTU – inhibits thyroid peroxidase + blocks peripheral T4→T3 conversion) and Methimazole/Carbimazole (inhibit thyroid peroxidase – organification and coupling steps). Thioureylene drugs used for hyperthyroidism (Graves’ disease).
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. What Is the Structural Difference Between Testosterone and Estradiol?
Testosterone (C19) contains a Δ4-3-keto group in Ring A and a 17β-hydroxyl group. Estradiol (C18) has an aromatized phenolic Ring A with a 3-hydroxyl group and lacks the C-19 angular methyl group, which is removed during aromatization by the enzyme Aromatase. The aromatic A ring is essential for binding to estrogen receptors.
Q2. Why Is Dexamethasone Much More Potent Than Hydrocortisone?
Dexamethasone has three key structural modifications compared to Hydrocortisone: a Δ1 double bond in Ring A that enhances glucocorticoid activity, a 9α-fluorine substitution that increases receptor affinity and anti-inflammatory potency, and a 16α-methyl group that eliminates mineralocorticoid activity. These modifications make dexamethasone approximately 25–30 times more potent as an anti-inflammatory agent with minimal sodium-retaining effects.
Q3. How Does Sildenafil Work?
Sildenafil inhibits phosphodiesterase type 5 (PDE5), the enzyme responsible for degrading cGMP in the corpus cavernosum. During sexual stimulation, nitric oxide (NO) increases cGMP levels, leading to smooth muscle relaxation and erection. By preventing cGMP breakdown, sildenafil enhances and prolongs the natural NO–cGMP pathway but does not initiate erection in the absence of sexual stimulation.
Q4. What Is Mifepristone and How Does It Work?
Mifepristone (RU-486) is a synthetic steroid that antagonizes progesterone receptors in the uterus. By blocking progesterone action, it disrupts maintenance of the endometrium, leading to detachment of the implanted embryo. It is commonly used in combination with Misoprostol for medical termination of early pregnancy.
Q5. How Do Antithyroid Drugs PTU and Methimazole Differ?
Propylthiouracil (PTU) and Methimazole both inhibit thyroid peroxidase (TPO), blocking iodination and coupling reactions in thyroid hormone synthesis. PTU additionally inhibits peripheral conversion of T4 to T3 by blocking type 1 deiodinase, making it preferred in thyroid storm. Methimazole is generally preferred for routine treatment of hyperthyroidism due to greater potency and longer duration of action.