Screening of Endocrine and Hormonal Drugs

Introduction, ablate-and-replace strategy & general principles of hormone bioassay

• Definition — Endocrine / hormonal drug screening is the systematic experimental evaluation of natural and synthetic hormone agonists, antagonists and modulators using in-vitro and in-vivo biological models to establish their potency, agonist/antagonist profile and relative activity against a defined reference standard.
• Ablate-and-replace — The classical evaluation backbone for any endocrine target: surgically remove the hormone-producing gland (adrenalectomy, ovariectomy, orchiectomy, thyroidectomy, hypophysectomy, parathyroidectomy) to abolish endogenous hormone, then substitute exogenous extract or synthetic hormone and measure the restored end-organ response; post-operative electrolyte/hormone support is essential (e.g. 1% NaCl after adrenalectomy).
• Bioassay principle — A bioassay quantifies the biological activity of an unknown hormone preparation by comparing the magnitude of a defined biological response it evokes against that of a reference standard of known potency under identical conditions.
• Reference standards — Each class runs against its own standard:
  • Estrogens → estradiol-17β / estradiol benzoate; progestogens → progesterone (± medroxyprogesterone acetate).
  • Androgens/anabolics → testosterone (s.c.) and methyltestosterone (oral); thyroid → L-thyroxine (T₄) and L-triiodothyronine (T₃), with propylthiouracil (PTU) as the goitrogen/peroxidase-inhibitor reference.
  • Glucocorticoids → hydrocortisone (cortisol) / dexamethasone (in-vitro receptor); mineralocorticoids → deoxycorticosterone acetate (DOCA) / aldosterone; oxytocics → oxytocin.
• Assay design — Log dose–response curves for standard and test are constructed and potency ratios with confidence limits derived; symmetrical parallel-line 2+2 and 3+3 point assays (occasionally 4-point) are standard; validity requires the log dose–response lines of standard and test to be linear and parallel; Latin-square randomisation where each animal receives repeated doses.
• Screening tiers — In-vitro — receptor-binding (radioligand competition), transactivation/reporter-gene, cell-proliferation and enzyme-induction assays; in-vivo — gland-ablation + replacement, target-organ weight change, secretory/histological transformation, and functional physiological endpoints (blood pressure, milk ejection, metabolic rate).