Introduction to Endocrine Pharmacology – Part II & Bioassay
This final unit completes endocrine pharmacology with the Sex Hormones (Androgens, Estrogens, Progestins), their therapeutic applications (hormone replacement, oral contraception), and drugs acting on the uterus (Oxytocics for labor induction, Tocolytics for premature labor). It concludes with Bioassay — the quantitative determination of drug potency using biological systems (living organisms or isolated tissues). Bioassay is a unique pharmacological tool that bridges laboratory measurement with clinical dosing.
Syllabus & Topics
- 1Androgens – Testosterone: Primary male sex hormone from Leydig cells. Actions: (1) Development of male secondary sexual characteristics (voice deepening, facial hair, muscle development). (2) Anabolic effects (↑protein synthesis, ↑muscle mass, ↑bone density). (3) Spermatogenesis (with FSH). (4) CNS effects (libido, aggression). Poorly effective orally (extensive first-pass). Preparations: IM (Testosterone enanthate/cypionate), Transdermal patches/gels, Oral (Methyltestosterone, Fluoxymesterone – but hepatotoxic).
- 2Anabolic Steroids: Nandrolone decanoate, Stanozolol – modified testosterone analogues with enhanced anabolic (muscle-building) and reduced androgenic (virilizing) ratio. Uses: chronic wasting diseases, severe anemia (↑erythropoietin), osteoporosis. Misuse in sports (doping) → banned by WADA. ADRs: hepatotoxicity (peliosis hepatis), atherosclerosis, testicular atrophy, virilization in women, premature epiphyseal closure in children.
- 3Anti-androgens: Flutamide, Bicalutamide (androgen receptor blockers – prostate cancer). Finasteride (5α-reductase inhibitor → blocks testosterone → DHT conversion – used for BPH and male pattern baldness). Cyproterone acetate (antiandrogen + progestational – used for hirsutism in women, precocious puberty).
- 4Estrogens – Pharmacology: Estradiol (E₂, most potent natural estrogen), Estrone (E₁), Estriol (E₃, weakest). Synthetic: Ethinylestradiol (oral contraceptive component), Diethylstilbestrol (DES – non-steroidal, historically used). Actions: female secondary sexual characteristics, endometrial growth (proliferative phase), bone protection (↓bone resorption), ↑HDL/↓LDL. Uses: HRT (postmenopausal), oral contraception, osteoporosis prevention.
- 5Anti-estrogens: Tamoxifen (SERM – Selective Estrogen Receptor Modulator: antagonist in breast, agonist in bone/uterus. Used in ER+ breast cancer). Raloxifene (SERM: agonist in bone, antagonist in breast + uterus – used for osteoporosis). Clomiphene (antiestrogen at hypothalamic level → blocks negative feedback → ↑FSH/LH → ovulation induction for infertility). Aromatase inhibitors (Letrozole, Anastrozole – block estrogen synthesis in postmenopausal breast cancer).
- 6Progesterone – Pharmacology: From corpus luteum. Actions: prepares endometrium for implantation (secretory phase), maintains pregnancy, thermogenic (↑body temperature), anti-estrogenic on endometrium. Synthetic progestins: Norethindrone, Levonorgestrel (used in OC pills, stronger progestational activity).
- 7Oral Contraceptives: Combined OC pills (Estrogen + Progestin): suppress ovulation (↓FSH/LH by negative feedback) + alter cervical mucus + make endometrium hostile. Progestin-only pills (minipill – for breastfeeding women, no estrogen-related risks). Emergency contraception: Levonorgestrel 1.5 mg single dose within 72 hours (Plan B). ADRs: DVT/thromboembolism (estrogen → ↑clotting factors), hypertension, headache, weight gain, breakthrough bleeding.
- 8Mifepristone (RU-486): Progesterone receptor antagonist + partial glucocorticoid antagonist. Used with Misoprostol (PGE₁ analogue) for medical termination of pregnancy (up to 9 weeks). Mechanism: blocks progesterone → endometrial shedding; Misoprostol → uterine contractions → expulsion.
- 9Drugs Acting on Uterus – Oxytocics: Oxytocin (posterior pituitary hormone – IV infusion for labor induction, IM for postpartum hemorrhage. Stimulates uterine smooth muscle contraction. ADR: water intoxication at high doses due to ADH-like activity). Ergometrine/Methylergometrine (ergot alkaloid – sustained uterine contraction → controls postpartum hemorrhage. NOT for induction – causes tetanic contraction). Prostaglandins: PGE₂ (Dinoprostone – cervical ripening), PGF₂α (Carboprost – refractory postpartum hemorrhage, mid-trimester abortion).
- 10Drugs Acting on Uterus – Tocolytics: Inhibit premature uterine contractions. β₂-agonists (Ritodrine, Isoxsuprine – relax uterine smooth muscle. ADR: tachycardia, hyperglycemia), MgSO₄ (↓Ca²⁺ entry → muscle relaxation, also neuroprotective for premature fetus), Nifedipine (CCB – relaxes uterine smooth muscle), Atosiban (oxytocin receptor antagonist).
- 11Bioassay – Principles: Estimation of the potency/concentration of a substance (drug/hormone/toxin) by measuring its biological effect on a living organism or isolated tissue. Necessary when: (1) Chemical assay cannot distinguish active from inactive forms. (2) Chemical composition is unknown (crude extracts). (3) Drug acts through complex biological pathways. Standard drug (S) vs Test preparation (T) compared using dose-response curves.
- 12Bioassay – Types: (1) Quantal assay (all-or-none response – e.g., death, convulsion. LD₅₀ determination). (2) Graded assay (measurable response – e.g., blood pressure change, blood glucose level). (3) Direct assay (measure dose of S and T producing identical response). (4) Indirect assay – 3-point assay (one dose of S, one dose of T) and 4-point assay (two doses each of S and T – more accurate, statistical validation possible).
- 13Bioassay of Insulin: Rabbit method – fasting rabbits injected with standard and test insulin preparations. Blood glucose measured at intervals. Equal hypoglycemic response indicates equal potency. 1 IU of insulin = amount required to produce convulsive hypoglycemia in a 2 kg fasting rabbit.
- 14Bioassay of Other Drugs: Oxytocin (isolated rat uterus – contraction measured on kymograph). Vasopressin (antidiuretic activity on rats – urine volume measurement; or pressor activity on blood pressure). ACTH (adrenal ascorbic acid depletion in rats). d-Tubocurarine (head-drop method in rabbits – neuromuscular block). Digitalis (lethal dose in pigeons/cats or Isolated frog heart). Histamine (guinea pig ileum contraction on organ bath). 5-HT (rat fundus strip contraction).
Learning Objectives
Frequently Asked Questions (FAQs)
Q1. How Do Combined Oral Contraceptives Prevent Pregnancy?
Combined oral contraceptives prevent pregnancy through three main mechanisms. First, estrogen and progestin suppress ovulation by exerting negative feedback on the hypothalamus and pituitary, reducing follicle-stimulating hormone (FSH) and preventing the luteinizing hormone (LH) surge required for ovulation. Second, progestin thickens cervical mucus, making it difficult for sperm to penetrate. Third, the endometrium becomes thin and atrophic, creating an environment unfavorable for implantation.
Q2. What Is the Difference Between Oxytocin and Ergometrine?
Oxytocin produces rhythmic uterine contractions similar to natural labor and is commonly used for induction or augmentation of labor. Ergometrine produces sustained, tetanic uterine contractions and is mainly used to control postpartum hemorrhage by compressing uterine blood vessels. Oxytocin acts on oxytocin receptors, while ergometrine acts on adrenergic and serotonin receptors in uterine smooth muscle.
Q3. Why Is Bioassay Still Important When Chemical Assays Exist?
Bioassay remains important because it measures the actual biological activity of a substance rather than only its chemical concentration. Chemical assays may detect inactive or degraded molecules, whereas bioassays confirm whether the compound produces the expected physiological effect. Bioassays are especially useful for biological products such as vaccines, sera, and hormones like Insulin, and for complex extracts where the active component is not fully characterized.
Q4. What Is a 4-Point Bioassay?
A four-point (2+2) bioassay compares two dose levels of a standard preparation (S₁ and S₂) with two dose levels of a test preparation (T₁ and T₂). These four responses are used to construct parallel log dose–response curves. The horizontal distance between the curves indicates the relative potency of the test compared with the standard. This method allows verification of parallelism and calculation of confidence limits, making it statistically reliable and widely recommended in pharmacopoeial standards.
Q5. What Is the Clomiphene Challenge and How Does It Work?
Clomiphene is a non-steroidal anti-estrogen that blocks estrogen receptors in the hypothalamus. This reduces estrogen’s negative feedback, increasing release of gonadotropin-releasing hormone (GnRH) and consequently raising pituitary secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The increased gonadotropins stimulate ovarian follicular development and ovulation, making clomiphene a first-line treatment for anovulatory infertility such as that seen in Polycystic Ovary Syndrome.